T. Kawazu

Molecular beam epitaxial growth of AlGaAs∕InGaAs∕GaAs planar superlattice structures on vicinal (111)B GaAs and their transport properties

AlGaAs∕InGaAs∕GaAs planar superlattice structures have been formed by depositing a very thin InGaAs layer onto vicinal (111)B GaAs surfaces, where the bunching of atomic steps has resulted in a corrugation of about 20–30nm in period and 2nm in height. The growth condition to form bunched steps with little irregularity is clarified. By selectively doping, electrons are introduced into such planar superlattices, and their transport parallel to and normal to the steps are studied. Clear Shubnikov–de Haas oscillations with specific features and quantized Hall plateaus are observed in both geometries, suggesting that electrons retain some of their quasi-two-dimensional characters. In-plane anisotropies of electron mobilities are studied, and discussed in terms of electron scatterings by step structures. Photoluminescence spectra are studied to evaluate the in-plane potential modulation and its inhomogeneities.

Self-assembly of InAs ring complexes on InP substrates by droplet epitaxy

We report the self-assembly of InAs ring complexes on InP (100) substrates by droplet epitaxy. Single-ring, ring-disk complex, and concentric double-ring structures were formed by controlling the As beam flux and substrate temperature. A clear photoluminescence signal was detected in a sample where InAs rings were embedded in InGaAs.

EFFECTS OF ANTIMONY FLUX ON MORPHOLOGY AND PHOTOLUMINESCENCE SPECTRA OF GaSb QUANTUM DOTS FORMED ON GaAs BY DROPLET EPITAXY

We investigated effects of the antimony flux on GaSb quantum dots (QDs) formed by droplet epitaxy. Ga droplets were first formed on GaAs and exposed to Sb4 molecular beam at 200 °C, where the flux PSb of Sb beam was varied from 2.4 to 12.8 × 10-7 Torr. The samples were then annealed for 2 minutes under the Sb flux. An atomic microscope study showed that the diameter of GaSb QDs increases and the density decreases, as the Sb flux PSb is increased. This indicates that the coalescence process of GaSb QDs occurs and is accelerated by the increase of the Sb flux. In a photoluminescence (PL) study, we observed a broad peak of GaSb QDs in all samples, while a strong luminescence of a wetting layer (WL)-like structure was found only in the samples prepared with the high Sb flux. This suggests that the PL of WL is controllable by adjusting the flux PSb of Sb beam.

Bias voltage dependence of two-step photocurrent in GaAs/AlGaAs quantum well solar cells

We investigated photoresponses of AlGaAs solar cells in which coupled GaAs quantum wells were embedded in the i-region of p-i-n diodes; we studied how the bias voltage Vb affects the normal photocurrent I generated by the visible light and a “two-step” photocurrent ΔI generated by the absorption of visible and infrared photons. We found that as Vb exceeds −0.2u2009V, ΔI rises and peaks at 0.6u2009V, while the normal photocurrent I falls to about half of its saturated level. These findings are discussed in terms of a rate equation model to show that ΔI is mainly determined by the balance of escape and recombination of photogenerated carriers.

GaAs/AlGaAs quantum wells with indirect-gap AlGaAs barriers for solar cell applications

We have fabricated GaAs/AlGaAs quantum well (QW) solar cells in which 3u2009nm-thick QWs and indirect-gap Al0.78Ga0.22As barriers are embedded, and we studied extraction processes of photogenerated carriers in this QW system. The photocurrent under 700u2009nm light illumination at voltages close to the open-circuit voltage shows only a small reduction, indicating that the carrier recombination inside QWs is largely suppressed. We attribute this result to an efficient extraction of electrons from the QWs through the X-valley of AlGaAs. The insertion of QWs is shown to be effective in extending the absorption wavelengths and in enhancing the photocurrent. The use of indirect-gap materials as barriers is found to enhance carrier extraction processes, and result in an improved performance of QW solar cells.

Temperature-dependent conductance of quasi-one-dimensional electrons in a novel constricted In0.1Ga0.9As/GaAs channel with corrugated interfaces

Transport properties of a novel quasi-ballistic quantum wire field-effect transistor are studied experimentally and then discussed in relation to a theory for dirty Tomonaga–Luttinger (T–L) liquids. The sample was prepared by constricting lithographically an epitaxially grown In0.1Ga0.9As/GaAsIn0.1Ga0.9As/GaAs quantum well channel, whose bottom interface is corrugated by a quasi-periodic array of multi-atomic steps of 20xa0nm in periodicity. A quasi-one-dimensional channel of about 200xa0nm in metallurgical width and 2μm in length was formed and its conductance parallel to the steps was measured at temperatures between 4 and 0.3xa0K as a function of gate voltage. Plateau-like structures substantially lower than 2e2/h2e2/h were observed. The conductance at each gate voltage decreases sensitively as temperature lowers until it gets nearly constant below a critical temperature. These tendencies are found to be qualitatively consistent with the theory of Ogata and Fukuyama for dirty T–L liquids. The temperature dependence above the critical temperature is found to fit quantitatively with the formula of Ogata and Fukuyama, if the parameters are suitably chosen.

Temperature-dependent conductance of quasi-one-dimensional electrons in a novel constricted In0.1Ga0.9As/GaAsIn0.1Ga0.9As/GaAs channel with corrugated interfaces

Transport properties of a novel quasi-ballistic quantum wire field-effect transistor are studied experimentally and then discussed in relation to a theory for dirty Tomonaga–Luttinger (T–L) liquids. The sample was prepared by constricting lithographically an epitaxially grown In0.1Ga0.9As/GaAsIn0.1Ga0.9As/GaAs quantum well channel, whose bottom interface is corrugated by a quasi-periodic array of multi-atomic steps of 20xa0nm in periodicity. A quasi-one-dimensional channel of about 200xa0nm in metallurgical width and 2μm in length was formed and its conductance parallel to the steps was measured at temperatures between 4 and 0.3xa0K as a function of gate voltage. Plateau-like structures substantially lower than 2e2/h2e2/h were observed. The conductance at each gate voltage decreases sensitively as temperature lowers until it gets nearly constant below a critical temperature. These tendencies are found to be qualitatively consistent with the theory of Ogata and Fukuyama for dirty T–L liquids. The temperature dependence above the critical temperature is found to fit quantitatively with the formula of Ogata and Fukuyama, if the parameters are suitably chosen.

Temperature-dependent conductance of quasi-one-dimensional electrons in a novel constricted channel with corrugated interfaces

Transport properties of a novel quasi-ballistic quantum wire field-effect transistor are studied experimentally and then discussed in relation to a theory for dirty Tomonaga–Luttinger (T–L) liquids. The sample was prepared by constricting lithographically an epitaxially grown In0.1Ga0.9As/GaAsIn0.1Ga0.9As/GaAs quantum well channel, whose bottom interface is corrugated by a quasi-periodic array of multi-atomic steps of 20xa0nm in periodicity. A quasi-one-dimensional channel of about 200xa0nm in metallurgical width and 2μm in length was formed and its conductance parallel to the steps was measured at temperatures between 4 and 0.3xa0K as a function of gate voltage. Plateau-like structures substantially lower than 2e2/h2e2/h were observed. The conductance at each gate voltage decreases sensitively as temperature lowers until it gets nearly constant below a critical temperature. These tendencies are found to be qualitatively consistent with the theory of Ogata and Fukuyama for dirty T–L liquids. The temperature dependence above the critical temperature is found to fit quantitatively with the formula of Ogata and Fukuyama, if the parameters are suitably chosen.

Molecular beam epitaxy growth of novel double-layer InAs quantum dot structures and their optical properties

We have designed and grown by molecular beam epitaxy (MBE) a novel sample with Schottky contact in which two sheets of InAs quantum dots (QDs) of different sizes are embedded at different positions of its depletion layer. Electronic states in these two respective QD layers are expected to be perturbed not only by the space-charge electric field but also by the carrier accumulation in QDs of both layers. We have investigated photoluminescence (PL) spectra of each dot layer under various bias conditions to clarify, in particular, how the local electric field is influenced by the accumulation of carriers in each dot layer, especially when two dot layers are closely spaced (25nm). The ground state transition energy of large dots shows an anomalous blue shift, which is as large as 11meV when a positive bias is applied. We show that the depopulation of charged dots is responsible for this anomaly.