MOVPE growth and characterization of (AlxGa1-x)2O3 thin films for high power devices

贝塔型氧化镓(β-Ga_2O_3)在高功率应用方面具有广阔的前景，其性能优于当前的关键技术，因为对于β-Ga_2O_3，预计会有相当强的电击穿场强。此外，与其他有希望的材料相比，它提供了潜在的低成本和大衬底尺寸的可控n型掺杂的大块晶体制备。高功率器件的性能直接取决于击穿电场的3次方以及载流子的迁移率。在β-Ga_2O_3中掺入铝可以调节带隙，从而调节击穿电场。因此，我们提出了一种基于金属-有机气相外延生长β-(Al_xGa_(1-x))_2O_3(AlGaO)薄膜的新方法，该方法能够在800℃以上的温度下生长，同时提高了铝在β-Ga_2O_3中的溶解度。为此，我们将首先生长出与目标β薄膜具有最小晶格失配的铝掺杂AlGa2O3体单晶。随后，MOVPE将在这些衬底上进行准同质外延生长高质量AlGaO薄膜的设计和优化，这要归功于对复杂材料表征的详细见解。综合运用原子力显微镜、电子显微镜、光电子显微镜、原位X射线衍射仪、电子衍射仪、椭圆偏振光谱仪和光电子能谱等技术，将有助于揭示AlGaO薄膜的生长模式、形貌、成分以及结构、电子、电学和光学性质。具体地说，我们将确定Al分布的限制因素以及Al在β-Ga2O3中不分相的最大可能掺杂量。然后，我们将探索在AlGaO系统中进行带隙和应变工程的可能性，研究AlGaO在β-Ga_2O_3系统中的表面形貌和界面，并进行电学和结构分析，以了解缺陷的形成过程和杂质的作用。我们的策略包括三个方面：(1)制备适合于外延生长的铝掺杂(高达15%)的块状β-Ga_2O_3晶体(直径为2厘米到2英寸)作为后续准同质外延生长的衬底，并对所获得的薄膜进行表征，以(2)优化生长和(3)评估与应用相关的性能。特别是，该项目的重点是外延AlGaO的制备和表征，其中最大限度地增加了铝的掺入，从而尽可能地增加了禁带和击穿电场。

Beta-type gallium oxide (β-Ga2O3) provides promising perspectives for high-power applications outperforming current key technology because for β-Ga2O3 a considerably stronger electrical breakdown field is predicted. In addition, it offers potentially low cost and large substrate size preparation from bulk crystals with controllable n-type doping in comparison to other promising materials. The performance of high-power devices directly depends on the breakdown field to the power of three as well as on the mobility of the charge carriers. The incorporation of aluminum into β-Ga2O3 allows tuning the band gap and consequently the breakdown field. Therefore, a suitable material growth method is required resulting in high-quality binary oxide thin films with optimized band gap and uncompromised materials properties.Hence, in this project we propose to develop a novel approach based on metal-organic vapor phase epitaxy (MOVPE) of β-(AlxGa1-x)2O3 (AlGaO) thin films on lattice-matched (100)-oriented β-Ga2O3 enabling the growth at temperatures above 800°C with an enhanced solubility of aluminum in β-Ga2O3. For this purpose, we will initially grow bulk aluminum-doped β-Ga2O3 single crystals exhibiting a minimal lattice mismatch with the targeted AlGaO films. Subsequently, the quasi-homoepitaxial growth of high-quality AlGaO thin films on these substrates by MOVPE will be engineered and optimized thanks to the detailed insights from sophisticated materials characterization. Our concerted, systematic use of atomic force, electron, and photoemission microscopy, in situ x-ray and electron diffraction, spectroscopic ellipsometry as well as photoelectron spectroscopy techniques will facilitate to unravel the growth mode, morphology, composition as well as the structural, electronic, electrical, and optical properties of the AlGaO thin films.Specifically, we will determine the limiting factors for Al distribution and its maximally possible incorporation into β-Ga2O3 without phase separation. Then, we will explore the possibilities for band gap and strain engineering in the AlGaO system, investigate the surface morphology as well as the interface of the AlGaO on β-Ga2O3 system, and perform electrical and structural analysis to understand the process of defect formation and the role of impurities. Our strategy is threefold: (1) preparation of epitaxy-ready aluminum-doped (up to 15%) bulk β-Ga2O3 crystals (2 cm to 2 inches in diameter) as substrates for the subsequent quasi-homoepitaxial growth of AlGaO thin layers and characterization of the obtained films to (2) optimize the growth and to (3) evaluate the application-relevant properties. Particularly, the project focuses on the preparation and characterization of epitaxial AlGaO with maximum aluminum incorporation resulting in the highest possible increase of the bandgap and the breakdown field.