Nanocrystalline Diamond Lateral Overgrowth for High Thermal Conductivity Contact to Unseeded Diamond Surface
Authors: Francis Daniel , Ji Xiaoyang , Vanjari Sai Charan , Tadjer Marko , Feygelson Tatyana , Lundh James Spencer , Masten Hannah , Spencer Joseph , Jacobs Alan G. , Anderson Travis J. , Hobart Karl D. , Pate Bradford , Kuball Martin , Ejeckam Felix 
Published: August 2024

We demonstrate diamond lateral overgrowth as a way of increasing the thermal conductivity of thin layers of diamond. The technique can be used as a way of growing diamond on top of semiconductors, creating a thin layer of high thermal conductivity diamond in direct contact with semiconductors and allowing for the encasement of GaN in high thermal conductivity diamond.
As we move to higher and higher power densities, the requirements for heat removal become extreme. The best passive way to remove heat from a semiconductor is to have a high thermal conductivity heat spreader near the heat source. Having diamond under the substrate is good. Having diamond under the epi is better and having diamond on top and bottom of the epi is better still.
As-grown thin layers of diamond on non-diamond substrates have nano-diamond seeds as the starting material. The diamond thermal conductivity is a function of the grain size (Especially for small grain material) [1]. If we can increase the grain of the diamond near the junction, we can increase the thermal conductivity near the junction and more efficiently spread the heat away from the source before rejecting it into the surrounding material.
In the experiment, we start with a bare silicon wafer seeded with nano-diamond seeds then grow about one micron of diamond. We coat it with a few nanometers of SiN. Then pattern the SiN layer with 2μm wide features to facilitate the coalescence of LOG-NCD over the SiN. Then we grow a 2μm thick microwave-plasma CVD NCD film (800W, 15Torr, 750°C, 0.3% CH4/H2) over the patterned SiN film initiating directly from the exposed areas of the host NCD. The diamond growth does not initiate on the patterned silicon nitride without seeds. The exposed diamond acts as a nucleating center initiating the diamond regrowth. The regrown diamond spreads from the diamond crystals exposed from the first growth. The edge crystals closest to the SiN patterned area expand over the silicon nitride forming diamond conformal to the SiN surface. This creates large crystals over the unseeded area. Figure 1 is a transmission electron microscope (TEM) image of the sample. The TEM, the image can be split into two regions. The left has no patterning, the called-out grain is 100nm wide. The right is patterned, and the diamond grains are between 1,000 to 2,000nm wide.
We measured the thermal conductivity of the diamond on this samples using the TDTR method. This method uses a laser many times larger in diameter than the thickness of the film. As such, we probe the out-of-plane thermal conductivity. The initial diamond growth near the silicon had an average thermal conductivity of 100W/mK. We measured the second micron of growth without patterns to have Tc of 130W/mK. The diamond directly above the patterns had Tc of 260W/mK with grains 10X larger than the un-patterned diamond.
Since the diamond grains are columnar, the difference in thermal conductivity between the patterned and un-patterned diamonds was 2X. Lateral measurements would likely show even larger changes in thermal conductivity. With grain sizes of 100 nm to 250 nm (as is the case with seeded diamond), the in-plane thermal conductivity will be dominated by the quality of the grain/grain interfaces and the lateral grain size dimensions rather than for the quality of the lattice. [1] We used lateral overgrowth to increase the grains to microns rather than 100s of nm. This means that we move into a regime where thermal conductivity is dominated by the quality of the diamond in the grains rather than the grain boundaries. As such the thermal conductivity is increased and can be further improved by improving the crystal quality - a parameter which is controlled by the diamond growth conditions. Typically, samples grown with more than 1% methane / hydrogen ratio show lower thermal conductivity [2]. However higher concentrations are important to establish the initial diamond film without damage to the underlying semiconductor. Here, the lateral overgrowth and methane concentration can be balanced to achieve both high thermal conductivity and low damage to the underlying substrate.
References:

J. Anaya, et. al “Control of the in-plane thermal conductivity of ultra-thin nanocrystalline diamond films through the grain and grain boundary properties”, Acta Materialia, Volume 103, 2016, Pages 141-152, ISSN 1359-6454

Gu, C., Jin, Z., Lu, X., Zou, G., Zhang, J. and Fang, R., 1997. The deposition of diamond film with high thermal conductivity. Thin Solid Films, 311(1-2), pp.124-127.

Figure 1