Incorporation of air-cavity into sapphire substrate and its effect on GaN growth and optical properties

We propose a new GaN growth scheme using a cavity engineered sapphire substrate (CES), in which a patterned cavity array was formed on the sapphire surface. Amorphous alumina film was deposited by atomic layer deposition on a photoresist patterned sapphire substrate, and subsequent high temperature annealing resulted in the formation of a cavity array surrounded by a crystallized sapphire shell. During the GaN growth on CES, the GaN film filled up the space between the cavities and then grew laterally over them, leading to a completely coalesced pit-free smooth surface. The incorporation of cavities was observed to reduce the stress in GaN film by ~30%. The output power of a light emitting diode (LED) on CES at an input current of 20 mA was measured to be 2.2 times higher than that on a planar sapphire substrate. It was also found that the dominant peak wavelength of the LEDs on CES showed a red-shift of 12 nm due to higher In incorporation associated with presumably lower surface temperature in the presence of air-cavity.