## 3 Results
### Etched Samples  
One of the sample preparation goals was to obtain a perfect-quality surface. First, the polished samples were subjected to etching using a mixture containing 48% HF (hydrofluoric acid), 30% H\({}_{2}\)O\({}_{2}\), and H\({}_{2}\)O. This procedure caused the photothermal signal to be wholly quenched for mixed samples, making it impossible to measure the spectra. A possible explanation for this will be discussed later in this article. Therefore, it was decided to grind the samples again and further etch them in a new solution. The surfaces of the specimens were ground and polished to use a mixture of K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\), 96% H\({}_{2}\)SO\({}_{4}\), and H\({}_{2}\)O for the etching. Figure 6 compares the amplitude spectra for the ground and etched Cd\({}_{0.95}\)Be\({}_{0.05}\)Te sample. The expectation was to obtain the spectra close in shape to the ideal crystal (black in Figure 2). The applied method of etching did not improve the surfaces of the sample. In the amplitude spectrum, an additional maximum still exists below the energy gap, which is not expected for an ideal sample. Although the phase has a different shape, it is worth noting that phase changes take place at the same energy values (indicated in the figure with vertical lines). This proves that the defects on the surface were present at a different depth of the damaged layer (by etching) or that additional effects had been achieved, e.g., with phenomena associated with the carriers. The same changes in phase are manifested differently in amplitude; with different maximums, it is also worth emphasizing that piezoelectric spectroscopy allows for studying the differences on both surfaces of the sample prepared with the same treatment procedure. Figures 7 and 8 present the experimental spectra of the amplitude and phase of Cd\({}_{0.95}\)Be\({}_{0.05}\)Te after the etching process was obtained for the illumination of different sur \begin{table}
\begin{tabular}{c c} \hline \hline \(\mathbf{x}\) & **Energy Gap (E\({}_{\mathrm{g}}\)), eV** \\ \hline
0 & 1.49 eV \\
0.01 & 1.53 eV \\
0.03 & 1.61 eV \\
0.05 & 1.64 eV \\
0.1 & 1.72 eV \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The experimental values of E\({}_{\mathrm{g}}\) for Cd\({}_{1-\mathrm{x}}\)Be\({}_{\mathrm{x}}\)Te. Figure 6: Amplitude (**a**) and phase (**b**) spectra for the ground (green lines) and etched (blue lines) Cd\({}_{0.95}\)Be\({}_{0.05}\)Te sample at 126 Hz of modulation frequency. faces of the sample. The sample was measured in rear and front configurations. The surfaces of the samples were assigned numbers 1 and 2 and then measured after illumination of surface 1, then 2. The aim was to examine and compare the surfaces after an identical treatment procedure. Complementarity, especially in the amplitude spectra, is evident. Whether rear or front detection is considered, the strong maximum associated with the surface defect is observed for the illumination at surface 2. Its intensity is much higher in comparison to the above bandgap region, which indicates that etching negatively affects the quality of the surfaces, and the defects strengthen the photothermal signal. In the case of illumination of surface 1, the amplitude spectra have a similar character with a smaller maximum in the sub-bandgap region. The maximum has a higher intensity for rear detection and is associated with subtracting the piston and drum effects. The phase spectra for the illumination of surface 2 show the changes at the same energy values despite the curves' different characters. The more significant differences are observed for phase spectra for front detection and the illumination at surface 1. It should be emphasized that both phase and amplitude must be analyzed to interpret piezoelectric spectra correctly. The same change of character was also observed for the rest of the samples, which proved that the chosen etching procedure introduced additional defects on the surfaces of the samples.