Shunsuke Ishizawa

InGaN/GaN Nanocolumn LEDs Emitting from Blue to Red

Self-assembled GaN nanocolumns were grown on sapphire and Si substrates by rf-plasma-assisted molecular-beam-epitaxy, clarifying the growth condition. The nanocolumn crystal showed a highly efficient photoluminescence (PL) emission at the room temperature, which intensity was 4 times stronger than that of a high-quality GaN substrate. InGaN/GaN quantum-disk nanocolumn LEDs were fabricated on n-type (111) Si substrates. For a macroscopic emission area of 500-&mgr;m-diameter, a broad electro-luminescence (EL) emission spectrum extending from the blue to the red region was observed. Microscopic EL measurement was performed for a 3-&mgr;m-diameter detection area, demonstrating a drastic spectral narrowing. In the microscopic EL spectrum, no blue shift of the emission wavelength was observed when the injection current increased. This suggests that the carrier localization or/and the piezo-electric field is minimized in nanocolumns. Selective growth of GaN nanocolumns was performed by use of patterned pre-deposited Al layers.

Selective-Area Growth of GaN Nanocolumns on Si(111) Substrates Using Nitrided Al Nanopatterns by RF-Plasma-Assisted Molecular-Beam Epitaxy

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 °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 °C, nanocolumn growth on the Si surface was completely suppressed.

Directional radiation beam from yellow-emitting InGaN-based nanocolumn LEDs with ordered bottom-up nanocolumn array

Yellow-emitting (572 nm) InGaN-based nanocolumn LEDs, consisting of orderly arranged bottom-up nanocolumns in a triangular lattice, were fabricated. We observed a spectral linewidth of 37.3 nm at 226 A/cm2, which was exceptionally narrow for yellow emission. The periodic arrangement of nanocolumns leads to the photonic crystal effect. We measured the angular dependence of the emission spectrum, using which the photonic band diagram was experimentally traced, evincing that the yellow light was diffracted at the photonic band edge. As a result, a highly directional radiation beam with a radiation angle of ±20° suitable for display applications was successfully demonstrated.

Spectrally-broadened multimode lasing based on structurally graded InGaN nanocolumn photonic crystals suitable for reduction of speckle contrast

Bottom-up grown structurally graded InGaN-based nanocolumn photonic crystals, in which nanocolumns were arranged in triangular lattice and the nanocolumn diameter changed one-dimensionally from 93 to 213 nm with a fixed lattice constant of 250 nm, were fabricated. The spatial distribution of the diameter resulted in random-laser-like operation under optical excitation. A broad multi-wavelength lasing spectrum with more than 10 peaks was obtained with a full width at half maximum of 27 nm at 505 nm wavelength as well as lowering of the polarization degree, which is expected to be suitable for speckle contrast reduction in laser projection display applications.

Photon correlation study of background suppressed single InGaN nanocolumns

We report on a linearly polarized non-classical light emission from a single InGaN/GaN nanocolumn, which is a site-controlled nanostructure allowing for pixel-like large-scale integration. We have developed a shadow mask technique to reduce background emissions arising from nitride deposits around single nanocolumns and defect states of GaN. The signal to background ratio is improved from to , which allows for detailed polarization-dependent measurement and photon-correlation measurements. Polarization-dependent measurements show that linearly polarized emissions arise from excitonic recombination involving a heavy-hole-like electronic state, corresponding to the bulk exciton of an in-plane polarized A exciton. The second-order coherence function at time zero g (2)(0) is 0.52 at 20 K without background correction. This value is explained in terms of a statistical mixture of a single-photon emission with residual weak background emissions, as well as efficient carrier injection from other localized states.

Single InGaN nanocolumn spectroscopy

We report on the spectroscopy of single InGaN/GaN nanocolumns that are site-controlled nanostructures allowing for pixel-like large-scale integration. A single nanocolumn shows several narrow photoluminescence peaks at around 600 nm at 20–90 K, the linewidth of which ranges between 0.3 and 10 meV. We have observed an interesting temperature dependence of the integrated intensity, the maximum being around 40–70 K, which suggests the existence of efficient carrier injection channels from localized states. The results show that the optical properties of single nanocolumns are favorable for the future large-scale integration of single-photon emitters and qubits.

GaN nanocolumn light-emitters, growth, and optical characterization

Selective area growth of GaN nanocolumn arrays on Si substrates was developed. Orange-emitting nanocolumns on GaN/Al2O3 templates were optically characterized, fabricating red-color InGaN-based nanocolumn LEDs. Successful monolithic integration of four emission-colors nanocolumn LEDs was demonstrated.

Triangle-lattice InGaN/GaN nanocolumn arrays exhibiting photonic crystal effect (Conference Presentation)

GaN nanocolumns are extensively studied as promising nano-materials for high-performance visible emitters because of their dislocation filtering and strain relaxation effects. The size and position of nanocolumns were precisely controlled using Ti-mask selective-area growth (SAG) by RF-MBE, fabricating uniform arrays of pn-junction InGaN/GaN nanocolumns. The periodic arrangement in the nanocolumn arrays led to nanocolumn photonic crystal (PhC) effect. It is however, necessary to integrate a wave-guiding scheme in the nanocolumn system to activate efficiently the PhCs. In the experiment, triangle-lattice GaN nanocolumn arrays with the lattice constant from 280 to 350 nm were grown, followed by the growth of InGaN/GaN superlattice buffer, MQW, and p-type GaN cladding layers. In the upper region of pn-junction nanocolumns from SL to p-GaN, the nanocolumn diameter increased and introduced the increase in the equivalent refractive index, which acts to confine the optical field there. Thus, the optical mode propagated laterally, interacting with the nanocolumn PhC. The diffraction at the photonic band edge resulted in high-directional beam radiations from the nanocolumn system. The photonic band edge was systematically investigated for various nanocolumn arrays with L=280–250 nm. The experimental photonic band diagram for the triangular-lattice pn-junction InGaN/GaN nanocolumn array exhibited a clear photonic band edge.

Emission Characteristics Based on Nanocolumn Photonic Crystal Effect of Orderly Arrayed InGaN/GaN Nanocolumns

In this paper, we discuss the emission characteristics based on the nanocolumn photonic crystal effect. The GaN nanocolumn arrays, at the top region of which InGaN/GaN MQWs were integrated, were optically pumped resulting in lasing oscillation, as shown in Fig. 2. Clear thresholds were observed in the light intensity vs. excitation density characteristics (see Fig.2 (c)). The lasing wavelength changed from 492.3 to 501.1 nm with increasing the column diameter (D) from 183 to 205 nm at the lattice constant (L) of 245 nm, as the result of the photonic band edge shift. Figure 3 shows the photonic band diagram of a nanocolumn system; note that the experimental lasing wavelength corresponded to the photonic band edge at Γ11. The normalized band edge wavelength L/λ is expressed as a function of the structural parameter of D/L 13) as shown in Fig. 4 (a), where the experimental lasing wavelength are plotted by circles.

InGaN-based nanocolumn light emitters in visible wavelength range

Light emission characteristics of bottom-up-grown GaN nanocolumns with InGaN/GaN multiple quantum wells (MQW) were investigated under high density optical excitations. Lasing emissions based on two-dimensional distributed feedback scheme in periodic structure of nanocolumns were successfully obtained in the visible wavelength range from 530 to 560 nm on the same substrate. Controllability of the wavelength by structural parameters of the nanocolumn array was demonstrated, suggesting realizability of monolithically integrated multiple-wavelength nanocolumn array lasers.