Van der Waals Ga2O3 functional materials epitaxy: Revolutionary power electronics

联合国政府间气候变化专门委员会(IPCC)在2021年8月的科学报告中强调了我们星球迫在眉睫的气候风险，称其为“人类的红色代码”。目前，几乎所有的能量转换电力电子产品都使用硅(Si)，它的效率相对较低，浪费了能量作为热量。事实上，全球72%的一次能源消耗被浪费了，其中20%可以通过新的电力电子产品来节省！使用GaN和SIC的宽带隙器件正在进入市场，但它们要么不能维持足够高的电压，要么太昂贵，无法广泛使用，例如在智能电网中。超宽带隙氧化镓(Ga2O_3)的效率远远超过窄禁带硅，已成为一种具有变革性的竞争者，具有低成本和1-2千伏甚至10千伏的电压能力，有可能将电力转换效率损失大幅降低&gt；100倍。考虑到Ga_2O_3的S高百利加品质因数，这一指标决定了一种材料对功率器件的益处，它甚至有可能在性能上超过当前宽带隙功率器件(GaN，SiC)的5-10倍以上。在这个项目中，我们瞄准了使用功能Ga2O_3的范德华外延作为一种变革性和革命性工具的高压功率器件，为低成本高压功率器件开辟了一个新的研究领域。这种形式的外延使用中间层，这减少了衬底-外延层的相互作用，使得外延层能够在晶片级生长到具有不同晶体结构的外来材料上，随后潜在地是层转移到其他有益的衬底，否则这是不可能的。如果成功，这将首次实现甚至是10千伏的低成本电力电子器件，例如用于智能电网，也就是说，这是一个很高的回报，但所采取的方法是高度投机性的：(I)范德华材料能在Ga2O3MOCVD室的反应性环境中幸存下来吗？(Ii)范德华材料的潜在损坏是否会影响后续器件的质量？(3)在生长过程中能否控制Ga_2O_3的S多型？(Iv)范德华生长的界面是否提供足够高的电子和声子传输通过它们，以及这些是否可以优化或缓解，例如使用生长条件或h-BN厚度，或在层转移期间？如果成功，后续的层转移将允许Ga2O3与许多低成本的高导热衬底(如聚AlN和聚钻石)进行异质集成，从而彻底改变器件的散热效果。使用p型衬底将为双极器件打开设计空间，甚至是超结，也就是已经改变了硅电力电子的器件概念，这些概念很少能够超越硅，如果我们成功了，将产生重大影响。横向设备将用于验证集成的有效性，高性能设备的商业化途径将通过我们与DyneX半导体的工业合作伙伴关系来探索。还将展示垂直器件(横向配置功率密度更高，对电力电子应用具有吸引力)。从可持续发展的角度来看，该计划也标志着循环经济中的一个关键里程碑；用于有源器件的范德华外延或外延层转移将最大限度地减少对Ga2O衬底的需求，将其转移到可持续性问题较少的衬底(元素Ga可能在100年内变得稀缺)，最大限度地减少电力设备中使用的镓的量。范德华外延异质集成Ga2O的成功演示还将为低成本、与其他超宽带隙材料(如单晶AlGaN、AlN、钻石)的晶片级集成铺平道路，为大幅降低电力电子效率低下对全球能源消耗的贡献提供了潜力。

The imminent climate risks for our planet have been highlighted by the UN's Intergovernmental Panel on Climate Change (IPCC) calling it "code red for humanity" in its scientific report in August 2021. Presently, nearly all energy conversion power electronics use Silicon (Si), which is relatively inefficient, wasting energy as heat. In fact, 72% of global primary energy consumption is wasted, of which 20% could be saved with new power electronics! Wide bandgap devices using GaN and SiC are entering the market, but they either do not sustain high enough voltages or are too expensive for widespread use, e.g., in smart grids. Ultrawide bandgap Gallium Oxide (Ga2O3), with efficiency far exceeding that of narrow bandgap Si, has emerged as a transformative contender with low cost and >1-2 kV, even 10 kV voltage capability, with potential for a massive >100x reduction in power conversion efficiency losses. Considering Ga2O3's high Baliga figure of merit, the metric determining how beneficial a material is for power devices, it has potential to even exceed current wide bandgap power devices (GaN, SiC), now replacing incumbent Si power electronics, by more than a factor of 5-10 in performance.In this project, we target high voltage power devices using van der Waals epitaxy of functional Ga2O3 as a transformative and revolutionary vehicle to open a new research field for low cost high voltage power devices. This form of epitaxy uses an intermediate layer, which reduces the substrate-epitaxial layer interaction, enabling growth of the epitaxial layer onto a foreign material with different crystal structure at the wafer-level, potentially followed by layer transfer to other beneficial substrates, which would otherwise not be possible. If successful this will enable for the first time even >10kV low cost power electronics devices, e.g. for smart grids, i.e., a high reward but the approach taken is highly speculative: (i) Do van der Waals materials survive the reactive ambient of a Ga2O3 MOCVD chamber? (ii) does potential damage to the van der Waals material impact the subsequent device quality? (iii) can we control Ga2O3's polytypes during growth? (iv) do van der Waals grown interfaces provide high enough electron and phonon transport through them, and can these be optimized or mitigated for e.g. using growth conditions or h-BN thickness, or during a layer transfer? If successful, subsequent layer transfer would allow heterogeneous integration of Ga2O3 with numerous low-cost high thermal conductivity substrates such as poly-AlN and poly-diamond to revolutionize device heat sinking. Use of p-type substrates would open the design space to bipolar devices, even superjunctions, i.e. device concepts which have transformed Si power electronics, concepts which have rarely being able to venture beyond Si, a major impact if we are successful. Lateral devices will be used to validate the effectiveness of the integration, with routes to possible commercialization of high performance devices to be explored through our industrial partnership with Dynex Semiconductor. Vertical devices (with improved power density over lateral configurations, attractive for power electronics applications) will also be demonstrated.The programme also marks a key milestone from a sustainability perspective, within a circular economy; van der Waals epitaxy or epilayer transfer used for the active devices will minimize the need for Ga2O3 substrates, to transfer to substrates with less sustainability issues (elemental Ga may become scarce within 100 years), minimizing the amount of Ga being used in power devices. Successful demonstration of heterogeneous integration of Ga2O3 by van der Waals epitaxy will also pave the way for low-cost, wafer-level integration with other ultrawide bandgap materials (e.g. single crystalline AlGaN, AlN, diamond), offering the potential to strongly reduce the contribution of inefficiency in power electronics to global energy consumption.