Electron g factors and optical properties of InAs quantum ellipsoids

The electronic structure, electron g factors and optical properties of InAs quantum ellipsoids are investigated, in the framework of the eight-band effective-mass approximation. It is found that the light-hole states come down in comparison with the heavy-hole states when the spheres are elongated, and become the lowest states of the valence band. Circularly polarized emissions under circularly polarized excitations may have opposite polarization factors to the exciting light. For InAs ellipsoids the length, which is smaller than 35 nm, is still in a strongly quantum-confined regime. The electron g factors of InAs spheres decrease with increasing radius, and are nearly 2 when the radius is very small. The quantization of the electron states quenches the orbital angular momentum of the states. Actually, as some of the three dimensions increase, the electron g factors decrease. As more dimensions increase, the g factors decrease more. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimension. The magnetic field along the z axis of the crystal structure causes linearly polarized emissions in the spheres, which emit unpolarized light in the absence of magnetic field.

Electronic structure and electron g factors of HgTe quantum dots

The electronic structure and electron g factors of HgTe quantum dots are investigated, in the framework of the eight-band effective-mass approximation. It is found that the electron states of quantum spheres have aspheric properties due to the interaction between the conduction band and valence band. The highest hole states are S (l = 0) states, when the radius is smaller than 9.4 nm. the same as the lowest electron states. Thus strong luminescence from H-Te quantum dots with radius smaller than 9.4 nm has been observed (Rogach et al 2001 Phys. Statits Solidi b 224 153). The bandgap of H-Te quantum spheres is calculated and compared with earlier experimental results (Harrison et al 2000 Pure Appl. Chem. 72 295). Due to the quantum confinement effect, the bandgap of the small HgTe quantum spheres is positive. The electron g factors of HgTe quantum spheres decrease with increasing radius and are nearly 2 when the radius is very small. The electron g factors of HgTe quantum ellipsoids are also investigated. We found that as some of the three dimensions increase, the electron g factors decrease. The more the dimensions increase, the more the g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimension.

Electronic states in InAs quantum spheres and ellipsoids

The eight-band effective-mass Hamiltonian of the free-standing narrow-gap InAs quantum ellipsoids is developed, and the electron and hole electronic structures as well as optical properties are calculated by using the model. The energies, wave functions and transition probabilities of quantum spheres as functions of the radius of quantum sphere R is presented. It is found that the energy levels do not vary as 1/R-2, which is caused by the coupling between the conduction and valence bands, and by the constant terms correspond to the spin-orbit splitting energy. The blueshifts of hole states depend strongly on the coupling from electron states, so that the order of hole states changes as has been predicted in experiment. The exciton binding energies are calculated, the calculated excitonic gaps as functions of the ground exciton transition energy are in good agreement with the photoluminescence measured spectra in details. Finally, the hole energy levels and the linear polarization factors in InAs quantum ellipsoids as functions of the aspect ratio are presented. The state 1S(Z up arrow)((1/2)) becomes the hole ground state when e is larger than 2.4. The saturation value of the linear polarization factors of the InAs long ellipsoids of diameter 2.0 nm is 0.86, in agreement with the experimental results.

Aluminium doping induced enhancement of p-d coupling in ZnO

Valence-band type Auger lines in Al doped and undoped ZnO were comparatively studied with the corresponding core level x-ray photoelectron spectrography (XPS) spectra as references. Then the shift trend of energy levels in the valence band was that p and p-s-d states move upwards but e and p-d states downwards with increasing Al concentration. The decreased energy of the Zn 3d state is larger than the increased energy of the 0 2p state, indicating the lowering of total energy. This may indicate that Al doping could induce the enhancement of p-d coupling in ZnO, which originates from stronger Al-O hybridization. The shifts of these states and the mechanism were confirmed by valence band XPS spectra and 0 K-edge x-ray absorption spectrography (XAS) spectra. Finally, some previously reported phenomena are explained based on the Al doping induced enhancement of p-d coupling.

Rashba spin splitting in biased semiconductor quantum wells

Rashba spin splitting (RSS) in biased semiconductor quantum wells is investigated theoretically based on eight-band k center dot p theory. We find that at large wave vectors, RSS is both nonmonotonic and anisotropic as a function of in-plane wave vector, in contrast to the widely used isotropic linear model. We derive an analytical expression for RSS, which can qualitatively reproduce such nonmonotonic behavior at large wave vectors. We also investigate numerically the dependence of RSS on the various band parameters and find that RSS increases with decreasing band gap and subband index, increasing valence band offset, external electric field, and well width. All these dependences can be qualitatively described by our analytical model.

Electronic structures of wurtzite ZnO and ZnO/MgZnO quantum well

The band structures of wurtzite ZnO are calculated using the empirical pseudopotential method (EPM). The 8 parameters of the Zn and O atom pesudopotential form factors with Schluter's formula are obtained. The effective mass parameters are extracted by using k.p Hamiltonian to fit the EPM results. The calculated band edge energies (E-g, E-A, E-B, and E-C) at Gamma point are in good agreement with experimental results. The ordering of ZnO at the top of valence band is found to be A(Gamma(7))-B(Gamma(9))-C(Gamma(7)) due to a negative spin-orbit (SO) splitting. Based on the band parameters obtained, the valence hole subbands of wurzite ZnO/MgxZn1-xO tensile-strained quantum wells (QWs) with different well widths and Mg compositions are calculated using 6-band k.p method. (c) 2005 Elsevier B.V. All rights reserved.

Violet/blue photoluminescence from CeO2 thin film

CeO2 thin film was fabricated by dual ion beam epitaxial technique. The violet/blue PL at room temperature and lower temperature was observed from the CeO2 thin film. After the analysis of crystal structure and valence in the compound was carried out by the XRD and XPS technique, it was inferred that the origin of CeO2 PL was due to the electrons transition from Ce4f band to O2p band and the defect level to O2p band. And these defects levels were located in the range of 1 eV around Ce4f band.

Study of optical properties in GaAs1-xSbx/GaAs single quantum wells

GaAsSb/GaAs single quantum wells (SQWs) grown by molecular beam epitaxy are studied by selectively-excited photoluminescence (SEPL) measurement. For the first time, we have simultaneously observed the PL, from both type I and type II transitions in GaAsSb/GaAs heterostructure in the SEPL. The two transitions exhibit different PL, behaviours under different excitation energy. As expected, the peak energy of type I emission remains constant in the whole excitation energy range we used, while type U transition shows a significant blue shift with increasing excitation energy. The observed blue shift is well explained in terms of electron-hole charge separation model at the interface. Time-resolved(TR) PL exhibits more type 11 characteristic of GaAsSb/GaAs QW. Moreover, the results of the excitation-power-dependent PL and TRPL provide more direct information on the type-II nature of the band alignment in GaAsSb/GaAs quantum-well structures. By combining the experimental results with some simple calculations, we have obtained the strained and unstrained valence band offsets of Q(v) = 1.145 and Q(v)(0) = 0. 76 in our samples, respectively.

Effect of InAs quantum dots on the Fermi level pinning of undoped-n(+) type GaAs surface studied by contactless electroreflectance

Self-assembled InAs quantum dots (QDs) have been fabricated by depositing 1.6, 1.8, 2.0 and 2.5 monolayer (ML) InAs on surfaces of the undoped-n(+) (UN+) type GaAs structure. Room temperature contactless electroreflectance (CER) was employed to study the built-in electric field and the surface Fermi level pinning of these QD-covered UN+ GaAs samples. The CER results show that 1.6 ML InAs QDs on GaAs do not modify the Fermi level, whereas for samples with more than 1.6 ML InAs coverage, the surface Fermi level is moved to the valence band maximum of GaAs by about 70 meV (which is independent of the InAs deposition thickness) compared to bare GaAs. It is concluded that the modification of InAs coverage on the Fermi level on the GaAs surface is due to the QDs, rather than to the wetting layer. (C) 2003 American Institute of Physics.

Pressure behavior of Te isoelectronic centers in ZnS : Te

ZnS:Te epilayers with Te concentration from 0.5% to 3.1% were studied by photoluminescence under hydrostatic pressure at 15 K. Two emission bands related to the isolated Te-1 and Te-2 pair isoelectronic centers were observed in the samples with Te concentrations of 0.5% and 0.65%. For the samples with Te concentrations of 1.4% and 3.1%, only the Te-2-related peak was observed. The pressure coefficients of all the Te-1-related bands were found to be unexpectedly much larger than that of the ZnS band gap. The pressure coefficients for all the Te-2-related bands are, however, rather smaller than that of ZnS band gap as usually observed. Analysis based on a Koster-Slater model indicates that an increase of the valence bandwidth with pressure is the main reason for the faster pressure shift of the Te-1 centers, and the huge difference in the pressure behavior of the Te-1 and Te-2 centers is due mainly to the difference in the pressure-induced enhancement of the impurity potential on the Te-1 and Te-2 centers. (C) 2002 American Institute of Physics.

Modulation spectroscopy of GaAs covered by InAs quantum dots

Contactless: electroreflectance has been employed at room temperature to study the Fermi level pinning at undoped-n(+) GaAs surfaces covered by 1.6 and 1.8 monolayer (ML) InAs quantum dots (QDs). It is shown that the 1.8 ML InAs QD moves the Fermi level at GaAs surface to the valence band maximum by about 70 meV compared to bare GaAs, whereas 1.6 ML InAs on GaAs does not modify the Fermi level, It is confirmed that the modification of the 1.8 ML InAs deposition on the Fermi level at GaAs surface is due to the QDs, which are surrounded by some oxidized InAs facets, rather than the wetting layer.

The PL "violet shift" of cerium dioxide on silicon

CeO2 thin film was fabricated by dual ion beam epitaxial technique. The phenomenon of PL violet shift at room temperature was observed, and the distance of shift was about 65 nm. After the analysis of crystal structure and valence in the compound were carried out by XRD and XPS technique, it was concluded that the PL shift was related with valence of cerium ion in the oxides. When the valence of cerium ion varied front tetravalence to trivalence, the PL peak position would move from blue region to violet region and the phenomenon of "violet shift" was observed.

Effects of piezoelectricity and spontaneous polarization on electronic and optical properties of wurtzite III-V nitride quantum wells

A theoretical model accounting for the macropolarization effects in wurtzite III-V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction-valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times. (C) 2001 American Institute of Physics.

Photoluminescence of rapid-thermal annealed Mg-doped GaN films

We report the investigation of temperature and excitation power dependence in photoluminescence spectroscopy measured in Mg-doped GaN epitaxial layers grown on sapphire by metalorganic chemical vapor deposition, The objective is to examine the effects of rapid-thermal annealing on Mg-related emissions. It is observed that the peak position of the 2.7-2.8 eV emission line is a function of the device temperature and annealing conditions, The phenomenon is attributed to Coulomb-potential fluctuations in the conduction and valence band edge and impurity levels due to the Mg-related complex dissociation. The blue shift of the 2.7-2.8 eV emission line with increasing excitation power provides clear evidence that a donor-acceptor recombination process underlies the observed emission spectrum. In addition, quenching of minor peaks at 3.2 and 3.3 eV are observed and their possible origin is discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.

Growth and photoluminescence of epitaxial CeO2 film on Si (111) substrate

A CeO2 film with a thickness of about 80nm was deposited by a mass-analysed low-energy dual ion beam deposition technique on an Si(111) substrate. Reflection high-energy electron diffraction and x-ray diffraction measurements showed that the film is a single crystal. The tetravalent state of Ce in the film was confirmed by x-ray photoelectron spectroscopy measurements, indicating that stoichiometric CeO2 was formed. Violet/blue light emission (379.5 nm) was observed at room temperature, which may be tentatively explained by charge transitions from the 4f band to the valence band of CeO2.