Low resistance Al/Ti/n-GaN ohmic contacts with improved surface morphology and thermal stability

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Abstract
The Al/Ti/n-GaN ohmic contact and multilayer variations on this contact are widely used because of the low contact resistivities they provide to n-GaN. However, there are few reports that reveal the influence of the atomic ratio of Al to Ti on the contact resistivity, thermal stability and surface morphology of the contacts. This study reveals that the ratio of Al to Ti strongly influences all of these characteristics. All contacts in this study had atomic ratios of Al to Ti higher than 1. Those contacts with ratios of Al to Ti less than 3 required higher annealing temperatures or longer annealing times to reach comparable contact resistivities compared to the more Al-rich contacts. On the other hand, the less Al-rich contacts provided several advantages. They exhibited smooth surface morphologies even after they were annealed at temperatures near 1000 °C, and they suffered much less severe degradation during long-term aging at 600 °C. These findings are explained by differences in the phases formed after annealing.