Characterization of Si 1-xCx : H Thin Films Deposited by PECVD for SiCOI Heterojuntion Fabrication

As heterojunções de SiC em isolante, conhecidas como SiCOI (SiC-on-insulator), ao combinar as vantagens do SiC e do substrato de Si mostram-se interessantes para aplicações em altas temperaturas, alta potência, altas freqüências, sensores inteligentes, e aplicações micromecânicas. Neste trabalho pesquisamos as propriedades de filmes finos de carbeto de silício amorfo hidrogenado (a-Si 1-x C x :H) depositado por PECVD em substratos de silício e silício coberto por uma camada isolante, antes e após o recozimento térmico para a cristalização do filme. Devido a seu grande interesse em aplicações optoeletrônicas foram pesquisadas quatro tipos de isolantes diferentes: SiO 2 térmico obtido a altas temperaturas (1100 C), SiO 2 , SiO x N y , e Si 3 N 4 obtidos por PECVD a baixa temperatura (320 C). A análise de infravermelho nas amostras como depositadas mostra que o filme depositado em substrato de silício coberto por Si 3 N 4 possui uma melhor coordenação entre os átomos de Si e C. O recozimento térmico realizado conduz à cristalização de todos os filmes amorfos depositados. As medidas Raman exibem a vibração referente ao C-C somente para os filmes depositados sobre isolantes PECVD, depois de recozidos. Os resultados da análise de DRX mostram que os filmes depositados em substrato de Si(100) e Si(100) coberto com SiO 2 térmico apresentam orientação cristalina preferencial na direção (100), enquanto os filmes depositados em substrato de silício coberto com isolantes PECVD apresentam uma banda larga associada com a difração do H-SiC(10L) e do 3C-SiC(111).


Introduction
In recent years, silicon carbide (SiC) has received considerable attention, due to the possibility of obtaining materials with a variety of topological structures, which affect the electronic density of states and the optical properties. 1 Indeed, beyond its good thermal stability and mechanical hardness, this material is versatile from structural considerations, electronic behavior as well as the photoluminescent response. 2,3 C, either in its amorphous or crystalline forms has been the subject of intensive investigation for several applications in the optoelectronic and microelectronic field such as, thin film light emitting diodes, 4 electroluminescent devices, 5 and heterojuntion bipolar transistors. 4,6Crystalline SiC is a prospective material for electronic devices.Its fundamental properties such as large band gap, high thermal conductivity, high electrical breakdown field strength, and high saturation drift velocity make it a suitable material for high power, high temperature and high frequency devices. 1,7t the same time, SiC grown on silicon, allows the integration of the high temperature capabilities of SiC with the electronic and micromachining possibilities of Si.However at temperatures higher than 500 K the SiC/ Si heterojunction starts leaking, resulting in current flow through the Si substrate.One method to avoid this problem is the deposition of SiC on SOI (silicon on insulator) substrates in order to obtain SiC-on-insulator (SiCOI) structures.These structures are desirable not only for low-cost, large-area substrates for high temperature and optoelectronic applications but also for high power, high frequency, smart-sensors, micromechanical devices that must operate in chemically and/ or physically aggressive environments, at high temperature and at high radiation ranges. 8,9arious growth methods have been developed and applied to prepare high quality SiC films.Plasmaenhanced chemical vapor deposition (PECVD) is understood to be a versatile and well-established technology.This technique offers the possibility of utilizing low temperature; designing new structures and changing the properties associated with films microstructure through the change of technological parameters.It also permits the growth of large-area films. 10n this paper we investigate the characteristics of SiC obtained by high temperature annealing of amorphous hydrogenated silicon carbide a-Si 1-x C x :H films deposited by PECVD on silicon and silicon covered with an insulator layer substrates.We analyze different kinds of insulator capping layers chosen due to their great interest for optoelectronic and microelectronic applications.

Experimental
The SiCOI heterojuntions were prepared on <100> oriented p-type Czochralski silicon, with average resistivity in the 8-12 Wcm range.First, the insulator films were grown on the Si substrate followed by the a-Si 1-x C x :H films deposition.All a-Si 1-x C x :H films were deposited by PECVD from CH 4 (32.4 sccm) and SiH 4 (3.6 sccm) mixtures, with H 2 dilution in the "starving plasma regime" at 300 o C with a power of 100 W. A parallel-plate radio frequency (RF, 13.56 MHz) PECVD reactor was utilized.
The PECVD insulator capping layers were deposited from: SiH 4 (15 sccm) and N 2 O(75 sccm) mixtures, at 320 o C, with a power of 200 W for the SiO 2 , SiH 4 (15 sccm) and N 2 O(37.5 sccm) mixtures, at 320 o C, with a power of 200 W for the SiO x N y , and SiH 4 (3 sccm) and N 2 (39 sccm) mixtures, at 320 o C, with a power of 100 W for the Si 3 N 4 .The thermal SiO 2 films were grown by dry oxidation at 1100 o C for 2 h 30 min.
Thermal annealing processing, at 1200 o C for 2 h under nitrogen flow, was performed in order to improve the atomic structure of the amorphous films and induce crystallization.The chemical bonding in the films, before and after thermal annealing, are investigated by Raman scattering with excitation provided by a He-Ne red laser operating at a wavelength of 633 nm, and by Fourier transform infrared spectrometry (FTIR) using a BioRad FTS 40 Fourier transform infrared spectrometer with a resolution of 8 cm -1 in the 400-4000 cm -1 wave number range.Either silicon or silicon covered with the concerned insulator capping layer substrates were used as references in both cases.The structural properties are studied by X-ray diffraction (XRD).These measurements were performed by a URD-6 GmbH diffraction system in the θ-2θ scan configuration utilizing the Cu Kα wavelength of 1.5418 Å.

FTIR analysis
Figure 1 shows the IR spectra for as-deposited films.This Figure reveals the absorption band attributed to Si-C stretching mode centered at 779 cm -1 for all the films, except for film deposited onto Si covered with Si 3 N 4 where the band is centered at 783 cm -1 .Other peak observed at ~1000 cm -1 is generally assigned to SiC-H n modes; however, as no traces of other C-H n bonds vibration modes are observed we believe that this peak is doe to some kind of Si-C vibration mode.In addition, a peak at 2100 cm -1 , related to Si-Hn stretching vibrations, is observed.The position, the larger area and the sharp increase of the Si-C related band, observed for the film deposited onto Si covered with Si 3 N 4 indicate that this film presents a better coordination among Si and C atoms.Carefully inspecting Figure 1, we can also observe that films deposited in these conditions do not present C-H n bonds.
After annealing all the spectra exhibit a shift of the Si-C peak position towards 798 cm -1 , a decrease in peak width and a change in line shape, as can be seen in Figure 2. The line shape of the Si-C related band, in the infrared spectra, for as-deposited films is of Gaussian type and after thermal annealing at 1200 °C it changes to Lorentzian type.This change in line shape as well as the decrease in peak width indicates the onset of crystallization.The observed change in line shape after annealing is consistent with a more uniform environment of the Si-C bond and further evidence of structural modification is given by the decrease in bandwidth, indicating increasing structural order due to the formation of microcrystallites.The peak shift is due to the reduction of Si-C bond length resulting in an increase of the effective force constant. 10he peaks corresponding to Si-H n (~2100 cm -1 ) bonds disappear after thermal annealing, indicating that these treatments cause the total hydrogen outdiffusion from the film together with crystallization.The other peaks observed are (400 to 500 cm -1 and 1000 to 1300 cm -1 ) related to the insulator capping layer.The negative peaks are associated with difference between the insulator capping layer substrate and the reference thickness as well as with the sputtering of the insulator capping layer at the beginning of the a-Si 1-x C x :H deposition due to the hydrogen plasma predominance.
The FTIR data analysis on Figures 1 and 2 are compatible with our previous reports indicating lower H incorporation and higher chemical and structural order, for a-Si 1-x C x :H films grown under "starving plasma regime" and H 2 dilution conditions. 11,12The results showed better material quality when compared with works reported by Jung et al., 13 where no H 2 dilution was used, which showed considerable C-H n and Si-H n bonds before and after thermal annealing.

Raman analysis
In Figure 3 the Raman spectra for the as-deposited films are shown.All the films reveal the second-order band related to crystalline silicon, located at 925-1060 cm -1 and there are not evidences of any band or peak corresponding to C-C or a-Si-Si bonds.
Raman measurements after the annealing steps are depicted in Figure 4. Films deposited onto bare Si and thermal-SiO 2 /Si do not present any C-C related band or peak.Giving evidences that films are understoichiometric configuration and thus all the carbon atoms are "used" to form silicon carbide.Spectra measured for films deposited on PECVD insulator/Si, after annealing, show a weak C-C signal splited into two subbands at ~1330 cm -1 and ~1595 cm -1 , corresponding to D line (disordered) and G line (graphite) respectively.For films deposited onto SiO x N y /Si, the C-C signal is larger.In all cases the