Hydrogen and Nitrogen Ambient Effects on Epitaxial Lateral Overgrowth (ELO) of GaN Via Metalorganic Vapor-Phase Epitaxy
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Figure 1 SEM images of GaN on Si02 stripe pattern along the < 1100> direction at a growth time of 30 min in (a) hydrogen ambient and (b) nitrogen ambient.
Figure 2 SEM images of GaN layer grown by ELO on SiO 2 stripe pattern along the direction (a) at a growth time of 120 min in hydrogen ambient and (b) at a growth time of 180 min in nitrogen ambient.
3 Am Figure 3 SEM images of GaN layer grown by ELO on SiO2 line pattern along the direction in the mixture ambient at growth times of (a) 30 min and (b) 120 min.
2. Experimental A conventional atmospheric MOVPE apparatus with a horizontal reactor was used. A 3-jim-thick undoped MOVPE-grown GaN on sapphire using a low-temperature GaN buffer layer was used as the substrate. A SiO 2 stripe pattern with a 4 jim window width and a 4 gm mask width was aligned along the or direction of the underlying GaN layer. After a 100-nm-thick SiO 2 film was deposited by a radio-frequency (RF) sputtering, the SiO 2 stripe pattern was fabricated by standard photolithographic processes and reactive ion etching (RIE). The growth temperature of GaN as measured by a thermocouple in the heating system was 1000°C. The growth rate of GaN at our standard conditions was 3.5pjm/hr. The ambient gas in the ELO process was hydrogen, nitrogen or their mixture (mixture ratio, hydrogen : nitrogen = I : 1), and was controlled by a carrier gas for metalorganic materials. However, because of using hydrogen for bubbling of metalorganic materials, small amount of hydrogen (4.2 %) was mixed with the ambient gases in all ELO processes. The growth rates of the lateral face and the c-facet of ELO-GaN were estimated from the fieldemission scanning electron microscopy (SEM) images and the growth times. In order to measure the dislocation density in the ELO-GaN layers, we observed the pits on an In0 .2Gao~sN layer (100 nm-thick) grown on an ELO-GaN layer. The growth pit density (GPD) on the InGaN layer is considered to correspond to the dislocation density of the underlying ELO-GaN layer [141. In order to investigate the crystallographic structure of the ELO-GaN layers, 0)-scan X-ray diffraction (XRD) measurements were performed on the ELO-GaN (0004) plane as a function of cp(t0: the rotation angle of the sample about its surface normal), and reciprocal space mapping measurements were also carried out using a high-resolution X-ray diffractometer (Philips X' Pert MRD). 3. Results and Discussions Figures 1(a) and (b) show SEM images of GaN on the stripe pattern in hydrogen ambient and nitrogen ambient at a growth time of 30 min, respectively. In hydrogen ambient, a (0001) facet is observed on top, and off-facets are observed on the side walls. On the other hand, the ELO-GaN grown on the stripe pattern in hydrogen ambient had triangular cross sections with only the {1101 1 facet on the side walls at the same growth time. The lateral overgrowth rate on the stripe pattern was faster than that on the stripe pattern [14]. The lateral overgrowth rate in nitrogen ambient is enhanced remarkably in contrast wi
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