Characterization and kinetic monitoring of the reactions between TixAly phases in Ti-Al based ohmic contacts on n-type GaN by Differential Scanning Calorimetry (DSC)

This work reports on DSC measurements performed on Ti-Al metallic layers stacks deposited on n+-GaN. The aim is to get better understanding of the mechanisms leading to ohmic contact formation during the annealing stage. Two exothermic DSC peaks were found : one below 500°C and the other one around 660°C. They can be respectively attributed to Al3Ti and Al2Ti compounds formation. Lowest contact resistance is well correlated with the presence of Al3Ti compound, corresponding to Al(200nm) / Ti(50nm) stoichiometric ratio. Subsequently, Al (200 nm) / Ti(50 nm) stacks on n+-GaN were comparatively annealed from 400 °C to 650 °C. Specific Contact Resistivity (SCR) values stay in the mid 10-5 Ω.cm² range for annealing temperatures between 450 °C and 650 °C. Such low-temperature annealed contacts on n+-GaN may open new device processing routes, simpler and cheaper, in which Ohmic and Schottky contacts are annealed together.