PHYSICAL REVIEW B VOLUME 48, NUMBER 23 15 DECEMBER 1993-I Impurity-induced phonon disordering in Cd& „Zn„Te ternary alloys D. N. Talwar Solid State Electronics Directorate, Wright Laboratories, Wright Patterson Air Force Base, Ohio 45433-6543 and Department of Physics, Indiana University of Pennsylvania, Indiana, Pennsylvania 15705-1087* Z. C. Feng Department of Physics, National University of Singapore, Singapore 0511 P. Becla Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 7 May 1993) A comprehensive study of impurity-induced phonon disordering in Cd& Zn Te alloys is reported for a variety of samples (with composition ranging from x=0.005 — 0. 5, and 1) by using far-infrared reflectivity and Raman-scattering spectroscopy. Substantial differences were noted among the various published values for the optical-phonon frequencies versus x. Contrary to an earlier Raman study on molecular-beam-epitaxy grown Cd& „Zn Te/GaAs films, our results yield increases in the numbers of both the CdTe- and ZnTe-like TO phonons with x. A modified random-element isodisplacement model provides excellent fits to the optical phonons. The effects of impurity-induced phonon disordering are studied, within the band mode region, by using an average-t-matrix formalism. Unlike earlier specula- tions in which a gap mode in ZnTe:Cd lies near — 140 — 145 cm ', our theory predicts it to be at a higher frequency, — 153 cm . Group-theoretical analysis suggests that the gap mode exhibits a triply degen- erate vibrational state and it can be detected both by IR absorption and Raman-scattering spectroscopy. I. INTRODUCTION Considerable efforts have been made in recent years to evaluate the basic properties of mixed II-VI compounds for applications in photovoltaic, photoconductive, and in- frared (IR) detection devices. Earlier, narrow-band-gap mercury cadmium telluride (MCT) was used as an epi- layer in fabricating high-performance optical devices. ' However, this material suffers from serious drawbacks re- lated to the poor lattice stability of its alloys. The lattice instability is caused primarily by the incorporation of Cd which weakens the Hg-Te bond. Since the material is not tolerant or resilient to stress it can cause substantial variations in stochiometery. Stress may also be caused by simple processes such as oxidation, or reactions with met- als, and/or by mechanical damage. The observed loss of Hg from MCT at elevated temperatures is thought to be as a result of one of the above processes. We mentioned that a small deviation (-0. 1%) in stochiometry may re- sult in native defects (e.g. , vacancies, antisites, etc. ) with concentrations as high as 10' cm . In MCT the native defects affect the transport properties and hence the de- vice performance of IR photodetectors. Since the epitax- ial layers used in IR detector arrays depend critically upon the quality of the surface and on the substrate, con- siderable interest has been stimulated to search for more stable materials for optical devices. Theoretical calculations ' made in recent years have indicated that the weak Hg-Te bond can be strengthened if alloyed with ZnTe instead of CdTe. However, the high segregation coef5cients and large mercury vapor pres- sures at high temperature associated with the liquidus have indicated unfavorable conditions for the growth of homogeneous single crystals of Hg& „Zn„Te over the ex- tended composition range. In evaluating the characteris- tics of other members of the II-VI series our total- energy calculations suggest that the Cd- Te bond in Cd, Zn Te is more stable than the Hg-Te bond in Hg& „Zn„Te. Like other ternary compounds, Cd & „Zn„Te offers a large tunability with x for both the lattice parameter and for the band gap. It is at the value x =0.04 that Cd& Zn Te is lattice matched to Hgo 78Cdo 22Te for the infrared detector applications. Al- though several studies' have been performed in recent years to understand the electronic properties (e.g. , band- structure, defect-induced deep-electronic states, etc. ) of Cd] „Zn Te very little attention is paid for the optical examination of its dynamical behavior. We strongly be- lieve that such a study of lattice dynamics is important to illustrate many fundamental aspects related to the crystal stability through the bonding mechanism. In ternary compounds an excellent illustration of lat- tice dynamics can be obtained by using infrared (IR) refIectance and Raman-scattering spectroscopy. In the 1970s, three research groups " studied the variation of optical phonons as a function of x in Cd& „Zn Te alloys at 300 and 80 K. All confirmed the two-mode behavior in Cd& „Zn„Te with nearly identical plots for the com- positional dependence of ZnTe- and CdTe-like TO and LO phonons. The two-mode behavior is typical of mixed crystals for which the optical-phonon bands of pure ma- terials do not overlap and where, near the extremes of the composition range, the 1ocalized mode of the minority ions lies well outside the restrahlen peak of the host crys- 0163-1829/93/48(23)/17064(8)/$06. 00 48 17 064 1993 The American Physical Society