Properties of Intrinsic and Doped Amorphous Silicon Produced by R.F. Sputtering

Abstract

The effect of varying the preparation conditions on the electrical and optical properties of a-Si films prepared by r.f. sputtering in pure noble gases was investigated. In particular the effect of varying the sputtering gas and gas pressure was considered. Other parameters varied were the target substrate distance, d.c. self-bias voltage on the target, the application of an axial magnetic field and the variation of the substrate temperature. The films were characterized by measuring the d.c. dark conductivity as a function of temperature over a range of about 170 K, determining the thermal activation energy, room-temperature photoconductivity, the optical gap and the composition of the films. The results suggest that the density of defect states can be reduced substantially by sputtering at high pressures, since the dark conductivity decreases and the photoconductivity, thermal activation energy and optical gap increase on increasing the sputtering pressure. For argon sputtered films the conductivity was decreased by more than four orders of magnitude to 10-7 (Ωcm)-1 by increasing the pressure from the conventionally used value of 5 mtorr to about 25-30 mtorr. Films sputtered at higher pressures exhibited even lower conductivities but were found to contain large quantities of oxygen (~20 at.%). Films sputtered in neon exhibited properties comparable to optimumly prepared hydrogenated silicon. By increasing the sputtering pressure the room temperature conductivity was decreased by more than six orders of magnitude to<10-9 (Ωcm)-1. Neon sputtered films were doped n-type by incorporation of tantalum by co-sputtering and arsenic and antimony by sputtering predoped targets. The results showed that the room-temperature conductivity could be varied systematically over range of nine orders of magnitude by substitutional doping.  Increasing the target substrate spacing or decreasing the d.c. self-bias voltage on the target also resulted in a reduction in the density of defect states. Increasing the substrate temperature also slightly reduced the density of defect states