Somchai Ratanathammaphan

InAs/GaAs self-organized quantum dots on (4 1 1)A GaAs by molecular beam epitaxy

Self-organized InAs QDs formation on (4 1 1)A GaAs and on controlled (1 0 0) GaAs substrates were studied by insitu RHEED observation, AFM measurement and PL spectroscopy. The transition from two-dimensional to threedimensional growth on (4 1 1)A was observed by RHEED pattern transformation to indicate the critical layer thickness of InAs QDs formation. The result is almost the same as that on (1 0 0). Improvement of QDs alignment having dot size uniformity on (4 1 1)A was observed by AFM measurement. Enhanced optical properties of QDs on (4 1 1)A was shown by low-temperature PL spectroscopy. Strong PL peak at 966 nm having FWHM of 35 nm was obtained from the 3stacked QDs on (4 1 1)A. The narrower PL FWHM of QDs peak on (4 1 1)A comes from the improved uniformity of QDs on (411)A. # 2001 Elsevier Science B.V. All rights reserved.

Improved quantum confinement of self-assembled high-density InAs quantum dot molecules in AlGaAs∕GaAs quantum well structures by molecular beam epitaxy

Self-assembled, multistack InAs quantum dot molecules (QDMs) were grown by a modified molecular beam epitaxial (MBE) technique, which involves multiple stacking and multiple cycling of the thin-capping-and-regrowth process, so as to obtain a large volume density of quantum dots on the sample. Furthermore, the high-density InAs QDMs were also grown sandwiched either between a double heterostructure (DHS) or between a quantum-well (QW) structure. It was found from microphotoluminescence (μ-PL) measurements that the QDMs sandwiched between these structures give broader PL spectra than those of the as-grown QDMs. The broadening of the PL spectra is associated with the poorer dot size uniformity, which arises from the long and complicated MBE growth processes. However, comparing between the QDMs in the DHS and in the QW structure, the latter give narrower PL spectra. The narrower PL spectra for the QDM-in-QW structure is attributed to the improved quantum confinement effect arising from the use of the QW.

Fabrication of Self-Assembled InGaAs Squarelike Nanoholes on GaAs(001) by Droplet Epitaxy

The fabrication of self-assembled InGaAs squarelike nanoholes on GaAs(001) substrates grown by droplet epitaxy using molecular beam epitaxy was reported. The formation mechanism is explained by the As4 diffusion in droplets during the supply of As4 flux. The effects of substrate temperature (300–390 °C) during the InGa droplet deposition on their dimension and density were investigated. The surface morphology of InGaAs nanoholes as well as their depth profile was examined by atomic force microscopy (AFM). The square shape is oriented along [110] and [110] crystallographic directions with slightly different profiles due to anisotropy behavior. The size uniformity of the squarelike nanoholes is well controlled with less deviation at higher substrate temperatures.

Effect of Na Content on the Physical Properties of Powders

The different Na contents (0 ≤ Na ≤ 0.35, based on mole of NaOH) of doped Ba0.5Sr0.5TiO3 (BST) powders synthesized via sol-gel process were studied. The substitution of Na

Nanometer-Scale In0.5Ga0.5As Ring-Like Structure Grown by Droplet Epitaxy

We have demonstrated the fabrication of InGaAs ring-like nanostructures by dropletepitaxy technique using molecular-beam epitaxy. Dependence on the substrate temperature and the amount of indium and gallium of the nanostructural properties was investigated. It was found that increasing substrate temperature resulted in larger InGaAs ring size but with lower density and that increasing In0.5Ga0.5 amount resulted in larger InGaAs ring size but with oscillating density. Photoluminescence results confirmed the high quality of the nanocrystal.

Selective growth of InAs/GaAs self-organized quantum dots by shadow mask technique

The 10-mm stripes of 3-stacked self-organized InAs QDs layers embedded in GaAs matrix were selectively grown on GaAs substrates by MBE with shadow mask technique. The shadow mask structure composed of a 1-mm GaAs mask layer and a 1.5-m mA l 0.5Ga0.5As spacer layer was prepared by LPE and MBE. The window stripes were lithographically defined and were opened by conventional chemical etching. The self-organized QDs formation was observed by monitoringthe 2D to 3D transition on an unpatterned GaAs substrate. PL spectrum from shadow mask sample confirms the QDs formation and reveals the different optical properties from QDs on the unpatterned substrate. The blue-shift of PL spectrum of QDs grown through shadow mask is due to the different dot sizes. # 2001 Elsevier Science

Raman and photoluminescence properties of type II GaSb/GaAs quantum dots on (001) Ge substrate

We investigate structural Raman and photoluminescence properties of type II GaSb/GaAs quantum dots (QDs) grown on (001) Ge substrate by molecular beam epitaxy. Array of self-assembled GaSb QDs having an areal density of ∼1.66 × 1010 dots/cm2 is obtained by a growth at relatively low substrate temperature (450 °C) on a GaAs surface segmented into anti-phase domains (APDs). Most of QDs form in one APD area. However, a few QDs can be observed at the APD boundaries. Raman spectroscopy is used to probe the strain in GaAs layer. Slight redshift of both LO and TO GaAs peaks are observed when GaSb QDs are buried into GaAs matrix. Optical properties of capped QDs are characterized by photoluminescence measurement at low temperatures (20 K and 30 K). Emission peaks of GaSb/GaAs QDs are found in the range of 1.0-1.3 eV at both temperatures. Slight redshift is observed when the laser excitation power is increased at 20 K while blueshift of QD peak is observed at 30 K. We attribute this abnormal behavior to the contribution of overlapped GaSb wetting layer peak in the PL emission as well as the feature of type II band structure.

Toward quantum state manipulation in twin InSb/GaAs quantum dots

This paper describes a route toward quantum state manipulation by using twin InSb/GaAs quantum dots (QDs) as building blocks. The discussion on the advantage of staggered (type-II) band alignment in InSb/GaAs heterojunction is given first. For the generation of coupled/entangled electronic state, symmetric twin is desired. However, asymmetric twin is typically expected from the fabrication point of view. The localized hole states might be tuned to delocalized ones by external electric and/or magnetic fields. In addition, control and read-out of the stored information in the state can be done via optical probe with laser spectroscopy. We present here an experimental realization of twin InSb/GaAs QDs by solid source molecular beam epitaxy. The InSb QDs are formed into both single and twin rectangular-based configurations.

Type-II recombination dynamics of tensile-strained GaP quantum dots in GaAs grown by droplet epitaxy

We use droplet epitaxy to create tensile-strained GaP quantum dots in a GaAs matrix. A strong biaxial tensile strain leads to the formation of a type-II band lineup with a transition energy lower than the bulk GaAs band gap. The luminescence transients exhibit highly non-exponential decay behavior with an average time constant of 11 ± 2 μs, which is more than three orders of magnitude longer than the lifetime of standard type-I quantum dots. The prolonged luminescence decay time for the GaP/GaAs dots confirms the formation of the type-II band alignment associated with the tensile strain.

Evolution of Ga droplets into GaSb quantum rings

We present the fabrication of GaSb quantum rings (QRs) on the GaAs (001) substrates by droplet epitaxy technique using solid-source molecular beam epitaxy (MBE). In droplet epitaxy process, Ga was deposited on GaAs surface to form liquid Ga droplets and then exposed to Sb flux for crystallization. The evolution of Ga droplets into GaSb QRs is discussed and tracked by means of reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM).