Václav Dědič a, Jan Franc a, Pavel Moravec a, Jakub Zázvorka a, Roman Grill a, Vladimír Šíma a, Miroslav Cieslar a, Utpal Roy b, Ralph B. James b
a Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, CZ- 121 16, Prague, Czech Republic
b Savannah River National Laboratory, Savannah River Site, Aiken, SC, 29808, USA
## Abstract
We studied the Vickers microhardness HV0.025 of CdZnTe and CdZnTeSe samples upon illumination with quasi-monochromatic light with wavelengths ranging between 750 and 1540 nm. We observed that light with a wavelength close to or above the bandgap of the samples results in a positive photo-plastic effect (hardening), and light in the spectral range well below the band gap can lead both to positive or negative effects depending on the parameters of deep levels in the bandgap. We performed measurements of spectrally dependent photoconductivity and compared them with HV0.025. Based on the analysis, we propose a complex qualitative model explaining the spectral dependence of HV0.025 by the charging and neutralization of deep levels by optical transitions between the valence and conduction bands and between the bands and deep levels in the band gap. Tuning of parameters of deep levels could be used to control the sign and magnitude of the photo-plastic effect.
## Introduction
The II-VI semiconductors of the CdTe group – CdTe, Cd1-xZnxTe with x∼0.1 (further referred to as CZT) and Cd1-xZnxTe1-ySey with x∼0.1 and y∼0.02–0.04 (CZTS) have been used in a number of applications like solar cells [1], room-temperature X-ray and gamma-ray detectors [2], substrates for the narrow gap (HgCd)Te epitaxial infrared detectors [3], electro-optical modulators, and other optical applications [4]. During the crystal growth and processing of the samples, an extensive number of defects can be generated due to the relatively low microhardness of the materials affecting their electronic properties and thus the performance of devices. Therefore, it is important to investigate external conditions and defect structure in the samples prior to and during the processing.
We have used the Vickers microhardness with additional illumination to characterize the photo-plastic effect (PPE) that has been extensively studied in II-VI semiconductors. A positive PPE has been reported for CdS [5] ZnSe [6], ZnO [7], ZnS [8], CdTe [9], CZT, and CZTS [10,11]. The applied light was in most cases spectrally tuned to the bandgap of the studied materials [[7], [8], [9]], where the effect is strongest due to the generation of a high concentration of free electrons and holes. Some authors [5,6] studied the spectral dependence of PPE including the generation of free carriers. The effect was also positive in all these cases. Negative PPE was observed in elementary semiconductor – Si [12,13] and in III-V semiconductor GaAs [14]. It was explained by a mechanism of radiation-enhanced dislocation glide (REDG). This mechanism is based on an increase of dislocation velocity by phonons emitted at non-radiative recombination of electrons with holes at dislocation sites.
Contrary to the previous findings we show in this contribution that CZTS can exhibit a spectrally dependent PPE that can be both a negative and a positive depending on parameters of deep levels in the band gap and wavelength of the light. We explain this phenomenon with a qualitative model including both charging and neutralization of dislocations or neighboring defects by optically induced transitions between the bands and deep levels. The observed spectrally dependent material softening has therefore a fundamentally different origin when compared to REDG mechanism. Tuning of parameters of deep levels thus could be used to control the sign and magnitude of the photo-plastic effect. We show that in samples with a certain structure of deep levels in the bandgap, it is possible to switch between optically induced softening and hardening of the material by a change of wavelength of the light.
The deep levels in CdTe and CZT have been studied by many research groups through various investigation methods including photo-induced current transient spectroscopy, deep-level transient spectroscopy, and photoluminescence. The research goal was to match the deep levels to native defects or impurities in the crystal lattice of the material and to modify the growth process (introduce a different dopant or adjust the growth parameters) to optimize the performance of radiation detectors. Castaldini et al. [15] and Mathew [16] have published results from extensive studies of the deep-level structure. Many deep levels can be found inside the bandgap of CdTe and CZT. The levels with a larger influence on trapping may be those with a higher trapping cross-section. Two main levels are found in most of the measurements, one being near the midgap (0.7–0.9 eV), whereas the other is in the range around 1.1 eV. The complexity of the issue of the deep level can be illustrated by the example of this particular PL band. Numerous studies done using low–temperature photoluminescence and/or cathodoluminescence (CL) since the 1950s. They have shown that a broad band in the spectral region around 1.1 eV is often observed in single crystals. The intensity and/or position of the “1.1 eV” PL band can be influenced by various treatments, e.g.: annealing in Cd vapor [17], plastic deformation [[18], [19], [20]], and dislocations induced by inclusions/precipitates [21,22]. A summary of defect levels in CZTS was presented in Ref. [23].
Deformation of the crystals is expected to form various types of dislocations. Hümmelgen and Schröter [24] found that the indentation of p–type CdTe generates glide dislocations marked as Te(g) and Cd(g). The dislocations-induced defect levels have ionization energies of 0.44 eV and 0.43 eV, respectively. The complementary energy is Eg – 0.44 eV ≈ 1.17 eV. The Y–line (1.47 eV) was assigned to the recombination of excitons bound to Te(g).
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/spectrally-dependent-positive-and-negative-photo-plastic-effects-in-cdznte-and-cdzntese.html