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

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.

The characterisation of clean metal surfaces

The most perfectly structured metal surface observed in practice is that of a field evaporated field-ion microscope specimen. This surface has been characterised by adopting various optical analogue techniques. Hence a relationship has been determined between the structure of a single plane on the surface of a field-ion emitter and the geometry of a binary zone plate. By relating the known focussing properties of such a zone plate to those obtained from the projected images of such planes in a field-ion micrograph, it is possible to extract new information regarding the local magnification of the image. Further to this, it has been shown that the entire system of planes comprising the field-ion imaging surface may be regarded as a moire pattern formed between over-lapping zone plates. The properties of such moire zone plates are first established in an analysis of the moire pattern formed between zone plates on a flat surface. When these ideas are applied to the field-ion image it becomes possible to deduce further information regarding the precise topography of the emitter. It has also become possible to simulate differently proJected field-ion images by overlapping suitably aberrated zone plates. Low-energy ion bombardment is an essential preliminary to much surface research as a means of producing chemically clean surfaces. Hence it is important to know the nature and distribution of the resultant lattice damage, and the extent to which it may be removed by annealing. The field-ion microscope has been used to investigate such damage because its characterisation lies on the atomic scale. The present study is concerned with the in situ sputtering of tungsten emitters using helium, neon, argon and xenon ions with energies in the range 100eV to 1keV, together with observations of the effect of annealing. The relevance of these results to surface cleaning schedules is discussed.