K. Tachibana

Nanometer-scale InGaN self-assembled quantum dots grown by metalorganic chemical vapor deposition

We have successfully grown nanometer-scale InGaN self-assembled quantum dots (QDs) on a GaN surface without any surfactants, using atmospheric-pressure metalorganic chemical vapor deposition. Atomic force microscopy shows that the average diameter of InGaN QDs is as small as 8.4 nm. Next, we have investigated the dependence of the QDs properties on the growth conditions: the amount of InGaN deposited and the growth temperature. Moreover, we have investigated the optical property of InGaN QDs, so that the strong emission was seen at 2.86 eV at room temperature.

Selective growth of InGaN quantum dot structures and their microphotoluminescence at room temperature

We have fabricated InGaN quantum dot (QD) structures on hexagonal pyramids of GaN, using metalorganic chemical vapor deposition with selective growth. Intense photoluminescence was observed from the sample at room temperature. To directly observe the emitting areas, microphotoluminescence intensity images with a spatial resolution of a few hundred nanometers were used. The images show the emission was only from the tops of the hexagonal pyramids. The width of the emitting areas is about 300 nm, which is comparable to the spatial resolution of the images. Such a narrow width of emission areas indicates that InGaN QDs are formed on the tops of pyramids.

High-density and size-controlled GaN self-assembled quantum dots grown by metalorganic chemical vapor deposition

GaN self-assembled quantum dots (QDs) with high quality and high density have been grown by low-pressure metalorganic chemical vapor deposition under very low V/III ratios. In depositing over a critical thickness of four monolayer GaN, we observed a transition from two-dimensional to three-dimensional growth mode. The density of the QDs could be changed between 109 and 1010 cm−2. The typical diameter and height of the QDs were 20 and 2 nm, respectively. The size of the QDs was controlled to a considerable extent by changing the growth temperature and V/III ratio. Moreover, we observed two photoluminescence peaks from both the QDs and the wetting layer at room temperature. This result clearly demonstrates that the GaN QDs were formed with the Stranski–Krastanow growth mode.

Room-temperature lasing oscillation in an InGaN self-assembled quantum dot laser

We have demonstrated laser action of an InGaN self-assembled quantum dot (QD) laser by optical pumping. We have grown the laser structure with the In0.2Ga0.8N QDs embedded in the active layer, using atmospheric-pressure metalorganic chemical vapor deposition. A clear threshold was observed in the relation between the excitation and emission intensity at room temperature. Above the threshold, the width of the emission peak was below 0.1 nm (resolution limit), and the emission was strongly polarized in the transverse electric mode. These results indicate that lasing oscillation in the InGaN self-assembled QD laser has been achieved at room temperature.

Lasing Emission from an In0.1Ga0.9N Vertical Cavity Surface Emitting Laser

Lasing action in an In0.1Ga0.9N vertical cavity surface emitting laser was successfully achieved, for the first time, at a wavelength of 381 nm. The 3λ vertical cavity comprising an In0.1Ga0.9N active region was grown on a GaN/Al0.34Ga0.66N quarter-wave reflector by metal organic chemical vapor deposition (MOCVD), and covered with a TiO2/SiO2 reflector by electron-beam evaporation. The laser was operated at 77 K under optical excitation. We have observed a significant narrowing of the emission spectrum from 2.5 nm below the threshold to 0.1 nm (resolution limit) above the threshold, which is a clear signature of lasing action.

Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots

Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots is reported. Two samples having different average size of quantum dots (QDs) have been investigated at the temperature of 3.5 K. The measurement has revealed that larger-QD sample shows longer photoluminescence (PL) decay time and smaller emission energy than smaller one. The dependence of radiative decay time of the samples on emission energy smoothly connects with each other reflecting the size distribution. The radiative decay time strongly increases by almost three orders magnitude, reaching microseconds, upon increasing the size of QDs. The increase of PL decay time with increasing the size of QDs is attributed to the reduction of oscillator-strength due to the strong built-in electric field in the GaN/AlN heterostructures.

Rectification properties of layered boron nitride films on silicon

Cubic boron nitride (c-BN)/turbostratic boron nitride (t-BN) layered films were deposited on n-type Si substrates, and their rectification properties were investigated. Rectification in a typical n-type/p-type diode was observed in the current–voltage characteristics of c-BN film with a thin t-BN initial layer. However, the rectification polarity was inverted in the double-layered film with thick t-BN, where conduction was found to be caused by Schottky and Frenkel–Poole emission conduction mechanisms, depending on the range of bias applied. In the case of a thick t-BN single-layered film, the Frenkel–Poole emission conduction mechanism governed the conduction.

Optical spectroscopy of self-assembled type II GaSb/GaAs quantum dot structures grown by molecular beam epitaxy

We report an optical spectroscopic study of GaSb/GaAs quantum dots (QDs) formed by the Stranski–Krastanow growth mode using molecular beam epitaxy. We identify the QD luminescence by photoluminescence obtained at different excitation energies and densities. We show that, for these structures, not only the spectral position of peaks, but also their relative intensities are critically dependent upon the density of photogenerated carriers. Photoluminescence excitation (PLE) measurements confirm our assignment of the QD related peaks and a feature ∼25–27 meV higher in energy than the PLE detection energy is discussed in terms of phonon relaxation.

Growth of InGaN self-assembled quantum dots and their application to lasers

We have successfully grown InGaN self assembled quantum dots (QDs) on a GaN layer, using atmospheric-pressure metalorganic chemical vapor deposition (MOCVD). The average diameter of the QDs was as small as 8.4 nm, and strong emission from the QDs was observed at room temperature. Next, we have investigated a structure in which InGaN QDs were stacked to increase the total QD density. InGaN QDs were formed even when the number of stacked layers was ten. As the number of layers increased, the photoluminescence (PL) intensity increased drastically. Moreover, we have fabricated a laser structure with InGaN QDs embedded into the active layer. A clear threshold of 6.0 mJ/cm/sup 2/ was observed in the dependence of the emission intensity on the excitation energy at room temperature under optical excitation. Above the threshold, the emission was strongly polarized in the transverse electric (TE) mode, and the linewidth of the emission spectra was reduced to below 0.1 nm (resolution limit). The peak wavelength was around 405 nm. These results indicate lasing action at room temperature.

Density Control of GaSb/GaAs Self-assembled Quantum Dots (〜25nm) Grown by Molecular Beam Epitaxy

We report the realization of quantum-sized GaSb dots, of small diameter (~25nm), on GaAs by molecular beam epitaxy in the Stranski-Krastanow growth mode. At the deposition of 3.1 mono-layer(ML) GaSb, the average diameter and height of GaSb quantum dot(QD) are 26 nm and 6.2 nm, respectively. In addition, the density control was systematically achieved between 2.6×109 to 1.2×1010cm-2 by carefully choosing the amount of GaSb deposited from 2.5 to 3.1ML. The growth mechanism are discussed in detail. These results are very useful in forming a QD of staggered band lineup (type-II) of possible use in novel device applications