Analysis of plasma-treated sapphire surface and its effect on crystallized Al2O3 quality

Abstract

GaN-based LEDs, which are a group III-nitride compound semiconductors, are emerging as next-generation lighting technologies due to their superior energy efficiency and loger luminescent life than conventional incandescent lamps and fluorescent lamps. Generally, a GaN-based compound semiconductors are grown on a sapphire substrate by a hetero-epiaxy. However, there are problem of high threading dislocation density due to lattice constant mismatch, wafer bow due to difference in the thermal expansion coefficients between substrate and GaN film, and the low light extraction efficiency due to difference in the refractive index. In order to improve these problems, researches have been carried out to insert a patterns on the sapphire substrate surface to enable lateral overgrowth of GaN film and increase diffuse reflection. Our research group has also developed a CES (cavity-engineered sapphire substrate) which inserts an air cavity arrays on a sapphire substrate surface. When GaN film is grown on a CES, the threading dislocation density decreased due to lateral overgrowth, and the light extraction efficiency increased because of the diffuse reflection. Also, the air cavity arrays reduce the stress inside the thin film, thereby reducing the wafer bow phenomenon. The CES is fabricated by crystallization of alpha-alumina through thermal treatment of amorphous-alumina deposited on a sapphire substrate. Therefore, in order to obtain a high-quality GaN film, the crystallinity of alumina layer is important. During the CES fabrication process, plasma treatment is performed before depositing the amorphous-alumina layer to remove the PR residue after the photolithography process and to obtain the desired shape pattern. This study was conducted to analyze the effect of plasma treatment on the surface of the sapphire substrate and the crystallinity of the crystallized alumina. Plasma treatment was performed by adjusting parameters. In this study, plasma treatment was carried out using oxygen (O2) and argon (Ar) gas. Also, experiments were conducted by controlling the power applied to the plate which the substrate is placed. Fluorine (F) impurities were detected in both the oxygen and argon plasma treatment and about 1% of argon impurities were detected in the case of argon plasma treatment. The oxygen ratio on the surface of the sapphire substrate was increased during the oxygen plasma treatment, and the opposite result was obtained in the case of the argon plasma treatment. Also, as the plate power increases, the stoichiometry of the sapphire substrate surface became more broken. As a result of analysis using AFM technique, the surface morphology and roughness of plasma-treated samples did not much change much, but the crystallized samples showed that nano-sized grains appeared on the surface, and the surface roughness also increased greatly. As a result of the crystallinity analysis, the crystallinity of both gamma-alumina and alpha-alumina was degraded in comparison with the samples that not plasma-treated. Therefore, this study can be used to experminetally examine the effect of plasma treatment on the sapphire substrate surface and the crystallized alumina.