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）在其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设备都使用硅（Si），这是相对低效的，浪费能量作为热量。事实上，全球72%的一次能源消耗被浪费，其中20%可以通过新的电力电子设备来节省！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，要么对于广泛使用来说太昂贵，例如，智能电网。超宽带隙氧化镓（Ga 2 O3）的效率远远超过窄带隙Si，已成为一种变革性的竞争者，具有低成本和>1-2 kV，甚至10 kV的电压能力，具有功率转换效率损失大幅度降低> 100倍的潜力。考虑到Ga 2 O3的高Baliga品质因数，即确定材料对功率器件的有益程度的度量，它甚至有可能超过当前的宽带隙功率器件（GaN，SiC），现在取代现有的硅电力电子，超过5-10倍的性能。在这个项目中，我们的目标是使用功能性Ga 2 O 3的货车德瓦尔斯外延的高压功率器件，作为一种变革性和革命性的工具，为低成本高压功率器件开辟一个新的研究领域。这种形式的外延使用中间层，其减少了衬底-外延层相互作用，使得外延层能够在晶片级生长到具有不同晶体结构的异质材料上，随后可能层转移到其他有益衬底，这在其他情况下是不可能的。如果成功，这将首次实现甚至> 10 kV的低成本电力电子设备，例如用于智能电网，即，高回报，但所采取的方法是高度投机性的：（i）货车德瓦耳斯材料在Ga 2 O 3 MOCVD室的反应性环境中存活吗？(ii)货车德瓦尔斯材料的潜在损坏是否会影响后续器械质量？(iii)我们能在生长过程中控制Ga 2 O3的多型体吗？(iv)货车德瓦耳斯生长的界面是否提供足够高的电子和声子通过它们的传输，并且这些可以被优化或减轻，例如使用生长条件或h-BN厚度，或在层转移期间？如果成功的话，随后的层转移将允许Ga 2 O3与许多低成本的高导热率衬底（如聚AlN和聚金刚石）的异质集成，以彻底改变器件的散热。p型衬底的使用将为双极器件，甚至是超结，即已经改变了Si功率电子器件的器件概念，这些概念很少能够超越Si，如果我们成功的话，这将产生重大影响。横向器件将用于验证集成的有效性，通过我们与Dynex Semiconductor的工业合作伙伴关系，探索高性能器件商业化的可能途径。垂直器件（与横向配置相比，功率密度更高，对电力电子应用有吸引力）也将得到展示。该计划也标志着循环经济中可持续发展角度的一个重要里程碑;用于有源器件的货车德瓦尔斯外延或外延层转移将最小化对Ga 2 O 3衬底的需要，以转移到具有较少可持续性问题的衬底（元素Ga可能在100年内变得稀缺），从而使功率器件中使用的Ga的量最小化。通过货车德瓦尔斯外延的Ga 2 O3异质集成的成功示范也将为与其他超宽带隙材料（例如单晶AlGaN、AlN、金刚石）的低成本、晶片级集成铺平道路，从而提供极大地减少电力电子中的低效率对全球能源消耗的贡献的潜力。

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.