Recent studies on photoconductive thin films of binary compounds

In this paper a review of recent progress achieved in the domain of MX2 films (M = Mo, W; X = Se, S) is presented. The MoS2 is essentially discussed. It is shown that the emerging interest in the use of MX2 thin films as absorbing layer in photovoltaic cells has induced significant improvements of the crystalline and optoelectrical properties of these films. Some years ago the films obtained were crystallized in the 2H-MoS2 structure but the size of their crystallites was small and the samples were poorly photoconductive. Recently many works have shown that, whatever the deposition technique used, textured films with large grains and good photoconductive properties could be obtained when a thin nickel layer is used. During the post annealing treatment, this thin nickel layer diffuses all over the thickness of the films. It is proposed that systematically the crystallization process of MX2 films is a two-step process. The primary crystallization corresponds to small crystallites formation and the secondary crystallization corresponds to large ordered domains growth by coalescence of the small crystallized domains. This secondary crystallization process is facilitated by the presence of Van der Waals surfaces parallel to the plane of the substrate. Moreover, this effect is strongly improved in the presence of nickel which allows the obtention of high quality films. The electrical properties of these films are interpreted with the help of grain boundary theories. (C) 1999 Elsevier Science B.V. All rights reserved.