Development of Polycrystalline Diamond Compatible with the Latest N-Polar GaN mm-Wave Technology

Abstract

Integration of diamond on GaN can ease the challenges associated with thermal management of GaN-based power amplifiers which need to base on highly scaled transistors to push toward higher frequencies at high powers for 5G networks. The integration of diamond was achieved by growing polycrystalline (PC) diamond on nitrogen-polar GaN. A standard 5 nm Si3N4 layer which forms the gate dielectric was used as an interlayer between diamond and the GaN channel for etching protection. Since diamond growth conditions involve high temperature and H2 plasma, it can easily decompose the underlying dielectric as well as the GaN channel and degrade the channel conductivity and hence the device performance. Due to the incompatibility of conventional growth recipes with thin dielectrics (<5 nm), a novel two-stage-three-step growth recipe was designed for PC diamond integration on top of nitrogen-polar GaN high-electron-mobility transistors in a H2/CH4-plasma environment. Using only H2 and CH4 in the chamber guarantees a higher-phase-purity diamond than chambers with added argon or nitrogen for lower substrate etching. This recipe maintains the performance of the two-dimensional-electron gas and provides a less columnar diamond structure with a larger grain size. Our observations were supported by scanning transmission electron microscopy and Hall mobility measurements using the van der Pauw technique before and after diamond growth. A mobility of ∼1250 cm2/V s, a sheet carrier concentration of ∼1.30 × 1013 cm–2, and a sheet resistivity of ∼380 Ω/□ were maintained after the growth of diamond. The anisotropy ratio has been decreased from 3.75 to 1.12 with this growth recipe. Using long channel devices, we measured the difference in the channel temperature, which decreased by more than 100 °C in the range of 10–24 W/mm power after the integration of the diamond on top of the device.