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.     

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.     

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.     

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.     

Study of the impurity photoconductivity and luminescence in ZnSe:Ni crystals in the visible spectral region

The photoconductivity and photoluminescence spectra of ZnSe:Ni crystals in the visible spectral region are studied. It is established that the high-temperature impurity photoconductivity of ZnSe:Ni crystals is controlled by the optical transitions of electrons from the ground state ³ T ₁(F) to high-energy excited states, with subsequent thermally activated transitions of electrons to the conduction band. A photoconductivity band associated with the photoionization of Ni impurities is revealed. The intracenter luminescence of ZnSe:Ni crystals is efficiently excited with light corresponding to the intrinsic absorption region of Ni²⁺ ions.     

Optical properties of Cd1−x ZnxTe (0<x<0.1) single crystals in the infrared spectral region

Frequency dependences of the transmittance T(ω) of Cd_1?x Zn_xTe (0<x<0.1) single crystals grown by a modified Bridgman method were studied using long-wavelength optical spectroscopy in the temperature range of 5–300 K. A sharp increase in absorption for energies smaller than the band gap was observed for unannealed samples of p-type conductivity. Moreover, with the temperature variation of the sample from 5 to 300 K, the dependences T(ω) intersect almost at the same wavelength. The theoretical frequency dependences T(ω) with allowance for substantial contribution of the intersubband transition of charge carriers in the valence band were calculated. The Fermi level position was evaluated for the samples at T=77.3 and 295 K. A nonmonotonic dependence of the Fermi level position on the composition was observed for x=0.040–0.047. The possible influence of tellurium precipitates on the transmittance decrease in Cd_1?x Zn_xTe in the wave number range of 3000?400 cm^?1 is discussed.     

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.     

Deep Radiation-Induced Defect Centers Created by a Fast Neutron Flux in CdZnTe Single Crystals

The effect of a fast neutron flux (Φ = 10¹⁴–10¹⁵ cm^–2) on the electrical and photoluminescence properties of p-CdZnTe single crystals is studied. Isothermal annealing is performed (T = 400–500 K), and the activation energy of the dissociation of radiation-induced defects is determined at E_D ≈ 0.75 eV.     

Defects of the structure of semiconductor superlattices grown on the basis of II–VI alloys

The specific features of defects of crystal lattices in multilayer device structures containing small-period (T ? 20 nm) superlattices of type I ZnSe/Cd _x Zn_{1 ? x} Se/ZnSe/.../ZnSe/(001)GaAs and type II ZnS/ZnSe_{1 ? x} S _x /ZnS/.../ZnS/(001)GaAs are studied by the methods of X-ray diffractometry and diffraction rocking pseudocurves, and partially by the X-ray topography. According to the data of quantitative analysis of the X-ray diffraction spectra, the periods of superlattices are in the range T _I = 11.3–16.1 nm (for the compositions Cd _x Zn_{1 ? x} Se with x ₁ = 0.047 and x ₂ = 0.107) for type I superlattices and T _II = 15.6–17.2 nm for type II superlattices (for the compositions ZnSe_{1 ? x} S _x with x ₁ = 0.20 and x ₂ = 0.10). The widths of diffraction peaks from both the ZnSe layers and small-period superlattices in the diffraction rocking pseudocurves considerably exceed their widths in the X-ray diffraction spectra. This fact proves that a pronounced plastic strain with the formation of series of rectilinear dislocations in the crossing slip systems took place in the studied device structures. In order to exclude the generation of dislocations in the growth processes, it is necessary to decrease the concentration of the solid solution to the values x < 0.047 for the first type of superlattices and to the values x ? 0.062 for the second type of superlattices, and to decrease the thickness of the ZnSe and ZnS layers.     

On the preparation and photoelectric properties of Tl1–x In1–x Sn x Se2 (x = 0.1–0.25) alloys

The technological conditions for growing single crystals of Tl_1–x In_1–x Sn _x Se₂ (x = 0.1–0.25) alloys are developed. The spectral distribution of the photoconductivity of the grown crystals at T = 300 K and thermally stimulated conductivity are studied. The effect of In³⁺cation substitution with Sn⁴⁺ in Tl_1–x In_1–x Sn _x Se₂ (x = 0.1–0.25) alloys on their photoelectric properties is shown.     

Diffusion of interstitial magnesium in dislocation-free silicon

The diffusion of magnesium impurity in the temperature range T = 600–800°C in dislocation-free single-crystal silicon wafers of p-type conductivity is studied. The surface layer of the wafer doped with magnesium by the ion implantation technique serves as the diffusion source. Implantation is carried out at an ion energy of 150 keV at doses of 5 × 10¹⁴ and 2 × 10¹⁵ cm^–2. The diffusion coefficient of interstitial magnesium donor centers (D _i ) is determined by measuring the depth of the p–n junction, which is formed in the sample due to annealing during the time t at a given T. As a result of the study, the dependence D _i (T) is found for the first time. The data show that the diffusion process occurs mainly by the interstitial mechanism.     

Photosensitivity of thin-film structures based on (CuInSe2)x(2ZnSe)1−x solid solutions

Polycrystalline (CuInSe₂)_x(2ZnSe)_1−x films (x=0.6–1.0) with p-type conductivity and a thickness of 0.5–0.9 μm were obtained by pulsed laser evaporation. It is shown that a chalcopyrite-sphalerite transition occurs in the above system for x=0.7. The obtained films were used to fabricate the photosensitive structure of the In/p-(CuInSe₂)_x(2ZnSe)_1−x and InSe(GaSe)/(CuInSe₂)_x(2ZnSe)_1−x types. Spectral dependences of photovoltaic-conversion quantum efficiency were studied, and the photosensitivity of the structures in relation to the type of energy barrier and the composition was analyzed. It is concluded that the structures under consideration can be used as broadband photovoltaic converters. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 5, 2000, pp. 576–581. Original Russian Text Copyright ? 2000 by V. Rud’, Yu. Rud’, Bekimbetov, Gremenok, Bodnar’, Rusak.     

Evaluation of mobility gaps and density of localized hole states in p-Ge/Ge1−x Six heterostructures in the quantum Hall effect mode

The temperature (0.1 K?T?20 K) and magnetic field (0 T?B?12 T) dependences of the longitudinal (ρ_xx) and Hall (ρ_xy) resistivities have been studied in detail for p-Ge/Ge_1?x Si_x (x=0.07) multilayer heterostructures with hole density p=(2.4–2.6)×10¹¹ cm^?2 and mobility μ=(1.1–1.7)×10⁴ cm² V^?1 s^?1. The energy spectrum parameters of two-dimensional (2D) hole gas in the quantum Hall effect mode have been determined. The mobility gap W=(2–2.5) meV and the background density of localized states g _c =(5–7)×10¹⁰ cm^?2 meV^?1 for the filling factors ν=1 and 2. The results are discussed in terms of long-range impurity potential models for selectively doped 2D systems.     

Features of conductivity of the intermetallic semiconductor n-ZrNiSn heavily doped with a Bi donor impurity

The crystal structure, distribution of the electron density of states, and the energy, kinetic, and magnetic properties of the intermetallic semiconductor n-ZrNiSn heavily doped with a Bi donor impurity have been investigated in the ranges T = 80?400 K, N _D ^Bi ?? 9.5 × 10¹⁹cm^?3 (x = 0.005)?1.9 × 10²¹ cm^?3 (x = 0.10), and H ?? 0.5 T. It has been established that such doping generates two types of donor-like structural defects in the crystal, which manifest themselves in both the dependence of the variation in the unit cell parameter a(x) and temperature dependence of resistivity ln??(1/T) of ZrNiSn_{1 ? x} Bi _x (x = 0.005). It is shown that ZrNiSn_{1 ? x} Bi _x is a new promising thermoelectric material, which converts thermal energy to electric energy much more effectively as compared to n-ZrNiSn. The results obtained are discussed within the Shklovskii-Efros model of a heavily doped and strongly compensated semiconductor.     

Electron-phonon processes in semiconductors at low temperatures

The thermal conductivity and thermopower are simultaneously studied in the semiconductors HgSe, HgTe, InP, GaAs, Cd _x H_{1 ? x} Te, and Ag₂S in which the effect of electron drag by phonons was detected at low temperatures T. It is found that x = 1–3 in the dependences K _ph ∝ T ^?x . It is shown that the underestimated values of x are caused by phonon scattering at certain defects. It is also found that the maxima of K _ph(T) and α_ph(T) coincide in all studied crystals, except for Ag₂S. For the first time, it is found that the strong effect α_ph ∝ T ^?3 is observed in Ag₂S, the α_ph maximum is at T = 16 K, and the K _ph maximum is at 27 K. The results obtained are in agreement with Callaway and Herring theory. The effect of magnetic field on the drag thermopower α_ph(T) in n-Cd _x H_1?x Te (in which the strong dependence α_ph ∝ T ^?3 and the strong effect of the magnetic field on it are detected) is considered for the first time. The results are compared with the Askerov theory. The force parameter A _ph(?) of the drag effect and its dependence on magnetic field are determined.     

Features of the conduction mechanisms of the n-HfNiSn semiconductor heavily doped with the Co acceptor impurity

The crystalline structure, electron density distribution, energy and kinetic parameters of a HfNi_{1 ? x} Co _x Sn semiconductor heavily doped with a Co acceptor impurity are studied in the ranges T = 80?C1620 K and N _A ^Co from 9.5 × 10²⁰ cm^?3 (at x = 0.05) to 7.6 × 10²¹ cm^?3 (at x = 0.40). It is shown that variations in the activation energy of hopping conduction ? ₃ ^?? (x) and the modulation amplitudes of continuous energy bands ? ₃ ^?? (x) are caused by the appearance of a donor source in the n-type HfNi_{1 ? x} Co _x Sn semiconductor. It is shown that the doping of n-HfNiSn with a Co acceptor impurity is accompanied by a change in the degree of compensation of the semiconductor due to the simultaneous generation of both structural acceptor-type defects during Ni atom substitution with Co atoms and structural donor-type defects during the partial occupation of Ni sites by Sn atoms. The results are discussed within the Shklovskii-Efros model for a heavily doped and compensated semiconductor.     

Charge carrier mobility in n-CdxHg1−x Te crystals subjected to dynamic ultrasonic stressing

The Hall mobility was studied in the n-Cd_xHg_1?x Te crystals subjected to dynamic ultrasonic stressing (W _US≤10⁴ W/m², f=5–7 MHz). It was found that, in field of the ultrasonic deformation, an increase in the carrier mobility in the impurity conduction region (T<120 K) and a decrease in the intrinsic conduction region (T>120 K) occurred in all tested samples. In this case, the magnitude of the sonic-stimulated variation in μ_H increases with decreasing structural perfection of a crystal. Different mechanisms of ultrasonic influence on μ_H with regard to scattering by optical phonons, ionized impurities, and alloy potential are analyzed, with the current flow conditions in the crystal taken into account. It is shown that, in the impurity conduction region, the main cause of the sonic-stimulated increase of the Hall mobility is the smoothing of the macroscopic intracrystalline potential that results from the inhomogeneity of the crystals. In the intrinsic conduction region, a decrease in mobility is caused by an increase in the intensity of scattering by the optical phonons.     

Effect of band inversion on the phonon spectra of the Hg1-x ZnxTe alloys

The narrow-gap II-VI and IV-VI alloys are convenient objects for studying the electron-phonon interaction. However, the concentration of free carriers in the IV-VI alloys is rather high (～10¹⁸ cm^?3), which complicates studying this effect, by the optical reflection method. The concentration of free carriers in the narrow-gap Hg_1-x Zn_xTe alloys is considerably lower (～10¹⁶ cm^?3). Therefore, the plasma component exerts less effect on the lattice reflection spectrum. The reflection spectra of Hg_1-x Zn_xTe crystals with x = 0.1–1 were studied in the far-IR region in the range of 30–700 cm^?1 at 40–300 K. Using the dispersion analysis and the Kramers-Kronig method, the frequencies of the TO phonons of the HgTe-like and ZnTe-like modes were determined depending on the composition. It is shown that the reconstructed phonon spectrum involves two modes. The temperature dependences of the frequencies of the TO phonons and the damping parameter were measured for the narrow-gap alloy with x = 0.1 in the range of 80–200 K. A decrease in the frequency of the TO phonon of the soft mode in the vicinity of the inversion point of the bands at T = 110 K was for the first time found by optical methods. The damping parameter slightly increases in the vicinity of this temperature. The result obtained qualitatively agreed with the theoretical model of Kawamura et al., which makes allowance for the effect of the electron-phonon interaction on the frequency of the soft mode in the IV-VI compounds.     

Influence of cation-vacancy imperfection on the electrical and photoelectric properties of the Cu1 − x Zn x InS2 alloy

The growth technology of single crystals of Cu_{1 ? x} Zn _x InS₂ alloys (x = 0–12) of n-type conductivity is developed. The formation mechanism of the alloy is investigated by X-ray structural analysis. It is shown that the single crystals have chalcopyrite structure, and the unit-cell parameters depend on the alloy composition. The temperature dependence of the electrical conductivity in the temperature range T = 27–300 K and the spectral distribution of the photoconductivity at T ≈ 30 K are investigated. Induced photoconductivity is found for CuInS₂-ZnIn₂S₄ with a content of ～8 and ～12 mol % ZnIn₂S₄ and thermally stimulated currents are investigated.     

Electrical Conductivity of V2O5–TeO2–Sb Glasses at Low Temperatures

Semiconducting glasses of the type 40TeO₂–(60 ? x) V₂O₅–xSb were prepared by rapid melt quenching and their dc electrical conductivity was measured in the temperature range 180–296 K. For these glassy samples, the dc electrical conductivity ranged from 2.26 × 10^?7  S cm^?1 to 1.11 × 10^?5 S cm^?1 at 296 K, indicating the conductivity is enhanced by increasing the V₂O₅ content. These experimental results could be explained on the basis of different mechanisms (based on polaron-hopping theory) in the different temperature regions. At temperatures above Θ _D/2 (where Θ _D is the Debye temperature), the non-adiabatic small polaron hopping (NASPH) model is consistent with the data, whereas at temperatures below Θ _D/2, a T ^?1/4 dependence of the conductivity indicative of the variable range hopping (VRH) mechanism is dominant. For all these glasses crossover from SPH to VRH conduction was observed at a characteristic temperature T _R ≤ Θ _D/2. In this study, the hopping carrier density and carrier mobility were determined at different temperatures. N (E _F), the density of states at (or near) the Fermi level, was also determined from the Mott variables; the results were dependent on V₂O₅ content.