...transport properties of amorphous hydrogenated boron carbide-蚂蚁淘生物

(PDF) The influence of hydrogen on the chemical, mechanical, optical/electronic, and electrical transport properties of amorphous hydrogenated boron carbideArticlePDF AvailableThe influence of hydrogen on the chemical, mechanical, optical/electronic, and electrical transport properties of amorphous hydrogenated boron carbideJuly 2015Journal of Applied Physics 118(3):035703DOI:10.1063/1.4927037Authors: Bradley J. NordellUniversity of Missouri - Kansas City Sudarshan KarkiUniversity of Oregon Thuong NguyenUniversity of Missouri - Kansas City Paul RulisUniversity of Missouri - Kansas CityShow all 12 authorsHide Download full-text PDFRead full-textDownload full-text PDFRead full-textDownload citation Copy link Link copied Read full-text Download citation Copy link Link copiedCitations (27)References (221)Figures (5)Abstract and FiguresBecause of its high electrical resistivity, low dielectric constant (κ), high thermal neutron capture cross section, and robust chemical, thermal, and mechanical properties, amorphous hydrogenated boron carbide (a-BxC:Hy) has garnered interest as a material for low-κ dielectric and solid-state neutron detection applications. Herein, we investigate the relationships between chemical structure (atomic concentration B, C, H, and O), physical/mechanical properties (density, porosity, hardness, and Young s modulus), electronic structure [band gap, Urbach energy (EU), and Tauc parameter (B1/2)], optical/dielectric properties (frequency-dependent dielectric constant), and electrical transport properties (resistivity and leakage current) through the analysis of a large series of a-BxC:Hy thin films grown by plasma-enhanced chemical vapor deposition from ortho-carborane. The resulting films exhibit a wide range of properties including H concentration from 10% to 45%, density from 0.9 to 2.3g/cm3, Young s modulus from 10 to 340GPa, band gap from 1.7 to 3.8eV, Urbach energy from 0.1 to 0.7eV, dielectric constant from 3.1 to 7.6, and electrical resistivity from 1010 to 1015 Ω cm. Hydrogen concentration is found to correlate directly with thin-film density, and both are used to map and explain the other material properties. Hardness and Young s modulus exhibit a direct power law relationship with density above ∼1.3g/cm3 (or below ∼35% H), below which they plateau, providing evidence for a rigidity percolation threshold. An increase in band gap and decrease in dielectric constant with increasing H concentration are explained by a decrease in network connectivity as well as mass/electron density. An increase in disorder, as measured by the parameters EU and B1/2, with increasing H concentration is explained by the release of strain in the network and associated decrease in structural disorder. All of these correlations in a-BxC:Hy are found to be very similar to those observed in amorphous hydrogenated silicon (a-Si:H), which suggests parallels between the influence of hydrogenation on their material properties and possible avenues for optimization. Finally, an increase in electrical resistivity with increasing H at 35 at. % H concentration is explained, not by disorder as in a-Si:H, but rather by a lower rate of hopping associated with a lower density of sites, assuming a variable range hopping mechanism interpreted in the framework of percolation theory. FTIR spectra for (a) ortho-carborane and (b) representative highdensity (B4, 2.3 g/cm 3 , a-B 4.6 C:H 0.7 ) and low-density (D18, 0.9 g/cm 3 , aB 3.5 CO 0.5 :H 3.5 ) films.… Hypothetical models illustrating the local physical structure of amorphous hydrogenated boron carbide (boron ¼ pink, carbon ¼ gray, and hydrogen ¼ white) in different density extremes: (a) low-density (0.9 g/cm 3 ) films with approximate stoichiometry of a-B 3.5 C:H 3 and pores with diameters on the order of 0.7 nm (compare to D18/D19), and (b) high-density (2.4 g/cm 3 ) non-porous films with stoichiometry of a-B 4.5 C:H (compare to B4).… Growth temperature/power response surface curves for: (a) Tauc optical band gap, E Tauc (n ¼ 2), (b) Tauc slope parameter, B 1/2 , (c) Urbach energy, E U , (d) total dielectric constant, j, (e) the electronic contribution to the dielectric constant, e 1 , and (f) the orientation/distortion contribution to the dielectric constant, j À e 1 .… Growth temperature/power response surface curves for: (a) B/C ratio, (b) at. % O, (c) at. % H, (d) density, (e) hardness, and (f) Young s modulus. The black circles represent actual data points and the colored surfaces the Kriging metamodel fit.… Representation of parameter space covered in present study by varying growth temperature and R.F. power while holding remaining growth conditions constant.… Figures - uploaded by Dhanadeep DuttaAuthor contentAll figure content in this area was uploaded by Dhanadeep DuttaContent may be subject to copyright. Discover the world s research20+ million members135+ million publications700k+ research projectsJoin for freePublic Full-text 1Content uploaded by Dhanadeep DuttaAuthor contentAll content in this area was uploaded by Dhanadeep Dutta on Oct 05, 2015 Content may be subject to copyright. The influence of hydrogen on the chemical, mechanical, optical/electronic, andelectrical transport properties of amorphous hydrogenated boron carbideBradley J. Nordell, Sudarshan Karki, Thuong D. Nguyen, Paul Rulis, A. N. Caruso, Sudhaunshu S. Purohit, HanLi, Sean W. King, Dhanadeep Dutta, David Gidley, William A. Lanford, and Michelle M. Paquette Citation: Journal of Applied Physics 118, 035703 (2015); doi: 10.1063/1.4927037 View online: http://dx.doi.org/10.1063/1.4927037 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/118/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effects of condensation reactions on the structural, mechanical, and electrical properties of plasma-depositedorganosilicon thin films from octamethylcyclotetrasiloxane J. Appl. Phys. 97, 113707 (2005); 10.1063/1.1923163 Dielectric properties of amorphous hydrogenated silicon carbide thin films grown by plasma-enhanced chemicalvapor deposition J. Appl. Phys. 93, 4066 (2003); 10.1063/1.1555676 Intermittent chemical vapor deposition of thick electrically conductive diamond-like amorphous carbon films usingi- C 4 H 10 / N 2 supermagnetron plasma J. Vac. Sci. Technol. A 20, 403 (2002); 10.1116/1.1446446 Influence of the plasma pressure on the microstructure and on the optical and mechanical properties ofamorphous carbon films deposited by direct current magnetron sputtering J. Vac. Sci. Technol. A 17, 2841 (1999); 10.1116/1.582022 Effects of deposition temperature on the properties of hydrogenated tetrahedral amorphous carbon J. Appl. Phys. 82, 4566 (1997); 10.1063/1.366193 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 The influence of hydrogen on the chemical, mechanical, optical/electronic,and electrical transport properties of amorphous hydrogenatedboron carbideBradley J. Nordell,1Sudarshan Karki,1Thuong D. Nguyen,1Paul Rulis,1A. N. Caruso,1Sudhaunshu S. Purohit,2Han Li,3Sean W. King,3Dhanadeep Dutta,4,a)David Gidley,4William A. Lanford,5and Michelle M. Paquette1,b)1Department of Physics and Astronomy, University of Missouri-Kansas City, Kansas City,Missouri 64110, USA2Department of Chemistry, University of Missouri-Kansas City, Kansas City, Missouri 64110, USA3Logic Technology Development, Intel Corporation, Hillsboro, Oregon 97124, USA4Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA5Department of Physics, University at Albany, Albany, New York 12222, USA(Received 27 April 2015; accepted 7 July 2015; published online 20 July 2015)Because of its high electrical resistivity, low dielectric constant (j), high thermal neutron capturecross section, and robust chemical, thermal, and mechanical properties, amorphous hydrogenatedboron carbide (a-BxC:Hy) has garnered interest as a material for low-jdielectric and solid-stateneutron detection applications. Herein, we investigate the relationships between chemical structure(atomic concentration B, C, H, and O), physical/mechanical properties (density, porosity, hardness,and Young’s modulus), electronic structure [band gap, Urbach energy (EU), and Tauc parameter(B1/2)], optical/dielectric properties (frequency-dependent dielectric constant), and electricaltransport properties (resistivity and leakage current) through the analysis of a large series ofa-BxC:Hythin ﬁlms grown by plasma-enhanced chemical vapor deposition from ortho-carborane.The resulting ﬁlms exhibit a wide range of properties including H concentration from 10% to 45%,density from 0.9 to 2.3 g/cm3, Young’s modulus from 10 to 340 GPa, band gap from 1.7 to 3.8 eV,Urbach energy from 0.1 to 0.7 eV, dielectric constant from 3.1 to 7.6, and electrical resistivity from1010to 1015Xcm. Hydrogen concentration is found to correlate directly with thin-ﬁlm density, andboth are used to map and explain the other material properties. Hardness and Young’s modulusexhibit a direct power law relationship with density above 1.3 g/cm3(or below 35% H), belowwhich they plateau, providing evidence for a rigidity percolation threshold. An increase in bandgap and decrease in dielectric constant with increasing H concentration are explained by a decreasein network connectivity as well as mass/electron density. An increase in disorder, as measured bythe parameters EUand B1/2, with increasing H concentration is explained by the release of strain inthe network and associated decrease in structural disorder. All of these correlations in a-BxC:Hyarefound to be very similar to those observed in amorphous hydrogenated silicon (a-Si:H), whichsuggests parallels between the inﬂuence of hydrogenation on their material properties and possibleavenues for optimization. Finally, an increase in electrical resistivity with increasing H at 35 at.% H concentration is explained, not by disorder as in a-Si:H, but rather by a lower rate of hoppingassociated with a lower density of sites, assuming a variable range hopping mechanism interpretedin the framework of percolation theory. VC2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4927037]I. INTRODUCTIONResearch on low-Zboron-rich boron carbide (BC) mate-rials continues to be motivated by their unique mechanical,thermal, chemical, optical, and electronic properties.1–6Inparticular, thin-ﬁlm boron carbide has emerged as a promisingmoderately high bandgap (2–4 eV) semi-insulating nanoelec-tronic material for direct-conversion solid-state neutron detec-tors7–11and low-dielectric-constant (low-j) intra/interlayerdielectrics,12,13as well as a candidate material for specializedcoatings.14–17For these and other applications to reach tech-nological maturity, a greater understanding of the relationshipbetween chemical structure and technologically relevant ma-terial properties, and how these can ultimately be controlledby processing conditions, is essential.A variety of techniques have been developed to fabricateboron-carbide-based solids,18,19with the resulting materialsspanning the BC(H) phase diagram5,20,21and potentialenergy landscape.22Growth techniques such as pressure andpressureless sintering, hot pressed pellet, and hot isostaticpressing,23–25which apply high growth temperatures(T 2200 C) and/or pressures (P 50 Torr), have been useda)Current address: Radiochemistry Division, Bhabha Atomic ResearchCentre, Trombay, Mumbai 400 085, India.b)Author to whom correspondence should be addressed. Electronic mail:paquettem@umkc.edu0021-8979/2015/118(3)/035703/16/$30.00 VC2015 AIP Publishing LLC118, 035703-1JOURNAL OF APPLIED PHYSICS 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 to produce single or polycrystalline BxC, typically with astoichiometry of B4C, exhibiting the extreme propertiesneeded for traditional applications such as armor and abra-sives. The problem with employing such bulk growth techni-ques for the specialized semiconductor device applicationsdescribed above is that they require harsh fabrication condi-tions, demonstrate limited tunability, and tend to yieldunfavorable electronic properties such as unacceptably highelectrical conductivity.26,27It has been demonstrated28–34that the use of thin-ﬁlm fabrication methods such as chemi-cal vapor deposition or physical vapor deposition can pro-duce BxC thin ﬁlms that maintain robust mechanical,thermal, and chemical properties, while improving on elec-trical transport, electronic, and/or optical properties (e.g.,resistivity and band gap), without having to resort to theextreme conditions required by traditional bulk growthtechniques. In particular, the plasma-enhanced chemicalvapor deposition (PECVD) of ﬁlms from ortho-carborane(o-C2B10H12), which typically yields amorphous hydrogen-ated boron carbide (a-BxC:Hy) when lower growth tempera-tures are used, has been shown to be suitable for producingﬁlms for device applications as well as conducive to tuningproperties over a wide range.35–44With tunability, however,comes complexity, and although the ability to vary proper-ties bodes well for optimizing a material system, there hasneither been an extensive study on just how variable theseproperties can be nor an emphasis on understanding theirphysical underpinnings. Such investigations will be critical ifboron-carbide-based materials are to meet the stringentmaterial requirements for next-generation technologies.The trajectory of PECVD amorphous hydrogenatedsilicon (a-Si:H) represents a compelling example of a tech-nological maturation process whereby many years of focusedstudy on a material led to its eventual integration into a num-ber of commercial applications, both anticipated and unfore-seen.45A key ﬁnding in the history of a-Si:H was realizingthe importance of hydrogenation in passivating danglingbonds and decreasing disorder. Within the amorphous siliconliterature, hundreds of papers have been and continue to bepublished on the topic of hydrogen alone, covering its inﬂu-ence on microstructure, electronic and optical properties,photodegradation, and more. The improvements in semicon-ducting properties that resulted from better understandingand optimizing a-Si:H allowed for its deployment in solarcells, active-matrix liquid crystal displays, and other large-area electronics.46,47Although a-BxC:Hyis still relativelynew on the electronic materials scene, we anticipate thatsimilar focused research efforts may set this material up forsimilar—if more modest—successes.Herein, we investigate the relationship between chemi-cal, mechanical, electronic/optical, and electrical transportproperties in thin-ﬁlm a-BxC:Hy. We apply a hybrid facto-rial/response surface experiment design to produce a largeseries of ﬁlms of varying chemical composition and proper-ties by modifying temperature and power in the PECVDgrowth of a-BxC:Hyfrom ortho-carborane. We measure awide range of properties, including B/C ratio, hydrogen andoxygen content, density, pore diameter, Young’s modulus,hardness, band gap, Urbach energy, Tauc slope parameter,dielectric constant (high- and low-frequency), electrical re-sistivity, and leakage current. From these results, we demon-strate that hydrogen concentration can serve as a usefulproxy for many important material properties in a-BxC:Hy.We analyze the effects of hydrogenation in the context of theaccumulated knowledge bank for a series of well-knownamorphous solids, a-Si:H, a-C:H, and a-SiC:H, and showhow parallel phenomena can guide us to pathways for mate-rial property optimization.II. EXPERIMENTAL DETAILSAll thin-ﬁlm a-BxC:Hysamples reported in this workwere prepared in a custom-built parallel-plate capacitivelycoupled PECVD system with ortho-carborane (o-C2B10H12)precursor and argon working gas. The reactor system, whichdiffers from commercial systems due to the use of aninverted, rotating substrate holder/heater, consists of a 12 cmdiameter upper rotating anode, acting as substrate holder,and 25 cm diameter lower cathode, separated from the anodeby 4.4 cm, acting as precursor dispersion and delivery system(also known as the showerhead) housed within an 80 l cham-ber operating at a base pressure of 5107Torr. The sub-strate heating system comprises an SHQ400 series rotatingsubstrate heater from AJA International controlled by a PTBSHQ-15A PID controller. The actual temperature of the sub-strate holder has been independently calibrated in referenceto the temperature controller readout. A motorized rotaryfeedthrough allows for substrate rotation of up to 20 rpm tofacilitate heating and deposition uniformity. R.F. power isdelivered at a standard frequency of 13.56 MHz via an ACG-5 XL R.F. plasma generator and MW-5 automatic matchingnetwork.The base of the showerhead is connected via feed-through to a solid precursor \"bubbler” (for sublimation) andﬂow rate control system. The bubbler consists of a glass nip-ple containing a stack of glass beads above which is loadedortho-carborane powder on top of a mesh screen. Heated Argas is ﬂowed through the bubbler to deliver sublimed ortho-carborane to the showerhead. The ﬂow rate of the argon iscontrolled by an MKS mass ﬂow controller (0–200 sccm),but the precise concentration of ortho-carborane within theargon/ortho-carborane mixture is unknown. The argon gaslines and showerhead are heated to 90 C and the bubbler to75 C. The ortho-carborane is sourced from Katchem Ltd.,resublimed in vacuo prior to use, and loaded into the bubblerunder inert conditions. Process argon gas is sourced fromAirgas Inc. (BIP grade) with a speciﬁed purity of 99.9999%( 10 ppb O2and 20 ppb H2O). High-capacity Restek O2and H2O ﬁlters are installed in line to further reduce oxygenand water impurity levels to the hundreds-of-ppt range.A series of a-BxC:Hyﬁlms were grown starting fromone-factor and two-factor designs by varying growth temper-ature and R.F. power, and ﬁlling in additional data points inthe regions of interest. The result was a response surface ma-trix of twenty-eight growths spanning growth temperaturesof 50–450 C and R.F. powers of 5–30 W (Table I, Fig. 1),with the remaining PECVD conditions held constant. Filmswere grown for thirty minutes at a pressure of 200 mTorr,035703-2 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 TABLE I. Summary of thin-ﬁlm properties for a series of a-BxC:Hysamples grown by plasma-enhanced chemical vapor deposition from ortho-carborane with varying growth temperature and power.aFilmTemp(C)Power(W)db(nm)Density(g/cm3)c%Hc%Bc%Cc%OcB/CHd(GPa)Ed(GPa)Pore diam.(nm)eETaucf(n¼2/3)(eV)E04f(eV) EUf(meV)B1/2f(eV cm)1/2e1b(1014Hz)jg(106Hz)qh(Xcm)D1 152 20 557 … … … … … … … … … 2.9/2.4 3.9 512 194 3.2 4.1 1 1012D2 232 20 795 1.50 34 55 11 0 4.9 … … … 2.7/2.0 3.2 513 282 3.4 … …D5 272 20 590 1.58 32 57 12 0 4.8 11 144 … 2.5/2.0 3.0 592 314 3.5 4.1 7 1012D8 312 20 577 1.64 29 59 12 0 4.9 13 183 … 2.4/1.8 2.7 607 334 3.9 4.6 1 1013D10 292 20 757 … … … … … … … … … 2.7/1.9 3.2 583 292 3.3 … 5 1013D11 52 20 550 0.98 42 47 11 11 4.3 … … 0.61 … … … … 2.3 3.2 1 1012D12 392 10 643 1.41 35 53 12 3 4.6 9 116 … 3.5/3.2 3.6 359 678 3.5 4.1 4 1014D13 112 30 403 1.40 36 53 11 1 4.6 1 12 … 3.4/3.0 3.3 479 793 3.6 4.0 1 1013D14 112 10 407 … … … … … … … … … 3.7/3.4 3.8 149 1249 2.9 … …D15 72 10 442 1.05 45 43 10 8 4.2 1 15 0.59 3.8/3.5 3.9 171 1230 2.4 3.8 1 1012D16 52 5 477 … … … … … … … … … 3.8/3.5 3.8 118 1650 2.3 3.4 1 1014D17 152 5 712 1.24 41 48 10 4 4.8 1 19 0.59 3.73.5 3.8 208 1043 2.9 4.5 1 1012D18 72 15 260 0.92 44 44 13 6 3.5 1 27 0.69 3.7/3.5 3.8 176 1640 2.3 4.3 5 1013D19 72 25 205 0.96 44 44 13 5 3.5 1 25 0.69 … … 109 … 2.5 3.1 1 1014D20 112 20 311 1.20 40 48 11 4 4.3 2 25 0.63 3.6/3.4 3.7 189 1428 2.9 4.1 …D21 152 25 325 1.48 38 50 10 5 4.8 … … … 3.6/3.4 3.7 182 1388 3.3 … …D22 192 15 319 1.27 40 47 11 3 4.3 … … 0.62 3.7/3.5 3.8 147 1422 3.0 4.1 …D23 192 30 362 1.56 31 58 12 0 4.8 10 146 … 3.1/3.0 3.0 451 459 3.7 4.1 …D24 232 10 467 1.32 39 51 11 1 4.8 … … 0.60 3.6/3.4 3.7 190 1136 3.2 4.0 …D25 272 30 345 1.67 27 60 13 2 4.7 15 195 … 2.9/3.0 3.1 473 314 4.5 4.8 4 1012D26 392 30 458 1.80 20 66 14 0 4.7 21 269 0.40 2.0/1.1 2.3 691 184 5.3 5.6 3 1011E1 272 5 552 1.46 38 52 10 4 4.7 … … … … … 359 230 3.4 4.2 …E2 272 10 549 1.44 38 51 11 9 4.5 … … … … … … … 3.4 4.6 1 1014E3 272 15 409 1.26 34 55 12 2 4.7 11 137 0.46 3.1/2.9 3.5 372 330 3.5 4.6 5 1014B1 300 30 1156 1.75 24 63 13 0 4.8 13 183 2.5/2.0 2.6 557 233 4.3 4.5 9 1012B3 400 30 721 2.13 16 68 15 0 4.5 24 311 2.1/1.8 2.3 695 198 6.3 6.8 1 1011B4 450 30 672 2.27 11 73 16 0 4.6 25 344 0.3 1.7/1.0 1.8 720 172 7.1 7.6 1 1010B6 350 30 924 1.83 19 65 16 5 4.0 19 258 0.3 2.3/2.1 2.5 550 565 4.7 5.1 …aAll other growth conditions were held constant at a pressure of 200 mTorr, total ﬂow rate of 50 sccm, partial ortho-carborane þargon ﬂow rate of 0.2, and growth time of 30 min, except in the case of B1–B6, whichwere grown for 45 min.bResults were obtained by ellipsometry.cResults were obtained by nuclear reaction analysis (atomic percentages of B, C, and H calculated relative to total BCH composition only; atomic percentage O calculated relative to BCOH composition).dResults were obtained by nanoindentation.eResults were obtained by positron annihilation lifetime spectroscopy.fResults were obtained by optical absorption spectroscopy.gResults were obtained by capacitance–voltage measurements (Hg probe).hResults were obtained by current–voltage measurements (Hg probe).035703-3 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 with a total ﬂow rate of 50 sccm and a partial precursor ﬂowrate of 0.2—that is, 40 sccm of argon was delivered directlyto the showerhead, while 10 sccm of argon was ﬁrst passedthrough the ortho-carborane prior to delivery to the shower-head. Batches of a-BxC:Hyﬁlms were grown under each setof conditions on two types of 15 15 mm substrates: 1–15 Xcm p-type Si(100) and Corning soda-lime glass microscopeslides. Select ﬁlms for FTIR measurements were grown onAl foil. Prior to being loaded into the PECVD chamber,substrates were cleaned by sonication in acetone for 60 min,followed by immersion in a standard Piranha 2:1 H2SO4:H2O2solution for 15 min, with subsequent rinsing usingdeionized water and drying using a Marangoni process.48The thickness and optical properties of the a-BxC:Hyﬁlms were measured using a J.A. Woollam alpha-SE spectro-scopic ellipsometer system with a 1.5–3.2 eV energy range.The CompleteEase software was used for data acquisitionand analysis/model ﬁtting. To extract thin-ﬁlm thickness andoptical properties, a Cauchy model with a graded layer wasused to model the measured w–Dtrajectories,49which gavelow mean-squared error (MSE) values ( 20). The high-frequency (4.6 1014Hz/1.96 eV) dielectric function,e¼e1þie2, which contains both real (e1) and imaginary (e2)components, was obtained from the measured index ofrefraction (n) and extinction coefﬁcient (k) values via therelationships e1¼n2k2and e2¼2nk.50The absolute atomic concentrations in the ﬁlms weredetermined by nuclear reaction analysis (NRA). The hydro-gen content was determined by the15N nuclear reactionmethod.51Brieﬂy, the ﬁlm is bombarded with15Nþþions,and the number of characteristic gamma rays from the15Nþ1H!12Cþ4He þc-ray nuclear reaction is recordedand used to determine the H content of the ﬁlm. The concen-trations for the other elements (B, C, N, and O) were deter-mined by bombarding the ﬁlm with a 1.2 MeV deuteronbeam and recording the number of particles from thefollowing nuclear reactions:11B(d,ao)9Be,12C(d,po)13C,14N(d,a1)12C, and16O(d,ao)14N.52,53Having determined theH, B, C, N, and O concentrations, the resulting compositionwas used to generate a RUMP simulation of the 2 MeVRutherford backscattering spectroscopy (RBS) spectrum,which was compared to the measured spectrum. Comparisonof the absolute RBS simulation with the measured RBS spec-trum provides a powerful check that there are no major errorsin the composition. Film densities were determined by multi-plying the elemental concentrations by the masses of theseelements and summing over all elements present to yield theareal density in g/cm2, which was then divided by theﬁlm thickness (measured by ellipsometry) to yield densityin g/cm3.The indentation modulus and hardness (H) of the ﬁlmswere determined using nanoindentation techniques.53,54Theindentation load–displacement curves were measured using aHysitron TriboIndenter equipped with a Berkovich diamondtip. A linear projection of the apparent modulus and hardnessat different indentation depths to vanishing depth was usedin this study to correct for substrate effects.55Calibration ofthe area function of the indenter tip and the machine compli-ance was performed on a fused silica specimen according tostandard procedure. For statistical purposes, at least ninerepeat measurements were made for each sample.Values for thin-ﬁlm pore diameter/volume were deter-mined using positron annihilation lifetime spectroscopy(PALS). The pore diameter of amorphous ﬁlms can beobtained from the relationship of antimatter (positronium,Ps) lifetime and the presence of voids, cracks, and porespresent in the amorphous network. The PALS beam was runon a subset of samples at a beam energy of 3.2 keV. At thisenergy, most of the incident positrons stop at the ﬁlm with-out penetrating into the substrate. The overall measure ofrelative porosity was calculated using the product IVsphere,where Iis the intensity of the Ps signal and Vis the speciﬁcpore volume calculated using a spherical model pore. Thespeciﬁc pore volume was determined from the Ps lifetime topore diameter conversion.56Attenuated total reﬂection Fourier transform infrared(ATR-FTIR) spectra were acquired using a ThermoScientiﬁc Nicolet iS10 spectrometer. All spectra were col-lected at room temperature in reﬂectance mode with a ger-manium crystal for a-BxC:Hysamples on aluminum foil,which contributes negligible background.The absorption coefﬁcients (a) for the a-BxC:Hyﬁlmswere obtained from ultraviolet–visible (UV–vis) transmissionspectroscopy measurements for thin ﬁlms on glass substrates(transparent from 1.5 to 4eV). The absorption coefﬁcient as afunction of energy was obtained from the Beer–Lambert lawfor solid ﬁlms,57T¼I/I0¼exp(ad), neglecting a correctionfor front or back surface reﬂection, where Tis transmission, Iis intensity of transmitted radiation, I0is intensity of incidentradiation, and dis ﬁlm thickness. From the absorption coefﬁ-cient data, the Tauc optical band gap (ETauc) and slopeparameter (B1/n) were determined using a Tauc analysis58inthe high-absorption-coefﬁcient region (a104–105cm1)based on the relationship (aE)n¼B(EETauc), with n¼2and 3. The isoabsorption gap, E04, was also determined as theenergy at which areaches 1 104cm1.59The Urbachenergy, EU, was determined from the absorption coefﬁcientdata extracted in the exponential low-absorption-coefﬁcientFIG. 1. Representation of parameter space covered in present study by vary-ing growth temperature and R.F. power while holding remaining growthconditions constant.035703-4 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 region (a103cm1) by ﬁtting the data to the expressiona(E)¼a0exp(E/EU).60,61Electrical measurements were done on metal–insulator–semiconductor (MIS) heterostructures, using an MDC mer-cury probe and controller station with a-BxC:Hy/Si (p-type)samples. The low-frequency (total) dielectric constant, j, wasdetermined from thin-ﬁlm capacitance measurementsobtained at 100 kHz using a Keithley 590 CV analyzer.Dielectric constant values were calculated with the parallel-plate capacitor formula, deﬁned for an MIS geometry in theaccumulation region by j¼Cd/e0A, where Cis measuredcapacitance, dis sample thickness, e0is the permittivity offree space, and Ais the Hg contact area. The current density(J) as a function of electric ﬁeld (E) was measured using aKeithley 2400 source meter (voltage source) and a Keithley6485 picoammeter (current sensor). The resistivity (q) wasextracted in the linear ohmic region (typically between 0 and10 V) of the MIS J–E curves via Ohm’s law, q¼E/J.III. RESULTSGrowth conditions and properties for the series ofa-BxC:Hyﬁlms spanning PECVD parameter space in thetemperature range of 50–450 C and R.F. power range of5–30 W are summarized in Table I. For each of the proper-ties, a response surface62,63curve was generated, and aKriging metamodel64,65was used as a qualitative tool to ﬁtand visualize the surface.A. Atomic composition and mechanical propertiesWe have analyzed the effect of growth conditions onthin-ﬁlm chemical composition and mechanical propertiesbased on response surface curves for a-BxC:Hyatomic con-centration, density, hardness, and Young’s modulus (Fig. 2).The boron-to-carbon ratio (B/C) ranges from 3.5 to 5 (repre-senting a stoichiometry range of B3.5CtoB5C) and is pri-marily dependent on growth temperature, increasing from3.5 at 50 Cto5at250 C, before decreasing againto 4.5 at 450 C [Fig. 2(a)]. The atomic concentration ofoxygen (at. % O) in these ﬁlms remains relatively low, rang-ing from 10% in the low temperature/power regime to 1% (below the limits of the NRA measurements) in thehigh temperature/power regime [Fig. 2(b)]. The atomicconcentration of hydrogen (at. % H) ranges from 10% to45% and appears to be correlated nearly equally with bothtemperature and power, being at a maximum in the low tem-perature/power regime and at a minimum in the high temper-ature/power regime [Fig. 2(c)]. Thin-ﬁlm density rangeswidely, from 0.9 to 2.3 g/cm3, with a response surface fol-lowing a near inverse trend to that for at. % H [Fig. 2(d)].A subset of predominantly low-density ﬁlms wasselected for PALS analysis to investigate porosity. Filmswith densities in the range of 0.9–1.8 g/cm3exhibited poresizes ranging from 0.69 to 0.40 (60.02) nm in diameter andPs intensities from 10% to 1.5%, yielding IV values of rela-tive porosity that nominally correlate with ﬁlm density (seesupplementary material for IV vs density data66). Moreover,no pore interconnection was observed for even the mostporous ﬁlms. A few higher density samples (1.8–2.3 g/cm3)did not present any discernible pores, which suggests thatthey have pore sizes 0.3 nm since this represents the limitof the PALS analysis.56The hardness (H) and Young’s mod-ulus (E) response surface curves [Figs. 2(e) and 2(f)] trendas a function of growth temperature and power similarly toFIG. 2. Growth temperature/power response surface curves for: (a) B/C ratio, (b) at. % O, (c) at. % H, (d) density, (e) hardness, and (f) Young’s modulus. Theblack circles represent actual data points and the colored surfaces the Kriging metamodel ﬁt.035703-5 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 density. At low densities ( 1.2 g/cm3), hardness ranges from1 to 2 GPa and Young’s modulus from 10 to 30 GPa, whereasfor higher densities in the range of 1.3 to 2.3 g/cm3, hardnessranges from 5 to 25 GPa and Young’s modulus from 100 to350 GPa.Chemical bonding information was obtained from FTIRspectroscopy. The spectra of representative high-density(2.3 g/cm3) and low-density (0.9 g/cm3) ﬁlms, with corre-sponding stoichiometries of a-B4.6C:H0.7and a-B3.5CO0.5:H3.5,respectively, are given in Fig. 3along with the spectrum of theortho-carborane precursor. We note that the intensities are notcalibrated, and therefore spectra can only be compared qualita-tively and not quantitatively. The a-BxC:Hyspectra exhibitsimilar features to those previously reported for other a-BC:Hﬁlms.44,67–69A peak at 3060–3070 cm1is attributed to anintra-icosahedral C–H stretching mode, and a peak at2560 cm1to an intra-icosahedral B–H stretching mode.70Both of these peaks are also present in the ortho-carboranespectrum,71which reinforces their assignment. The intensityof the B–H stretch conﬁrms that there is a signiﬁcant amountof hydrogenation of the carborane units in the a-BxC:Hyﬁlms.A number of weak peaks are also observed in the regionassociated with sp3hydrocarbon C–H stretching modes(2800–3000 cm1). Clear peak maxima at 2920 cm1and2850 cm1can be assigned to sp3CH2asymmetric andsymmetric stretching modes.72,73Their presence is consistentwith the proposed existence of bridging CH2-based hydrocar-bon groups.74Any differences between the spectra for thelow- and high-density ﬁlms in this region are very subtle, andwe cannot conﬁdently make additional assignments (e.g., theidentiﬁcation of CH3groups) or quantitative determinationswithout additional information. One feature that has beenobserved in spectra of a-B:H and a-BC:H is a very broadpeak at 2000 cm1assigned to a bridging B–H–B conﬁgu-ration,68,70but this is not observed here. A peak at1600 cm1is typically assigned to the three-center C–B–Cchain in crystalline boron carbide;75however, it is not clearthat this feature should be present in the amorphous solid,and the identity of this peak remains ambiguous. Peaks below1100 cm1are assigned to icosahedral vibrational modes.71The most distinct difference in this region for the twoa-BxC:Hyﬁlms is the presence of a strong peak at 820 cm1in the low density ﬁlms, which we cannot deﬁnitively assign.A strong peak at 3200cm1observed for the low-density ﬁlmis assigned to an O–H stretching vibration and is consistentwith the incorporation of some oxygen. It is plausible that amajority of this O–H is in the form of boric acid, as we alsoobserve a very strong peak at 1410 cm1and a second strongsharper peak at 1200 cm1for this same sample that can beassigned to this impurity.76,77A strong, broad peak between1100 and 1300 cm1often dominates a-BC:H spectra,although its energy and assignment vary. Shirai et al. haveproposed that a peak at 1100 cm1is due to a B–C stretch,speciﬁcally involving extra-icosahedral C as a networkmodiﬁer.78However, this is not consistent with the assign-ment by Lin and Feldman of a peak at 1280 cm1as anicosahedron-based mode79nor of the typical assignment ofthis peak in crystalline boron carbide.75Some of the mostdiagnostic hydrocarbon-based features can be found in the1300–1500 cm1region, such as methyl or methylene bend-ing modes.73,80–82However, while some of these featuresmay be observed (e.g., a distinct peak at 1315 cm1may beindicative of a CH2bending or C–C stretching mode),because of the number of overlapping peaks in this regionand the interference due to strong boric acid peaks, it is difﬁ-cult to make any deﬁnitive assignments or comparisonsbetween the carbon-rich and carbon-poor ﬁlms. While theFTIR analysis cannot provide an extremely detailed pictureof the local physical structure of a-BxC:Hy, it is generallyconsistent with our previous analysis based on solid-statenuclear magnetic resonance (SS-NMR) spectroscopy. Theseresults suggested that a-BxC:Hycan be considered as\"polymeric o-carborane,” and consists of partially hydrogen-ated partially cross-linked C2B10H12icosahedral units andbridging hydrocarbon chains.74A more thorough understand-ing of the structural details will require a deeper level ofanalysis, which is beyond the scope of this work.In sum, varying growth temperature and R.F. powergives rise to a range of a-BxC:Hyvariants with signiﬁcant dif-ferences in chemical composition and mechanical properties.In the low temperature/power growth regime, we obtain (rela-tively) soft, low-density ﬁlms that are both hydrogen andcarbon rich with approximate stoichiometry of a-B3.5C:H3,whereas in the high temperature/power regime, we obtainhard, high-density ﬁlms with lower carbon content and signif-icantly lower hydrogen content with approximate stoichiome-try of a-B4.5C:H. In Fig. 4, we provide hypothetical models torepresent these two extremes based on structural informationfrom FTIR and SS-NMR spectroscopy. These models arepurely illustrative and were generated by condensing aFIG. 3. FTIR spectra for (a) ortho-carborane and (b) representative high-density (B4, 2.3 g/cm3, a-B4.6C:H0.7) and low-density (D18, 0.9 g/cm3,a-B3.5CO0.5:H3.5) ﬁlms.035703-6 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 hypothesized set of chemical fragments into a simulation cellto produce a model with a given thin-ﬁlm density andstoichiometry.B. Electronic and optical propertiesOptical absorption spectroscopy is a particularly usefultechnique for investigating the electronic structure and disor-der in amorphous solids. In an amorphous solid, disorder-induced localized states give rise to exponential band tailsknown as Urbach tails. The point at which extended or delo-calized band states give way to localized states is deﬁned asthe mobility edge, and the separation between mobility edgesfor the valence band (VB) and conduction band (CB) deﬁnesthe mobility gap, analogous to the band gap, Eg, in crystallinesolids. The absorption spectrum in amorphous solids can thusbe represented by a superposition of optical transitions occur-ring between combinations of extended states, localizedstates, and mid-gap states.45,58Transitions occurring fromextended VB to extended CB states deﬁne the high-energyhigh-absorption-coefﬁcient region of the avs Espectrum,from which we can obtain optical band gap information.Tauc showed that this region of the spectrum can be modeledby a power law according to a¼B(EEg)n/E,58which canbe rearranged to (aE)1/2¼B1/2(EETauc)(nis typicallydeﬁned as 2 for amorphous solids), such that the Tauc opticalband gap, ETauc, and the Tauc parameter, B1/2, can beextracted from a plot of (aE)1/2vs Eas the intercept andslope of the linear region, respectively. Other methods havebeen employed to determine the optical band gap of amor-phous solids,59including the isoabsorption method,59,61,83where a gap is empirically deﬁned at a given absorption coef-ﬁcient value (typically E04at 1 104cm1), as well as theuse of alternative values for \"n” in the Tauc equation (typi-cally 3, as may be appropriate for boron-rich solids84,85). Theaccuracy of these methods depends on the speciﬁc form ofthe density of states in a given material and other factorsbeyond the scope of this paper. Transitions involving local-ized states contribute to the exponential lower-energy lower-absorption-coefﬁcient region of the avs Espectrum.Speciﬁcally, the exponential edge of the absorption curve hasbeen shown to be primarily deﬁned by transitions betweenlocalized VB states and extended CB states. This region ofthe curve can be modeled by a(E)¼a0exp(E/EU), wherethe decay factor, known as the Urbach energy (EU), is primar-ily a measure of the width of the valence band tail,61and canbe obtained as the slope of the curve for a plot of ln(a)vsln(E) [i.e., EU¼dln(a)/dln(E)]. Both the Urbach energy andthe Tauc slope parameter, B1/2, provide some measure of dis-order in a solid; however, they do not necessarily probe thesame type(s) of disorder nor do they have the same meaningin all materials.60,86We have extracted optical band gap values for a-BxC:Hyusing the Tauc method (n¼2 and 3) as well as the isoabsorp-tion method (E04), and the results are summarized in Table I.Representative absorption coefﬁcient spectra for a subset ofa-BxC:Hyﬁlms with annotated E04values and their corre-sponding Tauc plots (n¼2) are shown in Fig. 5. Both Taucanalyses (n¼2 and 3) yielded similarly linear ﬁts(R2¼0.99), but the ETaucvalues extracted using n¼3 weresystematically lower than those using n¼2 by an average of0.4 eV. In contrast, the E04method yielded band gap valuessystematically higher than those obtained by a Tauc analysiswith n¼2 by an average of 0.3 eV [Fig. 6(a)]. In addition tothe band gap values, we also obtained B1/2values from theTauc analysis (n¼2) as well as Urbach energy values usingan exponential ﬁtting of the absorption coefﬁcient curves inthe appropriate region.Figs. 7(a)–7(c) display the response surfaces for Taucband gap, Tauc slope parameter (in the form of B1/2), andUrbach energy. The optical band gap ranges from 3.8to 1.7 eV and B1/2from 1650 to 170 eV1/2cm1/2, bothdecreasing as a function of temperature and power, while theUrbach energy exhibits the opposite behavior, increasingfrom 0.1 to 0.7 eV. All three of the parameters are clearlycorrelated, as can be seen from Fig. 6. The optical band gaprange measured here is consistent with previous reports forortho-carborane-based a-BxC:Hyﬁlms, for which Egvalueshave been cited between 1.5 (Ref. 87) and 3.8 eV.88TheTauc slope parameter has been previously reported fora-B4C as 529 cm1/2eV1/2,84which is within the sameFIG. 4. Hypothetical models illustrating the local physical structure ofamorphous hydrogenated boron carbide (boron ¼pink, carb on ¼gray, andhydrogen ¼white) in different density extremes: (a) low-density (0.9 g/cm3)ﬁlms with approximate stoichiometry of a-B3.5C:H3and pores with diameterson the order of 0.7 nm (compare to D18/D19), and (b) high-density (2.4 g/cm3)non-porous ﬁlms with stoichiometry of a-B4.5C:H (compare to B4).035703-7 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 range as for the ﬁlms characterized here. The Tauc slope pa-rameter for a-BxC:Hyis further comparable to those found ina-Si:H,86,89,90a-Ge:H,86,91and a-SiC:H,92,93which rangeanywhere from 100 to 1200 cm1/2eV1/2. The authors arenot aware of any reports of EUvalues for BxC or a-BxC:Hyﬁlms. The Urbach energy range for a-BxC:Hy(0.1–0.7 eV) ismuch higher than that found in a-Si:H (0.05–0.1 eV)60aswell as the vast majority of amorphous solids,94although itis closer to that of a-C:H95or organic polymers,94which ison the order of hundreds of meV.The relative permittivity, er, of a solid is a complex fre-quency/energy-dependent quantity that represents the sum oforientation polarization (movement of permanent dipoles),distortion polarization (vibration of polar bonds), and elec-tronic polarization (displacement of electron cloud) contribu-tions to the total polarization response to an electric ﬁeld.96The macroscopic relative permittivity of a material can berelated to the microscopic properties of the individual molec-ular constituents by the Debye equation, (er1)/(erþ2) ¼[N(aeþadþl2/3kBT)]/3e0, where Nis the molecu-lar density, the aeand adterms represent the electronic anddistortion polarization contributions in the molecules,respectively, and the l2term takes into account the perma-nent dipole moments that give rise to the orientationpolarization contribution (kB¼Boltzmann constant,T¼temperature, and e0¼permittivity of free space).96Athigh (optical) frequencies, only aecontributes, at moderate(infrared) frequencies, we probe the sum of aeand adcontri-butions, and at low (microwave/radio) frequencies, all polar-ization contributions are at play. For our purposes, we takethe total dielectric constant, j, which represents the sum ofall polarization contributions, as the relative permittivitymeasured at 100 kHz, and the electronic contribution to thedielectric constant, e1, as the real part of the relative permit-tivity measured at high frequency (4.6 1014Hz). Thedifference between the two terms, je1, therefore repre-sents the combined orientation and distortion polarizationcontributions.The response surfaces for the total dielectric constant,the electronic contribution to the dielectric constant, and thedifference between the two (je1) for the series ofa-BxC:Hyﬁlms are given in Figs. 7(d)–7(f). The total dielec-tric constant ranges from 3.1 to 7.6, with the electronic con-tribution ranging from 2.3 to 7.1, both increasing with highergrowth temperature and power. Because a-BxC:Hycontainsvery low-Zatomic constituents (B, C, and H), which impliesa low mass and electron density, we expect e1to be low,which is found to be the case here. The range of e1valuesobserved is also consistent with those previously found inPECVD a-BxC:Hyﬁlms, wherein e1values of 2–4 wereobserved,35,49and in higher density crystalline BxC, whereine1values of 7 were measured.97Further, because a-BxC:Hycontains only low polarity bonds (B–B, B–C, B–H, andC–H), which implies low distortion and orientation polariza-tion contributions, we expect the electronic polarizationcontribution, e1, to dominate j, which is indeed observed.FIG. 5. Representative absorption coefﬁcient curves (a) and correspondingTauc plots (n¼2) (b) for a subset of a-BxC:Hysamples of varying densityand band gap illustrating extraction of E04and ETaucvalues. The oscillationsevident in the spectra are a result of thin-ﬁlm optical interference.FIG. 6. Correlations between (a) band gap values, ETauc[n¼2(䊉) andn¼3 (blue triangle)] and E04(䊊), and Urbach energy and (b) Tauc slope pa-rameter, B1/2, and Urbach energy, for a-BxC:Hyﬁlms.035703-8 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 This has also been shown to be true in the case of c-BxC, asdemonstrated by Samara et al., who reported a low distortioncontribution in the range of 0–2.97For the a-BxC:Hyﬁlms measured, e1makes up the ma-jority (60%–90%) contribution to the total j. The differ-ence between the two [je1, Fig. 7(f)] does not scale withe1or j, but rather exhibits an increase at low growth temper-ature and power, which results in a higher jthan might beexpected in this range. If we refer back to the NRA results[Table I, Fig. 2(b)], we see that the at. % O scales closelywith je1. Therefore, although ﬁlms grown at low temper-ature and power have low densities, which should yield alow total j, the presence of oxygen-containing functionalgroups with polar bonds such as B–O, and O–H is expectedto lead to an increase in the distortion and potentially orien-tation polarization contributions, which may explain all orpart of the higher je1contribution in this growth regime.C. Electrical transport propertiesThe response surface for electrical resistivity for theseries of a-BxC:Hyﬁlms is shown in Fig. 8(a), from whichwe can see that it varies across several orders of magnitudefrom 1010to 1015Xcm. Trends as a function of growth tem-perature and power are less evident than in the case of theother material properties investigated in this study. Previousresistivity measurements on carborane-based a-BxC:Hyﬁlmshave been reported in the range of 108–1010Xcm,10,28andtherefore the ﬁlms produced here are signiﬁcantly more insu-lating. Although resistivity is a useful fundamental materialproperty, from an application perspective, one of thekey electrical properties of interest is leakage current. FromFig. 8(b), we see that the leakage current generally trendswith resistivity at a moderately low electric ﬁeld of 0.5 MV/cm. At higher electric ﬁelds, the leakage current mechanismbegins to diverge from ohmic transport, and therefore thedirect resistivity/leakage current correlation can breakdown.A more detailed analysis of electrical properties and cur-rent–voltage behavior is beyond the scope of this paper.IV. DISCUSSIONA. Atomic composition, physical structure, andmechanical propertiesOne might ﬁrst consider the atomic structure of a-BxC:Hyas a base for understanding its material properties.Determining atomic structure is a particularly challengingproblem in the case of a-BxC:Hybecause of not only itsamorphous nature but also its molecule-based constituentsand the many conﬁgurations into which these can bearranged. We have previously proposed, based on solid-statenuclear magnetic resonance spectroscopy, that a-BxC:Hyconsists of partially hydrogenated C2B10H12icosahedralunits and hydrocarbon chains with varying degrees of cross-linking.74The FTIR results described here support these con-clusions. This picture, however, remains incomplete, andadditional rigorous studies will be needed to prove out thismodel and to ﬁll in the missing pieces. Despite the complex-ity and unknowns associated with the atomic structure ofa-BxC:Hy, we have found that hydrogen content appears tobe a relatively simple and efﬁcient proxy for mapping thin-ﬁlm properties. As seen in Fig. 9, the at. % H displays astrong correlation with a majority of the properties investi-gated in this study.FIG. 7. Growth temperature/power response surface curves for: (a) Tauc optical band gap, ETauc(n¼2), (b) Tauc slope parameter, B1/2, (c) Urbach energy,EU, (d) total dielectric constant, j, (e) the electronic contribution to the dielectric constant, e1, and (f) the orientation/distortion contribution to the dielectricconstant, je1.035703-9 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 In particular, we observe a distinct inverse linear trendbetween hydrogen concentration and thin-ﬁlm density [Fig.9(a)], where density decreases commensurate with increas-ing hydrogen content. Hypothetical models of a-BxC:Hyillustrating the hydrogen-rich/low-density and hydrogen-deﬁcient/high-density extremes were given in Fig. 4. Densitycan be reduced to the atomic mass of the constituents and therelative free volume in the solid (i.e., atoms/volume).Therefore, to a ﬁrst approximation, hydrogen content plays adirect role in determining thin-ﬁlm density because of bothits low atomic number (Z¼1), which decreases the averageatomic mass of the constituents, and its coordination numberof 1, which decreases the average network coordination andconsequently increases the free volume in the solid.98Relatively small (0.40 to 0.69 nm), isolated pores are alsoobserved in the ﬁlms, with the pore diameter appearing totrend with hydrogen content/density [Fig. 9(b)]. The increasein pore diameter observed with decreasing density representsa factor of ﬁve increase in pore volume, V.Similar hydrogen–density trends have been observed inamorphous hydrogenated silicon (a-Si:H), wherein greaterhydrogen content is correlated with lower density ﬁlms.However, in a-Si:H, hydrogen atoms have been found toreside in different chemical environments, including invacancies as SiH groups and on the surfaces of nanovoids asSiH2groups. As such, the relationship between hydrogencontent and density exhibits subtle variations with the hydro-gen concentration regime and the microstructural features ofthe a-Si:H network in that regime.99,100In amorphous hydro-genated carbon (a-C:H), the main structural feature control-ling density is not hydrogen content per se, but rather sp3/sp2fraction, where hydrogen tends to correlate with relative sp3concentration. At low hydrogen concentrations, increasingsp3carbon results in increasing ﬁlm density due to the higherdensity of the sp3carbon network relative to the sp2carbonnetwork. However, above a certain threshold, the trendreverses, and further increases in hydrogen/sp3carbonare associated with a decrease in density, which can beexplained by the additional sp3hydrocarbon groups nowacting as network modiﬁers (terminating groups) rather thannetwork formers, thereby increasing free volume.101Ina-SiC:H, hydrogen content has also been shown to correlatewith density, albeit often indirectly.92,102–104Similarly toa-Si:H, a-SiC:H exhibits micropores, which are associatedwith both SiH2and SiCH3groups to varying degrees depend-ing on growth conditions and stoichiometric regime.105Because the hydrogen can be tied up in many ways includingas SiH, SiH2,CH2bridging groups, and CH3terminalgroups, even more reﬁnements are introduced into the den-sity/hydrogen relationship.Like in a-Si:H, a-C:H, and a-SiC:H, more subtle micro-structural effects may also be at play for a-BxC:Hy, includingthe possibility of hydrocarbon groups acting as networkmodiﬁers vs network formers and contributing to the forma-tion of nanopores, or incorporating in different ways indifferent stoichiometric regimes, or even the presence ofB–H–B bridging groups,70often observed in a-B:H (but nothere so far). However, based on the linearity of the at. % Hvs density curve obtained for this set of a-BxC:Hysamples,we can conclude that either all of these samples fall within asingle structural phase/regime or that the existence of someof these more complex structural features does not signiﬁ-cantly skew the general trends. Overall, because hydrogencontent and density are intimately related, one or both prop-erties may be used to benchmark, and in many cases physi-cally explain, other material properties of interest.A strong correlation between at. % H (and consequentlydensity) and the mechanical properties of the a-BxC:Hyﬁlmsexists. Both hardness, H, and Young’s modulus, E, increasewith decreasing hydrogen content (or increasing density)[Figs. 9(c) and 9(d)]. Both properties exhibit similar behavior,where below 35% H (or above densities of 1.3 g/cm3),they follow a speciﬁc power law relationship, but above(below) this, the trend breaks down and a plateau is observed.Scaling relationships between Young’s modulus and densityof the form E/Dn, where nis a scaling exponent, have beenrecognized in many material systems.106,107The plateau canbe explained by rigidity percolation theory, which predictsthat below a critical density or degree of network coordina-tion, the number of degrees of freedom becomes equal to thenumber of constraints, whereupon a fundamental change inthe rigidity of the solid occurs.108–110FIG. 8. (a) Growth temperature/power response surface curve for electricalresistivity and (b) correlation between leakage current density (at 0.5 MV/cm) and resistivity.035703-10 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 B. Electronic and optical propertiesHydrogen content and density are also clearly correlatedwith the electronic structure of a-BxC:Hy. As these ﬁlmsbecome more hydrogen rich and less dense, the optical bandgap [Tauc (n¼2) and E04, shown in Figs. 9(e) and 9(f),respectively] and Tauc slope parameter [B1/2, Fig. 9(h)]increase, while the Urbach energy decreases [Fig. 9(g)]. Theband gap (Tauc/E04) increases linearly from 1.7 eV to 3.8 eVas a function of increasing hydrogen content. The same gen-eral correlation between hydrogen concentration and bandgap is observed in a-Si:H,60,90a-C:H,101and a-SiC:H;102however, as we will discuss below, the mechanisms differ.In a-BxC:Hy, the Urbach energy decreases linearly from 0.7to 0.1 eV as a function of increasing hydrogen concentration.A decrease in Urbach energy with increasing hydrogen con-centration is also observed in a-Si:H,60although the oppositetrend is observed in a-SiC:H102and a-C:H.94,95Lastly, theB1/2parameter increases with hydrogen content from 200to 1600 eV1/2cm1/2, however the relationship is not linearas in the case of Egand EU.A number of theoretical and empirical models exist thatrelate band gap to fundamental material properties. From achemical bonding point of view, as two atoms are broughttogether, bonding and antibonding molecular orbitals (MOs)are formed, creating an energy gap between the highestoccupied and lowest unoccupied frontier MOs. As additionalatoms are brought together to form an extended solid, thediscrete MO energy levels broaden into bands, effectivelydecreasing the gap between band edges.111The same reason-ing applies to molecule-based solids, where in a polymericmaterial, for example, the band gap decreases with increas-ing chain length as a greater number of monomers arebonded together. In a hydrogenated amorphous solid, therole of hydrogen—because it is one-fold-coordinate in thevast majority of circumstances—is inevitably to decrease theaverage coordination number of the network112(see, forexample, the direct correlation between coordination numberand at. % H in a-SiC:H104), unless dealing with an under-coordinated network.113Therefore, from the point of view ofthis simple bonding model, a decrease in hydrogen concen-tration and increase in density in a-BxC:Hy, commensurateFIG. 9. Correlations between a series of material properties and atomic concentration hydrogen in a-BxC:Hyﬁlms: (a) density, (b) pore diameter, (c) hardness,(d) Young’s modulus, (e) Tauc optical band gap (n¼2), (f) E04optical band gap, (g) Urbach energy, (h) Tauc slope parameter (n¼2), (i) high-frequencydielectric constant, (j) total (low-frequency) dielectric constant, (k) je1, and (l) electrical resistivity.035703-11 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 with greater connectivity in the solid (i.e., a higher averagecoordination number, or increased cross-linking betweenicosahedral \"monomer” units), would imply a lower bandgap.The factors contributing to band gap can be more com-plex, however, as exempliﬁed by the case of amorphoushydrogenated silicon. Pastawski et al. attributed the increasein band gap with hydrogen content to a decrease in siliconcoordination number,114which is the same explanation thatwe provide above for a-BxC:Hy. Papaconstantopoulos andEconomou, however, discussed the widening of the gap interms of local chemical environment and its effect onvalence/conduction band character (rather than merely theenergy between the two).115Cody et al. argued that the bandgap was actually related to disorder and only indirectly tohydrogen content, with greater disorder being correlatedwith a decreased band gap.60Mahan et al. have exploredhow different types of disorder in a-Si:H can individuallyinﬂuence Eg,116while Legesse et al. showed that differenttrends in Egexist for different hydrogen concentrationregimes,117which helps to rectify some of the differing inter-pretations. In all likelihood, most or all of these explanationshold some degree of validity. In amorphous hydrogenatedcarbon, again, we see different effects at play. In a-C:H, theband gap is deﬁned by the sp2-based porbitals. Band gap isstrongly correlated to sp2/sp3fraction and, importantly, tothe actual conﬁguration of the sp2carbon. Thus, any correla-tion with hydrogen is indirect.72,73In a-SiC:H, band gap isusually reported as a function of carbon fraction, where itwidens with increasing C up to a certain point (although, asSolomon notes, the properties of the samples dependstrongly upon growth conditions and trends become lostwhen data from different experiments are compared118).This increase in band gap is typically explained as thereplacement of Si–Si bonds by stronger Si–C bonds, whichresults in a lowering of the valence band edge and increasein band gap.102,119–121It has also been proposed that hydro-genation additionally contributes to the widening of the gap,albeit to a lesser degree.119,120Although we propose that, in a-BxC:Hy, an increase inhydrogen concentration (and decrease in density) leads to ahigher band gap because of the decrease in network connec-tivity/average coordination number, we do not neglect otherpossible contributions including the inﬂuence of speciﬁctypes of disorder or the change in valence/conduction bandcharacter due either directly to hydrogen bonding or indi-rectly to other simultaneous changes occurring in the solid(e.g., changes in hydrocarbon content and type). Additionalstudies of the density of states and of the disorder, bothexperimental and theoretical, will be needed to more conclu-sively address this issue.As mentioned previously, both the Urbach energy andTauc parameter are considered measures of disorder in amor-phous solids. Disorder can come in a number of ﬂavors,including thermal disorder (vibrational/dynamic), structuraldisorder (deviation of atoms away from a perfectly orderedlattice, manifest in the form of a variation in bond lengthsand angles), topological disorder (variation in structuralfeatures at a medium-range length scale, such as chain size,cluster size, and ring size distributions), and substitutionaldisorder (substitution of one type of atom for another, includ-ing voids and dangling bonds). In a-Si:H, the Urbachenergy—deﬁned in this material by the width of the valenceband exponential tail—is attributed predominantly to struc-tural disorder.60,86An inverse relationship between hydrogenconcentration and Urbach energy is observed, which hasbeen explained in terms of the ability of hydrogen to releasestrain in the amorphous network, thereby leading to adecrease in structural disorder and increase in EU. The sameholds in the case of a-Ge:H.122The meaning of the Urbach energy, and its potentialcorrelation with hydrogenation, is not universal, however. Ina-C:H, the relationship between hydrogen concentration andUrbach energy is opposite to that in a-Si:H, and the origin ofEUboth more complex and more controversial. Robertsonstates that, similarly to the relationship between band gapand sp2clustering, the Urbach energy is speciﬁcally relatedto the topological disorder arising from variations incluster sizes rather than structural disorder.73Fanchini andTagliaferro, on the other hand, maintain that EUis not relatedto disorder, but rather to the Gaussian width of the pband;however, this conclusion is based on phenomenologicalmodeling only.95Casiraghi et al.101show rather compel-lingly that the Urbach energy in a-C:H is related to bothstructural and topological disorder at low H concentration( 25%)—the same behavior that is observed in hydrogen-free amorphous carbon123—but that it is related to topologi-cal disorder alone at higher H concentration ( 25%),consistent with Robertson’s claims. The H concentrationitself, though, exhibits different relationships with topologi-cal and structural disorder. At low H concentration, bothstructural and topological disorder increase with increasingat. % H, whereas at high H concentration, topological disor-der continues to increase, but structural disorder decreases.At low H concentrations, it can be said that increasing hydro-gen content acts to break C–C bonds and thereby increasestructural disorder, whereas at higher H concentrations, itprimarily acts to release strain in the network and lowerstructural disorder. Therefore, even though in the high Hconcentration regime the same inverse correlation betweenstructural disorder and at. % H exists in both a-C:H anda-Si:H, the relationship between EUand at. % H is theopposite in a-C:H (positive rather than negative correlationwith at. % H) because now the Urbach energy is deﬁned bytopological rather than structural disorder.In a-SiC:H, EUhas been shown to increase with increas-ing C fraction (and thus indirectly with H concentration,which is typically correlated with C fraction), which is typi-cally explained by the fact that the incorporation of C intothe Si network introduces disorder by distorting bond lengthsand angles.102,120,121Others have argued, however, that EUis controlled by microstructure due to alloying rather thanstructural or compositional disorder.124Even if H were tohave an effect on disorder, it is difﬁcult to determine experi-mentally because the effect is convoluted with the inﬂuenceof C. Theoretical results are mixed, suggesting both anincrease and decrease in disorder as a function of hydrogena-tion.125–127We note that the relationship between Urbach035703-12 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 energy and band gap is opposite in the cases of a-SiC:H anda-Si:H, where in a-SiC:H, the two are directly correlated, butin a-Si:H, they are inversely correlated.In a-BxC:Hy, we see the same type of trend as in a-Si:H,with EUdecreasing with increasing H content (and increas-ing band gap) [Fig. 9(g)]. The simplest explanation is there-fore that the incorporation of H leads to a decrease instructural disorder commensurate with the formation of alower density, lower strain network. Further, although thea-BxC:Hyﬁlms with greater hydrogen content are also thosewith greater carbon content, we do not observe an increasein EUdue to the increase in carbon (see, for example, D18and D19), which suggests that the role of carbon incorpora-tion in the a-BxC:Hynetwork is fundamentally different thanin the a-SiC:H network. In a-BxC:Hy, presumably additionalhydrocarbon is involved in cross-linking icosahedra, whichmay even decrease the strain in the polymer compared todirect cross-linking between icosahedra. In a-SiC:H, theaddition of carbon serves as a more fundamental disruptionto the base Si network. Owing to the particularly complexnature of the bonding in a-BxC:Hy, and as evidenced bya-C:H, we must remain open to additional explanations fordisorder in this material, including different behaviors in dif-ferent regimes of B/C/H phase space. Even in a-Si:H andrelated materials, the Urbach energy can potentially be inﬂu-enced by more variables than simply the structural disorderdeﬁned by bond length and angle distortions.61,128,129The Tauc parameter, B1/2, is related to the oscillatorstrength of the optical transition, the deformation potential(related to the electron–phonon coupling), and the deviationof atom positions from perfect lattice coordinates, the latteroriginating from structural disorder (i.e., deviations in bondlengths and angles) in the case of amorphous materials.130Whereas EUis often said to be related to the width of thevalence band tail, the B1/2parameter has been said to berelated to the width of the conduction band tail.120In contrastto the Urbach energy, the magnitude of B1/2is inverselyproportional to disorder—that is, a lower value impliesincreased disorder. Although both EUand B1/2can be takenas measures of disorder, they do not necessarily representidentical effects, and the precise type of disorder probed bythe individual parameters is not fully understood. In manycases, the two parameters exhibit an inverse linear relation-ship,91,121implying that they do in fact probe a similarunderlying disorder feature. Zanatta et al.86,128highlightsome of the more subtle differences between the two, suchas sensitivity to substitutional (electronic) disorder vs. struc-tural disorder. In a-BxC:Hy,EUand B1/2are indeed inverselycorrelated and exhibit the same general trends with at. % H[Figs. 9(g) and 9(h)], however the correlation between thetwo is not perfectly linear [Fig. 6(b)], which suggests thatthey do not probe identical effects and perhaps demonstratea more complex interrelationship than in other solids. Amore detailed elucidation of the origin of the disorder param-eters in a-BxC:Hyis beyond the scope of this paper.Both the low- and high-frequency values of the dielec-tric constant (jand e1, respectively) exhibit an inverse linearcorrelation with at. % H, decreasing with an increase inhydrogen concentration (or decrease in density), as seen inFigs. 9(i) and 9(j). In the case of j, the correlation breaksdown somewhat above 30% H, owing we believe to theincreased oxygen content in this region and therefore inﬂatedje1contribution from polar oxygen-based bonds[Fig. 9(k)]. The electronic dielectric constant scales with theband gap as e1¼1þ[h2Ne2]/[m(Eg)2], where Nis the va-lence electron density, and mand eelectron mass andcharge, respectively.131Therefore, not only is e1intimatelyrelated to band gap, which—as discussed above—is inﬂu-enced by the degree of network connectivity in the solid, butit is also strongly dependent on mass density (number ofatoms/volume) and electron density (Zof constituentatoms).132,133The decrease in e1with increasing hydrogenconcentration/decreasing density can be rationalized by theeffect of hydrogen concentration on band gap, and also—more directly—by the relationship between higher hydrogencontent and lower mass and electron density, which aredirectly proportional to e1. The total dielectric constantencompasses e1, and therefore is expected to trend as e1,while also reﬂecting additional contributions from orienta-tion and distortion polarization components, which are repre-sented by the je1term. Due to the low polarity of thebonds in a-BC:H, the je1term is small in general, andtherefore jand e1exhibit similar magnitude. The je1termdoes show an increase with increasing hydrogen content, butwe propose that the majority of this effect has to do with theincorporation of oxygen in the network, and is not necessar-ily related to hydrogen content per se. A similar decrease indielectric constant (speciﬁcally, index of refraction, n) withhydrogen content is observed in the case of a-Si:H, whichcan also be explained by the signiﬁcant inﬂuence of H onmass and electron density.134Kageyama et al. highlight thesubtleties of the relationship between dielectric function andhydrogen in a-Si:H, and how it can be explained more pre-cisely by looking at how hydrogen is incorporated intomicrovoids.135It is possible that similar structural subtletiesare at play in a-BxC:Hy.C. Electrical transport propertiesHydrogen content also displays a correlation with elec-trical resistivity [Fig. 9(l)]. As the a-BxC:Hyﬁlms becomemore hydrogen rich, qincreases by several orders of magni-tude from 1010to 1015Xcm in the range of 10% to 35%H, at which point the data becomes scattered, and perhapsbegins to decrease. To interpret this result, we can considertwo different charge transport mechanisms that might occurin an amorphous semiconductor. First, if we assume chargetransport is dominated by transitions between localized statesabove/below the valence/conduction band mobility edges, asin the case of hopping conduction, we can relate conductivityto the transition probability for hopping between sites. Basedon the percolation theory model, which states that conduc-tion is dominated not by the average distance between sites,but by a critical distance at which hopping rates are at theirlowest while still allowing for a continuous transport paththrough the solid, conductivity (speciﬁcally, mobility) can bedeﬁned as proportional to exp(c/aN1/3), where cis a factorand Nis the concentration of sites, and is expected to035703-13 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38 increase with increasing N.136,137Therefore, assuming a cor-relation between concentration of sites and concentration ofatoms in the solid (i.e., density), we might expect thatdecreasing density would lead to decreasing conductivity(i.e., higher resistivity), as is indeed observed in a-BxC:HyatH concentrations 35%. Another way to look at the problemmight be to consider a multiple trapping and release mecha-nism, where conduction occurs via extended band states, butconductivity (speciﬁcally, mobility) is reduced through trap-ping by localized states. In this scenario, we can assume thatresistivity would be related to disorder, represented by theUrbach energy and Tauc slope parameter. A higher Urbachenergy implies a wider Urbach tail, and consequentlyincreased trapping by localized states and lower conductivity(higher resistivity). This reasoning is typically applied toamorphous silicon in relating narrower band tails to highermobilities and thus better device quality.60,138,139Becausethis relationship between Urbach energy and resistivity is notobserved in the regime of 10%–35% at. % H, this suggeststhat a hopping/percolation model better describes the data atlow hydrogen concentrations in a-BxC:Hy. This is reasonableconsidering the fact that at low H concentrations, the bandgap is at its lowest (2 eV), but the Urbach energy is at itshighest (700 meV), implying that localized tail states ﬁllthe majority of the gap. The reversal in trend between resis-tivity and at. % H observed at 35% H may suggest a transi-tion from a hopping to a multiple trapping mechanism. Inthis stoichiometric regime, the Urbach energy is much lower,and therefore the band tails much narrower. A similar transi-tion in transport mechanisms is proposed for a-SiC:H as afunction of C fraction, although not necessarily for identicalreasons.140V. CONCLUSIONSWe have investigated the relationship between chemi-cal, physical/mechanical, optical/electronic, and electricaltransport properties in amorphous hydrogenated boron car-bide, and have shown that hydrogenation has a clear andpowerful effect on material properties important for techno-logical applications. Through varying growth temperatureand power, we have demonstrated that a wide range of prop-erties can be obtained in PECVD ortho-carborane-baseda-BxC:Hyﬁlms, including atomic concentration hydrogenfrom 10% to 45%, density from 0.9 to 2.3 g/cm3, hardnessfrom 1 to 25 GPa, Young’s modulus from 10 to 340 GPa,dielectric constant (j) from 3.1 to 7.6, Tauc band gap(ETauc,n¼2) from 1.7 to 3.8 eV, Tauc slope parameter(B1/2) from 200 to 1600 cm1/2eV1/2, Urbach energy(EU) from 0.1 to 0.7 eV, and electrical resistivity from 1010to 1015Xcm. The low dielectric constant, excellent mechan-ical strength, low pore interconnectivity, and high resistivity/low leakage current in these ﬁlms are promising toward theiruse in insulating low-jdielectric applications. Further, thesensitive tunability of their optical, electronic, and electricaltransport properties suggests avenues for optimizationtoward other specialized electronic applications such asneutron detection.We have identiﬁed two extremes of material composi-tion within the growth regime studied: at low growth temper-ature and power, a hydrogen-rich, low-density ﬁlm isformed, and at high growth temperature and power, ahydrogen-deﬁcient, high-density ﬁlm is formed. Hydrogenand density demonstrate a very clear inverse linear correla-tion with each other, and map many of the other materialproperties. Lower density ﬁlms exhibit low hardness andYoung’s modulus, high band gap, low EU/high B1/2(i.e., lowdisorder), low dielectric constant, and high electrical resistiv-ity, while the opposite is true for the higher density ﬁlms.We have analyzed the trends in the context of classic amor-phous systems including a-Si:H, a-C:H, and a-SiC:H, andhave found that, despite its very different atomic structure(i.e., six-fold icosahedral coordinate vs four-fold tetrahedralcoordinate), a-BxC:Hyexhibits many similarities to thesesystems, especially a-Si:H. We propose that hydrogenationof the a-BxC:Hynetwork has several key effects: (1) itdecreases network connectivity and average coordinationnumber, which affects E,H,Eg,e1/j, and q; (2) it decreasesmass/electron density, which is also directly related to e1/j;and ﬁnally, (3) it relieves strain in the network, which resultsin a decrease in structural disorder as measured by EUandB1/2. Regarding (1), the identiﬁcation of a rigidity percola-tion threshold in a six-coordinate network is a novel ﬁndingthat provides an interesting basis for future studies.Regarding (2), we note that in a-BxC:Hy,EUand Egshow aninverse correlation like in a-Si:H, which is opposite to thedirect correlation between the two observed in a-C:H anda-SiC:H. Finally, we tentatively propose the possibility of atransition between hopping and multiple trapping transportmechanisms at a hydrogen concentration of 35% based onthe relationship between electrical resistivity and at. % H.This study has provided a snapshot of process–para-meter space for a-BxC:Hy, and—within a fairly narrow pro-cess space—this material has already demonstratedsigniﬁcant tunability as well as very clear correlationsbetween process, structure, and properties. Future work willinvestigate a wider range of process conditions, includingvariations in pressure and ﬂow rate, as well as delve deeperinto the analysis of atomic and electronic structure, and theirrelationship to mechanical, optical, and electrical properties.We anticipate that, as in the case of a-Si:H, rigorous effortsto understand and optimize this material will pay dividendstowards its integration into viable commercial technologies.ACKNOWLEDGMENTSThe authors gratefully acknowledge Intel Corporation(Contract No. 2012-IN-2313) and the Defense ThreatReduction Agency (Grant No. HDTRA1-10-1-0092) forﬁnancial support of this research.1D. Emin, Phys. Today 40(1), 55 (1987).2D. Emin, J. Solid State Chem. 179, 2791 (2006).3H. Werheit, J. Phys.: Conf. Ser. 176, 012019 (2009).4M. M. Balakrishnarajan, P. D. Pancharatna, and R. Hoffmann, New J.Chem. 31, 473 (2007).5V. Domnich, S. Reynaud, R. A. Haber, and M. Chhowalla, J. Am. Ceram.Soc. 94, 3605 (2011).035703-14 Nordell et al. J. Appl. Phys. 118, 035703 (2015) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:59.185.236.52 On: Mon, 05 Oct 2015 11:00:38Citations (27)References (221)... Since hydrogen is the lowest atomic number element (Z = 1), hydrogenation of dielectric materials to obtain low dielectric constant values has gained much interest. Materials such as a-Si:H, a-C:H, a-SiC:H, SiOC:H, and SiCN:H have been reported to exhibit lower dielectric values after hydrogenation [1,17]. Hydrogenation of amorphous boron carbide films using the plasma enhanced chemical vapor deposition (PECVD) technique exhibited dielectric values between 2.3-7.6 [17]. ...... Materials such as a-Si:H, a-C:H, a-SiC:H, SiOC:H, and SiCN:H have been reported to exhibit lower dielectric values after hydrogenation [1,17]. Hydrogenation of amorphous boron carbide films using the plasma enhanced chemical vapor deposition (PECVD) technique exhibited dielectric values between 2.3-7.6 [17]. Although dielectric properties of amorphous hydrogenated boron carbide films have been studied using the PECVD technique, focused research on the influence of other deposition techniques and growth parameters has not been evaluated. ...... The film deposited at 300 • C shows higher hydrogen content than other films. Hydrogen being the lowest atomic number element reduces the total mass density of the film and hence is known to reduce dielectric value of the film [17]. As the hydrogen concentration in the film is estimated to be less ( 10%), it was difficult to quantify the atomic hydrogen content in these films. ...Influence of Substrate Temperature on Electrical and Optical Properties of Hydrogenated Boron Carbide Thin Films Deposited by RF SputteringArticleFull-text availableFeb 2021 Shraddha Nehate Ashwin Kumar SaikumarKalpathy B. SundaramAmorphous hydrogenated boron carbide films were deposited on silicon and glass substrates using radio frequency sputtering. The substrate temperature was varied from room temperature to 300 °C. The substrate temperature during deposition was found to have significant effects on the electrical and optical properties of the deposited films. X-ray photoelectron spectroscopy (XPS) revealed an increase in sp2-bonded carbon in the films with increasing substrate temperature. Reflection electron energy loss spectroscopy (REELS) was performed in order to detect the presence of hydrogen in the films. Metal-insulator-metal (MIM) structure was developed using Al and hydrogenated boron carbide to measure dielectric value and resistivity. Deposited films exhibited lower dielectric values than pure boron carbide films. With higher substrate deposition temperature, a decreasing trend in dielectric value and resistivity of the films was observed. For different substrate temperatures, the dielectric value of films ranged from 6.5–3.5, and optical bandgap values were between 2.25–2.6 eV.ViewShow abstract... These icosahedral subunits are then-we believe (Paquette et al., 2011)-cross-linked either directly to each other ( Figure 1A) or via hydrocarbon linkers (Figure 1B), where the total amount of 1-fold-coordinate atomic hydrogen contained within the material correlates with decreased overall network coordination through the termination of icosahedral vertices and decreased cross-linking. By varying PECVD conditions, we have produced a substantial set of a-BC:H films with a vast range of densities, hydrogen concentrations, and effective network coordination (Figure 2A; Nordell et al., 2015Nordell et al., , 2016a. We have observed a clear threshold in Young s modulus [and other properties (Nordell et al., 2015(Nordell et al., , 2016a] as a function of density/hydrogen concentration ( Figure 2B), which we attribute to a rigidity transition. ...... By varying PECVD conditions, we have produced a substantial set of a-BC:H films with a vast range of densities, hydrogen concentrations, and effective network coordination (Figure 2A; Nordell et al., 2015Nordell et al., , 2016a. We have observed a clear threshold in Young s modulus [and other properties (Nordell et al., 2015(Nordell et al., , 2016a] as a function of density/hydrogen concentration ( Figure 2B), which we attribute to a rigidity transition. In the present study, we describe and compare a number of constraint counting strategies and show how a-BC:H adheres to traditional TCT predictions if we apply a superatom approach and treat individual icosahedra as independently constrained units. ...... In extending these constraint counting principles to amorphous hydrogenated boron carbide, we must first revisit the nature of the bonding in this material. The present study is based on multiple series of a-BC:H films (Nordell et al., 2015(Nordell et al., , 2016a, including some fabricated as part of this work, grown by plasma-enhanced chemical vapor deposition from the single molecular precursor, ortho-carborane (o-C 2 B 10 H 12 ). We can differentiate the resulting amorphous solid from the more well-known and heavily studied crystalline B x C variant, which is known to be comprised of B 12 or B 11 C icosahedra and C-C-C or C-B-C chains within a densely packed rhombohedral lattice (Domnich et al., 2011). ...Topological Constraint Theory Analysis of Rigidity Transition in Highly Coordinate Amorphous Hydrogenated Boron CarbideArticleFull-text availableOct 2019 Bradley J. Nordell Thuong NguyenAnthony N. Caruso Michelle PaquetteTopological constraint theory (TCT) has revealed itself to be a powerful tool in interpreting the behaviors of amorphous solids. The theory predicts a transition between a \"rigid” overconstrained network and a \"floppy” underconstrained network as a function of connectivity or average coordination number, 〈r〉. The predicted results have been shown experimentally for various glassy materials, the majority of these being based on 4-fold-coordinate networks such as chalcogenide and oxide glasses. Here, we demonstrate the broader applicability of topological constraint theory to uniquely coordinated amorphous hydrogenated boron carbide (a-BC:H), based on 6-fold-coordinate boron atoms arranged into partially hydrogenated interconnected 12-vertex icosahedra. We have produced a substantial set of plasma-enhanced chemical vapor deposited a-BC:H films with a large range of densities and network coordination, and demonstrate a clear threshold in Young s modulus as a function of 〈r〉, ascribed to a rigidity transition. We investigate constraint counting strategies in this material and show that by treating icosahedra as \"superatoms,” a rigidity transition is observed within the range of the theoretically predicted 〈r〉c value of 2.4 for covalent solids with bond-stretching and bond-bending forces. This experimental data set for a-BC:H is unique in that it represents a uniform change in connectivity with 〈r〉 and demonstrates a distinct rigidity transition with data points both above and below the transition threshold. Finally, we discuss how TCT can be applied to explain and optimize mechanical and dielectric properties in a-BC:H and related materials in the context of microelectronics applications.ViewShow abstract... The uncertainty in this approach to determining r is approximately ±5% [68]. Film thicknesses for the a-SiN x :H and a-Si:H samples were characterized by ultraviolet-visible (UV-VIS) spectroscopic ellipsometry using an absorbing model [69]. Additionally, the as-deposited film thicknesses were corroborated by fitting the undulations in the background for the transmission FTIR spectra [70]. ...... These differences are the result of two sets of independent measurements being performed on samples cleaved from different portions of the 300-mm-diameter wafer and represent both measurement-measurement variability and across wafer thickness and composition uniformity. Thickness and refractive index measurements via spectroscopic ellipsometry described in Ref. [69]. Refractive index values reported at 633 nm. ...Hydrogen effects on the thermal conductivity of delocalized vibrational modes in amorphous silicon nitride ( a − SiN x : H )ArticleFull-text availableMar 2021 Jeffrey L. Braun Sean W. King Eric HoglundPatrick E. HopkinsHydrogenated amorphous dielectric thin films are critical materials in a wide array of technologies. In this work, we present a thorough investigation of the thermal conductivity of hydrogenated amorphous silicon nitride (a−SiNx:H), a ubiquitously used material in which the stoichiometry plays a direct role in its functionality and application. In particular, through chemical, vibrational, and structural analysis in tandem with thermal conductivity measurements on chemically variant silicon nitride films, we show that hydrogen incorporation into silicon nitride disrupts the bonding among silicon and nitrogen atoms, and directly impacts the thermal conductivity, leading to as much as a factor of 2.5 variation in heat transfer. This variability, driven by the change in hydrogen content, is fundamentally related to the changes in the average atomic distances, as we experimentally measure with selected-area electron diffraction and computationally show with molecular dynamics simulations. This, combined with our evidence of chemical and spatial fluctuations on the order of average atomic pair distances, leads us to conclude that the vibrational heat transport in a−SiNx:H is primarily dominated by diffusonlike modes. The results presented in this work combined with our extensive review of prior reports on the thermal conductivity of a−SiNx:H films resolves discrepancies in decades of prior literature and facilitates a more universal understanding of the vibrational heat transport processes in hydrogenated amorphous silicon nitride.ViewShow abstract... Amorphous hydrogenated boron carbide (a-BC:H) is a desired option containing both proper mechanical properties and low dielectric constant. The a-BC:H films can be grown using plasma enhanced chemical vapor deposition (PECVD) with ortho-carborane precursors [4], which results in a wide range of variation in the structure and composition. Theoretical modeling of amorphous hydrogenated boron carbide has suggested that atomic structure of a-BC:H contains 12-vertex icosahedral clusters which are attributed to short range ordering (SRO, Fig. 2b) [5]. ...Direct Determination of Medium Range Ordering in Amorphous Hydrogenated Boron Carbide for Low-k Dielectric ApplicationsArticleJul 2020MICROSC MICROANAL Mehrdad Abbasi Soohyun Jerry ImJared JohnsonJinwoo HwangDirect Determination of Medium Range Ordering in Amorphous Hydrogenated Boron Carbide for Low-k Dielectric Applications - Mehrdad Abbasi Gharacheh, Soohyun Im, Jared Johnson, Gabriel Calderon Ortiz, Menglin Zhu, Nathan Oyler, Michelle Paquette, Paul Rulis, Ridwan Sakidja, Jinwoo HwangViewShow abstract... 15 While there have been numerous studies of the structural changes in boron carbides as a result of irradiation, [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] most studies involve hot-pressed/sintered or sputtered samples, rather than plasma enhanced chemical vapor deposition (PECVD) samples. The distinction being that PECVD synthesized boron carbide films may retain some fraction of their source molecule hydrogen, [31][32][33][34] and are not very likely to be exceedingly crystalline. Of the studies that do focus on PECVD synthesized boron carbide, very few studies have focused on the changes in electrical characteristics as a result of irradiation. ...Improved a -B sub 10 /sub C sub 2+x /sub H sub y /sub /Si p-n heterojunction performance after neutron irradiationArticleJan 2018 George Glenn Peterson Qing Su Yongqiang WangMichael NastasiThe impact of neutron irradiation, in the energy range of ~0.025 eV, on amorphous semiconducting partially dehydrogenated boron carbide (a-B10C2+xHy) on silicon p-n heterojunction diodes was investigated. The heterojunction devices were created by synthesizing a-B10C2+xHy via plasma enhanced chemical vapor deposition on n-type silicon. Unlike many electronic devices, the performance of the a-B10C2+xHy heterojunction diode improved with neutron irradiation, in spite of the large neutron cross section of 10B. There is also increased charge carrier lifetime of more than 200% with modest neutron irradiation of approximately 2.7×108 to 1.08×109 neutrons/cm2.ViewShow abstract... This is primarily a result of their high optical transparency and strong resistance to fluorine based dry etches. 37,264 Similarly, boron-based dielectrics have also gained significant interest as alternative or complementary hard masking materials 265,266 due to their significant dry etch rates in fluorine-based chemistries, 37,219,267 low wet etch rates in dilute HF, 223,268 and tunable bi-axial film stress 37 and mechanical properties. 264,269 The high resistance of high-k oxides and nitrides to fluorinated plasma chemistries has also led to significant interest for etch stopping applications in the final pattern transfer to the target material. ...Review—Beyond the Highs and Lows: A Perspective on the Future of Dielectrics Research for Nanoelectronic DevicesArticleOct 2019 Melanie Jenkins Dustin Austin J. F. Conley Sean W. KingHigh-dielectric constant (high-k) gate oxides and low-dielectric constant (low-k) interlayer dielectrics (ILD) have dominated thenanoelectronic materials research scene over the past two decades, but they have recently reached a state of maturity and perhapsthe limits of their scaling. Based on this, there is a need for a systematic review summarizing not only the historic research andachievements on high-k and low-k dielectrics, but also emerging device applications as well as an outlook of future challenges.We begin by first reviewing the factors that drove the emergence of low-k and high-k materials in nanoelectronics as ILD and gatedielectric materials, respectively, and the challenges and limits these materials ultimately approached in terms of permittivity scaling.We then illustrate that gate dielectric and ILD applications represent just a small fraction of the numerous dielectrics utilized inpresent day nanoelectronic products where permittivity scaling is now being increasingly demanded for materials such as dielectricspacers, trench isolation, and etch stopping layers.We conclude by examining the numerous new applications for dielectric materialsthat are emerging as the semiconductor industry transitions to novel patterning schemes, prepares for life post CMOS scaling, andexplores ways to natively embed device functionality in themetal interconnect. For the former, we specifically examine the \"colorful”requirements for the various enabling dielectric hardmask and spacer materials utilized in pitch division-multi-pattern processes andthen discuss the role that selective area deposition of dielectrics and metals could play in reducing the complexity of such patterningprocesses. For the latter, we review the use of both high-k and low-k dielectrics in various metal-insulator-metal (MIM) structuresas Fermi level de-pinning layers, tunnel diodes, and back-end-of-line (BEOL) compatible capacitive and resistive switching randomaccess memory (ReRAM) elements.We further examine how dielectrics can hinder or aid new forms of computing such as quantumand neuromorphic in reaching their full potential. In conclusion, we find that while the field of dielectrics has a long history, it remainsvibrant with numerous exciting new and old research vectors awaiting further exploration.© 2019 The Electrochemical Society.ViewShow abstractSingle-carrier charge collection in thin direct-conversion semiconductor neutron detector: A numerical simulationArticleMay 2021 Gyanendra Bhattarai Michelle PaquetteAnthony N. CarusoAlthough direct-conversion solid-state neutron detection has been investigated for over five decades, propelling this technology beyond the basic research stage remains an outstanding challenge. This challenge is due to the very small selection of neutron-sensitive isotopes and therefore lack of mature semiconductor materials available for this technology. Given these constraints, there is a reason to investigate materials with less-than-optimal charge transport properties, which could include low charge carrier mobility/lifetime and/or single-carrier transport (i.e., order of magnitude or greater difference between electron and hole mobility). Such materials are potentially best-suited to a thin-film configuration, which provides not only leniency in terms of charge transport requirements, but also processing flexibility and integration advantages. Single-carrier transport in detectors with thicknesses less than or comparable to radiation penetration depth can lead to partial and position-dependent charge collection effects not treated in the general case of direct-conversion neutron detection. Here, we have developed a theory to include the effect of single-carrier charge collection and the possible mismatch between carrier transit time and integration time to study the performance of thin neutron detectors. Taking a boron carbide (B4C) direct-conversion thermal neutron detector as an example, we use custom Monte Carlo simulations to study the effects of a range of mobility, lifetime, thickness, and integration time values on detection efficiency and pulse height spectra. We discuss the interplay between the traditional mobility–lifetime product ( μ τ ) metric and the integration time to carrier transit time ratio ( t i / t tr ), which takes into account mobility ( μ ) specifically, and their effect on detection efficiency. We describe the effect of these parameters on pulse height spectra and show how, although single-carrier transport leads to a loss of spectral resolution when signal current is fully integrated, using integration times shorter than carrier transit time allows for recovery of spectral features. We additionally present two methods to extract the mobility–lifetime product of a single-carrier device, with the first being based on the steady-state current as a function of electric field under a steady-state radiation detection mode, and the second being based on the shift of spectral peaks as a function of electric field under a single-particle radiation counting mode, both using modified Hecht equations that do not require either surface or uniform radiation absorption conditions. Finally, we discuss the performance of a hypothetical single-carrier 5 μm thick B4C neutron detector, which can provide a maximum intrinsic neutron detection efficiency of 14% with a set lower level discriminator value of 25% of the total energy deposited.ViewShow abstractFabrication of (a-nc) boron carbide thin films via chemical vapor deposition using ortho-carboraneArticleFull-text availableApr 2020 Rong TuXuan HU Jun li Song ZhangAmorphous-nanocrystalline (a-nc) boron carbide thin films were prepared by chemical vapor deposition (CVD) by using ortho-carborane as a single-source precursor for inertial confinement fusion (ICF) application. The effects of deposition temperature (Tdep) and total pressure (Ptot) on chemical composition, microstructure, stoichiometry and morphology of the boron carbide films were investigated. The TEM results show that the structure of the film is mainly composed of amorphous boron carbide with dispersive nano-grains, which will be able to improve the mechanical properties of the film with relatively low roughness. The hardness of the (a-nc) boron carbide film obtained in this study reached 20.6 GPa, and roughness of 3.21 nm. The deposited films sized 0.2–1.9 μm in thickness with B/C atomic ratio ranged from 0.14 to 3.29. The deposition rate decreased with increasing deposition temperature and Ptot, while B/C ratio increased.ViewShow abstractRole of generated free radicals in synthesis of amorphous hydrogenated boron carbide from orthocarborane using argon bombardment: A ReaxFF molecular dynamics studyArticleFeb 2020 Nirmal Baishnab Rajan Khadka Michelle Paquette Ridwan SakidjaViewEnergetics of Porous Amorphous Low-k SiOCH Dielectric FilmsArticleJul 2019J CHEM THERMODYN Jiewei ChenJason J. Calvin Sean W. King Alexandra NavrotskyThe trend toward dimension reduction in electronic devices has driven the research and development of low dielectric constant materials to solve resistance capacitance delays and power consumption issues. Amorphous porous hydrogenated silicon oxycarbides form a class of materials that have even lower dielectric constants than other silicon oxycarbides, achieved by introducing organic functional groups as well as porosity to alter the structure and decrease the density of conventional silica. However, these films tend to decompose to SiO2 during thermal annealing and nano-electronic fabrication processes. This apparent instability leads to the need to investigate how porosity can change the fundamental thermodynamic stability of this class of materials. We used high temperature oxidative molten salt solution calorimetry and cryogenic heat capacity measurements to directly determine enthalpies of formation, heat capacities, and standard entropies of a series of well-characterized porous SiOCH films. Thus, a full thermodynamic dataset has been obtained for these porous low-k films. All of these samples are either found to be unstable or metastable at or even below 298.15 K with respect to their crystalline counterparts and gaseous products. Thus, there appears little chance of forming thermodynamically stable amorphous porous SiOCH films and their persistence in fabrication and use is controlled by kinetic rather than thermodynamic factors.ViewShow abstractShow moreDielectric Barrier, Etch Stop, and Metal Capping Materials for State of the Art and beyond Metal InterconnectsArticleFull-text availableJan 2015 Sean W. KingOver the past decade, the primary focus for improving the performance of nano-electronic metal interconnect structures has been to reduce the impact of resistance-capacitance (RC) delays via utilizing insulating dielectrics with ever lower values of dielectric permittivity. The integration and implementation of such low dielectric constant (i.e. low-k) materials has been fraught with numerous challenges. For intermetal and interlayer (ILD) low-k dielectrics, these challenges have been largely associated to integration with metal interconnect fabrication processes and well documented and reviewed in the literature. Although equally important, less attention has been given to other low-k dielectrics utilized in metal interconnect structures that are commonly referred to as low-k dielectric barriers (DB), etch stops (ES), and/or Cu capping layers (CCL). These materials present numerous challenges as well for integration into metal interconnect fabrication processes. However, they also have more stringent integrated functionality requirements relative to low-k ILD materials that serve only a basic purpose of electrically isolating adjacent metal lines. In this article, we review the integration challenges and associated integrated functionality requirements for low-k DB/ES/CCL materials with a focus on the current status and future direction needed for these materials to facilitate both Moore s law (i.e. More Moore) and More than Moore scaling.ViewShow abstractCharge transport in disordered solids with applications in electronicsBookJan 2006S. BaranovskiThe field of charge conduction in disordered materials is a rapidly evolving area owing to current and potential applications of these materials in various electronic devices. This text aims to cover conduction in disordered solids from fundamental physical principles and theories, through practical material development with an emphasis on applications in all areas of electronic materials. International group of contributors. Presents basic physical concepts developed in this field in recent years in a uniform manner. Brings up-to-date, in a one-stop source, a key evolving area in the field of electronic materials.ViewShow abstractThe physics of amorphous solids [M]ArticleJan 2007R. J. ZallenAn in-depth study of non-crystalline solids in which the arrangement of the atoms do not have long-range order. Describes the way amorphous solids are formed, the phenomenology of the liquid-to-glass and glass- to-liquid transition, and the technological applications. Emphasizes modern approaches such as scaling, localization, and percolation. Includes extensive treatment of structural aspects of amorphous solids, ranging from metallic glasses, to chalcogenides, to organic polymers. Incorporates illustrations for the clarification of physics concepts. © 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim. All rights reserved.ViewShow abstractEffects of space exposure on ion-beam-deposited silicon-carbide and boron-carbide coatingsArticleJan 1998 Ritva A. M. Keski-KuhaTwo recently developed optical coatings, ion-beam-deposited silicon carbide and ion-beam-deposited boron carbide, are very attractive as coatings on optical components for instruments for space astronomy and earth sciences operating in the extreme-UV spectral region because of their high reflectivity, significantly higher than any conventional coating below 105 nm. To take full advantage of these coatings in space applications, it is important to establish their ability to withstand exposure to the residual atomic oxygen and other environmental effects at low-earth-orbit altitudes. The first two flights of the Surface Effects Sample Monitor experiments flown on the ORFEUS-SPAS and the CRISTA-SPAS Shuttle missions provided the opportunity to study the effects of space exposure on these materials. The results indicate a need to protect ion-beam-deposited silicon-carbide-coated optical components from environmental effects in a low-earth orbit. The boron-carbide thin-film coating is a more robust coating able to withstand short-term exposure to atomic oxygen in a low-earth-orbit environment.ViewShow abstractBiological ApplicationsChapterJul 2005 Barbara StuartProvides an introduction to those needing to use infrared spectroscopy for the first time, explaining the fundamental aspects of this technique, how to obtain a spectrum and how to analyse infrared data covering a wide range of applications. Includes instrumental and sampling techniques Covers biological and industrial applications Includes suitable questions and problems in each chapter to assist in the analysis and interpretation of representative infrared spectra Part of the ANTS (Analytical Techniques in the Sciences) Series.ViewShow abstractTopological Constraint Theory of GlassArticleMay 2011AM CERAM SOC BULL John C. MauroA microscopic physical description of the glassy state long has eluded even the top scientists in condensed matter physics because of the complicated noncrystalline nature of glass structure. Currently, many theorists turn to molecular dynamics or other atomistic simulations to determine the structure of various glass compositions. However, although available computing power has increased exponentially during the past several decades, it will be at least another 20 to 30 years before enough computing power is available for direct molecular dynamics simulations of glass on a realistic laboratory time scale. Fortunately, topological constraint theory provides another path forward. It focuses on the important microscopic physics governing the thermal, mechanical and rheological properties of glass, while filtering out unnecessary details that ultimately do not affect its macroscopic properties. Topological constraint theory has been successful in predicting the composition dependence of glass properties and can be used as a tool to enable the quantitative design of new glass compositions.ViewShow abstractEPMA and ellipsometric characterization of PECVD boron-carbon filmsArticleAug 1993A. I. KanaevS. Yu. RybakovM. N. CHURAEVAThe characteristics of a-B/C:H films, produced by PECVD on Si(100) surface using the novel harmless precursor-carborane have been studied. These films proved to be excellent for protection tokamaks inner surfaces, for they are heat-proof and chemically resistant. EPMA and ellipsometry used together enabled to determine not only the films composition and thickness, but also the density and some other parameters. A special computer program based on Yakovitz-Newbury method has been developed to study rather thin (approximately 100nm) films by EPMA. Spectroellipsometry was used to determine the wave dependence of the films optical constants. It has been established that B/C ratio grows from 1 to 3.6 with increasing in carborane pressure and does not depend on a substrate temperature and voltage applied. EPMA measurements performed in cooperation with ellipsometry showed the increasing microporosity (up to 40%) for the films exposed to the fluence of deuterium ions about 10(21)cm-2.ViewShow abstractLocal network structure of a-SiC:H and its correlation with dielectric functionArticleDec 2013J APPL PHYSShota KageyamaNobuyuki Matsuki Hiroyuki FujiwaraThe microscopic disordered structures of hydrogenated amorphous silicon carbide (a-Si 1−xCx:H) layers with different carbon contents have been determined based on the correlations between the dielectric function in the ultraviolet/visible region and the local bonding states studied by high-sensitivity infrared attenuated total reflection spectroscopy. We find that the microscopic structure of the a-Si 1−xCx:H layers fabricated by plasma-enhanced chemical vapor deposition shows a sharp structural transition at a boundary of x = 6.3 at. %. In the regime of x ≤ 6.3 at. %, (i) the amplitude of the a-SiC:H dielectric function reduces and (ii) the SiH2 content increases drastically with x, even though most of the C atoms are introduced into the tetrahedral sites without bonding with H. In the regime of x 6.3 at. %, on the other hand, (i) the amplitude of the dielectric function reduces further and (ii) the concentration of the sp3 CHn (n = 2,3) groups increases. Moreover, we obtained the direct evidence that the sp2 C bonding state in the a-SiC matrix exists in the configuration of C = CH2 and the generation of the graphite-like C = CH2 unit suppresses the band gap widening significantly. At high C contents of x 6.3 at. %, the a-SiC:H layers show quite porous structures due to the formation of microvoids terminated with the SiH2/CHn groups. By taking the SiH2/CHn microvoid generation in the network and the high-energy shift of the dielectric function by the local bonding states into account, the a-SiC:H dielectric function model has been established. From the analysis using this model, we have confirmed that the a-SiC:H optical properties in the ultraviolet/visible region are determined almost completely by the local network structures.ViewShow abstractOptical absorption and disorder in hydrogenated amorphous SiGe and SiC alloy systemsArticleJun 1987Sol Cell Raphael TsuP. Menna Archie Harvin MahanThe slope of depends on the product of the oscillator strength of the transition, the deformation potential and the mean deviation of the atomic coordinates. In a perfect crystal, this deviation is characterized by phonons, whereas in amorphous silicon (a-Si) and amorphous germanium (a-Ge) it is characterized by the mean bond angle distribution. We have used this slope to characterize disorder in a-SiGe:H and a-SiC:H alloy systems. The Urbach edge and gap states obtained by photothermal deflection spectroscopy are included for comparison.ViewShow abstractThe 1280 Cm−1 Absorption Line in Amorphous Hydrogenated Boron CarbideArticleJan 2011Mater Res Soc Symp ProcShu-Han Lin Bernard Joseph FeldmanWe report infrared absorption measurements that provide evidence for the presence of boron carbide icosahedra in amorphous hydrogenated boron carbide thin films. The infrared absorption spectra is dominated by an intense line at 1280 cm-1 with a FWHM of ≃320 cm-1. Similar lines have been previously reported in polycrystalline boron carbide, where boron carbide icosahedra make up the unit cell. In both systems, the linewidth narrows and the peak position shifts to higher energy with increasing carbon concentrations. From annealing studies of amorphous hydrogenated boron carbide, hydrogen plays a very small role in the 1280 cm-1 line. Finally, the integrated intensity of the 1280 cm-1 line is a sublinear function of the boron concentration, providing further evidence that the carbon concentration in these icosahedra increases as the carbon concentration of the film increases.ViewShow abstractShow moreAdvertisementRecommendationsDiscover moreProjectLIGO Project Sudarshan KarkiView projectProjectAdvanced ceramic functional materials Liaoyuan Wang Paul Rulis Wai-Yim ChingView projectArticleEffect of SiC on fabrication and properties of AlN/Mo/SiC composite ceramicsNovember 2011Q. DongZ. YangThe AlN-Mo-SiC composite ceramics with relative density of more than 98.5% with AlN, Mo and SiC as the starting materials could be prepared by spark plasma sintering (SPS) at the temperatures of 1700°C and under the pressure of 30 MPa. The phase structure and microstructure of different SiC content samples were observed by X-ray diffraction (XRD) and field-emission scanning electron microscope ... [Show full abstract] (FESEM) respectively. The dielectric properties and thermal conductivity were also tested. The experimental results showed that AlN/Mo/SiC composite ceramics with high relative density could be prepared by utilizing spark plasma sintering under a lower temperature than before; however, with the increase of SiC volume fraction, the relative density decreased sharply when the fraction of SiC volume reached 60%. With the addition of SiC, the electrical resistivity of composite ceramics could be controlled more accurately compared with AlN/Mo composite ceramics to avoid percolation. Under 26.5~40.0 GHz, dielectric constant and dielectric loss of composite ceramics increased with the increase of SiC volume fraction. The thermal conductivity of composite ceramics decreased with the increase of SiC volume fraction. When the relative density was higher than a threshold, its thermal conductivity was mainly determined by the volume fraction and distribution of SiC; however, when the relative density was lower than a certain critical value, it became a determining factor of thermal conductivity, which made the thermal conductivity decrease sharply with the decreasing density.Read moreArticleDirect observation of the M2 phase with its Mott transition in a VO$_2$ filmNovember 2016 · Applied Physics Letters Hoon Kim Tetiana SlusarDirk Wulferding[...] Jeehoon KimIn VO$_2$, the explicit origin of the insulator-to-metal transition is still disputable between Peierls and Mott insulators. Along with the controversy, its second monoclinic (M2) phase has received considerable attention due to the presence of electron correlation in undimerized vanadium ions. However, the origin of the M2 phase is still obscure. Here, we study a granular VO$_2$ film using ... [Show full abstract] conductive atomic force microscopy and Raman scattering. Upon the structural transition from monoclinic to rutile, we observe directly an intermediate state showing the coexistence of monoclinic M1 and M2 phases. The conductivity near the grain boundary in this regime is six times larger than that of the grain core, producing a donut-like landscape. Our results reveal an intra-grain percolation process, indicating that VO$_2$ with the M2 phase is a Mott insulator.Read moreArticleTuning the properties of a complex disordered material: Full factorial investigation of PECVD-grown...February 2016 · Materials Chemistry and Physics Bradley J. Nordell Thuong Nguyen Michelle Paquette[...]Christopher L. KeckA multiresponse 25 full factorial experiment is performed to investigate the effects of growth conditions (temperature, power, pressure, total flow rate, partial precursor flow rate) on the chemical, mechanical, dielectric, electronic, and charge transport properties of thin-film amorphous hydrogenated boron carbide (a-BxC:Hy) grown by plasma-enhanced chemical vapor deposition (PECVD) from ... [Show full abstract] ortho-carborane. The main and interaction effects are determined and discussed, and the relationships between properties are investigated via correlation analysis. The process condition with the strongest influence on growth rate is pressure, followed by partial precursor flow rate, with low pressure and high partial flow rate conditions yielding the highest growth rates. The atomic concentration of hydrogen (at.% H) and density are controlled primarily by temperature and power, with low temperature and power conditions leading to relatively soft, hydrogen-rich, low-density, porous films, and vice versa. The B/C ratio is controlled by temperature, power, pressure, and the power*pressure interaction, and is uncorrelated to hydrogen concentration. Thin-film dielectric and electronic structure properties, including high-frequency dielectric constant (ε1), low-frequency/total dielectric constant (κ), optical band gap (ETauc/E04), and Urbach energy (EU), are correlated strongly with at.% H, and weakly to moderately with B/C ratio. These properties are dominated by the influence of temperature, with a second significant influence from the power*pressure interaction. The interaction of power and pressure leads to two opposite growth regimes—high power and high pressure or low power and low pressure—that can produce a-BxC:Hy films with similar dielectric or electronic structure properties. Charge transport properties also show a correlation with at.% H and B/C, but not with the electronic structure and disorder parameters, which suggests a complicated relationship between the two. The range of properties measured highlights the potential of thin-film a-BxC:Hy for low-κ dielectric and neutron detection applications, and suggests clear pathways for future material property optimization.Read moreArticlePreparation and dielectric properties of AlN/Mo/TiB2 microwave attenuation materialsNovember 2015 Lli DongG.-X. DongX.-Q. Cheng[...]Y. ChenAlN/Mo/TiB2 microwave attenuation materials were prepared by spark plasma sintering (SPS). The effect of TiB2 content on the interface status, relative density, electrical resistivity and dielectric properties of the materials were investigated. The phase composition, microstructure, dielectric properties were investigated by XRD, SEM, network analyzer. The results show that the relative density ... [Show full abstract] of composite ceramic increases and then decreases with increase of TiB2 content; when TiB2 content is 15wt%, it can reach the highest relative density(98.71%). Adding TiB2 is contributed to increasing the dielectric constant and dielectric loss. The electrical resistivity result shows that the percolation threshold in TiB2 content of composite ceramic is 15wt%.Read moreDiscover the world s researchJoin ResearchGate to find the people and research you need to help your work.Join for free ResearchGate iOS AppGet it from the App Store now.InstallKeep up with your stats and moreAccess scientific knowledge from anywhere orDiscover by subject areaRecruit researchersJoin for freeLoginEmail Tip: Most researchers use their institutional email address as their ResearchGate loginPasswordForgot password? Keep me logged inLog inorContinue with GoogleWelcome back! Please log in.Email · HintTip: Most researchers use their institutional email address as their ResearchGate loginPasswordForgot password? Keep me logged inLog inorContinue with GoogleNo account? Sign upCompanyAbout usNewsCareersSupportHelp CenterBusiness solutionsAdvertisingRecruiting© 2008-2021 ResearchGate GmbH. All rights reserved.TermsPrivacyCopyrightImprint

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