PHYSICAL REVIEW 8 VOLUME 43, NUMBER 9 15 MARCH 1991-II Anomalous mode in the Raman and ir spectra of mercury telluride M. L. Bansal, * Alka Ingale, * and A. P. Roy Solid State Physics DiUision, Bhabha Atomic Research Centre, Bombay 400085, India (Received 7 May 1990) We discuss the origin of the low-frequency mode occurring at — 108 cm (at 77 K) in HgTe and Hg-rich Hg& Cd„Te. Through temperature- and wavelength-dependent Raman spectra and their analysis, we show that this feature is a combination phonon and acquires significant Raman (ir) ac- tivity as a consequence of a Fermi resonance with the TO phonon. Within this framework we are able to explain its TO-phonon-like behavior, and the variation of its frequency, width, and intensity with temperature. I. INTRODUCTION Mercury telluride and cadmium telluride are members of the alloy system Hg& Cd Te, an important material for ir applications. ' The special interest in HgTe and Hg-rich Hg& „Cd„Te arises because they provide a unique system where lattice vibrations and electronic ex- citations (interband and intraband) overlap in energy. Both the end members Hg Te and CdTe and, to a certain extent, their alloys have been investigated using ir reAectivity, ' Raman scattering, ' and neutron scatter- ing to understand their vibrational properties. In the ir spectra of HgTe (Ref. 2) and Hg& Cd Te (x (0.3), apart from the usual allowed TO phonon at 117 cm ', an additional mode has been observed below this frequency. Its oscillator strength increases and its frequency decreases with increasing temperature. Ra- man measurements ' that have been carried out on Hg Te at liquid-nitrogen temperature also reveal the presence of an additional feature at — 108 cm ', apart from the usual TO- and LO-phonon modes at 117 and 137 cm ', respec- tively. No detailed temperature-dependent Raman study has been reported in literature on HgTe. Swiatek et al. observed a similar structure in p-type HgTe. Grynberg et al. , on the basis of temperature-dependent ir work, found that this additional feature, which they call Q2 (we will follow the same notation), has TO-phonon character. Though no definitive assignment of this 02 mode has been made in Ref. 2, several possibilities are discussed. These include absorption due to combination phonon, an impurity-activated zone-edge phonon, a gap mode due to Hg substituting for Te, etc. Witowski and Grynberg ob- served a similar feature in HgSe. They have argued ' that in these materials I 8 ~I 8 electronic interband tran- sitions overlap with the phonon in energy, which can lead to resonant enhancement of the two-phonon cross sec- tion. Another school of thought' believes this mode to be a gap mode arising as a consequence of Hg substitut- ing for Te in HgTe. We show in this paper that none of the mechanisms stated above provides a satisfactory explanation of the occurrence of the 02 mode. With a view to understand- ing its origin, we have carried out temperature- and wavelength-dependent Raman measurements on HgTe. The experimental details and our main results are presented in Sec. II. We demonstrate by a careful analysis of the Raman data that the additional feature 02 can be understood in the framework of a mode coupling between the TO phonon and a difference phonon [coTo(q) — AT~(q) ]. In Sec. III, after briefiy presenting re- sults of the mode-coupling theory, we apply it to the present case of Hg Te and calculate the Raman line shapes. Section IV is devoted to a discussion and com- parison of our model with earlier work. Finally we sum- marize our main conclusions in Sec. V. II. EXPERIMENTAL RESULTS In order to rule out the effect of surface and sample quality on the intensity or frequency of the A2 mode, we have carried out investigations on the following three samples of (111)-oriented Hg Te. (a) Hg Te obtained from the Solid State Physics Labora- tory, New Delhi. The optical-quality surface was pro- duced with mechanical and chemomechanical (0. 1 vol. % Br& — methanol solution) polish. This was finally free etched in 0. 01 vol. % Br& — methanol solution and washed in methanol. (b) HgTe obtained from Purdue University. The sam- ple was polished with 0. 25-pm diamond powder and cleaned in ether and methanol to generate a rough sur- face. (c) HgTe same as sample (b) but with an optical-quality surface, produced with the same method as used for sam- ple (a). The Raman setup used for measurements is described in Ref. 5. The temperature was varied using a two-stage cryogenic refrigerator supplied by Air Products. Two lines at 4965 A (Ar laser) and 5309 A (Kr+ laser) with power — 100 mW were used to record the spectra at vari- ous temperatures. This choice of wavelengths was made with a view to ascertain whether the scattering mecha- nisms for the two modes (Q~ and the TO phonon) are the 0 same or different ones. 5309 A is very near the E, peak, 43 7020 1991 The American Physical Society