High mobility in LaAlO3/SrTiO3 heterostructures: origin, dimensionality, and perspectives

We have investigated the dimensionality and origin of the magnetotransport properties of LaAlO3 films epitaxially grown on TiO2-terminated SrTiO3(001) substrates. High-mobility conduction is observed at low deposition oxygen pressures (P(O2)<10(-5) mbar) and has a three-dimensional character. However, at higher P(O2) the conduction is dramatically suppressed and nonmetallic behavior appears. Experimental data strongly support an interpretation of these properties based on the creation of oxygen vacancies in the SrTiO3 substrates during the growth of the LaAlO3 layer. When grown on SrTiO3 substrates at low P(O2), other oxides generate the same high mobility as LaAlO3 films. This opens interesting prospects for all-oxide electronics.     

Upper limit to magnetism in LaAlO3/SrTiO3 heterostructures

Using polarized neutron reflectometry we measured the neutron spin-dependent reflectivity from four LaAlO(3)/SrTiO(3) superlattices. Our results imply that the upper limit for the magnetization averaged over the lateral dimensions of the sample induced by an 11 T magnetic field at 1.7 K is less than 2 G. SQUID magnetometry of the neutron superlattice samples sporadically finds an enhanced moment, possibly due to experimental artifacts. These observations set important restrictions on theories which imply a strongly enhanced magnetism at the interface between LaAlO(3) and SrTiO(3).     

Optical and magnetic characterisation of Co3+ and Ni3+ in LaAlO3: interplay between the spin state and Jahn-Teller effect

The coordination, the electronic structures and the spin of the ground state of Ni(3+) (3d(7)) and Co(3+) (3d(6)) introduced as impurities in LaAlO(3) are investigated through optical spectroscopy and magnetic measurements. The unusual trivalent valence state in both transition-metal ions was stabilised via a sol-gel process followed by high oxygen pressure treatments. We show that the crystal-field strength at the nearly O(h) transition-metal site in LaAlO(3) locates Ni(3+) and Co(3+) near the spin state crossover, yielding a low-spin ground state in both cases. We analyse how the interplay between the Jahn-Teller (JT) effect and the spin state affects the magnetic moment of the ion and its temperature dependence. The optical spectra reveal a JT effect associated with a low-spin ground state in Ni(3+) and with a thermally populated high-spin low-lying first excited state in Co(3+). The corresponding JT distortions are derived from structural correlations. We conclude that the JT effect is unable to stabilise the intermediate spin state in Co(3+). A low-spin ground state in thermal equilibrium with a high-spin low-lying first excited state is detected in diluted Co(3+)-doped LaAlO(3). These results are compared with those obtained in the parent pure compounds LaNiO(3) and LaCoO(3).     

Shubnikov-de Haas oscillations in SrTiO3/LaAlO3 interface

Quantum magnetic oscillations in SrTiO3/LaAlO3 interface are observed in the magnetoresistance. We study their frequency as a function of gate voltage and the evolution of their amplitude with temperature. The data are consistent with the Shubnikov-de Haas theory. The Hall resistivity ρ(xy) is nonlinear at low magnetic fields. ρ(xy) is fitted assuming multiple carrier contributions. We infer the density of the mobile charge carriers from the oscillations frequency and from Hall measurements. The comparison between these densities suggests multiple valley and spin degeneracy. The small amplitude of the oscillation is discussed in the framework of the multiple band scenario.     

Gate-controlled spin injection at LaAlO3/SrTiO3 interfaces

We report results of electrical spin injection at the high-mobility quasi-two-dimensional electron system (2-DES) that forms at the LaAlO3/SrTiO3 interface. In a nonlocal, three-terminal measurement geometry, we analyze the voltage variation associated with the precession of the injected spin accumulation driven by perpendicular or transverse magnetic fields (Hanle and inverted Hanle effect). The influence of bias and back-gate voltages reveals that the spin accumulation signal is amplified by resonant tunneling through localized states in the LaAlO3 strongly coupled to the 2-DES by tunneling transfer.     

Proposal of a one-dimensional electron gas in the steps at the LaAlO3-SrTiO3 interface

The two-dimensional electron gas at the interface between LaAlO(3) and SrTiO(3) has become one of the most fascinating and highly debated oxide systems of recent times. Here we propose that a one-dimensional electron gas can be engineered at the step edges of the LaAlO(3)/SrTiO(3) interface. These predictions are supported by first-principles calculations and electrostatic modeling which elucidate the origin of the one-dimensional electron gas as an electronic reconstruction to compensate a net surface charge in the step edge. The results suggest a novel route to increasing the functional density in these electronic interfaces.     

LaAlO3/SrTiO3界面的电子结构及光学性质的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,结合局域密度近似（LDA）研究了钙钛矿结构氧化物LaAlO3 /SrTiO3界面的电子结构及光学性质。能带结构分析表明当形成(AlO2)-/(TiO2)0界面时其禁带宽度为1.888 eV,呈现绝缘体的性质,当形成(LaO)+/(SrO)0界面时其禁带宽度为0.021 eV,呈现半导体或半金属性质。同时,对不同界面的光学性质也进行了研究,结果表明纯相的LaAlO3和SrTiO3的吸收系数、反射系数及能量损失谱强度明显高于由这两种单质形成不同界面的强度。     

Origin of metallic states at the heterointerface between the band insulators LaAlO3 and SrTiO3

We have studied the electronic structure at the heterointerface between the band insulators LaAlO3 and SrTiO3 using in situ photoemission spectroscopy. Our experimental results clearly reveal the formation of a notched structure on the SrTiO3 side due to band bending at the metallic LaAlO3/TiO2-SrTiO3 interface. The structure, however, is absent at the insulating LaAlO3/SrO-SrTiO3 interface. The present results indicate that the metallic states originate not from the charge transfer through the interface on a short-range scale but from the accumulation of carriers on a long-range scale.     

Structural comparison of n-type and p-type LaAlO3/SrTiO3 interfaces

Using a surface x-ray diffraction technique, we investigated the atomic structure of two types of interfaces between LaAlO3 and SrTiO3, that is, p-type (SrO/AlO2) and n-type (TiO2/LaO) interfaces. Our results demonstrate that the SrTiO3 in the sample with the n-type interface has a large polarized region, while that with the p-type interface has a limited polarized region. In addition, atomic intermixing was observed to extend deeper into the SrTiO3 substrate at the n-type interface compared to the p type. These differences result in distinct degrees of band bending, which likely contributes to the striking contrast in electrical conductivity between the two types of interfaces.     

Formation of Interfacial Layers in LaAlO3/Silicon during Film Deposition

We have studied the interracial reactions between amorphous LaAlO3 thin films and Si substrates, using high- resolution transmission electron microscopy and x-ray photoelectron spectroscopy. It has been shown that the interracial layer between LaAlO3 film and Si substrate chemical states show that the ratio of La 4d3/2 to Al 2p is SiLaxAlyOz. The depth distributions of La, Si and Al of the interfacial layer remains unchanged with the depth compared to that of the LaAlO3 film. Moreover, the Si content, in the interracial layer gradually decreases with increasing thickness of the interracial layer. These results strongly suggest that the Al element is not deficient in the interracial layer, as previously believed, and the formation of a SiLaxAlyOz interracial layer is mainly due to the diffusion of Si from the substrate during the LaAlO3 film deposition. With the understanding of the interracial layer formation, ones can control the interface characteristics to ensure the desired performances of devices using high-k oxides as gate dielectrics.     

Tuning the electrons at the LaAlO_3/SrTiO_3 interface: From growth to beyond growth

Recently, the quasi-two-dimensional electron gas （q2DEG） confined at the interface between LaAlO3 and SrTiO3 has attracted significant attention. In this paper, we briefly review experimental methods that have been used to tune the carrier density and mobility of this q2DEG. These methods can be classified into two categories： growth-related tuning （i.e. substrate, growth temperature, oxygen pressure, post-annealing, LaAlO3 thickness, stoichiometry, and capping layers） and post-growth tuning （i.e. electrostatic field gating, conductive atomic force microscopy and surface adsorbates）. Taken together, these methods enable the broad tuning of the electronic properties of this interface.     

铝酸镧单晶体中Ce3+的能级结构和荧光特性

采用提拉法成功生长了纯LaAlO₃和掺铈的LaAlO₃单晶体.测试了它们的远红外吸收谱，紫外吸收谱，荧光谱.根据吸收光谱确定了晶体中Ce³⁺的能级结构，利用这一能级结构模型较好地解释了Ce：LaAlO₃晶体的荧光光谱. 关键词： Ce：LaAlO3 吸收光谱 能级 荧光光谱     

Structural and electronic properties of the interface between the high-k Oxide LaAlO3 and Si(001)

The structural and electronic properties of the LaAlO(3)/Si(001) interface are determined using state-of-the-art electronic structure calculations. The atomic structure differs from previous proposals, but is reminiscent of La adsorption structures on silicon. A phase diagram of the interface stability is calculated as a function of oxygen and Al chemical potentials. We find that an electronically saturated interface is obtained only if Al atoms substitute some of the interfacial Si atoms. These findings raise serious doubts whether LaAlO3 can be used as an epitaxial gate dielectric.     

Influence of mechanical stresses on the charge state of the interface in the LaAlO3/(001)SrTiO3 heterostructures with a distorted stoichiometry

Physics of the Solid State - The medium-energy ion scattering (MEIS) spectroscopy was used to obtain the data on the structure and stoichiometry of interfaces in LaAlO3/SrTiO3 (LAO/STO)...     

Electric field effect in diluted magnetic insulator anatase Co: TiO2

An external electric field induced reversible modulation of a room temperature magnetic moment and coercive field is achieved in an epitaxial and insulating thin film of dilutely cobalt-doped anatase TiO2. This first demonstration of an electric field effect in any oxide-based diluted ferromagnet is realized in a high quality epitaxial heterostructure of PbZr(0.2)Ti(0.8)O(3)/Co: TiO(2)/SrRuO(3) grown on (001) LaAlO3. The observed effect, which is about 15% in strength in a given heterostructure, can be modulated over several cycles. Possible mechanisms for electric field induced modulation of insulating ferromagnetism are discussed.     

In situ photoemission study of Pr1-xCaxMnO3 epitaxial thin films with suppressed charge fluctuations

We have performed an in situ photoemission study of Pr1-xCaxMnO3 (PCMO) thin films grown on LaAlO3 (001) substrates and observed the effect of epitaxial strain on the electronic structure. We found that the chemical potential shifted monotonically with doping, unlike bulk PCMO, implying the disappearance of incommensurate charge fluctuations of bulk PCMO. In the valence-band spectra, we found a doping-induced energy shift toward the Fermi level (EF) but there was no spectral weight transfer, which was observed in bulk PCMO. The gap at EF was clearly seen in the experimental band dispersions determined by angle-resolved photoemission spectroscopy and could not be explained by the metallic band structure of the C-type antiferromagnetic state, probably due to localization of electrons along the ferromagnetic chain direction or due to another type of spin-orbital ordering.     

应力作用下GdTiO_3磁序相变的第一性原理研究

采用基于密度泛函理论的第一性原理方法,系统研究了GdTiO_3薄膜在压缩应力和拉伸应力作用下的磁序相变.计算结果表明:1)在LaAlO_3压缩衬底的作用下,GdTiO_3薄膜从铁磁基态转变为G型反铁磁基态.该结果不同于YTiO_3和LaTiO_3在LaAlO_3压缩衬底作用时都呈现A型反铁磁基态的情况.若进一步加大压缩应力,例如在(001)平面施加YTiO_3衬底,此时GdTiO_3薄膜基态才为A型反铁磁态.2)在LaScO_3和BaZrO_3拉伸衬底的作用下,GdTiO_3薄膜的基态仍是铁磁态,但是随着拉伸应力的增大,A型反铁磁态的能量和铁磁态的能量差逐渐缩小,即GdTiO_3薄膜的基态有转变为A型反铁磁态的趋势.3)在外加应力的作用下,GdTiO_3薄膜基态的磁序发生了相变,但是其绝缘性并没有变,说明GdTiO_3薄膜仍为Mott型绝缘体.     

高温超导衬底大尺寸LaAlO_3单晶体生长研究

铝酸镧(LaAlO3)单晶是超导研究领域使用最普遍的单晶体之一,详细介绍了采用下称重,CZ法(Czo-chralski)生长大尺寸(Φ80mm)LaAlO3单晶的工艺条件,并对影响晶体质量的主要因素进行了分析、讨论,给出了解决晶体质量问题的手段和有效途径。     

n-ZnO/LaAlO3/p-Si heterojunction for visible-blind UV detection

A visible-blind UV photodetector (PD) using a double heterojunction of n-ZnO/LaAlO3 (LAO)/p-Si was demonstrated. Inserted LAO layers exhibit electrical insulating properties and serve as blocking layers for photoexcited electrons from p-Si to n-ZnO, leading to an enhanced rectification ratio and a visible-blind UV detectivity of the n-ZnO/LAO/p-Si PDs due to the high potential barrier between LAO and p-Si layers (~2.0 eV). These results support the use of n-ZnO/LAO/p-Si PDs in the visible-blind UV PDs in a visible-light environment.     

Origin of charge density at LaAlO3 on SrTiO3 heterointerfaces: possibility of intrinsic doping

As discovered by Ohtomo and Hwang, a large sheet charge density with high mobility exists at the interface between SrTiO3 and LaAlO3. Based on transport, spectroscopic, and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.