Room temperature ageing of vapour deposited amorphous selenium thick films

Abstract

Many investigations of the structure of amorphous selenium have been reported [1] but a unique structure has not yet been agreed upon. The amorphous phase is considered to be composed of hexagonal chains or monoclinic rings or a mixture of" both. To determine the nature of the condensed species, experiments on ageing have been carried out [ 2 4 ] where the sample has been either in the bulk or thin film ( 200 nm thick) form and has been aged under controlled experimental conditions (for example, the sample has been kept either in the dark or in an atmosphere saturated with water vapour, etc.). However, there has been no report of amorphous-crystal l ine transition of amorphous selenium thick films upon ageing under ordinary (i.e. uncontrolled) experimental conditions. As a part of our programme of structural studies of amorphous semiconductors, we have recently observed by X-ray diffraction the effects of room temperature ageing on amorphous selenium films in the range of thickness 30 to 913/~m having different deposition rates. The films were prepared by evaporation in wLcuum ( ~ 5 x 10 -s tort) onto cleaned glass substrates at room temperature at the deposition rates of 7, 1 t and 12/~m min -1 which are rather fast compared to the ones usually reported in most of the publications on amorphous selenium. The films were aged at room temperature (25 -+ 2 ° C) under normal atmospheric pressure for a period of nearly three months and no special method was adopted to age the films. Rather the films were exposed in the laboratory to ordinary day (room light only) and night cycles by keeping them in a glass covered petri dish. X-ray diffraction patterns 66 were obtained with a Philips PW 1050, 1051 dif43 fractometer coupled to a Philips 1130 generator 30 92 and with LiF monochromated Cu-radiation.