A study of the electrical and optical properties of unhydrogenated neon-sputtered amorphous silicon

Abstract

Films of amorphous silicon (a-Si) were prepared by r.f. sputtering in a Ne plasma without the addition of hydrogen or a halogen. The d.c. dark
electrical conductivity, he optical gap and the photoconductivity of the
films were investigated for a range of preparation conditions, the sputtering gas pressure, P, the target-substrate spacing, d, the self-bias
voltage, Vsb, on the target and the substrate temperature, Ts. The dependence of the electrical and optical properties on these conditions
showed that various combinations of P, d and Vsb, at a constant Ts, giving
the same product (Pd/V sb) result in films with similar properties, provided that P, d and Vsb remain vithin a certain range.
Variation of Pd/Vsb between about 0.2 and 0.8 rrTorr.cm!V varied the
dark conductivity over about 4 orders of magnitude, the optical gap by
0.5 eV and the photoconductivity over 4-5 orders of magnitude. This is
attributed to controlling the density-of-states distribution in the mobility
gap. The temperature-dependence of photoconductivity and the
photoresponse of undoped films are in support of this conclusion. Films
prepared at relatively high (Pd/Vsb) values and Ts=300 ºc: exhibited low
dark-conductivity and high thermal activation energy, optical gap and
photoresponse, characteristic properties of a 'low density-of-states
material.
P-type doping with group-Ill elements (Al, B and Ga) by sputtering from
a composite target or from a predoped target (B-.doped) was investigated.
The systematic variation of room-temperature conductivity over many
orders of magnitude and a Fermi-level shift of about 0.7 eV towards the
valence-band edge suggest that substitutional doping had taken place.
The effects of preparation conditions on doping efficiency were also
investigated.
The post-deposition annealing of undoped and doped films were studied
for a temperature range from 250 ºC to 470 ºC. It was shown that annealing
enhanced the doping efficiency considerably, although it had little
effect on the basic material (a-Si) prepared at the optimum conditions
(Pd/Vsb=0.8 mTorr.cm/V and Ts=300 $ºC).
Preliminary experiments on devices imply potential applications of the
present material, such as p-n and MS junctions.

Date of Award	Jul 1984
Original language	English