Foundations of Ohmic Contact Formation on Aluminum Gallium Nitride/gallium Nitride Heterostructures

Aluminum gallium nitride (AlGaN)/gallium nitride (GaN) high electron mobility transistors (HEMTs) have tremendous potential for application in high frequencies, high temperatures and microwave and mm power amplifications. Fabrication of Ohmic contacts for such devices that meet the stringent low-resistance, high thermal stability and smooth surface morphology requirements has been challenging. In the cases where Ohmic behavior can be achieved, a full scientific understanding of the mechanism(s) through which Ohmic behavior is achieved, as well as the effects of metal interlayer thicknesses, surface preparation, annealing ambient, and interfacial reaction products, is yet to emerge. A such, the focus of this work centers on the investigation of design, processing and materials' issues in the development and optimization of stable Ohmic contacts to AlGaN/GaN heterostructure epilayers in order to begin to address some of these issues. Multilayer Ohmic contact schemes are designed, fabricated and characterized. Design of contact schemes is tailored to enable the identification of the roles constituent components play in achieving Ohmic behavior. The understanding gained from such students has allowed for controlling and optimization of interfacial and intermetallic reactions, and enabled the fabrication of low-resistance contacts with large processing window and high-temperature thermal stability. The outcome of this work lays the foundation for not only the design and fabrication of Ohmic contacts with excellent properties, but also the understanding of Ohmic contact formation mechanisms on AlGaN/GaN heterostructures.