Effects of Ge-Implantation on the Photoluminescence of CuGaSe 2 Thin Films
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Effects of Ge-Implantation on the Photoluminescence of CuGaSe2 Thin Films Serge Doka1, Marin Rusu1, Alex Meeder2, Ernest Arushanov3, Norbert Fabre4, Sebastian Fiechter1, Thomas Schedel-Niedrig1, Martha Ch. Lux-Steiner1. 1 Hahn-Meitner-Institut, Glienicker Str. 100, D-14109 Berlin, Germany, 2 SULFURCELL Solartechnik GmbH, Barbara-McClintock-Str. 11, 12489 Berlin, 3 Institute of Applied Physics, Academy of Sciences of the Republic Moldova, Academiei 5, Chisinau 277028, Moldova. 4 LAAS- CNRS, 7 Avenue du Colonel Roche, 31077 Toulouse Cedex 4, France. ABSTRACT Photoluminescence (PL) spectra of as grown Ga-rich, and Ga-rich plus Ge-doped and annealed CCSVT (Chemical Close-Spaced Vapor Transport) - CuGaSe2 thin films have been investigated. Visible (514.5 nm) and Ultra- Violet (351.1 nm) excitation energies of the laser have been used in order to determine intrinsic and extrinsic defects created due to the implantation as well as separating near surface from bulk recombination. Both visible and UV-PL spectra of the undoped films show the well known luminescence of Ga-rich CuGaSe2, which can be described by the widely accepted model of fluctuating potentials. Unlike the visible- and UV-PL emissions of Ge- implanted and annealed films differ strongly. Obviously, Ge-implantation in combination with the thermal treatment results in an extrinsic doping of the material producing so far unknown states in the CuGaSe2 band gap. Comparing the visible- and the UV-PL spectra we found an accumulation of these extrinsic doping levels in the near-surface-region of the films. INTRODUCTION Extensive studies have been performed on ternary chalcopyrite-like CuGaSe2 (CGSe) semiconductor as a promising material for solar cell applications due to its favorable direct band gap of 1.68 eV at RT and high optical absorption coefficient larger than 104 cm-1. In general chalcopyrite-based solar cells are natively doped via stoichiometry control [1], however, tuning the optical and electrical properties of CuGaSe2 chalcopyrites and, therefore, prompt the p-to-n conductivity-type conversion, encountered many difficulties theoretically explained by “doping pinning rule“ [2]. In order to overcome these difficulties, many attempts have been performed towards extrinsic doping using ion implantation [3]. Hence, the control and understanding of the doping mechanisms of extrinsic defects become a major task. In this work we present results of investigation of the visible photoluminescence (Vis PL) and ultra-violet photoluminescence (UV PL) of the Ge-implanted CuGaSe2 thin films in connection with the thermal treatment done after ion implantation. EXPERIMENTAL DETAILS CuGaSe2 thin films were prepared by CCSVT device grade and the corresponding solar cell devices have shown efficiency of 8.7 %. More detailed results on CGSe thin films preparation are reported elsewhere [4]. Ge ions were implanted into the thin films at accelerations energy of 150 KeV to a fluence of 1015 ions/cm-2. In order to eliminate damage that always accompanies ion implantation
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