Investigation of vertical compositional gradients in Cu(In,Ga)Se2 by highly spatially and spectrally resolved cathodoluminescence microscopy

Abstract

Polycrystalline Cu(In,Ga)Se2 (CIGS) thin films with thicknesses of 1.1μm, 2.4μm and 2.9μm were grown using an in-line co-evaporation process with a final Cu-poor composition. The different film thicknesses were achieved by a variation of the process speed under constant evaporation rates. We analyze CIGS thin films by means of highly spatially (<160nm) and spectrally resolved cathodoluminescence microscopy at low temperature (T=5K). The integral spectrum of the investigated samples is dominated by donor–acceptor-pair recombination around 1.15eV for the thinner samples and around 1.20eV for the thickest sample. The surface shows a Gaussian and therefore fully random distribution of the peak wavelength. An investigation of the cross-sections reveals a shift of the peak energies to lower energies towards the surface of the layers for all samples and thus visualizes the local vertical gradient of the Ga/(Ga+In) ratio of the samples via an optical method. The extent of this shift increases significantly from 13meV to 130meV with decreasing process speed.