The propagation of ultrasonic waves in mercury telluride

Abstract

The propagation of ultrasonic waves in single crystals of mercury telluride has been studied on a broad front between 1.2ºK and 380ºK. Measurements have been made by the pulse-soho technique for ultrasonic waves (frequency 10 MHz to 300 MHz) directed along the [100], [110] and [111] crystallographic directions. The large mercury telluride single crystals, required for the ultrasonic measurements, have been grown by the Bridgman technique from either stoichiometric melts or from off-stoichiometric, tellurium-rich, melts. The elastic constants C(_11), C(_12) an C(_44) of mercury telluride have been measured as a function of temperature between 1.2ºK and 300ºK, attention being paid to possible effects of non-stoichiometry in the crystals, and the results correlated with ultrasound attenuation data. Results are discussed in terms of the crystalline interatomic forces and are compared with those of other II-VI and III-V compounds with the zinc blende structure, together with group IV, elemental semiconductors and I-VII compounds: elastic properties of mercury telluride correspond closely to those of cubic zinc sulphide and fall into the general scheme presented by the related compounds. From the Szigeti relationship, the ionicity e* is estimated as 0.65 ± 0.05e and the fundamental lattice absorption (restrahlen) frequency as (4.1 ± 0.1) x 10(^12) Hz. The Debye temperature, calculated from the elastic constant data, is 141 ± 4 K. Anelastic properties of mercury telluride have been deduced from the temperature (1.2 K to 300 K), frequency (10 MHz to 300 MHz) and applied stress dependences of ultrasound attenuation measurements. The important ultrasound dissipation mechanisms include the viscous drag of lattice phonons and forced dislocation motion. Theoretical assessments of piezoelectric coupling on sound attenuation and thermoelastic loss, show that the effect of both are negligible. One of the main sound energy dissipation mechanisms is due to the lattice phonon-ultrasonic phonon interaction. Attenuation due to this effect exhibits characteristic features at low temperatures. These have been found in mercury telluride. The effect is larger than observed in other materials because the Debye velocity is lower and can therefore be observed at relatively low frequencies. Another loss mechanism arises from forced vibration of dislocation segments. At 4.2 K a maximum has been observed in the frequency dependence of attenuation. The results have been accounted for by the vibrating string model. The resonance frequency is 220 MHz. The dislocation drag coefficient is 2.3 x 10(^-5) dyn.sec.cm(^-2) at 4.2 K and loop length is about 3 x 10(^-4) cm. Data for the ultrasonic wave velocity and attenuation before and after annealing and under stress are in agreement with the dislocation mechanism. In the region 170 K to 260 K peaks are found in the attenuation which show characteristics of those of Bordoni. The activation energy is about 0.15 eV and the attempt frequency about 4 x 10(^9) Hz.