Transmission electron microscopy has been applied to characterize the structure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts on n-type GaN (∼1017 cm−3) epitaxial layers. The metals were deposited either by conventional electron-beam or thermal evaporation techniques, and then thermally annealed at 900 °C for 30 s in a N2 atmosphere. Before metal deposition, the GaN surface was treated by reactive ion etching. A thin polycrystalline cubic TiN layer epitaxially matched to the (0001) GaN surface was detected at the interface with the GaN substrate. This layer was studied in detail by electron diffraction and high resolution electron microscopy. The orientation relationship between the cubic TiN and the GaN was found to be: {111}TiN//{00.1}GaN, [110]TiN//[11.0]GaN, [112]TiN//[10.0]GaN. The formation of this cubic TiN layer results in an excess of N vacancies in the GaN close to the interface which is considered to be the reason for the low resistance of the contact.

We report on the thermal stability of Pt and Pd thin films on GaN up to 800 °C. We have found that for Pt thin films submicron size metal spheres form at temperatures as low as 600 °C and the film turns into discontinuous islands at 725 °C and above. Pd begins to form islands above 700 °C and areas of the film begin to delaminate due to thermally generated compressive stress. The general behavior of these films is that found for most metal films on ceramics, suggesting that metals on GaN do not react as they typically do on common elemental and compound semiconductors. This ceramic-like behavior must be considered when determining which metals may act as good electrical contacts for GaN, especially for high temperature applications or alloyed contacts.

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