Formation and characterization of 4-inch GaN-on-diamond substrates

This paper reports on the first demonstration of four-inch gallium nitride (GaN) on 100-micron CVD diamond substrates and the characterization of the interface between the GaN and the diamond. Currently, gallium nitride devices are used for microwave power amplification at frequencies of up to 100 GHz. The very high thermal conductivity of diamond enables the increase in power and improvement in lifetime and reliability of the amplifiers by efficiently removing the heat from the active region of devices fabricated on GaN-on-diamond substrates. While we have previously demonstrated and currently are producing 2-inch GaN-on-diamond wafers. Increasing the diameter of GaN-on-diamond substrate is both non-trivial and essential for entry into high-volume GaN electronics manufacturing. Since the primary significance of the GaN-on-diamond structure lies in its ability to efficiently remove the heat from the active regions, the state and quality of the bond between the GaN, the diamond, and any enabling adhesion layers are critical in the transmission of heat through the interface and the reliability of the completed devices. In this paper, in addition to the discussion of challenges associated with the scale-up, we characterize the interfacial bonding between the critical layers using a picosecond ultrasonic measurement technique. The measurements indicate excellent adhesion of the interlayer to both the GaN and to the diamond. The qualified substrates from this exercise were used in fabrication of devices that have demonstrated transition frequencies of up to 85 GHz. These findings should help to further the development of GaN-on-diamond technology on the path to commercialization for high-power, high-frequency amplifiers.