Morphological and compositional study of CBD-ZnSe thin films by microscopy techniques and angle resolved XPS

Abstract

A morphological and compositional study is carried out on ZnSe thin films obtained by the chemical bath deposition (CBD) method. SEM, TEM and AFM images are shown for the morphological characterisation. Angle-resolved XPS (ARXPS) measurements are used for the study of surface and subsurface composition of the films. The composition underneath is studied with XPS analysis of films eroded by sputtering. It is found that films have a mixed ZnSe-ZnO (or Zn(OH) 2) composition. The Zn/Se ratio in the film increases with depth, indicating that Zn is preferentially as ZnO-Zn(OH) 2 close to the film-substrate interface, and that the ZnSe proportion increases above. Such composition inhomogeneity is attributed to a change in the deposition mechanism during film growth. At first, it proceeds via reaction of adsorbed Zn and Se precursors, and then by deposition of ZnSe clusters formed in the bulk of the solution. Apparently the first mechanism is less efficient for the formation of a pure ZnSe film, at least under the experimental conditions used here, hence gives rise to higher concentration of Zn oxides close to the film-substrate interface. At longer times during the deposition process, the cluster precipitation mechanism predominates and, consequentially, the top layers of the film become richer in ZnSe but less compact. Other compounds detected by ARXPS are Se 0 occluded in the bulk of the film, and some SeO 2 at the surface. Annealing at 300°C results in structural and compositional changes which involve compaction of the films, the loss of the occluded Se 0, the increment of the SeO 2 overlayer and the transformation of Zn(OH) 2 into ZnO and/or Zn(O, Se) compounds.