Diamond
Diamond, an allotrope of carbon, is commonly associated with sparkling jewellery and gemstones – such as the amazing Hope diamond – and with the concept of extreme hardness, which originates in its cubic crystal structure. While diamond is often treated as a dielectric, it is in fact a semiconductor with an indirect band gap of 5.46 eV, exhibiting very high carrier mobilities. Hence, it lies in the focus of intense research for applications in power electronics as well. Thanks to the strong sp3 carbon-carbon bonds, diamond also exhibits the highest thermal conductivity amongst all materials, 2300 W/mK. This makes diamond the ideal heat spreading and sinking material for cooling electronics that operate at high power densities. Using diamond as a substrate for AlGaN/GaN HEMTs has been shown to enable a 3-fold increase in achievable power density. Diamond films and wafers can be gown via chemical vapour deposition (CVD). Typically, these are polycrystalline and exhibit a complex morphology and crystalline structure that changes with the thickness. Their properties can be on par with those of the natural diamond; however, they depend strongly on the growth conditions. The focus of our research is integrating synthetic diamond into the GaN technology for efficient heat sinking. Combining the extremely high thermal conductivity of diamond with the advantageous electronic properties of the AlGaN/GaN system, devices with outstanding performance can be fabricated that could revolutionise the field of telecommunication and power electronics.
Figure 1. Electron microscope image of diamond
To achieve this, various challenges must be overcome, for instance, the built-in stresses resulting from thermal expansion mismatch between GaN and diamond need to be mitigated, the thermal boundary resistance of the interface between GaN and diamond needs to be characterised and minimised without compromising mechanical stability. Understanding the effect of the complex crystalline structure on the thermal conductivity and other properties of diamond is also critically important. Thus, there is a plethora of intriguing problems to be solved in this research arena.
Figure 2. The evolution of morphology and crystalline structure with increasing diamond during CVD growth
Figure 3. Diamond membranes for determining thermal conductivities