Investigation of vertical compositional gradients in Cu(In,Ga)Se_2 by highly spatially and spectrally resolved cathodoluminescence microscopy

摘要

Polycrystalline Cu(In,Ga)Se_2 (CIGS) thin films with thicknesses of 1.1 μm, 2.4 μm and 2.9 μm 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 (<160 nm) and spectrally resolved cathodoluminescence microscopy at low temperature (T=5 K). The integral spectrum of the investigated samples is dominated by donor-acceptor-pair recombination around 1.15 eV for the thinner samples and around 120 eV 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 13 meV to 130 meV with decreasing process speed.