Structural and electrical characteristics of ultra-thin Si-doped GaN film regrown on patterned GaN/sapphire

We discuss the structural and electrical characteristics of an ultra-thin Si-doped GaN film, regrown on a patterned GaN/sapphire substrate with square-shaped patterns, at an extremely low growth rate. The Si-doped GaN regrowth (RG-GaN) films were grown by systematically varying V/III ratio (183, 241, 306, and 417) and growth temperature (880, 890, 900, and 910 ℃). Field-emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM) images of the RG-GaN film show that its surface morphology was improved with increasing V/III ratio up to 306, and then slightly degraded with further increasing the ratio. In addition, the surface morphology of the RG-GaN film was improved with increasing growth temperature up to 900 ℃, and then slightly degraded with further increasing the growth temperature. The root-mean-square value of the surface roughness evaluated from the AFM image of the RG-GaN film was measured to be 3.44 nm, which is much better than those of the previous reports. When the thickness of an epitaxial film is ultra-thin, its surface roughness is typically high because of poor surface planarization. However, the surface uniformity of the RG-GaN thin film in this work was significantly improved even with ultra-thin thickness. When the surface morphology of the Si-doped RG-GaN film was improved, its electrical resistance was effectively lowered.