Abstract
This study was undertaken to investigate the possibility of synthesis of nitride based semiconductors. To this end hydrogenated amorphous silicon nitride (a-SiNx:Hy) has been deposited as the starting material using PECVD (plasma enhanced chemical vapour deposition). Then the effects of implanting gallium into the a-SiNx:H target material have been studied with the aim of forming GaN compounds. Should this technique work, it opens the possibility of carrying out similar synthesis of Al, hi and other nitride based compounds. PECVD hydrogenated amorphous silicon nitride thin films are studied as a function of the ammonia/silane gas ratio. The power coupled to the plasma, pressure and the substrate temperature were held constant, while the gas flow ratio of NH3/SiH4 was varied. IV measurements on the metal- nitride-metal structures indicated that the conduction mechanism might be explained by Poole-Frenkel conduction. The composition of the a-SiNx:Hy films were analysed using Rutherford backscattering spectroscopy (RBS) and Elastic recoil detection analysis (ERDA). The Si content decreased in a logarithmic manner for 0 < NH3/SiH4 < 4 and saturated for higher NH3/SiH4, ratios at 27 at.%. The N content mirrored this trend and saturated to a maximum asymptotic value of 49 at.% for NH3/SiH4 > 4. Stress, refractive index and optical absorption studies were also conducted. A turning point for most of the properties was observed at a NH3/SiH4 ratio of 4. This corresponds to a N/Si ratio of about 1.4 with a hydrogen content of 22 at.% for the deposited films. Below this ratio, a-SiNx:H films have high growth rates, a refractive index between 1.9 and 2.7, a N/Si ratio between 0.5 and 1.5 and moderate values of compressive stress (~ 0.7 GPa). While, above this pivotal ratio, growth rates become significantly lower, the refractive index minimises to 1.8, N and Si concentrations in the films saturate and the compressive stress rapidly increases. Evidence has been found for Ga-N bonds by implanting gallium into amorphous silicon nitride films. The a-SiNx:H films grown at high gas ratios (R > 70) are highly stressed and the nitrogen content is saturated. They are, therefore, ideally suited to forming GaN bonds under the high energy conditions of Ga ion implantation, a phase that is not thermodynamically favourable. X-ray photoelectron spectroscopy (XPS), FTIR and RBS have been used to examine the bond structure, composition and the depth profile of the synthesised material. It has been found that the implanted Ga bonds with the N from the NH2, NH and SiN bonds and the released Si and H from these bonds combine to form additional SiH. From the RBS and XPS data, annealing at 200°C was shown to increase the thickness of the a-GaN and transform more of the target. The Ga profile moves deeper into the material and the stoichiometric phase of SiN that is thermodynamically stable, recovers. Annealing at a higher temperature (500°C) shows a significant reduction in the amount of H from the amorphous network (ERD), mainly from the NH bonds (FTIR), thus leaving the free N available to bond with the unbonded Ga in the material. Up to ~ 22 at.% Ga is present in the material and can be converted to GaN on annealing. Electron diffraction through the material shows no evidence of any crystallites in the synthesised material.