Conduction type conversion in ion etching of Au- and Ag-doped narrow-gap HgCdTe single crystal

Fundamental aspects of the p-to-n conductivity type conversion induced by ion etching in HgCdTe single crystals diffusion-doped with Au and Ag have been studied. A conversion mechanism is suggested, according to which interstitial mercury atoms rapidly diffuse from the surface source with high concentration and “kick out” impurity atoms from the cation sublattice into interstitial positions and thereby convert impurity atoms from the acceptor state to the donor state. It is shown that the structure of defects in the converted layer is unstable and the electrical parameters of this layer vary when stored at room temperature. The most probable mechanism of this process at room temperature is decomposition of an oversaturated solution of the impurity. A re-conversion to the p-type, observed in isochronous annealing of samples, is due to impurity diffusion into the converted layer from the unconverted bulk of the sample or from microscopic impurity inclusions.     

A Quasi-Classical Model of the Hubbard Gap in Lightly Compensated Semiconductors

A quasi-classical method for calculating the narrowing of the Hubbard gap between the A⁰ and A⁺ acceptor bands in a hole semiconductor or the D⁰ and D^– donor bands in an electron semiconductor is suggested. This narrowing gives rise to the phenomenon of a semiconductor transition from the insulator to metal state with an increase in doping level. The major (doping) impurity can be in one of three charge states (–1, 0, or +1), while the compensating impurity can be in states (+1) or (–1). The impurity distribution over the crystal is assumed to be random and the width of Hubbard bands (levels), to be much smaller than the gap between them. It is shown that narrowing of the Hubbard gap is due to the formation of electrically neutral acceptor (donor) states of the quasicontinuous band of allowed energies for holes (electrons) from excited states. This quasicontinuous band merges with the top of the valence band (v band) for acceptors or with the bottom of the conduction band (c band) for donors. In other words, the top of the v band for a p-type semiconductor or the bottom of the c band for an n-type semiconductor is shifted into the band gap. The value of this shift is determined by the maximum radius of the Bohr orbit of the excited state of an electrically neutral major impurity atom, which is no larger than half the average distance between nearest impurity atoms. As a result of the increasing dopant concentration, the both Hubbard energy levels become shallower and the gap between them narrows. Analytical formulas are derived to describe the thermally activated hopping transition of holes (electrons) between Hubbard bands. The calculated gap narrowing with increasing doping level, which manifests itself in a reduction in the activation energy ε₂ is consistent with available experimental data for lightly compensated p-Si crystals doped with boron and n-Ge crystals doped with antimony.     

Room-Temperature Mid-, and Far-Infrared Absorption and Electrical Properties of Intercalated GaSe and TlInS<Subscript>2</Subscript> Crystals

We report room-temperature measurements of the mid- and far-IR absorption throughout the 400 – 4000 cm^?1 and 10 – 700 cm^?1 spectral ranges and the resistivity of layered p-GaSe and p-TlInS2 intercalated with Li⁺. Intercalation was performed by immersing Bridgman grown crystals in 0.5 M solutions of LiCl in distilled water at ambient conditions. The crystal structure and the stoichiometry of the grown crystals were determined by X-ray diffraction and XRF methods. It is shown that intercalation does not change the frequency of the IR-, and Raman active low-frequency“rigid layer” mode (GaSe), the space symmetry group or the lattice parameters of the crystals. It was found that for both crystals, the resistivity versus time dependencies are nearly the same. Three ranges in the resistivity-intercalation time dependencies were explained qualitatively. The resistivity increase due to intercalation was explained by assuming that the intercalated lithium ions act as ionized donors and compensate the host p-type crystal. The highest degree of compensation for GaSe and TlInS₂ crystals was achieved after intercalation during 12 and 10 days, respectively.     

Electrical properties and low-temperature photolumincesence of Si-doped CdTe crystals

The CdTe:Si single crystals with Si concentration in the range of C _Si ⁰ =2×10¹⁸–5×10¹⁹ cm^?3 are grown by the Bridgman-Stockbarger method. The samples were of the n-and p-type with electrical conductivity σ=2×10^?1–8×10^?9 Ω^?1 cm^?1. Being heated in the temperature range 300–440 K, the p-CdTe crystals were annealed, and their conductivity decreased. The shape of the low-temperature (5–20 K) photoluminescence spectra of the samples are indicative of their high structural quality. The specific feature of the emission of the CdTe:Si crystals is its decrease in the intensity of all lines induced by donors as the samples are cut progressively closer to the ingot top. The results obtained indicate that the Si impurity, in contrast with Ge, Sn, and Pb, does not exhibit the compensating and stabilizing effect in CdTe.     

Quasi-local states of Sn in Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> according to the studies of galvanomagnetic effects in classical and quantizing magnetic fields

The magnetic-field dependences of the Hall coefficient and transverse magnetoresistance are studied experimentally and theoretically in p-Bi₂Te₃ crystals doped heavily with Sn and grown by the Czochralski method in the case of both classical and quantizing magnetic fields as high as 12 T with the magnetic-field orientation along the C ₃ trigonal axis. The Shubnikov-de Haas effect and quantum oscillations of the Hall coefficient were measured at temperatures of 4.2 and 11 K. The six-ellipsoid Drabble-Wolfe model of the energy spectrum and the magnetic-field dependence of the Hall coefficient are used as the basis for the method for determining the Hall factor and Hall mobility. New evidence is obtained in support of the existence of the narrow band of impurity Sn states occupied partially with electrons against the background of the light-hole band spectrum. The parameters of impurity states are estimated including their energy (E _Sn ≈ 15 meV), the broadening (Γ « kT), and the radius of localization of the impurity state (R ≈ 30 Å).     

Emission sensitization and mechanisms of electron-excitation migration in structures based on III-nitrides doped with rare-earth elements (Eu,Er, Sm)

The effect of doping with Eu, Er, and Sm rare-earth ions on the shape of the luminescence spectrum for heterostructures with GaN/In _x Ga_{1 ? x} N (0.1 < x < 0.4) quantum wells and from p-GaN〈Mg〉/n-GaN and p-AlGaN/n-GaN junctions is investigated. The results of measurements of the electroluminescence of these structures correlate with the previous data on photoluminescence and Mössbauer spectroscopy. It is shown that it is the GaN “yellow” (5000–6000 Å) band that plays the important role in the excitation of intracenter states in the structures with several GaN/InGaN quantum wells doped with Eu and Sm. In this case, Eu is most likely the sensitizer for Sm. Additional introduction of 3d metal (Fe⁵⁷) in p-GaN〈Mg〉/n-GaN:Eu results in the realization of intracenter transitions in Eu³⁺: ⁵ D ₀ → ⁷ F ₁ (6006 Å), ⁵ D ₀ → ⁷ F ₂ (6195 Å), ⁵ D ₀ → ⁷ F ₃ (6627 Å), and ⁵ D ₁ → ⁷ F ₄ (6327 Å) due to the occurrence of new, efficient channels of excitation transfer to intracenter states and in the effect of Fe on the local environment of rare-earth ions including due to the f–d hybridization enhancement.     

Specific features of the electrophysical parameters of NTD Si treated under different conditions of heat treatment

The effect of thermal annealing in the temperature range 800 ? T_ann ? 1200°C and of two cooling rates (v_cl = 1 and 15°C min^–1) upon a change in the concentration of charge carriers in the conduction band, their mobility, and tensoresistance in n-Si crystals doped by nuclear transmutation and doped with phosphorus impurity via the melt (during growth by the Czochralski method) was investigated. It is found that, after annealing of all of the crystals at T_ann = 1050–1100°C, the concentration of charge carriers is increased by a factor of 1.3–1.7 compared to the initial concentration (irrespective of the method of doping). The specific annealing-temperature-dependent effect of cooling with a rate of 15°C min^–1 on the properties of transmutation-doped n-Si:P crystals is detected.     

Impurity centers of rare-earth ions (Eu,Sm, Er) in GaN wurtzite crystals

Gallium nitride (GaN) was doped with Eu, Sm, and Er impurities using the diffusion method. The behavior of rare-earth impurities (the formation of donor or acceptor levels in the GaN band gap) correlates with the total concentration of defects, which is determined from optical measurements, and with the position of the Fermi level in starting and doped crystals. The intensity of emission lines, which are characteristic of the intracenter f-f transition of rare-earth ions, is controlled by the total defect concentration in the starting semiconductor matrix.     

Magnetism of III–V crystals doped with rare-earth elements

The procedure of measuring static magnetic susceptibility is used for the study of local ordering in III–V crystals containing rare-earth dopants. The dependences of the static magnetic susceptibility on the temperature and magnetic field show a slight paramagnetism at high temperatures due to quasi-molecular Ln₂O₃-type centers present in the III–V(Ln) crystals. In such centers, the pairs of Ln³⁺ ions are antiferromagnetically ordered by an exchange interaction via the oxygen valence electrons. At low temperatures, the crucial role is played by localization of electrons at shallow donors. Due to the s-f exchange interaction, such localization results either in paramagnetism of the crystals, due to transformation of the Ln₂O₃ molecules from the antiferromagnetically ordered state to an ferromagnetically ordered state, or in superparamagnetism, due the formation of bound spin polarons. In this case, the constants of the s-f exchange interaction determined from the temperature dependences of the static magnetic susceptibility are anomalously large. This circumstance probably results from the efficient compensation of spin correlations by the electron-vibration interaction.     

Simulation of Electronic Center Formation by Irradiation in Silicon Crystals

We present the results of a study on localized electronic centers formed in crystals by external influences (impurity introduction and irradiation). The main aim is to determine the nature of these centers in the forbidden gap of the energy states of the crystal lattice. For the case of semiconductors, silicon (Si) was applied as model material to determine the energy levels and concentration of radiation defects for application to both doped and other materials. This method relies on solving the appropriate equation describing the variation of the charge carrier concentration as a function of temperature n(T) for silicon crystals with two different energy levels and for a large set of N ₁, N ₂ (concentrations of electronic centers at each level), and n values. A total of almost 500 such combinations were found. For silicon, energy level values of ε ₁ = 0.22 eV and ε ₂ = 0.34 eV were used for the forbidden gap (with corresponding slopes determined from experimental temperature-dependent Hall-effect measurements) and compared with photoconductivity spectra. Additionally, it was shown that, for particular correlations among N ₁, N ₂, and n, curve slopes of ε ₁/2 = 0.11 eV, ε ₂/2 = 0.17 eV, and α = 1/2(ε ₁ + ε ₂) = 0.28 eV also apply. Comparison between experimental results for irradiation of silicon crystals by 3.5-MeV energy electrons and Co⁶⁰ γ-quanta revealed that the n(T) curve slopes do not always coincide with the actual energy levels (electronic centers).     

Fine Structure of the long-wavelength edge of exciton-phonon absorption and hyperbolic excitons in silicon carbide of 6<Emphasis Type="Italic">H</Emphasis> polytype

The fine structure of the long-wavelength edge of the polarization spectra of exciton-phonon absorption in moderate-purity n-type 6H-SiC crystals with a concentration of uncompensated donors N_D?N_A=(1.7–2.0)×10¹⁶ cm^?3 at T=1.7 K was studied. The analysis of new special features found at the absorption edge and the reliable detection of the onset of exciton-phonon steps related to the emission of phonons from acoustical and optical branches allowed highly accurate determination of a number of important parameters such as the band gap, the exciton band gap, the exciton binding energy, and the energies of spin-orbit and crystal-field splitting of an exciton. For the first time, transitions with the emission of LA phonons to the 1S exciton state with an M₁-type dispersion law were detected in E ∥ Z(C) polarization (the electric-field vector is parallel to the optical axis of the crystal). This observation supports the previously predicted “two-well” structure of the conduction band minimum in 6H-SiC.     

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.     

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.     

Spectra of low-temperature photoluminescence in thin polycrystalline CdTe films

The band of intrinsic (e–h) radiation emission by the subsurface potential barriers of crystal grains and the edge doublet band arising as LO-phonon replicas of the e–h band are observed in the spectra of the low-temperature (4.2 K) photoluminescence of fine-grained (with a grain size of d_cr ≤ 1 µm) CdTe films. Film doping with the In impurity results in quenching of the doublet band, while heat treatment leads to activation of the intrinsic band, a short-wavelength shift of the red boundary (ΔE_r = 16–29 meV) and the halfwidth modulation (Δ_A = 6–17 meV) of which correlate with the height of micropotential barriers and the temperature of recombining hot photocarriers.     

Photoluminescence of electron-hole plasma in semi-insulating undoped GaAs

The dependence of the photoluminescence spectrum of electron-hole plasma in semi-insulating undoped GaAs on the concentration of the background carbon impurity N_C(3×10¹⁵ cm^?3≤N_C≤4×10¹⁶ cm^?3) is studied at 77 K. It is established that the density of the electron-hole plasma, which is equal to n _e?h ≈1.1×10¹⁶ cm^?3 in crystals with the lowest impurity concentration at an excitation intensity of 6×10²² photons/(cm² s), decreases considerably as the value of N_C increases in the range mentioned above. A decrease in the density of the electron-hole plasma with increasing N_C is attributed to the effect of fluctuations in the carbon concentration N_C, which give rise to a nonuniform distribution of interacting charge carriers and to localization of holes in the tails of the density of states of the valence band.     

The influence of the energy of photoexcitation in the course of electron irradiation on defect formation in <Emphasis Type="Italic">n</Emphasis>-Si crystals

Effect of illumination of n-Si crystals in the course of irradiation with electrons on the nature of radiation defects is studied. The samples irradiated with 2-MeV electrons were subjected to isochronous annealing in the temperature range from 200 to 600°C. After each 20-min cycle of annealing, the electron concentration was measured by the Hall method in the temperature range 77–300 K. It is shown that, if the E centers are excited during irradiation with photons with the energy hν = 0.44 eV (the wavelength λ = 2.8 μm), divacancy phosphorus-containing defects of the PV ₂ type are formed in the n-Si crystals, which leads to an increase in the radiation resistance of the crystals under study. If negative vacancies V ^? are excited with photons with the energy hν = 0.28 eV (λ = 4.4 μm), the total number of radiation defects increases by a factor of 1.2.     

Electrical properties and structure of chalcogenide glasses containing bivalent tin

Two valence states of tin atoms are identified by Mössbauer and X-ray photoelectron spectroscopy in (As₂Se₃)_1?z (GeSe) _z?x (SnSe) _x glasses; it is shown that the presence of bivalent tin in the structural network of a glass does not give rise to impurity conductivity and impurity optical absorption. It is suggested to regard (As₂Se₃)_1?z (GeSe) _z?x (SnSe) _x and (As₂Se₃)_1?z (GeSe₂) _z?x (SnSe₂) _x glasses as semiconductor solid solutions whose electrical properties depend both on the electrical properties of the starting components and on the composition of the solid solutions.     

Radiative <Emphasis Type="Italic">d–d</Emphasis> transitions at tungsten centers in II–VI semiconductors

The luminescence spectra of W impurity centers in II–VI semiconductors, specifically, ZnSe, CdS, and CdSe, are studied. It is found that, if the electron system of 5d (W) centers is considered instead of the electron system of 3d (Cr) centers, the spectral characteristics of the impurity radiation are substantially changed. The electron transitions are identified in accordance with Tanabe–Sugano diagrams of crystal field theory. With consideration for the specific features of the spectra, it is established that, in the crystals under study, radiative transitions at 5d W centers occur between levels with different spins in the region of a weak crystal field.     

The influence of impurities on radiative recombination via <Emphasis Type="Italic">EL</Emphasis>2 centers in gallium arsenide single crystals

The influence of Cd and Se atoms on the quantum efficiency of emission induced by EL2 defects in GaAs single crystals is studied in detail. The diffusion of impurities under conditions of vacuum is compared to that in an As atmosphere. It is ascertained that the character and degree of variation in the quantum efficiency are governed by the vacancy type and concentration in the crystals and are related to the probability of formation of complexes that consist of EL2 centers and impurities.     

Photoconverters based on gallium arsenide diffused <Emphasis Type="Italic">p-n</Emphasis> junctions formed on a microprofile GaAs surface

The p-n junctions promising for photoconverters have been fabricated using diffusion from the gaseous phase and studied with the analysis of mass transport of the doping impurity (Zn) through the microprofile GaAs surface taken into account. Depending on the conditions of diffusion (the diffusant’s mass and diffusion duration), the formation of both a p-n junction in a microprofile and a planar p-n junction in the GaAs bulk with a heavily doped near-surface p ⁺ type layer is possible. Photoelectric characteristics of device structures with textured p-n junction and a thin wide-gap Al _x Ga_{1 ? x} As window obtained by liquid-phase epitaxy are reported.