Direct visualization of highly resistive areas in GaN by means of low-voltage scanning electron microscopy

Abstract

The damage-induced voltage alteration (DIVA) contrast mechanism in scanning electron microscope (SEM) at low electron energy has been presented as a fast and convenient method of direct visualization of increased resistivity induced by energetic ions irradiation in gallium nitride (GaN). Epitaxially grown GaN layers on sapphire covered with a metallic masks with etched windows were subjected to He2+ irradiations at 600 keV energy. The resulting two-dimensional damage profiles at the samples cross-sections were imaged at SEM at different e-beam currents and scan speeds. The gradual development of image contrast was observed with the increase of cumulative charge deposited by electron beam irradiation, to finally reach the saturation level of the contrast related to the local resistivity of the ion-irradiated part of GaN.The presented method allows one to directly visualize the ion-irradiated zone even for the lowest resistivity changes resulting from ion damage, i.e. all levels of insulation build-up in GaN upon irradiation with ions. Taking into account that it is not possible to apply the etch-stop technique by wet chemistry to GaN, it makes the presented technique the only available method of visualization of highly resistant and insulating regions in GaN-based electronic devices.Main aim of the presented work is to get a deeper insight into a DIVA contrast in GaN with the special emphasize to discuss the role of rastering speed and electron beam current, i.e. details of charge build-up ion the sample surface.