Environmental and thermal stability of Schottky contacts to N-type aluminum gallium nitride

With growing interest in AlGaN for W detectors and high-power/high-temperature electronic devices, the problem of forming quality Schottky contacts has become increasingly important. It was shown that wet chemical surface pre-treatments affect the as deposited Au/n-AlGaN Schottky diodes. However, these diodes improve over the course of days when exposed to air at room temperature, exhibiting reduced leakage currents, enhanced barrier heights, and reduced ideality factors. It was determined that the high as-deposited reverse leakage currents were beneath the contact area, rather than across the semiconductor surface. Metals that readily oxidize, as well as thick metallizations both affect the rate of aging and further suggest that the environmental aging occurs at the metal-semiconductor interface. Mild anneals revealed that the environmental change was partially reversible, which strongly suggests the passivation of electrically active defects at the metal-semiconductor interface. Therefore, oxidizing species, with enhancement in the presence of water vapor, diffuse to the metal-semiconductor interface and passivate electrically active defects that are reactivated upon mild anneals in N 2.; This study also examines two different forms of thermal stability; long-term anneals at moderate temperatures (accelerated aging) and high temperature rapid thermal processing (commonly used for ohmic contact formation). A GaN capped AlGaN/GaN heterostructure was used; the metallizations Re, Pt, Au, Ni, Ni/Au, Ni/Ga/Ni and Co were selected based on thermodynamic predictions and metal work functions. Excellent thermal stability based on reverse current densities was observed for non-encapsulated Pt/Au diodes subjected to long-term (200 h) anneals at 500°C. The metals tested on the AlGaN heterostructure for short-term anneals yield stable diodes up to 400°C for 60 s. Re contacts, show increases in reverse current densities between 400 and 600°C with a return to the as-deposited condition following anneals at 800°C for 60 s. Re contacts annealed at 800°C for 10 min saw the complete loss of the GaN cap. The Re metallization may provide a Schottky gate metallization that could be annealed in conjunction with the ohmic contacts.