Photoluminescence of Cu(In 1-X , Ga X )Se 2 Epitaxial Thin Films Grown by MOVPE
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Photoluminescence of Cu(In1-X,GaX)Se2 epitaxial thin films grown by MOVPE N. Rega*, S. Siebentritt, J. Albert, M. Lux Steiner Hahn Meitner Institut Berlin GmbH, Glienickerstr. 100, D-14109 Berlin ABSTRACT To study the intrinsic defect structure, detailed photoluminescence (PL) measurements of epitaxial Cu-rich Cu(In1-X,GaX)Se2 grown by metalorganic vapor phase epitaxy (MOVPE) were performed. Distinct emissions appear at an energy position 80-150meV below the bandgap. Excitation-power-dependent measurements showed blue shifts up to 18meV/decade for these emissions. The temperature dependent PL results in a defect energy for this emission of 5080meV due to a free-to-bound (FB) transition. This indicates that the emissions of this material are controlled by fluctuating potentials due to high compensation or compositional inhomogeneity. The compositional inhomogeneity is detected by x-ray diffraction (XRD) and occurs especially for epitaxial layers grown at 570°C. INTRODUCTION Chalcopyrites such as Cu(In1-X,GaX)Se2 are known as promising absorbers for thin film solar cells. They reach efficiencies up to 19.2%1. The integrated Ga content of their polycrystalline absorber is about 30% with a significant Ga gradient in the film. Solar cells made from pure CuInSe2 show a good performance, while pure CuGaSe2 can not reach the same efficiencies. For further optimization of the devices, more information is needed about the effect of Ga on the structure, the surface and the doping of Cu(In1-X,GaX)Se2. The doping of chalcopyrites is due to intrinsic defects such as vacancies, interstitials and antisites. PL measurements can provide ionization energies of “near-band-edge” defects and information about the compensation of the semiconductor. To obtain a deeper understanding of these defects, Cu(In1-X,GaX)Se2 epitaxial thin films were grown on GaAs(001) by metalorganic vapor phase epitaxy with varying Ga contents measured as X=[Ga]/[Ga]+[In] and were characterized by photoluminescence. This presentation extends our work on pure CuInSe22 and CuGaSe23 epitaxial layers. PREPARATION The Cu(In1-X,GaX)Se2 epitaxial layers were grown on “epi-ready” GaAs (001) wafers (Wafer Technologies Inc.) in a horizontal MOVPE reactor (AIXTRON 200) with a graphite substrate holder and an infrared heating system to allow fast temperature ramps. The EPICHEM metalorganic precursors were: tri methyl Ga, tri methyl In, ditertiary butyl Se and cyclopentadienyl Cu tri ethyl phosphine (CpCuTEP). During the process 5 l/min Pd-purified H2 was used as a carrier gas at a pressure of 30mbar. The growth temperature was 500°C or 570°C. The amount of deposited material was controlled by the flux of H2 through the precursors and by their vapor pressure. Due to its low vapor pressure, CpCuTEP has to be heated to 60°C4. The other precursors may be used at room temperature or below. The Cu-precursor was kept at maximum output and the Se-precursor was provided at 10 times the concentration of the other precursors. The composition of the films was controlled by the r
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