Session V18: Focus Session: Interfaces in Complex Oxides - Interface and Surface Conductivity in SrTiO3

Surface charges {\&} adsorbates tune the electron gas at the LaAlO$_{3}$/SrTiO$_{3 }$interface

The physical properties of the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface are substantially determined by the density and mobility of the electron gas at the buried interface. Here, we present two ways to tune the electron gas from the LaAlO$_{3}$ surface. First, the electron gas can be locally tuned by using biased atomic force microscope probe [1]. Our studies show that this is caused by the accumulation of charge on the LaAlO$_{3}$ surface [2], and can change the density by up to 3$\times $10$^{13}$ cm$^{-2}$[3], comparable to the maximum modulation achieved in normal field effect devices. Second, we found that the electron gas can be dramatically tuned by the surface adsorption of common polar solvents such as acetone, ethanol and water [4]. This strong surface-interface coupling provides a new technique for manipulating the interface-confined electrons. Most significantly, adsorbates induce an insulator to metal transition when the thickness of the LaAlO$_{3}$ is 3 unit cells, suggesting sensor applications with extremely large sensitivity. \\[4pt] [1] C. Cen \textit{et al.}, \textit{Nature Mater. }\textbf{7}, 298 (2008). \\[0pt] [2] Y. Xie \textit{et al.}, \textit{Nano Lett.} \textbf{10}, 2588 (2010). \\[0pt] [3] Y. Xie \textit{et al.}, \textit{Adv. Mater.} \textbf{23}, 1744(2011). \\[0pt] [4] Y. Xie \textit{et al.}, \textit{Nature Commun.} \textbf{2}, 494 (2011).   

Unexpected Two Dimensional Electron Gas at the LaAlO$_3$/SrTiO$_3$ (110) Interface

The observation of two dimensional electron gas (2DEG), superconductivity and magnetism at the interfaces of LaAlO$_3$/SrTiO$_3$ has further amplified the potential of complex oxides for novel electronics. These multifunctional properties are strongly believed to originate from the polarization discontinuity at the interface between the two oxides along (001) direction. In this scenario, the crystal orientation plays an important role and no conductivity would be expected for e.g., the interface between LaAlO$_3$ and (110)-oriented SrTiO$_3$, which should not have a polarization discontinuity. In this talk, we will show the observation of conductivity at the LaAlO$_3$/SrTiO$_3$ interface prepared on (110)-oriented SrTiO$_3$. The strong evidence for 2DEG at the interface of LaAlO$_3$/SrTiO$_3$ (110) is the observed thickness dependence of insulator-metal transition at 3-4 u.c of LaAlO$_3$. Directional dependent electrical transport is observed in resistance versus temperature measurements. We propose some models for explaining the observed phenomena.   

Temperature Dependent Strain Relaxation in LaAlO$_{3}$ Thin Films on SrTiO$_{3}$ Substrates

LaAlO$_{3}$/SrTiO$_{3}$ interface has attracted great interest due to discoveries of rich interfacial properties. Strain and strain relaxation in LaAlO$_{3}$ films on SrTiO$_{3}$ substrates directly impact the lattice distortions and defects at the interface and therefore will influence the interfacial properties. Combining grazing incident x-ray diffraction and reciprocal space mapping, we directly measured the in-plane lattice constants of LaAlO$_{3}$ films on SrTiO$_{3}$ with thickness ranging from 4 u.c. to 250 u.c. We found a strong relationship between the strain relaxation behavior and the growth temperature of the LaAlO$_{3}$ films. Cracks were observed when the strain relaxed rapidly, consistent with the fracture theory. The processing temperature-dependent strain relaxation significantly affects the LaAlO$_{3}$/SrTiO$_{3}$ interface properties.   

The origin of the 2DEG at oxide polar-non polar interfaces: electron reconstruction versus defect doping

In spite of intense research efforts, the microscopic mechanism underlying the formation of a 2DEG at the LaAlO3(LAO)/SrTiO3(STO) interface is still controversial. In particular, there is no consensus on whether the interface charge stems from an electronic reconstruction or from mobile electrons introduced by oxygen vacancies and/or cation doping. We present direct, atomic-column-resolved scanning transmission electron microscopy and electron energy loss measurements in LAO/STO and LaGaO3(LGO)/STO interfaces. We find that both the measured injected charge, the low concentration of point defects, and the lattice polarization are consistent with a model of electronic reconstruction in LAO/STO. On the other hand, LGO/STO interfaces grown in the same conditions show negligible polar lattice distortions and an injected charge incompatible with electronic reconstruction. Conductivity here is explained by the observed cation intermixing and the presence of oxygen vacancies.   

Computational study of oxygen vacancy defects in perovskite oxides, SrTiO$_{3}$ and LaAlO$_{3}$

Insulating perovskite oxides, SrTiO$_{3}$ (STO) and LaAlO$_{3}$ (LAO) have attracted a lot of attention due the observed two dimensional electron gas at the interface of these two insulating oxides. Oxygen vacancies, which are easily incorporated in these oxide systems during various processes such as growth, annealing and redox reactions, form an n-type defect in these systems. Hence they are responsible for modifications of various physical properties such as conductivity and optical properties to name a few. As such, there is still a lack of comprehensive theoretical understanding of these important defects in these materials. In this work we present first principles calculations of neutral (V$^{0})$ and charged (V$^{+}$, V$^{++})$ oxygen vacancies in STO and LAO. Density functional theory within the local density approximation proves insufficient in reliably predicting the defect levels due to their well know band gap problem. We therefore employ the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional to study these defects. Band gaps of 3.01 eV and 5.0 eV are obtained for bulk STO and LAO which are in excellent agreement to experimental values. In STO we find a localized V$^{0}$ defect state at 0.7 eV below the conduction band edge (CBE) while in LAO it is a deep lying level at 2.13 eV below the CBE. A band alignment of the defect levels in these two materials shows that upon stabilizing the singly ionized state, charge transfer could take place from the V$^{+}$ LAO state to the V$^{+}$ STO state which lies at 1.19 eV below the former.   

Oxygen vacancy shuttle across the LaAlO3/SrTiO3 interface

There have been several recent indications of a switchable piezo-response in oxide materials not associated with ferroelectric polarization. We explore a LaAlO3/SrTiO3 (LAO/STO) heterostructure as a model system to understand this behavior by considering oxygen vacancies as the origin. Using first-principles calculations based on a supercell approach and an adequate electrostatic model, we show that the oxygen vacancy formation energy has two local minima: one at the surface of LAO and another in STO few unit cell below the interface. Due to inbuilt electric field in the system, electrons forming the oxygen vacancies in LAO transfer to the interface leaving behind two holes at the vacancy site irrespective of their spatial position, whereas those in the STO exhibit such a behavior only when laying within a finite distance $\sim $1nm associated with the screening length of the electric field in STO. The two local minima may accommodate oxygen vacancies reversibly when the applied electric field changes its polarity producing a switchable piezo-response behavior in the LAO/STO system.   

Electronic phase separation at the LaAlO$_3$/SrTiO$_3$ interface

In the last few years we have seen a number of new properties to emerge at the interface between two insulating oxides. However the origin of these properties remains unclear and needs further investigation. In this talk I will discuss our recent studies on oxide interfaces in which we vary various parameters such as oxygen content and crystal orientation, and use different polar layers and heterostructures. Surprisingly, at the LaAlO$_3$/SrTiO$_3$ interface a remarkable combination of strong diamagnetism (superconductor like), paramagnetism and ferromagnetism can coexist with the quasi two dimensional electron gas (Q2DEG) when prepared under a more oxidizing condition. The ferromagnetic phase is stable even above room temperature and the diamagnetism below a relatively high temperature of 60 K. Our measurements show that the free surface of SrTiO$_3$ may be responsible for all these fascinating phenomena. The phenomena are explained due to the selective occupancy of interface/surface sub-bands of the nearly degenerate Ti orbital in the SrTiO$_3$. On changing the interface orientation, we unexpectedly can also obtain conductivity at the LaAlO$_3$/SrTiO$_3$ interface prepared on (110)-oriented SrTiO$_3$. The conductivity is found to be highly anisotropic, which is a novel feature for these oxide Q2DEGs and of potential interest for applications. I will discuss various possible models that can explain this intriguing observation and also review various interfaces prepared using different polar layers coupled to other functional materials which result in new phenomena.   

Effect of cation-nonstoichiometry in the SrTiO$_{3}$ layer on the interfacial conductance of LaAlO$_{3}$/SrTiO$_{3}$ heterostructures

A study of the interface conductivity in LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) heterostructures will be presented with focus on the defect chemical state of the SrTiO$_{3}$ close to the interface. SrTiO$_{3}$ layers with a controlled deviation from cation stoichiometry (Sr/Ti=1) were grown to investigate the effect of cation-nonstoichiometry in the SrTiO$_{3}$ layer on the resulting interface conductance of the corresponding LAO/STO heterostructures. The study comprises structural information and electrical properties derived from Hall measurements and high-temperature conductance measurements performed under equilibrium oxygen atmospheres. According to the model of defect chemistry the results give further evidence for the importance of charge compensation through defects on cation lattice sites.   

Nature of localized states induced by an oxygen vacancy in SrTiO$_3$

Combining the density function theory and model analysis, we study the nature of localized states induced by an oxygen vacancy (OV) in SrTiO$_3$. The key effect caused by an OV is that it introduces a local mixing between $3z^2-r^2$ and $4s$, $4p$ orbitals of Ti next to the vacancy. According to our analysis, the OV-induced localized state is highly one-dimensional and is mainly composed of Ti $3z^2-r^2$ orbitals along the Ti-OV-Ti axis (defined as the $z$-axis) and Ti $4s$, $4p$ at the OV site. Ti $t_{2g}$ based localized states are ruled out. We suggest the $4s$, $4p$ components of the OV induced localized state should be reflected in the x-ray adsorption spectroscopy when Ti $4s$, $4p$ are served as final states. Our analysis and conclusion apply to general transition metal perovskites.   

Oxygen deficiency on the p(1x1) structure of SrTiO$_{3}$ (001) surface

SrTiO$_{3}$(STO) is one of the most common substractes used for epitaxial film growth of oxide materials. However, the structure and stoichiometry of STO are still unclear even thought they may dictate the interface properties. We have studied the STO(001) surface structure by Low Energy Electron Diffraction(LEED) and angle-resolved X-ray Photoelectron Spectroscopy (AR-XPS). The unreconstructued $p$(1x1) STO(001) surface terminated with TiO$_{2}$ layer is obtained by simple chemical etching which is always accompanied by oxygen deficiency. Both LEED and AR-XPS results suggest that the surface has a considerable oxygen deficiency, but LEED I(V) analysis indicates the existence of more oxygen vacancies than that from AP-XPS results. The structureal analysis indicates a surface polarization due to a surface buckling combined with oxygen deficiency, which is comsistent with recent surface x-ray scattering results [1] with important implications on surface ferroelectric phenomena in STO. The annealing effect of the as-etched surface in oxygen atmosphere on surface oxgen deficiency as well as reconstruction will also be disussed.\\[4pt] [1] R. Herger et al., Phys. Rev. Lett. 98, 076102(2007).   

Strain effects on the electron effective mass in SrTiO$_3$

Strain can greatly affect the electronic properties of thin layers in epitaxial heterostructures, leading to improved transport properties. In SrTiO$_3$-based heterostructures, strain can lift the degeneracy and modify the order of the states near the conduction-band minimum. Using density functional theory with the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) we study the effects of strain on the energetic ordering and effective mass of the lowest-energy conduction-band states in SrTiO$_3$3. We predict that biaxial stress in the (001) or (110) planes results in the lowest-energy conduction-band state having significantly smaller electron mass in the in-plane directions compared to unstrained SrTiO$_3$, thus suggesting that pseudomorphic growth is a promising route to increasing the electron mobility in epitaxial films. We propose possible substrates that may lead to SrTiO$_3$ films with enhanced electron mobilities, and report deformation potentials that allow accurate prediction of conduction-band splittings for arbitrary strain configurations.   

Electrical Transport in SrTiO$_{3}$ Under Biaxial Strain

Mobility engineering with strain is widely used for conventional semiconductors, but has only recently been proposed for complex oxides such as SrTiO$_{3}$. The conduction band structure of SrTiO$_{3}$ is complicated with multiple degenerate bands derived from the Ti 3d orbitals. Strain is thus expected to have a significant effect by lifting this degeneracy and altering the occupancy and curvature of the bands. Indeed, a 300{\%} increase in the electron mobility with values exceeding 128,000 cm$^{2}$/Vs at 1.8 K was demonstrated in MBE-grown SrTiO$_{3}$ films subjected to \textit{uniaxial} compressive strain [1]. For heterostructure engineering, the effect of \textit{biaxial} strain is relevant. Here, the electron mobilities in SrTiO$_{3}$ subjected to biaxial strain are investigated through growth of coherent films on lattice-mismatched substrates. Lightly-doped (high-mobility) strained SrTiO$_{3}$ films below the critical thickness are insulating because of significant surface depletion, which increases with decreasing temperature due to the high dielectric constant of SrTiO$_{3}$. We show that highly-doped, low-mobility capping layers address this problem, but require a multilayer model to analyze the Hall data in terms of the mobility in the lightly doped layer. [1] B. Jalan et al., Appl. Phys. Lett. \textbf{98}, 132102 (2011)   

The Influence of Oxygen in Transition Metal Oxides

The existence of a 2D metallic state at the interface between LaAlO$_3$ and SrTiO$_3$ (LAO/STO) has generated much excitement. Its origin has been attributed to charge redistribution to avoid a dielectric catastrophe; however, oxygen vacancies in TiO$_2$-terminated STO can play a significant role in the electronic properties of the interface. In order to determine the nature and origin of the metallic phase, we have induced vacancies in TiO$_2$-terminated STO single crystal substrates by annealing under controlled vacuum conditions. We report resistivity, Hall effect, and thermopower measurements on these materials and discuss their implications for the nature of the 2D electron gas at the STO surface. We have explored the possibility of gate-tuning these systems in order to fabricate single-oxide based devices. This work was supported by the Ministerio de Ciencia e Innovaci\'on (Spain), grant MAT2010-16157, and the European Research Council, grant ERC-2010-StG 259082 2D THERMS.