EFFECT OF DOPANT ON THE UNIFORMITY OF InAsSELF-ORGANIZED QUANTUM DOTS

于2010-11-23批量导入

Proof of InAs/GaAs self-organized quantum dot lasing and the experimental determination of local Strain effect on the band structures

Confirmation of quantum dot lasing have been given by photoluminescence and electro-luminescence spectra. Energy levels of QD laser are distinctively resolved due to band filling effect, and the lasing energy of quantum dot laser is much lower than quantum well laser. The energy barrier at InAs/GaAs interface due to the built-in strain in self-organized system has been determined experimentally by deep level transient spectroscopy (DLTS). Such barrier has been predicted by previous theories and can be explained by the apexes appeared in the interface between InAs and GaAs caused by strain.

Self-assembled quantum dots, wires and quantum-dot lasers

Molecular beam epitaxy-grown self-assembled In(Ga)As/GaAs and InAs/InAlAs/InP quantum dots (QDs) and quantum wires (QWRs) have been studied. By adjusting growth conditions, surprising alignment. preferential elongation, and pronounced sequential coalescence of dots and wires under specific condition are realized. The lateral ordering of QDs and the vertical anti-correlation of QWRs are theoretically discussed. Room-temperature (RT) continuous-wave (CW) lasing at the wavelength of 960 nm with output power of 3.6 W from both uncoated facets is achieved fi-om vertical coupled InAs/GaAs QDs ensemble. The RT threshold current density is 218 A/cm(2). A RT CW output power of 0.6 W/facet ensures at least 3570 h lasing (only drops 0.83 dB). (C) 2001 Elsevier Science B.V, All rights reserved.

Electronic characteristics of InAs self-assembled quantum dots

Deep-level transient spectroscopy and photoluminescence studies have been carried out on structures containing self-assembled InAs quantum dots formed in GaAs matrices. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the deep-level transient spectroscopy technique. From analysis of deep-level transient spectroscopy measurements it follows that the quantum dots have electron levels 130 meV below the bottom of the GaAs conduction band and heavy-hole levels at 90 meV above the top of the GaAs valence band. Combining with the photoluminescence results, the band structures of InAs and GaAs have been determined. (C) 2000 Elsevier Science B.V. All rights reserved.

The strain distributions and carrier's confining potentials of self-organized InAs/GaAs quantum dot

On the basis of the finite element approach, we systematically investigated the strain field distribution of conical-shaped InAs/GaAs self-organized quantum dot using the two-dimensional axis-symmetric model. The normal strain, the hydrostatic strain and the biaxial strain components along the center axis path of the quantum dots are analyzed. The dependence of these strain components on volume, height-over-base ratio and cap layer (covered by cap layer or uncovered quantum dot) is investigated for the quantum grown on the (001) substrate. The dependence of the carriers' confining potentials on the three circumstances discussed above is also calculated in the framework of eight-band k (.) p theory. The numerical results are in good agreement with the experimental data of published literature.

High power continuous-wave operation of self-organized In(Ga)As/GaAs quantum dot lasers

Quantum dot (QD) lasers are expected to have superior properties over conventional quantum well lasers due to a delta-function like density of states resulting from three dimensional quantum confinements. QD lasers can only be realized till significant improvements in uniformity of QDs with free of defects and increasing QD density as well in recent years. In this paper, we first briefly give a review on the techniques for preparing QDs, and emphasis on strain induced self-organized quantum dot growth. Secondly, self-organized In(Ga)As/GaAs, InAlAs/GaAlAs and InAs/InAlAs Qds grown on both GaAs and InP substrates with different orientations by using MBE and the Stranski-Krastanow (SK) growth mode at our labs are presented. Under optimizing the growth conditions such as growth temperature, V/III ratio, the amount of InAs, InxGa1-xAs, InxAl1-xAs coverage, the composition x etc., controlling the thickness of the strained layers, for example, just slightly larger than the critical thickness and choosing the substrate orientation or patterned substrates as well, the sheet density of ODs can reach as high as 10(11) cm(-2), and the dot size distribution is controlled to be less than 10% (see Fig. 1). Those are very important to obtain the lower threshold current density (J(th)) of the QD Laser. How to improve the dot lateral ordering and the dot vertical alignment for realizing lasing from the ground states of the QDs and further reducing the Jth Of the QD lasers are also described in detail. Thirdly based on the optimization of the band engineering design for QD laser and the structure geometry and growth conditions of QDs, a 1W continuous-wave (cw) laser operation of a single composite sheet or vertically coupled In(Ga)As quantum dots in a GaAs matrix (see Fig. 2) and a larger than 10W semiconductor laser module consisted nineteen QD laser diodes are demonstrated. The lifetime of the QD laser with an emitting wavelength around 960nm and 0.613W cw operation at room temperature is over than 3000 hrs, at this point the output power was only reduced to 0.83db. This is the best result as we know at moment. Finally the future trends and perspectives of the QD laser are also discussed.

Asymmetric quantum-confined Stark effects of hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots

Quantum-confined Stark effects in GaAs/AlxGa1-xAs 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 an electric field in different directions. In our calculation, the effect of finite offset, valence-band mixing, the effects due to the different effective masses of electrons and holes in different regions, and the real quantum dot structures are all taken into account. The results show that the electron and hole energy levels and the optical transition energies can cause blueshifts when the electric field is applied along the opposite to the growth direction. Our calculated results are useful for the application of hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots to photoelectric devices. (c) 2005 American Institute of Physics.

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.

Experiments and their analysis for self-assembly growth of GaN quantum dots via MOCVD

In recent years, growth of GaN-based materials-related quantum dots has become a hot topic in semiconductor materials research. Considerable efforts have been devoted to growth of self-assembled quantum dots of GaN-based materials via MOCVD (Metal Organic Chemical Vapor Deposition) and there are a lot of relevant literatures. There is, however, few review papers for the topic. In this paper, different experimental methods for fabrication of quantum dots of GaN-based materials via MOCVD are critically reviewed and the experimental conditions and parameters, which may affect growth of the quantum dots, are analyzed, with an aim at providing some critical reference for the related future experiment research.

Type I-type II transition of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots

The photoluminescence (PL) of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots has been measured at 15 and 80 K under hydrostatic pressure. The lateral size of the dots ranges from 7 to 62 nm. The emissions from the dots with 26, 52 and 62 nm size have a blue shift under pressure, indicating that these quantum dots have the normal type-I structure with lowest conduction band at the Gamma -valley. However, the PL peak of dots with 7 nm diameter moves to lower energy with increasing pressure. It is a typical character for the X-related transition. Then these small dots have a type-II structure with the X-valley as the lowest conduction level. An envelope-function calculation confirms that the Gamma -like exciton transition energy will rise above the X-like transition energy in the In0.55Al0.45As/Al0.5Ga0.5As structure if the dot size is small enough.

Optical properties of self-assembled ternary In(GA/Al)As quantum dots on (100) and (N 1 1)B InP substrates

In this paper, we investigated the self-assembled quantum dots formed on (100) and (N11)B (N = 2, 3, 4, 5) InP substrates by molecular beam epitaxy (MBE). Two kinds of ternary QDs (In0.9Ga0.1As and In0.9Al0.1As QDs) are grown on the above substrates; Transmission electron microscopy (TEM) and photoluminescence (PL) results confirm QDs formation for all samples. The PL spectra reveal obvious differences in integral luminescence, peak position, full-width at half-maximum and peak shape between different oriented surfaces. Highest PL integral intensity is observed from QDs on (411)B surfaces, which shows a potential for improving the optical properties of QDs by using high-index surface. (C) 2000 Elsevier Science B.V. All rights reserved.

Pressure behaviour of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots with multi-modal distribution in size

The pressure behaviour of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots (QDs) has been studied at 15 K in the pressure range of 0-1.3 GPa. The atomic force microscopy image shows that the QDs have a multi-modal distribution in size. Three emission peaks were observed in the photoluminescence (PL) spectra, corresponding to the different QD families. The measured pressure coefficients are 82, 93 and 98 meV GPa(-1) for QDs with average lateral size of 26, 52 and 62 nm, respectively. The pressure coefficient of small QDs is about 17% smaller than that of bulk In0.55Al0.45As An envelope-function calculation was used to analyse the effect of pressure-induced change of barrier height, effective mass and dot size on the pressure coefficients of QDs. The Gamma-X state mixing was also included in the evaluation of the reduction of the pressure coefficients. The results indicate that both the pressure-induced increase of effective mass and Gamma-X mixing respond to the decrease of pressure coefficients, and the Gamma-X mixing is more important for small dots. The calculated Gamma-X interaction potentials are 15 and 10 meV for QDs with lateral size of 26 and 52 nm, respectively. A type-II alignment for the X conduction band is suggested according to the pressure dependence of the PL intensities. The valence-band offset was then estimated as 0.15 +/- 0.02.

Effects of rapid thermal annealing on self-assembled InGaAs/GaAs quantum dots superlattice

Postgrowth rapid thermal annealing was used to study the relaxation mechanism and optical properties of InGaAs/GaAs self-assembled quantum dots superlattice grown by molecular beam epitaxy. It is found that a significant narrowing of the luminescence linewidth (from 80 to 42 meV) occurs together with about 86 meV blue shift at annealing temperature up to 950 degrees C. Double crystal X-ray diffraction measurements show that the intensity of the satellite diffraction peak, which corresponds to the quantum dots superlattice, decreased with the increasing annealing temperature and disappeared at 750 degrees C, but recovered and increased again at higher annealing temperatures. This behavior can be explained by two competing relaxation mechanisms; interdiffusion and favored migration. The study indicates that a suitable annealing treatment can improve the structural properties of the quantum dots superlattice. (C) 2000 Elsevier Science B.V. All rights reserved.

Self-assembled InAs and In0.9Al0.1As quantum dots on (001)InP substrates grown by molecular beam epitaxy (MBE)

InAs and In0.9Al0.1As self-assembled quantum dots have been grown by Stranski-Krastanow growth mode on In0.52Al0.48As lattice-matched on (0 0 1)InP substrates by MBE. The ternary In0.9Al0.1As dots on InP was demonstrated for the first time. The structural and optical properties were characterized using TEM and PL, respectively. Experimental results show that, a larger critical thickness is required for In0.9Al0.1As dots formation than for InAs dots, the In0.9Al0.1As dots show larger sizes and less homogeneity; some ordering in alignment can be observed in both InAs and In0.9Al0.1As dots, and In0.9Al0.1As dots give narrower luminescence than InAs dots. (C) 1999 Elsevier Science 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.