title: Deposition and Characterisation of Amorphous and Nanocrystalline SiC
Creators: name: Wang, Li
Publisher: name: Griffith University
Year: 2010
Subjects: subject: amorphous
subject: nanocrystalline SiC
subject: methylsilane
subject: trimethylaluminium
subject: crystallisation
subject: hydrogen

description: In this thesis, the deposition of unintentionally doped amorphous and nanocrystalline SiC was demonstrated using a standard hot-wall low-pressure chemical vapour deposition reactor in a substrate temperature range of 600 to 850 oC, with methylsilane used as the single precursor. The pressure of methylsilane varied from 0.006 to 0.54 mbar. The Arrhenius activation energy of SiC deposition varied with methylsilane pressure, in a range of 2 to 2.8 eV. Surface reaction was found to be the rate-determining process at temperatures below 700 ºC; both surface reaction and mass transport processes control the rate-determining process in the temperature range of 700 to 850 ºC. The deposition rate of SiC was relatively insensitive to the substrate surface conditions. The crystallinity of deposited SiC improved with reduced methylsilane pressure and at elevated substrate temperature based on the results from x-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction, and the Fourier transform infrared spectroscopy. The deposition of Al-doped SiC films was demonstrated in a temperature range of 600 to 750 ºC, with methylsilane and trimethylaluminium used as precursors. The incorporation of trimethylaluminium caused in situ crystallisation of a-SiC film deposited on Si substrate at 600 ºC, which is much lower than the crystallisation temperatures usually required by other techniques. Hydrogen concentration ranged from 17 at. % to less than 1 at. % based on the secondary ion mass spectroscopy depth profile analyses, demonstrating that hydrogen concentration decreased both at an elevated substrate temperature and with increasing methylsilane pressure. The Si–C absorption band was the major peak found in the Fourier transform infrared spectra in the range of 400 to 4000 cm-1 for all SiC films. According to the chemical bonding analyses conducted by x-ray photoelectron spectroscopy, the fraction of Si and C atoms incorporated into the tetrahedral Si–C bonds ranged from 60 to 70 % in all a-SiC films. No contribution by Si–H/Si–Si bonds was identified. Si to C ratio was in the range of 0.88 to 0.99 in the unintentionally doped a-SiC. The presence of sp2 C–C/C–H bonds in the a-SiC indicates the network of a-SiC is neither fully chemically ordered nor does it completely follow the tetrahedral structure. By comparing samples deposited at 650 ºC and 600 ºC at a constant methylsilane pressure, it was seen that a-SiC deposited at higher temperatures had a higher Si–C bond percentage and less oxygen concentration, and its chemical composition was less sensitive to the substrate type. The Al concentration increased when the sample was placed further away from the gas inlet, increasing from about 4.1 at. % to 6.3 at. %. Al–Al metallic bonds were detected only in SiC film deposited on quartz substrate, which might indicate that Al concentration is beyond its solubility limit. The incorporation of Al reduced the fraction of sp2 C–C/C–H bonds and increased the fraction of sp3 C–C/C–H bonds. The optical energy gap and absorption coefficient were derived based on optical transmittance and reflectance measurements. The optical energy gap of unintentionally doped SiC was in the range of 1.6 to 4.2 eV, with the absorption coefficient varying in the range of 103 to 105 cm-1. For SiC films deposited at 600 ºC, the incorporation of Al not only narrowed the optical energy gap by 1.0 eV but also reduced the absorption coefficient by one order of magnitude. Both a decrease in methylsilane pressure and an increase in substrate temperature widen the optical energy gap of deposited SiC. The unintentionally doped SiC was n type conductive and Al-doped SiC was p type conductive, which were determined by hot-probe and capacitance-voltage techniques. The conductivity was derived using a modified four-point probe technique. It ranged from 2.7 × 10-7 S cm-1 to 2.1 × 10-3 S cm-1 for the unintentionally doped a-SiC, from 8.1 × 10-5 to 1.5 S cm-1 for the unintentionally doped nc-SiC, and from 7.0 × 10-3 S cm-1 to 1.0 × 101 S cm-1 for the Al-doped SiC. The linear I-V characteristics of the Al-doped SiC demonstrate that conduction was by a drift of holes in the valence band, and the temperature dependence of the conductivity in Al-doped sample could be modelled by acceptors 0.20 eV above the valence band edge with the mobility limited by ionic impurity scattering.