Kouji Yamano

Monolithic Integration of InGaN-Based Nanocolumn Light-Emitting Diodes with Different Emission Colors

We demonstrate the monolithic integration of green and orange InGaN-based nanocolumn light-emitting diodes (LEDs). Four nanocolumn LED crystals (LEDs 1 to 4), which consisted of regularly arranged InGaN-based nanocolumns in a triangular lattice of 400 nm lattice constant, were grown on the same GaN template on a (0001) sapphire substrate, with designed nanocolumn diameters D of 150, 190, 230, and 270 nm for LEDs 1–4, respectively. LEDs 1 to 3 operated under DC current injection at room temperature, emitting at 544, 583, and 597 nm, respectively. This experiment paves the way for the monolithic integration of three-primary-color nanocolumn LEDs.

Two-dimensional light confinement in periodic InGaN/GaN nanocolumn arrays and optically pumped blue stimulated emission

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.

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.

Raman Scattering from a Surface Phonon in GaN Nanowalls and Regularly‐Arrayed GaN Nanocolumns

We study Raman scattering of GaN nanowalls and regularly‐arrayed GaN nanocolumns, and observe a peak from surface phonon (SP) around 705 cm−1. The Raman intensity of SP decreases with increasing wall width in GaN nanowalls, while it increases with increasing column diameter in GaN nanocolumns. The Raman peak of SP has different polarization characteristics between GaN nanowalls and nanocolumns.

Second harmonic generation from photonic structured GaN nanowalls

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.

Raman scattering from surface phonons in GaN nanostructures

We study Raman scattering of GaN regularly-arrayed nanowalls, nanocolumns and nanorings, and observe a peak from surface phonons around 705 cm−1. The peak from the surface phonons in the nanocolumns is much broader than those in the nanowalls and nanorings, but is not observed when the diameter of column is thinner than 100 nm. This is explained in terms of quantization of the azimuthal wave vector on the circumference of columnar structure. In the nanorings, we observe a very weak peak from a surface phonon.

Regularly arranged InGaN-based nanocolumns and related device technology

By use of Ti-mask selective area growth (SAG) technique by rf-MBE for GaN [1], various regularly arranged InGaN-based nanocolumn arrays with triangular-lattice arrays were grown on the same substrate; the nanocolumn diameters of the nanocolumn arrays were changed from 137 nm to 270 nm at the same lattice constant of 400 nm; three-period InGaN(3nm)/GaN-MQWs were prepared at top regions of the nanocolumns. Change in the emission-color from blue to red with increasing the nanocolumn diameter was observed for the samples, as shown in Fig.1. Here, the photoluminescence peak wavelength shifted monotonically from 513 nm to 622 nm. The beam shadow effect by the neighboring nanocolumns introduces the change of the In incorporation ratio into the InGaN QWs, and the mechanism is discussed [2].

Nitride-based Nanocolumns and Applications

InGaN-based nanocolumn arrays were grown by selective area growth of rf-MBE. Using the nanocolumn arrays, the emission color control, green light stimulated emissions from 520 to 566 nm, and green emission LEDs were successfully demonstrated.

Green region stimulated emission in optically pumped InGaN/GaN MQW nanocolumn arrays of triangular lattice

520–565 nm wavelength stimulated emissions were successfully obtained under room temperature (RT) optical excitation for triangular-lattice GaN-based nanocolumn arrays, in which InGaN/GaN multiple-quantum-well (MQW) was integrated at the top of each nanocolumn. The GaN nanocolumn array was prepared with selective area growth (SAG) of rf-plasma assisted molecular beam epitaxy (rf-MBE); the diameter and height of the GaN nanocolumns were 100–300 nm and 0.2–2.0 µm, respectively. This experiment demonstrates prospects of GaN nanocolumns for application to green emitting devices.

InGaN-based nanocolumns for green light emitters

Uniform arrays of GaN nanocolumns periodically arranged in triangular-lattice, at the top regions of which InGaN/GaN multiple quantum wells (MQWs) were integrated, were grown on GaN templates by rf-MBE using Ti-mask selective area growth (SAG) technique [1]. Here the lattice constant of array L and nanocolumn diameter D were controlled from 200 to 300 nm and from 0.7L to 0.9L, respectively. Figure 1 shows top and birds-eye SEM views of a nanocolumn array with L = 275 nm and D = 210 nm; excellent