In view of the success of GaN-based photonic and electronic devices we have performed a study of long-term stability of ohmic contacts to GaN. The specific focus of the work was on thermally stable metallizations using transition metal nitrides (ZrN, NbN, TiN). Because of their high electrical conductivity and thermal stability these compounds are materials of choice for diffusion barriers in metallization systems. Moreover, transition metals have been shown effective in providing low-resistivity contacts to n-type GaN (by depleting its superficial film for nitrogen and creating thus a highly doped subcontact region) and p-type GaN (by extracting hydrogen from the subcontact region).
Our studies involved accelerated lifetime testing and thermal conductivity studies of metal/GaN ohmic contacts. The approach adopted for aging studies was based on the following premises. First, the degradation of an ohmic contact during the operation of the device is most likely caused by microstructural changes within the contact region. Therefore, the main issue in assessing the long-term stability was to identify the key interfacial transformation for good ohmic behaviour, as well as "the weakest point" of the contact microstructure. Aging was performed by means of annealing up to 150^oC over a period of 300 hours. Thermal conductivity measurements were made using a home-built scanning thermal microscope, enabling to couple the topographic image with the thermal conductivity information.
A correlation between aging characteristics and thermal conductivity measurements has been established. The difference between GaN and metallization conductivity of 6 - 10 W/m/K for the most reliable ohmic contacts has been found. These contacts were characterised by featureless surface morphology after lifetime test.

Work partially supported by grants from European Commission G5RD-CT-2001-00566-DENIS and the State Committee for Scientific Research 7T11B 009 20.

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