Mg acceptor energy levels in AlxInyGa1-x-yN quaternary alloys: An approach to overcome the p-type doping bottleneck in nitrides

The Mg-Ga acceptor energy levels in GaN and random Al8In4Ga20N32 quaternary alloys are calculated using the first-principles band-structure method. We show that due to wave function localization, the MgGa acceptor energy level in the alloy is significantly lower than that of GaN, although the two materials have nearly identical band gaps. Our study demonstrates that forming AlxInyGa1-x-yN quaternary alloys can be a useful approach to lower acceptor ionization energy in the nitrides and thus provides an approach to overcome the p-type doping difficulty in the nitride system.

Theoretical analysis of gate voltage-controlled subband states in an AlxGa1-xN/GaN heterostructure

The subband structure and inter-subband transition as a function of gate voltage are determined by solving the Schrodinger and Poisson equations self-consistently in an AlxGa1-xN/GaN heterostructure. Different aluminum mole fraction and thickness of AlxGa1-xN barrier are considered. Calculation results show that energy difference between the first and second subband covers a wide range (from several tens to hundreds milli-electron volt) by applying different gate voltage, which corresponds to the midinfrared and long-wave infrared wavelength scope. Furthermore, such a modulation on the subband transition energy is much more pronounced for the structure with thin barrier. When the applied positive gate voltage is increased, the triangle well formed at the interface turns to be deeper and narrower, which enhances the confinement for electrons. As a result, the overlap between electron wave function at two subbands increases, and thus the optical intersubband transition also enhances its intensity. This tendency is in good agreement with the available data in the literature. (c) 2005 Elsevier B.V. All rights reserved.

Photoluminescence pressure coefficients of InAs/GaAs quantum dots

We have investigated the ground exciton energy pressure coefficients of self-assembled InAs/GaAs quantum dots by calculating 21 systems with different quantum dot shape, size, and alloying profile using the atomistic empirical pseudopotential method. Our results confirm the experimentally observed significant reductions of the exciton energy pressure coefficients from the bulk values. We show that the nonlinear pressure coefficients of the bulk InAs and GaAs are responsible for these reductions, and the percentage of the electron wave function on top of GaAs atoms is responsible for the variation of this reduction. We also find a pressure coefficient versus exciton energy relationship which agrees quantitatively with the experimental results. We find linear relationships which can be used to get the information of the electron wave functions from exciton energy pressure coefficient measurements.

Excitonic energy of vertically stacked self-assmbled InAs quantum dots

We have studied the exciton states in vertically stacked self-assembled quantum disks within the effective mass approximation. The energy spectrum of the electron and hole is calculated using the transfer matrix formalism in the adiabatic approximation. The Coulomb interaction between the electron and the hole is treated accurately by the direct diagonalization of the Hamiltonian matrix. The effect of the vertical alignment of the disks on the ground energy of heavy- and light-hole exciton is presented and discussed. The binding energy is discussed in terms of the probability of the ground wave function. The ground energy of heavy- and light-hole excitons as a function of the magnetic field is presented and the effect of the disk size (the radius of disks) on the exciton energy is discussed.

Unveiling the morphology of buried In(Ga)As nanostructures by selective wet chemical etching: From quantum dots to quantum rings

The three-dimensional morphology of In(Ga)As nanostructures embedded in a GaAs matrix is investigated by combining atomic force microscopy and removal of the GaAs cap layer by selective wet etching. This method is used to investigate how the morphology of In(Ga)As quantum dots changes upon GaAs capping and subsequent in situ etching with AsBr3. A wave function calculation based on the experimentally determined morphologies suggests that quantum dots transform into quantum rings during in situ etching. (c) 2007 American Institute of Physics.

Electronic structure of InSb quantum ellipsoids in an external magnetic field

The shape dependence of electronic structure, electron g factors in the presence of the external magnetic field of InSb quantum ellipsoids are investigated in the framework of eight-band effective-mass approximation. It is found that as the increasing aspect ratio e, the electron states with P character split into three doublets for the different physical interaction and the light-hole states with S character come up to the top of valence bands at e = 2.6 in comparison with the heavy-hole states. In the presence of the external magnetic field, the energy splits of electron states are different for their wave function distribution direction, and the hole ground state remain optical active for a suitable aspect ratio. The electron g factors of InSb spheres decrease with increasing radius, and have the value of about two for the smallest radius, about -47.2 for sufficiently larger radius, similar to the bulk material case. Actually, the electron g factors decrease as any one of the three dimensions increase. The more dimensions increase, the more g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimensions. (c) 2006 Elsevier B.V. All rights reserved.

Design of shallow acceptors in ZnO: First-principles band-structure calculations

p-type doping is a great challenge for the full utilization of ZnO as short-wavelength optoelectronic material. Due to a large electronegative characteristic of oxygen, the ionization energy of acceptors in ZnO is usually too high. By analyzing the defect wave-function character, we propose several approaches to lower the acceptor ionization energy by codoping acceptors with donor or isovalent atoms. Using the first-principles band-structure method, we show that the acceptor transition energies of V-Zn-O-O can be reduced by introducing F-O next to V-Zn to reduce electronic potential, whereas the acceptor transition energy of N-O-nZn(Zn) (n=1-4) can be reduced if we replace Zn by isovalent Mg or Be to reduce the anion and cation kinetic p-d repulsion, as well as the electronic potential.

Exciton states of vertically stacked self-assembled InAs quantum disks in an axial magnetic field

We have studied the exciton states of vertically stacked self-assembled quantum disks within the effective mass approximation. The ground energies of a heavy-hole and a light-hole excitons as functions of the vertical disk separation are presented and discussed. The transition energy of a heavy-hole ground-state exciton is calculated and compared with the experimental data. The binding energies are discussed in terms of the probability of ground wave function. The ground energies of a heavy-hole and a light-hole excitons as functions of the applied axial magnetic field are calculated and the effect of disk size (radius of disks) on exciton energies is discussed. (c) 2006 Elsevier B.V. All rights reserved.

Time-resolved photoluminescence spectra of self-assembled InAs/GaAs quantum dots

Two types of InAs self-assembled Quantum dots (QDs) were prepared by Molecular beam epitaxy. Atomic force microscopy (AFM) measurements showed that, compared to QDs grown on GaAs substrate, QDs grown on InGaAs layer has a significantly enhanced density. The short spacing (several nanometer) among QDs stimulates strong coupling and leads to a large red-shift of the 1.3 mu m photoluminescence (PL) peak. We study systematically the dependence of PL lifetime on the QDs size, density and temperature (1). We found that, below 50 K, the PL lifetime is insensitive to temperature, which is interpreted from the localization effects. As T increases, the PL lifetime increases, which can be explained from the competition between the carrier redistribution and thermal emission at higher temperature. The increase of carriers in QDs migrated from barriers and wetting layer (WL), and the redistribution of carriers among QDs enhance the PL lifetime as T increases. The thermal emission and non-radiative recombination have effects to reduce the PL lifetime at higher T. As a result, the radiative recombination lifetime is determined by the wave function overlapping of electrons and holes in QDs, and QDs with different densities have different PL lifetime dependence on the QDs size. (c) 2005 Elsevier B.V. All rights reserved.

Quantum fluctuation of dissipative mesoscopic capacitance coupling circuit

The quantum wave function and the corresponding energy levels of the dissipative mesoscopic capacitance coupling circuits are obtained by using unitary and linear transformations. The quantum fluctuation of charge and current in an arbitrary eigenstate of the system have been also given. The results show that the fluctuation of charge and current depends on not only the eigenstate but also the electronic device parameters.

InxGa1-xAs/AlyGa1-yAs/AlzGa1-zAs asymmetric step quantum-well middle wavelength infrared detectors

InxGa1-xAs/AlyGa1-yAs/AlzGa1-zAs asymmetric step quantum-well middle wavelength (3-5 mum) infrared detectors are fabricated. The components display photovoltaic-type photocurrent response as well as the bias-controlled modulation of the peak wavelength of the main response, which is ascribed to the Stark shifts of the intersubband transitions from the local ground states to the extended first excited states in the quantum wells, at the 3-5.3 mum infrared atmospheric transmission window. The blackbody detectivity (D-bb*) of the detectors reaches to about 1.0x10(10) cm Hz(1/2)/W at 77 K under bias of +/-7 V. By expanding the electron wave function in terms of normalized plane wave basis within the framework of the effective-mass envelope-function theory, the linear Stark effects of the intersubband transitions between the ground and first excited states in the asymmetric step well are calculated. The obtained results agree well with the corresponding experimental measurements. (C) 2001 American Institute of Physics.

Longitudinal optic phonon-plasmon coupling in delta-doped metamorphic InAlAs/InGaAs high-electron-mobility transistor structures on GaAs substrates

InAlAs/InGaAs metamorphic high-electron-mobility transistor structures with different spacer layers on GaAs substrates are characterized by Raman measurements. The influence of In0.52Al0.48As spacer thickness on longitudinal optic phonon-plasmon coupling is investigated. It is found that the intensity of GaAs-like longitudinal optic phonon, which couples with collective intersubband transitions of two-dimensional electron gas, is strongly affected by the different subband energy spacings, subband electron concentrations, and wave function distributions, which are determined by different spacer thicknesses. (C) 2001 American Institute of Physics.

Variational calculations of ionized-donor-bound excitons in GaAs-AlxGa1-xAs quantum wells

Using a two-parameter wave function, we calculate variationally the binding energy of an exciton bound to an ionized donor impurity (D+,X) in GaAs-AlxGa1-xAs quantum wells for the values of the well width from 10 to 300 Angstrom, when the dopant is located in the center of the well and at the edge of the well. The theoretical results confirm that the previous experimental speculation proposed by Reynolds tit al. [Phys. Rev. B 40, 6210 (1989)] is the binding energy of D+,X for the dopant at the edge of the well. in addition, we also calculate the center-of-mass wave function of the exciton and the average interparticle distances. The results are discussed in detail.

Binding energy of ionized-donor-bound excitons in the GaAs-AlxGa1-xAs quantum wells

The binding energy of an exciton bound to an ionized donor impurity (D+,X) located st the center or the edge in GaAs-AlxGa1-xAs quantum wells is calculated variationally for the well width from 10 to 300 Angstrom by using a two-parameter wave function, The theoretical results are discussed and compared with the previous experimental results.

Time-resolved photoluminescence spectra of self-assembled InAs/GaAs quantum dots

Two types of InAs self-assembled Quantum dots (QDs) were prepared by Molecular beam epitaxy. Atomic force microscopy (AFM) measurements showed that, compared to QDs grown on GaAs substrate, QDs grown on InGaAs layer has a significantly enhanced density. The short spacing (several nanometer) among QDs stimulates strong coupling and leads to a large red-shift of the 1.3 mu m photoluminescence (PL) peak. We study systematically the dependence of PL lifetime on the QDs size, density and temperature (1). We found that, below 50 K, the PL lifetime is insensitive to temperature, which is interpreted from the localization effects. As T increases, the PL lifetime increases, which can be explained from the competition between the carrier redistribution and thermal emission at higher temperature. The increase of carriers in QDs migrated from barriers and wetting layer (WL), and the redistribution of carriers among QDs enhance the PL lifetime as T increases. The thermal emission and non-radiative recombination have effects to reduce the PL lifetime at higher T. As a result, the radiative recombination lifetime is determined by the wave function overlapping of electrons and holes in QDs, and QDs with different densities have different PL lifetime dependence on the QDs size. (c) 2005 Elsevier B.V. All rights reserved.