Ohmic contacts to AlGaN/GaN heterostructures which have low contact resistance and good surface morphology are required for the development of commercial high power, high frequency transistors in the GaN system. The development of such contacts would be helped by a better understanding of the effect of microstructure on electrical behavior, which is studied here. Au/Ni/Al/Ti/AlGaN/GaN ohmic contact structures were rapid thermal annealed in argon for 60 s at temperatures in the range 550–900 °C. The variation of contact resistance with anneal temperature was correlated with the phase distribution observed by transmission electron microscopy (TEM). A combination of TEM techniques was required to determine the resulting microstructure, including energy filtered TEM, high resolution electron microscopy and energy dispersive x-ray spectrum imaging. Contacts with the lowest resistance were formed after 700 °C annealing. Very little consumption of the 30 nm AlGaN layer was observed. An unidentified phase containing Al, Ti and Au is present at the interface in the samples with low specific contact resistance. The identification of the observed thin interfacial phases (including TiN and AlN) is discussed, along with the effects of oxidation and possible mechanisms of ohmic contact formation.

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This paper describes the microstructure of ohmic contacts to an AlGaN/GaN heterostructure, of interest for high power transistors, and an analysis of V-defects in an InGaN/GaN multi-quantum well (MQW) light-emitting structure. A combination of different transmission electron microscopy (TEM) techniques has been employed, as they provide complementary information. These include bright field and dark field TEM, high-resolution electron microscopy, X-ray mapping, energy filtered TEM and high angle annular dark field. A full determination of the phase distribution in the ohmic contacts was achieved. The onset of low contact resistance was found to correspond with the formation of an interfacial layer containing both TiN and AlN, and of an intermetallic layer containing Al, Ti and Au in contact with it. The MQW structures were capped with a p-type GaN layer, and TEM and ADF studies of the samples show a number of V-defects 100-200 nm apart along the MQW. Each V-defect incorporates a pure edge (b = 1/3 <11-20>) dislocation, which runs through its apex up to the free surface. The defects contain GaN with no InGaN layers, suggesting the V-pits have been filled in by the capping layer.

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.

Although developed to improve spatial resolution, a TEM spherical aberration corrector or 'image corrector' has the added benefit of eliminating the artifact of delocalization in HR-TEM imaging. This greatly increases the power of HRTEM imaging and is especially important for clear imaging of surfaces and interfaces. When such a corrector is combined with a source monochromator, ultra high resolution at low accelerating voltages also becomes feasible. These technologies can also be added to an environmental TEM (ETEM) to achieve unprecedented resolution on TEM samples in gaseous environments. Some recent results illustrating the power of image corrected TEM and how it is being used at the cutting edge of materials science are presented.