The equilibrium conductance of LaAlO3/SrTiO3 (LAO/STO)-heterointerfaces was investigated at high temperatures (950 K-1100 K) as a function of ambient oxygen partial pressure (pO2). Metallic LAO/STO-interfaces were obtained for LAO grown on STO single crystals as well as on STO-buffered (La,Sr)(Al,Ta)O3 substrates. For both structures, the high temperature sheet carrier density nS of the LAO/STO-interface saturates at a value of about 1 × 1014 cm−2 for reducing conditions, which indicates the presence of interfacial donor states. A significant decrease of nS is observed at high oxygen partial pressures. According to the defect chemistry model of donor-doped STO, this behavior for oxidizing conditions can be attributed to the formation of Sr-vacancies as charge compensating defects.
Experimental evidence of strong in-plane anisotropy in electrical properties of the confined electron gas at the SrTiO3–LaAlO3 interface on top of (LaAlO3)0.3(Sr2AlTaO3)0.7 substrates is provided by detailed transport measurements. Structured measurement geometries in multiple directions are used to show dependence of the sheet resistance with the in-plane angle θ, which is fitted with a sine function with a period of 180°. The carrier density remains constant and a directional dependence of the carrier mobility of more than one order of magnitude is determined with respect to the orientation of the unit cell height steps present at the SrTiO3–LaAlO3 interface.
La0.67Sr33MnO3 (LSMO) thin films under compressive strain have an orthorhombic symmetry with (110)o and (001)o in-plane orientations. (The subscript o denotes the orthorhombic symmetry.) Here, we grew LSMO on cubic (LaAlO3)0.3-(Sr2AlTaO6)0.7 (LSAT) substrates and observed a uniaxial contribution to the magnetic anisotropy which is related to the orthorhombic crystal structure. Since the lattice mismatch is equal in the two directions, the general understanding of anisotropy in LSMO, which relates the uniaxial anisotropy to differences in strain, cannot explain the results. These findings suggest that the oxygen octahedra rotations associated with the orthorhombic structure, possibly resulting in different Mn-O-Mn bond angles and therefore a change in magnetic coupling between the [110]o and [001]o directions, determine the anisotropy. We expect these findings to lead to a better understanding of the microscopic origin of the magnetocrystalline anisotropy in LSMO.
The electrical properties of the metallic interface in LaAlO3/SrTiO3 (LAO/STO) bilayers are investigated with focus on the role of cationic defects in thin film STO. Systematic growth-control of the STO thin film cation stoichiometry (defect-engineering) yields a relation between cationic defects in the STO layer and electronic properties of the bilayer-interface. Hall measurements reveal a stoichiometry-effect primarily on the electron mobility. The results indicate an enhancement of scattering processes in as-grown non-stoichiometric samples indicating an increased density of defects. Furthermore, we discuss the thermodynamic processes and defect-exchange reactions at the LAO/STO-bilayer interface determined in high temperature equilibrium. By quenching defined defect states from high temperature equilibrium, we finally connect equilibrium thermodynamics with room temperature transport. The results are consistent with the defect-chemistry model suggested for LAO/STO interfaces. Moreover, they reveal an additional healing process of extended defects in thin film STO.
engineering can be used to tune the specifi c thermoelectric material properties.Detailed thermoelectric characterization of Na x CoO 2 thin fi lms has been hindered by the chemical instability in ambient conditions. [ 7,8 ] However, a recently developed method to obtain chemically stable, single-phase Na x CoO 2 thin fi lms by pulsed laser deposition due to the in situ deposition of an amorphous AlO x capping layer enables us to exploit the intrinsic properties of these thermoelectric thin fi lms. [ 8 ] Here, we show that by structural engineering in chemically stable Na x CoO 2 thin fi lms the thermoelectric properties can be controlled and enhanced as compared to bulk samples. By changing the single crystalline substrate material we can control the structural properties and as a consequence the electronic and thermal properties of the thermoelectric thin fi lms. Tuning of the grain size within the Na x CoO 2 thin fi lms signifi cantly infl uences the achievable Seebeck coeffi cient. We demonstrate that preservation of the crystallinity in these thin fi lms with enhanced Seebeck coeffi cient results in minimal reduction of the electrical conductivity and, therefore, leads to a doubling of the thermoelectric power factor at room temperature.Here, structural engineering is applied as a tool to obtain improved control over the thermoelectric properties of Na x CoO 2 thin fi lms, which is unique for epitaxial thin fi lms and cannot be obtained in single crystal or polycrystalline samples. To study this effect, Na x CoO 2 thin fi lms were grown by pulsed laser deposition (PLD) on various single crystal substrates. All Na x CoO 2 thin fi lms were deposited under the same conditions and have a thickness of 60 nm. Independent of the substrate material and structure, all thin fi lms showed a preferred growth orientation with the (00l) direction parallel to the surface normal.Previously it was shown that the crystallinity of Na x CoO 2 thin fi lms does not strongly depend on the deposition temperature, [ 8 ] and an optimum deposition temperature of 430 °C was determined. However, the effect of oxygen deposition pressure on the crystallinity was not systematically studied yet. Here, we observe a signifi cant decrease in crystallinity when the deposition pressure was reduced by one and two orders of magnitude from the previously reported value of 0.4 mbar, [ 8 ] resulting in an increased resistivity by a factor of fi ve, together with a strong reduction of the Seebeck coeffi cient. Based on these results we can conclude that, although the deposition pressure can clearly be used to tune the crystallinity of these Na x CoO 2 thin fi lms, it will not provide the required enhanced control over the thermoelectric properties. Therefore the optimized deposition parameters [ 8 ] are used, which result in a combination of the optimum crystallinity and thermoelectric properties. Furthermore, all thin fi lms have been cooled down after growth in 1 atm. of oxygen at a rate of 10 °C min -1 to optimize the oxidation level.
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