Hydrogen and Nitrogen Ambient Effects on Epitaxial Lateral Overgrowth (ELO) of GaN VIA Metalorganic Vapor-Phase Epitaxy
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1 Downloaded from https://www.cambridge.org/core. IP address: 185.46.84.221, on 28 Jan 2020 at 04:41:19, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300002325
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Figure 1 SEM images of GaN on SiO2 stripe pattern along the direction at a growth time of 30 min in (a) hydrogen ambient and (b) nitrogen ambient.
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Figure 2 SEM images of GaN layer grown by ELO on SiO2 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.
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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 Downloaded from https://www.cambridge.org/core. IP address: 185.46.84.221, on 28 Jan 2020 at 04:41:19, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300002325
2. Experimental A conventional atmospheric MOVPE apparatus with a horizontal reactor was used. A 3-µm-thick undoped MOVPE-grown GaN on sapphire using a low-temperature GaN buffer layer was used as the substrate. A SiO2 stripe pattern with a 4 µm window width and a 4 µm mask width was aligned along the or direction of the underlying GaN layer. After a 100-nm-thick SiO2 film was deposited by a radio-frequency (RF) sputtering, the SiO2 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 3.5µm/hr. The ambient gas in the ELO process was hydrogen, nitrogen or their mixture (mixture ratio, hydrogen : nitrogen = 1 : 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 In0.2Ga0.8N 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 [14]. In order to investigate the crystallographic structure of the ELO-GaN layers, ω-scan X-ray diffraction (XRD) measurements were performed on the ELO-GaN (0004) plane as a function of φ (φ: 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 a
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