Kouichi Akahane

Highly packed InGaAs quantum dots on GaAs(311)B

We have fabricated highly packed and ordered In0.4Ga0.6As quantum dots (QDs) array on GaAs(311)B substrate without coalescence of QDs. Reflection high-energy electron diffraction and Auger spectra suggest the inhomogeneous distribution of In and Ga in QD. In concentration near the surface of QD is larger than that of the inside, and the inhomogeneous distribution of In and Ga in QDs prevents QDs from merging.

Self-organized InGaAs quantum dots on GaAs (311)B studied by conductive atomic force microscope tip

We have used conductive atomic force microscope (AFM) tips in order to probe the local electronic properties of InGaAs quantum dots (QDs) grown on GaAs (311)B and (001) substrates by atomic H-assisted molecular beam epitaxy. Highly doped Si and Si3N4 AFM tips coated with a metal such as Au and Ti which warrant electrical conductivity were used to measure the current–voltage (I–V) characteristics of QDs of varying sizes and of any other arbitrary positions on the surface such as the wetting layer. In the case of QDs formed on (001) substrates, it was found that the local surface potentials of larger QDs were lower than the small QDs due to the effect of surface states. On the other hand, noticeable differences were not observed for the QDs formed on (311)B substrates. The local surface potential was similar on each QD and in fact over the whole (311)B surface, and a complex phase separation and strain-relief mechanism were thought to be responsible the observed QDs assembly on (311)B. Last, a resonant tunn...

Self-organized InGaAs quantum dots grown on GaAs (3 1 1)B substrate studied by conductive atomic force microscope technique

We have used conductive atomic force microscope (AFM) in a high vacuum in order to investigate the electronic properties of self-organized InxGa1–xAs quantum dots (QDs) on GaAs (3 1 1)B substrates. The QDs were fabricated by atomic H-assisted molecular beam epitaxy, and Si AFM tips coated with Au, which warrants electrical conductivity were used to measure both the topographic and current images of QDs surface simultaneously. The conductive AFM measurements were performed in vacuum at room temperature and at lowered temperatures. With this technique, the current–voltage (I–V ) characteristics of QDs of varying sizes, and of any other arbitrary positions on the QDs surface can also be studied by using the same conductive AFM tip. It was found that the center of a QD is more conductive than its periphery, and the surface in between the QDs is highly resistive. The differences in the conductance are thought to be due to the local modification of surface bending associated with the surface states. Further, we have shown that the conductance becomes spatially uniform at all points over the packed and ordered QDs at low temperatures, which could be explained by lateral coupling of these strained QDs. r 2002 Elsevier Science B.V. All rights reserved. PACS: 73.21.La; 73.23.� b; 73.63.Kv

Formation of lateral-two-dimensional ordering in self-assembled InGaAs quantum dot on high index substrates

Abstract The in-plane strain relaxation properties on InGaAs/GaAs (3 1 1) B were investigated by means of reciprocal space mapping using triple axis X-ray diffractometry to study the origin of two-dimensional ordering of InGaAs quantum dots (QDs) on a GaAs (3 1 1) B substrate. From the X-ray data, the strain relaxation anisotropy in the InGaAs layer was observed to be larger on the GaAs (3 1 1) B substrate than on (0 0 1) . It is proposed that the large strain anisotropy is responsible for the QD ordering observed on the (3 1 1) B surface.

Formation of extended states in disordered two-dimensional In0.4Ga0.6As/GaAs(311)B quantum dot superlattices

Formation of extended states or minibands in two-dimensional (2D) In0.4Ga0.6As/GaAs(311)B quantum dot superlattices (QDSLs) is directly demonstrated in time-resolved photoluminescence measurements. At a low excitation density of 1 W/cm2, photoluminescence transients with ∼15 ps rise time and ∼25 ps decay time are observed. Both rise and decay times are found to increase with increasing excitation density. The excitons in 2D QDSLs exhibit different relaxation and recombination behaviors as compared to those in quantum wells and quantum dots. A physical model treating 2D QDSLs as disordered systems containing localized and extended states can successfully interpret all of the experimental observations.

Isolated and close-packed In0.4Ga0.6As/GaAs (311)B quantum dots

Abstract Remarkably different characteristics were found on isolated and close-packed self-assembled In 0.4 Ga 0.6 As/GaAs (311)B quantum dots (QDs). Sub-peaks superimposed on the regular Gaussian photoluminescence (PL) peak were observed in a close-packed QD array, in contrast to the single Gaussian PL peak in an isolated one. A tunneling-current peak in an isolated QD array changes into several equidistant features in a close-packed one. The possible mechanisms of the fine structures are discussed.

Spatial alignment evolution of self-assembled In0.4Ga0.6As island arrays grown on GaAs (3 1 1)B surface by atomic hydrogen-assisted molecular beam epitaxy

Abstract Self-assembled In0.4Ga0.6As island arrays have been grown on (3xa01xa01)B GaAs substrates by using atomic hydrogen-assisted molecular beam epitaxy (H-MBE). The evolution process of surface morphology with deposition has been analyzed by atomic force microscopy (AFM) and the development of lateral ordering has been highlighted by two-dimensional fast Fourier transformation (2DFFT) analysis of the AFM images. It is revealed that the InGaAs islands are arranged in nearly perfect two-dimensional (2D) square-like lattice with two sides parallel to [0xa01xa0−1] and [−2xa03xa03] azimuths. Such an alignment of islands is coincident with the anisotropy of bulk elastic modulus of the GaAs (3xa01xa01)B substrate.

Distinctly different two-dimensional ordering alignments of InGaAs island arrays on GaAs(311)B and AlGaAs(311)B surfaces

Abstract By using the atomic force microscopy image, fast Fourier transformation and auto-correlation spectrum analysis, we have systematically investigated the spatial alignment of self-assembled In 0.4 Ga 0.6 As islands on the GaAs(3xa01xa01)B and the AlGaAs(3xa01xa01)B surfaces grown by atomic hydrogen assisted molecular beam epitaxy . Surprisingly, it is revealed that InGaAs island arrays are aligned in very different patterns on these two surfaces. They are aligned in a well-defined square two-dimensional (2D) lattice when formed on GaAs surface, but in a nearly perfect hexagonal 2D lattice when formed on AlGaAs surface, despite the fact that both AlGaAs and GaAs have the same lattice mismatch with InGaAs. This striking difference is preliminarily related to the enhanced bond strength of Al in surface, which not only changes the surface energy, but also slows down column III atom migration on the growing surface.

Strikingly well-defined two-dimensional ordered arrays of In0.4Ga0.6As quantum dots grown on GaAs (3 1 1)B surface

Abstract Self-assembled In 0.4 Ga 0.6 As quantum dots (QDs) on GaAs (3xa01xa01)B surface has been grown by atomic hydrogen-assisted molecular beam epitaxy (H-MBE). The lateral ordering and size homogenization of islands is studied by using atomic force microscopy (AFM), fast Fourier transformation (FFT) and auto correlation (AC) spectra. It is revealed that strikingly well-defined square two-dimensional (2D) lattice of InGaAs dots are formed on the GaAs (3xa01xa01)B surface. This remarkable self-organization behavior is explained by the special ordering mechanism in our material system, originating from the high elastic anisotropy of the GaAs as well as the chosen (3xa01xa01)B growth orientation.

Ingaas quantum dots on GaAs(311)B substrates confined in AlGaAs barrier layers

Abstract We investigated the growth of QDs on AlGaAs/GaAs(3xa01xa01)B by atomic hydrogen assisted molecular beam epitaxy. In the growth of QDs directly on AlGaAs, the QDs size fluctuation increased with increasing Al composition of the AlGaAs underlying layer. However, the uniformity of QDs size and the ordering were improved by introduction of a thin GaAs spacer layer. It was concluded that there is a strong interaction between the InGaAs-strained and underlying layers. The strong photoluminescence intensity of QDs confined in AlGaAs with GaAs spacer layers was maintained up to room temperature. Hence, the carrier collection efficiency into the QDs was improved and the escape of carriers from the QDs was suppressed by introducing higher potential AlGaAs barrier layers.