Electronic states of InAs/GaAs quantum ring

In the framework of effective mass envelope function theory, the electronic states of the InAs/GaAs quantum ring are studied. Our model can be used to calculate the electronic states of quantum wells, quantum wires, and quantum dots. In calculations, the effects due to the different effective masses of electrons in rings and out rings are included. The energy levels of the electron are calculated in the different shapes of rings. The results indicate that the inner radius of rings sensitively changes the electronic states. The energy levels of the electron are not sensitively dependent on the outer radius for large rings. If decreasing the inner and outer radii simultaneously, one may increase the energy spacing between energy levels and keep the ground state energy level unchanged. If changing one of two radii (inner or outer radius), the ground state energy level and the energy spacing will change simultaneously. These results are useful for designing and fabricating the double colors detector by intraband and interband translations. The single electron states are useful for studying the electron correlations and the effects of magnetic fields in quantum rings. Our calculated results are consistent with the recent experimental data of nanoscopic semiconductor rings. (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.

The study on high efficient AlxGa1-xAs/GaAs solar cells

The investigation of AlxGa1-xAs/GaAs solar cells is carried out by means of both metalorganic chemical vapor deposition (MOCVD) and liquid-phase epitaxial (LPE) technique. The measurements of illuminated I-V characteristics, dark I-V characteristics and quantum efficiencies were performed for the GaAs solar cells made in author's laboratory. The measuring results revealed that the quality of materials in GaAs solar cell's structures is the key factor for getting high-efficient GaAs solar cells, but the effect of post-growth technology on the performances of GaAs solar cells is also very strong. The 21.95% (AM0, 2 x 2cm(2), 25 degreesC) high conversion efficiency in a typical GaAs solar cell has been achieved owing to improving the quality of materials as well as optimizing the post-growth technology of devices. (C) 2001 Elsevier Science B.V. All rights reserved.

Investigation of defects in low-temperature-grown GaAs using optical transient spectroscopy

Optical transient current spectroscopy (OTCS) has been used to investigate defects in the low-temperature-grown GaAs after postgrowth rapid thermal annealing (RTA). Two samples A and B were grown at 220 degreesC and 360 degreesC on (001) GaAs substrates, respectively. After growth, samples were subjected to 30s RTA in the range of 500-800 degreesC. Before annealing, X-ray diffraction measurements show that the concentrations of the excess arsenic for samples A and B are 2.5 x 10(19) and 1 x 10(19) cm(-3), respectively. It is found that there are strong negative decay signals in the optical transient current (OTC) for the annealed sample A. Due to the influence of OTC strong negative decay signals, it is impossible to identify deep levels clearly from OTCS. For a comparison, three deep levels can be identified for sample B before annealing. They are two shallower deep levels and the so-called As-Ga antisite defect. At the annealing temperature of 600 degreesC, there are still three deep levels. However, their structures are different from those in the as-grown sample. OTC strong negative decay signals are also observed for the annealed sample B. It is argued that OTC negative decay signals are related to arsenic clusters. (C) 2000 Elsevier Science B.V. All rights reserved.

Influence of dual incorporation of In and N on the luminescence of GaInNAs/GaAs single quantum wells

The optical properties of above- and below-band-edge transitions have been investigated by incorporating In atoms into GaNAs/GaAs single quantum wells. The experimental results show that with increasing In concentration the interband luminescence is improved and the luminescence intensity below the band edge in GaInNAs/GaAs decreases significantly. An interpretation is given that N atoms are preferable to form a covalent bond with In than with Ga atoms in a GaInNAs alloy, due to the compensation of the atomic-size difference between In and N atoms on the GaAs substrate. The photoreflectance spectra of the GaInNAs/GaAs single quantum well support the assignment of an intrinsic mechanism to the high-energy luminescence peak. (C) 2000 American Institute of Physics. [S0003- 6951(00)01752-6].

Effects of interdiffusion on the luminescence of InAs/GaAs quantum dots covered by InGaAs overgrowth layer

We have studied the effects of postgrowth rapid thermal annealing on the optical properties of 3-nm-height InAs/GaAs quantum dots covered by 3-nm-thick InxGa1-xAs (x = 0, 0.1, and 0.2) overgrowth layer. At higher annealing temperature (T greater than or equal to 750 degreesC), the photoluminescence peak of InGaAs layer has been observed at lower-energy side of the InAs quantum-dot peak. In addition, the blueshift in photoluminescence (PL) emission energy is found to he similar for all samples with increasing the annealing temperature from 650 to 850 degreesC. However, the trend of narrowing of photoluminescence linewidth is significantly different for InAs quantum dots with different In mole fractions in InGaAs overgrowth layer. These results suggest that the intermixing in the lateral direction plays an important role in helping to understand the modification of optical properties induced by rapid thermal annealing. (C) 2000 Elsevier Science B.V. All rights reserved.

Theoretical analysis and experimental study of Fourier transformation of Franz-Keldysh oscillations in GaAs

Fourier transformation (FT) has been used in the theoretical line shape analysis of Franz-Keldysh oscillations (FKOs) in detail by numerical simulations. FKOs from the surface-intrinsic-n(+) GaAs structure were obtained in photoreflectance (PR) measurements with various modulation light intensities and with different strengths of bias light illumination, which were used to change the static electric field in the intrinsic layer of the sample. The FT spectra of the PR spectra, including the real part, imaginary part, and the modulus, were very consistent with the theoretical line shapes. The ratio of the square root of the reduced mass (root mu (L)/root mu (H)) and the ratio of transition strength of the electron heavy hole to the electron light hole were obtained from the PT spectra. In addition, the electric field in the intrinsic layer of the sample without and with bias illumination and the modulation field induced by photomodulation were also obtained. (C) 2000 American Institute of Physics. [S0021-8979(00)02123-X].

Quantum-confined Stark effects of InAs/GaAs self-assembled quantum dot

Quantum-confined Stark effects in InAs/GaAs self-assembled quantum dots are investigated theoretically in the framework of effective-mass envelope function theory. The electron and hole energy levels and optical transition energies are calculated in the presence of perpendicular and parallel electric field. In our calculation, the effect of finite offset, valence band mixing, and strain are all taken into account. The results show that the perpendicular electric field weakly affects the electron ground state and hole energy levels. The energy levels are affected strongly by the parallel electric field. For the electron, the energy difference between the ground state and the first excited state decreases as electric field increases. The optical transition energies have clear redshifts in electric field. The theoretical results agree well with the available experimental data. Our calculated results are useful for the application of quantum dots to photoelectric devices. (C) 2000 American Institute of Physics. [S0021-8979(00)11001-7].

1.35 mu m photoluminescence from In0.5Ga0.5As/GaAs islands grown by molecular beam epitaxy via cycled (InAs)(1)/(GaAs)(1) monolayer deposition

1.35 mum photoluminescence (PL) with a narrow linewidth of only 19.2 meV at room temperature has been achieved in In0.5Ga0.5As islands structure grown on GaAs (1 0 0) substrate by solid-source molecular beam epitaxy. Atomic force microscopy (AFM) measurement reveals that the 16-ML-thick In0.5Ga0.5As islands show quite uniform InGaAs mounds morphology along the [ 1(1) over bar 0] direction with a periodicity of about 90 nm in the [1 1 0] direction. Compared with the In0.5Ga0.5As alloy quantum well (QW) of the same width, the In0.5Ga0.5As islands structure always shows a lower PL peak energy and narrower full-width at half-maximum (FWHM), also a stronger PL intensity at low excitation power and more efficient confinement of the carriers. Our results provide important information for optimizing the epitaxial structures of 1.3 mum wavelength quantum dots devices. (C) 2000 Elsevier Science B.V. All rights reserved.

Photoluminescence studies on pseudomorphic delta-doped AlGaAs/InGaAs/GaAs quantum wells

Photoluminescence (PL) measurements were performed on several series of single-side Si-doped pseudomorphic high electron mobility transistors (p-HEMTs) quantum well (QW) samples, with different spacer layer widths, well widths and Si delta -doped concentrations , under different temperatures and excitation power densities. The dynamic competitive luminescence mechanism between the radiations of e2-hh1 and e1-hh1 was discussed in detail. The confining potential, subband energies, corresponding envelope functions, subband occupations and transferring efficiency etc., were calculated by self-consistent finite differential method at different temperatures in comparison with the present experiment results. The relative variation of the integrated luminescence intensity of the two transitions (e1-hh1 and e2-hh1) was found to be dependent on the temperature and the structure's properties, e. g. spacer layer width, dopant concentration and well width.

Photoluminescence of InAs quantum dots grown on GaAs surface

InAs quantum dots (QDs) grown on GaAs surface are investigated. The observed abnormal photoluminescence (PL) properties, including extremely sharp high-energy peaks, almost temperature-independent linewidth, and fast thermal quenching, are discussed in terms of the strong quantum confinement effects due to the absence of a cap layer and the lack of carrier redistribution channel caused by the small number of QDs capable of contributing to PL and the high-density surface defects. (C) 2000 American Institute of Physics. [S0003-6951(00)01244-4].

Temperature dependence of electron redistribution in modulation-doped InAs/GaAs quantum dots

In this work we report the photoluminescence (PL) and interband absorption study of Si-modulation-doped multilayer InAs/GaAs quantum dots grown by molecular beam epitaxy (MBE) on (100) oriented GaAs substrates. Low-temperature PL shows a distinctive double-peak feature. Power-dependent PL and transmission electron microscopy (TEM) confirm that they stem from the ground states emission of islands of bimodal size distribution. Temperature-dependent PL study indicates that the family of small dots is ensemble effect dominated while the family of large dots is likely to be dominated by the intrinsic property of single quantum dots (QDs). The temperature-dependent PL and interband absorption measurements are discussed in terms of thermalized redistribution of the carriers among groups of QDs of different sizes in the ensemble. (C) 2000 Elsevier Science B.V. All rights reserved.

Effects of seed layer on the realization of larger self-assembled coherent InAs/GaAs quantum dots

The size and shape evolution of self-assembled InAs quantum dots (QDs) influenced by 2.0 ML InAs seed layer has been systematically investigated for 2.0, 2.5, and 2.9 ML deposition on GaAs(100) substrate. Based on comparisons with the formation of large incoherent InAs islands on single-layer samples at late growth stage, the larger coherent InAs quantum dots at 2.9 ML deposition has been observed on the second InAs layer. A simple model analysis accounting for the surface strain distribution influenced by buried islands gives a stronger increment of critical QD diameter for dislocation nucleation on the second layer in comparison with the single-layer samples. Additionally, the inhibition of dislocation nucleation in InGaAs/GaAs large islands can also be explained by our theoretical results. (C) 2000 American Institute of Physics. [S0021-8979(00)08922-2].

Formation of InAs quantum dots on low-temperature GaAs epi-layer

InAs self-organized quantum dots (QDs) grown on annealed low-temperature GaAs (LT-GaAs) epi-layers and on normal temperature GaAs buffer layers have been compared by transmission electron microscopy (TEM) and photoluminescence (PL) measurements. TEM evidences that self-organized QDs were formed with a smaller size and larger density than that on normal GaAs buffer layers. It is discussed that local tensile surface strain regions that are preferred sites for InAs islands nucleation are increased in the case of the LT-GaAs buffer layers due to exhibiting As precipitates. The PL spectra show a blue-shifted peak energy with narrower linewidth revealing the improvement of optical properties of the QDs grown on LT-GaAs epi-layers. It suggests us a new way to improve the uniformity and change the energy band structure of the InAs self-organized QDs by carefully controlling the surface stress states of the LT-GaAs buffers on which the QDs are formed. (C) 2000 Elsevier Science B.V. All rights reserved.

Influence of InxGa1-xAs (0 <= x <= 0.3) cap layer on structural and optical properties of self-assembled InAs/GaAs quantum dots

Optical and structural properties of self-organized InAs/GaAs quantum dots (QDs) with InxGa1-xAs or GaAs cover layers grown by molecular beam epitaxy (MBE) have been characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and photoluminescence (PL) measurements. The TEM and AFM images show that the surface stress of the InAs QDs was suppressed by overgrowth of a InxGa1-xAs covering layer on the top of the QDs and the uniformity of the QDs preserved. PL measurements reveal that red shifts of the PL emission due to the reduction of the surface strain of the InAs islands was observed and the temperature sensitivity of the PL emission energy was suppressed by overgrowth of InxGa1-xAs layers compared to that by overgrowth of GaAs layers.