Rheotaxial growth of semiconductor thin films

Abstract

The efficiency of thin film solar cells depends to a large extent on the crystalline grain size. The increase of the latter can be obtained by rheotaxy, i.e. the growth of thin films on a liquid metallic layer. In this work we report the rheotaxial growth of CuInSe, thin films on liquid indium-coated glass substrates. The films were deposited onto bare glass and liquid indium coated (100 nm thickness) glass substrates by co-evaporation of copper, indium and selenium. Moreover, in order to study the influence of the indium film thickness, a deposition on a stepped indium layer with thicknesses between 10 and 100 nm was performed. The morphology, composition and crystalline properties of the deposition CuInSe, thin films have been studied in relation to the deposition parameters. The deposition rate of the rheotaxial films is much higher than that of the films deposited on glass, and increases with increasing indium layer thickness. The films are polycrystalline with grain sizes in the 0.1-0.3 pm range for films grown on glass and in the 0.332 pm range for rheotaxial films. The grain size shows an important dependence on the growth temperature and the indium layer thickness. The films grown on glass have compositions near stoichiometry, but the rheotaxial films show a higher In concentration which cannot be explained solely by the incorporation of the In layer in the growing film. The X-ray diffraction spectra indicate that the films are constituted by the crystalline phases of CuInSe, and by several binary compounds of Cu, In and Se, depending on the growth temperature. Rheotaxial films with only CuInSe, phases can be obtained by increasing the Cu flux with respect to the one used for films deposited on glass, or by decreasing the indium layer thickness.