Alizadeh M. X-ray-induced conductivity ZnSe single crystals as detectors of ionizing radiation

The thesis is devoted to the experimental and theoretical studies of the optical - electrical and spectral - luminescent properties of physical processes occurring in high- resistance wide-band gap (WBG) semiconductor single-crystal zinc selenide (ZnSe) under X-ray excitation and UV excitation. A wide range of experimental methods were used in the work: photo- (PL) and X- ray luminescence (XRL) studies, lux-luminescence characteristics (LLC) of luminescence, lux-ampere characteristics (LAC), and current-voltage characteristics (I – V) of conduction, current relaxation of conduction (RC), phosphorescence (P), thermally stimulated luminescence (TSL) and thermally stimulated conductivity (TSC), dose dependences of luminescence and conductivity at different X-ray intensity (X-) and UV excitation in the wide temperature range (8 – 430 K). Similarly, the nature of the obtained experimental data and interpretation of the research results, a theoretical and kinetic model of the dipole recombination center (Dipole-center) for the 630nm band (1.92eV) was proposed, which explains the possibility of implementing two recombination mechanisms at one complex luminescence center. On the basis of the experimental studies of LAC and LLC at different types and intensities of irradiation, the processes that cause the nonlinearity of these characteristics are analyzed. The I – V of the XRC and the PC showed that the general character of the I – V, which does not depend on exciting type and on temperature in ZnSe crystals - they are nonlinear. Two new processes were considered: an increase in the average thermal electron velocity under the action of an electric field and the selectivity of the direction of electron velocity during delocalization from traps. Based on the Maxwell equation, a velocity distribution function was obtained for moving particles with a constant velocity and a ratio for the average and root-mean-square velocity of the particles. It has been made the supplementation of the Poole-Frenkel effect, a formula is obtained for the phase transition and the phase space, which takes into account the selectivity of the direction of the electron velocity during delocalization from traps. It has been established that by the results obtained dose dependences of TSL and TSC luminescence and conductivity, between X-excitation and UV excitation - X- excitation is more informative to investigate of the effect of traps on the kinetics of conductivity and luminescence. It was found that the amplitude of the scintillation pulse and the amplitude of the current pulse change during X-irradiation. The initial delay in the rise of conduction current as compared to the rise in luminescence at low temperatures is explained by the intense filling of traps with free charge carriers in the first seconds of irradiation. Using the determined energy spectrum of the traps and the temperature of the maxima of the TSL and TSC peaks for the crystal, we can calculate the corresponding frequency factors and localization cross sections for free charge carriers on these traps. Particular emphasis in the thesis is on the study of ZnSe with X-excitation and UV excitation. In this regard, some problems and some issues of creating and using high- resistance wide-gap semiconductors as X-ray and gamma-radiation detectors at various radiation doses and in different temperature regimes are of particular importance.