Strain-driven carrier-type switching of surface two-dimensional electron and hole gases in a KTaO₃ thin film

Abstract

Since the discovery of a two-dimensional (2D) electron gas at the LaAlO/SrTiO interface, 2D carrier gases at such oxide interfaces and surfaces have attracted great attention because they can host many important phenomena and may produce novel functional devices. Here, we show through first-principles investigations that the surface 2D electron and hole gases in a KTaO (KTO) thin film can be tuned by applying biaxial stress. When increasing compressive in-plane strain, the 2D carrier concentrations decrease down to zero and then a new pair of surface 2D electron and hole gases appears in which the carrier types are switched to the opposite ones. Our analysis indicates that this carrier-type switching occurs because the increasing compressive strain reverses the slope of monolayer-resolved electrostatic potential along the [001] direction. We also present strain-dependent carrier concentrations and effective masses, and explore their thickness dependence. It is further shown that the 2D carrier gases and their strain-driven carrier-type switching across the KTO layer still remain true in the presence of overlayers and epitaxial substrates. These phenomena should be useful to design novel functional devices.

Strain-enhanced giant Rashba spin splitting in ultrathin KTaO3 films for spin-polarized photocurrents

“…It should be pointed out that the electron concentration in the 2DEG is equivalent to the hole concentration in the 2DHG. 32 There is a critical strain 3 s ¼ À2%, as shown in Fig. 1(d).…”

“…41 The effective value U eff ¼ 3 eV is employed for Ta 5d states in this work, as it is well established that such a value is appropriate to describe these strongly-correlated states. 32 An Monkhorst-Pack k-point grid of 4 Â 4 Â 1 is used for reciprocal space integration, and the plane wave energy cutoff is set to 500 eV. Our convergence standard requires that the Hellmann-Feynman force on each atom is less than 0.01 eVÅ À1 and the absolute total energy difference between two successive consistent loops is smaller than 1 Â 10 À5 eV.…”

“…Recent studies have demonstrated that the strain can affect the formation and migration of oxygen vacancies in KTO 31 and induce electron-hole interchanging of the two opposite surface 2D carrier gases in KTO ultrathin lm. 32 It has been also shown, experimentally and theoretically, that the KTO (001) 1 Â 1 surfaces terminated with KO and TaO 2 are most stable compared to others. 33,34 Therefore, structurally stable ultrathin KTO (001) lms with the two stable surfaces can be manipulated to realize giant Rashba spin effects.…”

Coexisting two-dimensional electron and hole gases highly confined at the interfaces of undoped KTaO3-sandwiching heterostructures

“…The band bending due to the intrinsic electric field can be attributed mainly to two origins, namely the polar discontinuity at the interfaces (the divergence is avoided effectively) [2] and the biaxial-stress-induced electric polarization within the KTO layer (the in-plane a is set to a STO and the z-axis lattice constant is optimized) [32]. The polar discontinuity at the interface including (TaO 2 ) + causes a surface charge 0.5e/a 2 at most, and the polarization within the KTO layer can contribute at most -0.476e/a 2 at the interface, but actually they are both renormalized by the structural optimization and electron reconstruction.…”

“…The on-site Coulomb interaction in the 5d states of transition-metal ions is corrected by using the DFT+U method [31], where U is the Hubbard parameter. The U eff = 3 eV is used for Ta 5d state [32], and 4.36 eV for Ti 3d states [33], because it is well established that the U eff values are good to describing these strongly-correlated electronic states, in addition to excellent bulk lattice constants, for STO/BTO and KTO [32,33]. A Monkhorst-Pack k-point grid of (4×4×1) is used for the reciprocal space integration and the plane wave energy cutoff is set to 500 eV.…”