# Electrical and Optical Properties of Laser-Induced Structural Modifications in PbSe Films
## 3 Results and Discussion
### Electrical and Optical Characterization  
The film's electrical characterization, including the measurement of electrical resistance and current, was carried out before and after sample structure laser modification. The measurement results are shown in Table 1. From the data obtained, it can be seen that with an external voltage in the area equal to 0.6 V, the current value in the modified region increased by 62.8%, compared with similar measurements of the untreated film. Under an external voltage of the measured area equal to 4 V, the current value for the modified areas increased by 40% in comparison with similar measurements of the untreated film. The PbSe film optical characteristics study was carried out to explain the features of the band structure and material band gap \(E_{g}\). The measured characteristics were used to determine the optical constants: refractive index \(n\) and extinction coefficient k [19]. The samples were opaque for the visible wavelength range. Therefore, the film reflectance spectra were measured before and after laser irradiation (Figure 6). For the film-modified \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**External Voltage**} & \multicolumn{2}{c}{**Untreated Film**} & \multicolumn{2}{c}{**Film after Laser Modification**} & \multicolumn{2}{c}{**Comparison: before and after Irradiation**} \\ \cline{2-6}  & **R, Ohm** & **I, nA** & **R, Ohm** & **I, nA** & **dR, \%** & **dI, \%** \\ \hline U = 4 V & 218 & 100 & 208 & 140 & \(-\)4.59\% & 40.00\% \\ U = 0.6 V & 1400 & 86 & 1200 & 140 & \(-\)14.29\% & 62.79\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrical characteristics of PbSe films. Figure 5: PbSe film optical microscopy a in a bright field of reflected light: (**a**) untreated, (**b**) after laser modification; PbSe film pictures obtained with the SEM secondary electron detector: (**c**) untreated, (**d**) after laser modification. sections in the selected spectral range, a sharp decrease in reflection was observed compared with untreated film. In terms of their optical properties, the modified regions approached an absolutely black body (Figure 6b). The untreated and after laser modification film optical constants were calculated for a wavelength of 405 nm and presented in Figure 7. The absorption coefficient was calculated by the expression: \(\alpha=\mathrm{A}/\mathrm{d}\), where A- absorption, d- film thickness. The extinction coefficient was calculated through \(a\) by the expression: \(k=a\lambda/(4\pi\gamma)\), where \(\lambda\) is the wavelength. The refractive index was calculated from the expression:  
\[n=[(1+R)+\{(1+R)^{2}-(1-R)^{2}(1+k^{2})]^{0.5}\}/(1-R)\]  
Figure 6: Dependence of (**a**) reflection, (**b**) absorption from wavelength for different PbSe films processing: untreated (black curve), laser modification (red curve). Figure 7: PbSe films optical characteristics at a wavelength 405 nm for untreated film (green) and film after laser modification (cyan): refractive index \(n\), extinction coefficient \(\mathrm{k}\), absorption coefficient \(\alpha\). where \(R\)--reflection. Films laser modification resulted in a refractive index decrease and an increase in the absorption coefficient and extinction coefficient, which may be associated with the valence and conduction bands modification as a result of a high temperature gradient. The regimes were also determined under which the effect of PbSe films photodarkening can be observed (Table 2). The power density of the acting radiation \(q\) was determined by Formula (1), where d is the laser spot diameter, equal to 10 \(\mu\)m. Using Formulas (2) and (3), we determined the exposure time t on the sample and the energy density \(w\), where \(\
u\) is the scanning speed.