Structural and Optical Characterization of Cu x Ga y Se 2 Thin Films under Excitation with Above and Below Band Gap Lase
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Structural and Optical Characterization of CuxGaySe2 Thin Films under Above and Below Band Gap Laser Light
V3.7.1
Εxcitation with
C. Xue1, D. Papadimitriou1, Y.S. Raptis1, T. Riedle2, N. Esser2, W. Richter2, S. Siebentritt3, S. Nishiwaki3, J. Albert3, M.Ch. Lux-Steiner3 1
National Technical University of Athens, Department of Physics, GR-15780 Athens, Greece. Technical University of Berlin, Institute of Solid State Physics, Hardenbergstr. 36, D-10623 Berlin, Germany. 3 Hahn-Meitner Institute, Glienickerstr. 100, D-14109 Berlin, Germany. 2
ABSTRACT CuxGaySe2 MOCVD and PVD grown films were structurally and optically characterized by Raman, Micro-Raman and photoluminescence spectroscopy. Defect related photoluminescence excitation with wavelengths varying across the material band gap reveals: a) in Cu-rich CuGaSe2 films, three band edge splitting due to the spin-orbit interaction and the crystal field, and donoracceptor pair recombination between a shallow donor and two different acceptor levels, and b) in Ga-rich CuGaSe2 films, donor-acceptor pair transitions between quasi-continua of donor and acceptor levels related to potential fluctuations. Raman spectra of CuxGaySe2 films, excited by laser light near and below the material band gap, show intense modes at 197cm-1, 187cm-1, and 277cm-1, which can be used as indicators of crystallinity and Ga-content of the films. Polarization- and angular- dependent micro-Raman spectra of MOCVD CuGaSe2 indicate that CuxSey-crystallites, dispersed on the surface of Cu-rich films, are grown oriented with their caxis perpendicular to the film surface. INTRODUCTION Ternary chalcopyrite semiconductors of type Cu-III-VI2 (III=Ga, In, and VI=S, Se) have attracted a lot of attention because of their potential applications in solar cell technology. CuGaSe2, as a higher band-gap chalcopyrite (Eg=1.68 eV at 300K), can be used, in combination with CuInSe2 (Eg≈1eV), in the growth of alloyed materials1 with band-gap energies over a wide spectral range. For good cell performance, the properties of its constituting materials have to be well understood. In this work, the structural and optical properties of CuGaSe2-films grown on Glass/Mo-substrates by commercially attractive PVD techniques were studied with reference to the properties of CuGaSe2 epitaxial layers grown by MOCVD on GaAs (100). EXPERIMENT All the studied samples were grown at Hahn-Meitner-Institute in Berlin as described elsewhere2-3. Photoluminescence (PL) emission was excited in the temperature range 300-20 K by 514.5 nm(2.4 eV) Ar+-laser and Ar+-laser pumped Ti:Sapphire-laser operated in the spectral range 1.51-1.77 eV. Raman spectra were excited by laser light near (647.1 and 676.4 nm Kr+laser) and below the material band gap (Ar+-pumped Ti-Sapphire laser). The emitted light was spectrally analyzed by a double grating monochromator (SPEX 1403) with cooled PMT-detector. In addition, the CuxSey-phase formed on the surface of Cu-rich films was studied by micro-
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