Naochika Horio

Subpicosecond exciton spin relaxation in GaN

The spin-relaxation process of A-band exciton in GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The spin-relaxation times at 150−225K are 0.47−0.25ps. These are at least one order of magnitude shorter than those of the other III-V compound semiconductors. The spin-relaxation time τs is found to be proportional to T−1.4, where T is the temperature.

Control of Emission Wavelength of GaInN Single Quantum Well, Light Emitting Diodes Grown by Metalorganic Chemical Vapor Deposition in a Split-Flow Reactor

We investigated the dependence of the electroluminescence (EL) characteristics on the layer thickness and the solid-phase composition of AlGaN clad layers in GaInN single-quantum-well light-emitting diodes (LEDs), and how to control the emission wavelength by the device structure. GaInN well layers were sandwiched by AlGaN clad layers, and GaInN inter layers were inserted between GaN n-contact and AlGaN n-clad layers. A wavelength shift of about 50 nm was achieved by changing either the Al solid-phase composition or the thickness of the clad layer. Due to an increase of the lattice mismatch between the clad and the well layers, the emission wavelength increased and the peak intensity decreased. Compared with the performance without an inter layer, the peak intensity increased 3.8 times. Moreover, by changing the growth temperature of the well layer, the LED wavelength can be varied from blue to red. Though the difference of the emission wavelength was 40 nm in the radial direction of the wafer, a luminous intensity of 1 cd for yellowish LEDs was achieved.

Metalorganic Chemical Vapor Deposition Growth of GaN Using a Split-flow Reactor

We developed a split-flow reactor for growing epitaxial films in an atmospheric metalorganic chemical vapor deposition (MOCVD), where the reactant gas stream is separated from the wall by a buffer gas stream. This reduces the use of reactant gases, thereby minimizing operating costs. Wall deposits were also minimized. The reactor was fan-shaped with an inclined upper wall. To reduce the interdiffusion of the reactant and buffer streams, the substrate was placed upstream, near the gas inlet. The downward inclined upper wall maintains a constant cross sectional area and therefore constant velocity in the radial direction. The result is that the flow is stabilized. We demonstrated this reactor by growing undoped GaN epitaxial films on a two-inch substrate using a two-step growth process, where the epitaxial layer is grown on a buffer layer. The highest quality GaN films were grown using a buffer stream of H2, but the lowest consumption of the reactant gases was achieved using a buffer stream of N2.

Picosecond spin relaxation of acceptor‐bound exciton in wurtzite GaN

The spin relaxation process of acceptor‐bound excitons in wurtzite GaN is observed by spin‐dependent pump and probe reflectance measurement with subpicosecond time resolution. The time evolutions measured at 15–50 K have a single exponential component corresponding to spin relaxation times of 1.40 – 1.14 ps. The spin relaxation time, τs, is found to be proportional to T−0.175, where T is the temperature. This temperature dependence is quite weak compared with that of A‐band free excitons showing τs ∝ T−1.41 at 150 – 225K.

Sub-picosecond exciton spin-relaxation in GaN

Exciton spin relaxations in bulk GaN were directly observed with sub-picoseconds time resolution. The obtained spin relaxation times of A-band free exciton are 0.47 ps - 0.25 ps at 150 K - 225 K. The spin relaxation time of the acceptor bound exciton at 15K is measured to be 1.1 ps. These are at least one order of magnitude shorter than those of the other III-V compound semiconductors. The spin relaxation time of A-band free exciton is found to be proportional to T -1.4, where T is the temperature. The fact that the spin relaxation time in GaN is shorter than that in GaAs, in spite of the small spin-orbit splitting, suggests that the spin relaxation is dominated by the defect-assisted Elliot-Yafet process.

Sub‐picosecond Spin Relaxation in GaN

The spin relaxation process of A‐band exciton in GaN is observed for the first time, to our knowledge, by spin dependent pump and probe reflectance measurement with sub‐picosecond’s time resolution. The spin relaxation times at 150–225 K are measured to be 0.47–0.25 ps. These are at least one order of magnitude shorter than those of the other III–V compound semiconductors. The spin relaxation time, τs, is found to be proportional to T−1.4, where T is the temperature.

Time-resolved photoluminescence study of InGaN MQW with a p-contact layer

To clarify the influence of diffused Mg impurities on the carrier recombination in InGaN multiple quantum wells (MQWs), we have performed a systematic study using time-resolved photoluminescence (PL) measurements. It was found that the MQWs with a p-contact layer and the MQWs with a nondoped GaN layer had almost the same carrier lifetime and PL intensity below 200 K. However, the MQWs with the p-contact layer had shorter carrier lifetime and lower PL intensity than the MQWs with the nondoped GaN layer above 200 K. This degradation of the PL for the MQWs with the p-contact layer can be attributed to nonradiative recombination caused by the diffusion of Mg impurities from the p-contact layer into MQWs.