Ab initio calculations for the neutral and charged O vacancy in sapphire

The energetics, lattice relaxation, and the defect-induced states of st single O vacancy in alpha-Al2O3 are studied by means of supercell total-energy calculations using a first-principles method based on density-functional theory. The supercell model with 120 atoms in a hexagonal lattice is sufficiently large to give realistic results for an isolated single vacancy (square). Self-consistent calculations are performed for each assumed configuration of lattice relaxation involving the nearest-neighbor Al atoms and the next-nearest-neighbor O atoms of the vacancy site. Total-energy data thus accumulated are used to construct an energy hypersurface. A theoretical zero-temperature vacancy formation energy of 5.83 eV is obtained. Our results show a large relaxation of Al (O) atoms away from the vacancy site by about 16% (8%) of the original Al-square (O-square) distances. The relaxation of the neighboring Al atoms has a much weaker energy dependence than the O atoms. The O vacancy introduces a deep and doubly occupied defect level, or an F center in the gap, and three unoccupied defect levels near the conduction band edge, the positions of the latter are sensitive to the degree of relaxation. The defect state wave functions are found to be not so localized, but extend up to the boundary of the supercell. Defect-induced levels are also found in the valence-band region below the O 2s and the O 2p bands. Also investigated is the case of a singly occupied defect level (an F+ center). This is done by reducing both the total number of electrons in the supercell and the background positive charge by one electron in the self-consistent electronic structure calculations. The optical transitions between the occupied and excited states of the: F and F+ centers are also investigated and found to be anisotropic in agreement with optical data.

Electronic structure of quantum spheres and quantum wires

The electronic structures of quantum spheres and quantum wires are studied in the framework of the effective-mass theory. The spin-orbital coupling (SOC) effect is taken into account. On the basis of the zero SOC limit and strong SOC limit the hole quantum energy levels as functions of SOC parameter lambda are obtained. There is a fan region in which the ground and low-lying excited states approach those in the strong SOC limit as lambda increases. Besides, some theoretical results on the corrugated superlattices (CSL) are given.

Electronic structures of the zinc-blende GaN/Ga1-xAlxN compressively strained superlattices and quantum wells

The electronic structures of the zinc-blende GaN/Ga0.85Al0.15N compressively strained superlattices and quantum wells are investigated using a 6 x 6 Hamiltonian model (including the heavy hole, light hole and spin-orbit splitting band). The energy bands, wavefunctions and optical transition matrix elements are calculated. It is found that the light hole couples with the spin-orbit splitting state even at the k=0 point, resulting in the hybrid states. The heavy hole remains a pure heavy hole state at k=0. The optical transitions from the hybrid valence states to the conduction states are determined by the transitions of the light hole and spin-orbit splitting states to the conduction states. The transitions from the heavy hole, light hole and spin-orbit splitting states to the conduction states obey the selection rule Delta n=0. The band structures obtained in this work will be valuable in designing GaN/GaAlN based optoelectronic devices. (C) 1996 Academic Press Limited

Verification of EL2 electronic absorption effect on charge transfer in semi-insulating GaAs

The electronic absorption of EL2 centers has been clarified to be related to the electron acid hole photoionizations, and the transition from its ground state to metastable state, respectively. Under an illumination with a selected photon energy in the near infrared region, these three processes with different optical cross sections will show different kinetics against the illumination time. It has recently been shown that the photosensitivity (measured under 1.25 eV illumination) of the local vibrational mode absorption induced by some deep defect centers in SI-GaAs is a consequence of the electron and hole photoionizations of EL2. This paper directly measures the kinetics of the electronic transition associated with EL2 under 1.25 eV illumination, which implies the expected charge transfer among different charge states of the EL2 center. A calculation based on a simple rate equation model is in good agreement with the experimental results.

Self-consistent calculation of electronic states in asymmetric double barrier structure

With contributions from both three-dimensional (3D) electrons in heavily doped contacts and 2D electrons in the accumulation layer, a self-consistent calculation based on effective mass theory is presented for studying the anomalous behaviour of the quasi-bound levels in the accumulation layer and that in the central well of an asymmetric double barrier structure (DBS). By choosing the thickness of the incident barrier properly, it is revealed that these two quasi-bound levels may merge into a unique bound level in the off-resonance regime which shows a very good 2D nature in contrast to the conventional picture for level crossing. An evident intrinsic I-V bistability is also shown. It is noticeable that the effect of charge build-up in the central well is so strong that the electric field in the incident barrier even decreases when the applied bias increases within the resonant region.

STATE MIXING IN INAS/GAAS QUANTUM DOTS AT THE PRESSURE-INDUCED GAMMA-X CROSSING

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Light-excited structural instability of a-Si : H.

With the accumulation of experimental data, it has been recognized by many that the light-induced metastable change of a-Si:H, Staebler-Wronski effect (SWE), may be related to a structural instability of the whole a-Si:H network. However, direct evidence of such a structural change is still lacking. In the present paper, the efforts of our laboratory in this direction will be reviewed, including the light-induced changes of Si-H bond absorption, low frequency dielectric response, and an apparent photo-dilation effect.

Photovoltage and luminescence study of stacked InAs/GaAs self-organized quantum dots

The ground and excited state excitonic transitions of stacked InAs self-organized quantum dots (QDs) in a laser diode structure are studied. The interband absorption transitions of QDs are investigated by non-destructive PV spectra, indicating that the strongest absorption is related to the excited states with a high density and coincides with the photon energy of lasing emission. The temperature and excitation (electric injection) intensity dependences of photoluminescence and electroluminescence indicate the influence of state filling effect on the luminescence of threefold stacked QDs. The results indicate that different coupling channels exist between electronic states in both vertical and lateral directions.

Electronic and vibrational Raman scattering in resonance with yellow luminescence transitions in GaN on sapphire substrate

We analyze low-temperature Raman and photoluminescence spectra of MBE-grown GaN layers on sapphire. Strong and sharp Raman peaks are observed in the low frequency region. These peaks, which are enhanced by excitation in resonance with yellow luminescence transitions, are attributed to electronic transitions related to shallow donor levels in hexagonal GaN. It is proposed that a low frequency Raman peak at 11.7 meV is caused by a pseudo-local vibration mode related to defects involved in yellow luminescence transitions. The dependence of the photoluminescence spectra on temperature gives additional information about the residual impurities in these GaN layers.

Investigation of excited states in 22Mg via resonant elastic scattering of 21Na+p and its astrophysical implications

The excited states in 22Mg have been investigated by the resonant elastic scattering of 21Na + p.A 4.0 MeV/nucleon 21Na beam was separated by the Center for Nuclear Study (CNS) radioactive ion beam separator (CRIB) and then used to bombard a thick (CH2)n target. The energy spectra of recoiled protons were measured at scattering angles of θc.m. ≈ 172◦, 146◦, and 134◦, respectively. A wide energy-range of excitation function in 22Mg (up to Ex ∼ 8.9 MeV) was obtained simultaneously with a thick-target method, and a state at 7.06 MeV was newly observed. The resonant parameters were deduced from an R-matrix analysis of the center-of-mass (c.m.) differential cross-section data with a SAMMY-M6-BETA code. The astrophysical resonant reaction rate for the 18Ne(α,p)21Na reactionwas recalculated based on the present parameters.Generally speaking,the present rates are much smaller than the previous ones.

Experimental study of two-proton correlated emission from S-29 excited states

An experiment of a S-29 beam bombarding a Au-197 target at an energy of 49.2 MeV/u has been performed to study the two-proton correlated emission from S-29 excited states. Complete-kinematics measurements were carried out in the experiment. The relative momentum, opening angle, and relative energy of two protons, as well as the invariant mass of the final system, were deduced by relativistic-kinematics reconstruction. The Si-27-p-p coincident events were picked out under strict conditions and the phenomenon of p-p correlations was observed among these events. The mechanisms of two-proton emission were analyzed in a simple schematic model, in which the extreme decay modes like He-2 cluster emission, three-body phase-space decay, and two-body sequential emission were taken into account. Associated with the Monte Carlo simulations, the present results show that two protons emitted from the excited states between 9.6 MeV and 10.4 MeV exhibit the features of He-2 cluster decay with a branching ratio of 29(-11)(+10)%.

Investigation of excited states in Mg-22 via resonant elastic scattering of Na-21+p and its astrophysical implications

The excited states in 22Mg have been investigated by the resonant elastic scattering of 21Na + p. A 4.0 MeV/nucleon 21Na beam was separated by the Center for Nuclear Study (CNS) radioactive ion beam separator (CRIB) and then used to bombard a thick (CH2)n target. The energy spectra of recoiled protons were measured at scattering angles of θc.m. ≈ 172◦ , 146◦, and 134◦, respectively. A wide energy-range of excitation function in 22Mg (up to Ex ∼ 8.9 MeV) was obtained simultaneously with a thick-target method, and a state at 7.06 MeV was newly observed. The resonant parameters were deduced from an R-matrix analysis of the center-of-mass (c.m.) differential cross-section data with a SAMMY-M6-BETA code. The astrophysical resonant reaction rate for the 18Ne(α,p)21Na reactionwas recalculated based on the present parameters. Generally speaking, the present rates are much smaller than the previous ones.

Doubly excited 2s2p P-1,3(1) resonances in photoionization of helium

The multi-configuration Dirac Fock (MCDF) method is implemented to study doubly excited 2s2p P-1,3(1) resonances of the helium atom and the interference between photoionization and photo excitation autoionization processes. In order to reproduce the total photoionization sprectra, the excited energies from the ground 1s(2) S-1(0) state to the doubly excited 2s2p P-1,3(1) states and the relevant Auger decay rates and widths are calculated in detail. Further more, the interference profile determined by the so-called Fano parameters q and rho(2) is also reproduced. Good agreement is found between the present results and other available theoretical and experimental results. This indeed shows a promising way to investigate the Fano resonances in photoionization of atoms within the MCDF scheme, although there are some discrepancies in the present calculations of the 2s2p P-3(1) state.