Structural analysis of the semiconductor-semimetal alloyCd1−xHgxTeby infrare

Abstract

Far-infrared reflectivity measurements have been made on traveling-heater-method-grown Cd 1Ϫx Hg x Te bulk single crystals. The spectra at 300, 77, and 25 K display two bands with fine structure, which cannot be explained by the usual two-oscillator model. These lattice spectra are shown to originate from four Cd-Te and Hg-Te vibrational modes in five basic Cd(4Ϫn)Hg(n)Te (nϭ0,1,2,3,4) cells consisting of cations tetrahedrally distributed around a shared Te anion. The mode frequencies are determined by the cation configuration in each cell. They are assumed to be independent of alloy composition, which determines only the mode strengths. The temperature dependence of the Cd-Te and Hg-Te vibrations revealed differences. The Cd-Te frequencies decrease with temperature, as is typical for semiconductor compounds. For the Hg-Te vibrations the temperature dependence is more complicated. Such temperature behavior was explained by influence of the anharmonicity corrections, which change sign with variation of frequency. Dispersion analysis of the dielectric function Љ(w) restored from the reflection spectra of the alloys at 25 K by Kramers-Kronig analysis revealed a peculiarity at 137 cm Ϫ1 , which appears with nearly constant strength over the wide compositional range x ϭ0.1-0.8. This mode has the same temperature dependence as the Cd-Te vibrational modes. The most probable explanation of this mode is associated with the alloy disorder or with multiphonon processes.

Statistical strained-tetrahedron model of local ternary zinc blende crystal structures

“…Thus only 6, 4, 4 intense + 2 weak phonon lines, respectively. More than 8 lines are observed when point defects occur (antisites, vacancies, …), responsible for the extra lines (as reported for HgCdTe [17]). Thus FIR admits the detection of defects: vacancies, intersites, antisites, H-loading deformations, quantifying amount of impurity ions [18].…”

Model of the two well potential for Hg-atoms in the Hg1 - xCdx Te alloy lattice

“…An additional mode caused by introducing of the hydrogen atoms in the tetrahedra in the crystal lattice of CdTe was observed in the FIR-spectra obtained using synchrotron radiation as the source [11]. The problem of the additional phonon modes (APM) in the 100-115 cm −1 range has existed for the Hg 1− x Cd x Te alloys for the last forty years [13][14][15][16][17][18][19][20][21][22]. The additional mode is observed too in HgTe crystals in this same range [23,24].…”

“…The most comprehensive experimental investigation of the Hg 1− x Cd x Te phonon spectra was undertaken by Kozyrev et al [17], where the composition dependence of the subtle structure of two sub-bands HgTe-like and CdTe-like were observed and analysed in detail and an interpretation in terms of five based cells (tetrahedra) was presented. Nevertheless, the region below 118 cm −1 was excluded from consideration in [17].…”

“…Nevertheless, the region below 118 cm −1 was excluded from consideration in [17]. However, several experiments have indicated that in the range of about 100-115 cm −1 additional lines are located [12][13][14][15][16][17][18].…”

Additional phonon modes related to intrinsic defects in CdHgTe

“…The line shape asymmetry of Hg 1-x Cd x Te -phonon spectra was observed also. In the paper by S. Kozyrev et al [3], the range 100 cm -1 -115cm -1 is excluded from consideration. Since the 70th, the range of 30 cm -1 -115 cm -1 was discussed in the literature [4][5][6][7].…”