Electric-Field-Induced Band Control of Perovskite Oxide Epitaxial Thin Films using Piezoelectric PMN-PT Substrate: NdNiO3 and La:BaSnO3

Abstract

Perovskite oxide have attracted significant attention due to a wealth of physical phenomena, such as ferroelectricity and superconductivity. Due to the extreme sensitivity of those materials systems to external stimuli, unique features can be achieved by controlling various knobs. There are two types of band control in perovskite oxides: band-width control and band-filling control. Band-width control modulates the overlapping of the valence bands, while band-filling control changes the electron concentration or the chemical potential. PMN-PT ((1-x) Pb(Mg1/3Nb2/3)O3-x PbTiO3), which has both piezoelectricity and ferroelectricity, is a good candidate for both of these controls. The converse piezoelectric strain of PMN-PT is suitable for pure strain engineering and band-width study of perovskite oxides. In addition, using the switchable polarization of PMN-PT can not only alter the carrier concentration of perovskite oxides, but also induce non-volatile and reversible states in the perovskite oxides. Rare-earth nickelates (RNiO3) are a fascinating family of compounds exhibit a sharp metal-insulator transition due to strong electron correlations. The band-width control through external stress has been demonstrated as a useful knob to modulate metal-insulator transition (MIT) in these materials. However, lattice mismatch strain using different substrates have been widely utilized to investigate the effect of strain on MIT temperature of NNO so far but the results were inconsistent in the previous literatures. The first part will discuss dynamic modulation of MIT based on electric field-controlled pure strain in high-quality NdNiO3 (NNO) thin films utilizing converse-piezoelectric effect of (001)-cut PMN-PT single crystal substrates. The structural quality of NNO thin films has been significantly improved by inserting SrTiO3 (STO) buffer layers. Moreover, the MIT temperature (TMI) in NNO is downward shifted by ~3.3 K in response of 0.25% in-plane compressive strain, which indicates less effective TMI modulation of field-induced strain than substrate-induced strain. Perovskite alkaline-earth stannates (ASnO3; A = Ba, Sr, and Ca), which have high optical transparency in the visible range and the high electrical conductivity, have recently attracted much attention as an earth-abundant alternative to tin-doped indium oxide (ITO). The second part focuses on the dynamic modulation of electrical properties in La-doped BaSnO3 (LBSO) thin films through ferroelectric-field-effect of (001)-cut PMN-PT single crystal substrates. The structural quality of LBSO thin films has been significantly improved by inserting SrTiO3 (STO) buffer layers. Upon the polarization switching in the PMN-PT, the resistivity of the LBSO thin films could be reversibly modified, resulting in a large non-volatile resistivity modulation by ~ 70 % at room temperature. From the resistivity modulation, the effect of piezoelectric strain and the role of acceptor-like defect states are discussed.