Takaaki Mano

Fabrication of InGaAs quantum dots on GaAs(0 0 1) by droplet epitaxy

Abstract We have proposed a new self-organized growth method for InGaAs quantum dots (QDs) using droplet epitaxy with highly dense Ga droplets. During the crystallization process of InGaAs in droplet epitaxy, the highly dense Ga droplets effectively prevented the two-dimensional growth of InGaAs. Phase-separation during annealing for the InAs–GaAs system resulted in the formation of high-quality InGaAs QDs in the upper part of the sample with a flat surface.

New Self-Organized Growth Method for InGaAs Quantum Dots on GaAs(001) Using Droplet Epitaxy

Quantum dot (QD) systems for InGaAs/GaAs without a wetting layer have been fabricated on GaAs (001) surfaces by a new self-organized growth method using droplet epitaxy with highly dense Ga droplets. Droplets of InGa alloy with highly dense Ga droplets have been formed by supplying 1) Ga, 2) In and 3) Ga molecular beams, sequentially. These highly dense Ga droplets have successfully prevented the two-dimensional growth of InGaAs during crystallization under As flux supply. In the plan-view transmission electron microscope image, the InGaAs QDs with the density of 7×109 cm-2 are observed. These QDs show a very sharp photoluminescence peak (full width half maximum (FWHM): 21.6 meV) at 946 nm.

InAs nanocrystal growth on Si (100)

We have tried the growth of InAs dots on Si surfaces terminated with hydrogen atoms. It has been confirmed with cross-section transmission electron microscopy (XTEM) and field emission scanning electron microscopy (FESEM) that high quality InAs nanocrystals can be grown with a high density on Si (100) surfaces terminated with hydrogen atoms. The growth of InAs nanocrystals is initiated with formation of In islands followed by diffusion of As atoms into the In dots. The band discontinuity of the InAs/Si interface has been evaluated using the results of synchrotron radiation photoelectron spectroscopy (SRPES). The offsets for valence band and conduction band are estimated to be 0.31 eV and 0.44 eV, respectively. This result indicates that both holes and electrons can be well confined into InAs nanocrystals.

Characterization of strain distribution in quantum dots by X-ray diffraction

Abstract Structural properties of InAs quantum dots grown on Si (1xa00xa00) substrate have been investigated with G-GIXD reciprocal space mapping. Although both the volume (37xa0nm in diameter) and the surface coverage (about 11%) of the InAs dots are small, strong diffraction spots have been observed. To investigate the origin of the peak broadening, we compared the experimental data with structural factors calculated with FEM. We have found that the size of diffraction spots from the InAs dots is determined not only by their size and within-a-dot strain distribution, but also by the dot-to-dot strain distribution.

Indium segregation in the fabrication of InGaAs concave disks by heterogeneous droplet epitaxy

Abstract We have studied the effects of As 4 flux intensity during crystallization process and sample temperature during annealing process on the structural and optical properties of InGaAs (quantum dots) QDs fabricated by heterogeneous droplet epitaxy method. The QDs formation mechanism including In segregation and InAs–GaAs intermixing was investigated. We found the very wide optimum growth conditions, such as photoluminescence peak energy is constant, intensity is maximum value and full-width at half-maximum is minimum value.

InAs Quantum Dots Growth by Modified Droplet Epitaxy Using Sulfur Termination.

We have applied the modified droplet epitaxy method using sulfur termination to fabricate InAs quantum dots on GaAs(001) substrates for the first time. It is observed that the S-terminated surface effectively prevents the two-dimensional growth of InAs, forming InAs nanocrystals. From the magneto-photoluminescence measurements of this structure, three-dimensional quantum confinement effect was confirmed. This modified droplet epitaxy method is promising for the fabrication of quantum dots, not only in the lattice-matched system but also in the lattice-mismatched system.

High-resolution core-level photoemission study on GaAs(111)B surfaces

GaAs(111)B surfaces grown by molecular-beam epitaxy have been investigated in situ using synchrotron radiation photoemission spectroscopy. For the GaAs(111)B-2×2 phase, the Gau20093d core-level spectra consists of one dominating component while the Asu20093d spectra are decomposed into four components. This analysis is fully consistent with an As-trimer model where As-trimers in T4 or H3 position exist on top of As atoms in the uppermost double layer. On the other hand, the core-level spectra for the 19×19 surface disagree with the hexagonal ring structural model.

Automated angle-scanning photoemission end-station with molecular beam epitaxy at KEK-PF BL-1C

In order to satisfy demands to study the electronic structure of quantum nanostructures, a VUV beamline and a highresolution and high-throughput photoemission end-station combined with a molecular beam epitaxy (MBE) system have been constructed at the BL-1C of the Photon Factory. An angle-resolved photoemission spectrometer, having high energy- and angular-resolutions; VG Microtech ARUPS10, was installed. The total energy resolution of 31 meV at the 60 eV of photon energy is achieved. For the automated angle-scanning photoemission, the electron spectrometer mounted on a two-axis goniometer can be rotated in vacuum by the computer-controlled stepping motors. Another distinctive feature of this end-station is a connection to a MBE chamber in ultahigh vacuum (UHV). In this system, MBE-grown samples can be transferred into the photoemission chamber without breaking UHV. Photoemission spectra of MBE-grown GaAs(0 0 1) surfaces were measured with high-resolution and bulk and surface components are clearly resolved. r 2001 Elsevier Science B.V. All rights reserved.

Magneto-photoluminescence study of InGaAs quantum dots fabricated by droplet epitaxy

Abstract We have successfully measured magneto-photoluminescence of InGaAs quantum dots (QDs) fabricated by droplet epitaxy with highly dense Ga droplets, termed separated-phase enhance epitaxy with droplets (SPEED). In the low magnetic field region, the PL peak energy shift increases linearly with square of the magnetic field. From the estimated Bohr radii for the QDs, it is found that the size of the QDs in the lateral direction is 2.7-times larger than that in the vertical direction. Moreover, using high magnetic region for estimating the detailed lateral size, the cyclotron radius around 25 T in the QDs becomes equal to the lateral size of the QDs. The cyclotron radius of 5.1 nm at 25 T suggests that the size of the InGaAs QDs in lateral direction might be as small as about 10 nm.

Band discontinuity in the GaAs/AlAs interface studied by in situ photoemission spectroscopy

In order to investigate the valence-band discontinuity of the GaAs/AlAs interface, the thickness dependence of the photoemission spectra of a GaAs layer in situ deposited on AlAs by molecular-beam epitaxy has been studied. Although the interface is atomically abrupt, the electronic structure in the interface region displays Al1−xGaxAs alloy-like behaviors. The valence-band maximum as well as the Ga 3d core level show a gradual shift as a function of GaAs layer thickness of less than 2 nm (8 monolayers), which indicates that interface formation needs about 2 nm thickness for the electronic structure of the GaAs layer to become that of bulk GaAs.