**1. Introduction**

The quality of the crystals used in the construction of X-ray and gamma radiation detectors affects their efficiency and sensitivity; hence, there is a need to know about their lattice disorder, defects, and both radial and axial homogeneity. Good homogeneity and low defect density lead to significant charge transport properties, low leakage currents, and no conductive short circuits between the detector contacts. The substitution of the native element with a foreign atom within the crystal always leads to undesired effects like disordered structure, defect generation, etc.

Bulk form CdTe and Cd1−xZnxTe mixed compounds are applied in X-ray and gammaray detectors [1–4], in electrooptic and photorefractive devices [5], and as substrates for epitaxy [6]. Cd1−xZnxTe (0.1 ≤ x ≤ 0.2) crystals have been studied intensively over the last twenty years as materials for gamma-ray detection application and also spectroscopic X-ray imaging [7].

Intensive investigations of Cd1−xZnxTe have also been carried out in recent years. Lately, C. Zhou et al., studied the extended defects in CdZnTe crystal [8], and the effects of Al-rich AlN transition layers on the performance of CdZnTe films for solar-blind photodetector were investigated by J. Gu [9]. Effects of deep-level traps on the transport properties of high-flux X-ray CdZnTe detectors were investigated by Y. Li et al. [10].

Great attention is also paid to the surface preparation of the samples [11–14]. Surface phenomena play a large role in obtaining crystals in all phases of processing and often affect the parameters of an electronic component. The sub-surface layer plays an important

**Citation:** Zakrzewski, J.; Strzałkowski, K.; Boumhamdi, M.; Marasek, A.; Abouais, A.; Kami ´nski, D.M. Photothermal Determination of the Surface Treatment of Cd1-xBexTe Mixed Crystals. *Appl. Sci.* **2023**, *13*, 2113. https://doi.org/10.3390/ app13042113

Academic Editors: Li Pei, Jia Shi, Hua Bai, Yunhui Mei and Pingjuan Niu

Received: 22 December 2022 Revised: 31 January 2023 Accepted: 2 February 2023 Published: 7 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

role, which in high-purity semiconductor devices could become significant due to miniaturization. The effects of the inductively coupled Ar plasma etching on the performance of (111) face CdZnTe detector were investigated by B. Song [15], who proposed and analyzed the influence of different solutions for etching. The proposed method could remove the damaged layer caused by mechanical polishing. Still, it also led to the surface composition deviating from the stoichiometric ratio and forming a Te-rich surface. They noticed that the Te-rich layer is a highly conductive region, which results in a large surface leakage current and affects the detection performance. Zhang et al. [16] proposed a new chemical mechanical polishing method (silica, hydrogen peroxide, and citric acid), effectively reducing surface roughness. Min et al. [17] analyzed the effect of hydrogen plasma on CdZnTe and showed that hydrogen plasma could fill the Cd vacancy defect on the surface and reduce the leakage current.

Photothermal spectroscopy was also used to investigate the surface treatment procedures and their influence on the surface quality. Zakrzewski et al. [18] applied piezoelectric phase spectra to determine both the energy gaps (Eg) and the thermal diffusivities of CdxZn1−xSe mixed crystals. The authors showed and interpreted the different amplitude and phase spectra characters depending on the surface preparation procedure for Cd0.3Zn0.7Se and Cd0.5Zn0.5Se samples. The mechanical (grounding, polishing) and chemical (etching) procedures of surface preparation and their impact on both the amplitude and phase of photothermal spectra were observed for Zn1−x−yBexMnySe compounds [19]. The surface defects in the samples' sub-surface damaged layer were also reported for ZnSe binary crystal [20].

There are many methods of surface quality testing, e.g., different imaging methods. Piezoelectric photothermal spectroscopy is a cheap and fast method. Other photothermal methods that use microphones or pyroelectrics as detectors are not sensitive (do not show) to effects related to surface states. This is an advantage and a disadvantage; in the case of thermal diffusivity studies in the frequency domain, there is no signal coming from the subsurface layer, and it is easier to interpret the obtained results (there are well-known models).

Still, more investigations and literature data on Cd1−xBexTe mixed crystals are needed. One article has been published so far [21], which determines thermal properties using the pyroelectric method in the frequency domain. The present work investigates the optical and thermal properties using piezoelectric photothermal spectroscopy. Adding beryllium to the CdTe matrix should give a similar effect in tuning the energy bandgap or lattice constant as in the case of zinc. This fact should be interesting for detecting purposes and producing better substrates for infrared sensors.

Here we report the first experimental data describing the properties of crystals grown in our laboratory. Piezoelectric photothermal spectroscopy was applied to characterize Cd1−xBexTe mixed crystals. This work aimed to investigate the optical properties of Cd1−xBexTe mixed crystals and the effect of surface treatment on the amplitude and phase spectra. On that basis, we aimed to find a procedure and the best solution for obtaining excellent surface quality among the samples.
