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 e ects of changes in the microstructure on the contact resistance. To study this, various Al±Ti-based Ohmic metallizations to n-GaN and Al x Ga 1¡x N=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 identi®ed. 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 di raction patterns and with chemical information obtained using energy-dispersive X-ray analysis or energy-®ltered TEM. Lattice imaging provides essential information both on the identi®cation of phases and on their morphology and distribution, and the FFT technique maximizes the accuracy of d-spacing measurements. The bene®ts and di culties of these techniques are described, and TEM results illustrating the range of observed interfacial phases, including TiN, AlN and AlTi 2 N, are presented. Whether variations in the speci®c contact resistance can be explained by changes in the metal±nitride interfacial phase is then discussed.

Formation mechanism of Ohmic contacts on AlGaN∕GaN heterostructure: Electrical and microstructural characterizations

“…9. 2,5,8 The ͗110͘ TiN ʈ ͗11− 20͘ GaN relationship was nevertheless maintained, suggesting that large TiN crystals did not epitaxially grow on GaN. 10͑b͒ had a discontinuous porous structure ͑a void is indicated in the image͒; a large protrusion into the GaN beneath the metal was revealed.…”

“…Ti reacts with GaN and forms TiN ͑Refs. 3,5,20,21 The investi-a͒ Author to whom correspondence should be addressed. This reaction extracts N from GaN and generates N-vacancies in the GaN layer.…”

Investigation of the Ti/Al/Pt/Au and Ti/Au contact materials on the N‐face surface of oxygen doped GaN

“…The formation of high quality ohmic contacts that have low specific contact resistance (ρ c ) and excellent reliability is important for the fabrication of high performance GaNbased vertical LEDs [3]. For conventional LEDs, n-ohmic contacts are easily formed on Ga-face n-GaN using Ti/Albased schemes [4][5][6][7]. The achievement of acceptable device characteristics relies heavily on developing low ρ c ohmic metallization schemes [8].…”

Interfacial reactions of Ti/n-GaN contacts at elevated temperature

“…However, the nature of these metal/ GaN interactions is not well understood. 4 It also remains unclear just how the changes in interface morphology and chemistry affect the electrical properties of the contact.…”