1. Laser modification of the structure
PbSe films that were not subjected to heat treatment in an oven were used as reference samples. To study the laser irradiation effect on the such film’s structure modification, samples optical characterization was carried out before and after exposure to radiation. According to the results of scanning electron microscopy (SEM), the untreated sample had an unstructured surface (Fig. 3a, c), and the film itself contained inhomogeneities with a size of about 1.0 μm.
Structure modification after laser irradiation is shown in Figure 3 (b, d). Between the tracks, it can be seen that the film was not subjected to thermal action from the laser irradiation zone and retained the characteristics of the untreated sample (Fig. 3b). Within the track, local modification regions were formed, spaced at the same distance from each other (Fig. 3d). The mechanism of laser modification is associated with film local heating to a softening and melting temperature, followed by material redistribution entrained by a moving heat source with a high temperature gradient. At some point in time, the film within the track becomes thin, which is represented in the SEM image as periodic dark areas. Light, denser regions are film thickenings that form during sharp heating when the laser spot approaches and cools when it is removed [13]. As a result of the film material redistribution, concentric regions with local elevations are formed in the track center, and depressions are formed on its periphery. The structure morphology and the periodicity of light and dark regions depended on the scanning rate, laser spot size and the incident radiation power density. These parameters determined the heat source size, its maximum temperature and the heating and cooling rate.
2. Electrical and optical characterization
The films 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.
Table 1. Electrical characteristics of PbSe films
External voltage
|
Untreated film
|
Film after laser modification
|
Comparison: before and after irradiation
|
R, Ом
|
I, нА
|
R, Ом
|
I, нА
|
dR, %
|
dI, %
|
U = 4 V
|
218
|
100
|
208
|
140
|
-4,59%
|
40,00%
|
U = 0,6 V
|
1400
|
86
|
1200
|
140
|
-14,29%
|
62,79%
|
The PbSe films optical characteristics study was carried out to explain the features of the band structure and material band gap Eg. The measured characteristics were used to determine the optical constants: refractive index n, extinction coefficient k [18]. The samples were opaque for the visible wavelength range. Therefore, the films reflectance spectra were measured before and after laser irradiation (Fig. 4). For the film modified sections in the selected spectral range, a sharp decrease in reflection was observed compared to untreated film. In terms of their optical properties, the modified regions approached an absolutely black body (Fig. 4. b).
The untreated and after laser modification film optical constants were calculated for a wavelength 405 nm and presented in Figure 5. The absorption coefficient was calculated by the expression: α = A/d, where A- absorption, d- film thickness. The extinction coefficient was calculated through 𝑎 by the expression: k = 𝑎𝜆/(4𝜋), where 𝜆 is the wavelength.
The refractive index was calculated from the expression: n = [(1 + R) + {(1 + R)2 − (1 − R)2(1 + k2)}0.5]/(1 − R), where R - reflection.
Films laser modification resulted in a refractive index decrease, 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 PbSe film optical band gap was determined under various processing conditions. As a result of two straight sections extrapolation, the values Eg = 1.33 and 1.50 eV were determined (Fig. 6). The obtained values are in good agreement with the previously published results of the scientific group T.S. Shyju et al. [18], where the band gap varied from 1.46 to 1.62 eV. The decrease in the band gap is associated with an increase in the probability of charge carriers’ passage from defect states in the modified regions of the film induced by thermal processes.
3. Modification mechanism
Based on the above results, it can be made an assumption about the mechanism of film structure laser modification (Fig. 7). As a result of laser action on the film material, the concentration of defect centers increased, which became secondary sources of free charge carriers in the film. For this reason, after sample irradiation, the current increased and the resistance decreased. Also, a decrease in the band gap indicated an increase in the probability of electron–hole pair transfer from defect states.