A structural analysis of the Pd/GaN ohmic contact annealing behavior
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L11.51.1
A structural analysis of the Pd/GaN ohmic contact annealing behavior C.C. Kim1, P. Ruteranaa),2, and J.H. Je3 1
Samsung Electro-Mechanics Co., 314, Maetan-3dong, Suwon, 442-743, Korea
2
LERMAT, FRE 2149 CNRS, ISMRA, 6, Boulevard Maréchal Juin, 14050 CAEN, FRANCE 3 Synchrotron X-ray Laboratory, Department of Materials Science and Engineering, Pohang University, Pohang 790-794, South Korea
Abstract For ohmic contact on p GaN, palladium is one of the best candidates showing ohmic characteristics already without annealing. To be realized in devices, it is necessary to know the behavior of the ohmic contacts at accelerated conditions, especially for high temperatures and power. We report on the structural evolution of palladium layers (30 nm) deposited on GaN (0001) by electron beam evaporation without intentional annealing. They were next cut into various pieces which were individually submitted to rapid thermal annealing at 400, 500, 600, 700 and 800°C for 10 sec. We investigate the differences in the microstructure and the location of interfacial phases and their relationships as determined by X-ray diffraction and transmission electron microscopy, we then suggest the formation mechanism based on the relationship. It is shown that the interface is disrupted at annealing above 600°C and by 800°C only very small patches of Pd are still present, however they area completely imbedded in a matrix of intermetallic phases (gallides) formed by the reaction with GaN. Introduction Gallium based nitrides are direct wide-band-gap semiconductors which have a large potential for high-temperature and high-power applications1. However, this wide-band-gap makes the device fabrication difficult for ohmic contacts, especially for p type material. Many extensive studies have been made for developing optimized ohmic contact systems.2-4 Pd is one of the most competing candidates, showing a promising ohmic characteristics at room temperature3. In order for Pd to be used as a reliable metal contact on GaN, it is essential to understand the thermal stability and metallurgy of the contact. In spite of the increasing importance of Pd/GaN system, the detailed microstructure of the Pd layer and the high temperature structure are not yet understood. For device applications at high power and high temperature, it is therefore important to study the structural evolution of metal overlayers and their reactions at elevated temperatures. In this work, we investigated the structural evolution of Pd/GaN (0001) heteroepitaxy during post annealing process. We reveal the existence of interfacial, epitaxial, Pd grains in the as-deposited Pd film that was evaporated on GaN(0001) at room temperature. During subsequent annealing up to 600°C, the grains of Pd grow and we explain the origin of the Pd epitaxy on GaN(0001) to a six-to-seven matched interface structure, wherein six-Ga atomic distances in GaN match to seven-Pd atomic distances. At high temperature annealings (~700°C), the Pd film transforms, by the Pd-Ga reaction to Ga2Pd5 and Ga5Pd gall
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