Václav Dědič a, Jan Franc a, Pavel Moravec a, Roman Grill a, Hassan Elhadidy a, Vladimír Šíma a, Miroslav Cieslar a, Utpal N. Roy b, Ralph B. James b
a Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, CZ, 121 16 Prague 2, Czech Republic
b Savannah River National Laboratory, Savannah River Site, Aiken, SC 29808, USA
## Abstract
We studied the influence of applied bias on of the Vickers microhardness HV 0.025 of CdZnTeSe and CdZnTe semi-insulating samples without and with the illumination of light at 870 nm. We observed that a small applied bias results in the hardening of the material. The effect is strongest at a bias of ± 0.5 V and is further strengthened by additional illumination. We suggest that the observed positive electro and electro-photo plastic effects in CdZnTeSe and CdZnTe can be explained by an increase in the concentration of free electrons and holes injected from contacts in a biased sample or generated by illumination. These free carriers can be trapped at dislocations and induce a reconstruction of bonds at the dislocation core. The necessity to break the bonds before dislocation glide results in an increase of microhardness.
## Introduction
Electro-plasticity is frequently observed as a decrease in the hardness of a material because of the flow of electric current. It is a common property of many metals and has found several applications in industrial processing and manufacturing [1], [2], [3], [4]. It was observed for the first time from irradiation of a Zn crystal with electron beams [5]. Despite the clear success in applications, a theoretical explanation of the phenomenon is lacking [6]. The theoretical models include the hypothesis of action from a direct force on moving dislocations (electron wind) created by the electric current [7], an inertial dislocation depinning mechanism [8], magneto-plastic mechanism [9], and Joule heating [10]. The electric current densities at which the effect is observed in metals are typically very high (∼103-104 A/cm2).
In semiconductors, electro-plasticity was also observed in low-resistivity Ge and Si. In Ref. [11] Westbrook and Gillmann observed that the resistance of crystals to deformation by indentation lowers by 30 % when a small bias (0.05–10 V) is applied parallel to the surface or perpendicular to the surface between the indenter and the material. In Ref. [12] the hardness of the near-surface region of ZnO was studied as a function of the surface charge. The observed dependence was explained by the transition of electrons between donor levels and unoccupied states in the conduction band, which enables dislocation motion. The hardness of materials can also be influenced by illumination [13], [14]. In CdTe a positive photo-plastic effect (hardening under illumination) was reported in Refs. [15], [16].
CdTe and Cd1−xZnxTe (x = 0.1–0.2) found extensive applications in the generation of electric current in solar cells [17], medical applications for the detection of hard X-ray and gamma-ray radiation [18], electro-optical modulators and other optical applications [19]. CdZnTeSe is currently under development as a future material in hard X-ray and gamma-ray detectors because the addition of Se was found to limit the formation of sub-grain boundaries and its networks, significantly reduce the Zn segregation, and improve the crystal’s compositional homogeneity as compared to Cd1−xZnxTe [20].
The mechanical characteristics of the materials play an important role because both point defects and dislocations can be generated during material processing (thermal annealing, grinding, lapping, polishing, mounting, and other handling). These defects then can influence the performance of fabricated devices. Metal contacts are also used in most device structures. It is therefore important to understand how the optical and electric fields are able to influence the mechanical properties of CdTe-based compounds. We investigated the photo-plastic effect in CdZnTe and CdZnTeSe and its spectral dependence (see Ref. [21]). Hardening of the studied materials was observed upon illumination (positive photo-plastic effect). Materials used for radiation detectors must be semi-insulating to suppress the dark current. Research of their mechanical properties is of particular importance because the concentration of free carriers can change dramatically under bias and illumination and effects connected with trapping and recombination can be strong.
The purpose of the current investigation is to study the influence of an applied bias on the microhardness of CdZnTe and CdZnTeSe (electromechanical effect) and the combination of an applied bias with optical illumination (i.e., the influence of a combined electro-mechanical and photo effect).
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/electro-and-electro-photo-plasticity-of-cdzntese-and-cdznte.html