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Graphene was directly grown on r-plane (1-102), c-plane (0001), and a-plane (11-20) sapphires by low pressure chemical vapor deposition without the use of a metal catalyst. The growth temperature was systematically changed between 1090 and 1210 °C to investigate the effects of the crystal orientation of sapphire on the graphene growth. It was found that the growth rate of graphene on r-plane sapphire was very fast compared to that of the samples grown on other orientations. The surface catalytic effect of r-plane sapphire promotes the smooth and flat growth of single-layer graphene. The surface of the r-plane sapphire was kept smooth even at a high temperature of 1210 °C because a quick coverage of graphene protects the surface of the sapphire from thermal decomposition and roughening.

In this study, the direct growth of multilayer graphene from amorphous carbon on a sapphire (0001) substrate by precipitation using a nickel catalyst layer and a tungsten capping layer was examined. The findings revealed that a tungsten carbide layer was formed on top of the catalyst, and this suppressed the diffusion of carbon atoms upwards towards the surface. This caused the graphene layer to precipitate below the catalyst layer rather than above it. Under optimized growth conditions, Raman spectroscopy indicated that a high-quality graphene layer was formed with a low D/G peak intensity ratio of 0.10.

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In order to advance the mass production of graphene devices, it is beneficial to avoid the difficulty graphene transfer process. Direct precipitation of graphene using a tungsten capping layer is convenient for this purpose, and is quite simple and compatible with conventional semiconductor fabrication processes. In this study, multilayer graphene was directly precipitated on a wafer of GaN-based blue LEDs to form a transparent electrode. The fabricated LED exhibited superior I-V characteristics and emitted blue luminescence around the probe of the electrode.

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