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

热管理和散热已成为下一代电子和光子器件的主要技术挑战。任何电子设备产生的热量都必须有效地消散，以提高性能，可靠性并防止过早失效。因此，迫切需要具有高热导率的新型电子材料。目前，只有非常有限数量的具有成本效益且可靠的高导热率材料可用于电子设备，包括用于被动冷却。理想的材料是金刚石，其热导率高达2300 W/mK。然而，它的生产成本很高，而且金刚石的热膨胀系数与大多数半导体之间存在不匹配。铜（~400 W/mK）及其合金（例如与钨的合金）和铝（~200 W/mK）仍然是当前电子器件中最广泛使用的散热材料。最近的理论预测表明，BAs具有与金刚石相当的热导率，这重新点燃了对BAs系统的兴趣。2018年，三个研究小组独立报道了BAs微晶的生长，其热导率接近金刚石。已经证明，由于其独特的声子带结构和界面匹配，BA-微晶冷却衬底允许在GaN晶体管中表现出显著更低的热点温度，超出了使用金刚石和碳化硅衬底时的那些。这说明了在电子设备的热管理中使用BA的潜力，然而，目前的BA晶体的尺寸仅为几mm。此外，由于其有益的电子特性，BAs不仅对GaN等电子产品的被动冷却有吸引力，而且本身也是一种非常有前途的新型材料，可用于电子和光伏器件的变革。现在，实现这种新型材料潜力的主要挑战是开发一种可扩展的高质量BAs层技术。氮化硼（BN）存在于几种结构多型体中。六方氮化硼（hBN）多型体，石墨状，是结晶学上最稳定的相，目前是最广泛开发的多型体。层状晶体结构使hBN成为主要的2D材料之一。然而，更令人感兴趣的是较少探索的BN -锌-尖晶石（cBN）的立方结构多型体。cBN不具有层状结构，并且可以更容易地与标准半导体器件异质结构集成。立方氮化硼是一种具有更大带隙能量（~6.4 eV）的半导体材料，这使其成为一种非常重要的新型材料，具有潜在的深紫外（DUV）发光和电力电子应用前景。cBN还具有非常好的各向同性导热性，因此在散热装置中具有高潜力。最近展示了第一个cBN块状微晶。然而，cBN层的可扩展技术尚未开发。该项目将为硼基半导体开发一种变革性的可扩展技术，有望彻底改变电力电子和光子学领域。硼基材料，包括砷化硼（BAs）、立方氮化硼（cBN）和高度失配的BNAs合金层，将使光子学能够实现从红外（IR）到深紫外（DUV）的宽光学范围，并将使层具有高热导率。高击穿场强将使其在电力电子领域得到应用。我们的愿景是，分子束外延（MBE）提供了最有前途的路线，可扩展的立方硼基半导体的生长。这将是全球第一个使用MBE作为主要生长方法实现可扩展的高热导率硼基层的项目。

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.