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].

Two-dimensional ordering of self-assembled InAs quantum dots grown on (311)B InP substrate

Self-assembled InAs quantum dots (QDs) in InAlAs grown on (001) and (311)B InP substrates by molecular beam epitaxy (MBE) have been comparatively investigated. A correlated study of atomic force microscopy (AFM) and photoluminescence (PL) disclosed that InAs QDs grown on high-index InP substrates can lead to high density and uniformity. By introducing a lattice-matched InAlGaAs overlayer on InAlAs buffer, still more dense and uniform InAs QDs were obtained in comparison with InAs QDs formed with only InAlAs matrix. Moreover, two-dimensional well-ordered InAs dots with regular shape grown on (311)B InP substrates are reported for the first time. We explained this exceptional phenomenon from strain energy combined with kinetics point of view. (C) 2000 Elsevier Science B.V. All rights reserved.

Self-assembled InAs quantum wires on InP(001)

Self-assembled InAs quantum wires (QWRs) embedded in In0.52Al0.48As, In0.53Ga0.47As, and (In0.52Al0.48As)(n)/(In0.53Ga0.47As)(m)-short-period-lattice matrices on InP(001) were fabricated with molecular beam epitaxy (MBE). These QWR lines are along [110], x 4 direction in the 2 x 4 reconstructed (001) surface as revealed with reflection high-energy electron diffraction (RHEED). Alignment of quantum wires in different layers in the InAs/spacer multilayer structures depends on the composition of spacer layers. (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].

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.

Structural and optical properties of self-assembled InAs/GaAs quantum dots covered by InxGa1-xAs (0 <= x <= 0.3)

Optical and structural investigations of InAs quantum dots (QDs) covered by InxGa1-xAs (0 less than or equal to x less than or equal to 0.3) overgrowth layer have been systematically reported. The decrease of strain in the growth direction of InAs quantum dots covered by InGaAs layer instead of GaAs is demonstrated by transmission electron microscopy experiments. In addition, the atomic force microscopy measurement shows that the surface of InAs islands with 3-nm-thick In0.2Ga0.8As becomes flatter. However, the InGaAs islands nucleate on the top of quantum dots during the process of InAs islands covered with In0.3Ga0.7As. The significant redshift of the photoluminescence peak energy and reduction of photoluminescence linewidth of InAs quantum dots covered by InGaAs are observed. The energy gap change of InAs QDs covered by InGaAs could be explained in terms of reducing strain, suppressing compositional mixing, and increasing island height. (C) 2000 American Institute of Physics. [S0021-8979(00)04018-4].

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.

Study of self-assembled InAs quantum dots grown on low temperature GaAs epi-layer

InAs self-organized quantum dots (QDs) grown on annealed low temperature GaAs (LT-GaAs) epi-layer were investigated by transmission electron microscopy (TEM) and photoluminescence (PL) measurement. TEM showed that QDs formed on annealed LT-GaAs epi-layer have a smaller size and a higher density than QDs formed on normal GaAs buffer layer. In addition, the PL spectra analysis showed that the LT-GaAs epi-layer resulted in a blue shift in peak energy, and a narrower linewidth in the PL peak. The differences were attributed to the point defects and As precipitates in annealed LT-GaAs epi-layer for the point defects and As precipitates change the strain field of the surface. The results provide a method to improve the uniformity and change the energy band structure of the QDs by controlling the defects in the LT-GaAs epi-layer.

Structural and photoluminescence properties of In-0.9(Ga/Al)(0.1)As self-assembled quantum dots on InP substrate

Self-assembled In0.9Ga0.1As, In0.9Al0.1As, and InAs quantum dots (QD) were fabricated in an InAlAs matrix lattice-matched to an InP substrate by molecular beam epitaxy. Preliminary characterizations were performed using transmission electron microscopy, photoluminescence, and reflection high-energy electron diffraction. Experimental results reveal clear differences in QD formation, size distribution, and luminescence between the InAs and In-0.9(Ga/Al)(0.1)As samples, which show the potential of introducing ternary compositions to adjust the structural and optical properties of QDs on an InP substrate. (C) 2000 American Institute of Physics. [S0021-8979(00)10213-0].

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.

Photoluminescence studies of type-II self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots grown on (311)A GaAs substrate

We investigated the photoluminescence (PL) of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots (QDs) grown on (311)A GaAs substrate. The PL peak at 10 K shifts to lower energy by about 30 meV when the excitation power decreases by two orders of magnitude. It has a red-shift under pressure, that is the character of X-like transition. Moreover, its peak energy is smaller than the indirect gap of bulk Al0.5Ga0.5As and In0.55Al0.45As. We then attribute that peak to the type-II transition between electrons in X valley of Al0.5Ga0.5As and heavy holes in In0.55Al0.45As QDs. A new peak appears at the higher energy when temperature is increased above 70 K. It shifts to higher energy with increasing pressure, corresponding to the transition from conduction Gamma band to valence band in QDs. The measurements demonstrate that our In0.55Al0.45As/Al0.5Ga0.5As quantum dots are type-II QDs with X-like conduction-band minimum. To interpret the second X-related peak emerged under pressure, we discuss the X-valley split in QDs briefly. (C) 2000 American Institute of Physics. [S0003-6951(00)04622-2].

Effect of In-mole-fraction in InGaAs overgrowth layer on self-assembled InAs/GaAs quantum dots

We have studied the optical and structural properties of InAs/GaAs QDs covered by InxGa1-xAs (0 less than or equal to x less than or equal to 0.3) layer using transmission electron microscopy, photoluminescence (PL) spectra and atomic force microscopy. We find that the strain reduces in the growth direction of InAs islands covered by InGaAs instead of GaAs layer. Significant redshift of PL peak energy and narrowing of PL linewidth are observed for the InAs QDs covered by 3 nm thick InGaAs layer. In addition, atomic force microscopy measurements indicate that the InGaAs islands will nucleate on top of InAs quantum dots, when 3 nm In0.3Ga0.7As overgrowth layer is deposited. This result can well explain the PL intensify degradation and linewidth increment of quantum dots with a higher In-mole-fraction InGaAs layer. The energy gap change of InAs QDs covered by InGaAs may be explained in terms of reducing strain, suppressing compositional mixing and increasing island height. (C) 2000 Elsevier Science B.V. All rights reserved.

InAs self-assembled nanostructures grown on InP(001)

The 6-period stacked layers of self-assembled InAs quasi-quantum wires(qQWRs) and quantum dots(QDs) embedded into InAlAs on InP(001) substrates have been prepared by solid molecular beam epitaxy. The structures are characterized by atomic force microscopy(AFM) and transmission electron microscopy(TEM). From AFM we have observed for the first time that InAs qQWRs and QDs coexist, and we explained this phenomenon from the view of the energy related to the islands. Cross-sectional TEM shows that InAs qQWRs are vertically aligned every other layer along the growth direction [001], which disagrees with conventional vertical self-alignment of InAs QDs on GaAs substrate.