Nobuyuki Koguchi

New MBE growth method for InSb quantum well boxes

Abstract We propose a new MBE growth method for InSb microcrystals on CdTe which has a nearly equal lattice constant to InSb. The average size of the InSb microcrystals was about 150 nm × 200 nm × 70 nm. This method is based on the Sb incorporation into In droplets and thought to be useful for fabricating quantum well boxes.

Growth of GaAs Epitaxial Microcrystals on an S-Terminated GaAs Substrate by Successive Irradiation of Ga and As Molecular Beams

Numerous GaAs epitaxial microcrystals with an average base size of 250 A×430 A with (111) facets were fabricated on a sulfur-terminated (S-terminated) GaAs (001) substrate with successive irradiation of Ga and As molecular beams. The growth of GaAs microcrystals on the S-terminated substrate was caused by a vapor-liquid-solid (VLS) mechanism. This phenomenon originated in the inertness for the adhesion of Ga and As molecules and nearly equal lattice constants of the S-terminated GaAs surface and GaAs surface. This method, called droplet epitaxy, is thought to show promise as a growth method for fabricating GaAs quantum well boxes.

Optical transitions in quantum ring complexes

High Magnetic Field Center, National Institute for Materials Science, Sakura 3-13, Tsukuba 305-0003, Japan(Dated: February 2, 2008)Making use of a droplet-epitaxial technique, we realize nanometer-sized quantum ring complexes, consistingof a well-defined inner ring and an outer ring. Electronic structure inherent in the unique quantum system isanalyzed using a micro-photoluminescence technique. One advantage of our growth method is that it presentsthe possibility of varying the ring geometry. Two samples are prepared and studied: a single-wall ring and aconcentric double-ring. For both samples, highly efficient photoluminescence emitted from a single quantumstructure is detected. The spectra show discrete resonance lines, which reflect the quantized nature of the ring-type electronic states. In the concentric double–ring, the carrier confinement in the inner ring and that in theouter ring are identified distinctly as split lines. The obse rved spectra are interpreted on the basis of singleelectron effective mass calculations.

Fabrication of Multiple Concentric Nanoring Structures

We present the fabrication of GaAs/AlGaAs Multiple (from three to five) concentric nanoring structures by an innovative growth method based on droplet epitaxy and characterized by short time As supply to the Ga droplets at different substrate temperatures. The formation mechanism has been interpreted on the basis of a detailed ex situ and in situ characterization of nanostructure morphology and surface reconstruction. We introduce design criteria which will allow to obtain concentric quantum ring structures of the desired complexity.

Fabrication of ZnO quantum dots embedded in an amorphous oxide layer

ZnO quantum dots (QDs) have been fabricated by the growth of SiO2/ZnO films/Si substrate and subsequent rapid-thermal annealing in a N2 ambient. Transmission electron microscopy (TEM) results show that the ZnO QDs 3–7 nm in size are formed and embedded in the amorphous silicon oxide interfacial layer when annealed at 850 °C. Photoluminescence (PL) at room temperature from the 850 °C-annealed samples reveals only high-energy emission at about 3.37 eV, while PL at 10 K shows a broad spectra with a tail up to about 3.5 eV. The TEM and PL results indicate that the broad spectra are caused by the presence of the ZnO QDs and hence by the quantum confinement effect.

New selective molecular-beam epitaxial growth method for direct formation of GaAs quantum dots

Numerous GaAs epitaxial microcrystals having an average base size of 700 A×700 A surrounded mainly by (111) and (110) facets were fabricated on a sulfer‐terminated (S‐terminated) GaAs (001) substrate by sequentially supplying Ga and As molecular beams. The S‐terminated GaAs (001) surface was produced by exposing the surface to a sulfur vapor in the molecular‐beam epitaxy system immediately after obtaining a Ga‐stabilized surface. The growth of GaAs microcrystals on the S‐terminated substrate is caused by a vapor–liquid–solid mechanism. The process consists of forming Ga droplets on the inert surface and reacting the droplets with As to produce GaAs microcrystals. This method termed droplet epitaxy is thought to be a promising growth method for fabricating the GaAs quantum dots.

Near room temperature droplet epitaxy for fabrication of InAs quantum dots

By using the droplet epitaxy method, we succeed in fabricating the InAs quantum dots (QDs) with the spatial density of 4×1010∕cm2 and an average lateral size of 20nm on GaAs (001) at the droplets deposition temperature of 50°C and subsequent annealing process under As4 flux. These QDs shows the intense photoluminescence even at room temperature indicating that high-quality InAs QDs can be fabricated by near room temperature droplets deposition and crystallization process by 400°C in situ annealing.

Self-assembly of laterally aligned GaAs quantum dot pairs

The authors report the fabrication of self-assembled, strain-free GaAs∕Al0.27Ga0.73As quantum dot pairs which are laterally aligned in the growth plane, utilizing the droplet epitaxy technique and the anisotropic surface potentials of the GaAs (100) surface for the migration of Ga adatoms. Photoluminescence spectra from a single quantum dot pair, consisting of a doublet, have been observed. Finite element energy level calculations of a model quantum dot pair are also presented.

Temperature dependence of the photoluminescence of InGaAs/GaAs quantum dot structures without wetting layer

We analyze the photoluminescence temperature behavior of InGaAs/GaAs quantum dots grown by heterogeneous droplet epitaxy. Morphologically, these dots are nanocrystal InGaAs inclusions in the GaAs matrix, with a concave disk shape and, more important, no wetting layer is connecting the dots. The photoluminescence of the dots does not show any of the typical of the Stranski–Krastanov dots temperature properties, such as sigmoidal peak energy position and linewidth narrowing. We demonstrate that such behavior stems from the lacking of the thermally activated dot–dot coupling channel provided by the wetting layer thus preventing the establishment of a common quasiequilibrium in the whole dot ensemble.

Coupled quantum dot―ring structures by droplet epitaxy

The fabrication, by pure self-assembly, of GaAs/AlGaAs dot-ring quantum nanostructures is presented. The growth is performed via droplet epitaxy, which allows for the fine control, through As flux and substrate temperature, of the crystallization kinetics of nanometer scale metallic Ga reservoirs deposited on the surface. Such a procedure permits the combination of quantum dots and quantum rings into a single, multi-functional, complex quantum nanostructure.