Identification of interfacial layers in Ohmic contacts to n-type GaN and Al_xGa_1-xN/GaN heterostructures using high-resolution electron microscopy

Abstract

The optimization of Ohmic contacts to high-power GaN-based electronic devices would be greatly helped by a better understanding of the effects of changes in the microstructure on the contact resistance. To study this, various Al-Ti-based Ohmie metallizations to n-GaN and Al_xGa_1-xN/GaN heterostructures have been examined by transmission electron microscopy (TEM) after annealing in Ar. Although other factors are also important, reaction layers at the metal-nitride interface play a key role in such contacts and hence need to be identified. By far the most convenient means of characterizing such reaction layers is to take fast Fourier transforms (FFTs) of high-quality lattice images and then to compare the measured lattice spacings and angles with selected-area diffraction patterns and with chemical information obtained using energy-dispersive X-ray analysis or energy-filtered TEM. Lattice imaging provides essential information both on the identification of phases and on their morphology and distribution, and the FFT techniaue maximizes the aceuracy of d-spacing measurements. The benefits and difficulties of these techniaues are described, and TEM results illustrating the range of observed interfacial phases, including TiN, AlN and AlTi₂N, are presented. Whether variations in the specific contact resistance can be explained by changes in the metal-nitride interfacial phase is then discussed.