Scanning electron microscopy and cathodoluminescence study of the epitaxial lateral overgrowth (ELO) process for gallium nitride

Traditional epitaxial growth of GaN by metalorganic vapor phase epitaxy (MO VPE) on mismatched substrates such as sapphire or SiC produces a columnar m aterial consisting of many hexagonal grains similar to 0.2-1.0 mu m in diam eter. The epitaxial-lateral-overgrowth (ELO) process for GaN creates a new material: single-crystal GaN. We have studied the ELO process for GaN grown by MOVPE in a vertical flow rotating substrate reactor. Characterization c onsisted of plan-view SEM: and vertical-cross-section TEM studies, which re vealed a large reduction in dislocation density in the overgrown regions of the GaN. Panchromatic and monochromatic cathodoluminescence images and spe ctra were used to study the spatial Variation of the optical properties wit hin the GaN ELO samples. The effects of growth temperature and stripe mater ial on the overgrown layers were examined. Through the use of a higher subs trate temperature during growth and the use of a SN, stripe material, the o vergrown crystal shape has a smooth 2D top surface with vertical sidewalls. Applying a second ELO step, rotated by 60 degrees, over a fully coalesced ELO layer yields a further reduction of defects in GaN overgrown surfaces.