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The two-dimensional electron gas (2DEG) at oxides interfaces and surfaces has attracted large attention in physics and research due to its unique electronic properties and possible application in optoelectronics and nanoelectronics. The origin of 2DEGes at oxide interfaces has been attributed to the well known polar catastrophe mechanism. On the other hand, recently a 2DEG was also found on a clean SrTiO3(001) surface where it is formed due to oxygen vacancies. However, these 2DEG systems have been until now found mostly on atomically perfect crystalline samples usually grown by pulsed laser deposition or molecular beam epitaxy i.e. samples which are difficult to be prepared and require specific experimental conditions. Here, we report on the fabrication of SrTiO3 thin films deposited by magnetron sputtering which is suitable for mass-production of samples adapted for nanoelectronic applications. The characterization of their structural and electronic properties was done and compared to those of SrTiO3 single crystals. XRD patterns and SEM micrography show that the deposited films are amorphous and their structure changes to polycrystalline by heating them at 900 {deg}C. Photoemission spectroscopy (XPS and UPS) was used to study the electronic properties of the films and the crystal. In both, we observe the 2DEG system at Fermi level and the formation of Ti3+ states after heating the surface at 900 {deg}C.
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We studied the structural, magnetic and transport properties of LaAlO3/EuTiO3/SrTiO3 heterostructures grown by Pulsed Laser Deposition. The samples have been characterized in-situ by electron diffraction and scanning probe mi-croscopy and ex-situ by transport measurements and x-ray absorption spectroscopy. LaAlO3/EuTiO3/SrTiO3 films show a ferromagnetic transition at T<7.5 K, related to the ordering of Eu2+ spins, even in samples characterized by just two EuTiO3 unit cells. A finite metallic conductivity is observed only in the case of samples composed by one or two EuTiO3 unit cells and, simultaneously, by a LaAlO3 thickness equal or above 4 unit cells. The role of ferromagnetic EuTiO3 on the transport properties of delta-doped LaAlO3/EuTiO3/SrTiO3 is critically discussed.
Comparison between single- and the poly-crystalline structures provides essential information on the role of long-range translational symmetry and grain boundaries. In particular, by comparing single- and poly-crystalline transition metal oxides (TMOs), one can study intriguing physical phenomena such as electronic and ionic conduction at the grain boundaries, phonon propagation, and various domain properties. In order to make an accurate comparison, however, both single- and poly-crystalline samples should have the same quality, e.g., stoichiometry, crystallinity, thickness, etc. Here, by studying the surface properties of atomically flat poly-crystalline SrTiO3 (STO), we propose an approach to simultaneously fabricate both single- and poly-crystalline epitaxial TMO thin films on STO substrates. In order to grow TMOs epitaxially with atomic precision, an atomically flat, single-terminated surface of the substrate is a prerequisite. We first examined (100), (110), and (111) oriented single-crystalline STO surfaces, which required different annealing conditions to achieve atomically flat surfaces, depending on the surface energy. A poly-crystalline STO surface was then prepared at the optimum condition for which all the domains with different crystallographic orientations could be successfully flattened. Based on our atomically flat poly-crystalline STO substrates, we envision expansion of the studies regarding theTMOdomains and grain boundaries.
Novel properties arising at interfaces between transition metal oxides, particularly the conductivity at the interface of LaAlO3 (LAO) and SrTiO3 (STO) band insulators, have generated new paradigms, challenges, and opportunities in condensed matter physics. Conventional transport measurements have established that intrinsic conductivity appears in LAO/STO interfaces when the LAO film matches or exceeds a critical thickness of 4 unit cells (uc). Recently, a number of experiments raise important questions about the role of the LAO film, the influence of photons, and the effective differences between vacuum/STO and LAO/STO, both above and below the standard critical thickness. Here, using angle-resolved photoemission spectroscopy (ARPES) on in situ prepared samples, as well as resonant inelastic x-ray scattering (RIXS), we study how the metallic STO surface state evolves during the growth of a crystalline LAO film. In all the samples, the character of the conduction bands, their carrier densities, the Ti3+ crystal fields, and the responses to photon irradiation bear strong similarities. However, LAO/STO interfaces exhibit intrinsic instability toward in-plane folding of the Fermi surface at and above the 4-uc thickness threshold. This ordering distinguishes these heterostructures from bare STO and sub-critical-thickness LAO/STO and coincides with the onset of unique properties such as magnetism and built-in conductivity.
Using density-functional-theory (DFT) calculations with the HSE06 hybrid functional, we accurately evaluate the critical thickness of LaAlO3 film for the intrinsic doping in LaAlO3/SrTiO3 (LAO/STO) heterstructures. The calculated critical thickness of 6 unit-cell (uc) layers suggests to rule out the intrinsic doping mechanism. We also calculate the density of oxygen vacancies on the LAO surface at varying LAO thicknesses, preparation oxygen pressures and temperatures by using the condition of chemical equilibrium and DFT calculations. We find that once LAO thickness >=3 uc high-density (~ 10^14 cm^-2 ) oxygen vacancies will inevitably exist on the LAO surface of the LAO/STO heterstructures even though the samples are grown under high oxygen pressure. The oxygen vacancies are stabilized by releasing the electrostatic energy in the LAO film.
The structural and electronic properties of thermally reduced SrTiO3(100) single crystals have been investigated using a probe with real- and reciprocal-space sensitivity: a synchrotron radiation microsopic setup which offers the possibility of Scanning Photoemission Microscopy and Angle Resolved Photoelectron Spectroscopy (ARPES) down to the nanometric scale. We have spectroscopically imaged the chemical composition of samples which present reproducible and suitable low-energy electron diffraction patterns after following well-established thermal reduction protocols. At the micrometric scale, Ca-rich areas have been directly imaged using high-energy resolution core level photoemission. Moreover, we have monitored the effect of Ca segregation on different features of the SrTiO3(100) electronic band structure, measuring ARPES inside, outside and at the interface of surface inhomogeneities with the identified Ca-rich areas. In particular, the interaction of Ca with the well-known intragap localized state, previously attributed to oxygen vacancies, has been investigated. Moreover, the combination of direct imaging and spectroscopic techniques with high spatial resolution has clarified the long-standing dilemma related to the bulk or surface character of Ca segregation in SrTiO3. Our results present solid evidence that the penetration depth of Ca segregation is very small. In contrast to what has been previously proposed, the origin of long-range surface reconstructions can unlikely be associated to Ca due to strong local variations of its surface concentration.
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