PREPARATION OF CU(IN,GA)SE-2 POLYCRYSTALLINE THIN-FILMS BY 2-STAGE SELENIZATION PROCESSES USING H2SE-AR GAS

In this study, various two-stage deposition processes have been invest igated in order to produce device-quality of Cu(In, Ga)Se-2 chalcopyri te thin films. In principle, these techniques involved the preparation of various metallic precursors (by co-evaporation and sequential depo sition) and the subsequent reaction of these precursors with a control led H2Se-Ar atmosphere. In the first approach, conventional co-evapora tion 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 p resence of broad x-ray peaks (indicative of poor crystallinity). The i mprovement in crystallinity brought about by H2Se-Ar treatment was cri tically influenced by the selenization parameters (especially the reac tion temperature and H2Se gas concentrations). Virtually no improvemen t in the material's quality was observed at selenization temperatures below 450 degrees C. Optimum material properties (single-phase materia l of high crystallinity) were obtained when these co-evaporated precur sors were exposed to 10 vol% H2Se at final selenization temperatures o f 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 (morph ological irregularities and separated CuInSe2 and CuGaSe2 phases were present). However, reaction of multilayers with 10% H2Se in Ar at fina l temperatures of 450 degrees C resulted in single-phase material with uniform and dense surface morphologies. Photoluminescence studies ind icated, in all cases, the; presence of one broad donor-acceptor-pair t ransition. However, in the cases of selenized co-evaporated Cu-In-Ga-S e alloys and sequentially evaporated multilayers, this emission line s hifted to higher energies, which indicated that Ga is present in the n ear-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 proc esses) and the control of the Ga concentration gradient through the sa mples are important technological advantages of these two-stage proces sing techniques.