Читать книгу All sciences. №6, 2022. International Scientific Journal - Ibratjon Xatamovich Aliyev - Страница 6

In this paper, a spatially oscillating photovoltaic current (POFT) in the direction [100] in the SbSI ferroelectric is detected and investigated when illuminated with polarized light in the direction [010].

Antimony sulfoiodide (SbSI) belongs to the class of chalcogenides of metals of the fifth group A^VB^VIC^II, where A-Sb; Bi; B-S, Se, Te; C-CL, Br, I. SbSI and SbSI_xBr_1-x crystals are biaxial, have a large double refraction, below temperature. Curie T_c=22⁰C SbSI crystals belong to the mm2 class and have rhombic symmetry. During the phase transformation, the center of symmetry disappears, therefore, SbSI crystals become ferroelectrics below the transition point.

The phase transition at 22⁰C was registered for the first time by Fatuzzo [5] with a change in the temperature dependence of the dielectric constant. Crystals have pronounced semiconductor properties, their photovoltaic properties are well studied [1].

Measurements were carried out for SbSI single crystals in the ferroelectric phase at a temperature of T = 133 K. The crystal was illuminated by plane polarized light using a xenon lamp and a ZMR monochromator. The stationary photovoltaic current J was measured by the method previously described [1]. In accordance with the SbSI symmetry (point group mm²), when measuring J_z (z is the direction of spontaneous polarization) and illuminating the crystal in the x and y directions, POFT does not occur. The expression for the photovoltaic current Jz when illuminated in the x and y directions, respectively, has the form:

where I is the light intensity, β is the angle between the plane of polarization of light and the z axis. In Fig.1, curve 1 represents the experimental angular dependence of J_z (β) for λ=600 nm when illuminated along [100]. From the comparison of the experimental angular dependences of J_z (β) with (4) and (5), the numerical values of αιjκ or photovoltaic coefficients were estimated

Taking into account pleochroism and anisotropy of light reflection in SbSI [6], the following values were obtained:

K₃₁₄*10^—8; K₃₂₃*10^—8; K₃₃ (2—3) *10^—8A*cm* (W) ^-1. Thus, in SbSI, the photovoltaic coefficients K₃₁, K₃₂, K₃₃ are more than an order of magnitude higher than the corresponding coefficients in LiNbO3: Fe.

Fig.1. Dependence of the photovoltaic current Jz (1) at l = 600 nm and J_x (2) at l = 460 on the orientation of the plane of polarization of light in SbSI.

According to (2), for SbSI, the photovoltaic current components are spatially oscillating. However, when the crystal is illuminated in the region of strong absorption in the direction of the x or y axes and when condition (3) is met, currents flow along the surfaces (100) and (010), respectively.

where β is the angle between the plane of polarization of light and the z axis. According to [1,7] for SbSI, the strong absorption condition (3) should be fulfilled already at λ470 nm. To observe the POFT under conditions of strong absorption, silver electrodes in the form of bands parallel to the axis of spontaneous polarization z were sprayed onto the face of the cinacoid (010). Using these electrodes, when the crystal was illuminated in the direction [010] by polarized light with λ=460 nm, the current J_x curve 2 was measured and the current J_z was measured in the long-wavelength region (λ=600nm, curve 1).The angular dependence of the measured current satisfies (5), while the Oh current in this region cannot be observed at all due to violation of condition (3) and spatial oscillation. Figure 2 shows the spectral J_z (curve 1), J_x (curve 2), attributed to the unit of incident energy, as well as the spectral dependence

constructed taking into account the dispersion of n0, pe and the absorption coefficient α* in the [010] direction.

Angular dependence J_x (β) in the form of curve 2, which agrees well with (7) at K₁₅= (2—4) ·10^—9A·cm· (W) ^-1 (λ=460nm).

Fig. 2. Spectral dependence of J_z (1), J_x (2) and L=l₀a* (3).

While the spectral dependence measured earlier in is monotonic, the spectral dependence of Jx detects a sharp maximum near L₁. Thus, the decline of J_x in the long-wave region, where L <<1, is due to POFT. The decline of J_x in the short wave region, where L> 1, is interesting.Since the AF effect is not related to the lifetime of nonequilibrium carriers, it is possible that this short-wave decline of Jx is due to a decrease in K₁₅ and, consequently, mobility in the direction [100].