EPR Studies of E-Beam and Gamma Irradiated ZnGeP 2 : A Nonlinear Optical Material for the Infrared
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M.H. RAKOWSKY*, W.J. LAUDERDALE*, R. A. MANTZ', R. PANDEY** AND P.J. DREVINSKY1 *Frank J. Seiler Research Laboratory, USAF Academy, CO 80840-6272 tPhillips Laboratory, Edwards AFB, CA 93524-7680 "**Department of Physics, Michigan Technological University, Houghton, MI 49931 lPhillips Laboratory, Hanscom AFB, MA 01731-2090
ABSTRACT Zinc germanium phosphide is a nonlinear optical material for efficient frequency conversion in the mid-IR spectral region. One challenge in the development of ZnGeP 2 is to reduce the near band edge absorption in the 0.7 to 2.5 micron region. Several methods have been used to ascertain the origin of this absorbance. One method, electron paramagnetic resonance (EPR), has been used to characterize the paramagnetic native acceptor in ZnGeP 2 by studying as-grown, thermally annealed, electron-beam irradiated, and gamma irradiated single crystals. Each of the these processing routines improves the optical transparency with a concurrent decrease in the concentration of paramagnetic centers and defect site symmetry as seen in the EPR spectra. High energy e-beam and gamma irradiation may cause compensation, movement, or creation of new defects. In addition, nuclear magnetic resonance (NMR) has been used to further characterize several samples of as-grown, annealed and polycrystalline ZnGeP2 . Preliminary calculations of the defect energetics have been conducted using atomistic simulation techniques employing the shell model to describe the lattice. Interionic potentials between the constituent ions were obtained by performing quantum cluster calculations on ZnGeP 2. INTRODUCTION Materials with nonlinear optical (NLO) properties are of great interest because of their optoelectronic applications in communications, data storage, optical switching, and frequency conversion 1' 2 . The importance of zinc germanium phosphide is discussed elsewhere in this volume3 . In our work we are interested in the characterization of defects in the material which may cause the sub-bandgap absorption. The structure of the ternary chalcopyrite is shown in Fig. 1. In the ordered ternary phase each phosphorus is bound to two zinc and two germanium atoms in nearly tetrahedral geometry 4. The EPR of the as-grown single crystal ZnGeP 2 sample grown by horizontal gradient freeze techniques is consistent with this geometry 5. Recent results suggest there are two possible models for the deep level acceptor observed by EPR, a zinc vacancy (Vzn) or a zinc ion on a germanium site (Znc.)'. In addition, EPR experiments, including an angular dependent electron nuclear double resonance (ENDOR) study of single-crystal ZnGeP 2, are being conducted to further characterize the paramagnetic acceptor. EPR and ENDOR are powerful techniques for determining the structure of paramagnetic defects. It has been suggested that this defect may be the same deep level acceptor designated ALl that gives rise to a dominant near-infrared 735 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
absorption band 5' 6. However, the role
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