FIB micro-milled sapphire for GaN maskless epitaxial lateral overgrowth: a systematic study on patterning geometry

A systematic study of the GaN epitaxial lateral overgrowth (ELO) of the focused ion beam (FIB) patterned sapphire substrate is presented. The FIB technology with its atom at a time removal principle is a flexible and high-resolution maskless processing technique, unique to fabricate trenches of the highest quality without surface damage, debris, and cracks, therefore, best suited to investigate the improvement of the maskless GaN ELO on the patterned sapphire. Arrays of the square (size varying from 1 x 1 mu m(2) to 5 x 5 mu m(2)) and rectangular (size varying from 20 x 1 mu m(2) to 20 x 5 mu m(2)) trenches with the inter-trench distance varying from 1 to 5 mu m, aligned parallel/perpendicular to sapphire < 11 (2) over bar0 > direction were overgrown and investigated structurally and electrically using Raman scattering, atomic force microscopy (AFM), and electron beam induced current (EBIC). To find out the optimized ELO of the array, the threading dislocation density and the remaining strain in the ELO epilayer was analyzed. The epilayer improvement was demonstrated in a correlation between the trench size, the array geometry (the trench shape and inter-trench distance), and the epilayer thickness. A homogeneous overgrowth of the entire array was observed for the arrays of the largest trenches and the reduced intertrench distance.