Optical an photoelectric properties odf ZnSe:Ti crystals

The photoconductivity and photoluminescence spectra of ZnSe:Ti crystals in the visible and infrared spectral regions are studied. It is established that the high-temperature impurity-induced photoconductivity of ZnSe:Ti crystals is defined by the optical transitions of electrons from the ³ A ₂(F) ground state to highenergy excited states, with the subsequent thermal transitions of electrons to the conduction band. The efficient excitation of intracenter luminescence of ZnSe:Ti crystals is achieved by light from the region of the intrinsic absorption of Ti²⁺ ions.     

Effect of iron impurities on the photoluminescence and photoconductivity of ZnSe crystals in the visible spectral region

The photoconductivity and photoluminescence of ZnSe:Fe crystals in the visible spectra region are studied. The scheme of optical transitions within Fe²⁺ impurity centers is established. It is shown that the high-temperature photoconductivity of ZnSe:Fe crystals is controlled by optical transitions of electrons from the ⁵ E(F) ground state to the higher levels of excited states of Fe²⁺ ions, with subsequent thermal activation of the electrons to the conduction band. Efficient excitation of intracenter luminescence of ZnSe:Fe crystals is attained with light corresponding to the region of intrinsic absorption in Fe²⁺ ions.     

Electrical properties of ZnSe crystals doped with transition elements

The conductivity and photoconductivity of ZnSe crystals doped with transition elements are studied. It is shown that the doping of ZnSe crystals with 3d impurity elements is not accompanied by the appearance of electrically active levels of these impurities. At the same time, the introduction of these impurities into the cation sublattice brings about the formation of electrically active intrinsic defects. It is established that ZnSe crystals doped with Ti, V, Cr, Fe, Co, or Ni exhibit high-temperature impurity photoconductivity. Photoconductivity mechanisms in the crystals are proposed. From the position of the first ionization photoconductivity band, the energies of ground states of 3d ²⁺ ions in ZnSe crystals are determined.     

Effect of nonstoichiometry and doping on the photoconductivity spectra of GeSe layered crystals

The polarization photoconductivity spectra of Bi-doped nonstoichiometric GeSe layered crystals grown by static sublimation were investigated. Two strongly polarized maxima at the photon energies hν_max = 1.35 eV (E ∥ a) and 1.44 eV (E∥ b) due to the V ₁ ^V → V ₁ ^c and Δ ₂ ^v → Δ ₁ ^c optical transitions, respectively, were found in the spectra of nominally undoped GeSe crystals near the intrinsic absorption edge at 293 K. In the low-temperature region, an exciton photoconductivity band peaked at hν_max=1.32 eV, which is due to exciton dissociation at the cation vacancies, was revealed. With an increase in excess Se in crystals, a sharp increase in the intensity of the exciton peak in the photoconductivity spectra was observed. It is shown that doping of GeSe crystals with donor Bi impurity is an effective tool of the control of their electrical and photoelectric properties. Although introduction of Bi into germanium monoselenide does not lead to the conductivity conversion from the p to n type, a sharp increase in the resistivity is observed, the crystals become photosensitive, and a strong impurity band peaked at 1.11 eV arises in the photoconductivity spectra.     

Preparation and optical properties of the co-doped ZnTe single crystals

The ZnTe:Co single crystals are obtained by diffusion Co-doping. The spectra of optical density in the range of 2.4–0.38 eV are investigated. It is found that Co-doping of the crystals shifts the absorption edge to the low-energy region. The similarity of optical absorption spectra of the ZnTe:Co and ZnS:Co crystals is established. The observed absorption lines of ZnTe:Co are attributed to the optical transitions of electrons from the level of the ground state ⁴ A ₂(F) of the Co²⁺ ion to the levels of excited states ⁴ T ₁(P), ⁴ T ₁(F), and ⁴ T ₂(F) split by the spin-orbit interaction.     

Power broadening and nonlinear FIR magneto-photoconductivity in n-GaAs

The saturation of the photoconductivity due to 1s-2p₊ shallow donor transitions in n-GaAs has been investigated using a high power cw-FIR-laser. Magnetic field strengths were chosen in such a way that 2p₊ energy levels were either below or above the N=0 conduction band Landau level. In the former case 1s-p₊ transitions are found to be inhomogeneously broadened with a saturation intensity as low as 0.84 mW/cm², giving an effective lifetime of the 2p₊ state of 1.5 μs. Above the band edge the integrated photoconductivity does not saturate though the intensity-normalized peak photosignal decreases and the linewidth increases with raising intensity. This strange behaviour is tentatively attributed to optical excitations of 2p₊ electrons to higher lying electron Landau states.     

Energy states of a Cr2+ ion in ZnSe crystals

ZnSe:Cr single crystals fabricated by diffusion doping are studied. The optical density spectra in the energy range 0.4–3 eV are investigated. The chromium concentration in the investigated crystals is determined by the absorption-edge shift. The energy states of the Cr²⁺ ion in ZnSe crystals are calculated. The nature of optical transitions determining the optical properties of ZnSe:Cr single crystals in the visible and IR spectral regions is established. It is shown that effective excitation of the IR luminescence of ZnSe:Cr crystals is implemented by light from the fundamental absorption region of Cr²⁺ ions.     

Photoconductivity and luminescence of CuInSe2 single crystals at a high level of optical excitation

The luminance-current and spectral characteristics of photoluminescence of the CuInSe₂ single crystals are studied. The superlinear portion of the excitation-intensity dependence of photoconductivity at low excitation intensities in compensated p-CuInSe₂ crystals is explained on the basis of a recombination model. The emission band that peaked at 0.98 eV in the n-CuInSe₂ photoluminescence spectrum corresponds to radiative recombination of electrons at the donor level with a depth of 0.04 eV. The maximum in the band intensity corresponds to the energy gap between the trap level and the valence band.     

Optical absorption of vanadium in ZnSe single crystals

The study is concerned with ZnSe:V single crystals produced by diffusion doping. The optical density spectra are recorded in the photon energy range from 0.4 to 3 eV. From the shift of the absorption edge, the vanadium concentration in the crystals is determined. The nature of optical transitions controlling the optical properties of the ZnSe:V single crystals in the visible and infrared spectral regions is identified. The diffusion profile of the vanadium impurity is established from measurements of the relative optical density of the crystals in the visible spectral region. The vanadium diffusion coefficients in ZnSe crystals at temperatures of 1120–1320 K are calculated; at 1320 K, the vanadium diffusion coefficient is 10^?9 cm² s^?1.     

Photoconductivity storage in Ga1−xAlxAs alloys at low temperatures

Intentionally undoped n-type and high purity Ga_1?xAl_xAs alloys with compositions in the range 0.19≤x≤0.78 are found to show a long life time photoconductivity effect at low temperatures (T < 80 K) when irradiated with white light filtered through a Ge filter and also when the light source is removed after photoexcitation. For the direct gap materials (0≤x≤ 0.43), it is shown that the deep level in the alloys, which controls the electrical properties of the crystals, captures and emits electrons via the first higher energy subsidiary conduction band inima L although the Γ minimum is the lowest in energy. These indirect electron transitions by the deep level, via the L minima, which is found to have an acceptor like nature, provide a natural explanation of the photoconductivity storage at low temperatures. For indirect gap materials (x > 0.43), when X minima are the lowest energy subsidiary minima, the photoconductivity storage at low temperatures is due to the double acceptor nature of the deep level.     

Specific features of recombination processes in CdTe films produced in different temperature conditions of growth and subsequent annealing

The steady-state and kinetic characteristics of photoconductivity and photoluminescence and the thermally stimulated conductivity spectra of the GdTe layers deposited by vacuum evaporation onto heated substrates are studied in relation to the substrate temperature. The measurements are carried out at temperatures, illuminations, and wavelengths ranging from 4.2 to 400 K, from 10¹⁰ to 10²³ photon/cm², and from0.4 to 2.5 μm, respectively. A certain optimal range of substrate temperatures T _s ≈ 450–550°C, at which the as-prepared layers exhibit a high resistivity, a high photosensitivity, and the best structural quality, is established. In the spectra of these layers, a new luminescence band at hv _m = 1.09 eV is observed along with the known photoluminescence band at hv _m = 1.42 eV. It is established that this new band is due to intracenter transitions rather than recombination transitions. The nature of radiative recombination centers in the layers is discussed. It is suggested that the d electrons of cations can be involved in the formation of chemical bonds of local centers in CdTe.     

Optical absorption and diffusion of iron in ZnSe single crystals

ZnSe:Fe single crystals obtained by the diffusion doping are studied. The spectra of optical density in the range of energies of 0.4–3 eV are studied. The iron concentration in the studied crystals is determined using the magnitude of the shift of the absorption edge. The nature of the optical transitions determining the optical properties of ZnSe:Fe single crystals in the visible and IR regions of the spectrum is identified. The diffusion profile of Fe impurity is determined by measuring the relative optical density of crystals in the visible spectral region. The diffusivities of Fe in the ZnSe crystals are calculated for temperatures of 1120–1320 K. At 1270 K, the diffusivity of Fe is 3 × 10⁻¹⁰ cm²/s.     

DX centers in II-VI semiconductors and heterojunctions

Measurements of the photoconductivity and Hall effect in Ga-doped ZnSe indicate that Ga donors form DX states in ZnSe. When the photocarriers remain in the ZnSe:Ga layer, the photoconductivity is persistent up to T_a= 100K, due to a barrier to recapture the photocarriers, E_c ≈ 0.3eV. Under certain growth conditions, there is a large conduction band offset at the heterojunction with the GaAs substrate. The photocarriers are trapped at the interface, causing an enhancement of the annealing temperature to T_a≈350K. We discuss the implications of these results to device applications.     

Renormalization of the band gap in highly photoexcited type-II ZnSe/BeTe structures

For the type-II ZnSe/BeTe heterostructures, a large (～0.1 eV) red shift of the edge of interband recombination in the ZnSe layers is observed at high densities of spatially separated photoexcited electrons and holes (～10¹³ cm^?2). The observed magnitude of renormalization of the band gap exceeds the magnitudes predicted by the multiparticle theory for dense type-I electron-hole systems at the same concentrations of two-dimensional charge carriers. Numerical calculations show that macroscopic electric fields induced by separated charges have a profound effect on the energy of direct transitions in type-II structures, resulting in an additional decrease in the energy of the transitions. In wide structures, where the ZnSe layer thickness is ? 15 nm, the renormalization effect is less pronounced. This is attributed to incomplete spatial separation of photoexcited charge carriers in the case of profound band bending and, thus, to the less-pronounced effect of electric fields.     

The effect of composition on the properties and defect structure of the CdS-Ga2S3 solid solution

The effect of excess CdS in gallium thiogallate (CdGa₂S₄) on the spectrum of defect states in its band gap is investigated. Comparative investigations of Raman, photoluminescence, and cathodoluminescence spectra, as well as of the kinetics of photoconductivity in starting crystals (type A) and crystals obtained in conditions with excess CdS (type B), were carried out. It was found that the main types of defects in type A crystals are Cd and S vacancies, antisite Ga_Cd donor defects, as well as I_S defects, which are caused by the incorporation of carrier gas. In type B crystals, along with I_S, the main defects are antisite acceptor defects Cd_Ga as well as interstitial atoms Cd_i and S_i. It is demonstrated that the emission characteristics of the crystals investigated are determined by associations of the above defects. A luminescence band peaked at hν_m=0.971 eV was observed for the first time. This band is related to intracenter transitions in the d shell of Cd of the Cd_Ga defect, which is split due to the effects of the crystalline field and spin-orbit interaction, in the presence of S_i.     

Optical absorption and chromium diffusion in ZnSe single crystals

ZnSe:Cr single crystals were obtained using diffusion-related doping with chromium. The diffusion of chromium was performed in an atmosphere of saturated zinc vapors, and the metallic Cr layer deposited on the crystal surface was used as the source. Lines corresponding to chromium absorption at 2.766, 2.717, and 2.406 eV were observed in the optical-density spectrum at 77 K. The highest chromium concentration in the crystals was determined from infrared absorptance in the region of 0.72 eV and was found to be equal to 8 × 10¹⁹ cm^?3. It is shown that the diffusion profile of the chromium impurity can be determined by measuring the optical density of the crystals in the visible region of the spectrum. The diffusion coefficients D of chromium in ZnSe crystals at temperatures of 1073–1273 K are calculated. An analysis of the temperature dependence D(T) made it possible to determine the coefficients in the Arrhenius equation: D₀ = 4.7 × 10¹⁰ cm²/s and E = 4.45 eV.     

Study of the impurity photoconductivity in <Emphasis Type="Italic">p</Emphasis>-InSb using epitaxial <Emphasis Type="Italic">p</Emphasis> <Superscript>+</Superscript> contacts

The optical absorption coefficient α in p⁺-InSb layers (with hole concentrations of p ≈ 1 × 10¹⁷–1.2 × 10¹⁹ cm^–3), grown by liquid-phase epitaxy on p-InSb substrates, is measured in the spectral range of 5-12 µm at 90 K, and the impurity photoconductivity is measured (at 60 and 90 K) in p⁺–p structures. It is found that a in the p⁺ layers reaches a value of 7000 cm^–1 (at p ≈ 2 × 10¹⁹ cm^–1). It is shown that the measured substrate value of (α ≈1–3 cm^–1) is overestimated in comparison with estimates (α ≈ 0.1 cm^–1) based on comparing the photoconductivity data. This discrepancy is explained by the fact that the optical transitions of holes responsible for photoconductivity are obscured by the excitation of electrons to the conduction band. The photoionization cross section for these transitions does not exceed 1 × 10^–15 cm².     

Study of the recombination process at crystallite boundaries in CuIn1 − x Ga x Se2 (CIGS) films by microwave photoconductivity

The loss kinetics of photogenerated charge carriers in thin polycrystalline chalcopyrite CuIn_1?x Ga _x Se₂ (CIGS) films has been studied by microwave photoconductivity (at 36 GHz). The films were synthesized using the ampoule method and three variants of physical vapor deposition with subsequent selenization: magnetron sputtering, thermal deposition, and modified thermal deposition with intermetallic precursors. The photoconductivity was excited by 8-ns nitrogen laser pulses with maximum intensity of 4 × 10¹⁴ photons/cm per pulse. Measurements were performed in the temperature range 148–293 K. The photoresponse amplitude is found to depend linearly on the sizes of coherent-scattering regions in the film grains, which were calculated from X-ray diffraction data. The photoresponse decay obeys hyperbolic law. The photoresponse half-decay time increases with a decrease in both temperature and light intensity. It is shown that the recombination of free holes with trapped electrons is very efficient near the crystallite boundaries.     

Native and impurity defects in ZnSe:In single crystals prepared by free growth

The optical absorption and photoluminescence spectra and the Hall effect were studied in ZnSe:In single crystals. The presence of electrically active In _Zn ⁺ donor centers responsible for the impurity absorption and electrical conduction of crystals is established. It is shown that the conduction compensation in the ZnSe:In crystals is effected by cationic vacancies. The In _Zn ⁺ donors and cationic vacancies form associative defects responsible for long-wavelength ZnSe:In luminescence. A high crystal conductivity (～5 Ω^?1 cm^?1) is achieved as a result of ZnSe:In annealing in the zinc melt, which results in the extraction of cationic vacancies. The electron mobility in high-conductivity crystals is limited by scattering at the LO phonons and macrodefects formed due to the reduction of In solubility in crystals by their annealing in zinc.     

Preparation and optical properties of Co-doped ZnSe single crystals

ZnSe single crystals doped via Co diffusion are investigated. The diffusion was carried out from metal Co in He or Ar atmosphere. The spectra of optical density in the wavelength range 0.4–2 μm are investigated. It is found that the absorption edge shifts as the concentration of doping impurities increases. This shift is caused by the formation of the Zn_{1 ? x} Co_xSe alloy. The diffusion profile of the Co dopant is determined via measurement of the relative optical density of the crystals in the visible spectral region. The Co diffusivities (D) in the ZnSe crystals at T = 1103–1273 K are calculated. The analysis of the temperature dependence D(T) made it possible to determine the coefficients in the Arrhenius equation, namely, D ₀ = 3.4 × 10⁶ cm²/s and E ₀ = 3.8 eV.