Charge transfer mechanism for the formation of metallic states at the KTaO3/SrTiO3interface

Abstract

The electronic and optical properties of the KTaO 3 /SrTiO 3 heterointerface are analyzed by the full-potential linearized augmented plane-wave approach of density functional theory. Optimization of the atomic positions points at subordinate changes in the crystal structure and chemical bonding near the interface, which is due to a minimal lattice mismatch. The creation of metallic interface states thus is not affected by structural relaxation but can be explained by charge transfer between transition metal and oxygen atoms. It is to be expected that a charge transfer is likewise important for related interfaces such as LaAlO 3 /SrTiO 3 . The KTaO 3 /SrTiO 3 system is ideal for disentangling the complex behavior of metallic interface states, since almost no structural relaxation takes place. During the past decade, heterointerfaces based on perovskite transition metal oxides have attracted a great deal of attention due to the discovery of remarkable electronic and magnetic properties. They are of special importance for device applications, such as field effect transistors, bipolar transistors, and light emitting diodes. 1,2 Ohtomo and Hwang 3 have reported on metallicity at the interface between LaAlO 3 and SrTiO 3 , two insulators with sizable band gaps of 5.6 and 3.2 eV, respectively. The creation of metallic states is intimately connected to the structural relaxation. 4 The high mobility of the charge carriers at this interface has initiated many investigations, 5,6 as did the discovery of superconductivity at low temperature. 7 However, despite huge experimental and theoretical efforts, many aspects of the physical picture of the LaAlO 3 /SrTiO 3 (LAO/STO) interface remain open. [8][9][10][11][12][13] To a large extent, this appears to be the result of a complex interplay between structural and electronic reconstructions as well as defects. 14 An essential ingredient of the electronic reconstruction is the transfer of charge between different orbitals, i.e., alterations of the orbital occupations, which can modify the electronic properties of transition metal oxides significantly. 15 An interesting system to disentangle the effects of the charge transfer from other parameters is the KTaO 3 /SrTiO 3 (KTO/STO) heterointerface. KTO has an insulting band gap of 3.75 eV and does not show the ferroelectric phase transition that is common for perovskites. 16,17 The band gap of STO amounts to 3.2 eV. Its electronic, transport, ferroelectric, catalytic, and dielectric properties have been widely studied. [18][19][20][21] Interestingly, the lattice match of KTO with STO is much better than that of LAO with STO. As a consequence, KTO is a promising material for application as an insulator in STO-based field-effect devices. Recently, Kalabukhov et al. 22 have investigated the KTO/STO interface experimentally and found close relations to the LAO/STO interface, even though at the (KO) − (TiO 2 ) 0 contact 0.5 holes per unit cell are transferred, while at the (LaO) + (TiO2) 0 contact 0.5 electrons are transferred.The elect...

High charge carrier density at the NaTaO3/SrTiO3 hetero-interface

“…The formation of metallic states at the (KO) À /(TiO 2 ) 0 IF has been explained in terms of charge transfer between the Ti and O atoms, an effect which should also be important for related IFs such as LAO/STO. 19 However, the 2DEG in this case is found to have a charge carrier density of 0.5 Â 10 13 cm À2 , which is considerably less than in LAO/STO, for example. Therefore, we are interested in the question whether the chemical similarity of Na and K (same valence) can be exploited to improve the charge carrier density, while maintaining the less involved mechanism for the creation of the 2DEG.…”

“…Besides the higher charge carrier density, the calculated electronic structure is fully in line with previous findings for the KTO/STO heterostructure. 19 This particularly is true for the mechanism leading to the creation of metallic states.…”

High mobility of the strongly confined hole gas in AgTaO3/SrTiO3

“…Both are not the case for the closely related (KO) À /(TiO 2 ) 0 and (NaO) À /(TiO 2 ) 0 p-type IFs, where the A site cations K and Na do not contribute to the metallic IF states. 10,11 In addition, at the M point the O p x ; p y orbitals dominate. As discussed before, the O 2p hole density in the TiO 2 IF layer is much higher than the Ag 4d and O 2p hole densities in the AgO IF layer.…”

“…10 The p-type IFs in KTaO 3 /STO, NaTaO 3 / STO, and LaGaO 3 /STO show a metallic behavior. [11][12][13] In polar heterostructures (HSs), a general issue is the thickness of the two-dimensional gas induced at the IF, which is usually some 1-2 nm. 14,15 For example, it has been argued that an ultra small thickness of the two-dimensional hole gas is important for enhancing the spin Hall and Rashba effects.…”

The metallic interface between the two band insulators LaGaO3 and SrTiO3

“…25 This code is suitable for studying IF systems. 26,27 For calculating the band structure, the exchangecorrelation potential is parametrized in the local density approximation. Whereas the core states are treated fully relativistically, the scalar relativistic approximation (i.e., neglection of spin orbit coupling) is applied to the valence states.…”