Surface zeta potential and diamond seeding on gallium nitride films

With the high breakdown voltage and current handling ability of GaN, AlGaN/GaN on SiC HEMT structures are the current benchmark for high-power, high-frequency applications[1].  However, in such devices the GaN epilayer and particularly the SiC substrate, with thermal conductivity of around 400 W/mK, limit the heat extraction leading to de-rating of the maximum power dissipation[2]. Through replacement of the substrate and capping of the transistor channel with diamond of thermal conductivity of up to 2000 W/mK, large decreases in the thermal resistance should therefore be achievable allowing full utilisation of the properties of GaN based devices[3].

The growth of pinhole free, thin film diamond on non-diamond substrates requires the use of a nucleation enhancement step. One of the most commonly used techniques involves seeding the substrate with nanodiamond particles, resulting in high nucleation densities of the order of 1011 cm-2[4]. As attachment of the particle to the substrate is dependent on both the zeta potential of the surface and the particles, it is essential to measure the zeta potential of the surface and tailor the surface groups of the seeds to reach such nucleation densities. In the present study we have measured the surface zeta potential of the GaN surface. Using such knowledge, diamond films have been successfully grown atop GaN on sapphire wafers, without the addition of a thermally resistant intermediate dielectric layer to aid growth as used within previous studies[1].  Films were grown at 850 °C, under 5% methane admixture (CH4/H2) conditions to a thickness of ~150 nm, as judged by in-situ laser interferometry. SEM characterization of the resulting samples revealed continuous films over the 15 by 15 mm samples, free of pinholes, and highly crystalline. The dataset contains 7 files. The txt file is the raw data for pH vs zeta potential for both faces of GaN ( Ga- and N- face). The AFM dataset is in a folder named AFM and the data for each sample namely, Ga-face unseeded, N- face unseeded, G-face seeded with H-terminated diamond, N-face seeded with H-terminated diamond, Ga-face seeded with O-terminated diamond and N-face seeded with O-terminated diamond are in their respective folders. Any AFM analyesis software like WSXM can be used to open and analyse the data.

References
1.     J. W. Pomeroy, M. Bernardoni, D. C. Dumka, D. M. Fanning and M. Kuball, Applied Physics Letters 104 (8), 083513 (2014).
2.     J. Pomeroy, M. Bernardoni, A. Sarua, A. Manoi, D. C. Dumka, D. M. Fanning and M. Kuball, presented at the 2013 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS), 2013 (unpublished).
3.     O. A. Williams, Diamond and Related Materials 20 (5-6), 621-640 (2011).
Oliver A. Williams, Olivier Douhéret, Michael Daenen, Ken Haenen, Eiji Ōsawa, Makoto Takahashi, Chemical Physics Letters 445, 255 (2007)

Research results based upon these data are published at https://doi.org/10.1021/acsomega.7b01069