Erbium in silicon carbide crystals: EPR and high-temperature luminescence

Erbium ions have been incorporated for the first time in bulk 6H-SiC crystals during growth, and they were unambiguously identified from the ¹⁶⁷Er EPR hyperfine structure. High-temperature luminescence of erbium ions at a wavelength of 1.54 μm has been detected. The observed luminescence exhibits an increase in intensity with increasing temperature. The observation of Er luminescence in 6H-SiC offers a promising potential for development of semiconductor light-emitting devices at a wavelength within the fiber-optics transparency window. Fiz. Tverd. Tela (St. Petersburg) 41, 38–40 (January 1999)     

ODMR study of recombination processes in ionic crystals and silicon carbide

The paper presents the results of optically detected magnetic resonance (ODMR) study of tunneling and photostimulated recombination processes in irradiated ionic crystals, observation of resonance effects using optically pumped F-centres, and investigation of spin-dependent recombination processes in silicon carbide doped with boron, aluminium, gallium and scandium.     

Observation of the triplet metastable state of shallow donor pairs in AlN crystals with a negative-U behavior: a high-frequency EPR and ENDOR Study

Theoretical predictions about the n-type conductivity in nitride semiconductors are discussed in the light of results of a high-frequency EPR an ENDOR study. It is shown that two types of effective-mass-like, shallow donors with a delocalized wave function exist in unintentionally doped AlN. The experiments demonstrate how the transformation from a shallow donor to a deep (DX) center takes place and how the deep DX center can be reconverted into a shallow donor forming a spin triplet and singlet states.     

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.     

Dependence of the energy of the resonance states of an acceptor in silicon on the host isotopic mass

It has been found that the energy of the transitions of acceptors to resonance states under a spin-split valence subband depends on the isotope content of the silicon matrix. An estimate of the isotope effect for the spin-orbit splitting of the valence band of silicon has been obtained.     

Effect of uniaxial stress on shallow acceptor states in silicon and germanium

Summary Energies and wave functions of shallow acceptor states under uniaxial stress have been computed in silicon and germanium. Calculations have been carried out up to high stress values within the effective-mass approximation. The full cubic crystal symmetry has been taken into account. The splittings of the ground state and of the odd-parity excited states, the relative intensities of optical transitions, as well as the energies of odd states for high stress values have been found to be in agreement with the results of optical measurements. However, results for even-parity excited states cannot as yet be compared with present experiments.

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Riassunto Le energie e le funzioni d'onda degli stati accettori poco prodondi sotto tensione uniassiale sono state calcolate per silicio e germanio. I calcoli sono stati fatti fino ad alti valori di tensione nell'approssimazione della massa efficace. è stata presa in considerazione la simmetria cubica completa del cristallo. Gli sdoppiamenti dello stato fondamentale, degli stati eccitati a parità dispari, le intensità relative delle transizioni ottiche, cosí come le energie degli stati dispari per alti valori di tensione sono in accordo con i risultati delle misurazioni ottiche. Tuttavia, i risultati per gli stati eccitati a parità pari non possono ancora essere confrontati con gli esperimenti attuali.

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Реэюме Вычисляются энергии и волновые функции мелких акцепторных состояний при одноосном напряжении в кремнии и германии. Вычисления проводятся в рамках приближения эффективной массы вплоть до больших значений напряжений. Учитывается полная кубическая симметрия кристалла. Получается, что расщепления основного состояния и нечетных возбужденных состояний, относительные интенсивности оптических переходов, а также энергии нечетных состояний при больших значениях напряжений согласуются с результатами оптических измерений. Однако, результаты для четных возбужденных состояний не могут быть еще сравнены с имеющимися экспериметами.

     

Structure of the ground state of electrons and holes in a silicon quantum dot with a shallow donor

The results of the calculation of electronic wave functions and spectra of electrons and holes in silicon quantum dots with a shallow impurity center of the donor type are presented. The fine splitting of energy levels induced by the Coulomb interaction of charge carriers with the donor and by the spin-orbit splitting in the valence band is obtained in the framework of the envelope function approximation. It is shown that the maximum removal of degeneracy is accomplished in the case of an asymmetric location of the donor inside the quantum dot when the directions of the donor radius vector do not coincide with the principal crystallographic directions.     

Interaction of nickel donor and acceptor excitons with defect vibrations in ZnSe:Ni crystals

Defect vibrations in ZnSe:Ni crystals induced by the Ni impurity with charge of ±1 relative to the neutral state are calculated by a recursive method in the shell model. This leads to an interpretation of vibronic structures in electroabsorption spectra for donor and acceptor Ni excitons in ZnSe:Ni crystals. Fiz. Tverd. Tela (St. Petersburg) 39, 2147–2151 (December 1997)     

Interstitial Mn as a new donor in GaP and GaAs: an EPR study

We report the observation of a new electron paramagnetic resonance centre in neutron-irradiated GaP and a similar new EPR centre in Mn-doped GaAs. Both centres have been identified as interstitial Mn and act as a donor. To our knowledge this is the first observation by EPR of an interstitial transition-metal impurity in a III-V compound. The implication of this new finding on the issue of Mn diffusion is discussed.     

Dynamics of photorefractive response in sillenite crystals with double-ionized donor centers and shallow traps

The dynamics of the photorefractive response in the Bi₁₂TiO₂₀ crystal with double-ionized donor centers and shallow traps is analyzed. Consideration is given to the time dependences of the recording of a photorefractive grating in the absence of an external electric field, its storage in the dark, and its development upon the application of an external field and turning on of a reading light beam.     

Probing the wave function of shallow Li and Na donors in ZnO nanoparticles

Electron paramagnetic resonance and electron nuclear double resonance (ENDOR) experiments on ZnO nanoparticles reveal the presence of shallow donors related to interstitial Li and Na atoms. The shallow character of the wave function is evidenced by the multitude of 67Zn ENDOR lines and further by the hyperfine interactions with the 7Li and 23Na nuclei that are much smaller than for atomic lithium and sodium. In the case of the Li-doped nanoparticles, an increase of the hyperfine interaction with the 7Li nucleus and with the 1H nuclei in the Zn(OH)(2) capping layer is observed when reducing the size of the nanoparticles. This effect is caused by the confinement of the shallow-donor 1s-type wave function that has a Bohr radius of about 1.5 nm, i.e., comparable to the dimension of the nanoparticles.     

EPR identification of the triplet ground state and photoinduced population inversion for a Si-C divacancy in silicon carbide

It is shown that intrinsic defects responsible for the semi-insulating properties of SiC represent Si-C divacancies in a neutral state (V_Si-V_C)⁰, which have the triplet ground state. The energy level scheme and the mechanism of creating the photoinduced population inversion of the triplet sublevels of the divacancy ground state are determined. It is concluded that there is a singlet excited state through which spin polarization is accomplished, and this fact opens the possibility of detecting magnetic resonance on single divacancies.     

The relaxation rate of the magnetic moment of a shallow acceptor center as a function of impurity concentration in silicon

A study is made into the temperature dependence of residual polarization of negative muons in crystalline silicon with the concentration of impurity of the n-and p-types ranging from 8.7×10¹³ to 4.1× 10¹⁸ cm^?3. The measurements are performed in a magnetic field of 1000 G transverse to the muon spin, in the temperature range from 4.2 to 300 K. The form of the temperature dependence of the relaxation rate v of the magnetic moment of the μAl⁰ acceptor in silicon is determined. For a nondegenerate semiconductor, the relaxation rate depends on temperature as v ∝ T ^q (q ≈ 3). A variation in the behavior of the temperature dependence and a multiple increase in the relaxation rate are observed in the range of impurity concentration in excess of 10¹⁸ cm^?3. The importance of phonon scattering and spin-exchange scattering of free charge carriers by an acceptor from the standpoint of relaxation of the acceptor magnetic moment is discussed. The constant of hyperfine interaction in an acceptor center formed by an atom of aluminum in silicon is estimated for the first time: |A _hf (Al)/2π| ～ 2.5×10⁶s^?1.     

Evidence for an excited level of the neutral indium acceptor in silicon

On the basis of the dependence of the photoluminescence on pump power, indium concentration and temperature we have determined that an excited level of the neutral indium acceptor exists 4.1 ± 0.1 meV above the ground state.     

An EPR study of the orientational transition in smectic liquid crystals in a magnetic field

Spin probe EPR spectroscopy is used to detect an orientational transition in the smectic mesophase in 4-n-octyl-4'-cyanobiphenyl (8CB) liquid crystal under the action of an intense magnetic field (5.87 T).     

EPR and ENDOR study of an oxygen-vacancy-associated Ti center in RbTiOPO4 crystals

The dominant Ti³⁺ trapped electron center in flux-grown RbTiOPO₄ (RTP) crystals has been characterized using electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR). This center is produced during an X-ray irradiation at room temperature when a Ti⁴⁺ ion traps an electron and becomes a Ti³⁺ ion, and is best studied in the 30–40 K range. The EPR spectrum contains a three-line hyperfine pattern from two nearly equivalent neighboring ³¹P nuclei, along with hyperfine lines from the ⁴⁷Ti and ⁴⁹Ti nuclei. The g matrix, determined from the angular dependence of the EPR spectrum, has principal values of 1.819, 1.889, and 1.947. Hyperfine matrices for four ³¹P nuclei are obtained from the angular dependence of the ENDOR spectrum. The proposed model for this defect is a Ti³⁺ ion adjacent to an oxygen vacancy at an OT position. Analogies are made to a similar Ti³⁺ center in KTiOPO₄ (KTP) crystals.     

Quantitative x-ray topographic analysis of the defects of 6H-SiC single crystals and homoepitaxial silicon carbide

Planar dislocation pileups (PDPs) and curvilinear dislocation segments (CDSs) are considered as indicators of the local elastic shear stress fields (LESSFs) that existed in the growing single crystals at the time of stabilization of their dislocation structure. Calculations using the theory of dislocations with the experimental parameters of PDPs and CDSs measured from the x-ray topograms (taken by the Lang and divergent-polychromatic-beam (DPB) methods) give values of the LESSFs in the range (0.2–1.5)×10⁶ Pa for thin single-crystal wafers of SiC (6H) grown by sublimation in a graphite container. A strong nonuniform bending of the single-crystal wafers is observed; for the x-ray topographic study of the dislocation structure in these wafers the DPB method is preferable to the Lang method on account of its low sensitivity to bending. Zh. Tekh. Fiz. 69, 64–67 (July 1999)     

Synthesis of epitaxial silicon carbide films through the substitution of atoms in the silicon crystal lattice: A review

A review of recent advances in the field of epitaxial growth of SiC films on Si by means of a new method of epitaxial substitution of film atoms for substrate atoms has been presented. The basic statements of the theory of the new method used for synthesizing SiC on Si have been considered and extensive experimental data have been reported. The elastic energy relaxation mechanism implemented during the growth of epitaxial SiC films on Si by means of the new method of substitution of atoms has been described. This method consists in substituting a part of carbon atoms for silicon matrix atoms with the formation of silicon carbide molecules. It has been found experimentally that the substitution for matrix atoms occurs gradually without destroying the crystalline structure of the matrix. The orientation of the film is determined by the “old” crystalline structure of the initial silicon matrix rather than by the silicon substrate surface only, as is the case where conventional methods are used for growing the films. The new growth method has been compared with the classical mechanisms of thin film growth. The structure and composition of the grown SiC layers have been described in detail. A new mechanism of first-order phase transformations in solids with a chemical reaction through an intermediate state promoting the formation of a new-phase nuclei has been discussed. The mechanism providing the occurrence of a wide class of heterogeneous chemical reactions between the gas phase and a solid has been elucidated using the example of the chemical interaction of the CO gas with the single-crystal Si matrix. It has been shown that this mechanism makes it possible to grow a new type of templates, i.e., substrates with buffer transition layers for growing wide-band-gap semiconductor films on silicon. A number of heteroepitaxial films of wide-band-gap semiconductors, such as SiC, AlN, GaN, and AlGaN on silicon, whose quality is sufficient for the fabrication of a wide class of micro- and optoelectronic devices, have been grown on the SiC/Si substrate grown by solid-phase epitaxy.