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面临的主要技术挑战。任何电子设备产生的热量都必须得到有效的散热，以提高性能、可靠性和防止过早故障。迫切需要具有高导热系数的新型电子材料。目前，可用于电子器件，包括用于被动冷却的高导热材料，其成本效益和可靠性非常有限。理想的材料是导热系数高达2300W/MK的金刚石。然而，它的生产成本很高，而且钻石的热膨胀系数与大多数半导体之间存在不匹配。铜(~400W/MK)及其合金，例如钨和铝(~200W/MK)仍然是当前电子器件中使用最广泛的散热材料。砷化硼(BAS)是一种禁带宽度为~1.5 eV的半导体。最近的理论预测重新点燃了人们对BAS系统的兴趣，BAS具有超高的热导率，可与钻石相媲美。2018年，三个小组独立报告了热导率接近钻石的BaS微晶的生长。已经证明，由于其独特的声子带结构和界面匹配，准微晶冷却衬底允许在GaN晶体管中显示出比使用金刚石和碳化硅衬底时更低的热点温度。这说明了在电子设备的热管理中使用BaS的潜力，然而，目前的BaAs晶体的尺寸只有几毫米。此外，由于其有利的电学性质，BaS不仅对GaN等电子设备的被动冷却具有吸引力，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转型。目前，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氮化硼薄膜技术。氮化硼(BN)以几种结构类型存在。六方氮化硼(HBN)多晶型，类石墨，是热力学上最稳定的相，也是目前研究最广泛的多晶型。层状晶体结构使hBN成为主要的2D材料之一。然而，更令人感兴趣的是很少被探索的BN-锌-闪石(CBN)立方结构多型体。CBN不具有层状结构，可以更容易地与标准半导体器件异质结构集成。立方氮化硼是一种禁带宽度大得多的半导体材料，其禁带宽度约为6.4 eV，是潜在的深紫外光发光和电力电子应用的重要新材料。CBN还具有非常好的各向同性导热系数，因此在热沉器件中具有很大的潜力。最近首次发现了CBN块状微晶。然而，CBN层的可扩展技术还没有开发出来。该项目将开发一种用于硼基半导体的变革性可扩展技术，这有望给电力电子和光子学领域带来革命性的变化。以硼为基础的材料，包括砷化硼(BAS)、立方氮化硼(CBN)和高度失配的BNAs合金层，将使光子学的光学范围从红外(IR)到深紫外光(DUV)广泛，并将允许具有高导热系数的层。较高的击穿电场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)为立方硼基半导体的可扩展生长提供了最有前途的途径。这将是世界上第一个使用分子束外延作为主要生长方法的可伸缩高导热硼基薄膜的项目。

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.