光電所所訊

Regularly-patterned Multi-section GaN Nanorod Arrays Grown with a Pulsed Growth Technique

Professor C. C. (Chih-Chung) Yang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所楊志忠教授

The growth of regularly-patterned multi-section GaN nanorod (NR) arrays based on a pulsed growth technique with metalorganic chemical vapor deposition is demonstrated. Such an NR with multiple sections of different cross-sectional sizes is formed by tapering a uniform cross section to another through the decrease of Ga supply duration stepwise for reducing the size of the catalytic Ga droplet. Contrast line structures are observed in either a scanning electron microscopy or a transmission electron microscopy image of an NR. Such a contrast line-marker corresponds to a thin Ga-rich layer formed at the beginning of GaN precipitation of a pulsed-growth cycle and illustrates the boundary between two successive growth cycles in pulsed growth. By analyzing the geometry variation of the contrast line-markers, the morphology evolution in the growth of a multi-section NR, including a tapering process, can be traced. Such a morphology variation is controlled by the size of the catalytic Ga droplet and its coverage range on the slant facets at the top of an NR. The comparison of emission spectrum between single-, two-, and three-section GaN NRs with sidewall InGaN/GaN quantum wells (QWs) indicates that a multi-section NR can lead to a significantly broader sidewall emission spectrum.

Fig. 1 (a)-(d): Tilted SEM images of a single-section (sample S), two 2-section (samples T1 and T2), and one 3-section (sample T3) n-GaN NR arrays. (e)-(h): Tilted SEM images after three periods of InGaN/GaN QWs are grown on the sidewalls of the n-GaN NRs in samples S and T1-T3, respectively. (i)-(l): Cross-sectional SEM images of the QW NR arrays in samples S and T1-T3, respectively. Numbers 1-3 are marked for indicating the locations of local cathodoluminescence measurements.

Fig. 2 (a)-(d): Local cathodoluminescence spectra at the three locations for samples S and T1-T3, respectively.

Fig. 3 Normalized photoluminescence spectra of the QW NR arrays in samples S and T1-T3.

Coupling Behaviors of a Radiating Dipole with the Surface Plasmon Induced on a Metal Protrusion

Professor Y. W. Kiang’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所江衍偉教授

The coupling behaviors of a radiating dipole with localized surface plasmon (LSP) resonance modes induced on an Ag protrusion in a GaN layer are numerically studied with an algorithm including the feedback effect of LSP resonance to modify the emission behavior of the radiating dipole. Two spectral ranges (400 – 570 and 570 – 800 nm) of radiated power enhancement can be observed. The radiated power enhancement in the long-wavelength (short-wavelength) range is generated through the coupling of the radiating dipole with the dipole and quadrupole (higher-order) LSP resonance modes induced on the protrusion. Because the LSP electromagnetic fields of those higher-order modes are mainly distributed around the tip of the protrusion, which is close to the radiating dipole, the radiated power enhancement is quite effective in the blue-green range even though the corresponding metal absorption can be higher than that of a lower-order LSP mode. Compared with the configurations of surface metal nanoparticle (NP) and embedded metal NP, the protrusion structure has the advantages of relatively higher radiated power enhancement in the blue-green spectral range and relatively lower fabrication cost from a practical application viewpoint.

Fig. 1. Problem geometry of numerical simulation. A truncated-cone-shaped Ag protrusion with the base radius r, height h, and inclination angle θ in GaN is connected to the upper half-space of Ag. The vertical distance between the protrusion tip and the designated QW, which consists of a radiating dipole array in the x-y plane, is d. The lateral position of a radiating dipole is defined as s.

Fig. 2. Normalized radiated powers as functions of wavelength by fixing the distance between the protrusion tip and the radiating dipole, d, at 30 nm and varying the protrusion length, h. The lateral distance of the radiating dipole s is set at s=0. The radiating dipole is oriented along the x-axis. The dashed curve is plotted for the normalized total radiated power of a radiating dipole located in GaN 100 nm away from an air/GaN interface.