Yu. V. Rud

Optical properties of laser-evaporated CuGaSe2 films near and above the fundamental absorption edge

The absorption coefficient and fundamental transition energies of highly oriented, single-phase, laser-evaporated CuGaSe2 thin films were determined using interferential reflectivity and transmission in the system air-film-substrate-air in the near-IR and visible wavelength range 400–1700 nm. Optical constants of CuGaSe2 films were calculated by solving the system of transcendental equations with the assumption of interference. From the detailed analysis of the structure of the fundamental absorption edge, values for the direct allowed valence to conduction band splitting owing to the crystal field and spin-orbit interaction were estimated.

Properties of Dopants in ZnGeP 2 , CdGeAs 2 , AgGaS 2 and AgGaSe 2

Ternary-chalcopyrite structure ZnGeP 2 , CdGeAs 2 (II-IV-V 2 ) and AgGaS 2 , AgGaSe 2 (I-III-VI 2 ) compounds are currently of technological interest. They show the most promise for practical nonlinear optical applications in the areas of high-efficiency optical parametric oscillators and frequency up-converters for the infrared (ir) range as well as for widespectral-range optoelectronic devices. (See also the article by Schunemann, Schepler, and Budni in this issue.) However extensive realization of their potential has still not been achieved. One of the principal difficulties in the way to obtaining high-device-quality ZnGeP 2 , CdGeAs 2 , AgGaS 2 , and AgGaSe 2 single crystals is undesired optical absorption in their transparency range near the fundamental band edge induced by lattice-related defects. This article summarizes selected aspects of dopant-incorporation techniques of these crystals including dopant choice of dopant material and monitoring of dopant incorporation as done in our laboratory. In general for the ternary chalcopyrite compounds, doping-incorporation processes are more complicated in comparison to those of binary zinc-blende III-V compounds. The most common sources of dominant incorporation of acceptors and donors in as-grown chalcopyrites are believed to appear from (1) nonstoichiometric melts as well as by doping with different elements during the growth process and (2) incomplete removal of disorder on the cation sublattice during subsequent cooling. Furthermore the chalcopyrite structure II-IV-V 2 undergoes a disorder-order phase transition upon cooling through approximately 1220 K for ZnGeP 2 and 900 K for CdGeAs 2 . At these transition temperatures, solidification can be complicated also by supercooling phenomena, and the crystals transform from the cubic zinc-blende structure (where Zn atoms randomly fill cation sites) to the ordered chalcopyrite structure (e.g., when Zn and Ge occupy alternating cation sites in ZnGeP 2 ).

Growing FeIn2S4 single crystals and fabrication of photosensitive structures on their basis

Bulk single crystals of FeIn2S4 have been grown for the first time by the method of directional crystallization of an almost stoichiometric melt. The first photosensitive structures of the In(Al)/FeIn2S4 have been fabricated. Using the crystals grown, the first spectra of photosensitivity of the new structures have been obtained at T = 300 K. Based on an analysis of the photosensitivity spectra, it has been established that the edge absorption of FeIn2S4 is formed by indirect and direct interband transitions, and the values of the energy gap corresponding to these transitions have been estimated. A conclusion was made on the possibility of applying the structures obtained in wideband photoconverters.

Photosensitive structures on single crystals of MnIn2S4: Preparation and properties

Uniform single crystals of a ternary compound MnIn2S4 have been grown by the method of directional crystallization of an almost stoichiometric melt. The problem of the creation of photosensitive structures has been solved and the first studies of the spectra of photosensitivity have been performed, which made it possible to draw a conclusion on the nature of interband optical transitions in this semiconductor and to estimate the width of the energy gap for the direct and indirect transitions. The possibility of the application of the new structures in wideband photoconverters of optical radiation is noted.

Production and investigation of AgIn5S8/(InSe, GaSe) heterojunctions

It is shown that photosensitive AgIn5S8/(InSe, GaSe) heterojunctions can be fabricated using bulk crystals grown from the melt and from the vapor phase or polycrystalline thin films of the ternary compound prepared by pulsed laser evaporation. The spectral curves of the photosensitivity of the heterojunctions are investigated as a function of the photodetection geometry. It is concluded that the resulting structure have a promising potential as wideband and selective photodetectors.

High power gain for stimulated Raman amplification in CuAlS2

The spontaneous Raman spectra of the chalcopyrite structure crystal CuAlS2, which is promising for nonlinear optical applications, has been investigated at 8 and 300 K. The main aim of this study is to compare the absolute spontaneous Raman scattering efficiency in CuAlS2 crystals with that of their isomorphous analog, zinc‐blende structure GaP crystals, known as one of the most efficient materials for Raman amplification. Observation of a high value of absolute scattering efficiency S/L dΩ (where S is the fraction of incident power that scatters into the solid angle d Ω and L is the optical path length with S/L dΩ=9.5×10−5 cm−1 sr−1), together with relatively narrow linewidth (Γ=5.1 cm−1, full width at half maximum at room temperature and Γ=1.5 cm−1 at 8 K for the strongest Γ1 phonon mode of CuAlS2 at 314 cm−1) indicate that CuAlS2 has the highest value of the stimulated Raman gain coefficient gs/I where I is the incident laser power density. The calculated value of this gain is gs/I=2.1×10−6 cm−1/W at 3...

Optical and electrical properties of porous gallium arsenide

Photoluminescence (PL), Raman scattering, and carrier transport have been studied for the first time in porous GaAs prepared on (111) oriented wafers of n-type crystalline GaAs (faces A and B). Peaks of the main PL band from faces A and B were observed at 1.82 and 1.88 eV, respectively. The electron drift mobility was found to be ∼4×10−4 cm2 V−1 s−1. The nanocrystallite size in porous GaAs was determined both from PL spectra and from the Raman shift. The obtained values are close or equal to 6–8 nm.

Physical properties of CuxAg1−xIn5S8 single crystals and related surface-barrier structures

The electrical properties and photoluminescence spectra of single crystals of the ternary compounds CuIn5S8, AgIn5S8 and their solid solutions have been investigated. We have determined the type of conductivity, the mobility, charge carrier concentrations and energies of the radiative transitions in these materials. We have fabricated surface-barrier structures from these single crystals and measured the voltaic photosensitivity.

Photoelectric phenomena in ZnO:Al-p-Si heterostructures

Al-doped zinc-oxide (ZnO:Al) films are obtained by magnetron sputtering. Based on an investigation of electrical properties of the films, it is shown that the electron density in these films is as high as 5×1020 cm−3 and is practically constant in the temperature range 77–300 K, which indicates high efficiency of doping ZnO with an Al impurity. It is found that the deposition of thin films (d≈1 µm) on the p-Si(111) surface leads to the formation of heterostructures with the highest photosensitivity of ∼400 V/W at T=300 K, which oscillates in the spectral range 1.3–3.5 eV. With the oblique incidence of linearly polarized radiation, induced pleochroism emerges in such heterostructures. The magnitude of pleochroism oscillates in the range 5–40% (θ≈75°), which is associated with the interference phenomena in the ZnO films. The prospects of using the heterostructures obtained as highly selective photosensors of natural and linearly polarized radiation are considered.

Photopleochroism of GaPxAs1-x Surface-Barrier Structures

The proposed work reported here constitutes the first investigation of the polarimetric effects in III-V surface-barrier structures of metal-semiconductor (Au-n-GaPxAS1-x, Au-n-GaP) type, illuminated with linearly polarized radiation (LPR) at oblique incidence on the entrance plane (Au). As the angle θ increases the coefficient of photopleochroism increases as P~θ2. The maximum azimutal photosensitivity of the Au-n-GaPxAs1-x (x=0.45–1.0) structures is \varPhiI=0.15–0.20 A/W deg at θ=80° (300 K).