## 4 Discussion
The recombination of charge carriers is a primary factor limiting the performance of CdTe devices, such as solar cells and detectors [32]. While detectors are often fabricated from single crystals, thin films are primarily used for commercial solar  
Figure 4: PL spectra of CdTe for three surfaces with RMS roughness values of (**a**) 307 nm, (**b**) 467 nm, and (**c**) 602 nm. The colored lines correspond to the experimental data (blue), fitted curve (red), and individual Gaussian contributions (other colors). cells because of processing costs and the lack of large-diameter wafers of bulk CdTe [33]. Typically, a significant portion of charge carrier recombination in CdTe solar cells occurs at or near the surface, with surface recombination velocities often exceeding 10\({}^{5}\) cm/s [32; 34]. However, work has shown that the dominant recombination losses occur in the bulk of the absorber. This is because of charged grain boundaries and interfacial carrier recombination, which limit open-circuit voltage (V\({}_{\text{OC}}\)) and efficiency [35; 36]. Depositing buffer layers on the CdTe surface can modify these properties and potentially reduce recombination [36]. In this work, TRPL measurements were not conducted. A comprehensive literature search revealed no studies investigating the influence of surface roughness on the electrical properties of CdTe single crystal using TRPL. However, the literature does provide examples of TRPL being employed to characterize surface recombination velocity (SRV) in passivated CdTe surfaces [32]. It is well established that increased surface roughness correlates with a higher SRV [37]. Given that SRV is directly proportional to the density of surface states [18], a greater roughness implies a higher density of surface states. This correlation is consistent with the observed increase in PL FWHM from our results. On the other hand, Figure 5 shows the 1 LO and 2 LO peaks of RRS spectroscopy. The open circles correspond to the average experimental data in Figure 3. The intensity of the 2 LO phonon has been normalized to that of the 1 LO peak. The principal peaks (blue lines) were fitted using Lorentzian functions centered at 165.7 and 332.5 cm\({}^{-1}\) for the 1 LO and 2 LO phonon modes, respectively. In the base of 1 LO phonon, a small peak located at 151 cm\({}^{-1}\) was identified and is attributed to the surface optical (SO) phonon mode. An SO mode arises from the vibration of atoms in the near-surface region. Its frequency falls between the bulk TO and LO phonon frequencies of the material [27; 28]. The SO mode is difficult to observe because of weak scattering; however, its intensity is amplified under RRS conditions. Thus, the TO mode around 141 cm\({}^{-1}\) and the SO mode around 151 cm\({}^{-1}\) were well identified with vertical dashed lines. Two Lorentzian functions centered on these positions were used to fit them. Notably, the TO mode is symmetry-forbidden in bulk CdTe(001) but becomes Raman-active because of surface-induced symmetry breaking [38]. Figure 5: Experimental 1 LO and 2 LO data (open circles) and spectra fittings (red line) correspond to decomposition on elementary contributions (colored lines). Besides the TO and the SO phonon modes, a shoulder at the high-frequency side of the 1 LO peak is observed. Similar Raman scattering was reported in polycrystalline CdTe thin film. This scattering has been attributed to the contribution of the 2 LO-SO coupling mode [27]. The Raman fitting (red line) and observation of the SO mode in our CdTe single crystal indicate that the high-frequency shoulder of the 1 LO peak is caused by multi-phonon processes. These processes involved the 2 LO-SO coupling phonon modes. Moreover, the broadening shape of the base of the 2 LO peak can be perfectly fitted by the multi-phonon coupling of the SO mode with the 1 LO phonon and its overtones.