PREPARATION AND CHARACTERISATION OF HYDROGENATED R. F. SPUTTERED AMORPHOUS SILICON CARBIDE FILMS

Abstract

Hydrogenated amorphous silicon carbide films were deposited on silicon substrates using the r.f. sputtering technique in a hydrogen and argon ambient with a 99.99 % pure polycrystalline silicon carbide target This project aimed to study the structural and electrical properties of the sputtered films. The thickness of the deposited films ranged between 0.25 to 1.6 µm. The film's deposition rate was found to depend on the deposition conditions, such as the chamber pressure, the substrate temperature, the r.f. power and the hydrogen partial pressure. The film's refractive index was in the range of 1.85 to 3.6 and it varied with the deposition conditions. Thermal annealing reduced the refractive index of all the samples. The amorphous nature of the films was confirmed by the infrared spectroscopy which revealed two major modes of vibration, i.e., C-Hn stretching and Si-Hn stretching. The hydrogen content in the films was determined from the Si-Hn absorption peak and was found to increase with the hydrogen partial pressure. The stoichiometry of the films was studied using the X-ray photoelectron spectroscopy. It was found to be close to the target composition and did not change significantly with deposition conditions. Symmetrical C=C bonds were observed in the Raman spectra. The optical bandgap increased with an increase in the hydrogen partial pressure, but decreased with an increase in the r.f. power. The I-V characteristics showed that the conduction at low field was ohmic in nature. The d.c. conductvity was in the range of 10-13 -10-6 ?-1 cm-1 and was found to be related to the hydrogen content, NSi-H. A higher value of NSi-H invariably resulted in a lower conductivity. The temperature dependence of the d.c. conductivity followed the Arrhenius relation. The activation energy increased from 0.16 to 0.24 eV when the hydrogen partial pressure was increased from 0.15 to 0.6 Pa. In the high field region, the dominant mechanism was the Frenkel-Poole emission. The dynamic dielectric constant was calculated from the Frenkel-Poole plot to be between 2 to 5. The barrier height associated with the trap was between 0.32-0.43 eV. The a.c. characteristics in the low frequency region (10-5 -10 Hz) was obtained as the frequency dependence of the complex susceptibility. Current transient behaviour was observed and found to satisfy the Curie Von Schweilder power law, I?t-n. The complex susceptibility was then obtained by performing Fourier transform on the I-t data. Dielectric loss peaks were· observed at a low frequency of 5x10-3 Hz. In the high frequency regime (1 kHz-10MHz), the a.c. characteristics was obtained as the frequency dependence of the capacitance and conductance. The capacitance was flat at low frequency but decreased slightly at high frequency. The conductance exhibited a loss peak at 0.2 MHz. A significant deep depletion region was observed in the high frequency C-V characteristics, which was attributed to leakages in our films. The interface trap density was estimated using the conductance method and was found to be between 5x1011 and 1 x1012 cm-2 eV-1 near the midgap of the silicon substrate.