Abstract
Quasi-two-dimensional electron gas has been realized at the polar-nonpolar interface of several insulating oxide heterostructures. However, its hole counterpart remains elusive. In an attempt to find a novel system that exhibits quasi-two-dimensional hole gas (q-2DHG) at the heterointerface, we adopt to materials search, first based on phenomenology followed by a comprehensive set of calculations based on first-principles density functional theory. Our studies show the epitaxial growth of cubic Ca0.5TaO3 on TiO2 terminated substrate display (q-2DHG). The hole gas emanates from the O 2p orbitals of the TiO2 layers of the substrate. On the other hand, an electron gas is formed at the (001) TaO2 top surface, thereby representing the heterostructure as a coupled quantum well system. The partial filling of the Ta 5dt2g conduction band indicates electron reconstruction, in agreement with the polar catastrophe model. Besides, a critical thickness of three monolayers is deduced from the calculations for the formation of q-2DHG in the Ca0.5TaO3/SrTiO3 heterostructure, which is consistent with the model prediction based on the modern theory of polarization. With both cubic systems, Ca0.5TaO3 and SrTiO3, having a similar underlying symmetry and minimal lattice mismatch, epitaxial growth with an abrupt interface can be well anticipated. Such a single-tier oxide heterostructure composed of separated confined hole-electron subsystems is expected to provide a platform to unravel exciting physics and also for functional devices related to oxide electronics.
Highlights
The origin of quasi-two-dimensional electron gas at the interface of polar and nonpolar insulating oxides has been attributed to several mechanisms, such as oxygen defects, intersite cation disorder, atomic reconstruction, and electronic reconstruction
To look for an appropriate oxide that can be epitaxially grown on SrTiO3, we adhere to the fundamental assumptions of the polar catastrophe model, i.e., the overlayer oxide must be polar and must have minimal lattice mismatch with the substrate
Finding that the lattice mismatch is minimal and that a p-type interface can be formed in Ca0.5TaO3/SrTiO3, we performed a comprehensive set of calculations based on first-principles density functional theory
Summary
The origin of quasi-two-dimensional electron gas at the interface of polar and nonpolar insulating oxides has been attributed to several mechanisms, such as oxygen defects, intersite cation disorder, atomic reconstruction, and electronic reconstruction.1–9 Of these, the most fascinating is the electronic reconstruction at the heterointerface due to the polar catastrophe model. From these sets of calculations, it becomes evident that GGA-PBE consistently underestimates the Eg of band insulators, such as SrTiO3 and Ca0.5TaO3, by discrepancies exist between the theoretical and experimental values, the calculations provide qualitatively good scitation.org/journal/apm results.
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