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.

Study of rapid carrier capture and relaxation in InAs/GaAs heterostructures

The rapid carrier capture and relaxation processes in InAs/GaAs quantum dots were studied at 77K by using a simple degenerate pump-probe technique. A rising process was observed in the transient reflectivity, following the initial fast relaxation associated with GaAs bulk matrix, and this rising process was assigned to be related to the carrier capture from the GaAs barriers to InAs layers. The assignment was modeled using Kramers-Kronig relation. By analyzing the rising process observed in the transient reflectivity, the carrier capture time constants were obtained. The measured capture times decrease with the increase of carrier concentration.

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].

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.

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.

X-ray diffraction and optical characterization of interdiffusion in self-assembled InAs/GaAs quantum-dot superlattices

We report on the characterization of thermally induced interdiffusion in InAs/GaAs quantum-dot superlattices with high-resolution x-ray diffraction and photoluminescence techniques. The dynamical theory is employed to simulate the measured x-ray diffraction rocking curves of the InAs/GaAs quantum-dot superlattices annealed at different temperatures. Excellent agreement between the experimental curves and the simulations is achieved when the composition, thickness, and stress variations caused by interdiffusion are taken in account. It is found that the significant In-Ga intermixing occurs even in the as-grown InAs/GaAs quantum dots. The diffusion coefficients at different temperatures are estimated. (C) 2000 American Institute of Physics. [S0003-6951(00)02440-2].

Temperature dependence of photoluminescence of an n-i-p-i GaAs superlattice

Both the photoluminescence peaks corresponding to the vertical transitions and the nonvertical transitions in an n-i-p-i GaAs superlattice are clearly observed. The redshifts of the two peaks with increasing temperature are: discussed in terms of the temperature-dependent carrier separation effect.

Fourier transformation study of the Franz-Keldysh Oscillation in SIN+ GaAs structures

Fourier transformation (FT) method has been used in the theoretical lineshape analysis of the Franz-Keldysh Oscillation (FKO) in detail by numerical simulation. The FKO of a set of GaAs SIN+ samples was obtained in photoreflectance measurements. The FT spectra of a part of the samples,including of the real part,imaginary part, and mode of the FT,are well consistent with the theoretical lineshapes. The ratio of the square root of the reduced mass of the light hole (LH) to the heavy hole (HH), root mu(1)/root mu(h), obtained in the analysis was in the range of 0.805 to 0.816 for different samples. In addition,the built-in electric field F-1, and the modulation field delta F = F-1 - F-2 induced by photo-modulation were also obtained in the analysis. However,for a few samples great difference was found in the lineshape of the real part and imaginary part of their FT spectra from the theoretical lineshape. In this case the mode of the FT spectra still can be used to obtain useful information.

Self-assembled InAs/GaAs quantum dots and quantum dot laser

Systematic study of molecular beam epitaxy-grown self-assembled In(Ga)As/GaAs, In-AlAs/AlGaAs/GaAs, and InAs/InAlAs/InP quantum dots (QDs) is demonstrated. By adjusting growth conditions, surprising alignment, preferential elongation, and pronounced sequential coalescence of dots under the specific condition are realized. Room-temperature (RT) continuous-wave (CW) lasing at the wavelength of 960 nm with output power of 1 W is achieved from vertical coupled InAs/GaAs QDs ensemble. The RT threshold current density is 218 A/cm(2). An RT CW output power of 0.53 W ensures at least 3 000 h lasing (only drops 0.83 db). This is one of the best results ever reported.