Growth of GaN Nanocolumns and Their Coalescence Overgrowth Using Metalorganic Chemical Vapor Deposition and the Characterization Study
Date Issued
2009
Date
2009
Author(s)
Tang, Tsung-Yi
Abstract
In this dissertation, the fabrication of high-quality GaN template is demonstrated by using nanocolumn (NC) coalescence overgrowth. First, the NC coalescence overgrowth by metalorganic chemical vapor deposition (MOCVD) is performed on NCs grown by molecular beam epitaxy (MBE). Plan-view scanning electron microscopy (SEM) shows coalesced surface morphology, although hexagonal structures are still visible in the images. The cross-section cathodoluminescence (CL) image shows more efficient emission in the overgrowth layer than from the NC layer. The plan-view CL image demonstrates that the emitted light is mainly from the hexagonal structures. The photoluminescence measurement result indicates that the emission efficiency of the overgrown layer is even higher than that of an un-doped GaN thin film of high quality. The presence of hexagonal structures correlates to surface roughness values in the range of several nm.esides the NCs grown by MBE, the MOCVD growth method of flow-rate modulation is applied to grow periodic and uniform NCs on the templates patterned with nanoimprint lithography. After NC coalescence overgrowth, although domain structures of a tens-micron scale in the overgrown layer can be identified with cathodoluminescence measurement, from atomic force microscopy (AFM) measurement, the surface roughness of the overgrown layer in an area of 5um*5um is as small as 0.411 nm, which is only one-half that of the high-quality GaN thin-film template directly grown on sapphire substrate (the control sample). Based on the AFM and depth-dependent X-ray diffraction measurements, near the surface of the overgrown layer, the dislocation density is reduced to the order of 107 cm-2, which is one order of magnitude lower than that of the control sample and 2-3 orders of magnitude lower than those of ordinary GaN templates for fabricating light-emitting diode. Also, the lateral domain size, reaching a level of ~2.7 um, becomes three times larger than the control sample. Meanwhile, the ratio of photoluminescence intensity at room temperature over that at low temperature of the overgrown sample is at least six times higher than that of the control sample. Although the strain in nanocolumns is almost completely released, a stress of ~0.66 GPa is rebuilt when the coalescence overgrowth is implemented.urthermore, the overgrowth quality dependence on NC cross section size and NC spacing size is studied in detail. Generally, a smaller NC dimension and spacing size lead to higher overgrowth quality, including lower threading dislocation (TD) density and larger lateral domain size. From the measurement results of cross-section transmission electron microscopy (TEM), it is found that the TD density in an NC depends on the patterned hole size for NC growth. Also, the TD formation at the beginning of coalescence overgrowth is related to the NC spacing size. Although the TD density at the bottom of the overgrown layer is weakly dependent on NC and spacing sizes, at its top surface, the TD density strongly relies on NC size. Among the overgrowth samples of different NC diameters and spacing sizes with a fixed NC diameter/spacing ratio, the one with the smallest size and spacing leads to the lowest TD density, the largest lateral domain size, and the highest photoluminescence efficiency. Also, the optical and crystal qualities at the surfaces of all the overgrowth samples are superior to those of a GaN template. inally, the QW and LED structures are grown on the overgrown templates. The emission enhancement results of blue and green-emitting InGaN/GaN QW and LED structures based on NC growth and coalescence overgrowth are presented. Significant enhancements (up to ~80 % output intensity increase in a blue LED) are demonstrated. For LED application, the TD density reduction in an overgrown GaN template can more effectively enhance the emission efficiency of a blue LED, when compared with a green LED.
Subjects
GaN
nanocolumn
threading dislocation
LED
strain
Type
thesis
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