Takuya Kawazu

Self-Assembled Growth of GaSb Type II Quantum Ring Structures

We demonstrate the self-assembled growth of GaSb quantum ring structures by molecular beam epitaxy. GaSb rings with the internal and external diameters of about 20 nm and 60 nm are successfully formed on GaAs by a growth procedure different from that for InAs rings reported earlier. The shape of GaSb structures can be controlled from a ring-like to an elongated disk-like geometry by changing the amount of deposited GaSb. A possible growth mechanism of GaSb rings is discussed. Photoluminescence spectra of the rings are presented and their features are discussed in terms of the type II band alignment, in which only holes are confined in the ring.

Optical properties of GaSb/GaAs type-ІІ quantum dots grown by droplet epitaxy

We study the optical properties of GaSb/GaAs type-ІІ quantum dots (QDs) on a GaAs substrate grown by droplet epitaxy. Ga droplets are formed on GaAs and then exposed to Sb flux to be clad by large granular crystals of Sb. Then the sample was annealed at 380 °C to enhance the reaction of Ga droplets with Sb and to evaporate the excess granular layer. In photoluminescence (PL) measurements, the peaks of the QDs and wetting layer (WL) are observed. The PL intensity of the QDs is much stronger than that of the WL, where the ratio IQD/IWL of the integral intensities is about 13.3. The PL peaks shift toward higher energies with increasing excitation energy, suggesting that the band lineups exhibit a type-ІІ staggered alignment. In addition, we investigate the temperature dependences of the PL peak energy and intensity.

Effects of Sb/As intermixing on optical properties of GaSb type-II quantum dots in GaAs grown by droplet epitaxy

Optical properties of GaSb type-II quantum dots (QDs) in GaAs were studied and compared with a theoretical model to clarify how the spatial overlap of holes in the dot and electrons outside is affected by the interdiffusion of Sb and As. GaSb QDs were grown in a GaAs substrate by droplet epitaxy and annealed at the temperature Ta=650–850 °C to induce the Sb/As intermixing. Photoluminescence (PL) studies showed that the integrated PL intensity I decreases to less than 1/10 as Ta is raised from 650 to 750 °C, while I increases by three orders of magnitude with the increase of Ta from 750 to 850 °C. This behavior is explained by the overlap Θ between electron and hole wave functions; in an initial stage of the interdiffusion, the mixing occurs only near the dot/matrix boundary, leading to the decrease in the overlap Θ, since electrons are more repelled by the dot. In later stages, however, the hole confinement and the electron repulsion in the dot both weaken, leading to the increase in the overlap Θ.

Formation of Ultra-low Density (

We have studied a self-assembled growth technique to form ultra-low density InAs quantum dots on GaAs by molecular beam epitaxy. After growing a GaAs layer under a particular condition, we have deposited an InAs layer of far less than the critical thickness and performed an annealing process. By optimizing these process steps, the density of dots is successfully controlled over a wide range from 104 to 108 cm-2, at which the average interdot distance gets as long as 100 µm. Photoluminescence spectra of low dot density samples have shown discrete single-dot features even under a macroscopic optical excitation. These dots are found to be formed preferentially on GaAs mounds especially when the dot density is around 2.5×105 cm-2.

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The drain current Id vs gate voltage Vg characteristics in GaAs/n-AlGaAs heterojunction field effect transistors (FETs) with embedded InAs quantum dots (QDs) are studied in detail. It is found that the presence or absence of the current hysteresis can be controlled by the initial gate voltage before the gate scan. It is also found that those with positive shift of current hysteresis is observed in FETs with 5-nm-high QDs, while the negative shift is observed in those with 3-nm-high QDs. This property can be explained by the fact that electrons trapped in QDs escape more easily from 5-nm-high QDs than in 3-nm-high QWs. We thus conclude that the current hysteresis or the threshold voltage (Vth) in quantum-trap FETs with embedded InAs QDs can be controlled by engineering charging states of QDs via the gate or the geometrical shape of QDs. A Vth shift-controllable floating dot memory device is suggested.

cm-2) Self-Organized InAs Quantum Dots on GaAs by a Modified Molecular Beam Epitaxy Method

We investigated the effect of a wetting layer (WL) on the optical properties of GaSb type-II quantum dots (QDs) in GaAs. GaSb QDs were grown by droplet epitaxy, where Ga droplets are first formed on GaAs and then exposed to Sb flux, followed by an annealing step. By adjusting the annealing temperature, we fabricated GaSb QDs with and without a well-defined WL-like structure. Photoluminescence (PL) measurements showed that the high-temperature-annealed samples exhibit a strong PL of the WL, whereas the WL luminescence is quite weak for the samples annealed at low temperatures. As the measurement temperature T increases, the PL energy decreases for the GaSb QDs without the WL. In contrast, the PL energy of the GaSb QDs with the WL has little dependence on T. These PL energy shifts are explained by considering the effects of the hole population and energy-dependent absorption.

Control of Current Hysteresis Effects in a GaAs/n-AlGaAs Quantum Trap Field Effect Transistor with Embedded InAs Quantum Dots.

Electronic states in type-II GaSb quantum dots (QDs) in GaAs are studied theoretically and compared with experiments to clarify how the spatial overlap of holes in the dot and electrons outside is affected by the interface grading caused by the interdiffusion of Sb and As. Cone-shaped QDs with the initial size of 6 nm height and 20 nm radius are analyzed. The wavefunctions of electrons and holes and their overlap Θ are calculated. In an initial stage of grading, when the grading length Lg is below 1.2 nm, the mixing occurs only near the dot/matrix interface, leading to a decrease in Θ, since holes are squeezed into a smaller volume whereas electrons are more repelled by the dot. In later stages where Lg exceeds 1.2 nm, however, the hole confinement and the electron repulsion by the dot both weaken, leading to an increase in Θ; this accounts for a recent finding that the annealing of GaSb QDs induces a blue shift and an intensity increase in photoluminescence spectra.

Growth and optical properties of GaSb/GaAs type-II quantum dots with and without wetting layer

We form self-assembled GaSb type-II nanorods on a vicinal (111)B GaAs substrate by molecular beam epitaxy and study their optical anisotropy. The GaSb nanorods are elongated and aligned along the [−1 0 1] direction, where the average length, width, and height are about 84, 30, and 2.5 nm. In polarized photoluminescence (PL) measurements, the peak of the GaSb nanorods is observed at about 1.1 eV, where the PL intensity is largest for the [−1 0 1] polarization and smallest for the polarization perpendicular to it. The degree of polarization is more than 20% and depends on the recombination energy. By comparing with a theoretical model based on 4 × 4 Luttinger-Kohn Hamiltonian, we find that the experimental results are explained by considering the Sb/As inter-diffusion and the nanorod height distribution.

Effects of Interface Grading on Electronic States and Optical Transitions in GaSb Type-II Quantum Dots in GaAs

We investigate the growth of GaSb and AlSb quantum dots (QDs) on high-index GaAs substrates at various substrate temperatures (Ts). The GaSb QD density on GaAs(311)A is about 1.5?2 times larger than that on GaAs(100) substrates. Small and dense AlSb QDs are formed by AlSb deposition on GaAs(311)A substrates at relatively high temperatures (Ts ? 430 ?C) with QD densities as high as 29 ? 1010 cm?2. In contrast, no QDs are observed for low-temperature AlSb deposition (Ts ? 410 ?C). In contrast to InAs QDs, GaSb and AlSb QDs are easily formed even on GaAs(111)A surfaces. We use a rate equation to analyze the experimental data and discuss QD density and size and their Ts dependences.

Optical anisotropy of GaSb type-II nanorods on vicinal (111)B GaAs

The scattering processes of two-dimensional electrons are studied in selectively doped n-AlGaAs∕GaAs heterojunctions where high density InAlAs anti dots are embedded in the vicinity of the GaAs channel. Mobilities μ are measured as a function of the electron concentrations Ns in two samples where the In1−xAlxAs antidots are grown with different Al contents (x∼0.75 and 0.5). It is found that the Ns dependence of μ is strongly dependent on the Al content x of the embedded InAlAs dots, although their shapes and densities are almost same. The experimental data are well explained by theoretical models based on the surface profiles of the InAlAs dot layers.