Electron statistics in CdF2 semiconductor crystals with DX centers

The degree of compensation and ionization energy of two-electron DX centers in CdF₂: In and CdF₂: Ga semiconductors are determined by studying the statistical distribution of electrons localized on impurity levels. The sharp temperature dependence of the concentration of neutral donors observed in CdF₂: Ga over the temperature range T = 250–400 K is explained by a high compensation degree, K ≥ 0.996. Thus, all Ga ions introduced into a CdF₂ crystal lattice during crystal growth form shallow donor levels. However, the concentration of Ga ions that can form bistable DX centers is rather low and is close to the concentration of electrons injected into the crystal during additive coloring. In CdF₂: In crystals, the degree of compensation is smaller than that in CdF₂: Ga but is sufficiently high and the number of bistable DX centers is not limited. It is concluded that an extremely narrow impurity band forms in the CdF₂: Ga semiconductor. For a total charged-impurity concentration of ～10²⁰ cm^?3, the width of the impurity band in CdF₂: Ga is not likely to exceed ～0.02 eV.     

Experimental observation of “configuration” modes of bistable centers in CdF2:In crystals

New “configuration” modes, which were predicted by us for CdF₂:In crystals, have been revealed at the frequencies ν₁ ≈ 32.4 cm^?1 and ν₂ ≈ 96.3 cm^?1 for deep and shallow impurity states, respectively. The frequencies of these oscillations exactly correspond to the potential-energy curves calculated by us for shallow and deep states of In with regard to the reduced mass M = 2m ₁ m ₂/(m ₁ + 2m ₂) of the In ion (m ₁) and two F ions (2m ₂) per primitive fluorite cell. This correspondence confirms the correct choice of the height of the potential barrier between the impurity states of In in CdF₂ (0.02 eV), which was used in the calculations. The dielectric contributions of the noted modes were determined, which made it possible to calculate the concentrations of In impurity ions in the deep (N ₁) and shallow (N ₂) states. The obtained ratio N ₂/N ₁ ≈ 2 directly indicates that photoionization of deep In centers leads to the formation of a doubled number of shallow centers and that two electrons are localized in the deep state of the In ion; such behavior is characteristic of DX centers. A photoinduced increase in the real (ε′) and imaginary (ε″) parts of the dielectric constant has been found (at a frequency of 25 cm^?1, Δε′ ≈ 0.2 and Δε″ ≈ 0.06). These changes correspond to the changes in the dielectric contributions of the configuration modes under illumination. A photoinduced decrease in the lattice reflection of CdF₂:In, related to the impurity lattice modes, has also been revealed.     

Microwave measurements of electrical conductivity of CdF<Subscript>2</Subscript> semiconductor crystals

The electrical conductivity of CdF₂ semiconductor crystals is measured using the microwave intracavity technique at a frequency of ～35 GHz. The crystals are activated with yttrium donor impurities and indium and gallium ions forming bistable one-electron donor impurity and two-electron DX centers. The conclusion is drawn that the concentration of electrons in the conduction band of CdF₂: Ga crystals has an anomalously high value. This confirms the results obtained in earlier NMR investigations of CdF₂ semiconductor crystals at room temperature.     

Compensation effect in undoped polycrystalline CdTe synthesized under nonequilibrium conditions

The compensation effect has been revealed in undoped polycrystalline CdTe synthesized during rapid crystallization. The revealed effect leads to an increase in the electrical resistivity to 10⁸–10¹⁰ Ω cm at a background impurity concentration of ～1015 cm^?3 (Ga_Cd and Cl_Te donors, unidentified acceptors). For some samples, this effect is accompanied by the appearance of persistent photoconductivity, which disappears at a temperature of ～200 K. It has been shown that all the polycrystals studied are characterized by a three-level compensation mechanism in which the fundamental properties of the material are determined by deep donors and/or acceptors with a concentration of 10¹² cm^?3. Depending on the specific growth conditions, the electrical resistivity at room temperature is determined by deep centers with activation energies of 0.59 ± 0.10 and 0.71 ± 0.10 eV, which are supposedly related to intrinsic point defects, and deep centers with activation energies of 0.4 ± 0.1 eV, which belong to the DX center formed by the Ga_Cd donor.     

Ionization mechanisms of aluminum acceptor impurity in silicon

Processes of ionization of shallow acceptor centers (ACs) in silicon are studied. In crystalline silicon samples with phosphorus (1.6×10¹³, 2.7×10¹³, and 2.3×10¹⁵cm^?3) and boron (1.3×10¹⁵cm^?3) impurities, _μAl impurity atoms were produced by implantation of negative muons. It is found that thermal ionization is the main mechanism for ionizing the Al acceptor impurity in both p-type and n-type silicon with an impurity concentration of ?10¹⁵cm^?3 at T>45 K. The thermal ionization rate of Al ACs in Si varies from ～10⁵ to ～10⁶s^?1 in the temperature range 45–55 K.     

Magnetic moment relaxation of a shallow acceptor center in heavily doped silicon

Results of studying the temperature dependence of the residual polarization of negative muons in crystalline silicon with germanium (9×10 ¹⁹ cm ^?3 ) and boron (4.1×10 ¹⁸ , 1.34×10 ¹⁹ , and 4.9×10 ¹⁹ cm ^?3 ) impurities are presented. It is found that, similarly to n-and p-type silicon samples with impurity concentrations up to ～10 ¹⁷ cm ^?3 , the relaxation rate ν of the magnetic moment of a _μ Al acceptor in silicon with a high impurity concentration of germanium (9×10 ¹⁹ cm ^?3 ) depends on temperature as ν～T ^q , q≈3 at T=(5–30) K. An increase in the absolute value of the relaxation rate and a weakening of its temperature dependence are observed in samples of degenerate silicon in the given temperature range. Based on the experimental data obtained, the conclusion is made that the spin-exchange scattering of free charge carriers makes a significant contribution to the magnetic moment relaxation of a shallow acceptor center in degenerate silicon at T?30 K. Estimates are obtained for the effective cross section of the spin-exchange scattering of holes (σ _h ) and electrons (σ _e ) from an Al acceptor center in Si: σ _h ～10^?13 cm² and σ _e ～8×10^?15 cm² at the acceptor (donor) impurity concentration n _a (n _d )～4×10¹⁸ cm^?3.     

Energy barrier between states of a bistable center in photochromic crystals CdF2:Ga and CdF2:In

The establishment of thermal equilibrium between photoinduced (shallow) and ground (deep) states of bistable DX centers in photochromic crystals CdF₂:In and CdF₂:Ga, which are used as real-time holographic media, is studied based on the notions of chemical kinetics. Two mechanisms of mutual transformation of shallow and deep centers—the tunnel mechanism and the mechanism with the participation of free charge carriers—are considered. Equations describing the decay of a photoinduced shallow state are obtained. These equations take into account the distribution of electrons between the photoinduced and ground states and the conduction band. Analysis of the experimental kinetic curves of the decay of photoexcited shallow centers makes it possible to determine the activation energies and barrier height for thermally activated processes of mutual transformation of shallow and deep centers. In CdF₂:In and CdF₂:Ga, this barrier, which determines the decay kinetics of holograms, lies above the bottom of the conduction band by ～10 and ～500 meV, respectively.     

Photodielectric effect and “Configuration” modes of bistable centers in the CdF<Subscript>2</Subscript>: Ga crystal

A photoinduced increase in the real (?′) and imaginary (?″) parts of the permittivity (Δ?′ ≈ 0.23 and Δ?″ ≈ 0.10 at a frequency of 15 cm^?1) is revealed experimentally. This photodielectric effect is adequately described by the predicted configuration modes at the frequencies gv ₁ = 354 cm^?1 and gv ₂ = 123 cm^?1, which correspond to the potential-energy curves previously calculated for deep and shallow impurity states in CdF₂: Ga crystals. The dielectric contributions of these modes are determined, and the corresponding concentrations of Ga ions in deep (N ₁) and shallow (N ₂) impurity states are calculated. It is found that, unlike the CdF₂: In crystals, the changes in the quantities ?′ and ?″ before and after illumination of the CdF₂: Ga crystals are predominantly determined by the change in the contribution from the configuration mode of the shallow state, because the contribution from the configuration mode of the deep state is very small. A photoinduced decrease in the lattice reflection in the CdF₂: Ga crystals due to the change in the dielectric contribution from the impurity mode of the lattice is predicted.     

Electrical properties of n-type vapor-grown gallium nitride

Hall measurements are reported for undoped and Zn-doped vapor-grown single crystal GaN on (0001) Al₂O₃ layers with 298 K carrier concentrations (n-type) between 1·4×10¹⁷cm^?3 and 9×10¹⁹ cm^?3. Then n～10¹⁷ cm^?3 crystals (undoped) have mobilities up to μ～440 cm²/V sec at 298 K. Their conduction behavior can be described by a two-donor model between 150 and 1225 K and by impurity band transport below 150 K. Crystals with n≥8×10¹⁸ cm^?3 show metallic conduction with no appreciable variation in n or μ between 10 and 298 K.Results of mass spectrographic analyses indicate that the total level of impurities detected is too low to account for the observed electron concentration at the n～10¹⁹ cm^?3 level, and suggest the presence of a high concentration of native donors in these crystals. No significant reduction in carrier concentration was achieved with Zn doping up to concentrations of ～10²⁰ cm^?3 under the growth conditions of the present work, and no evidence was found to indicate that high conductivity p-type behavior may be achieved in GaN. The influence of factors such as growth rate, crystalline perfection and vapor phase composition during growth on the properties of the layers is described.     

Photoinduced submillimeter-wave amplitude diffraction grating in a semiconducting CdF<Subscript>2</Subscript>:Ga crystal

It is shown that exposure of an additively colored CdF₂:Ga crystal with bistable DX centers that is slowly cooled to 150 K to blue-green light through a slotted mask produces a submillimeter-wave diffraction grating, which persists for a long time at temperatures of 160–240 K. It is also shown that the diffraction grating induced in a sample is an amplitude grating. The absorption of submillimeter waves in illuminated regions of the sample is associated with the conductivity due to the transition of impurity centers to a metastable donor state. In the n-i-n-i-type conducting structure obtained, the conductivity of n-type regions at 225 K amounts to σ ′ ≈ 0.24 Ω^?1 cm^?1.     

Photoluminescent studies of the condensed phase in phosphorus-doped silicon

Photoluminescent studies give evidence for the existence of the electron—hole droplet in phosphorus-doped silicon in the impurity concentration range 9.0 × 10¹⁵cm^?3 ? N_D ? 4.3 × 10¹⁹cm^?3.     

Transport and magnetic properties of multiwall carbon nanotubes before and after bromination

Bulk samples of oriented carbon nanotubes were prepared by electric arc evaporation of graphite in a helium environment. The temperature dependence of the conductivity σ(T), as well as the temperature and field dependences of the magnetic susceptibility χ(T, B) and magnetoresistance ρ(B, T), was measured for both the pristine and brominated samples. The pristine samples exhibit an anisotropy in the conductivity σ_∥(T)/σ_⊥>50, which disappears in the brominated samples. The χ(T, B) data were used to estimate the carrier concentration n ₀ in the samples: n _0ini ～3×10¹⁰ cm^?2 for the pristine sample, and n _0Br～10¹¹ cm^\t—2 for the brominated sample. Estimation of the total carrier concentration n=n _e+n _p from the data on ρ(B, T) yields n _ini=4×10¹⁷ cm^?3 (or 1.3×10¹⁰ cm^?2) and n _Br=2×10¹⁸ cm^?3 (or 6.7×10¹⁰ cm^?2). These estimates are in good agreement with one another and indicate an approximately fourfold increase in carrier concentration in samples after bromination.     

New mechanism of impurity conduction in lightly doped crystalline noncompensated silicon

It is found that the plastic deformation of lightly doped crystalline silicon samples (N<6×10¹⁶ cm^?3) with a low compensation (K～3×10^?2) gives rise to nonohmic conduction σ_M in electric fields that differs radically from conventional hopping conduction via the ground states of impurities (σ₃). The values of σ_M can exceed values of σ₃ by a factor of 10³?10⁵. The value of σ_M and its dependence on the electric (E) and magnetic (H) fields can be controlled by varying the density of dislocations and the mode of thermal sample treatment. A strong anisotropy of σ_M is observed in samples with oriented dislocations: the conductivities along and across dislocations can differ by a factor of 10⁴. The results are explained by the occurrence of conduction via the H^?-like states of impurities concentrated in the vicinity of dislocations. The levels of these states lie between the upper and the lower impurity Hubbard bands.     

Charge exchange and excitation cross sections at collisions of alpha particles with be-like impurity oxygen ions in a plasma

The charge exchange and excitation cross sections at collisions of alphas with O⁴⁺(1s ²2s ²) impurity atoms in a hot plasma for striking energies E _c varying from 20 keV to 2 MeV are determined for the first time. The cross sections are calculated using the method of close-coupling equations with 13 singlet four-electron quasi-molecular states taken as a basis. The partial cross sections of charge transfer to the 1s, 2s, and 2p states of a He⁺ ion and for O⁴⁺(1s ²2s ²) → O⁴⁺(1s ²2lnl’) (n = 2, 3) electronic excitation of an oxygen ion are found. The maximal value of the charge exchange total cross section roughly equals 2.2 × 10^?16 cm² at E _c ≈ 0.7 MeV. The excitation total cross section has a maximum of ≈ 7.7 × 10^?16 cm² at E _c ≈ 80 keV for single-electron excitation and ≈6.5 × 10^?16 cm² at E _c ≈ 0.7 MeV for two-electron excitation.     

Polycrystalline GaSb films prepared by the coevaporation technique

Gallium antimonide (GaSb) films were deposited onto fused silica and n-Si (100) substrates by coevaporating Ga and Sb from appropriate evaporation sources. The films were polycrystalline in nature. The size and the shape of the grains varied with the change in the substrate temperature during deposition. The average surface roughness of the films was estimated to be 10 nm. Grain boundary trap states varied between 2×10¹² and 2.2×10¹² cm^?2 while barrier height at the grain boundaries varied between 0.09 eV and 0.10 eV for films deposited at higher temperatures. Stress in the films decreased for films deposited at higher temperatures. XPS studies indicated two strong peaks located at ～543 eV and ～1121 eV for Sb 3d_3/2 and Ga 2p_3/2 core-level spectra, respectively. The PL spectra measured at 300 K was dominated by a strong peak located ～0.55 eV followed by two low intensity peaks ～0.63 eV and 0.67 eV. A typical n-Si/GaSb photovoltaic cell fabricated here indicated V _oc～311 mV and J～29.45 mA/cm², the density of donors (N _d)～3.87×10¹⁵ cm^?3, built in potential (V _bi)～0.48 V and carrier life time (τ)～28.5 ms. Impedance spectroscopy measurements indicated a dielectric relaxation time ～100 μs.     

Infrared plasma reflectivity minimum in heavily doped n-Si

This paper presents a numerical analysis of infrared (IR) plasma reflectivity minimum in ultra heavily doped (UHD) n-Si (impurity concentration N up to 6 × 10²¹cm⁻³) by using a self-consistent method (SCM) and a complex physical model. The necessity of taking into account the dependence of effective mass on impurity concentration is shown. The scattering on defects (N_def = 5 × 10¹⁷ cm⁻³) and dislocation (N_dis = 5 × 10¹¹ cm⁻²) is included. The approximate relation for the wavelength λ_m(N) of the reflectivity minimum is given. The results obtained are compared with the experimental results for n-Si and satisfactory agreement is found.     

DX-Like behavior of Se-impurity centers in gasb under hydrostatic pressure

Abstract

Hall coefficient (R_H) and electrical resistivity measurements have been performed as a function of temperature (between 77 K and 300 K) and under hydrostatic pressures (up to 15 kbar) on a set of Se-doped GaSb samples with impurity concentrations in the range 8×10¹⁷ cm^?3 - 1×10¹⁸ cm^?3. With increasing pressure at 300 K, the electrons are strongly trapped into a resonant impurity level. The pressure induced occupation of this level leads to time-dependent effects at T<120 K. The activated thermal electron emission over a potential barrier E<sb>B = 300×30 meV gives clear evidence for a large lattice relaxation around the impurity centers characteristic for DX-like behavior.     

Effects of the overlap between wave functions of impurity centers on the activation energy of hopping conduction

The hopping conductivity σ₃ has been studied in samples of slightly counterdoped crystalline Si: B with a boron concentration of 2×10¹⁶ cm^?3<N<10¹⁷ cm^?3 and a compensation of 10^?4≤K≤10^?2. It is found that at K≤10^?3 the activation energy ε₃ is not lower (as it must be according to classical notions at finite K) but larger than the value ε_N=e ² N ^1/3/κ, where e is the electronic charge and κ is the dielectric constant. With decreasing N, the energy ε₃ drops slower and, with decreasing K, grows faster than follows from the standard theory. At K≤10^?4, ε₃ is higher than ε _N by a factor of 1.5–2. The result is explained by the effect of the overlap between wave functions of neighboring impurity centers on the structure of the impurity band.     

Exciton spectra and electrical conductivity of epitaxial silicon-doped GaN layers

Optical spectra and electrical conductivity of silicon-doped epitaxial gallium nitride layers with uncompensated donor concentrations N _D — N _A up to 4.8 × 10¹⁹ cm^?3 at T ≈ 5 K have been studied. As follows from the current-voltage characteristics, at a doping level of ～3 × 10¹⁸ cm^?3 an impurity band is formed and an increase of donor concentration by one more order of magnitude leads to the merging of the impurity band with the conduction band. The transformation of exciton reflection spectra suggests that the formation of the impurity band triggers effective exciton screening at low temperatures. In a sample with N _D — N _A = 3.4 × 10¹⁸ cm^?3, luminescence spectra are still produced by radiation of free and bound excitons. In a sample with N _D — N _A = 4.8 × 10¹⁹ cm^?3, Coulomb interaction is already completely suppressed, with the luminescence spectrum consisting of bands deriving from impurity-band-valence band and conduction-band-valence band radiative transitions.     

Effect of surface states on the amphoteric behavior of Sn in vapor epitaxial GaAs

The amphoteric behavior of Sn, a commonly-used dopant in AsCl₃GaH₂ vapor epitaxy, is examined for Sn concentration from 5 × 10¹⁴ to 5 × 10¹⁷cm^?3. The compensation ratio $\text{(}\text{N}{}_{\mathrm{A}}\text{N}{}_{\mathrm{D}}\text{)}$ remains constant at 0.23 for low concentrations and begins to increase in the 10¹⁶cm^?3 range. This behavior can be explained quantitatively with non-equilibrium impurity incorporation model which takes into account 3 × 10¹¹ cm^?2 surface states.