JOURNAL Ot,'MATERIALS SCIENCE 19 (1984) 113-120 Phase transitions in cuprous sulphide evaporated thin films M. LEON Department of Applied Physics, Autonomous University of Madrid, Cantoblanco, Madrid-34, Spain N. TE RAO Laboratory of Physical Metallurgy, Cathofic University of Louvain, B-1348 Louvain-/a-Neuve, Belgium F. RUEDA Department of Applied Physics, Autonomous University of Madrid, Cantoblanco, Madrid-34, Spain The films of CuxS(1 ~<x ~< 2), prepared by evaporation in vacuum and condensation of stoichiometric sulphides of copper, change their phases by heat treatment in vacuum. Cu2S chalcocite appears as the most stable phase in the range 270 to 450 ~ C and the other sulphur rich phases transform into chalcocite presumably by a loss of sulphur. By electron diffraction we have confirmed the low temperature monoclinic chalcocite phase, the high temperature hexagonal chalcocite phase, the monoclinic djurleite phase, the cubic digenite low and high temperature forms and the hexagonal covellite phase. The low temperature digenite phase presents a superstructure which is similar to that observed by Kazinets et aL on digenite prepared by evaporation in vacuum on a single crystal substrate of NaCI at temperatures of 350 to 400 ~ C. Electron diffraction data of chalcocite (Cu2S), djurleite (Cu 1.93S),digenite (CUl.sS)are presented. 1. Introduction The study of thin films of cuprous sulphide CuxS in the range 1 <~x ~< 2 is of interest for their appli- cations, particularly the compound CuzS chalco- cite. The main application of chalcocite at present is in thin film solar cells of the CuzS-CdS type where it is used as the active part of the hetero- junction. By increasing the knowledge of struc- tures and behaviours of cuprous sulphide thin films obtained by evaporation in high vacuum the improvement of the cell characteristics may be stimulated. Electron diffraction work on CuxS films obtained by vacuum evaporation have been done by Rastogi et al. [1] and Kazinets etal. [2], using glass or NaCI single crystals as substrates. The first authors studied the effect of the glass substrate temperature on the film phase composition observ- ing Cu-rich phases for lower temperatures. At room temperature they found the orthorhombic and hexagonal phase of chalcocite and metallic copper at 300K of substrate temperature , while hexagonal chalcocite and djurleite at 400K and djurleite and Cui.93S at 475 K, respectively. Our present results observed on specimens prepared on amorphous soluble substrates at room temperature do not agree with the analogous ones of these authors. Kazinets et al. [2] have used single crystals of NaCI at 350 to 400~ as substrates and have evaporated CUl.aS. They observed an epitaxial rela- tion and a new structure phase of digenite, stable at room temperature, that we have also observed with amorphous substrates. 2. Experimental conditions The films studied have been obtained by evapora- tion of chalcocite, djurleite and anilite synthesized 0022-2461/84503.00+.12 9 Chapman and Hall Ltd. 113