This paper gives a thorough treatment of a new effect of non-steady-state photo-electromotive-force (emf). The effect consists of an alternating electric current arising in a short-circuited bulk sample of a photoconductor illuminated by a vibrating sinusoidal pattern. As a detailed theoretical analysis shows, the frequency transfer function of the effect is identical with that of a differentiating RC circuit having a time constant equal to the characteristic time of a space-charge grating formation within the sample volume τsc. For high excitation frequencies (ω≳τ−1sc), the photo-emf signal peaks at spatial frequency K=L−1D (LD is the mean diffusion length of photoinduced carriers) and the photovoltage amplitude in an open-circuit sample can be as high as kBT/e times the number of fringes of the interference pattern in the interelectrode spacing. The basic conclusions of the theoretical analysis of the effect are supported by the experimental evidence obtained for cubic photoconducting Bi12SiO20 crystals.
The main suggestions in practical applications of dynamic photorefractive holographic recording for interferometry, processing of 2D pictures, operations with laser beams, and holographic memory systems are reviewed. Specific photorefractive techniques underlying these applications, and in particular, methods of continous and nondestructive hologram reconstruction, nonlinearities in recording mechanisms, and basic limitations due to geometrical factors, Bragg regime of diffraction, and optical noise are considered in detail. The paper is preceded by two introductory sections devoted to fundamentals of dynamic holography and phase conjugation in volume media, and to specific features of' recording and reconstruction of the photorefractive holograms.
Epitaxial layers of α-Ga2O3 with different Sn doping levels were grown by halide vapor phase epitaxy on sapphire. The films had shallow donor concentrations ranging from 1017 to 4.8 × 1019 cm−3. Deep level transient spectroscopy of the lowest doped samples revealed dominant A traps with level Ec − 0.6 eV and B traps near Ec − 1.1 eV. With increasing shallow donor concentration, the density of the A traps increased, and new traps C (Ec − 0.85 eV) and D (Ec − 0.23 eV) emerged. Photocapacitance spectra showed the presence of deep traps with optical ionization energy of ∼2 and 2.7 eV and prominent persistent photocapacitance at low temperature, surviving heating to temperatures above room temperature. The diffusion length of nonequilibrium charge carriers was 0.15 µm, and microcathodoluminescence spectra showed peaks in the range 339–540 nm, but no band-edge emission.
Films of α-Ga2O3 doped with Sn were grown by halide vapor phase epitaxy (HVPE) on planar and patterned sapphire substrates. For planar substrates, with the same high Sn flow, the total concentration of donors was varying from 1017 cm−3 to high 1018 cm−3. The donor centers were shallow states with activation energies 35–60 meV, centers with levels near Ec–(0.1–0.14) eV (E1), and centers with levels near Ec–(0.35–0.4) eV (E2). Deeper electron traps with levels near Ec−0.6 eV (A), near Ec−0.8 eV (B), Ec−1 eV (C) were detected in capacitance or current transient spectroscopy measurements. Annealing of heavily compensated films in molecular hydrogen flow at 500 °C for 0.5 h strongly increased the concentration of the E1 states and increased the density of the E2 and A traps. For films grown on patterned substrates the growth started by the formation of the orthorhombic α-phase in the valleys of the sapphire pattern that was overgrown by the regions of laterally propagating α-phase. No improvement of the crystalline quality of the layers when using patterned substrates was detected. The electric properties, the deep traps spectra, and the effects of hydrogen treatment were similar to the case of planar samples.
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