Mixed-size diamond seeding for low-thermal-barrier growth of CVD diamond onto GaN and AlN

Abstract

We report a method of growing a diamond layer via chemical vapour deposition (CVD) utilizing a mixture of microdiamond and nanodiamond seeding to give a low effective thermal boundary resistance (TBReff) for heat-spreading applications in high-frequency, high-power electronic devices. CVD diamond was deposited onto thin layers of both GaN and AlN on Si substrates, comparing conventional nanodiamond seeding with a two-step process involving sequential seeding with microdiamond then nanodiamond. Thermal properties were determined using transient thermoreflectance (TTR), and the samples were also analysed with SEM and X-ray tomography. While diamond growth directly onto GaN proved to be unsuccessful due to poor adhesion, films grown on AlN were adherent and robust. The twostep mixed-seeding method gave TBReff values <6 m 2 K GW -1 that were 30 times smaller than for films grown under identical conditions but using nanodiamond seeding alone. Such remarkably low thermal barriers obtained with the mixed-seeding process offer a promising route for fabrication of high-power GaN HEMTs using diamond as a heat spreader with an AlN interlayer.

Diamond/GaN HEMTs: Where from and Where to?

“…This results in an extremely rough surface that increases the scattering of the phonons. Smith et al [76] and Waller et al [74] reported TBR GaN/diamond as high as 220 m 2 •K/GW. In both works the GaN surface was seeded using a two-step electrostatic spray technique (see more details in Section 4.1.3.1).…”

“…At the same time the bottleneck of the heat extraction was recognized to be the TBR between GaN and diamond (TBR GaN/diamond ) [72] and most of following research focused on decreasing it, whether by decreasing the dielectric thickness, by using a different dielectric, or by optimizing the diamond nucleation layer [10,[72][73][74][75][76][77][78][79][80][81][82][83][84]. The impact of the thickness of the GaN buffer layer on the R th of the HEMT devices [85][86][87][88][89][90] and the effects of the stress caused by the difference in the CTEs of GaN and diamond [91][92][93][94][95][96] were also evaluated by different research groups.…”

“…It should be mentioned that in the experiments by Mandal et al [84] and Smith et al [76] the TBR values refer to the interface between the diamond and the AlN substrate: as a consequence, no direct comparison can be made with the TBRGaN/diamond previously re- Seeding the pre-treated AlN surfaces with H-terminated detonation nanodiamond (DND) seeds resulted in an average TBR of 16 m 2 •K/GW. A breakthrough was recently reported by Smith and co-workers [76], who used a two-step electrostatic spray technique to seed 130 nm-thick AlN films deposited on Si substrates.…”

Diamond as the heat spreader for the thermal dissipation of GaN-based electronic devices

“…The interface thermal properties between GaN and NCD diamond were improved by Smith et al using a two-step mixed-seeding method [86]. It was found that the mixture of microdiamond and nanodiamond seeding led to a low TBR eff .…”

Boron-Doped Diamond/GaN Heterojunction—The Influence of the Low-Temperature Deposition

“…This results in an expansion of the surface of GaN substrate; the irregular topography of the substrate significantly increases the adhesion of the diamond layer to the GaN surface. This is critical due to cooling cracks or small gaps opening up at the diamond/GaN interface, resulting in partial or full delamination and negligible thermal conduction through the interface, which was shown by Smith et al [ 10 ]. In view of the fact that the lack of a covalent bond between Ga-C can lead to delamination of the diamond layer, solutions that increase the adhesion of the layer to GaN are very desirable.…”