Diamond on GaN: Hetero Interface and Thermal Transport Study
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III-Nitride based high electron mobility transistors (HEMT) have been in the forefront of the 5G LTE revolution and continue to show promise for next-generation terahertz (THz) communications and radar detection systems. GaN-based HEMT devices possess wide bandwidth and high breakdown voltage due to their high electron velocity, high-density two-dimensional electron gas (2DEG), and large breakdown field. More recently, AlGaN/GaN-based transistors for terahertz (THz) emitters and detectors have been reported that can operate from 0.75 THz to 2.1 THz. Furthermore, AlGaN/GaN HEMTs grown on silicon are reported to have switching speeds as high as ∼150 GHz, while the power densities can exceed 12 W/mm, as well as under harsh operational conditions. The maximum radio frequency (RF) power densities for GaN-based HEMTs reported to date is 41.4 Wmm-1 at 4 GHz. However, the currently fielded and commercially available HEMTs are operating at only 5-6 Wmm-1. This massive gap in performance is due to power loss because of self-heating in the HEMT device, which degrades the power added efficiency (PAE) and eventually causes device failure. This self-heating effect is quite severe and can cause a temperature rise in the device as high as 350˚C while operating at 7.8 Wmm-1. Creating a low thermally resistive pathway using chemical vapor deposition (CVD) diamond at very close proximity to the self-heating source could effectively dissipate heat, thereby mitigating the self-heating problem. The challenge here is to find the thinnest possible dielectric adhesion layer with low thermal boundary resistance (TBR) to facilitate diamond growth on the HEMT and to protect it from the harsh CVD diamond growth environment.
In-situ SiNx using metal organic chemical vapor deposition (MOCVD) has been developed and optimized for use as a dielectric adhesion layer to facilitate diamond growth. The effect of reactant gas stoichiometry of in-situ SiNx passivation on structural and electrical properties of MOCVD grown AlGaN/GaN HEMT structures on 100 mm Si (111) is reported. A systematic study on the effect of constituent gas flows on surface morphology and growth rate is reported. X-ray reflectometry and atomic force microscopy is performed to determine the surface morphology and thickness of the near-surface layers. Conformal coverage of SiNx with abrupt SiNx -III-Nitride interface is confirmed by transmission electron microscopy. When the growth rate of the in-situ SiNx is less than a critical threshold of 10 nm/hr, the AlGaN barrier layer is significantly etched. The charge density of the 2DEG induced at the AlGaN/GaN interface due to polarization and surface state filling is evaluated with Hg-probe C-V profile and calculations based on strain relaxation. Sheet charge density, electron mobility, and sheet resistance were determined by measuring Cloverleaf Hall structures. Passivated samples with growth rates higher than the critical threshold show excellent suppression of strain relaxation in the barrier layer. In addition to the strain induced carrier density, surface state filling with in-situ SiNx passivation contributes 8-12% of the total sheet charge density. Increased sheet charge density as high as 1.07E13 cm-2, mobility up to 2500 cm-2V-1s-1 and sheet resistance as low as 275 Ω/sq is observed for the in-situ passivated samples.
Integration of diamond and AlGaN/GaN HEMTs terminated with an in-situ grown SiNx interface layer via MOCVD is also investigated. The effect of diamond growth on the structure and interface properties of the HEMT is studied using high-resolution x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy and scanning transmission electron microscopy (STEM). No structural or physical damage is observed to the HEMT device layers as a result of the hot filament chemical vapor deposited diamond fabrication process. TEM cross-section confirms the smooth and abrupt interface of in-situ SiNx/AlGaN/GaN before and after the diamond growth, with no detectable carbon diffusion into the GaN buffer layer. However, selective degradation of the in-situ SiNx dielectric adhesion layer was observed at the SiNx/diamond interface. Using time domain thermoreflectance (TDTR) the effective isotropic thermal conductivity of the diamond was determined to be up to 176 – 35/ + 40 W/m-K. The effective thermal boundary resistance of the diamond/GaN interface (including the SiNx and additional layers) was as low as 31 - 2.6/+ 2.5 m2K/GW.