In this work, we synthesize a 43 nm thick layer of amorphous Al₂O₃ on transfer-free p-type hydrogen-intercalated quasi-free-standing (QFS) epitaxial chemical vapor deposition graphene on semi-insulating vanadium-compensated on-axis 6H-SiC(0001), using trimethylaluminum as the source of aluminum and deionized water or oxygen plasma as the source of oxygen. With a combination of surface-sensitive X-ray reflectometry (angular range of 0–6°), spectroscopic ellipsometry (angle of incidence of 49° and λ = 490 nm), and scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy, we draw depth profiles of the physical density, refractive index, and elemental composition to reveal that the physical properties of the oxide are inhomogeneous. Its density increases from the graphene interface (ρ = ∼1.5 g/cm³) toward the surface (ρ = ∼3.0 g/cm³). Characteristically, it takes 26 nm for the oxide density to equal its reference value on a test silicon substrate. We demonstrate that an additional postdeposition annealing step in an oxygen-radical-reach environment densifies the oxide and elevates its oxygen content, thus suggesting that the a-Al₂O₃ is oxygen deficient. We reason that the structural inhomogeneity induces the negative polarization effect and the associated reduction of intrinsic hole density observed in a-Al₂O₃-passivated p-type QFS graphene on hexagonal SiC(0001).

中文翻译：

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6H-SiC(0001)上氧化铝钝化准自支撑外延石墨烯双极化机制的起源

在这项工作中，我们在半绝缘钒补偿轴上的无转移p型插氢准自支撑（QFS）外延化学气相沉积石墨烯上合成了43 nm厚的非晶Al ₂ O ₃层6H-SiC(0001)，采用三甲基铝作为铝源，去离子水或氧等离子体作为氧源。结合表面敏感 X 射线反射仪（角度范围 0–6°）、光谱椭偏仪（入射角 49° 且 λ = 490 nm）以及扫描透射电子显微镜与能量色散 X 射线相结合通过光谱学，我们绘制了物理密度、折射率和元素组成的深度剖面，以揭示氧化物的物理性质是不均匀的。其密度从石墨烯界面（ρ = ∼1.5 g/cm ³）向表面（ρ = ∼3.0 g/cm ³）增加。典型地，在测试硅衬底上，氧化物密度需要 26 nm 才能等于其参考值。我们证明，在氧自由基到达环境中进行额外的沉积后退火步骤可以使氧化物致密化并提高其氧含量，从而表明a-Al ₂ O ₃是缺氧的。我们推断，结构不均匀性会导致负极化效应，并导致在六方 SiC(0001) 上的 a-Al ₂ O ₃钝化 p 型 QFS 石墨烯中观察到的固有空穴密度降低。