Mechanism of surface reaction in the deposition process of<i>a</i>-Si:H by rf glow discharge

Abstract

Hydrogenated amorphous Si (a-Si:H) films are deposited by the rf glow discharge of ${\mathrm{SiH}}_{4}$. The mechanism of the surface reaction of the incident ${\mathrm{SiH}}_{3}$ radical produced by the glow discharge to form the a-Si:H network was studied in this work. The hydrogen concentration was investigated by IR absorption in a-Si:H films deposited under various conditions of the substrate temperature ${\mathit{T}}_{\mathit{s}}$ and the ${\mathrm{SiH}}_{4}$ partial pressure ${\mathit{P}}_{\mathit{s}}$ in the reactant gases. The hydrogen concentration in the films was divided into SiH and ${\mathrm{SiH}}_{2}$ configurations by spectral deconvolution. While the SiH concentration ${\mathit{S}}_{1}$ was not influenced very much by variation of the deposition conditions, the ${\mathrm{SiH}}_{2}$ concentration ${\mathit{S}}_{2}$ was remarkably influenced. ${\mathit{S}}_{2}$ increased with an increase in the deposition rate of the film at a different rate by the variation of ${\mathit{T}}_{\mathit{s}}$ and ${\mathit{P}}_{\mathit{s}}$. The following reaction scheme of the radicals adsorbed on the growing surface is proposed to explain these changes in ${\mathit{S}}_{2}$ and ${\mathit{S}}_{1}$. The dehydrogenation and Si-Si bond formation into the network are made by two kinds of processes, the fast process and the slow process. The fast process occurs predominantly in high-rate depositions and incorporates ${\mathrm{SiH}}_{2}$ into the network. The slow process occurs predominantly in low-rate depositions and incorporates some SiH. The fast process takes place by interactions of two adsorbed radicals and the slow process takes place at steplike sites after migration on the surface.The observed variations in ${\mathit{S}}_{2}$ with ${\mathit{T}}_{\mathit{s}}$ and ${\mathit{P}}_{\mathit{s}}$ are analyzed by this model and the rate constants of each process are determined. The activation energy in the slow process is obtained to be 0.3 eV, which is considered to be due to the migration of the ${\mathrm{SiH}}_{3}$ radical on the growing surface. The data of the ${\mathit{S}}_{1}$ variation are also analyzed and the minimum concentration of H atom is found to be about 10 at. %, in agreement with experiments. Variation of the dangling-bond density with deposition condition is found to have similarities with that of ${\mathit{S}}_{1}$ and ${\mathit{S}}_{2}$. The features of the surface reaction mechanism are successfully explained by this model.