InAs self-assembled quantum dots grown on an InP (311)B substrate by molecular beam epitaxy

Self-assembled InAs quantum dots (QDs) have been grown by solid-source molecular beam epitaxy on a (311)B InP substrate. Transmission electron microscopy clearly shows that a high density of smaller InAs islands can be obtained by using such a high index substrate. After introducing a lattice-matched underlying In0.52Al0.24Ga0.24As layer, the InAs QDs are much more uniform in size and form two-dimensional well ordered arrays. The photoluminescence (PL) spectra also confirm that the InAs QDs grown on underlying In0.52Al0.24Ga0.24As have a better quality than those grown in the In0.52Al0.48As matrix. A simple calculation indicates that the redshift of the PL peak energy mainly results from InAs QDs on underlying In0.52Al0.24Ga0.24As of large size. (C) 2001 American Institute of Physics.

Structural anisotropy and optical properties of InxGa1-xAs quantum dots on GaAs(001)

InAs and InxGa1-xAs (x = 0.2 and 0.5) self-organized quantum dots (QDs) were fabricated on GaAs(0 0 1) by molecular beam epitaxy (MBE) and characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), acid photoluminescence polarization spectrum (PLP). Both structural and optical properties of InxGa1-xAs QD layer are apparently different from those of InAs QD layer. AFM shows that InxGa1-xAs QDs tend to be aligned along the [1 (1) over bar 0] direction, while InAs QDs are distributed randomly. TEM demonstrates that there is strain modulation along [1 1 0] in the InxGa1-xAs QD layers. PLP shows that In0.5Ga0.5As islands present optical anisotropy along [1 1 0] and [1 (1) over bar 0] due to structural and strain field anisotropy for the islands. (C) 2001 Elsevier Science B.V. All rights reserved.

Substrate dependence of InGaAs quantum dots grown by molecular beam epitaxy

A systematic study of self-organized In0.5Ga0.5As quantum dots (QDs) and islands grown by molecular beam epitaxy on (100) and (n11) A/B GaAs substrates is given, where n varies from 1 to 5. Low-temperature photoluminescence results show that the properties of the dots have a strong dependence on the substrate orientation as revealed by atomic force microscopy, consistent with the differences in size, shape, and distribution of QDs on different substrates. From (100) to (111) surface, the photoluminescence peak position of dots on B surfaces is found to blueshift more than that on A surfaces. QDs are also formed on (511) A surface. The positional distribution of these dots exhibits a wavy shape, which is related to the corrugated structure of this surface. Two kinds of islands are formed on (111) A surface, but further work is needed to explain the mechanism of these islands. (C) 2001 American Vacuum Society.

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].

Study of self-assembled InAs quantum dot structure covered by InxGa1-xAs(0 <= x <= 0.3) capping layer

InAs self-assembled quantum dots(QDs) covered by 3-nm-thick InxGa1-xAs(0 less than or equal tox less than or equal to0.3) capping layer have been grown on GaAs(100) substrate. Transmission electron microscopy shows that InGaAs layer reduces the strain in the InAs islands,and atomic force microscopy evidences the deposition of InGaAs on the top of InAs islands when x = 0.3.The significant redshift of the photoluminescence (PL) peak energy and the reduction of PL linewidth of InAs quantum dots covered by InGaAs are observed. In addition,InGaAs overgrowth layer suppresses the temperature sensitivity of PL peak energy. Based on our analysis, the strain-reduction and the size distribution of the InAs QDs are the main cause of the redshift and temperature insensitivity of the PL respectively.

Oblique alignment of columns of self-organized Ge/Si(001) islands in multilayer structure

Ge/Si multilayer structures with a bimodal distribution of the island spacing in the first layer have been investigated by atomic-force microscopy and transmission electron microscopy. Besides the vertical alignment, some oblique alignments of stacked islands are observed. The presence of the elastic interaction between islands is responsible for the oblique alignment of stacked islands. (C) 2000 American Institute of Physics. [S0003-6951(00)04644-1].

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.

Room temperature 1.55 mu m emission from InAs quantum dots grown on (001)InP substrate by molecular beam epitaxy

Self-assembled InAs nanostructures on (0 0 1)InP substrate have been grown by molecular beam epitaxy (MBE) and evaluated by transmission electron microscopy (TEM) and photoluminescence (PL). It is found that the morphologies of InAs nanostructures depend strongly on the underlying alloy. Through introducing a lattice-matched underlying InAlGaAs layer on InAlAs buffer layer, the InAs quantum dots (QDs) can be much more uniform in size and great improvement in PL properties can be attained at the same time. In particular, 1.55 mu m luminescence at room temperature (RT) can be realized in InAs QDs deposited on (0 0 1)InP substrate with underlying InAlGaAs layer. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

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].

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].