Evaluation of the sound velocities of cadmium zinc telluride crystals by time-domain Brillouin scattering technique

Cadmium zinc telluride (Cd${}_x$Zn${}_{1-x}$Te) is frequently used material for the production of nuclear radiation detectors and solar cells. Therefore, the characterization of its quality by different methods is very important. This work is devoted to the development of a technique for mapping the poly-crystalline structure of Cd${}_x$Zn${}_{1-x}$Te, alternative to the X-ray technique usually used for this goal. In comparison with X-ray, the non-destructive and contactless picosecond laser ultrasonic technique provides numerous advantages, including simplicity of the analysis of the results and possibility to work with very compact laser spots (about 1 µm) probing the sample. As a step to achieve this goal, picosecond laser ultrasonics is applied in this work to study the samples in the form of 8 mm-diameter discs of 1 mm thickness including several connected disoriented crystals of Cd${}_x$Zn${}_{1-x}$Te. The experimental study is performed using classical pump-probe experimental set-up based on Tsunami femtosecond laser and delay line with moving retro-reflector. The sound velocities of the samples have been evaluated from frequency spectra of obtained signals, using the literature data on the index of refraction. Then, the value of the longitudinal sound velocity in the direction perpendicular to the sample surface is traced as a function of Zn concentration. Finally, measured sound velocities are compared with their available theoretical evaluation, using statistical analysis.