Fabrication and characterization of lateral polar GaN structures for second harmonic generation

The growth, fabrication, and properties of GaN/AlN/sapphire with periodically poled surface polarity for second harmonic generation are investigated. The periodic inversion of the surface polarity is achieved by the growth of a thin AlN buffer layer and subsequent partial removal by using either wet etching with potassium hydroxide (KOH) or reactive-ion etching (RIE). GaN growth on these substrates by MOCVD leads to Gapolar GaN on the AlN buffer and N-polar GaN on the bare sapphire. Using atomic force microscopy and scanning electron microscopy, it is demonstrated that a sufficient combination of H2 and NH3 surface treatment before the growth of the GaN layers removes surface defects introduced by RIE etching. Thus, films with comparable quality and properties independent of the etching technique could be grown. However, in contrast to RIE etching, the interfaces between the Ga-polar and N-polar GaN is rough if KOH etching is applied. Thus, it is concluded that MOCVD in combination with RIE etched AlN/sapphire substrates can be a versatile process to fabricate GaN with periodically poled surface polarity as desired for UV light generation via frequency doubling.