Optical and EPR Study of Defects in Cadmium Germanium Arsenide
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Optical and EPR Study of Defects in Cadmium Germanium Arsenide Lihua Bai,1 N. Y. Garces,1 Nanying Yang,1 P. G. Schunemann,2 S. D. Setzler,2 T. M. Pollak,2 L. E. Halliburton,1 and N. C. Giles1 1 Physics Department, West Virginia University, Morgantown, WV 26506-6315, U.S.A. 2 BAE Systems, Nashua, NH 03061-0868, U.S.A. ABSTRACT Bulk crystals of CdGeAs2 have been characterized using photoluminescence (PL), optical absorption, Hall effect, and electron paramagnetic resonance (EPR) techniques. An absorption band near 5.5 microns at room temperature is observed in all of the p-type samples we have studied. A correlation between the magnitude of this optical absorption and the excess hole concentration at room temperature is established. Also, an EPR signal is found to correlate with the strength of this absorption band. PL data are consistent with an increased concentration of shallow acceptors being present in high-absorption samples. From the EPR data, we suggest that a model for the paramagnetic defect associated with the absorption at 5.5 microns may be an acceptor on an anion site.
INTRODUCTION Cadmium germanium arsenide (CdGeAs2) is a II-IV-V2 nonlinear optical material belonging to the chalcopyrite family. Single crystals of this material have a band-gap energy of 0.57 eV at room temperature, a high nonlinear optical coefficient, and sufficient birefringence to allow phase matching. Furthermore, they can be grown in large sizes. Bulk crystals are used in mid-infrared frequency-conversion applications involving high-power pump lasers [1-3]. However, there is a discrete absorption band near 5.5 microns that hinders device performance. This absorption, although present in crystals at room temperature, can be reduced significantly by cooling the sample. It is generally accepted that this absorption is a transition between the two upper valence bands [4,5]. An unidentified shallow acceptor defect is believed to be responsible for the empty states (i.e., holes) in the upper valence band, thus compensation or removal of the shallow acceptor is needed in order to further reduce or eliminate the unwanted absorption. This would allow frequency conversion devices to operate at higher powers. An EPR spectrum has been previously reported [6] for CdGeAs2 crystals, although a specific model for the responsible paramagnetic center was not established. Also, previous reports have noted a broad emission peaking near 0.35-0.38 eV and a sharper emission near 0.55 eV as the dominant PL bands [7,8]. We have performed a correlation study of over 35 CdGeAs2 samples using photoluminescence, optical absorption, Hall effect, and electron paramagnetic resonance. These samples were grown at different times and represent a wide range of absorption coefficient values at 5.5 microns. We show that the EPR signal intensity correlates with the absorption at 5.5 microns, and thus with the concentration of shallow acceptors. On the basis of our study, we are able to suggest a defect model for this unwanted shallow acceptor in CdGeAs2.
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