Contribution of Surface Defects to the Interface Conductivity of SrTiO_3/LaAlO_3

Based on the first-principles method,the structural stability and the contribution of point defects such as O,Sr or Ti vacancies on two-dimensional electron gas of n-and p-type LaAlO_3/SrTiO_3 interfaces are investigated.The results show that O vacancies at p-type interfaces have much lower formation energies,and Sr or Ti vacancies at n-type interfaces are more stable than the ones at p-type interfaces under O-rich conditions.The calculated densities of states indicate that O vacancies act as donors and give a significant compensation to hole carriers,resulting in insulating behavior at p-type interfaces.In contrast,Sr or Ti vacancies tend to trap electrons and behave as acceptors.Sr vacancies are the most stable defects at high oxygen partial pressures,and the Sr vacancies rather than Ti vacancies are responsible for the insulator-metal transition of n-type interface.The calculated results can be helpful to understand the tuned electronic properties of LaAlO_2/SrTiO_3 heterointerfaces.     

Sr掺杂对La_(1-x)Sr_xMnO_3/LaAlO_3/SrTiO_3界面电子结构的影响

采用基于密度泛函理论的第一性原理计算方法,系统地研究了La_(1-x)Sr_xMnO_3层中Sr的掺杂方式和掺杂量对4La_(1-x)Sr_xMnO_3/3LaAlO_3/4SrTiO_3(LSMO/LAO/STO)异质结构原子和电子结构的影响.结果表明:对于相同的Sr掺杂量,掺杂方式的差异对体系电子结构的影响微弱,不会导致体系发生金属-绝缘体转变;掺杂量的不同对体系电子结构有着显著的影响,当Sr的掺杂量较少时,LAO/STO界面处存在着准二维电子气,当Sr的掺杂量高于1/3时,LAO/STO界面处准二维电子气消失.我们相信,Sr的引入以及通过Sr掺杂量的改变可以对LSMO覆盖层极化进行调控,这也是导致体系LAO/STO界面处金属-绝缘体转变的可能原因,进一步为极化灾变机制导致的界面处电子重构提供了证据.     

Enhanced tunability of two-dimensional electron gas on SrTiO3 through heterostructuring

Two-dimensional electron gases (2DEGs) on the SrTiO₃ (STO) surface or in STO-based heterostructures have exhibited many intriguing phenomena, which are strongly dependent on the 2DEG-carrier density. We report that the tunability of the 2DEG-carrier density is significantly enhanced by adding a monolayer LaTiO₃ (LTO) onto the STO. Ultraviolet (UV) irradiation induced maximum carrier density of the 2DEG in LTO/STO is increased by a factor of ~4 times, compared to that of the bare STO. By oxygen gas exposure, it becomes 10 times smaller than that of the bare STO. This enhanced tunability is attributed to the drastic surface property change of a polar LTO layer by UV irradiation and O₂ exposure. This indicates that the 2DEG controllability in LTO/STO is more reliable than that on the bare STO driven by defects, such an oxygen vacancy.     

静电纺丝纳米ZrO_2:Ti纤维的制备与发光性质的研究

通过静电纺丝和高温煅烧相结合的方法制备了一维钛掺杂氧化锆的纳米纤维。复合纤维在1 000℃煅烧后,由于模板剂、聚乙烯吡咯烷酮(PVP)在高温下的分解,纤维直径大大减小,扫描电镜观察最终得到直径100 nm左右的单斜相纤维。利用稳态光谱和时间分辨光谱技术室温下研究了纤维的光致发光性质。结果表明发射主峰在467 nm的宽带,在350 nm处有一个小肩峰。两种不同的发光行为可以通过ZrO2∶Ti纳米纤维中存在的两种情况的氧空穴作为电子俘获中心来解释。一种由电荷补偿产生的氧空位,俘获电子,产生467 nm处Ti参与的发光现象。另外一种ZrO2表面缺陷导致的氧空穴,对应于350 nm的发光现象,没有发现长余辉现象。测得的激发态寿命短于体材料而长于普通的纳米晶,可能是由于一维纳米结构的独特性质导致的。     

Effects of oxygen vacancy concentration and temperature on memristive behavior of SrRuO_3/Nb:SrTiO_3 junctions

Metal/semiconductor memristive heterostructures have potential applications in nonvolatile memory and computing devices. To enhance the performance of the memristive devices, it requires a comprehensive engineering to the metal/semiconductor interfaces. Here in this paper, we discuss the effects of oxygen vacancies and temperature on the memristive behaviors of perovskite-oxide Schottky junctions, each consisting of SrRuO_3 thin films epitaxially grown on Nb:SrTiO_3 substrates. The oxygen partial pressure and laser fluence are controlled during the film growth to tune the oxygen defects in SrRuO_3 films, and the Schottky barrier height can be controlled by both the temperature and oxygen vacancies. The resistive switching measurements demonstrate that the largest resistance switching ratio can be obtained by controlling oxygen vacancy concentration at lower temperature. It suggests that reducing Schottky barrier height can enhance the resistive switching performance of the SrRuO_3/Nb:SrTiO_3 heterostructures. This work can conduce to the development of high-performance metal-oxide/semiconductor memristive devices.     

The study of optical properties of amorphous thin films of mixed oxides In<Subscript>2</Subscript>O<Subscript>3</Subscript>—SnO<Subscript>2</Subscript> system,deposited by co-evaporation

A discussion of optical properties of mixed oxides In₂O₃—SnO₂ system is presented. Film thickness, substrate temperature, composition (in molar %) and annealing have a profound effect on the structure and optical properties of these films. Initially the increase in band gap with the increase of SnO₂ content in In₂O₃ is due to the increase in carrier density as a result of donor electrons from tin. The decrease in band gap above the critical Sn content is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in band gap with film thickness is caused by the increase in free carrier density which is generated by (i) Sn atom substitution of In atom, giving out one extra electron and (ii) oxygen vacancy acting as two electrons donor. The decrease in band gap with substrate temperature and annealing is due either to the severe deficiency of oxygen, which deteriorate the film properties and reduce the mobility of the carriers, or to the formation of indium species of lower oxidation state (In²⁺).     

含氧空位锐钛矿TiO2光学性质的第一性原理研究

为了研究锐钛矿TiO2晶体中氧宅位对光学性质的影响,利用基于局域密度泛函理论框架下的广义梯度近似平面波超软赝势方法.用第一性原理对含氧空位锐钛矿TiO2晶体进行了结构优化处理.计算了完整的和含氧空位锐钛矿TiO2晶体的电子态密度、复数折射率、介电甬数及吸收光谱的偏振特性.二者比较发现,引入氧空位后,锐钛矿TiO2的电子结构发生了变化,电子总态密度的费米面进入了导带,引起了莫特相变;含氧空位的锐钛矿TiO2晶体的Ti 3d态大约在-6.097 eV处出现了新的劈裂峰值.两种模型的介电函数虚部、吸收光谱以及复折射率的实部,它们的峰值位置一一对应.说明它们之间存在着内在的联系,这些都与电子态密度分布直接相关.理论分析和计算发现二者的介电函数虚部和吸收光谱峰值位置移动是弛豫效应影响的结果,同时,峰值大小变化是电子跃迁的几率来决定的.晶体的各向异性是由晶体结构的对称性决定的.     

SrTiO3薄膜氧缺陷的第一性原理研究

在广义梯度近似(GGA)下,利用超软赝势对真空条件下SrTiO3超晶胞的体系能量、原子间电子云重叠布局数和电子态密度等进行了自恰计算.结果显示,对有氧缺陷的超晶胞优化后,晶格参数的几何平均值增大了2.711%,这表明在高温条件下外延生长STO薄膜时,产生了氧缺陷,并且氧空位易处于薄膜表层;另外,表层氧缺陷使表层Ti原子明显的发生偏心位移,两个Ti原子的核间距由0.3905 nm增大至0.4234 nm,b轴上的氧原子则向中心靠近了0.0108 nm、并沿c轴方向上突了0.0027 nm,使STO晶体产生自发极化,氧缺陷还使STO的电子态密度的能隙增宽了1.75 eV,达到2.48 eV,从而使STO晶体由顺电相转向铁电相.     

First-principles study of ferromagnetism in Ti-doped ZnO with oxygen vacancy

Using first-principles density functional calculations, we have investigated the electronic structures of Ti-doped ZnO (Ti is in 4+ oxidation state) with and without oxygen vacancy. The ferromagnetic property is identified in the presence of oxygen vacancy despite Ti being nonmagnetic in its natural phase. The ferromagnetism originates from the charge transferring from donor derived-defect band to unoccupied Ti-3d states and the hybridization between Ti-3d and O-2p band near the Fermi level. On increasing the oxygen vacancy concentration, a transition from a long-ranged magnetic order to a short-ranged interaction is found and the oxygen vacancies prefer to distribute non-uniformly in Ti-doped ZnO.     

Effect of oxygen vacancy defect on the magnetic properties of Co-doped ZnO

The influence of oxygen vacancy on the magnetism of Co-doped ZnO has been investigated by the first-principles calculations.It is suggested that oxygen vacancy and its location play crucial roles on the magnetic properties of Co-doped ZnO.The exchange coupling mechanism should account for the magnetism in Co-doped ZnO with oxygen vacancy and the oxygen vacancy is likely to be close to the Co atom.The oxygen vacancy (doping electrons) might be available for carrier mediation but is localized with a certain length and can strengthen the ferromagnetic exchange interaction between Co atoms.     

氧空位对共掺型锐钛矿TiO2能带结构的影响

基于电子密度泛函理论计算,系统研究了氧空位对V-C(p型)、Cr-N(n型)、V-N和Cr-C(电中性)四种共掺型锐钛矿TiO₂的几何和电子结构的影响．结果表明在最稳定构型中氧空位处在与金属杂质原子相连的位置．由于氧空位的出现,共掺TiO₂禁带中一些未占据的杂质态被填充满电子,掺杂体系的导带带边位置几乎不变,这有利于减少光生载流子复合,且不影响光解水过程中的产氢反应,在厌氧条件下很容易在V-N共掺TiO₂中引入氧空位．     

Electronic properties of anatase TiO2 doped by lanthanides: A DFT+U study

The effects of mono-doping of 4f lanthanides with and without oxygen vacancy defect on the electronic structures of anatase TiO₂ have been studied by first-principles calculations with DFT+U (DFT with Hubbard U correction) to treat the strong correlation of Ti 3d electrons and lanthanides 4f electrons. Our results revealed that dopant Ce is easy to incorporate into the TiO₂ host by substituting Ti due to its lower substitutional energy (∼−2.0 eV), but the band gap of the system almost keeps intact after doping. The Ce 4f states are located at the bottom of conduction band, which mainly originates from Ti 3d states. The magnetic moment of doped Ce disappears due to electron transfer from Ce to the nearest O atoms. For Pr and Gd doping, their substitutional energies are similar and close to zero, indicating that both of them may also incorporate into the TiO₂ host. For Pr doping, some 4f spin-down states are located next to the bottom of the conduction band and narrow the band gap of the doping system. However, for Gd doping, the 4f states are located in deep valence band and there is no intermediate band in the band gap. The magnetic moment of dopant Gd is close to the value of isolated Gd atom (∼7 μB), indicating no overlapping between Gd 4f with other orbitals. For Eu, it is hard to incorporate into the TiO₂ host due to its very higher substitutional energy. The results also indicated that oxygen vacancy defect may enhance the adsorption of the visible light in Ln-doped TiO₂ system.     

Ce和O空位共掺杂TiO_2的电子结构与光学性质

采用基于密度泛函理论加U的计算方法,研究了Ce和O空位单(共)掺杂锐钛矿相TiO_2的电子结构和光吸收性质.计算结果表明,Ce和O空位共掺杂TiO_2的带隙中出现了杂质能级,且带隙窄化为2.67 eV,明显比纯TiO_2和Ce,O空位单掺杂TiO_2的要小,因而可提高TiO_2对可见光的响应能力,使TiO_2的光吸收范围增加.光吸收谱显示,掺杂后TiO_2的光吸收边发生了显著红移;在400.0—677.1 nm的可见光区,共掺杂体系的光吸收强度显著高于纯TiO_2和Ce单掺杂TiO_2,而略低于O空位单掺杂TiO_2.此外,Ce掺杂TiO_2中引入O空位后,TiO_2的导带边从-0.27 eV变化为-0.32 eV,这表明TiO_2的导带边的还原能力得到了加强.计算结果为Ce和O空位共掺杂TiO_2在可见光光解水方面的进一步研究提供了有力的理论依据.     

A first-principles study of the effect of oxygen vacancy on rutile Ti1−xCdxO2

A first-principles study has been performed to understand the effect of oxygen vacancy on the electronic properties of cadmium doped rutile TiO₂. We observe that Cd incorporation on rutile TiO₂ induces Cd p-states on the top of the valence band which is consistent with an earlier result of Zhang et al. (2008) [5]. Furthermore, by creating an oxygen vacancy, some new states are induced, which originate from the Ti 3d electrons at the middle of the band gap and spread up to the conduction band. Therefore, the band gap of the material reduces significantly, making it suitable to act as a better photocatalyst.     

钛原子位置对立方BaTiO3电子结构的影响

本文基于密度泛函理论(DFT)的第一原理方法,计算了Ti原子位置对BaTiO3电子结构的影响.Ti的位置变化导致晶格畸变,使电子结构发生变化;从能带结构、能态密度(DOS)、电子密度、Mulliken布居等计算结果分析表明,导带和价带主要由Ti的3d电子和O的2p电子,Ti原子位置的变化,使Ti的3d电子能量分布上移,而O的2p电子能量下移;Ti位置变化,Ti的3d电子与的sp电子形成的杂化轨道更趋向离子化,以致于使OI出现了正电荷,表明发生了的2p电子向Ti的转移;O原子电子的转移使得Ti原子在导带的3d电子能量降低,与O原子在价带的2p电子能量重叠,禁带消失;随着畸变程度提高,转移逐渐增强,使禁带宽度逐渐减小,直至完全消失.     

Role of Ti 3d carriers in mediating the ferromagnetism of Co∶TiO2 anatase thin films

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co∶TiO(2) anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of ～1 and ～10 nm, respectively. We obtain direct evidence of metallic Ti(3+) states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal Ti(3+) electrons at the Fermi level (E(F)) and high-spin Co(2+) electrons occurring away from E(F). The results show the importance of the charge neutrality condition: Co(2+)+V(O)(2-)+2Ti(4+)?Co(2+)+2Ti(3+) (V(O) is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.     

Calculations of two dimensional electron gas distributions in AlGaN/GaN material system

This paper presents calculating results of the two-dimensional electron gas （2DEG） distributions in AlGaN/GaN material system by solving the Schroedinger and Poisson equations self-consistently. Due to high 2DEG density in the AlGaN/GaN heterojunction interface, the exchange correlation potential should be considered among the potential energy item of Schroedinger equation. Analysis of the exchange correlation potential is given. The dependencies of the conduction band edge, 2DEG density on the Al mole fraction are presented. The polarization fields have strong influence on 2DEG density in the AlGaN/GaN heterojunction, so the dependency of the conduction band edge on the polarization is also given.     

Calculations of two dimensional electron gas distributions in AlGaN/GaN material system

This paper presents calculating results of the two-dimensional electron gas (2DEG) distributions in AlGaN/GaN material system by solving the Schr\"{o}dinger and Poisson equations self-consistently. Due to high 2DEG density in the AlGaN/GaN heterojunction interface, the exchange correlation potential should be considered among the potential energy item of Schr\"{o}dinger equation. Analysis of the exchange correlation potential is given. The dependencies of the conduction band edge, 2DEG density on the Al mole fraction are presented. The polarization fields have strong influence on 2DEG density in the AlGaN/GaN heterojunction, so the dependency of the conduction band edge on the polarization is also given.     

Electric gating of the multichannel conduction in LaAlO_3/SrTiO_3 superlattices

The electric gating on the transport properties of two-dimensional electron gas(2DEG) at the interface of LaAlO₃/SrTiO₃(LAO/STO) heterostructure has attracted great research interest due to its potential application in fieldeffect devices. Most of previous works of gate effect were focused on the LAO/STO heterostructure containing only one conductive interface. Here, we systematically investigated the gate effect on high-quality LAO/STO superlattices(SLs)fabricated on the TiO₂-terminated(001) STO substrates. In addition to the good metallicity of all SLs, we found that there are two types of charge carriers, the majority carriers and the minority carriers, coexisting in the SLs. The sheet resistance of the SLs with a fixed thickness of the LAO layer increases monotonically as the thickness of the STO layer increases. This is derived from the dependence of the minority carrier density on the thickness of STO. Unlike the LAO/STO heterostructure in which minority and majority carriers are simultaneously modulated by the gate effect, the minority carriers in the SLs can be tuned more significantly by the electric gating while the density of majority carriers is almost invariable. Thus, we consider that the minority carriers may mainly exist in the first interface near the STO substrate that is more sensitive to the back-gate voltage, and the majority carriers exist in the post-deposited STO layers. The SL structure provides the space separation for the multichannel conduction in the 2 DEG, which opens an avenue for the design of field-effect devices based on LAO/STO heterostructure.     

Optical study of the free-carrier response of LaTiO3/SrTiO3 superlattices

We used infrared spectroscopic ellipsometry to investigate the electronic properties of LaTiO_{3}/SrTiO_{3} superlattices (SLs). Our results indicated that, independent of the SL periodicity and individual layer thickness, the SLs exhibited a Drude metallic response with sheet carrier density per interface approximately 3x10;{14} cm;{-2}. This is probably due to the leakage of d electrons at interfaces from the Mott insulator LaTiO3 to the band insulator SrTiO3. We observed a carrier relaxation time approximately 35 fs and mobility approximately 35 cm;{2} V-1 s;{-1} at 10 K, and an unusual temperature dependence of carrier density that was attributed to the dielectric screening of quantum paraelectric SrTiO3.