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.

The effect of substrate orientation on the morphology of InAs nanostructures on (001) and (11n)A/B(n=1-5) InP substrates

Strained InAs nanostructures have been grown by solid-source molecular beam epitaxy in In0.52Al0.48As matrix on different InP substrate surfaces ((0 0 1) and (1 1 n)A/B (n = 1 - 5)). The morphology of the nanostructures was characterized using atomic force microscopy (AFM). The AFM results reveal interesting differences in the size, shape, and alignment of the nanostructures between different oriented surfaces. It was found that some faceted nanostructures tend to form on A-type surfaces, the shape and the alignment of these nanostructures show clear dependence on the substrate orientation. Samples grown on (0 0 1) and B-type surfaces showed preferentially dense round dots. Dots formed on (1 1 3)B, (1 1 3)B and (1 1 5)B surfaces have a higher dot density and size homogeneity, which shows a potential for the production of high-quality and customized self-assembled quantum dots for photonics applications. (C) 2000 Elsevier Science B.V. All rights reserved.

Structural and optical characterization of InAs nanostructures grown on (001) and high index InP substrates

The effects of InP substrate orientations on self-assembled InAs quantum dots (QDs) have been investigated by molecular beam epitaxy (MBE). A comparison between atomic force microscopy (AFM) and photoluminescence (PL) spectra shows that a high density of smaller InAs islands can be obtained by using such high index substrates. On the other hand, by introducing a lattice-matched underlying In0.52Al0.24Ga0.24As layer, the InAs QDs can be much more uniform in size and have a great improvement in PL properties. More importantly, 1.55-mu m luminescence at room temperature (RT) can be realized in InAs QDs deposited on (001) InP substrate with underlying In0.52Al0.24Ga0.24As layer. (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.

Arsenic clusters on the surface of vertically aligned InAs islands on GaAs substrate by annealing

The formation of arsenic clusters in a system of vertically aligned InAs quantum islands on GaAs during thermal annealing under As overpressure has been investigated by transmission electron microscopy (TEM) and Raman scattering. Semicoherent arsenic clusters, identified by TEM examination, have been formed on the surface of the GaAs capping layer. The existence of arsenic precipitates is also confirmed by Raman spectra, showing new peaks from the annealed specimen at 256 and 199 cm(-1). These peaks have been ascribed to A(1g) and E-g Raman active phonons of crystalline arsenic. The phenomenon can be understood by a model of strain-induced selected growth under As overpressure. (C) 1999 American Institute of Physics. [S0003-6951(99)02045-8].

Effect of defects on optical phonon Raman spectra in SiC nanorods

Fabricated one-dimensional (1D) materials often have abundant structural defects. Experimental observation and numerical calculation indicate that the broken translation symmetry due to structural defects may play a more important role than the quantum confinement effect in the Raman features of optical phonons in polar semiconductor quantum wires such as SiC nanorods, (C) 1999 Elsevier Science Ltd. 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.

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.

Size quantization effects in InAs self-assembled islands on InP(001) at the onset of 2D-to-3D transition

Photoluminescence spectroscopy has been used to investigate self-assembled InAs islands in InAlAs grown on InP(0 0 1) by molecular beam epitaxy, in correlation with transmission electron microscopy. The nominal deposition of 3.6 monolayers of InAs at 470 degrees C achieves the onset stage of coherent island formation. In addition to one strong emission around 0.74 eV, the sample displaces several emission peaks at 0.87, 0.92. 0.98, and 1.04 eV. Fully developed islands that coexist with semi-finished disk islands account for the multipeak emission. These results provide strong evidence of size quantization effects in InAs islands. (C) 1999 Elsevier Science B.V. All rights reserved.

Growth mode and strain relaxation of InAs on InP (111)A grown by molecular beam epitaxy

Growth mode and strain relaxation of molecular-beam-epitaxy grown InAs/InAlAs/InP (111)A system have been investigated using reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy, and photoluminescence measurements. In direct contrast to the well-studied InAs/GaAs system, our experimental results show that the InAs grown on InAlAs/InP (111)A follows the Stranski-Krastanov mode. Both self-organized InAs quantum dots and relaxed InAs islands are formed depending on the InAs coverage. Intense luminescence signals from both the InAs quantum dots and wetting layer are observed. The luminescence efficiency of (111)A samples is comparable to that of (001) samples, suggesting the feasibility of fabricating quantum dot optoelectronic devices on InP (111)A surfaces. (C) 1999 American Institute of Physics. [S0003-6951(99)01010-4].

Analysis of atomic force microscopic results of InAs islands formed by molecular beam epitaxy

Atomic force microscopy (AFM) measurements of nanometer-sized islands formed by 2 monolayers of InAs by molecular beam epitaxy have been carried out and the scan line of individual islands was extracted from raw AFM data for investigation. It is found that the base widths of nanometer-sized islands obtained by AFM are not reliable due to the finite size and shape of the contacting probe. A simple model is proposed to analyze the deviation of the measured value From the real value of the base width of InAs islands. (C) 1998 Elsevier Science B.V. All rights reserved.

Asymmetry in the vertically aligned growth induced InAs islands in GaAs

A 10-InAs-island-layer vertically coupled quantum dot structure on (001) GaAs was grown and investigated by molecular beam epitaxy and transmission electron microscopy. The result shows that the vertically aligned InAs islands are asymmetrical along the two < 110 > directions on the (001) growth plane. Such an asymmetry in the vertically coupled quantum dot structure can be explained with the chemical polarity in the III-V compound semiconductors.

Photoluminescence from surface-oxidized Ge nanoclusters

Photoluminescence from gas-evaporated Ge nanoclusters consisting of a crystalline core encased in an oxide shell are presented. An as-grown sample shows room temperature luminescence with separate peaks around 357 and 580 nm. Prolonged air exposure of the clusters reduces the Ge core dimensions, and the emission initially at 580 nm shifts to 420 nm; however, the violet luminescence at 357 nm displays no difference. These results indicate that there are two mechanisms involved with light emission from Ge nanoclusters, visible light emission associated with the quantum confinement effect, and violet light emission correlated to luminescent centers. (C) 1998 Elsevier Science B.V.

Photoluminescence from Ge clusters embedded in porous silicon

Visible photoluminescence (PL) and Raman spectra of Ge clusters embedded in porous silicon (PS) have been studied. The as-prepared sample shows redshifted and enhanced room temperature PL relative to reference PS. This result can be explained by the quantum confinement effect on excitons in Ge clusters and tunnel of excitons from Si units of the PS skeleton to Ge clusters. One year storage in dry air results in a pronounced decrease in PL intensity but blue-shifted in contrast to reference PS. This phenomenon correlates to the size decrease of macerated Ce clusters and occurrence of "quantum depletion" in Ge clusters. Consequently, only excitons in Si units contribute to PL. (C) 1998 American Institute of Physics.

Optical anisotropy of InAs monolayer in (311)-oriented GaAs matrix

In-plane optical anisotropy which comes from the heavy hole and the light hole transitions in an InAs monolayer inserted in (311)-oriented GaAs matrix is observed by reflectance difference spectroscopy. The observed steplike density of states demonstrates that the InAs layer behaves like a two-dimensional quantum well rather than isolated quantum dots. The magnitude of the anisotropy is in good agreement with the intrinsic anisotropy of (311) orientation quantum wells, indicating that there is little structural or strain anisotropy of the InAs layer grown on (311)-oriented GaAs surface.