Photoluminescence study of hydrogen donors in ZnO

A photoluminescence study of hydrogenated ZnO bulk crystals is presented. Two excitonic recombination lines at 3362.8 and 3360.1 meV are assigned to hydrogen shallow donors. Experimental evidence is presented that the corresponding donor to the line at 3362.8 meV, previously labeled I₄, originates from hydrogen trapped within the oxygen vacancy, H_O. The line at 3360.1 meV was found to be due to hydrogen located at the bond-centered lattice site, H_BC. The corresponding shallow donor has an ionization energy of 53 meV.     

Effect of correlation between the shallow and deep metastable level subsystems on the excitonic photoluminescence spectra in <Emphasis Type="Italic">n</Emphasis>-GaAs

The excitonic photoluminescence spectra of GaAs epitaxial layers are studied. Changes in the relative arrangement of shallow and deep centers in the tetrahedral lattice are shown to bring about changes in the decay kinetics and the shape of the (D⁰, x) emission line (corresponding to an exciton bound to a shallow neutral donor). This change in the excitonic photoluminescence spectra is caused by dispersion in the exciton binding energy of shallow donors E_D, the dispersion being a result of the influence of the subsystem of deep metastable defects in n-GaAs crystals.     

On the interface states and series resistance profiles of (Ni/Au)–Al0.22Ga0.78N/AlN/GaN heterostructures before and after 60Co (γ-ray) irradiation

The values of interface states (N _SS) and series resistance (R _S) of (Ni/Au)–Al_0.22Ga_0.78N/AlN/GaN heterostructures were obtained from admittance and current–voltage measurements before and after 250 kGy ⁶⁰Co irradiation. The analyses of these data indicate that the values of capacitance and conductance decrease, as the R _S increases with increasing dose rate due to the generation of N _SS. The increase in R _S with increasing dose rate was attributed to two main models. According to the first model, it has been attributed to a direct decrease in the donor concentration in semiconductor material as a result of the elimination of shallow donor states. According to the second model, it is a result of irradiation because of the formation of deep acceptor centers in the semiconductor bulk, and electrons from the shallow donor centers are captured by these acceptors.     

First-principles study on electronic structures of BaWO4 crystals containing F-type color centers

The electronic structures of BaWO₄ crystals containing F-type color centers are studied within the framework of the fully relativistic self-consistent Dirac-Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded that F and F⁺ color centers have donor energy level in the forbidden band. The optical transition energies are 2.449 and 3.101 eV, which correspond to the 507 and 400 nm absorption bands, respectively. It is predicted that 400-550 nm absorption bands originate from the F and F⁺ color centers in BaWO₄ crystals.     

Thin-film aggregate color centers as media for frequency domain optical storage

Thin films of aggregate color centers have been produced by electron and ion irradiation of bulk alkali halide crystals. The transverse spatial distribution of the centers was controlled on a submicron scale using electron lithography. Photochemical hole burning has been accomplished for the first time in a thin film of color centers, using the 6070 Å zero-phononN ₁ line produced by ion and electron irradiation of NaF crystals.     

CaWO4晶体中F型色心电子结构的研究

运用相对论的密度泛函离散变分法(DV-Xα)研究了CaWO₄晶体中F型色心的电子结构. 计算结果表明，F和F⁺心在禁带中引入了新的施主能级；分析了晶体内可能存在的光学跃迁模式，并通过过渡态的方法计算了F，F⁺心跃迁到导带底的能量分别为1.92eV和2.42eV. 因此，从理论上推断了F和F⁺心在CaWO₄晶体中可能引起650nm和515nm的吸收，由此说明CaWO₄晶体中650nm和515nm吸收带起源于晶体中的F和F⁺心. 关键词： 4晶体')" href="#">CaWO₄晶体 +心')" href="#">F和F⁺心 DV-Xα     

Electronic structure and spatial distribution of the spin density of shallow nitrogen donors in the SiC lattice

The discovery of unique magnetooptical properties of paramagnetic centers in silicon carbide, which make it possible to control spins of small arrays of centers of atomic sizes to single centers at room temperatures, using the techniques of optical detection of the magnetic resonance, posed a number of problems, among which one of the main ones is the creation of conditions under which spin relaxation effects are minimized. As studies of properties of spin nitrogen-vacancy centers in diamond showed, the main contribution to spin relaxation is made by the interaction with nitrogen donors, being a major impurity in diamond. A similar problem exists for silicon carbide, since nitrogen donors are also basic background impurities. The objective of this work is to study the spatial distribution of the spin density of nitrogen donors in two basic silicon carbide polytypes, i.e., 4H-SiC and 6H-SiC, to use this information for minimizing the interaction of nitrogen donors with paramagnetic centers in silicon carbide. The results of the study are analyzed by magnetic resonance methods; the spin density distribution on the nearest coordination spheres of nitrogen donors occupying carbon sites in silicon carbide is determined. It is concluded that paramagnetic centers in the 4H-SiC polytype, including silicon vacancies, can be more stable to the interactions with unpaired donor electrons, since electrons are not localized on the coordination sphere closest to the paramagnetic center in this case.     

On the role of group I elements in ZnO

Diffusion of Li, Na and K into single-crystal substrates of ZnO was performed. We compare the results with ZnO epitaxial films doped with the respective elements during growth. The diffused and in-situ doped layers were studied using mass spectroscopy and low temperature photoluminescence spectroscopy. Li and Na are known to produce deep acceptor centers which give rise to shallow donor to deep acceptor recombinations in the visible spectral region. We will demonstrate that shallow acceptors are also introduced, having binding energies around 300 meV. A donor–acceptor pair recombination (zero phonon line at 3.05 eV) with LO phonon replica is observed. We further investigated bulk ZnO crystals which contained the deep Li acceptor by thermal treatments under H₂ atmospheres. With increasing annealing temperature shallow donors are introduced as monitored by EPR while the EPR signal of the neutral Li acceptors decreases. Quite unexpectedly, the shallow Li acceptor centre which is not present in the as-grown state is also created. PACS 71.55.Gs; 78.55.Et; 76.30.Da     

Probing of the shallow donor and acceptor wave functions in silicon carbide and silicon through an EPR study of crystals with a modified isotopic composition

The spatial distributions of the unpaired-electron wave functions of shallow N donors in SiC crystals and of shallow P and As donors in silicon crystals were determined by studying crystals with a modified content of the ²⁹Si and ¹³C isotopes having a nonzero nuclear magnetic moment. As follows from the present EPR and available ENDOR data, the distribution of donor electrons in SiC depends substantially on the polytype and position in the lattice; indeed, in 4H-SiC, the unpaired electrons occupy primarily the Si s and p orbitals, whereas in 6H-SiC these electrons reside primarily in the s orbitals of C. The electron distributions for the N donor in the hexagonal position, which has a shallow level close to that obtained for this material in the effective-mass approximation, and for the donor occupying the quasi-cubic position differ substantially. The EPR spectrum of N in quasi-cubic positions was observed to have a hyperfine structure originating from a comparatively strong coupling with the first two coordination shells of Si and C, which were unambiguously identified. The effective-mass approximation breaks down close to the N donor occupying the quasi-cubic position, and the donor structure and the donor electron distribution become less symmetric. In silicon, reduction of the ²⁹Si content brought about a substantial narrowing of the EPR line of the shallow P and As donors and an increase in the EPR signal intensity, as well as a noticeable increase in the spin-lattice relaxation time T₁. This offers the possibility of selectively studying these spectra by optically exciting a region of the crystal in order to shorten T₁ and thereby precluding EPR signal saturation only in the illuminated part of the material. This method may be used to advantage in developing materials for quantum computers based on donors in silicon and SiC.     

Photoluminescence and EPR of porous silicon formed on <Emphasis Type="Italic">n</Emphasis><Superscript>+</Superscript> and <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript> single crystals implanted with boron and phosphorus ions

The effect of combined doping by shallow donor and acceptor impurities on boosting the quantum yield of porous-silicon photoluminescence (PL) in the visible and near IR range was studied using phosphorus and boron ion implantation. Nonuniform doping of samples and subsequent oxidizing annealing were performed before and after porous silicon was formed on silicon single crystals strongly doped by arsenic or boron up to ≈10¹⁹ cm^?3. The concentration of known P_b centers of nonradiative recombination was controlled by electron paramagnetic resonance. It is shown that there is an optimal joined content of shallow donors and acceptors that provides a maximum PL intensity in the vicinity of the red part of the visible spectrum. According to estimates, the PL quantum yield in the transitional n ⁺⁺-p ⁺ or p ⁺⁺-n ⁺ layer of porous silicon increases by two orders of magnitude as compared to that in porous silicon formed on silicon not subjected to ion irradiation.     

Impurity centers in ZnSe:Na crystals

The influence of sodium impurity on photoluminescence (PL) spectra of ZnSe crystals doped in a growth process from a Se+Na melt is investigated. It is shown that the introduction of the impurity results in emergence of emission bands in the PL spectra due to the recombination of exciton impurity complexes associated with both donors and hydrogen-like acceptors. Apart from that, four bands generated by donor-acceptor pairs recombination and a band produced by electronic transitions from the conduction band to a shallow acceptor are discussed. As a result of the analysis it is concluded that Na impurity forms in ZnSe lattice Na_Zn hydrogen-like acceptors with activation energy of 105±3 meV, Na_i donor centers with activation energy of 18±3 meV, as well as Na_ZnV_Se and Na_iNa_Zn associative donors with activation energy of 35±3 and 52±9 meV, respectively.     

n‐type conductivity in sublimation‐grown AlN bulk crystals

AlN crystals grown by physical vapour‐phase transport in the presence of a SiC doping source possess n‐type conductivity. The net donor concentration attains up to mid 10¹⁷ cm^–3. The investigation reveals shallow donors forming an impurity band and acceptor‐like electron traps at about 0.5 eV below the conduction band edge. Thermal electron emission from these traps is responsible for the observed n‐type conductivity. The shallow donors are suggested to be due to Si atoms on Al sites. The majority of them is assumed to be compensated by deep acceptors in the lower half of the band gap. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

EPR and ENDOR Studies of Shallow Donors in SiC

Recent progress in the investigation of the electronic structure of the shallow nitrogen (N) and phosphorus (P) donors in 3C–, 4H– and 6H–SiC is reviewed with focus on the applications of magnetic resonance including electron paramagnetic resonance (EPR) and other pulsed methods such as electron spin echo, pulsed electron nuclear double resonance (ENDOR), electron spin-echo envelope modulation and two-dimensional EPR. EPR and ENDOR studies of the ²⁹Si and ¹³C hyperfine interactions of the shallow N donors and their spin localization in the lattice are discussed. The use of high-frequency EPR in combination with other pulsed magnetic resonance techniques for identification of low-temperature P-related centers in P-doped 3C–, 4H– and 6H–SiC and for determination of the valley–orbit splitting of the shallow N and P donors are presented and discussed.     

Shallow thermal donors in nitrogen-doped silicon single crystals

Czochralski-grown nitrogen-doped silicon crystals contain shallow thermal donors (STD) which are not present in reference crystals. In the course of annealing at 600 or 650°C, the STD concentration reaches saturation and this concentration scales with nitrogen content N as N^1/2. This implies that an STD includes only one nitrogen atom and that the most likely model of the STD defect is the NO_m complex of an interstitial nitrogen atom with m oxygen atoms. The number m is estimated as, on the average, m=3 from data on the temperature dependence of the equilibrium constant for the complex formation reaction     

Optical spectra of (CdF2)0.9(InF3)0.1 semiconductor solid solutions

The differences in the optical spectra of CdF₂:In semiconductors with bistable DX centers (concentrated (CdF₂)_0.9(InF₃)_0.1 solid solutions) and “standard” samples with a lower impurity concentration used to record holograms are discussed. In contrast to the standard samples, in which complete decay of two-electron DX states and transfer of electrons to shallow donor levels may occur at low temperatures, long-term irradiation of a (CdF₂)_0.9(InF₃)_0.1 solid solution by UV or visible light leads to decay of no more than 20% deep centers. The experimental data and estimates of the statistical distribution of electrons over energy levels in this crystal give the total electron concentration, neutral donor concentration, and concentration of deep two-electron centers to be ～5 × 10¹⁸ cm^?3, ～9 × 10¹⁷ cm^?3, and more than 1 × 10²⁰ cm^?3, respectively. These estimates show that the majority of impurity ions are located in clusters and can form only deep two-electron states in CdF₂ crystals with a high indium content. In this case, In³⁺ ions in a limited concentration (In³⁺ (～9 × 10¹⁷ cm^?3) are statistically distributed in the “unperturbed” CdF₂ lattice and, as in low-concentrated samples, form DX centers, which possess both shallow hydrogen-like and deep two-electron states.     

Deep impurity states and intrinsic defects in photorefractive Cd1−x FexTe crystals

AbstractStudies were made of the low-temperature optical and photoelectric properties of Cd_1−x Fe_xTe crystals (x=0.0038) which provided information on the optical quality of these crystals and the nature of their inhomogeneity, and also revealed deep Fe²⁺ impurity centers and singly charged acceptor complexes. It was established that these complexes, which include doubly charged cadmium vacancies and ionized donors, are anisotropic. It was shown that their anisotropy is determined by the nature of the donor atom and its position in the crystal lattice (at a cationic or anionic site). Since these crystals contain real deep impurity centers and acceptor complexes, a mechanism is proposed for the photorefractive effect in these crystals. It was observed for the first time that the photorefractive properties of CdTe crystals containing impurity 3d elements may exhibit anisotropy unrelated to that of the electrooptic effect. Fiz. Tverd. Tela (St. Petersburg) 40, 1216–1220 (July 1998)     

Thermal decay of photochromic color centers in CaF2, SrF2, and BaF2 crystals doped by La and Y impurities

The absorption spectra of photochromic centers in CaF₂, SrF₂, and BaF₂ crystals doped by La and Y impurities and thermal decay of the centers in the temperature range 80–600 K are investigated. Under low-temperature x-ray irradiation, ionized photochromic color (PC⁺) centers are generated in La- and Y-doped CaF₂ crystals and in a La-doped SrF₂ crystal. It is revealed that, upon heating of the CaF₂-LaF₃ crystal, PC⁺ centers are transformed into photochromic color (PC) centers. In the SrF₂-YF₃ crystal irradiated at room temperature, photochromic color centers are generated as well. All color centers decay at a temperature of approximately 600 K. After irradiation of the BaF₂-YF₃ crystal at a temperature of 80 K, absorption bands are observed at energies of 2.25 and 3.60 eV, which are related to neither PC centers nor PC⁺ centers.     

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.     

Energy levels from lattice defects in AgGaS2

The luminescence of single crystals of AgGaS₂ from shallow centers is studied. As-grown crystals and crystals annealed in S, Ga, or in vacuum, were used. We show that the S vacancy causes a donor level at 50 meV (probably due to a charge state of V_S) and that the cation vacancy (perhaps Ag) introduces an acceptor level at 110 meV. Two other levels are found: a donor level at 30 meV and an acceptor level at 200 meV. Their origin is unknown.     

Elektronenspin-Resonanz von Stickstoffzentren in Alkalihalogenid-Kristallen

Paramagnetic nitrogen centers are produced in nitrate doped single crystals of KCl, KBr, KJ, and NaCl by introduction of F-centers. The electron spin resonance is studied at low temperature. The hyperfine structure of the lines is caused by interaction with two N¹⁴ nuclei. The angle dependence of the resonance spectra is measured by rotating the crystals about several crystallographic axes. The results can be fitted to a spin-Hamiltonian with orthorhombic symmetry. The components of theg-tensor and theA-tensor are determined. The centers are believed to be N ₂ ^? -ions in negative ion vacancies.