Shape evolution of Ge/Si(001) islands induced by strain-driven alloying

The shape evolution of Ge/Si(001) islands grown by ultrahigh vacuum chemical vapor deposition were investigated by atomic force microscopy at different deposition rates. We find that, at low deposition rates, the evolution of islands follows the conventional pathway by which the islands form the pyramid islands, evolve into dome islands, and dislocate at a superdome shape with increasing coverage. While at a high deposition rate of 3 monolayers per minute, the dome islands evolve towards the pyramids by a reduction of the contact angle. The presence of the atomic intermixing between the Ge islands and Si substrate at high deposition rate is responsible for the reverse evolution. (C) 2001 American Institute of Physics.

Multiplicity factor and diffraction geometry factor for single crystal X-ray diffraction analysis and measurement of phase content in cubic GaN/GaAs(001) epilayers

On the basis of integrated intensity of rocking curves, the multiplicity factor and the diffraction geometry factor for single crystal X-ray diffraction (XRD) analysis were proposed and a general formula for calculating the content of mixed phases was obtained. With a multifunction four-circle X-ray double-crystal diffractometer, pole figures of cubic (002), {111} and hexagonal {1010} and reciprocal space mapping were measured to investigate the distributive character of mixed phases and to obtain their multiplicity factors and diffraction geometry factors. The contents of cubic twins and hexagonal inclusions were calculated by the integrated intensities of rocking curves of cubic (002), cubic twin {111}, hexagonal {1010} and {1011}.

Different transfer paths for thermally activated electrons and holes in self-organized Ge/Si(001) islands in a multilayer structure

We investigated the temperature dependence (10-250 K) of the photoluminescence (PL) emission spectrum of self-organized Ge/Si(001) islands in a multilayer structure. With elevated temperature, we find that the thermally activated holes and electrons are gathered by the Ge islands in different ways. The holes drift from the wetting layer into the islands, while the electrons, confined in Si due to type-II band alignment, leak into the Ge islands by the electrostatic interaction with the holes accumulated there. It results in an increase of the integrated intensity of island-related PL at a certain temperature range and a reduction of the phonon energy in the phonon-assisted PL of the islands by involving a type-I transition into a type-II transition. (C) 2001 American Institute of Physics.

Influence of growth conditions on self-assembled InAs nanostructures 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 and PL properties of InAs nanostructures depend strongly on the growth condition. For the same buffer layer, elongated InAs quantum wires (QWRs) and no isotropic InAs quantum dots (QDs) can be obtained using different growth conditions. At the same time, for InAs quantum dots, PL spectra also show several emission peaks related to different islands size. Theoretical calculation indicated that there are size quantization effects in InAs islands. (C) 2001 Elsevier Science B.V. All rights reserved.

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.

X-ray evidence for Ge/Si(001) island columns in multilayer structure

A Ge/Si(0 0 1) multilayer structure is investigated by cross-sectional transmission electron microscopy, atomic force microscopy and double crystal X-lay diffraction. We find that the multilayer-structure-related satellite peaks in the rocking curve exhibit a similar nonuniform broadening and rye fit the zero-order peak with two Lorentz lineshapes. The ratio of the integrated intensity of two peaks is approximately equal with the anal ratio of the top Ge layer deposited between the areas that are and are not occupied by islands. It proves the existence of vertical-aligned island columns from the viewpoint of macroscopic dimension. (C) 2001 Elsevier Science B.V. All rights reserved.

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].

Defects and morphologies in In0.8Ga0.2As/InAlAs/InP(001) for high electron-mobility transistors

Defects and morphologies are presented in this paper as revealed with transmission electron microscope (TEM) in the In(0.8)G(0.2)As/InAlAs heterostructure on InP(001) for high-electron-mobility transistors application. Most of the misfit dislocation lines are 60 degrees type and they deviate < 110 > at some angles to either side according to their Burges vectors. The misfit dislocation lines deviating [-110] are divided into two types according to whether their edge component b(eg) of Burges vectors in [001] pointing up or down. If b(eg) points up in the growth direction, there is the local periodical strain modulation along the dislocation line. In addition, the periodical modulation in height along [-110] on the In(0.8)G(0.2)As surface is observed, this surface morphology is not associated with the relaxation of mismatch strain.

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.

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.

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.

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.

Anomalous strains in the cubic-phase GaN films grown on GaAs (001) by metalorganic chemical vapor deposition

Strains in cubic GaN films grown on GaAs (001) were measured by a triple-axis x-ray diffraction method. Residual strains in the as-grown epitaxial films were in compression, contrary to the predicted tensile strains caused by large lattice mismatch between epilayers and GaAs substrates (20%). It was also found that the relief of strains in the GaN films has a complicated dependence on the growth conditions. We interpreted this as the interaction between the lattice mismatch and thermal mismatch stresses. The fully relaxed lattice constants of cubic GaN are determined to be 4.5038 +/- 0.0009 Angstrom, which is in excellent agreement with the theoretical prediction of 4.503 Angstrom. (C) 2000 American Institute of Physics. [S0021-8979(00)07918-4].

X-ray double-crystal characterization of the strain relaxation in GaAs/GaNxAs1-x/GaAs(001) sandwiched structures

An X-ray diffraction method, estimating the strain relaxation in an ultrathin layer, has been discussed by using kinematic and dynamical X-ray diffraction (XRD) theory. The characteristic parameter Delta Omega, used as the criterion of the strain relaxation in ultrathin layers, is deduced theoretically. It reveals that Delta Omega should be independent of the layer thickness in a coherently strained layer. By this method, we characterized our ultrathin GaNxAs1-x samples with N contents up to 5%. XRD measurements show that our GaNxAs1-x layers are coherently strained on GaAs even for such a large amount of N. Furthermore, a series of GaNxAs1-x samples with same N contents but different layer thicknesses were also characterized. It was found that the critical thickness (L-c) of GaNAs in the GaAs/GaNAs/GaAs structures determined by XRD measurement was 10 times smaller than the theoretical predictions based on the Matthews and Blakeslee model. This result was also confirmed by in situ observation of reflection high-energy electron diffraction (RHEED) and photoluminescence (PL) measurements. RHEED observation showed that the growth mode of GaNAs layer changed from 2D- to 3D-mode as the layer thickness exceeded L-c. PL measurements showed that the optical properties of GaNAs layers deteriorated rapidly as the layer thickness exceeded L-c. (C) 2000 Elsevier Science B.V. All rights reserved.