Growth and Device Process of Regularly-patterned GaN Nanorod Light-emitting Diode Arrays
Date Issued
2015
Date
2015
Author(s)
Tu, Charng-Gan
Abstract
In this dissertation, we first demonstrate the growth of high-quality GaN nanorods (NRs) by metal-organic chemical vapor deposition (MOCVD) on patterned GaN/sapphire growth template. With nano-imprint lithography technique and selective area epitaxy, we can obtain regularly-patterned GaN NR array. The growth of GaN NR array starts with a hole-filling process, followed by NR growth with pulsed growth mode through switching group III supply (TMGa) and group V supply (NH3) on and off alternatively. Regularly-patterned GaN NR array of uniform geometry are formed. InGaN/GaN quantum wells (QWs) can be deposited on the c-plane top faces, m-plane sidewalls, and {1-101}-plane slant facets on c-oriented NR array with highest (lowest) growth rate in the c-plane ({1-101}-plane). After regrowth of p-GaN on NR array with n-cores and QWs deposition, an NR light-emitting diode (LED) array can be implemented. Then, we demonstrate the growth and process of a regularly-patterned NR-LED array with its emission from sidewall non-polar QWs. A pyramidal un-doped GaN structure is intentionally formed at the NR top for minimizing the current flow through this portion of the NR such that the injection current can be effectively guided to the sidewall m-plane InGaN/GaN QWs for emission excitation by a conformal transparent conductor (GaZnO). The injected current density at a given applied voltage of the NR-LED device is similar tothat of a planar c-plane or m-plane LED. The blue-shift trend of NR LED output spectrum with increasing injection current is caused by the non-uniform distributions of QW width and indium content along the height on a sidewall. The photoluminescence (PL) spectral shift under reversed bias confirms that the emission of the fabricated NR-LED originates from non-polar QWs. Next, the growth of regularly-patterned multi-section GaN NR arrays based on a pulsed growth technique 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. Line-markers are observed in either a scanning electron microscopy (SEM) or a transmission electron microscopy (TEM) image of an NR for illustrating the boundaries between two successive growth cycles in pulsed growth. A line-marker corresponds to a thin Ga-rich layer formed at the beginning of GaN precipitation of a pulsed-growth cycle. By analysing the geometry variation of the 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 indicates that a multi-section NR can lead to a significantly broader sidewall emission spectrum. Finally, we demonstrate the growth of a two-section, core-shell, InGaN/GaN QWs NRLED device. A two-section n-GaN NR is grown through a tapering process for forming two uniform NR sections of different cross-sectional sizes. The cathodoluminescence (CL), PL, and electroluminescence (EL) characterization results of the two-section NR structure are compared with those of a single-section NR sample, which is prepared under the similar condition to that for the first uniform NR section of the two-section sample. All the CL, PL, and EL spectra of the two-section sample are red-shifted from those of the single-section sample. Also, the emitted spectral widths of the two-section sample become significantly larger than their counterparts of the single-section sample. Such variations are attributed to the higher indium incorporation in the sidewall QWs of the two-section sample due to the stronger strain relaxation in an NR section of a smaller cross-sectional size and the more constituent atom supply from the larger gap volume between neighboring NRs.
Subjects
metal-organic chemical vapor deposition
pulsed growth mode
GaN nanorods
core-shell nanorod light-emitting diodes
multi-section nanorod
Type
thesis
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