Silicon Carbide Thin Films Research Articles

Concentration quenching and thermal activation of the luminescence from terbium‐doped a ‐SiC:H and c ‐AlN thin films

AbstractThe effect of terbium (Tb) doping on the photoluminescence (PL) of crystalline aluminum nitride (c ‐AlN) and amorphous hydrogenated silicon carbide (a ‐SiC:H) thin films has been investigated for different Tb atomic concentrations. The samples were prepared by DC and RF magnetron reactive sputtering techniques covering the concentration range of Tb from 0.5 to 11 at.%. The Tb‐related light emission versus the Tb concentration is reported for annealing temperatures of 450 °C, 750 °C and 1000 °C. In the low concentration region the intensity exhibits a linear increase and its slope is enhanced with the annealing temperature giving an activation energy of 0.106 eV in an Arrhenius plot. In the high concentration region an exponential decay is recorded which is almost independent on the host material, its structure and the annealing process (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermodynamics of the formation of face-centered-cubic silicon nanocrystals in silicon-rich SiC thin films annealed using rapid thermal annealing

Silicon nanocrystals (Si-NCs) in silicon carbide (SiC) matrix fabricated by rapid thermal annealing were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Two structures of Si-NCs were found in the thin films. Si-NCs in the surface layer had face-centered cubic (fcc) structure, whereas those in the inner layer had cubic diamond (cd) structure. The thermodynamics for the formation of fcc-structured Si-NCs was demonstrated. We suggested that two main factors, Gibbs free energy per atom and surface energy, determined what structure of Si-NC nucleus to be formed. Specifically, reducing the Gibbs free energy per atom of fcc-Si crystal and lowering the surface energy of fcc-Si crystal would be helpful for the formation of fcc-structured Si-NC nucleus. In addition, the effects of carbon and substrate on the formation of Si-NCs were discussed. The work suggested that fcc-structured Si-NCs were readily generated in the surface layer of Si-rich SiC thin films.

Si-rich a-Si1−xCx thin films by d.c. magnetron co-sputtering of silicon and silicon carbide: Structural and optical properties

Si-rich hydrogenated amorphous silicon carbide (a-Si1−xCx:H) thin films with different carbon fractions were elaborated by a d.c. magnetron sputtering system. Their structural and optical properties were investigated by UV–visible spectrophotometry, infrared spectroscopy (FTIR), secondary ion mass spectroscopy (SIMS), Raman spectroscopy and photoluminescence (PL). The results show that the increase in the carbon fraction induces an increase in the optical gap (Eg) up to a maximum value of 2.53eV, corresponding to a carbon fraction (x) of 0.25, and then Eg decreases to 1.76eV corresponding to a carbon fraction x=0.33.

Pressure dependent structural and optical properties of silicon carbide thin films deposited by hot wire chemical vapor deposition from pure silane and methane gases

Silicon carbide (SiC) thin films were deposited using hot wire chemical vapor deposition technique from silane (SiH4) and methane (CH4) gas precursors. The effect of deposition pressure on structural and optical properties of SiC films was investigated. Various spectroscopic methods including Fourier transform infrared spectroscopy, Raman scattering spectroscopy, Auger electron spectroscopy, and UV–Vis–NIR spectroscopy were used to study these properties. Films deposited at low deposition pressure were Si-rich, and were embedded with nano-crystals of silicon. These films showed strong absorption in the visible region and had low energy band gaps. Near stoichiometric SiC film, were formed at intermediate deposition pressure and these films were transparent in the visible region and exhibited a wide optical band gap. High deposition pressures caused inhomogeneity in the film as reflected by the increase in disorder parameter and low refractive index of the films. This was shown to be due to formation of sp 2 carbon clusters in the film structure.

Behaviour of amorphous silicon carbide in Au/a-SiC/Si heterostructures prepared by PECVD technology using two different RF modes

Amorphous silicon carbide a-SiC (or more accurate a-Si1−xCx) thin films were prepared within Au/a-Si1−xCx/c-Si(p)/Al structures by PECVD deposition technique. Thin silicon carbide films on (100) silicon substrates were deposited using two different RF modes, i.e. placing the substrates either on electrically powered or grounded electrode. FTIR spectroscopy method was used for characterization of chemical bond states in the structure. Morphology of surface was analysed by AFM. C–V measurements on the prepared heterostructures were performed to evaluate the charge transport properties within the Au/a-Si1−xCx/c-Si(p)/Al structures. Temperature dependences of forward (FW) current–voltage (I–V) characteristics of Au/a-Si1−xCx/c-Si(p)/Al structures are shown and analysed in this paper. Parameters as saturation current and activation energies were calculated from forward biased I–V curves.

Preparation of Al-doped hydrogenated nanocrystalline cubic silicon carbide by VHF-PECVD for heterojunction emitter of n-type crystalline silicon solar cells

Al-doped p-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) thin films were successfully deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) using dimethylaluminum hydride (DMAH) as an Al source. A high dark conductivity of 6.88×10−4S/cm was obtained for a 27nm-thick film under high H2/MMS and plasma power conditions. We applied this optimized film to a heterojunction emitter of n-type c-Si solar cells. The solar cell showed high fill factor of 0.756. The internal quantum efficiency in the short wavelength region was improved to 0.87 with decreasing the thickness of a buffer layer.

Amorphous Silicon Carbide Thin Films Deposited by Magnetron Co-Sputtering: Effect of Applied Power and Deposition Pressure on Film Characteristics

In this work, the effect of deposition pressure and total power applied to Si and C targets on the a-SiC film characteristics was investigated by profilometry, X-ray diffraction (XRD), Raman spectroscopy, and Rutherford-Backscattering Spectrometry (RBS). Profilometry results indicate an increase in the film thickness by increasing both the deposition pressure and total power. Both Crich and Si-rich a-SiC films were grown only by varying the deposition pressure or total power. Raman spectra suggest that the films contain amorphous phases because a-SiC clusters together with a-Si and a-C clusters were observed. The XRD analysis confirmed the amorphous structure of these films.

Room temperature photoluminescence spectrum modeling of hydrogenated amorphous silicon carbide thin films by a joint density of tail states approach and its application to plasma deposited hydrogenated amorphous silicon carbide thin films

Room temperature photoluminescence (PL) spectrum of hydrogenated amorphous silicon carbide (a-SiCx:H) thin films was modeled by a joint density of tail states approach. In the frame of these analyses, the density of tail states was defined in terms of empirical Gaussian functions for conduction and valance bands. The PL spectrum was represented in terms of an integral of joint density of states functions and Fermi distribution function. The analyses were performed for various values of energy band gap, Fermi energy and disorder parameter, which is a parameter that represents the width of the energy band tails. Finally, the model was applied to the measured room temperature PL spectra of a-SiCx:H thin films deposited by plasma enhanced chemical vapor deposition system, with various carbon contents, which were determined by X-ray photoelectron spectroscopy measurements. The energy band gap and disorder parameters of the conduction and valance band tails were determined and compared with the optical energies and Urbach energies, obtained by UV–Visible transmittance measurements. As a result of the analyses, it was observed that the proposed model sufficiently represents the room temperature PL spectra of a-SiCx:H thin films.

Prospects of chemical vapor grown silicon carbide thin films using halogen-free single sources in nuclear reactor applications: A review

Abstract

Cyclic indentation behavior of metal–ceramic nanolayered composites

The indentation behavior of metal/ceramic nanolayered composites is studied numerically using the finite element method. Attention is devoted to cyclic response under fixed maximum and minimum loads, with the primary objective of examining the evolving plastic deformation in the ductile metal constrained by the hard ceramic layers. The axisymmetric model consists of alternating aluminum (Al) and silicon carbide (SiC) thin films on a silicon (Si) substrate, with the Al/SiC layered structure indented by a conical diamond indenter. It is found that, unlike the homogeneous material where indentation unloading consists of purely elastic recovery, in the multilayered material plastic deformation in the Al layers continues to occur upon unloading and subsequent loading/unloading operations. With each additional cycle the indenter penetrates deeper into the composite. The modeling results are in qualitative agreement with the actual cyclic nanoindentation experiment conducted on the Al/SiC nanolayers.

Fabrication of Silicon Carbide Thin Film as a Stabilizing Layer for Improving the Stability of Porous Silicon Photodiodes

This paper investigates the effect of silicon carbide (SiC) thin film as a stabilizing layer on porous silicon (PS) in metal–semiconductor–metal (MSM) photodiodes. Photo-assisted pulsed electrochemical etching method was used to produce the PS layers with large pore density and small crystallite size. As–prepared PS surface was modified with acetylene gas flow in a thermal process (750°C) in order to replace the hydrogen termination by Si–C bonds which is more stable. During the thermal carbonization, carbon atoms penetrated into the silicon lattice forming a thin (~4 nm) SiC layer. Because of high inertness of silicon carbide thin film, thermally carbonized porous silicon layer (TC-PS) was found to be more stable than the freshly prepared PS surfaces. A small reduction in specific surface area was found after carbonization which is due to the small size of acetylene molecules. The FTIR measurements confirmed the presence of SiC bonds in the TC-PS sample. The photocurrent of the fabricated photodiodes based on as-grown PS was lowered under prolonged green laser radiation (532nm, 5mW), but devices based on TC-PS showed more stable I-V characteristics under the same condition even for 120 min of laser exposure.

Si<sub>x</sub>C<sub>y</sub> Thin Films Deposited at Low Temperature by DC Dual Magnetron Sputtering: Effect of Power Supplied to Si and C Cathode Targets on Film Physicochemical Properties

A DC dual magnetron sputtering system with graphite (C) and silicon (Si) targets was used to grow stoichiometric and non-stoichiometric silicon carbide (SixCy) thin films at low temperature. Two independently DC power sources were used to enable the total discharge power be shared, under certain proportions, between the Si and C magnetron cathodes. The motivation was to control the sputtering rate of each target so as to vary the stoichiometric ratio x/y of the deposited films. The species content, thickness and chemical bonds of as-deposited SixCy films were studied by Rutherford backscattering spectroscopy (RBS), profilometry analysis and Fourier transform infrared absorption (FTIR), respectively. Overall, the present work reveals a new reliable plasma sputtering technique for low temperature growth of amorphous SixCy thin films with the capability of tuning the degree of formation of a-SiC, a-Si and a-C bonds in the film bulk.

Theoretical Study of Thermoelectric Properties of SiC Nanowires

We present, for the first time, simulations of thermoelectric properties of silicon carbide (SiC) nanowires as a function of the wire cross section at high temperature (500K), based on non-equilibrium classical molecular dynamics simulations for the lattice thermal transport and non-equilibrium green's function for the electrical transport. Our calculations show that figure of merit (ZT) was increasing with decreasing cross section area: ZT of SiC nanowire at 2x2 nm2 has maximum value in the range of 0.65 - 0.89 at 500K, which is 7 - 8 times larger than maximum ZT of SiC thin film value (0.125 at 973 K). These results show that SiC may be a promising material for thermoelectric applications operating at high temperature.

A narrow process window for the preparation of polytypes of microcrystalline silicon carbide thin films by hot-wire CVD method

Effects of deposition conditions on the structure of microcrystalline silicon carbide (μc-SiC) films prepared by hot-wire chemical vapor deposition (hot-wire CVD) method have been investigated. It is found from X-ray diffraction patterns of the film that a diffraction peak from crystallites from hexagonal polytypes of SiC is observed in addition to those of 3C-SiC crystallites. This result is obtained in the film under a narrow deposition conditions of SiH3CH3 gas pressure of 8Pa, the H2 gas pressure of 80–300Pa and the total gas pressure of 40–300Pa under fixed substrate and filament temperatures employed in this study. Furthermore, the grain size of hexagonal crystallites (about 20nm) on c-Si substrates becomes larger than that of 3C-SiC crystallites (about 10nm) for the films deposited under the total gas pressure of 36–88Pa. The fact that microcrystalline hexagonal SiC can be deposited under limited deposition conditions could be interpreted in the context of a result for c-SiC polytypes prepared by thermal CVD method.

Properties of thin silicon carbide films prepared by rapid thermal annealing

We report experimental results related to the structural and electrical properties of thin SiC films. Thin carbon films with thicknesses 50 Å and 300 Å were deposited by R.F. sputtering and processed by rapid thermal annealing (RTA) for 3 min at temperatures of 800 °C and 1400 °C in a vacuum chamber at 2×10−5 Torr. The thin films properties were studied by Raman spectroscopy and electrical cross-conductance.

Effects of the Substrate on Piezoresistive Properties of Silicon Carbide Thin Films

Silicon carbide (SiC), in bulk or film form, has been shown as a promising material to replace the silicon as sensing element in devices for harsh environments. This has motivated several studies on growth and characterization of SiC thin films on different substrates such as silicon, silicon dioxide (SiO2), aluminum nitride (AlN) and silicon nitride (Si3N4), among others. However, less attention has been given to the investigation on how substrates affect the piezoresistive properties of SiC thin films. In this work, we have investigated the effect of substrates on the piezoresistive properties of SiC thin films produced by magnetron sputtering. Three types of substrates were utilized: (100) monocrystalline silicon, AlN on silicon and thermally oxidized silicon. Test structures were fabricated using photolithography, metallization lift-off and RIE (reactive ion etching) processes in order to perform piezoresistive characterization of the SiC samples produced.

Nanocrystalline silicon and silicon quantum dots formation within amorphous silicon carbide by plasma enhanced chemical vapour deposition method controlling the Argon dilution of the process gases

Structural and optical properties of the amorphous silicon carbide (a-SiC:H) thin films deposited by radio frequency plasma enhanced chemical vapour deposition method from a mixture of silane (SiH4) and methane (CH4) diluted in argon (Ar) have been studied with variation of Ar dilution from 94% to 98.4%. It is observed that nanocrystalline silicon starts to form within the a-SiC:H matrix by increasing the dilution to 96%. With further increase in Ar dilution to 98% formation of the silicon nanocrystals (nc-Si) with variable size is enhanced. The optical band gap (Eg) of the a-SiC:H film decreases from 2.0eV to 1.9eV with increase in Ar dilution from 96% to 98% as the a-SiC:H films gradually become Si rich. On increasing the Ar dilution further to 98.4% leads to the appearance of crystalline silicon quantum dots (c-Si q-dots) of nearly uniform size of 3.5nm. The quantum confinement effect is apparent from the sharp increase in the Eg value to 2.6eV. The phase transformation phenomenon from nc-Si within the a-SiC:H films to Si q-dot were further studied by high resolution transmission electron microscopy and the grazing angle X-ray diffraction spectra. A relaxation in the lattice strain has been observed with the formation of Si q-dots.

Electromagnetic energy absorption potential and microwave heating capacity of SiC thin films in the 1–16 GHz frequency range

We report on the electromagnetic (EM) absorption potential and microwave heating capacity of amorphous hydrogenated silicon carbide thin films (a-SiC:H) in the 1–16GHz frequency domain. a-SiC:H thin films with typical thickness of 1μm were deposited by plasma enhanced chemical vapor deposition on [100] undoped silicon substrates, and exhibit a deep EM absorption – up to 96% of the total EM energy irradiation – which is systematically converted into heat. Two-wavelength pyrometer tests show that temperatures exceeding 2000K can be reached in a very short time, less than 100s exposure to microwaves, showing a promising potential for specific microwave heating applications.

Studies on SiC, DLC and TiO2 thin films as piezoresistive sensor materials for high temperature application

The use of thin films as sensing elements for microsensor applications has been shown very attractive due to their low-cost fabrication, potential for integration with standard CMOS technologies and possibility of deposition on different substrate types. In particular, piezoresistive sensors based on thin films have been commonly developed because can be easily implemented using microfabrication processes and present the best relation between sensitivity and system complexity, which showing great advantages in term of device integration. In our previous works (Fraga et al. 2010, 2011a), we studied undoped and nitrogen-doped PECVD a-SiC thin films as alternative materials to replace the silicon piezoresistors in strain and pressure sensors for harsh environments. Here, we focused our attention on the piezoresistive properties of sputtered silicon carbide (SiC), diamond-like carbon (DLC) and titanium dioxide (TiO2) thin films. These materials were evaluated in terms of sensitivity or gauge factor and of the influence of the temperature on this sensitivity, allowing a preliminary analysis of the applicability of these thin films in high temperature piezoresistive sensors.

Annealing effect on optical and electronic properties of silicon rich amorphous silicon-carbide films

A series of Si-rich amorphous silicon carbide (a-SiC:H) thin films were deposited in conventional plasma enhanced chemical vapor deposition system with various gas ratio R = [CH4]/[SiH4]. The microstructural, optical and electronic properties of as-deposited films were investigated in this study. It was found that optical band gap was linearly proportional to carbon content in the films and it could be controlled in a range of 1.8–2.4 eV by changing the gas ratio, R. Both dark and photo conductivities in room temperature were decreased with the increasing of carbon content in the films, and the photosensitivity reached as high as 104 for the film with the optical band gap of 1.96 eV. The as-deposited samples were subsequently annealed at the temperatures of 900°C and 1000°C. The formation of nanocrystalline silicon (nc-Si) dots in amorphous silicon carbide (a-SiC) host matrix was shown. The dark conductivity was enhanced by five orders of magnitude after annealing compared with that of as-deposited films. The result of temperature-dependent conductivity suggested that the property of carrier transport was dominated by conduction process between the extended states. Furthermore, room temperature electroluminescence (EL) was achieved from nc-Si/SiC system and the possible mechanism of radiative recombination mechanism was discussed.