Electronic properties of amorphous and microcrystalline silicon prepared in a microwave plasma from SiF₄

Abstract

The paper is concerned with a systematic investigation of electronic, optical and structural properties of hydrogenated Si films prepared by decomposing a mixture of SiF₄, H₂ and (initially) Ar in a microwave plasma (2·45 GHz). The temperature dependence of the dark conductivity and drift mobility, as well as measurements of photoconductivity, H content, optical absorption and electron diffraction have been used to characterize the specimens. Two related reactor geometries were studied. The first, system A, included the coaxial tube arrangement for gas flow through the cavity also used by Shibata et al. The electronic quality of the initial material was greatly improved by two modifications: firstly removal of the inner tube to allow interaction of the SiF₄, and H₂ in the cavity and secondly exclusion of the Ar in the mixture to prevent defects from Ar-ion bombardment. With this arrangement, system B, good electronic properties were obtained (nμτ≈10⁻⁵ cm² V⁻¹). System B results show a transition to electronically viable microcrystalline Si as the pressure in the reactor is reduced below P = 0·20 Torr. Analysis of the Urbach edge parameter E, suggests that the optimized amorphous material, deposited at P = 0·25 Torr still contains a small microcrystalline volume fraction. Drift mobility experiments show a decrease in electron mobility and a remarkable increase in the hole mobility, associated with the narrowing of the hole tail states to about 0·09 eV. All experiments so far were carried out at a hydrogen flow rate of 2s.c.c.m., giving typical values of the H content C_H of 8–10 at.%. By reducing the flow rate from 2 to 1 s.c.c.m. it is shown that C_H can be controllably reduced to about 4 at.%, whilst maintaining acceptable electronic properties. The light degradation of photoconductivity has been studied for C_H between 8 and 4 at.%, showing that the rate of degradation is reduced by decreasing C_H. The results are consistent with a bond-switching mechanism whereby the catalytic action of H stabilizes the broken bonds.