菊池 昭彦

We report observations of random laser action in self-organized GaN nanocolumns. We have measured three samples with different filling fractions and investigated the dependence of the lasing property on the random configuration of nanocolumns. Numerical calculations based on a finite-difference time-domain method have also been performed and the comparison with the experimental results shows a clear relationship between the strength of light localization and the occurrence of random laser action. (C) 2010 American Institute of Physics. [doi:10.1063/1.3495993]

The relaxation of lattice-mismatched strain by deep postetching was systematically investigated for InGaN/GaN multiple quantum wells (MQWs). A planar heterojunction wafer, which included an In(0.21)Ga(0.79)N (3.2 nm)/GaN (14.8 nm) MQW, was etched by inductively coupled plasma dry etching, to fabricate high-density nanopillar, nanostripe, and nanohole arrays. The etching depth was 570 nm for all nanostructures. The diameter of the nanopillars was varied from 50 to 300 nm, then the mesa stripe width of the nanostripes and the diameter of the nanoholes were varied from 100 nm to 440 nm and 50 nm to 310 nm, respectively. The effect of strain relaxation on various optical properties was investigated. For example, in an array of nanopillars with diameter 130 nm and interval 250 nm, a large blueshift in the photoluminescence (PL) emission peak from 510 nm (as-grown) to 459 nm occurred at room temperature (RT). PL internal quantum efficiency (defined by the ratio of PL integral intensity at 300 K to that at 4.2 K) was enhanced from 34% (as-grown) to 60%, and the PL decay time at 4.2 K was reduced from 22 ns (as-grown) to 4.2 ns. These results clearly indicate the reduction of lattice-mismatched strain by postetching, which enhanced strain reduction with decreasing nanopillar diameter down to a diameter of 130 nm, where the strain reduction became saturated. The dependence of RT-PL decay time on nanopillar diameter was measured, and the surface nonradiative recombination velocity was estimated to be 5.8x10(2) cm/s. This relatively slow rate indicates a little etching damage. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3369434]

Triangular GaN nanocolumn arrays, in which InGaN/GaN multiple quantum wells were integrated at the top region of each nanocolumn, were grown by Ti-mask selective-area growth of RF-plasma-assisted molecular beam epitaxy. In the nanocolumn arrays, a strong enhancement in light intensity at the specific wavelength of 479.4 nm was observed under low optical excitation, and then optically pumped stimulated emission occurred at near the specific wavelength (476.3 nm) under high optical excitation with a threshold excitation density of 400 kW/cm(2). The light response of the nanocolumn array was numerically analysed by two-dimensional (2D) finite-difference time-domain and 2D plane-wave expansion methods, proving that the stimulated emission occurred in the 2D distributed feedback scheme.

We present frequency dependence of Anderson localization of light in finite-sized two-dimensional random systems. We have investigated time variation of total energy within a system composed of dielectric columns, by adopting parameters of actual nano-sized semiconductor crystals with a high filling fraction of the columns. We have succeeded in obtaining single parameter scaling of the localization effect. In addition, we show that the frequency dependence of the localization in the system depends on the degree of wave interference due to Bragg-like diffraction, rather than on the magnitude of the light scattering cross section for a single column.

We fabricated periodic IncaN-based hexagonal nanoring arrays by rf-molecular beam epitaxy (rf-MBB) with Ti-mask selective area growth (SAG). Multiple photoluminescence peaks from InGaN-based hexagonal nanorings were observed in room temperature photoluminescence (RT-PL) measurements with 325 nm He-co laser. We employed a simple plane wave model and a two-dimensional finite difference time domain (2D-FDTD) method in the numerical analysis to investigate these multiple pearks. Experimental data was in good coincidence with calculated data, evincing that whispering gallery modes (WGMS) supported in the InGaN-based hexagonal nanorings were responsible for the multipeaks.

The optical properties of InGaN/GaN quantum wells, which were nanopatterned into cylindrical shapes with diameters of 2 mu m, 1 mu m, or 500 nm by chemically assisted ion beam etching, were investigated. Photoluminescence (PL) and time-resolved PL measurements suggest inhomogeneous relaxation of the lattice-mismatch induced strain in the InGaN layers. By comparing to a strain distribution simulation, we found that partial stain relaxation occurs at the free side wall, but strain remains in the middle of the pillar structures. The strain relaxation leads to an enhanced radiative recombination rate by a factor of 4-8. On the other hand, nonradiative recombination processes are not strongly affected, even by postgrowth etching. Those characteristics are clearly reflected in the doughnut-shape emission patterns observed by optical microscopy.

InGaN-based nanocolumn LEDs from ultraviolet to red were fabricated through self-assembly of GaN nanocolumns by rf-MBE. Ti-mask selective area growth (SAG) was employed to fabricate uniform arrays of InGaN/GaN nanocolumns, and the photoluminescence observation of the arrays demonstrated that the emission color changed from blue to red with increased nanocolumn diameter. © 2009 OSA.

We have measured the optical properties of wurtzite InN nanocolumns and film by photoluminescence (PL) Measurements at temperatures from 5 to 300 K and analyzed the PL spectra by fitting with the free-electron recombination bound (FERB) model. For the top-linked InN nanocolumns, we observed strong PL intensity compared to the InN film sample. The PL spectra were asymmetrical with low-energy tails and a red-shift of the PL peak energy position was observed with increasing temperature. However, For the separated InN nanocolumns, we observed weak PL intensity and symmetrical PL spectra. Analyzing the spectra shape of the top-linked InN nanocolumns at 5 K using the FERB model, we evaluated the intrinsic bandgap energy and carrier density of InN nanocolumns to be 0.69 eV and 2.5 x 10(17) cm(-3), respectively. (C) 2009 Published by Elsevier B.V.

Fast and slow photocarrier dynamics in indium nitride films have been investigated using femtosecond transient measurements at room temperature. The behavior of the decay dynamics is found to be strongly dependent on the excitation energy and the background carrier density. We have found that the slow decay component disappears under optimized conditions due to equilibrium between band filling and bandgap renormalization effects. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3272916]

Two-dimensional (2D) light diffraction in a uniform array of GaN nanocolumns arranged in a rectangular lattice dramatically enhanced the light intensity at a specific wavelength, indicating the function of 2D distributed feedback (DFB). Here a GaN rectangular-lattice nanocolumn array, which integrated InGaN/GaN multiple quantum wells (MQWs) in the top region of the nanocolumns, was grown by rf-plasma-assisted molecular beam epitaxy (rf-MBE). At a specific wavelength of 471.1 nm, the first observation of stimulated emission from 2D-DFB in an InGaN-based nanocolumn array was obtained. The specific wavelength is calculated by the 2D finite-difference time domain (2D-FDTD) method on the assumption of a refractive index dispersion of GaN; a simple expression for specific wavelength, which is a function of the array period L and the hexagon side length S of each nanocolumn, is proposed, which is convenient for producing a simple design of a GaN nanocolumn array structure in a square lattice. (C) 2009 Optical Society of America

GaN nanorings with InGaN/GaN multiple quantum wells (MQWs) were successfully fabricated on a GaN template by RF-plasma-assisted molecular-beam epitaxy (RF-MBE) using a titanium (Ti) mask selective-area growth (SAG) technique. We employed focused ion beam (FIB) milling to make patterns on the Ti-mask (ring groove patterns). 3-period InGaN/GaN MQWs integrated into the GaN nanorings exhibited a clear room-temperature photoluminescence (RT-PL) spectrum with a peak wavelength of 462 nm and the full width at half maximum (FWHM) of the RT-PL spectrum was 145 meV. In addition, we fabricated GaN double nanorings and hexagonal nanorings with InGaN/GaN MQWs by controlling the amount of FIB etching, and the growth conditions. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have investigated how the morphology and diameter of GaN nanocolumn affect the optical properties of exciton and biexciton. We measured photoluminescence (PL) of excitons in GaN nanocohinms at low temperature and found that the PL spectra depend on the column diameter. Changing the distance between the sample and the collimatmg lens, it was established that this phenomenon is due to the z-polarized PL radiated from the side surface. This result proves that the nano-scale morphology produces a change in the optical properties. In addition, to elucidate the mechanism causing the increase of the biewciton binding energy observed in a previous experiment, we calculated the exciton binding energies by the variational method and also estimated their sizes. From the calculated results we can expect that the only biexcitons experience spatial confinement effect even when the nanocohimn width is much larger than the exciton Bohr radius. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.

The Ti-mask selective-area growth (SAG) of GaN nanocolumns was performed at the growth temperature of 900 degrees C, while decreasing the supplied nitrogen flow rate (Q(N2)) from 3.5 to 0.5 sccm. Highly uniform arrays of GaN nanocolumns were demonstrated. At low Q(N2), both the desorption and diffusion of Ga from/on the nitrided Ti mask were accelerated, which sufficiently suppressed the crystal nucleation on the Ti-mask surface, and hence the SAG of the GaN nanocolumns was achieved even when the spacing between the nanocolumns was several hundred nm. The enhancement of Ga desorption with decreasing Q(N2) brought about a reduction in the growth rate of GaN nanocolumns from 1.05 to 0.15 mu m/h. The lateral growth rate of the GaN nanocolumns rapidly increased above the critical Q(N2) value of 1.5 sccm and became 45 nm/h at Q(N2) of 3.5 sccm. For low Q(N2) values less than 1.5 sccm, the lateral growth rate became sufficiently low, approximately 8 nm/h; this contributes to well-controlled SAG of GaN, where the underlying nanomask patterns are well traced. (C) 2008 Elsevier B.V. All rights reserved.

Ultrafast and slow photo-earner dynamics in InN films were investigeted using femtosecond transient measurements. We found the slow component disappears under the optimized condition due to the equilibrium of band filling and bandgap renormalization effects

We observed large enhancement of reflected second harmonic generation (SHG) using the one-dimensional photonic effect in regularly arranged InGaN/GaN single-quantum-well nanowalls. Using the effect when both fundamental and SH resonate with the photonic mode, we obtained enhancement of about 40 times compared with conditions far from resonance.

We show pseudogap maps of Anderson localization of light adopting the parameters of self-organized nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate the parametric dependence of the localization effect. To obtain the maps, we simulated light propagation in open random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We found that the shape of the pseudogaps is close to the one of bandgaps in photonic crystals. We conclude that strong localization of light occurs because of interference by average Bragg diffraction, not strong Mie resonant peaks.

We analyzed photoluminescence (PL) spectra of In-rich In(x)Al(1-x)N nanocolumns (NCs; x = 0.80) and film (x = 0.82) using the free-electron recombination to bound (FERB) model. The FERB model fits well to the PL spectra of the In-rich In(x)Al(1-x)N film and NCs, which implies that light-emitting properties of In-rich In(x)Al(1-x)N are similar to those of InN. We obtained bandgap energies and electron carrier densities from the results of fitting the PL spectra.

Photo-excited carrier relaxation dynamics in InN films were investigated using femtosecond transient measurements. We measured two samples with different background carrier densities, and discussed the influence of the carrier densities on the relaxation dynamics. We also found that the slow decay component disappears under the optimized condition because of the effects of equilibrium band filling and bandgap renormalization.

GaN nanocolumns are one-dimensional high-quality dislocation-free nano-crystals. The effect of Be-doping on GaN nanocolumns was investigated using InGaN/GaN nanocolumn LED structure grown by RF-plasma-assisted molecular-beam epitaxy. Be was doped into the upper cladding region of the nanocolumn light-emitting diodes (LEDs). Be is expected to be a p-type dopant of GaN with a shallow accepter level, but p-type conductivity was not obtained in these experiments. However, we observed a remarkable enhancement of the lateral growth of the GaN nanocolumn by Be doping; that is, the surface of the Be-doped region became smooth and continuous with increasing Be doping concentration. The root mean square (RMS) surface roughness was reduced from 35 to 10 nm with increasing Be cell temperature from 710 to 975 °C. Cross-sectional TEM observation revealed that no threading dislocations were generated in the observed area at the conjunction boundary between neighboring nanocolumns during lateral growth. This suggests that a high-quality over-growth layer with a low threading dislocation density can be realized on Be-doped GaN nanocolumns. (Figure Presented) © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

The selective-area growth (SAG) of GaN nanocolumns by RF-plasma-assisted molecular-beam epitaxy is demonstrated by the use of Ti mask patterns on (111) Si substrates. At the growth temperature of 935 degrees C, the GaN nanocolumns grew only on the stripe windows where Si was exposed, and the growth of nanocolumns on the Ti surface was completely suppressed. The SAG of GaN occurred above approximately 900 degrees C, but below that no SAG occurred because GaN crystals nucleated on the Ti mask. When the window width was 120 nm, a single-nanocolumn was aligned along the Ti window.

We report the fabrication of GaN/AlGaN nanocolumn on n-(111) si substrates by RF-MBE for the first time. Clear diode characteristics with a turn-on voltage of 4 V and an ultraviolet emission spectrum with a peak wavelength of 354 nm were observed at room temperature. When the Al composition of p-Al xGa 1-xN was changed from 8.8% to 25.1%, the high Al content led a narrowing of the FWHM compared withthat for low Al contents due to the suppression of carrier overflow. We measured the electroluminescence (EL) under dc and pulsed operations. The integrated EL intensity under the pulsed operation was 3 times as strong as that under the dc operation at 100 mA due to the reduced generation of heat. The thermal resistance was estimated to be 40 °C/W from the EL peak wavelength difference between the dc and pulsed operations. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

We measured photoluminescence (PL) from excitons and biexcitons in GaN nanocolumns at low temperature and found that the PL spectra of excitons depend on the nanocolumn diameter. Taking into account the polaritonic effect of the excitons, calculation of the PL spectra revealed that the dependence on diameters causes a difference of PL intensity from side surfaces of the nanocolumns. At high excitation intensities, we also observed biexciton emissions and found that the biexciton binding energies are higher than those in bulk samples. Although the mechanism for the increase in the binding energy is not clear at present, we suppose that it arises from a spatial confinement effect due to the nanocolumn morphology. (C) 2007 Elsevier B.V. All rights reserved.

InxAl1-xN nanocolumns (0.71≤xIn≤1. 00) were fabricated on Si (111) substrates by RF-MBE. The room temperature photoluminescence (RT-PL) in optical communication wavelengths from 0.95 to 1.94 μm with changing xIn was observed. InN/InAlN heterostructures were also fabricated. ©2008 IEEE.

Periodical GaN nanocolumn arrays were grown on (111) Si substrates with AlN patterns by RF-MBE. Positional controls of nanocolumns were realized by artificial control of GaN nucleation using AlN nano-dot and AlN nano-hole patterns. ©2008 IEEE.

The IQE of InGaN quantum-wells emit from green to red is not so high due to spatial separation of electrons and holes by strain-induced piezo-electric-field. The strain relaxation in lnGaN-QWs by nano etching is discussed. ©2008 IEEE.

The selective area growth of GaN nanocolumns was performed on a Si (111) substrate using predeposited Al nanopatterns by rf-plasma-assisted molecular-beam epitaxy. The nanopatterns consisted of Al dots with 20-65 nm diameter and 9-27 nm thickness that were arranged in a 300-nm-period triangular lattice array and prepared by electron-beam lithography and a liftoff process. The shape of the At pattern tended to change during the growth process before nanocolumn growth, but its significant deformation was suppressed by nitridation at temperatures below 860 degrees C. The configuration of the GaN nanocolumns grown on the nitrided Al patterns depended on the pattern configuration; a sufficient volume of nitrided Al was required for nanocolumns to grow perpendicular to the substrate surface.

The selective-area growth of GaN nanocolumns using predeposited Al nanopatterns (a 300-nm-period triangular lattice of 85-nm-diameter Al nanodots) on Si(111) substrates by rf-plasma-assisted molecular-beam epitaxy (rf-MBE) was demonstrated. GaN nanocolumns were grown at the edge of each nitrided Al dot after nitridation, forming a nanotubular structure in the growth temperature range from 941 to 966 degrees C. The size fluctuation of the sidewall thickness in the nanotubular structure was less than that of the diameters of nanocolumns grown on the Si surface outside the nitrided Al nanopatterns. At a high growth temperature of 966 degrees C, nanocolumn growth on the Si surface was completely suppressed. C) 2008 The Japan Society of Applied Physics.

GaN nanocolumns have excellent optical characteristics owing to their dislocation-free nature. GaN/A1GaN nanocolumn ultraviolet light-emitting diodes were demonstrated on n-(111) silicon by RF-plasma-assisted molecular beam epitaxy for the first time. Clear diode characteristics and ultraviolet emission with a peak wavelength of 354 nm were observed under continuous current injection at room temperature.

The selective area growth of GaN nanocolumns was performed on a Si (111) substrate using predeposited Al nanopatterns by rf-plasma-assisted molecular-beam epitaxy. The nanopatterns consisted of Al dots with 20-65 nm diameter and 9-27 nm thickness that were arranged in a 300-nm-period triangular lattice array and prepared by electronbeam lithography and a liftoff process. The shape of the Al pattern tended to change during the growth process before nanocolumn growth, but its significant deformation was suppressed by nitridation at temperatures below 860°C. The configuration of the GaN nanocolumns grown on the nitrided Al patterns depended on the pattern configuration a sufficient volume of nitrided Al was required for nanocolumns to grow perpendicular to the substrate surface. (Figure Presented). © 2008 Wiley-VCH Verlag GmbH &amp Co. KGaA.

GaN nanocolumns are one-dimensional high-quality dislocation-free nano-crystals. The effect of Be-doping on GaN nanocolumns was investigated using InGaN/GaN nanocolumn LED structure grown by RF-plasma-assisted molecular-beam epitaxy. Be was doped into the upper cladding region of the nanocolumn light-emitting diodes (LEDs). Be is expected to be a p-type dopant of GaN with a shallow accepter level, but p-type conductivity was not obtained in these experiments. However, we observed a remarkable enhancement of the lateral growth of the GaN nanocolumn by Be doping that is, the surface of the Be-doped region became smooth and continuous with increasing Be doping concentration. The root mean square (RMS) surface roughness was reduced from 35 to 10 nm with increasing Be cell temperature from 710 to 975 °C. Cross-sectional TEM observation revealed that no threading dislocations were generated in the observed area at the conjunction boundary between neighboring nanocolumns during lateral growth. This suggests that a high-quality over-growth layer with a low threading dislocation density can be realized on Be-doped GaN nanocolumns. (Figure Presented) © 2008 Wiley-VCH Verlag GmbH &amp Co. KGaA.