Direct low-temperature bonding of AlGaN/GaN thin film devices onto diamond substrates

This chapter presents a direct low-temperature bonding technology for the fabrication of AlGaN/GaN diodes and transistors on poly- and single crystalline diamond substrates. We explain how an aluminum nitride layer reacts with water to form a 30 nm thick electrically insulating and mechanically strong bond with excellent thermal properties. Based on a dissolution and crystallization reaction of various aluminum compounds, an adaptive aluminum hydroxide bond layer without voids is created at the bonding interface. An experimental analysis of AlGaN/GaN Schottky diodes on silicon, poly-, and single crystalline diamond demonstrates a large increase of the saturation currents on diamond, and a thermal resistance of ~ 10-100 m2K/GW is calculated from thermal simulations. A thermal resistance of ~ 15-30 m2K/GW is expected from theoretical considerations based on the 30 nm thickness and an expected thermal conductivity of 1-2 W/m K. 3 GHz load-pull measurements demonstrate a 15% higher Pout on single crystalline diamond as compared to silicon at similar power added efficiency. Additionally, similar performance is measured for a 2- and a 6-finger transistor on diamond, which shows that thermal crosstalk between device fingers is mitigated. A disadvantage in our current technology is identified in the thermally poor stress relief layers grown on Si to accommodate mechanical stress and improve the electrical breakdown voltage.