Electronic characteristics of InAs/GaAs self-assembled quantum dots by deep level transient spectroscopy

A deep level transient spectroscopy technique has been used to determine the emission activation energies and capture barriers for electrons and holes in InAs self-assembled quantum dots embedded in GaAs. The ground electron and hole energies relative to their respective energy band edges of GaAs are 0.13 and 0.09 eV. Measurements show that the capture cross section of quantum dots is thermally activated. The capture barrier of quantum dots for electrons and holes are 0.30 and 0.26 eV, respectively. The results fit well with the results of photoluminescence spectroscopy measurements. (C) 2000 Elsevier Science B.V. All rights reserved.

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.

Effect of Si overgrowth on the structural and luminescence properties of Ge islands on Si(100)

The effect of Si overgrowth on the structural and luminescence properties of strained Ge layer grown on Si(1 0 0) is studied. Capping Si leads to the dissolution of Ge island apex and reduced island height. The structural changes in island shape, especially in chemical composition during Si overgrowth have a large effect on the PL properties. The integrated PL intensity of Ge layer increases and there are large blue shifts in peak energies after capping Si. The PL spectra from buried Ge layer are consistent with type-II band alignment in SiGe/Si. We show that the PL properties from buried Ge layer may be tailored by modifying the cap layer growth conditions as well as post-growth annealing. (C) 1999 Elsevier Science B.V. All rights reserved.

Two-dimensional ordering of self-assembled InxGal-xAs quantum dots grown on GaAs(311)B surfaces

Two-dimensional (2D) ordering of self-assembled InxGa1-xAs quantum dots (QDs) fabricated on GaAs(311)B surface by molecular beam epitaxy (MBE) are reported. The QDs are aligned into rows deferring from the direction of the misorientation of the substrate, and strongly dependent on the mole In content x of InxGa1-xAs solid solution. The ordering alignment deteriorates significantly as the In content is increased to above 0.5. The 2D ordering can be described as a centered rectangular unit mesh with the two sides parallel to [01 (1) over bar] and [(2) over bar 33], respectively. Their relative arrangement seems to be determined by a combination of the strongly repulsive elastic interaction between neighbouring islands and the minimization of the strain energy of the whole system. The ordering also helps to improve the size homogeneity of the InGaAs islands. Photoluminescence (PL) result demonstrates that QDs grown on (311)B have the narrowest linewidth and the strongest integrated intensity, compared to those grown on (100) and other high-index planes under the same condition. (C) 1999 Elsevier Science B.V. All rights reserved.

Effect of Si doping on cubic GaN films grown on GaAs(100)

Epitaxial layers of cubic GaN have been grown by metalorganic vapor-phase epitaxy (MOVPE) with Si-doping carrier concentration ranging from 3 x 10(18) to 2.4 x 10(20)/cm(3). Si-doping decreased the yellow emission of GaN. However, the heavily doped n-type material has been found to induce phase transformation. As the Si-doping concentration increases, the hexagonal GaN nanoparticles increase. Judged from the linewidth of X-ray rocking curve, Si-doping increases the density of dislocations and stacking faults. Based on these observations, a model is proposed to interpret the phase transformation induced by the generated microdefects, such as dislocations and precipitates, and induced stacking faults under heavy Si-doping. (C) 1999 Elsevier Science B.V. All rights reserved.

Two-dimensional ordering of self-assembled InxGa1-xAs quantum dots grown on GaAs(311)B surfaces

The two-dimensional (2D) ordering of self-assembled InxGa1-xAs quantum dots (QDs) fabricated on GaAs(3 1 1)B surface by molecular beam epitaxy (MBE) are reported. The QDs are aligned into rows differing from the direction of the misorientation of the substrate, and strongly dependent on the mole In content x of InxGa1-As-x solid solution. The ordering alignment deteriorates significantly as the In content is increased to above 0.5. The 2D ordering can be described as a centered rectangular unit mesh with the two sides parallel to [0 1 (1) over bar] and [(2) over bar 3 3], respectively. Their relative arrangement seems to be determined by a combination of the strongly repulsive elastic interaction between the neighboring islands and the minimization of the strain energy of the whole system. The ordering also helps to improve the size homogeneity of the InGaAs islands. The photoluminescence (PL) result demonstrates that QDs grown on (3 1 1)B have the narrowest linewidth and the strongest integrated intensity, compared to those on (1 0 0) and other high-index planes under the same condition. (C) 1999 Elsevier Science B.V. All rights reserved.

Fabrication of InGaAs quantum dots with an underlying InGaAlAs layer on GaAs(100) and high index substrates by molecular beam epitaxy

In this paper, InGaAs quantum dots with an adjusting InGaAlAs layer underneath are grown on (n 1 1)A/B (n = 2-5) and the reference (1 0 0) substrates by molecular beam epitaxy. Small and dense InGaAs quantum dots are formed on (1 0 0) and (n 1 1)B substrates. A comparative study by atomic force microscopy shows that the alignment and uniformity for InGaAs quantum dots are greatly improved on(5 1 1)B but deteriorated on (3 1 1)B surface, demonstrating the great influence of the buried InGaAlAs layer. There is an increase in photoluminescence intensity and a decrease in the full-width at half-maximum when n varies from 2 to 5. Quantum dots formed on (3 1 1)A and (5 1 1)A surfaces are large and random in distribution, and no emission from these dots can be detected. (C) 1999 Elsevier Science B.V. All rights reserved.

Self-organization of wire-like InAs nanostructures on InP

The initial InAs growth on InP(1 0 0) during molecular beam epitaxy has been investigated. The as-grown islands were shaped like nanowires and formed dense arrays over the entire surface in the 3-6 monolayer InAs deposition range. The wires were oriented along the [(1) over bar 1 0] direction. Transmission electron microscopy images confirm that the wires are coherently grown on the substrates. Our results suggest that the coherent wire-shaped island formation may be a possible method to fabricate self-organized InAs nanowires. (C) 1999 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.

Electrical properties of GaN deposited on nitridated sapphire by molecular beam epitaxy using NH3 cracked on the growing surface

We have found that GaN epilayers grown by NH3-source molecular beam epitaxy (MBE) contain hydrogen. Dependent on the hydrogen concentration, GaN on (0001) sapphire can be either under biaxially compressive strain or under biaxially tensile strain. Furthermore, we notice that background electrons in GaN increase with hydrogen incorporation. X-ray photoelectron spectroscopy (XPS) measurements of the N1s region indicate that hydrogen is bound to nitrogen. So, the microdefect Ga...H-N is an effective nitrogen vacancy in GaN, and it may be a donor partly answering for the background electrons. (C) 1999 Elsevier Science B.V. All rights reserved.

Strain-induced morphological evolution and preferential interdiffusion in SiGe epitaxial film on Si(100) during high-temperature annealing

Strain relaxation in initially flat SiGe film on Si(1 0 0) during rapid thermal annealing is studied. The surface roughens after high-temperature annealing, which has been attributed to the intrinsic strain in the epilayers. It is interesting to find that high-temperature annealing also results in roughened interface, indicating the occurrence of preferential interdiffusion. It is suggested that the roughening at the surface makes the intrinsic strain in the epilayer as well as the substrate unequally distributed, causing preferential interdiffusion at the SiGe/Si interface during high-temperature annealing. (C) 1999 Elsevier Science B.V. All rights reserved.

Self-organization of the InGaAs GaAs quantum dots superlattice

The mechanism of self-organization of quantum dots (QDs) during the growth of InGaAs/GaAs multilayers on GaAs (1 0 0) was investigated with cross-sectional transmission electron microscopy (XTEM), and double-crystal X-ray diffraction (DCXD). We found that the QDs spacing in the first layer can affect the vertical alignment of QDs. There seems to exist one critical lateral QD spacing, below which merging of QDs with different initial size is found to be the dominant mechanism leading to perfect vertical alignment. Once the critical value of QDs spacing is reached, the InGaAs QDs of the first layer are simply reproduced in the upper layers. The X-ray rocking curve clearly shows two sets of satellite peaks, which correspond to the QDs superlattice, and multi-quantum wells (QW) formed by the wetting layers around QDs. (C) 1999 Elsevier Science B.V. All rights reserved.

Scanning electron microscope studies of cubic AlxGa1-xN films grown on GaAs(100) by metal organic vapor phase epitaxy (MOVPE)

Cubic AlGaN films were grown on GaAs(100) substrates by MOVPE. Scanning electron microscope and photoluminescence were used to analyze the surface morphology and the crystalline quality of the epitaxial layers. We found that both NH, and TEGa fluxes have a strong effect on the surface morphology of AlGaN films. A model for the lateral growth mechanism is presented to qualitatively explain this effect. The content of hexagonal AlGaN in the cubic AlGaN films was also related to the NH3 flux. (C) 1999 Elsevier Science B.V. All rights reserved.

Structural and optical characterization of InAs nanostructures grown on high-index InP substrates

The structural and optical properties of InAs layers grown on high-index InP surfaces by molecular beam epitaxy are investigated in order to understand the self-organization of quantum dots and quantum wires on novel index surfaces. Four different InP substrate orientations have been examined, namely, (1 1 1)B, (3 1 1)A, and (3 1 1)B and (1 0 0). A rich variety of InAs nanostructures is formed on the surfaces. Quantum wire-like morphology is observed on the (1 0 0) surface, and evident island formation is found on (1 1 1)A and (3 1 1)B by atomic force microscopy. The photoluminescence spectra of InP (1 1 1)A and (3 1 1)B samples show typical QD features with PL peaks in the wavelength range 1.3-1.55 mu m with comparable efficiency. These results suggest that the high-index substrates are promising candidates for production of high-quality self-organized QD materials for device applications. (C) 1999 Elsevier Science B.V. All rights reserved.

Structural characterization of InGaAs/GaAs quantum dots superlattice infrared photodetector structures

InGaAs/GaAs quantum dots (QDs) superlattice grown by molecular beam epitaxy (MBE) at different substrate temperatures for fabricating 8-12 mu m infrared photodetector were characterized by transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL). High-quality QDs superlattice can be achieved by higher growth temperature. Cross-sectional TEM shows the QDs in the successive layers are vertically aligned along growth direction. Interaction of partial vertically aligned columns leads to a perfect vertical ordering. With increasing number of bilayers, the average QDs size becomes larger in height and rapidly saturates at a certain value, while average lateral length nearly preserves initial size. This change leads to the formation of QDs homogeneous in size and of a particular shape. The observed self-organizations are attributed to the effect of strain distribution at QDs on the kinetic growth process. DCXRD measurement shows two sets of satellite peaks which corresponds to QDs superlattice and multi quantum wells formed by the wetting layers. Kinematical simulations of the wetting layers indicate that the formation of QDs is associated with a decrease of the effective indium content in the wetting layers. (C) 1999 Elsevier Science B.V. All rights reserved.