While there is a great wealth of data for thermal transport in synthetic diamond, there remains much to be learned about the impacts of grain structure and associated defects and impurities within a few microns of the nucleation region in films grown using chemical vapor deposition. Measurements of the inhomogeneous and anisotropic thermal conductivity in films thinner than 10 μm have previously been complicated by the presence of the substrate thermal boundary resistance. Here, we study thermal conduction in suspended films of polycrystalline diamond, with thicknesses ranging between 0.5 and 5.6 μm, using time-domain thermoreflectance. Measurements on both sides of the films facilitate extraction of the thickness-dependent in-plane (κr) and through-plane (κz) thermal conductivities in the vicinity of the coalescence and high-quality regions. The columnar grain structure makes the conductivity highly anisotropic, with κz being nearly three to five times as large as κr, a contrast higher than that reported previously for thicker films. In the vicinity of the high-quality region, κr and κz range from 77 ± 10 W/m-K and 210 ± 50 W/m-K for the 1 μm thick film to 130 ± 20 W/m-K and 710 ± 120 W/m-K for the 5.6 μm thick film, respectively. The data are interpreted using a model relating the anisotropy to the scattering on the boundaries of columnar grains and the evolution of the grain size considering their nucleation density and spatial rate of growth. This study aids in the reduction in the near-interfacial resistance of diamond films and efforts to fabricate diamond composites with silicon and GaN for power electronics.

The electron field emission properties of the (111)1×1:H surface of natural semiconducting (p-type) diamond have been examined with simultaneous field emission and photoemission measurements. We find that the origin of the field emission is due to the electron tunneling from the valence band and show that the shape of the field emission energy distributions can be described by the theory of semiconductor field emission. Analysis of our results demonstrate that the combination of field emission and photoemission is a powerful technique for the study of the electron emission properties of materials.

Reduced performance in Gallium Nitride (GaN)-based high electron mobility transistors (HEMTs) as a result of self-heating has been well-documented. To mitigate this effect, the incorporation of high thermal conductivity diamond heat spreading films or substrates has been proposed. A mid-process integration scheme, termed “gate-after-diamond,” is shown to improve the thermal budget for NCD deposition and enables scalable, large-area diamond coating without degrading the Schottky gate metal. The optimization of this process step is presented in this work. Nanocrystalline (NCD)-capped devices had a 20% lower channel temperature at equivalent power dissipation. Improved electrical characteristics were also observed, notably improved on-resistance and breakdown voltage, and reduced gate leakage.

Top-side integration of nanocrystalline diamond films in the fabrication sequence of AlGaN/GaN high electron mobility transistors is demonstrated. Reliable oxygen plasma etching of the diamond capping layer, required for a diamond-before-gate process, was implemented by using a sacrificial SiN "dummy" gate. Hall characterization showed minimal ($6%) reduction in sheet carrier density and commensurate increase in sheet resistance, while maintaining mobility and on-state drain current density. Off-state drain current and threshold voltage were increased, likely by fluorination of the AlGaN surface after removal of the sacrificial gate, even though a 20 nm thick Al 2 O 3 layer was used as a SF 6 -plasma etch stop. Pulsed I DS and on-resistance were improved, indicating that a 10 nm SiN/ 500 nm NCD could offer improved AlGaN surface passivation compared to a more conventional 100 nm thick PECVD SiN film.

No abstract