Electron effective mass and resonant polaron effect in CdTe/CdMgTe quantum wells

Cyclotron resonance in CdTe/CdMgTe quantum wells (QWs) was studied. Due to the polaron effect the zero-field effective mass is strongly influenced by the QW width. The experimental data have been described theoretically by taking into account electron-phonon coupling and the nonparabolicity of the conduction band. The subband structure was calculated self-consistently. The best fit was obtained for an electron-phonon coupling constant alpha = 0.3 and bare electron mass of m(b) = 0.092m(0).

Strong three-level resonant magnetopolaron effect due to the intersubband coupling in heavily modulation-doped GaAs/AlxGa1-xAs single quantum wells at high magnetic-fields

Electron cyclotron resonance CR) measurements have been carried out in magnetic fields up to 32 T to study electron-phonon interaction in two heavily modulation-delta -doped GaAs/Al0.3Ga0.7As single-quantum-well samples. No measurable resonant magnetopolaron effects were observed in either sample in the region of the GaAs longitudinal optical (LO) phonons. However, when the CR frequency is above LO phonon frequency, omega (LO)=E-LO/(h) over bar, at high magnetic fields (B>27 T), electron CR exhibits a strong avoided-level-crossing splitting for both samples at frequencies close to (omega (LO)+ (E-2-E-1)1 (h) over bar, where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large with the minimum separation of 40 cm(-1) occurring at around 30.5 T. A detailed theoretical analysis, which includes a self-consistent calculation of the band structure and the effects of electron-phonon interaction on the CR, shows that this type of splitting is due to a three-level resonance between the second Landau level of the first electron subband and the lowest Landau level of the second subband plus one GaAs LO phonon. The absence of occupation effects in the final states and weak screening or this three-level process yields large energy separation even in the presence of high electron densities. Excellent agreement between the theory and the experimental results is obtained.

Optical properties of InGaAs quantum dots formed on InAlAs wetting layer

We have fabricated a new self-assembled quantum dot system where InGaAs dots are formed on InAlAs wetting layer and embedded in GaAs matrix. The low-temperature photoluminescence and atomic force microscopy measurements confirm the realization of the structure. In contrast to traditional InAs/Ga(Al)As quantum dots, the temperature dependence of the photoluminescence of the dots in such a structure exhibits an electronically decoupled feature due to a higher energy level of the wetting layer which keeps the dots more isolated from each other. (C) 2001 Published by Elsevier Science B.V.

Different transfer paths for thermally activated electrons and holes in self-organized Ge/Si(001) islands in a multilayer structure

We investigated the temperature dependence (10-250 K) of the photoluminescence (PL) emission spectrum of self-organized Ge/Si(001) islands in a multilayer structure. With elevated temperature, we find that the thermally activated holes and electrons are gathered by the Ge islands in different ways. The holes drift from the wetting layer into the islands, while the electrons, confined in Si due to type-II band alignment, leak into the Ge islands by the electrostatic interaction with the holes accumulated there. It results in an increase of the integrated intensity of island-related PL at a certain temperature range and a reduction of the phonon energy in the phonon-assisted PL of the islands by involving a type-I transition into a type-II transition. (C) 2001 American Institute of Physics.

Pressure dependence of Mn2+ fluorescence in ZnS : Mn2+ nanoparticles

The photoluminescence of Mn2+ in ZnS:Mn2+ nanoparticles with an average size of 4.5 nm has been measured under hydrostatic pressure from 0 to 6 GPa. The emission position is red-shifted at a rate of -33.3+/-0.6meV/GPa, which is in good agreement with the calculated value of -30.4meV/GPa using the crystal field theory. (C) 2000 Elsevier Science B.V. All rights reserved.

Temperature dependence of polaron cyclotron resonance mass in GaAs/AlGaAs heterostructures

The temperature dependence of polaron cyclotron resonance mass in GaAs/AlGaAs heterostructures is reinvestigated theoretically. By taking into account the electron-longitudinal-optic phonon interaction with temperature-dependent many-body effects, the conduction band non-parabolicity, and the influence of nonzero magnetic field, a good agreement with experiment is obtained.

Blocking of the polaron effect and spin-split cyclotron resonance in a two-dimensional electron gas

Cyclotron resonance (CR) of high density GaAs quantum wells exhibits well-resolved spin splitting above the LO-phonon frequency. The spin-up and spin-down CR frequencies are reversed relative to the order expected from simple band nonparabolicity. We demonstrate that this is a consequence of the blocking of the polaron interaction which is a sensitive function of the filling of the Landau levels.

Raman-forbidden mode and oxygen ordering in Bi2Sr2-xLaxCuO6+gamma single crystals annealed in oxygen

A Raman-forbidden phonon mode at about 840 cm(-1) is observed popularly on the surface of pun and La-doped Bi2Sr2-xLaxCuO6+y (0 less than or equal to x less than or equal to 0.8) single crystals annealed in oxygen. A remarkable excitation dependence of this additional line is found. Based on the properties of the structure of the Bi-O layer with excess oxygen atoms and the similarity in the appearance of the Raman-forbidden modes between RBa2Cu3Ox (R = Y, Nd, Gd, Pr) and Bi2Sr2-xLaxCuO6+y systems, we attribute the manifestation of this additional line to the ordering of the interstitial oxygen in the Bi-O layers. Our results provide Raman evidences for confirming that the ordering of the movable oxygen may exist universally in high-temperature superconductors.

Raman scattering of nanocrystalline silicon embedded in SiO2

Raman scattering of nanocrystalline silicon embedded in SiO2 matrix is systematically investigated. It is found that the Raman spectra can be well fitted by 5 Lorentzian lines in the Raman shift range of 100-600 cm(-1). The two-phonon scattering is also observed in the range of 600-1100 cm(-1) The experimental results indicate that the silicon crystallites in the films consist of nanocrystalline phase and amorphous phase; both can contribute to the Raman scattering. Besides the red-shift of the first order optical phonon modes with the decreasing size of silicon nanocrystallites, we have also found an enhancement effect on the second order Raman scattering, and the size effect on their Raman shift.

Nitrogen vacancy scattering in n-GaN grown by metal-organic vapor phase epitaxy

Electron mobility limited by nitrogen vacancy scattering was taken into account to evaluate the quality of n-type GaN grown by metal-organic vapor phase epitaxy. The nitrogen vacancy scattering potential used for our mobility calculation has to satisfy two requirements: such potential is (1) spatially short range, and (2) finite and not divergent at the vacancy core. A square-well potential was adopted to calculate the mobility, because it satisfies not only these two requirements, but also simplifies the calculation. As a result, the estimated mobility shows a T-1/2 temperature dependence, and is very sensitive to the potential well width. After introducing the nitrogen vacancy scattering, we obtained the best fitting between the calculated and experimental results for our high quality sample, and it was found that the measured mobility is dominated by ion impurity and dislocation scatterings at the low temperatures, but dominated by optical phonon and nitrogen vacancy scatterings at the high temperatures. (C) 2000 American Institute of Physics. [S0003-6951(00)04112-7].

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-delta-doped GaAs/Al0.3Ga0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E-LO at B > 27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E-LO + (E-2 - E-1), where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm(-1) around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LI, of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence: of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Cyclotron resonance mass of magnetopolaron in a polar crystal slab at finite temperatures

Using the Green function method, we have studied the cyclotron resonance of an electron interacting with bulk longitudinal optical(BO) phonons as well as surface optical(SO) phonons in a polar crystal slab at finite temperatures. It is found that the temperature dependence of magnetopolaron depends strongly on the strength of the magnetic field. The numerical results show that the cyclotron resonance mass of polaron in a slab is an increasing or decreasing function of temperature when the magnetic field is lower or higher than the resonant magnetic field region, respectively. The magnetic field and slab width dependence of cyclotron resonance mass are also studied in this paper. (C) 1999 Elsevier Science B.V. All rights reserved.

Optical properties of nanocrystalline silicon embedded in SiO2

Nanocrystalline silicon embedded SiO2 matrix has been formed by annealing the a-SiOx films fabricated by plasma enhanced chemical vapor deposition technique. Absorption and photoluminescence spectra of, the films have been studied in conjunction with micro-Raman scattering spectra. It is found that absorption presents an exponential dependence of absorption coefficient to photon energy in the range of 1.5-3.0 eV, and a sub-band appears in the range of 1.0-1.5 eV. The exponential absorption is due to the indirect band-to-band transition of electrons in silicon nanocrystallites, while the sub-band absorption is ascribed to transitions between surfaces and/or defect states of the silicon nanocrystallites. The existence of Stokes shift between absorption and photoluminescence suggests that the phonon-assisted luminescence would he enhanced due to the quantum confinement effects.

Raman scattering studies of III-V semiconductors grown on [n11]-oriented substrates

The room temperature Raman spectra of the Ga(0.5)Al(0.5)AS and the In0.52Al0.48As epilayer grown on [n11]-oriented substrates were measured in various back scatterng geometries, The relative intensity of TO modes and LO modes in those samples shows a regular Variation with differently oriented substrates in the experiments. By comparing experimental data with Raman scattering selection rules for the zincblende structure epilayer grown on [n11]-oriented substrates, it was found that the present calculations are in good agreement with the experimental results.

Analytical study of the layered XY model with VCE method

Motivated by experiments on liquid-crystal films, we study the development of specific heat anomaly of finite layer system. With the VCE method, we introduce the strong surface interaction into the layered XY model and get the results of the forth-order analytical expansion. The results show that when the strong surface interaction becomes strong enough, the order trend defeats the quantum noise and the specific heat peak moves abnormally to the high temperature with the number of layers decreasing.