Thermal and mechanical properties of single- and double-layer amorphous Se1−x Te x photoreceptors

Abstract

Single and multilayer photoreceptor devices based on various compositions of amorphous selenium alloyed with tellurium and arsenic have been tested recently in a number of laboratories for use in laser and LEDtype printers employing the xerographic process, as well as for use in vidicon image tubes. Monolayer a-Sel_xTex alloys form one of the most widely used commercial xerographic photoreceptors and hence have been extensively examined for electrical and optical properties by a number of research organizations. There is, however, very little information available on the thermal stability and mechanical durability of these amorphous semiconductors, particularly in machine operating conditions where they are subjected to temperature variations, humidity changes, mechanical wear and scratching dUring the cleaning cycle and other environmental influences. In fact it was the proneness to crystallization and the weakness to mechanical wear and tear which led to the abandonment of pure a-Se as a viable xerographic photoreceptor in the 1960s. It was replaced by a-Se alloyed with 0.5 to 1% As which has a higher resistance to crystallization. The purpose of this letter is to provide some useful comparative data on the thermal and mechanical behaviour of singleand double-layer photoreceptor films from differential scanning calorimetry (DSC) and microhardness measurements. Inasmuch as a-As 2 Se 3 is another commercial amorphous photoconductor competing directly with the a-Se~_xTex system, we will also give data on its thermal and mechanical characteristics. Singleand double-layer a-Se~_xTe~ photoreceptor films were fabricated by vacuum evaporation of vitreous pellets from long stainless steel boats on to heated (~ 70 ° C) aluminium substrates. The stainless steel coating plant pumped by a conventional silicone diffusion pump (10-6torr) was equipped with a set of independently controlled boats and a shutter system to enable the fabrication ofmultilayers. Pure selenium pellets of xerographic grade (99.999%) were loaded into one boat and a mixture of selenium and prealloyed Se: Te pellets in correct proportions into another. The two sources were then evaporated sequentially without breaking the vacuum. Typical coating rates were 1 to 2 #m rain~ and the film thicknesses were 50 to 70/tm. Preparation and characterization of monolayer xerographic photoreceptors have already been reported [1]. Typical composition variation with distance in a singleand two doublelayer photoreceptors are shown in Fig. 1. In doublelayer photoreceptor structures the thin a-Se:Te top coating serves as the photogeneration layer (PGL) whereas the thicker a-Se coating between the PGL and the aluminium substrate acts as the charge transport layer (TL). The tellurium concentration profile was obtained from scanning electron microprobe (SEM) analysis. Films of a-As2Se3 were also prepared by vacuum evaporation on to heated aluminium substrates. The substrate temperature in the latter case was ~ 185 ° C. All the samples were well aged at room temperature to allow the physical properties to stabilize via structural relaxation processes. DSC studies were carried out on a DuPont differential thermal analyser, Type 990, equipped with the DSC cell attachment as described previously and also by others [1]. The films were stripped of their substrates and placed in Smallaluminium pans and then sealed. An empty pan Was used as a reference. A typical specimen weight was in the range 5 to 20 rag. The observed DSC thermograms were unaffected when the samples were preilluminated under highintensity light or dark-rested inthe pan for many days. The microhardness tests were carried Out on a Leitz (Wetzlar, Germany) miniload hardness tester with a 25g load which was the minimum available. The equipment operates a diamond pyramidal indenter for Vickers indentations on the sample under the selected load. For the softest films (a-Se) the measured indent diameters were ~ 35/~m which implies that the indentation depth was less than the film thickness. The quoted Vickers hardness number, Hv, for each sample is a mean over a number of random indentations. The variation was within ~ 10%. Fig. 2 shows three typical DSC thermograms at a heating rate of 5°Cmin -~ on (a) pure a-Se, (b) a-Se: 10wt % Te/Se double-layer and (c) a-Se: 10wt % Te single-layer photoreceptor films. The various characteristic temperatures are defined on thermogram (a). It is apparent from Fig. 2 that both the glass transition temperature, Tg, and the peak crystallization rate temperature, To, are shifted to higher temperatures with 10wt % Te alloying. Because of the broadening of the exotherms with tellurium addition the crystallization onset temperatures, Tff and To, however, are only slightly affected. By applying thermoanalytical methods developed for non-isothermal crystallization transformations based on nucleation and growth, we were able to show that the crystallization