Nanocrystalline diamond thin film integration in AlGaN/GaN high electron mobility transistors and 4H-SiC heterojunction diodes
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The extremely high thermal conductivity and mechanical hardness of diamond would make it the natural choice for device substrates when large area wafer production becomes possible. Until this milestone is achieved, people could utilize nanocrystalline diamond (NCD) thin films grown by chemical vapor deposition (CVD). A topside thermal contact could be pivotal for providing stable device characteristics in the high power, high temperature, and high switching frequency device operating regime that next-generation power converter circuits will mandate. This work explores thermal and electrical benefits offered by NCD films to wide bandgap semiconductor devices. Reduction of self-heating effects by integrating NCD thin films near the device channel of AlGaN/GaN high electron mobility transistors (HEMTs) is presented. The NCD layers provide a high thermal conductivity path for the reduction of hot electron dispersion, a phenomenon caused by self-heating and detrimental to the continuous operation of GaN devices in power switching circuits. Recent advances in diamond doping have made it possible to think of this material as a very wide bandgap semiconductor (5.5 eV for ideal diamond). A few unique properties, such as negative electron affinity (χ = -0.2 eV for H-terminated diamond), make this material very interesting. Using H-terminated NCD, a heterojunction with 4H-SiC has been developed. Undoped and B-doped NCD were deposited on both n<super>-</super> and p<super>-</super> 4H-SiC epilayers. Different metals were studied to provide an Ohmic contact to the NCD layer. I-V measurements on p<super>+</super> NCD / n<super>-</super> 4H-SiC p-n junctions indicated Schottky rectifying behavior with a turn-on voltage of around 0.2 V. The current increased over 8 orders of magnitude with an ideality factor of 1.17 at 30 °C. Ideal energy-band diagrams suggested a possible conduction mechanism for electron transport from the SiC conduction band to either the valence band or Boron acceptor level of the NCD film. Cathodoluminescence and thermally stimulated current methods were employed to study the deep level assisted conduction in this heterojunction. Applications as a simultaneous UV-transparent optical and Schottky electrical contact to 4H-SiC are discussed.