Effect of iodine impurity on relaxation of photoexcited silver chloride

The time and temperature dependences of relaxation of excited AgCl and AgCl:I crystals is studied by the method of photostimulated flash of luminescence. The presence of iodine impurity in silver chloride gives rise to hole recombination (luminescence) centers and hole traps in the band gap. It is shown that the main contribution to the decrease in the concentration of electrons localized at deep traps is made by the recombination of electrons with holes released thermally from shallow localization levels (iodine-related centers). Estimation of activation energy for the relaxation process showed that these energies for the AgCl and AgCl:I samples under study are the same within the experimental error and are equal to E _{rel 1} = 0.01 ± 0.0005 eV for the initial stage of relaxation and E _{rel 2} = 0.09 ± 0.005 eV for the final state. This fact indicates that the majority of hole traps involved in the relaxation process in AgCl are related to iodine impurity. In the course of thermal relaxation in AgCl, relocalization of nonequilibrium charge carriers from shallow levels to deep levels is observed. The depth of the corresponding trap is E _arl = 0.174 ± 0.03 eV.     

Relaxation of photoexcited silver chloride

Relocalization of excess carriers from shallow levels to deep levels is observed in addition to carrier recombination during thermal relaxation of photoexcited silver chloride. Experimental dependences of the parameters of the photostimulated luminescence flash (released light sum and kinetic coefficient) on the relaxation time and temperature are explained within a three-level model including a recombination (luminescence) level, a deep level with energy ΔE ≈ 1.8 eV, and a shallow level with energy ΔE ≈ 0.03 eV with respect to the conduction-band bottom. It was shown that Ag⁺ ion adsorption caused a decrease in the relocalization activation energy from 0.17 to 0.03 eV, which is attributed to the surface nature of the centers responsible for the initial relaxation stage.     

Structurally complex two-hole and two-electron slow traps with bikinetic properties in <Emphasis Type="Italic">p</Emphasis>-ZnTe and <Emphasis Type="Italic">n</Emphasis>-ZnS crystals

Thermal-activation and photoactivation methods were used to ascertain the existence of two-hole traps in p-ZnTe crystals and two-electron traps in n-ZnS. It was found that these traps have a large number of energy states that are grouped in two series of levels: E_V+(0.46–0.66) eV and E_V+(0.06–0.26) eV in p-ZnTe and E_C?(0.6–0.65) eV and E_C?(0.14–0.18) eV in n-ZnS. Both the hole and the electron traps belong to the class of slow traps with bikinetic properties. These traps feature normal kinetic properties in the state with a single trapped charge carrier and feature anomalous kinetic properties in the state with two charge carriers. Multiple-parameter models allowing for a relation of traps in p-ZnTe and n-ZnS to the vacancy-impurity pairs distributed according to their interatomic distances and localized in the region of microinhomogeneities with collective electric fields that repel the majority charge carriers are suggested. The main special features of behavior of electron and hole traps are explained consistently using the above models.     

Determination of deformation potential constants for <Emphasis Type="Italic">n</Emphasis>-and <Emphasis Type="Italic">p</Emphasis>-Si from the concentration anharmonicity

The contribution of charge carriers to the fourth-order modulus of elasticity β for n-and p-type silicon under uniaxial tension along the [110] direction was analyzed in the approximation of small strains. The effect of concentration on β was measured using spontaneous excitation of Lamb waves in bent plates with different doping levels. Experimental curves were used to determine the deformation potential constants of the conduction band Ξ_u=7±1 eV and the averaged value of the deformation potential of the valence band \(\sqrt[4]{{\left\langle {\Phi ^4 } \right\rangle }} = 5.6 \pm 0.8eV\) at room temperature.     

Deep levels in the band gap of GaN layers irradiated with protons

Deep-level transient spectroscopy was used to study the parameters of deep levels in the band gap of epitaxial n-GaN layers after irradiaton of the Schottky barriers with 1-MeV protons to a dose of 10¹² cm^?2. A deep level EP1 with an activation energy of 0.085 eV was introduced by irradiation into the upper half of the GaN band gap. The introduction rate of the corresponding defect was found to depend on the bias voltage applied to the Schottky barrier during irradiation.     

Tuning the Electronic Properties of Graphene Oxide Nanoribbons Through Different Oxygen Doping Configurations

The electronic properties of armchair graphene oxide nanoribbons (AGONRs) with different doped oxygen configurations are studied based on density functional theory using first principle calculations. The electronic properties of the AGONRs are tuned by different oxygen configurations for top edges, center, bottom edges and fifth width. The AGONRs for top-edge O doping configuration are indirect band gap semiconductors with an energy gap of 1.268 eV involving hybridization among C-2p and O-2s, 2p electrons and electrical conductivity of oxygen atoms. The center and bottom edges are direct band gap semiconductors with 1.317 eV and 1.151 eV, respectively. The valence band is contributed from C-2p, O-2p and H-1s for top-edge O doping. The electronic properties of AGONRs are changed due to localization in ?2.94 eV of O-2p states. The center O-doped AGONRs are n-type semiconductors with Fermi levels near the conduction band bottom. This is due to hybridization among C-2s, 2p and O-2p electrons. However, bottom-edge O-doped AGONRs are p-type semiconductors, due to the electrical conductivity of oxygen atoms. The fifth-width O-doped AGONRs are indirect band gap semiconductors with an energy gap of 0.375 eV. The projected density of states shows that the localization and hybridization between C-2 s, 2p, O-2p and H-1s electronic states are rising in the conduction band and valence band from the projected density of states. The localization is induced by O-2p electronic states at a Fermi level.     

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.     

Impact ionization luminescence of InGaN/AlGaN/GaN p-n-heterostructures

The luminescence spectra of InGaN/AlGaN/GaN p-n heterostructures with reverse bias sufficient for impact ionization are investigated. The injection luminescence of light-emitting diodes with such structures was examined earlier. A strong electric field is present in the InGaN active layer of the heterostructures, and for small reverse bias the tunneling component of the current predominates. Avalanche breakdown commences at voltages V _th>8–10 V, i.e., ∼3E _g , (E _g is the width of the band gap) in the absence of lightly doped structures. The luminescence spectra have a short-wavelength edge corresponding to the width of the GaN band gap (3.40 eV) and maxima in the region 2.60–2.80 eV corresponding to the maxima of the injection luminescence spectra in the active layer. The long-wavelength edge of the spectra in the region 1.7–1.8 eV may be associated with deep recombination levels. Mechanisms of recombination of the hot electron-hole plasma in the strong electric fields of the p-n heterostructures are discussed. Fiz. Tekh. Poluprovodn. 32, 63–67 (January 1998)     

Shallow and Deep Centers in As-Grown and Annealed MgZnO/ZnO Structures with Quantum Wells

Shallow and deep centers in ZnO(P)/MgZnO/ZnO/MgZnO/ZnO(Ga) structures grown by pulsed laser deposition on sapphire were studied before and after annealing in oxygen atmosphere at high temperatures of 850°C to 950°C. In both as-grown and annealed structures, microcathodoluminescence spectra in the near-bandgap region demonstrate a blue-shift by 0.13 eV compared with bulk ZnO films, indicating carrier confinement in the MgZnO/ZnO/MgZnO quantum well (QW). Annealing strongly decreases the concentration of shallow uncompensated donors from ~10¹⁷ cm⁻³ to ~10¹⁶ cm⁻³ and makes it possible to probe the region of the QW by capacitance–voltage (C–V) profiling. This profiling confirms charge accumulation in the QW. The dominant electron traps in the as-grown films are the well-known traps with activation energies of 0.3 eV and 0.8 eV. After annealing, the electron traps observed in the structure have activation energies of 0.14 eV, 0.33 eV, and 0.57 eV, with the Fermi level in the n-ZnO(P) pinned by the 0.14-eV traps. The annealing also introduces deep compensating defects that decrease the intensity of band-edge luminescence and produce a deep luminescence defect band at 2.2 eV. In addition, a defect vibrational band becomes visible in Raman spectra near 650 cm⁻¹. No conversion to p-type conductivity was detected. The results are compared with the data for the structures successfully converted to p-type, and possible reasons for the observed differences are discussed.     

Surface states on the <Emphasis Type="Italic">n</Emphasis>-InN-electrolyte interface

Dependences of differential capacitance of the electrolyte-n-InN (0001) contact on the bias voltage are studied. Their analysis of the basis of a model similar to a model of the MIS structure shows that the energy spectrum of surface states of InN above the conduction band bottom can be represented by two, relatively narrow, bands of deep levels described by the Gaussian distribution. Parameters of these bands are as follows: the average energy counted from the conduction band bottom, ΔE ₁ ≈ 0.15 eV and ΔE ₂ ≈ 0.9 eV; and the mean-square deviation, ΔE ₁ ≈ 0.15–0.25 eV and ΔE ₂ ≈ 0.05–0.1 eV. The total density of states in the bands are (1–2.5) × 10¹² and (0.2–4) × 10¹² cm^–2.     

Features of high-temperature electroluminescence in an LED <Emphasis Type="Italic">n</Emphasis>-GaSb/<Emphasis Type="Italic">n</Emphasis>-InGaAsSb/<Emphasis Type="Italic">p</Emphasis>-AlGaAsSb heterostructure with high potential barriers

The electroluminescent properties of a light-emitting diode n-GaSb/n-InGaAsSb/p-AlGaAsSb heterostructure with high potential barriers are studied in the temperature range of 290–470 K. An atypical temperature increase in the power of the long-wavelength luminescence band with an energy of 0.3 eV is experimentally observed. As the temperature increases to 470 K, the optical radiation power increases by a factor of 1.5–2. To explain the extraordinary temperature dependence of the radiation power, the recombination and carrier transport processes are theoretically analyzed in the heterostructure under study.     

Changes in the conductivity of lead-selenide thin films after plasma etching

The conductivity of epitaxial n- and p-PbSe thin films after dry etching in radio-frequency highdensity low-pressure inductively coupled argon plasma at a bombarding-ion energy of 200 eV is studied. It is shown that the observed changes in the conductivity can be adequately interpreted in the context of the classical model of the generation of donor-type radiation defects and that the processes of post-irradiation vacuum annealing result in the removal of such defects. The mean free path of charge carriers in p-PbSe films is determined within the context of the Fuchs–Sondheimer theory. It is found that, at room temperature, this parameter is 16 and 32 nm for the specularity parameter 0 and 0.5, respectively.     

Anti-stokes luminescence of Zn<Subscript>0.75</Subscript>Cd<Subscript>0.25</Subscript>S microcrystals annealed in the presence of oxygen

The nature of anti-Stokes luminescence centers excited in the temperature range 77–300 K by light with wavelengths from 620 to 710 nm and intensities in the range 10¹⁵–10¹⁶ photons/cm² in Zn_0.75Cd_0.25S microcrystals annealed in the presence of oxygen has been investigated. It is shown that the centers of two-photon excitation of this luminescence are clusters of native metal oxides (ZnO) _n and (CdO) _n , whose energy levels in the Zn_0.75Cd_0.25S band gap are 1.70–1.95 eV below the bottom of the conduction band.     

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.     

A discrete element small-signal equivalent circuit for forward-biased p+n diodes containing deep levels

A discrete element small-signal equivalent circuit model for p-n diodes containing deep defect levels is developed, by extending an existing model for undamaged devices. With the aid of a simple analytical expression which accurately describes the forward bias d.c. current, the enhanced small-signal conductance due to carrier recombination in the depletion region is included in the model. The influence of trapped charge on the space charge capacitance is incorporated using a simplified version of the analysis of Beguwala and Crowell. The predictions of the model are verified by experimental data from silicon p⁺n diodes, in which deep levels have been induced by electron irradiation. It is shown that the deep level activation energies may be estimated from the forward bias capacitance-voltage characteristics, yielding values which agree well with those obtained by established techniques.     

Thermal emission rates and capture cross sections of majority carriers at vanadium centers in silicon

The thermal emission rates and capture cross sections of majority carriers on the vandium associated centers in the depletion region of reverse biased silicon p-n junctions have been measured by the dark capacitance transient method. The three vanduim associated levels observed, two donor levels and a deep acceptor level, belong to the same vandium center. Least square fits of the emission data give the following emission rates; e_nl^t = 1.047 × 10⁶T² exp [?0.179±0.004 eV/kT], e_n0^t = 3.55 × 10⁷T² exp [?0.426±0.004 eV/kT] and e_p-2^t = 1.514 × 10⁶T² exp [?0.450±0.003 eV/kT]. The activation energy of the hole emission rate at the lower donor level is about 0.1 eV larger than the equilibrium thermal activation energy. The capture cross sections are σ_n0 = 3 × 10^?17cm² and σ_p0 = 8 × 10^?16cm² for the electron capture process at the deep acceptor level and the hole capture process at the upper donor level, respectively. The hole capture cross section on the lower donor level (σ_p-1) depends significantly on temperature. The large temperature dependence of the hole capture cross section can be expected due to the nonradiative multiphonon emission process.     

Comparative study of admittance spectroscopy and DLTS in determining trap levels of CdTe-ZnTe heterojunctions

A study of deep levels in the depletion region of an n⁺-CdTe-p-ZnTe heterojunction has been made using admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS) methods. Four trap level signals were observed by both As and DLTS methods. Activation energies, trap densities and capture cross sections of the shallowest and the deepest trap levels of 0.12 and 1.0 eV, determined from AS data, agree well with DLTS results. These are identified as hole traps by DLTS. A reasonable agreement is also found in case of the electron trap level at 0.24 eV. The trap level with an activation energy of 0.58 eV observed from AS has been shown to be a hole type and exhibits a complex behavior in DLTS. The mechanism of the 0.58 eV trap level is discussed in this paper.     

On the origin of the luminescence band with hνm=1.5133 eV in gallium arsenide

The excitation-level dependence of intensities of the luminescence bands with hν_m=1.5133, 1.5141, and 1.5153 eV in semi-insulating GaAs crystals at 4.2 K was examined. The dependences obtained for all three bands are identical. The analysis of these results indicates that, in this material, the luminescence band with hν_m=1.5133 eV is related to the annihilation of the exciton-impurity complexes D ⁺ X (excitons X being bound to ionized shallow donors D ⁺).     

硅中钛的若干物理性质

本文报道研究扩散掺钛的硅中深能级的结果。用DLTS法观测到三个与钛有关的深能级,即在n-Si(Ti)中有二个电子陷阱,能级位置分别为Ec0.23eV和Ec0.53eV,在p-Si(Ti)中有一个空穴陷阱,能级位置为Ev+0.32eV。详细的电容瞬态研究得到了这些能级在一定测试温度范围内的热激活能和俘获截面以及其它有关参量。本文还就测量结果对能级的键合性质和钉扎于那一能带做了讨论。     

Mechanism of reverse current in the Al/p-InP schottky diodes

Reverse current-voltage characteristics of the Al/p-InP Schottky diodes based on Zn-doped InP epilayers were measured in relation to bias and temperature. Temperature dependence of reverse current is characterized by the activation energies of 0.75 and 0.51 eV in the high-temperature region and at temperatures T<280 K, respectively. Results are explained by the phonon-assisted tunneling generation of charge carriers from the surface states of a semiconductor with regard to the Frenkel emission mechanism. It is found that, in the low-temperature region, tunneling occurs via the centers with energy levels of 0.51 eV. Comparing experimental results with theory, we estimated electric-field strength in the barrier at (5–13)×10⁷ V/m and the surface density of the hole charge in the boundary layer of the semiconductor.