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在SrTiO3基氧化物异质结中，由于Ti 3d轨道的影响，界面处的二维电子气 (2DEG) 室温迁移率较低，这限制了它们在半导体器件中的应用。而SnO2的导带底由Sn的5s轨道组成，因此，基于SnO2的异质结界面具有形成室温下高迁移率2DEG的潜力。本文采用第一性原理计算的方法，对 (001) HfO2/SnO2异质结构的界面电子结构进行了系统研究。计算结果表明，无缺陷的HfO2/SnO2异质结具有半导体的能带结构，其界面附近不会形成2DEG。当异质结界面附近的SnO2侧存在氧空位时，体系出现跳跃导电，但其界面附近仍不会出现2DEG。当氧空位处于异质结的HfO2表面时，氧空位的存在使表面产生缺陷态。缺陷态的电子处于局域态，并不参与导电，但缺陷态与异质结构的导带底 (由界面贡献) 发生交叠，使异质结表面缺陷态中的电子转移至界面，并在异质结界面附近形成2DEG。此外，对于HfO2层较薄的HfO2/SnO2异质结构，比如HfO2层为7个单胞 (约2.37 nm) 厚时，HfO2表面吸附的H原子向异质结提供电子，这些电子部分转移到界面附近的导带，形成2DEG。随着HfO2层厚度的增加，表面电子转移到界面的几率逐渐下降，使得界面处的电子浓度逐渐降低。In SrTiO3-based oxide heterostructures, the mobility of the two-dimensional electron gas (2DEG) at the interface is relatively low at room temperature due to the influence of Ti 3d orbitals, which limits their application in semiconductor devices. In contrast, the conduction band bottom of SnO2 is composed of Sn 5s orbitals, and it has been demonstrated that bulk SnO2 exhibits high carrier mobility at room temperature. Therefore, SnO2-based heterostructure interfaces have the potential to form 2DEG with high mobility at room temperature. In this paper, we construct a heterostructure (HfO2)7/(SnO2)13 with supercell in (001) plane and systematically studied the electronic structure of the heterostructure using first-principles calculations. The calculation results show that the defect-free (HfO2)7/(SnO2)13 heterostructure has a band structure similar to that of a semiconductor, and there is no 2DEG near the interface of the heterostructure. However, the conduction band bottom is primarily contributed by non-degenerate Sn 5s orbitals at this situation. In the in-plane supercell of the (HfO2)7/(SnO2)13 heterostructure, each layer contains 8 oxygen atoms (the thickness of 1 unit cell defined as a layer). When an oxygen atom in a layer on the SnO2 side near the interface of the heterostructure is removed, the presence of the oxygen vacancy leads to the formation of a defect band below the conduction band. This will lead to hopping conductivity in the heterostructure. However, 2DEG still does not appear near the heterostructure interface. When the oxygen vacancy is located at the surface layer of the HfO2 in the supercell structure, the presence of the oxygen vacancy leads to the formation of a defect state at the surface. The electrons in the defect state are localized and do not contribute to conductivity. However, the defect band overlaps with the conduction band from the interface, causing the electrons on the surface of HfO2 to tunnel towards the interface. In this scenario, the 2DEG emerges in the vicinity of the heterostructure interface. In addition, for HfO2/SnO2 heterostructures with thinner HfO2 layers, such as HfO2 layers with a thickness of 7 unit cells (about 2.37 nm), the H atoms adsorbed on the HfO2 surface provide electrons to the heterostructure. Some of these electrons transfer to the conduction band near the interface, leading to the formation of a 2DEG in that region. Meanwhile, the remaining electrons stay on the surface, forming a conductive layer with a thickness of approximately 2 unit cells. As the thickness of the HfO2 layers increases, the probability of electrons transferring from the surface to the interface gradually decreases, resulting in a gradual decrease in the electron density at the interface.
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