Nondestructive Compositional and Defect Characterization of CdZnTe Alloys Using Photoluminescence Spectroscopy
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NONDESTRUCTIVE COMPOSITIONAL AND DEFECT CHARACTERIZATION OF CdZnTe ALLOYS USING PHOTOLUMINESCENCE SPECTROSCOPY W.M. DUNCAN, R.J. KOESTNER, J.H. TREGILGAS, H.-Y. LIU and M.-C. CHEN Central Research Laboratories, Texas Instruments, Inc., Dallas, TX 75265 ABSTRACT Results from high resolution helium temperature photoluminescence (PL) spectroscopy have been correlated to precision lattice constant and transport measurements and to theoretical band gap versus composition behavior. It is found that low temperature PL spectra provide precise determination (+/- 0.02%) of ZnTe mole fraction as well as carrier type, relative impurity concentration and point defect properties of these substrates. In addition helium and room temperature PL results are correlated to determine the accuracy of room temperature measurements for composition determination. INTRODUCTION Alloys of CdZnTe are important substrate materials for growth of epitaxial thin films of HgCdTe for infrared detector and focal plane array applications. In order to attain thin films of high crystalline quality, it is known that close lattice match is needed between the film and substrate. For matching to nominally 10 pm HgCdTe, substrates of CdZnTe alloys containing a few percent of ZnTe are needed. Although photoluminescence has been used widely for evaluation of II-VI binary materials [1] only more recently has CdZnTe films [2,3] and bulk crystals been studied [4]. In the current work the photoluminescence optical properties of a large sampling of CdZnTe ingots with ZnTe fractions of 0 to about 8% been studied and correlated to electrical and precision X-ray lattice constant measurements. EXPERIMENTAL The crystals of CdZnTe studied in this work were prepared in sealed ampoules by the horizontal Bridgman method. Stoichiometric melt charges were precompounded from the elements. Unseeded crystals 3-4 kg in mass were grown. Etch pit counts range from 5 x 10' to 2 x 10' cm- 2 in these crystals. Unoriented samples were rough cut from the boules, chemomechanically polished and etched in Br/methanol prior to study. Samples typically were taken at 3 inch intervals along the 15 inch boules. Precision lattice parameter measurements were made using a Siemens D-500 powder defractometer. Silicon was used as an internal standard. Compositions were calculated based on the JCPDS [5] lattice constants for pure ZnTe (6.1026 A) and CdTe (6.4810 A). Photoluminescence measurements were made at room temperature and at 4.2 K with samples immersed in liquid helium. Although different spectrometers were used for analyzing the emission at room and helium temperatures, nearly identical excitation conditions and backscattering sampling geometries were used for both cases. Samples were 2 excited using 5145 A argon ion radiation at a power density of approximately 0.2 W/cm (6mW). The room temperature emission was analyzed with a conventional scanning dispersive monochromator and cooled GaAs photocathode photomultiplier whereas the low temperature spectra were analyzed interferometrically [6] us
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