Phonons in Mixed II-VI Compound Semiconductors
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PHONONS IN MIXED II-VI COMPOUND SEMICONDUCTORS D. N. Talwar*, Alan C. Coleman*, P. M. Amirtharaj!, S. Perkowitz#, Z. C. Feng# and P. Becla@ Department of Physics, Indiana University of Pennsylvania, Indiana, PA, 15705 U.S. Army Center for Night Vision and Electro-Optics, Fort Belvoir, VA, 22060 # Department of Physics, Emory University, Atlanta, GA, 30322 @MIT Francis Bitter Magnetic Laboratory, Cambridge, MA, 02139
ABSTRACT We have analysed the far infrared reflectivity and Raman scattering spectra of phonons in Cdl.xMnxTe, Znl.xMnxTe, Hgl.xMnxTe, and Hgl-xZnxTe for low composition of x. Unlike CdMnTe and ZnMnTe, weak impurity induced peaks are observed in HgZnTe and HgMnTe, near - 96 and 98 cm- 1 , well above the LA-phonon band of HgTe. Comprehensive calculation of lattice dynamics based on a semi-empirical Green's function theory has provided convincing arguments to support the above features as gap modes of Zn and Mn in HgTe. The theory also predicts the possibility of resonance modes, above the TA cutoff of HgTe, in HgMnTe and HgZnTe to be observed by IR spectroscopy. INTRODUCTION Over the past years, the variable band gap II-VI alloy system Hgl.xCdxTe has been investigated extensively for the fabrication of high performance infrared (IR) detection devices.1, 2 The material, however, suffers from serious technological problems related to lattice, surface and interface instabilities caused primarily by the incorporation of Cd which weakens the Hg-Te bond.3 -5 Such problems can sustain a large variation in stochiometry and transport properties as a result of treatments as diverse as oxidation, mechanical damage, and reaction with metals. These concerns with the HgCdTe system have, 69 therefore, stimulated interest to search for alternative materials for IR detection devices. The ternary compounds Hgl-xZnxTe, Cdl-xMnxTe, Znl-xMnxTe and Hgt-,MnxTe (x < 0.35), because of their relative stability and high figure of merit, are regarded as potential candidates appropriate to replace HgCdTe for IR detector applications. Fourier transform infrared absorption and Raman scattering spectroscopy generally provide information of the vibrational behavior of impurities and defects in 1I-VI ternary alloys. 10,11 The analyses of these experiments, based on reliable theoretical models, can play important roles both in
Mat. Res. Soc. Symp. Proc. Vol. 209. ยง1991 Materials Research Society
452
probing the nature of "impurity-host bonding" and to search for realistic lattice dynamical and perturbation schemes. 12,13 The purpose of this paper is to analyse the existing data of phonon mode behavior in HgMnTe, HgZnTe, ZnMnTe and CdMnTe systems by using simple and realistic lattice dynamical model calculations. We predict, on the basis of a semi-empirical Green's function theory, some new results of impurity activated modes for HgMnTe and HgZnTe in the low phonon frequency (acoustical) region. 13 Although limited data is available for such vibrational features in mixed semiconductors, the calculations are strongly supported, by the
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