THE WSE2 TUNGSTEN-OXIDE INTERFACE - STRUCTURE AND PHOTOLUMINESCENCE

A ''mixed'' surface of WSe2, which contains both the parallel-to c and perpendicular-to c facets was produced by mechanical indentation, fol lowed by photoelectrochemical etching, and chemical cleaning. This sur face was shown to exhibit low surface recombination velocity, and cons equently high solar to electrical conversion efficiency, in the past. Using O-18 as a tracer for nuclear activation analysis it is shown tha t, after the above surface treatment, the surface of the parallel-to c facet is covered with a thin (3 - 5 monolayers) film of tungsten oxid e. The structure of the crystalline oxide phases that comprise the tun gsten-oxide/WSe2 interface are investigated with powder X-ray diffract ion. X-ray photoelectron spectroscopy is used to study the chemical bo nding at the interface. A model is proposed to explain the absence of surface recombination at this interface. A strong above the bandgap lu minescence is observed from parallel-to c facets of ''mixed'' surfaces , and from crystal edges (parallel-to c) which were allowed to ''age'' for a few weeks in the air. The origin of this luminescence was inves tigated in some detail, and a mechanism, involving partially oxidized parallel-to c surface which possibly leads to quantum confinement of e xcitons in parallel-to c edges of WSe2, is proposed.