Different Thermal Stabilities of Cation Point Defects in LaAlO_3 Bulk and Films

Using the first-principles method, we investigate the thermal stability of cation point defects in LaAlO_3 bulk and films. The calculated densities of states indicate that cation vacancies and antisites act as acceptors. The formation energies show that cation vacancies are energetically favorable in bulk LaAlO_3 under O-rich conditions,while the Al_(La) antisites are stable in reducing atmosphere. However, the same behavior does not appear in the case of LaAlO_3 films. For LaO-terminated LaAlO_3 films, La or Al vacancies remain energetically favorable under O-rich and O-deficient conditions. For an AlO_2-terminated surface, under O-rich condition the La interstitial atom is repelled from the outmost layer after optimization, which releases more stress leading to the decrease of total energy of the system. An Al interstitial atom has a smaller radius so that it can stay in distorted films and becomes more stable under O-deficient conditions, and the Al interstitial atoms can be another possible carrier source contribution to the conductivity of n-type interface under an ultrahigh vacuum. La and Al antisites have similar formation energy regardless of oxygen pressure. The results would be helpful to understand the defect structures of LaAlO_3-related materials.     

Atomic and electronic structures of Mg-doped perfect SrTiO3 and crystals containing oxygen vacancies from first principles

The electronic properties of Mg-doped perfect SrTiO₃ and crystals containing oxygen vacancies systems are investigated by first principles calculation. Dopant formation energy results show that the Mg atoms preferentially enter the Sr site in SrTiO₃. Substitution of Ti by Mg brings some acceptor levels, which introduces the p-type conductivity of SrTiO₃. Creation of oxygen vacancies in SrTiO₃ introduces donor levels, which can contribute to the n-type conductivity. In SrTiO₃ containing oxygen vacancies system, a self-compensation effect will occur when Ti is substituted by Mg, and the system undergoes n-type to p-type transition.     

不同组分厚度比的LaMnO3/SrTiO3异质界面电子结构和磁性的第一性原理研究

基于密度泛函理论的第一性原理计算, 研究了(LaMnO₃)_n/(SrTiO₃)_m(LMO/STO)异质界面的离子弛豫、电子结构和磁性质. 研究表明, 不同组分厚度比及界面类型时, 离子弛豫程度各不相同, 并且界面处的电子性质受此影响较大. 对于n型界面, 当LMO的厚度达到6个单胞层后, 电子会从LMO转移到STO, 转移的电子占据界面层Ti原子的3d电子轨道, 界面处出现二维电子气. 对于n型界面(LMO)_n/(STO)₂, 随着LMO厚度数n的增加, 由离子弛豫造成的结构畸变减小, 而界面处Ti原子周围电子的态密度和自旋极化却增大, 表明高厚度比的n型界面有利于产生高迁移率的二维电子气和自旋极化. 而对于p型(LMO)₂/(STO)₈界面, 在STO一侧基本没有结构畸变, 界面处无电子转移和自旋极化现象. 通过计算平均静电势发现n型和p型界面处的势差大小相差2 eV, 解释了p型界面不容易发生电荷转移的原因.     

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.     

Vacancy induced magnetism in SrTiO3

Vacancy-induced magnetism in perovskite SrTiO₃ is investigated by ab initio calculations and magnetic measurements. The calculations of the generalized gradient approximation (GGA), the local density approximation (LDA) and the local density approximation with on-site effect U (LDA+U) methods show that stoichiometric SrTiO₃ is nonmagnetic. The GGA calculated results indicate that Ti or O vacancy could induce magnetism rather than Sr vacancy. The LDA and LDA+U calculations show that the Ti vacancy could induce magnetism, while Sr and O vacancies couldn't. The experimental results confirm that SrTiO₃ nanocrystalline powders exhibit room-temperature ferromagnetism (FM) and the magnetic moment results from cation vacancies.     

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.     

Evolution of the interfacial structure of LaAlO3 on SrTiO3

The evolution of the atomic structure of LaAlO_{3} grown on SrTiO_{3} was investigated using surface x-ray diffraction in conjunction with model-independent, phase-retrieval algorithms between two and five monolayers film thickness. A depolarizing buckling is observed between cation and oxygen positions in response to the electric field of polar LaAlO_{3}, which decreases with increasing film thickness. We explain this in terms of competition between elastic strain energy, electrostatic energy, and electronic reconstructions. Based on these structures, the threshold for formation of a two-dimensional electron system at a film thickness of 4 monolayers is quantitatively explained. The findings are also qualitatively reproduced by density-functional-theory calculations.     

Polar discontinuity doping of the LaVO_{3}/SrTiO_{3} interface

We have investigated the transport properties of LaVO_{3}/SrTiO_{3} Mott-insulator-band-insulator heterointerfaces for various configurations. The (001)-oriented n-type VO_{2}/LaO/TiO_{2} polar discontinuity is conducting, exhibiting a LaVO3 thickness-dependent metal-insulator transition and low temperature anomalous Hall effect. The (001) p-type VO_{2}/SrO/TiO_{2} interface, formed by inserting a single layer of bulk metallic SrVO3 or SrO, drives the interface insulating. The (110) heterointerface is also insulating, indicating interface conduction arising from electronic reconstructions.     

空位缺陷对β-AgVO_3电子结构和光吸收性能的影响

基于密度泛函的第一性原理研究了Ag空位、O空位和Ag-O双空位对β-AgVO_3的电子结构及光学性质的影响.采用广义梯度近似平面波超软赝势GGA+U方法,对不同缺陷体系的形成能、能带结构、电子态密度、差分电荷密度和吸收光谱进行了计算和分析.通过比较不同Ag空位和O空位的形成能,确定了β-AgVO_3中主要形成Ag3空位和O1空位,并且Ag空位较O空位更容易形成.Ag3空位和O1空位的存在都使得β-AgVO_3带隙有一定程度的减小;Ag3空位使β-AgVO_3呈现p-型半导体性质,而O1空位和Ag3-O1双空位使β-AgVO_3呈现n-型半导体性质.Ag3和O1空位对晶体在可见光范围内的光吸收影响较小.     

Conversion of p-type to n-type conductivity in undoped ZnO films by increasing operating temperature

ZnO thin films with the thickness of about 15 nm on (0 0 0 1) sapphire substrates were prepared by pulsed laser deposition. X-ray photoelectron spectroscopy indicated that both as-grown and post-annealed ZnO thin films were oxygen-rich. H₂ sensing measurements of the films indicated that the conductivity type of both the unannealed and annealed ZnO films converted from p-type to n-type in process of increasing the operating temperature. However, the two films showed different conversion temperatures. The origin of the p-type conductivity in the unannealed and annealed ZnO films should be attributed to oxygen related defects and zinc vacancies related defects, respectively. The conversion of the conductivity type was due to the annealing out of the correlated defects. Moreover, p-type ZnO films can work at lower temperature than n-type ZnO films without obvious sensitivity loss.     

Defect properties of Ti-doped Cr2O3

The defects in Cr_2−xTi_xO₃ (x = 0, 0.2 and 0.3) were studied by a combination of X-ray diffraction, density and electrical conductivity measurements supported by atomistic simulation. The results are consistent with the Ti being dissolved as Ti⁴⁺ compensated by Cr vacancies which associate to form complex defects of lower energy. Ti doping gives n-type semiconductivity due to a small concentration of Ti³⁺ in equilibrium with the complexes.     

Defect-induced strong ferromagnetism in Cr-doped In2O3 from first-principles theory

We demonstrate by means of first-principles calculations that the high Curie temperature observed in Cr-doped In₂O₃ is mediated by intrinsic p-type defects, namely In vacancies or O interstitials. Charge transfer from Cr 3d states to the hole states formed by these defects makes Cr ions in the mixed valence state, giving rise to a strong ferromagnetic coupling. Calculated formation energies of various defects also show that doping Cr in In₂O₃ could greatly lower the formation energies of p-type intrinsic defects even in oxygen-deficient growth conditions. These results advance our understanding of the underlying physics of diluted magnetic oxides.     

First-principles investigation of the intrinsic defects in Ti3SiC2

First-principles calculations have been carried out to investigate intrinsic defects including vacancies, interstitials, antisite defects, Frenkel and Schottky defects in the 312 MAX phase Ti₃SiC₂. The formation energies of defects are obtained according to the elemental chemical potentials which are determined by the phase stability conditions. The most stable self-interstitials are all found in the hexahedral position surrounded by two Ti(2) and three Si atoms. For the entire elemental chemical potential range considered, our results demonstrated that Si and C related defects, including vacancies, interstitials and Frenkel defects are the most dominant defects. Besides, the present calculations also reveal that the formation energies of C and Si Frenkel defects are much lower than those of all Schottky defects considered. In addition, the calculated profiles of densities of states for the defective Ti₃SiC₂ indicate that these defects should have great influence on its thermal and electrical properties.     

First-principles study of polarization and piezoelectricity behavior in tetragonal PbTiO_3-based superlattices

Using first-principles calculation, the contribution of A-site and B-site atoms to polarization and piezoelectricity d33 in the tetragonal Pb TiO_3/KNbO_3 and PbTiO_3/LaAlO_3 superlattices is investigated in this paper. It is shown that PbTiO_3/KNbO_3 superlattice has larger polarization and d33 than PbTiO_3/LaAlO_3 superlattice, because there is stronger charge transfer between A(B)-site atoms and oxygen atom in PbTiO_3/KNbO_3 superlattice. InPbTiO_3/KNbO_3 superlattice,B-site atoms(Ti, Nb) make larger contribution to the total polarization and d33 than the A-site atoms(Pb, K) because of the strong covalent interactions between the transition metal(Ti, Nb) and the oxygen atoms, while piezoelectricity in PbTiO_3/LaAlO_3 superlattice mainly ascribes to piezoelectric contribution of Pb atom and Ti atom in PbTiO_3 component.Furthermore, by calculating the proportion of the piezoelectric contribution from Pb TiO_3 component in superlattices, we find there is different response of strain to piezoelectric contribution from Pb TiO_3 component in two superlattices but still with a value larger than 50%. InPbTiO_3/KNbO_3 superlattice, the c-axis strain reduces the proportion, especially under tensile condition. Meanwhile in PbTiO_3/LaAlO_3 superlattice, Pb TiO_3 plays a leading role to the total d33, especially under compressive condition, and the proportion decreases as the tensile strain increases.     

First-principles study of intrinsic defect properties in hexagonal BN bilayer and monolayer

The formation energies and transition energy levels of intrinsic defects in hexagonal BN (h-BN) bilayer and monolayer have been studied by first-principles calculations based on density functional theory. Our calculated results predict that excellent intrinsic p-type and n-type conductivities are very difficult to be realized in h-BN bilayer and monolayer. This is because of the high formation energies of acceptor-like defects (≥4.6 eV ) and the rather deep transition energy levels of donor-like defects (≥2.0 eV ). In order to obtain h-BN layers with more efficient p-type and n-type conductivity, extrinsic doping using foreign impurities is necessary.     

Temperature-dependent rectifying and photovoltaic characteristics of an oxygen-deficient Bi_2Sr_2Co_2O_y/Si heterojunction

A Bi2Sr2Co2Oy/Si heterojunction is obtained by growing a layer of p-type oxygen-deficient Bi2Sr2Co2Oy film on a commercial n-type silicon wafer by pulsed laser deposition. Its rectifying and photovoltaic properties are studied in a wide temperature range from 20 K to 300 K. The transport mechanism under the forward bias can be attributed to a trap- filled space-charge-limited current conduction mechanism. Under the irradiation of a 532-nm continuous wave laser, a clear photovoltaic effect is observed and the magnitude ofphotovoltage increases as the temperature decreases, The results demonstrate the potential application of a Bi2SrzCo2Oy-based heterojunction in the photoelectronic devices.     

Zn空位对Al-P共掺杂ZnO电子结构影响的第一性原理计算

采用第一性原理平面波赝势法计算ZnO（Al,P）体系的晶格参数和电子结构,重点分析Zn空位对体系晶体结构、形成能、态密度的影响.计算结果表明：Al和P共掺杂过程中,Al_Zn-P_Zn有更低的形成能,能带分析呈现n型.并随着Zn空位浓度的增大使得掺杂后的晶胞体积减小,晶格常数c先增大后减小.存在Zn空位的掺杂体系形成能比Al_Zn-P_O掺杂体系低,体系较稳定.能带分析呈现p型趋势.Al和P以1∶2的比例掺杂时,体系的形成能降低,体系更稳定;同时,比较1个V_Zn和2个V_Zn的Al_Zn-P_Zn共掺杂体系的能带结构发现,随着Zn空位浓度增大,带隙增大,体系p型化特征增强.Al_Zn-2P_Zn共掺杂体系带隙减小为0.56 eV,更有利于提高其导电性质.然而出现2V_Zn后,带隙增大为0.73 eV,小于本征ZnO带隙,p型化程度更强烈;此外态密度分析表明2V_Zn的Al_Zn-2P_Zn共掺杂使得态密度更加分散,更多的电子穿过费米能级使得p型化更明显.因此,将Al/P按1∶2的比例共掺且Zn空位增至2个时,可以获得导电性能更好的p型ZnO.     

Characterisation of atomic-scale defects in ODS ferritic alloys by positron annihilation

The vacancy-type defects and their local chemical environment in different ODS alloys produced in the USA (14YWT), China (K5) and Russia (ODS EP-450) are studied. The Angular Correlation of Annihilation Radiation (ACAR), which is one of the positron annihilation spectroscopy method, was used. It was shown that in all alloys, except 14YWT, the dominant type of positron traps are vacancy-like defects, localised in matrix or associated with dislocations and/or interfaces of the incoherent particles. In the case of 14YWT alloy, which contains Y–Ti–O nanoclusters of a high density, the positrons confine and annihilate at O-vacancy pairs or complexes within nanoclusters. It is testified by enhanced electron density in annihilation sites and neighbourhood of Ti and Y atoms. These results, obtained by the ACAR method, indicate that the vacancies play an essential role in the formation of nanoclusters in ODS 14YWT alloy as it was theoretical predicted by first-principle calculations.     

First-Principles Study of Intrinsic Point Defects of Monolayer GeS

The properties of six kinds of intrinsic point defects in monolayer GeS are systematically investigated using the“transfer to real state”model,based on density functional theory.We find that Ge vacancy is the dominant intrinsic acceptor defect,due to its shallow acceptor transition energy level and lowest formation energy,which is primarily responsible for the intrinsic p-type conductivity of monolayer GeS,and effectively explains the native p-type conductivity of GeS observed in experiment.The shallow acceptor transition level derives from the local structural distortion induced by Coulomb repulsion between the charged vacancy center and its surrounding anions.Furthermore,with respect to growth conditions,Ge vacancies will be compensated by fewer n-type intrinsic defects under Ge-poor growth conditions.Our results have established the physical origin of the intrinsic p-type conductivity in monolayer GeS,as well as expanding the understanding of defect properties in lowdimensional semiconductor materials.     

SrTiO3薄膜生长初期Sr，Ti，O原子分子反应机理的理论研究

在激光分子束外延(LMBE)生长SrTiO₃(STO)薄膜过程中,激光闪蒸出的Sr,Ti,O原子的微观反应过程及粒子形态是STO薄膜生长初期形成的关键.采用密度泛函理论中的广义梯度近似(DFT/GGA)方法,在PW91/DNP 水平上研究了Sr,Ti,O原子在真空中的优先反应过程和形态,计算研究了SrO,TiO₂和STO分子形成的反应机理,获得了相应的中间体和过渡态及反应活化能,并运用前线轨道理论分析了STO分子的形成机理.对比计算了STO分子可能的几何构型,得 关键词： 3薄膜')" href="#">SrTiO₃薄膜 反应机理 活化能