LAUSR

Wide bandgap semiconductors, particularly gallium-nitride (GaN) based devices, are of great interest for high-power electronics because of superior material properties such as wide bandgap (3.44 eV), high thermal conductivity (~1.5 x Si), high critical electric field (~10 x Si), and high saturation velocity (~3 x Si). Developments in growth techniques such as hydride vapor pressure epitaxy (HVPE) and ammonothermal methods, have led to the availability of freestanding GaN substrates with defect densities lower than ~106 cm-2, which can then be used to grow epitaxial GaN-on-GaN layers with reduced defect density (< 104 cm-2). Despite these advances, the random presence of defects is still liable to cause degraded device performance and unreliable device behavior [1-2]. This work has investigated stresstesting of GaN-on-GaN vertical devices that were grown on HVPE substrates from two different sources and one ammonothermal substrate. An unintentionally-doped (UID) GaN drift layer with a thickness range of 2-2.4 microns was grown by metal-organic chemical vapor deposition, followed by overgrowth with Mg-doped p-GaN layers with thicknesses of 300-500 nm. The devices were studied using X-ray topography (XRT), SEM and TEM, and the substrate morphology was correlated with the electrical stresstest results. Samples suitable for cross-sectional TEM observation were prepared by FIB milling using a FEI NOVA 200 dual-beam system, with initial thinning done at 30 keV and final thinning done at 5 keV. Scanning electron micrographs were also recorded with the FEI NOVA 200 during progressive milling. A Philips-FEI CM-200 FEG transmission electron microscope (TEM) operated at 200 keV was used for imaging.