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Register a new userInterface chemical and electronic properties of LaAlO3/SrVO3 heterostructures
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We have studied the chemical and electronic properties of LaAlO3/SrVO3 ultrathin films by combining hard x-ray photoemission spectroscopy and transport measurements. We compare single SrVO3 (SVO) ultrathin films and SrVO3 buried below a polar LaAlO3 (LAO) thin layer, both epitaxially grown on SrTiO3. While ultrathin films (4 unit cells) of SVO do show insulating behavior over the entire temperature range, the LAO/SVO interface has a resistivity minimum at 250 K. When increasing the SVO layer thickness, the minimum is observed to shift to higher temperatures, but the resistivity stays always smaller than that of comparable SVO single films. Hard x-ray photoemission spectroscopy reveals a surface or interface related V5+ component in the V 2p spectra for SVO films and LAO/SVO heterostructures, respectively, attributed to a strongly oxidized component. This chemical reconstruction is weaker in LAO/SVO heterostructures compared to single SVO films. We show that this dead layer in SVO ultrathin films has to be considered when the film thickness reaches the few unit-cells limit and propose solutions on how to prevent this detrimental effect.
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Depositing disordered Al on top of SrTiO$_3$ is a cheap and easy way to create a two-dimensional electron system in the SrTiO$_3$ surface layers. To facilitate future device applications we passivate the heterostructure by a disordered LaAlO$_3$ capping layer to study the electronic properties by complementary x-ray photoemission spectroscopy and transport measurements on the very same samples. We also tune the electronic interface properties by adjusting the oxygen pressure during film growth.
The structure and interface characteristics of (LaVO3)6m(SrVO3)m superlattices deposited on (100)-SrTiO3 (STO) substrate were studied using Transmission Electron Microscopy (TEM). Cross-section TEM studies revealed that both LaVO3 (LVO) and SrVO3 (SVO) layers are good single crystal quality and epitaxially grown with respect to the substrate. It is evidenced that LVO layers are made of two orientational variants of a distorted perovskite compatible with bulk LaVO3 while SVO layers suffers from a tetragonal distortion due to the substrate induced stain. Electron Energy Loss Spectroscopy (EELS) investigations indicate changes in the fine structure of the V L23 edge, related to a valence change between the LaVO3 and SrVO3 layers.
When insulator LaAlO3 is grown by epitaxy onto a TiO2-terminated {100} surface of insulator SrTiO3, the resulting system has a metallic character. This phenomenon has been associated with an electrostatic frustration at the interface, as {100} surfaces of SrTiO3 are neutral while those of LaAlO3 are polar, but its microscopic mechanism is not quite understood. Here, we present a structural characterisation of this interface by aberration-corrected transmission electron microscopy. The unit cells at the interface appear elongated: we discuss this distortion in terms of electrostatic charge and extra carriers at the interface.
We have made very thin films of LaAlO3 on TiO2 terminated SrTiO3 and have measured the properties of the resulting interface in various ways. Transport measurements show a maximum sheet carrier density of 1016 cm-2 and a mobility around 104 cm2 V-1 s-1. In situ ultraviolet photoelectron spectroscopy (UPS) indicates that for these samples a finite density of states exists at the Fermi level. From the oxygen pressure dependence measured in both transport as well as the UPS, we detail, as reported previously by us, that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process used to make the heterointerfaces. Under the conditions studied the effect of LaAlO3 on the carrier density is found to be minimal.
The hysteretic piezoelectric response in LaAlO3/SrTiO3 heterostructures can provide important insights into the mechanism for interfacial conductance and its metastability under various conditions. We have performed a variety of nonlocal piezoelectric force microscopy experiments on 3 unit cell LaAlO3/SrTiO3 heterostructures. A hysteretic piezoresponse is observed under various environmental and driving conditions. The hysteresis is suppressed when either the sample is placed in vacuum or the interface is electrically grounded. We present a simple physical model which can account for the observed phenomena.
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