Thermal Stability of Cu/Pd₂Si/, Cu/ and Au/Ni/ Schottky Contacts to AlGaN/GaN Heterostructures

The thermal stability of Schottky contacts to AlGaN/GaN heterostructures was evaluated by subjecting planar diodes to high-temperature annealing in a vacuum. The anodes of the diodes consisted of a single Cu/ layer or a stack of Cu/Pd2Si/ or Au/Ni/ layers. Annealing steps were performed subsequently at incremental temperatures, up to 900 C-degrees. The current-voltage (I - V) characteristics of the diodes were measured before and after each annealing step. The microstructure of the pristine contacts and the thermally treated contacts was examined through transmission electron microscopy (TEM). It was found that the leakage current in the diodes with Cu/ anodes increased significantly after annealing at 700 C-degrees. A thin interlayer was observed at the interface. The leakage current in the diode with Au/Ni/ Schottky contact, although with the lowest initial value among investigated devices, changed noticeably after each annealing step and was distinctly increased after completion of the annealing sequence at 900 C-degrees. The intermixing of the initial Au/Ni/ bilayer stack was revealed. In contrast, the electrical properties of the Cu/Pd2Si/ Schottky contacts were not changed after the complete annealing cycle, including annealing at 900 C-degrees. The double-layer stack was preserved after annealing. The Au-free Cu/Pd2Si/ Schottky contacts to AlGaN/GaN heterostructure revealed excellent thermal stability which is a significant improvement over the Cu/ contacts.