## 2 Materials and Methods  
Crystals investigated in this work were grown by using the vertical Bridgman technique. Before the growth process, starting mixtures were prepared from high-purity CdTe (6 N) and Be (99.9%) powders and put into a graphite crucible. The crucible was placed into a growth chamber, sealed, and evacuated. After evacuation, the external pressure of argon gas (around 90 atm.) was applied to reduce evaporation of the material during the growing. The temperature in the hot zone was set to 1600 K, with a stability better than 0.1 K. We used a 4 mm/h pulling rate to grow the crystal within two days. Typically, our crystals grown by the Bridgman high-temperature and high-pressure technique were 4-6 cm in length and 1 cm in diameter. We could cut 15-20 plates 1-1.5 mm thick from one crystal rod. Four specimens of beryllium content (in ingot) were measured for x = 0.01, 0.03,0.05, and 0.1. Pure CdTe crystals were also measured for the same procedure of surface preparation. More details concerning the growth procedure can be found elsewhere [22]. The beryllium concentration in the samples was determined after crystal milling (in agate mortar) by applying the powder X-ray diffraction technique (Empyrean diffractometer, Malvern Panalytical) with a Cu anode used as a source of CuK\(\alpha\) X-ray radiation (\(\lambda\) = 1.5406 A). More details about the measurement were presented in previous work [23]. The diffraction data were fitted using the ReX v. 0.91 Rietveld analysis software. The following parameters were fitted: scale factor, 2-theta offset, background coefficients, and cell constant a for the CdTe structure. In this case, the beryllium in the CdTe was treated as a solid solution of the first type, and thus the main changes were mainly observed for the cell dimension. All U factors were isotropic and set to 0.5. The reflection shape was described by the pseudo-Voight function. For analysis, the necessary crystal structure file of CdTe (cif) was downloaded from the American Mineralogist Crystal Structure Database [24]. The XRD patterns of example CdBeTe mixed crystals (CdTe and Cd\({}_{0.9}\)Be\({}_{0.1}\)Te) powdered are presented in Figure 1a. Characteristic diffraction reflections from (111), (220), (311), (400), (331), (420), and (511) planes of the cubic phase (type of zinc blende structure) are visible on the patterns for both CdTe (black) and Cd\({}_{0.9}\)Be\({}_{0.1}\)Te (red) samples. In the case of a mixed crystal, the diffraction peaks are shifted towards higher 2 \(\theta\) in comparison to the position of these signals in the CdTe. Such behavior demonstrates the presence of mixed crystals, so the beryllium was successfully incorporated into the crystal lattice of CdTe. The absolute values of the lattice constant were calculated by the Rietveld method and the results that were obtained as the function of the composition are displayed in Figure 1b. Linear dependence of the lattice constant versus Be content confirms the composition of the grown crystals. The cut samples were ground in a suspension of water and Al\({}_{2}\)O\({}_{3}\) powder on flat glass parallel plates. The grain diameter of the grinding powder was approximately 10 \(\upmu\)m. Then, the samples were polished with diamond paste on polishing discs until a mirror surface was obtained. The grain diameter in the polishing paste was between 0.1 and 1 \(\upmu\)m.  
The samples were subjected to two different etching processes. First, the samples were etched using a mixture containing 48% HF (hydrofluoric acid), 30% H\({}_{2}\)O\({}_{2}\), and H\({}_{2}\)O. Digestion took place at room temperature. The samples were subjected to photothermal tests. Then, the surfaces of the samples were ground and polished again to use a mixture of K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\), 96%H\({}_{2}\)SO\({}_{4}\), and H\({}_{2}\)O for etching. The etching took place at a temperature of about 80 \({}^{\circ}\)C. After etching, the samples were briefly immersed in a 50% NaOH solution, then rinsed in water, and finally in ethyl alcohol.