Growth and characterization of epitaxial films of ZnGeP 2 .
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M8.46.1
Growth and characterization of epitaxial films of ZnGeP2. G.A. Verozubova1, A. I. Gribenyukov1, M.C. Ohmer2, N.C. Fernelius2, J.T. Goldstein2. 1 Institute of Optical Monitoring SB RAS, 10/3 Akademicheskii Tomsk, 634055,Russia 2 Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLPSO, Wright-Patterson Air Force Base, Dayton, OH, USA ABSTRACT Thermodynamic analysis of the vapor phase over ZnGeP2 in Zn-Ge-P-Cl system has been carried out. The analysis showed that this system can be used for the vapor growth of ZnGeP2. Homoepitaxial layers of ZnGeP2 were grown in a closed system using chemical vapor transport. Electrical and photoluminescence properties of the layers were studied, and crystal lattice parameters were measured. Comparison of properties for bulk and vapor grown ZnGeP2 crystals were carried out. It was found that the vapor grown crystals have more perfect structure than the bulk ones, particularly, they have significantly lower vacancy concentration. INTRODUCTION ZnGeP2, a semiconductor with chalcopyrite structure, has a wide transparency range (0.7-12 µm), high nonlinear optical coefficient, and adequate birefringence for phase matching. Together with high hardness and high thermal conductivity these properties make material very attractive for incorporation in non-linear optical devices used in solution of a number of applied problems of spectroscopy and laser physics. Volume and surface imperfections existing in ZnGeP2 preclude the use of these crystals in high power laser systems. Bulk ZnGeP2 crystals grown from the melt show a defect-related absorption region extending from the fundamental absorption edge to approximately 2.5 µm. One model [1] explains the absorption in terms of high concentrations of zinc and phosphorus vacancies. The vacancy excess is attributed to the loss of the volatile zinc and phosphorus during synthesis and growth. Another model suggests that substitutional defect GeZn is responsible for 2-µm absorption [2]. Scattering from micro-precipitates [3] has also been suggested as a possible additional optical loss mechanism. Physical properties of crystals grown from the vapor can differ sometimes advantageously from those of the bulk melt ones. Particularly, homoepitaxial layers of this material grown from the vapor can improve surface properties of ZnGeP2 crystals. In this paper we report initial studies on growth and characterization of ZnGeP2 epitaxial films. EXPERIMENTAL DETAILS ZnGeP2 was synthesized from high purity elements using a modified two-temperature technique, yielding over 500 grams of polycrystalline ZnGeP2 material from each process run [4]. To produce bulk crystals for substrates a Vertical Bridgman seeded method was used. The temperatures of the upper and lower zones were 1060oC and 1000oC, respectively. The temperature gradient was about 2-3ºC/cm. The pulling rate was 0.5 mm/h. The grown single crystal ingots were annealed at 600°C, cut along the (508) or (100) plane, and then the substrates were mechanically polished.
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