Takayuki Kiba

Temperature-Dependent Operation of GaAs Quantum Nanodisk LEDs with Asymmetric AlGaAs Barriers

Quantum dot photonic devices such as light-emitting diodes (LEDs), laser diodes (LDs), high-speed modulators, and semiconductor optical amplifiers are attractive because of their small threshold voltages, low power consumption, and temperature stability. We have studied and developed a defect-less top-down dry fabrication process for GaAs quantum nanodisk (QND) LEDs with diameters of less than 20 nm and thicknesses of 8 nm by employing a bionanotemplate, neutral beam etching, and asymmetric AlGaAs/GaAs regrowth via metalorganic vapor phase epitaxy. AlxGa1-xAs barriers with high (Al0.3Ga0.7As) and low (Al0.17Ga0.83As) aluminum contents were used between QNDs in the in-plane and vertical directions, respectively. This permits the control of the deep band energy offset between GaAs QNDs and AlxGa1-xAs barriers, leading to stable room-temperature operation. The temperature dependence of the optical properties of the QND LED was measured by electroluminescence, and we found that the energies and their transient behaviors were strongly affected by the band offset energies between the QNDs and the aluminum-rich barriers. Therefore, we could enhance the optical performances of our symmetric QND LED with low-Al-content barriers by developing asymmetric AlxGa1-x As barriers with low-Al-content vertical barriers and high-Al-content in-plane barriers.

Nanometer scale fabrication and optical response of InGaN/GaN quantum disks.

In this work, we demonstrate homogeneously distributed In0.3Ga0.7N/GaN quantum disks (QDs), with an average diameter below 10 nm and a high density of 2.1xa0×xa010(11) cm(-2), embedded in 20 nm tall nanopillars. The scalable top-down fabrication process involves the use of self-assembled ferritin bio-templates as the etch mask, spin coated on top of a strained In0.3Ga0.7N/GaN single quantum well (SQW) structure, followed by a neutral beam etch (NBE) method. The small dimensions of the iron cores inside ferritin and nearly damage-free process enabled by the NBE jointly contribute to the observation of photoluminescence (PL) from strain-relaxed In0.3Ga0.7N/GaN QDs at 6 K. The large blueshift of the peak wavelength by over 70 nm manifests a strong reduction of the quantum-confined Stark effect (QCSE) within the QD structure, which also agrees well with the theoretical prediction using a 3D Schrödinger equation solver. The current results hence pave the way towards the realization of large-scale III-N quantum structures using the combination of bio-templates and NBE, which is vital for the development of next-generation lighting and communication devices.

Temperature-dependent spin injection dynamics in InGaAs/GaAs quantum well-dot tunnel-coupled nanostructures

Time-resolved optical spin orientation spectroscopy was employed to investigate the temperature-dependent electron spin injection in In0.1Ga0.9As quantum well (QW) and In0.5Ga0.5As quantum dots (QDs) tunnel-coupled nanostructures with 4, 6, and 8u2009nm-thick GaAs barriers. The fast picosecond-ranged spin injection from QW to QD excited states (ES) was observed to speed up with temperature, as induced by pronounced longitudinal-optical (LO)-phonon-involved multiple scattering process, which contributes to a thermally stable and almost fully spin-conserving injection within 5–180u2009K. The LO-phonon coupling was also found to cause accelerated electron spin relaxation of QD ES at elevated temperature, mainly via hyperfine interaction with random nuclear field.

Power-dependent spin amplification in (In, Ga)As/GaAs quantum well via Pauli blocking by tunnel-coupled quantum dot ensembles

Power-dependent time-resolved optical spin orientation measurements were performed on In0.1Ga0.9As quantum well (QW) and In0.5Ga0.5As quantum dot (QD) tunnel-coupled structures with an 8-nm-thick GaAs barrier. A fast transient increase of electron spin polarization was observed at the QW ground state after circular-polarized pulse excitation. The temporal maximum of polarization increased with increasing pumping fluence owing to enhanced spin blocking in the QDs, yielding a highest amplification of 174% with respect to the initial spin polarization. Further elevation of the laser power gradually quenched the polarization dynamics, which was induced by saturated spin filling of both the QDs and the QW phase spaces.

Metal nanolayer deposited highly stable Ag thin films and their optical properties

We have reported that deposition of several-nm-thick Al surface layer on Ag thin films improved thermal stability remarkably. In the present study, we investigated the influence of the deposition of Al nanolayer on the reflectance of the Al/Ag multilayer. 100-nm-thick Ag film and 1–5-nm-thick Al surface layer were deposited successively by vacuum evaporation, and their optical properties were measured. The reflectance of the samples on which a 1- or 3-nm-thick Al layer was deposited on the Ag film was almost the same as that of the Ag single-layer film. However, the samples on which a 5-nm-thick Al layer was deposited on the Ag film showed decreased reflectance. It was considered that the deposited Al nanolayer was fully oxidized in the former samples, but metallic Al and oxidized Al coexisted in the latter sample, which coincided with the simulated reflectance data. As a result, we revealed that thermally stable Ag films formed by 1- or 3-nm-thick Al nanolayer deposition have also high optical reflectance.

Reactive sputter deposition of nickel oxide thin films at liquid nitrogen temperature

Nickel oxide thin films were reactively sputter deposited in Ar + H2O atmosphere on substrates cooled with liquid nitrogen. H2O gas was injected toward the substrate surface at various flow rates from 0 to 1.5 cm3/min. A nickel metal target is considered to maintain the metallic state because excess H2O molecules condensed on the liquid nitrogen tank. The films deposited at liquid nitrogen temperature were found to be mixtures of nickel metal and nickel oxide. Electrochromic transmittance changes between 14 and 26% were observed for the film in a KOH aqueous electrolyte.

High-rate sputter deposition of electrochromic nickel oxide thin films using substrate cooling and water vapor injection

Nickel oxide is a promising electrochromic (EC) material because it changes color upon electrochemical oxidation and reduction. In this study, the authors developed a reactive sputtering technique using water vapor as a reactive gas to deposit highly hydrated oxide thin films. They used an Ar+H2O gas mixture, with H2O injected directly onto the substrate surface. Subsequently, the authors studied the effects of substrate cooling on the mechanism of Ni oxide thin film formation and how this process affected the EC properties of the resulting Ni oxide thin films. At substrate temperatures of −80 and −120u2009°C, amorphous hydrated Ni oxide thin films were deposited with a high deposition rate of approximately 35u2009nm/min, which was greater than that of metallic Ni films. It was surmised that the metallic target mode was achieved at low substrate temperatures in the Ar+H2O atmosphere. The Ni oxide films exhibited good EC properties with a large transmittance change. Consequently, substrate cooling and water vapor injection were found to be effective in the high-rate deposition of Ni oxide thin films with good EC properties.Nickel oxide is a promising electrochromic (EC) material because it changes color upon electrochemical oxidation and reduction. In this study, the authors developed a reactive sputtering technique using water vapor as a reactive gas to deposit highly hydrated oxide thin films. They used an Ar+H2O gas mixture, with H2O injected directly onto the substrate surface. Subsequently, the authors studied the effects of substrate cooling on the mechanism of Ni oxide thin film formation and how this process affected the EC properties of the resulting Ni oxide thin films. At substrate temperatures of −80 and −120u2009°C, amorphous hydrated Ni oxide thin films were deposited with a high deposition rate of approximately 35u2009nm/min, which was greater than that of metallic Ni films. It was surmised that the metallic target mode was achieved at low substrate temperatures in the Ar+H2O atmosphere. The Ni oxide films exhibited good EC properties with a large transmittance change. Consequently, substrate cooling and water vapor ...

Indium-saving effect and physical properties of transparent conductive multilayers

Indium-free transparent conductive multilayer structures consisting of top and bottom MoO3 layers and an Ag interlayer (MoO3/Ag/MoO3; MAM) are deposited onto glass substrates by vacuum evaporation. The transmittance and sheet resistance of the structures are evaluated, and the optimum structure is determined to be MAM (20/14/30 nm) as it shows the best figure of merit (FOM), which is used as the index for transparent conductive films, with a value of 6.2 × 10-3 Ω-1. To further improve the performance of the films, we attempt to fabricate a multilayer consisting of MoO3 and indium zinc oxide (IZO), based on previous results. The obtained IAM (30/14/50 nm) multilayer shows an FOM higher than that of the MAM, with a value of 32 × 10-3 Ω-1. Moreover, it reduces the amount of required indium as compared with the IZO/Ag/IZO multilayer.

Morphological and Electrochemical Properties of Bilayered Copper Oxide Nanostructures Directly Grown on Transparent Conductive Oxides by a Simple Wet-Chemical Process

We successfully synthesized copper oxide (CuO) nanostructures using an aqueous solution of copper acetate monohydrate without any surfactant at 90°C, and analyzed their morphological evolutions and electrochemical properties with different growth times. All samples consisted of polycrystalline single-phase monoclinic CuO. The CuO directly grown on transparent conductive oxide (TCO) substrates had a bilayered structure, with the bottom layer consisting of compact nanostructures formed from heterogeneous nucleation on the surface of the TCO substrate and the top layer comprising microspheres formed from homogeneous nucleation in the solution. The entirely binder-free bilayered CuO nanostructures prepared by the simple wet-chemical method exhibited an impressive specific capacitance with good cycling stability over 1000 cycles.

Temperature-dependent radiative and non-radiative dynamics of photo-excited carriers in extremely high-density and small InGaN nanodisks fabricated by neutral-beam etching using bio-nano-templates

Temperature-dependent radiative and non-radiative dynamics of photoexcited carriers were studied in In0.3Ga0.7N nanodisks (NDs) fabricated from quantum wells (QWs) by neutral-beam etching using bio-nano-templates. The NDs had a diameter of 5u2009nm, a thickness of 2 and 3u2009nm, and a sheet density of 2u2009×u20091011u2009cm–2. The radiative decay time, reflecting the displacement between the electron and hole wavefunctions, is about 0.2u2009ns; this value is almost constant as a function of temperature in the NDs and not dependent on their thickness. We observed non-exponential decay curves of photoluminescence (PL) in the NDs, particularly at temperatures above 150u2009K. The thermal activation energies of PL quenching in the NDs are revealed to be about 110u2009meV, corresponding to the barrier heights of the valence bands in the disks. Therefore, hole escape is deemed responsible for the PL quenching, while thermal activation energies of 12u2009meV due to the trapping of carriers by defects were dominant in the mother QWs. The above-mentioned non-exponential PL decay curves can be attributed to variations in the rate of hole escape in the NDs because of fluctuations in the valence-band barrier height, which, in turn, is possibly due to compositional fluctuations in the QWs. We found that non-radiative trapping, characteristic of the original QW, also exists in about 1% of the NDs in a form that is not masked by other newly formable defects. Therefore, we suggest that additional defect formation is not significant during our ND fabrication process.Temperature-dependent radiative and non-radiative dynamics of photoexcited carriers were studied in In0.3Ga0.7N nanodisks (NDs) fabricated from quantum wells (QWs) by neutral-beam etching using bio-nano-templates. The NDs had a diameter of 5u2009nm, a thickness of 2 and 3u2009nm, and a sheet density of 2u2009×u20091011u2009cm–2. The radiative decay time, reflecting the displacement between the electron and hole wavefunctions, is about 0.2u2009ns; this value is almost constant as a function of temperature in the NDs and not dependent on their thickness. We observed non-exponential decay curves of photoluminescence (PL) in the NDs, particularly at temperatures above 150u2009K. The thermal activation energies of PL quenching in the NDs are revealed to be about 110u2009meV, corresponding to the barrier heights of the valence bands in the disks. Therefore, hole escape is deemed responsible for the PL quenching, while thermal activation energies of 12u2009meV due to the trapping of carriers by defects were dominant in the mother QWs. The above-me...