Preparation of Cu(In, Ga)Se2 polycrystalline thin films by two-stage selenization processes using H2Se-Ar gas

In this study, various two-stage deposition processes have been investigated in order to produce device-quality of Cu(In, Ga)Se-2 chalcopyrite thin films. In principle, these techniques involved the preparation of various metallic precursors (by co-evaporation and sequential deposition) and the subsequent reaction of these precursors with a controlled H2Se-Ar atmosphere. In the first approach, conventional co-evaporation processes were used to prepare metallic Cu-In-Ga-Se precursors at substrate temperatures as low as 200 degrees C. This process produced uniform and dense films, but x-ray diffraction studies revealed the presence of broad x-ray peaks (indicative of poor crystallinity). The improvement in crystallinity brought about by H2Se-Ar treatment was critically influenced by the selenization parameters (especially the reaction temperature and H2Se gas concentrations). Virtually no improvement in the material's quality was observed at selenization temperatures below 450 degrees C. Optimum material properties (single-phase material of high crystallinity) were obtained when these co-evaporated precursors were exposed to 10 vol% H2Se at final selenization temperatures of 450 degrees C. In the second approach, sequentially deposited triple layers (Ga/Cu/In, Ga/In/Cu and Cu/Ga/In) and multilayers (Ga/Cu/In/Ga/Cu/In) were reacted with H2Se-Ar. In general, selenization of triple layers resulted in Cu(In, Ga)Se-2 films with poor crystallinity (morphological irregularities and separated CuInSe2 and CuGaSe2 phases were present). However, reaction of multilayers with 10% H2Se in Ar at final temperatures of 450 degrees C resulted in single-phase material with uniform and dense surface morphologies. Photoluminescence studies indicated, in all cases, the; presence of one broad donor-acceptor-pair transition. However, in the cases of selenized co-evaporated Cu-In-Ga-Se alloys and sequentially evaporated multilayers, this emission line shifted to higher energies, which indicated that Ga is present in the near-surface region of these specific samples. The production of single-phase Gu(In, Ga)Se-2 films at relatively low processing temperatures (450 degrees C rather than the 650 degrees C used in conventional processes) and the control of the Ga concentration gradient through the samples are important technological advantages of these two-stage processing techniques.