Impedance-based interpretations in 2-dimensional electron gas conduction formed in the LaAlO3/SrxCa1−xTiO3/SrTiO3 system

Frequency-dependent impedance spectroscopy was applied to the 2-dimensioanl conduction transport in the LaAlO₃/Sr_xCa_1−xTiO₃/SrTiO₃ system. The 2-dimensional conduction modifies the electrical/dielectric responses of the LaAlO₃/Sr_xCa_1−xTiO₃/SrTiO₃ depending on the magnitude of the interfacial 2-dimensional resistance. The high conduction of the 2-dimensional electron gas (2DEG) layer can be described using a metallic resistor in series with two parallel RC circuits. However, the high resistance of the 2-dimensional layer drives the composite system from a finite low resistor in parallel with the surrounding dielectrics composed of LaAlO₃ and SrTiO₃ materials to a dielectric capacitor. This change in the resistance of the 2-dimensional layers modifies the overall impedance enabled by the presence of the interfacial layer due to Sr_xCa_1−xTiO₃, which alters the charge transport of the 2-dimensional layer from metallic to semiconducting conduction. A noticeable change is observed in the capacitance Bode plots, indicating highly amplified dielectric constants compared with the pristine SrTiO₃ substrates and Sr_xCa_1−xTiO₃ with a greater Ca content.     

Stable pH sensitivity of LaAlO3/SrTiO3 interfacial electronic gas

Probing pH value is essential for many applications. The request of developing stable and highly sensitive pH sensor with small size are increased recently, this is a challenge to the traditional glass pH sensors. This work investigated the pH sensing performance of the novel two-dimensional electronic gas (2DEG) at the LaAlO₃/SrTiO₃ heterostructure interface discovered recently. The experimental results demonstrated that the devices host excellent sensing ability to the pH value of aqueous solutions. Quite stable output current is realized for a given pH value, and the output current is linearly dependent on pH value as required by sensor applications. This work would prompt the application research of LaAlO₃/SrTiO₃ heterostructure and design of new generation of advanced pH sensors.     

The ferroelectric field effect on the two-dimensional electron gas at LaAlO3/SrTiO3 (001) interface: Insights from first principle simulations

We perform first-principles calculations to explore the possibility of tuning the two-dimensional electron gas at the LaAlO₃/SrTiO₃ (001) interface through BaTiO₃ substrate. A metal-to-insulator transition is found at the interface as the polarization of BaTiO₃ reverses. Through the potential analysis of the LaAlO₃/SrTiO₃/BaTiO₃ superstructure, we find that the intrinsic electric field of LaAlO₃ is significantly suppressed as the polarization points away from the LaAlO₃/SrTiO₃ interface, while it is enhanced with the polarization pointing to the interface. The ferroelectric field control of the intrinsic electric field, and therefore the electronic reconstructions at the interface, originating from the screening of polarization charges, opens the way to the development of novel nanoscale electronic devices.     

Band offsets of epitaxial LaAlO3/TiO2 interface determined by X-ray photoelectron spectroscopy

Room-temperature ferromagnetism in doped anatase TiO₂ has previously been observed, ferromagnetic semiconductor heterostructures based on anatase TiO₂ can thus provide a new opportunity to study spin-dependent transport phenomena at room temperature. An accurate determination of barrier heights or band offsets at the TiO₂-based heterojunctions is of great importance for the spintronics application with semiconductors. X-ray photoelectron spectroscopy with high-energy resolution was used to determine the band offsets of epitaxial LaAlO₃/TiO₂ heterojunction on SrTiO₃(001) substrate fabricated by pulsed laser deposition. Results showed an upward band bending of 0.48(0.03) eV when the film thickness of the overlayer LaAlO₃ above 4 unit cells. The valence band offset obtained is about 0.35(0.16) eV. Assuming bulk band gaps for the LaAlO₃ and TiO₂ epitaxial films, the associated conduction band offset is about 2.95(0.16) eV. These results show that LaAlO₃ can be an ideal tunneling barrier for TiO₂-based heterojunctions.     

Structural relaxation and metal-insulator transition at the interface between SrTiO3 and LaAlO3

The electronic structure of interfaces between LaAlO₃ and SrTiO₃ is studied using local spin density approximation (LSDA) with intra-atomic Coulomb repulsion (LSDA + U). We find that the nature of the interface metallic states is strongly affected by the type of the structure (sandwich or bilayer) and by the termination surface of LaAlO₃. In all structures the atomic relaxation plays a crucial role in the electronic properties of the system. While in sandwiches the structural relaxation produces a significant polarization in SrTiO₃ and a splitting of Ti 3d_xy orbitals, in AlO₂-terminated bilayers the relaxation occurs primarily in LaAlO₃ and results in an insulator-metal transition which has been observed experimentally with increasing thickness of the LaAlO₃ layer.     

Instability,intermixing and electronic structure at the epitaxial LaAlO3/SrTiO3(001) heterojunction

The question of stability against diffusional mixing at the prototypical LaAlO₃/SrTiO₃(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using on-axis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as potential energy gradients and valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO₃ or SrTiO₃, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Significantly, we find it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface—electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO₃ below the interface as being the cause of the observed conductivity.     

Spatial charge distribution and conductivities of the LaAlO3/SrTiO3 interfaces: A theoretical study

The electronic properties of the charge carriers at the LaAlO₃/SrTiO₃ interfaces are investigated by first principles studies. For the n-type interface, the carriers are located only on the SrTiO₃ side. For the p-type interface, the carriers are highly localized at the interface. A critical thickness of the LaAlO₃ overlayer exists, below which, the interface is insulating. Moreover, we show that the effective masses and mobilities of the carriers are spatially anisotropic and have a strong disparity for the two types of carriers. These results are consistent with experimental observations and are explained by the band structures and alignments of the consisting oxides and their interaction at the interfaces.     

Gate‐controlled metal–insulator transition in the LaAlO3/SrTiO3 system with sub‐critical LaAlO3 thickness

We studied the electrical conduction in the LaAlO₃/SrTiO₃ (LAO/STO) interface electron system with a sub‐critical LAO layer thickness of ～3.5 unit cells (uc). It was found that the true dividing point between metallic and insulating behaviour without gating lies near the LAO thickness of 3.5 uc. Our marginally metallic 3.5 uc sample showed a sharp transition to insulating state at temperatures which strongly depended on the applied negative back‐gate voltage. The superior gate‐controllability of the sample was attributed to its sheet carrier density which was an order of magnitude lower than those of conducting LAO/STO samples with 4 uc or more of LAO layers. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

氧压对SrTiO3和SrNb0.2Ti0.8O3薄膜晶格参数的影响及激光感生热电电压效应

采用脉冲激光沉积技术制备了SrTiO₃和SrNb_0.2Ti_0.8O₃薄膜.X射线衍射分析表明在LaAlO₃(100)单晶平衬底上生长的SrTiO₃及SrNb_0.2Ti_0.8O₃薄膜是沿［001］取向的近外延生长.随着氧压在一定范围内逐渐增大,SrTiO₃薄膜的晶格参数减小,而SrNb_0.2Ti_0.8O₃薄膜的晶格参数先减小后增大.同时摸索出制备具有二维电子气超晶格(SrTiO₃/SrNb_0.2Ti_0.8O₃)_L的最佳氧压为1.0×10^-2Pa.另外在LaAlO₃(100)倾斜衬底上制备的SrNb_0.2Ti_0.8O₃薄膜中观察到激光感生热电电压效应. 关键词： 0.2Ti_0.8O₃薄膜')" href="#">SrNb_0.2Ti_0.8O₃薄膜 晶格参数 激光感生热电电压 脉冲激光沉积     

Strain-Gradient-Induced Modulation of Carrier Density and Mobility at the LaAlO3/SrTiO3 Heterointerface

A strain gradient induced by mechanical bending on a SrTiO₃ substrate is demonstrated, and has a pronounced influence on the carrier density and mobility of the interfacial 2D electron gas at the LaAlO₃/SrTiO₃ heterointerface. Tensile and compressive strain gradients represent two states of upward and downward bending. Under the tensile strain gradient, the carrier density decreases and the mobility has about 200% increase. Conversely, under the compressive strain gradient, the mobility decreases and the carrier density increases by up to 107%. These results demonstrate a range of opportunities to modulate the carrier density and mobility at oxide heterointerface and open up a promising way for further research on application of oxide devices.     

Growth and characterization of La2/3Ca1/3MnO3 thin films on ‘silicon on insulator’ substrates

La_2/3Ca_1/3MnO₃ thin films have been grown on SrTiO₃, LaAlO₃, and yttria-stabilized zirconia buffered silicon-on-insulator (SOI) substrates by the pulsed laser deposition technique. While full cube-on-cube epitaxy was achieved on the SrTiO₃ and LaAlO₃ substrates, a coexistence of the cube-on-cube and cube-on-diagonal epitaxy was observed in the the manganite films on SOI substrates. Besides the intrinsic four-fold magnetocrystalline anisotropy, a uniaxial anisotropy also exists in the films, which is determined by the demagnetization field and the mismatch-induced strain. A tensile strain leads to an easy plane, while a compressive strain favors an easy axis. The different magnetization configurations in the films on different substrates are the reason for their varied transport and magnetic properties. Due to a combined effect of these magnetic anisotropy, the magnetization in the two crystallography domains in the film on SOI tends to lie in the film plane but align in their respective easy axes. There are always large spin angles across the domain boundaries. As a result, a quite large low-field magnetoresistance (LFMR) based on spin-dependent tunnelling was observed. It shows a resistance change of ∼20% at 50 K in a magnetic field ∼700 Oe, which is promising for real applications. PACS 75.47.Lx; 72.25.Mk     

Electro- and Magnetotransport near a LaAlO3/SrTiO3 Interphase Boundary

Comparison of the channel spectra of medium energy ion scattering, visualized for LaAlO₃/(001)SrTiO₃ heterostructures with a thickness of the lanthanum aluminate layer of one to six unit cells, indicates that the lanthanum aluminate layer grows coherently on a TiO₂-terminated surface of a strontium titanate substrate. The resistance of the interphase boundary in the heterostructure with a thickness of the LaAlO₃ layer of six unit cells decreased with temperature. At T < 100 K, the positive magnetoresistance and Hall mobility of electrons increased sharply with decreasing temperature.

     

Electronic Properties of a Two-Dimensional Electron Gas at the Interface between Transition Metal Complex Oxides

The structural and electronic properties of heterostructures based on transition metal oxides containing strongly correlated electrons are compared. The investigated structures are LaAlO₃/SrTiO₃ (LAO/STO), LaAlO₃/BaTiO₃ (LAO/BTO), and BaTiO₃/SrTiO₃ (BTO/STO). The role of structural relaxation in the formation of a two-dimensional electron gas at the interface of two dielectrics is revealed. The contribution from different orbitals and atoms to conductivity is analyzed, along with the correlation between structural distortions induced by the dipole moment in an LAO layer and conductivity.     

TbMnO3 epitaxial thin films by pulsed-laser deposition

Epitaxial TbMnO₃ films have been fabricated on SrTiO₃(001) and LaAlO₃(001) substrates by pulsed laser deposition (PLD), the structure and surface morphology of the films were characterized by X-ray diffraction with Cu K_α radiation and atomic force microscopy. The electrical transport and magnetic properties of the TbMnO₃ films and bulk were examined, the resistivity and the forbidden band width E_g change with epitaxial orientation, semiconductor transport properties are found in the films and bulk, the average of the E_g of the films on SrTiO₃ and on LaAlO₃ is equal to the E_g of the bulk. The two TMO films have different magnetization mode, the magnetization of the film on SrTiO₃ have an analogy to that of TbMnO₃ single crystal.     

Characterization of epitaxial lanthanum lutetium oxide thin films prepared by pulsed-laser deposition

Epitaxial thin films of LaLuO₃ were deposited on SrTiO₃(100) and SrRuO₃/SrTiO₃(100) or SrRuO₃/LaAlO₃(100) substrates using pulsed-laser deposition. They were investigated by means of Rutherford backscattering spectrometry, X-ray diffraction, transmission electron microscopy and atomic force microscopy. Smooth, c-axis oriented films with a channeling minimum yield of 3% were obtained. The electrical characterization of Au/LaLuO₃/SrRuO₃/SrTiO₃(100) and Au/LaLuO₃/SrRuO₃/LaAlO₃(100) metal–insulator–metal capacitor stacks revealed a dielectric constant of κ>45 and a breakdown field of 2 MV/cm for 100 nm thick epitaxial LaLuO₃ films. PACS 73.61.Ng; 73.40.Rw; 77.22.Ch; 77.55.+f     

Formation of the ferroelectric domains in epitaxial PbTiO3 thin films grown by metalorganic chemical vapor deposition

3 thin films have been prepared by metalorganic chemical vapor deposition under reduced pressure. The formation of ferroelectric domains in films grown on SrTiO₃ and LaAlO₃ substrates was investigated by synchrotron radiation and Rutherford backscattering spectroscopy. Single-domain (3000-Å thick) and multi-domain (4500-Å thick) PbTiO₃ films were produced on SrTiO₃. For multi-domain PbTiO₃ film, the c-domain presented epitaxial structure with its c-axis perpendicular to the substrate surface, while a-domains aligned four-fold symmetrically with c-domains by 2.79^° off the c-axis of c-domains. In the film, the measured lattice constants (a, b and c) of the a- and c-domains were different from each other, indicating that the films suffered a modulated strain during domain formation. In contrast, both the a and c domains of films on LaAlO₃ were alternatively aligned on substrate with the a-axis of the a-domain and the c-axis of c-domains perpendicular to the substrate surface. Two-dimensional distribution of these domains is proposed and the formation of these kinds of domains is discussed. The surface morphology and phase transition process of single and multi domain PbTiO₃ film on SrTiO₃ were studied by atomic force microscope (AFM) and high temperature X-ray diffraction, respectively. Received: 15 August 1996/Accepted: 21 January 1997     

Combinatorial discovery of anomalous substrate effect on the photochemical properties of transition metal-doped epitaxial SrTiO3 heterostructures

Epitaxial Sr(V_xCr_yTi_1−x−y)O₃ (0≤x+y≤0.05) ternary composition spreads were grown on two different single crystal substrates, LaAlO₃ and Nb-doped SrTiO₃, by use of combinatorial laser molecular beam epitaxy with a specially patterned slide masking plate. The photocatalytic activity on the composition spreads was evaluated by the photo-reduction of Ag⁺ in an AgNO₃ aqueous solution to deposit Ag metal on the spreads. The V-doping effect was found to depend greatly on the substrate: the photodeposition of Ag was much enhanced in the composition region of SrV_0.05Ti_0.95O₃ only on the Nb-doped SrTiO₃, but not on the LaAlO₃ and non-doped SrTiO₃.     

Phase separation in strained cation- and anion-deficient Nd0.52Sr0.48MnO3 films

The magnetic and transport properties of anion- and cation-deficient Nd_0.52Sr_0.48MnO₃ films with different thicknesses, as well as of two films from this system grown on different SrTiO₃ and LaAlO₃ substrates, are studied. Below Curie temperature T _C, the films with different thicknesses exhibit phase separation: they represent magnetic clusters (drops) embedded in a nonconducting paramagnetic (at T > T _N, where T _N is the Néel temperature) or antiferromagnetic (T < T _N) matrix. The temperature dependences of the resistivity of the films are well described in terms of the polaron mechanism of conduction. In external magnetic field H = 0.01 T, the drops may reach 15 nm in size. They consist of magnetic polarons with a small radius (1–2 nm). The drops are shown to interact with each other in the films. Because of competition between drop-drop dipole interaction and the magnetic energy, the drops disintegrate into droplets with a size comparable to that of a magnetic polaron in a field of 1 T. An explanation is given for the discrepancy between our results and the frequently observed growth of the drops with a rise in the external magnetic field. As the film gets thicker, the fraction of the ferromagnetic phase grows with thickness nonlinearly. In the film grown on SrTiO₃ (compressed by 0.9%), the characteristic Néel and Curie temperatures are lower than in the film grown on LaAlO₃. The diameters of ferromagnetic drops (both maximal at H = 0.01 T and minimal at H = 1 T) turn out to be roughly the same as in the films with different thicknesses.     

Resistance switching effect in LaAlO3/Nb-doped SrTiO3 heterostructure

The authors report on the fabrication and electronic transport property of LaAlO₃/Nb-doped SrTiO₃ heterostructure. The current–voltage curves of this heterostructure show hysteresis and a remarkable resistance switching behavior, which increase dramatically with decreasing temperature. Multiresistance states were realized by voltage pulses with different amplitudes and polarities and the ratio of the electrical pulse induced resistance change is larger than 10⁴. More interestingly, the relaxation of junction current after switching follows the Curie–von Schweidler law J∝t ⁻ⁿ with an exponential increase of n with temperature. The results were discussed in terms of the trap-controlled space charge limited conduction process via defects near the interface of the heterostructure.     

Pulsed laser deposition and characterization of nitrogen-substituted SrTiO3 thin films

Nitrogen-substituted cubic perovskite-type SrTiO₃ thin films were deposited in a one-step process using pulsed reactive crossed beam laser ablation (PRCLA) and RF-plasma assisted pulsed laser deposition (RF-PLD). Both techniques yield preferentially oriented films on SrTiO₃(0 0 1), LaAlO₃(0 0 1) and MgO(0 0 1) substrates with the unit cell parameters within 0.390(5) < a < 0.394(9) nm. The nitrogen content is higher in films deposited by PRCLA (0.84-2.40 at.%) as compared to films deposited by RF-PLD with nitrogen plasma (0.10-0.66 at.%). PRCLA with an ammonia gas pulse leads to a higher nitrogen content compared to the films grown with a nitrogen gas pulse, while films deposited by RF-PLD with ammonia plasma reveal only minor nitrogen contents (<0.10 at.%). The amount of the incorporated nitrogen can be tuned by adjusting the deposition parameters. Films deposited by PRCLA have a lower roughness of 1-3 nm compared to 12-18 nm for the films grown by RF-PLD. PRCLA yields partially reduced films, which exhibit electronic conductivity, while films deposited by RF-PLD are insulating. There is also a pronounced influence of the substrate material on the resistivity of the films deposited by PRCLA: films grown on SrTiO₃ substrates exhibit a metallic-like behaviour, while the corresponding films grown on MgO and LaAlO₃ substrates reveal a metal-to-semiconductor/insulator transition. Nitrogen incorporation into the SrTiO₃ films results in an increased optical absorption at 370-500 nm which is associated with N(2p) localized states with the energy about 0.7 eV higher than the valence band energy in strontium titanate. The optical band gap energies in the studied N-substituted SrTiO₃ films are 3.35-3.40 eV.