Boron-based semiconductors - the next generation of high thermal conductivity materials

Thermal management and heat dissipation have become the main technological challenges for the next generation of electronic and photonic devices. Heat generated by any electronic device must be effectively dissipated to improve performance, reliability and prevent premature failures. There is an urgent need for novel electronic materials with a high thermal conductivity.

Presently there are only a very limited number of cost-effective and reliable high thermal conductivity materials which can be used in electronic devices, including for passive cooling. The ideal material is diamond, with a thermal conductivity as large as 2300 W/mK. However, it is costly to produce, and there is a mismatch between diamond's coefficient of thermal expansion and majority of semiconductors. Copper (~400 W/mK) and its alloys for example with tungsten, and aluminium (~200 W/mK) remain the most widely used materials for heat dissipation in current electronic devices.

Boron arsenide (BAs) is a semiconductor with a band gap of ~1.5 eV. Interest in the BAs system has been reignited by recent theoretical predictions that BAs has an ultrahigh thermal conductivity, comparable to that of diamond. In 2018 three groups independently reported the growth of BAs microcrystals with a thermal conductivity close to diamond. It has been demonstrated that BAs-microcrystal cooling substrates allow to exhibit substantially lower hot-spot temperatures in GaN transistors due to their unique phonon band structures and interface matching, beyond those when using diamond and silicon carbide substrates. This illustrates the potential for using BAs in the thermal management of electronics, however, present BAs crystals are only a few mm in size. Furthermore, due to its beneficial electronic properties, BAs is not only attractive for passive cooling of electronics such as GaN, but also by itself a very promising novel material to be transformative for electronic and photovoltaic devices. Now the main challenge in realising the potential of this novel material is to develop a scalable technology of high-quality BAs layers.

Boron nitride (BN) exists in several structural polytypes. Hexagonal boron nitride (hBN) polytype, graphite-like, is thermodynamically the most stable phase and presently the most widely explored polytype. The lamellar crystal structure made hBN one of a major 2D material. However, of even greater interest is the much less explored cubic structural polytype of BN - zinc-blende (cBN). cBN does not have a laminar structure and could be more easily integrated with standard semiconductor device heterostructures. Cubic boron nitride is a semiconductor with much larger bandgap energy of ~6.4 eV, which makes it a very important new material for potential deep ultraviolet (DUV) light-emitting and power electronic applications. cBN also has a very good isotropic thermal conductivity and therefore has high potential in heat sink devices. The first cBN bulk microcrystals were recently demonstrated. However, a scalable technology for cBN layers is not yet developed.

This project will develop a transformative scalable technology for the boron-based semiconductors, which promise to revolutionize the areas of power electronics and photonics. Boron-based materials, including boron arsenide (BAs), cubic boron nitride (cBN) and highly mismatched BNAs alloy layers, will enable a wide optical range from infrared (IR) to deep ultraviolet (DUV) for photonics and will allow layers with high thermal conductivity. High breakdown fields will allow their applications in power electronics. Our vision is that molecular beam epitaxy (MBE) provides the most promising route to the scalable growth of the cubic boron-based semiconductors. This will be the first project world-wide enabling scalable high thermal conductivity boron-based layers using MBE as main growth method.