Ellipsometric characterization of surface oxidation in polycrystalline Zn 3P 2 thin films

Abstract

Zn 3P 2 is II–V semiconductor that has been studied as a light-absorbing material in highly efficient photovoltaic heterojunctions and in photodetectors. The optical properties of the films, as measured by spectroscopic ellipsometry, indicate the existence of two absorption peaks at 2.7 eV and 4.0 eV, as in the case of monocrystalline material. These peaks have a soft shape because of the polycrystalline character, roughness and surface oxide of the film. The spectral ellipsometric analysis, after surface etching, revealed notable evolutions of the optical spectra. Real-time ellipsometric analysis of the surface evolution, at a fixed wavelength, was performed in air at room temperature after surface etching. The results indicate the formation of a very thin and porous native zinc oxide layer with an exponentially decreasing growth rate. The results are discussed in terms of surface changes, such as oxidation, loss of stoichiometry and porosity which are of great interest for discerning the kinetics of surface degradation, with a view to the applications of these films in optoelectronic devices.