Role of thin Ti layer in formation mechanism of low temperature-annealed Ti/Al-based ohmic contact on AlGaN/GaN heterostructure

To obtain Ti/Al-based ohmic contacts on AlGaN/GaN heterostructures with a low resistance, it is crucial to understand their formation mechanism. Such a mechanism is still unclear, especially when the annealing temperature is as low as 500 °C and the Ti layer is thin, typically with a thickness of 2.7 nm. We investigated the sputtered Ti/Al/W ohmic contacts on the AlGaN/GaN heterostructure on-Si grown by metal–organic chemical vapor deposition, using scanning transmission electron microscopy, energy dispersive x-ray spectrometry, and x-ray diffraction. The microstructural analysis of the samples revealed that as-deposited Ti removes oxygen from the AlGaN surface before annealing; such phenomenon represents the capital mechanism of oxygen-removal in a low temperature annealing process. After annealing, Ti 0.5 Al 0.5 N and possibly TiGa 3 compounds are formed, and these formations suggest that AlGaN is decomposed by Ti. As a result, the interface between metal alloy and AlGaN turns into an intact and intimate one. Furthermore, we found that the contact resistance correlates with the Al/O atomic ratio at the metal/AlGaN interface, and the Al/O ratio becomes maximum when the Ti thickness is 2.7 nm. From the findings, an optimal formation mechanism of ohmic contacts is proposed.