Dislocation structures of low-angle boundaries in Nb-doped SrTiO3 bicrystals

Dislocation core structures in low-angle boundaries of Nb-doped SrTiO₃ bicrystals were investigated by high-resolution electron microscopy. Bicrystals with tilt angles of 2°, 4°, 6° and 8° with respect to the [001] zone axis were prepared by joining two single crystals at 1873 K. All of the boundaries consisted of a regular array of dislocations whose spacing gradually decreased with an increase in tilt angle. Except for the 2° tilt-angle boundary, the dislocation cores exhibited a dissociation from a[010] into two partials of a/2[010] on (100). Furthermore, two kinds of dislocation core structures were observed; Sr–Sr atomic columns and Ti–O atomic columns inside the cores. In addition, it was found that the positioning of adjacent cores along the boundary tended to change from a linear form to a zig-zagg shape as the tilt angle was increased from 4° to 8°. In the case of the linear array, dislocation core structures including Sr–Sr columns or Ti–O columns alternately appear. In contrast, only one core structure was observed in the zig-zagged array. On the other hand, the dislocation cores in the 2°-tilt-angle boundary had another type of dissociation with a/2[110] or a/2[111] partials, which included the twist component at a tilt axis of [001].     

The electrode effect in reduced semiconducting Nb-doped SrTiO3 ceramics

Complex-plane impedance analysis was used to study the electrode effect in reduced semiconducting SrTiO₃ ceramics. The electrodes investigated were In, In-Ga, Ag and Ni. A real-axis intercept and a semicircular arc were observed for the In, In-Ga and Ag electrodes. But only a real-axis intercept was observed for the Ni electrode. Based on the observations, the electrode effect was modelled, and an equivalent circuit was proposed.     

Nb掺杂浓度对SrTiO3的电子结构和光学性能的影响

用第一性原理计算不同Nb掺杂浓度的n型Nb掺杂SrTiO3,研究了Nb掺杂浓度对SrTiO3的形成焓、电子结构和光学性能的影响.在Nb掺杂SrTiO3中Nb替位Ti原子,与实验结果一致.Nb掺杂SrTiO3的费米能级进入导带底部,Nb掺杂SrTiO3呈现n型半导体特征.从微观角度分析了Nb掺杂浓度对导电性的影响,1.11at％ Nb掺杂SrTiO3在可见光范围有强吸收,是一种有潜在应用的光催化材料;而1.67at％和2.5at％Nb掺杂SrTiO3是潜在的透明导电材料.     

Fabrication and characterization of epitaxial SrTiO3/Nb-doped SrTiO3 superlattices by double ECR ion beam sputter deposition

Epitaxial oxide superlattices (SLs) of SrTiO₃ (STO) and Nb-doped SrTiO₃(STNO) were fabricated on LaAlO₃ (LAO) (001) substrates by an ion beam sputter deposition (IBSD) system having double electron cyclotron resonance (ECR) ion guns. The [STO_x/STNO_y]₁₀ SLs were epitaxially grown at different stacking sequences (x = 6 nm, y = 1–6 nm) and maintained the periodicity z of 10. Structural properties and surface morphology are found to be strongly dependent on the STNO sublayer thickness (y). Highly strained SLs with two-dimensional growth mode is observed at smaller STNO sublayer thickness (y = 1 nm). With increasing to a critical thickness (y = 4 nm), the SLs are freely strained and transformed to three-dimensional growth mode. The results demonstrate that the double ECR-IBSD is a versatile technique for the growth of high-quality oxide SLs.     

BaTiO3 thin film prepared by coating-pyrolysis process on Nb-doped SrTiO3 substrate

Epitaxial BaTiO3 thin film was prepared on Nb-doped SrTiO3 substrate by coating-pyrolysis process using a mixed solution of barium and titanium naphthenates. The amorphous film pyrolyzed at 470 °C was crystallized with high orientation after heat-treatment at 850 °C under low oxygen partial pressure. X-ray diffraction /2 and scans indicated that the BaTiO3 film was epitaxial relationship with Nb-doped SrTiO3 substrate. The dielectric constant was approximately 230 at 103 Hz and was monotonically decreased with increasing of frequency at room temperature.     

Formation and investigation of p–n diode structures based on lanthanum manganites and Nb-doped SrTiO3

High quality La_2 / 3Ba_1 / 3MnO₃ (LBMO), La_2 / 3Ca_1 / 3MnO₃ (LCaMO) and La_2 / 3Ce_1 / 3MnO₃ (LCeMO) thin films were grown on Nb 0.1 wt.% doped conducting SrTiO₃(100) (STON) substrates. Asymmetric current–voltage relations measured for the LBMO/STON, LCaMO/STON and LCeMO/STON heterostructures at T = 78/300 K certified hole-doping of the manganite films. The diffusion voltage, corresponding to a steep current increase at forward bias has been estimated. The LCaMO/STON heterojunction showed possible impact of interfacial strain on the rectifying behavior, meanwhile, the LaCeMO/STON heterostructures demonstrated evidence of phase separation of the manganite film at the interface.     

Electronic properties and surface reactivity of SrO-terminated SrTiO3 and SrO-terminated iron-doped SrTiO3

Surface reactivity and near-surface electronic properties of SrO-terminated SrTiO₃ and iron doped SrTiO₃ were studied with first principle methods. We have investigated the density of states (DOS) of bulk SrTiO₃ and compared it to DOS of iron-doped SrTiO₃ with different oxidation states of iron corresponding to varying oxygen vacancy content within the bulk material. The obtained bulk DOS was compared to near-surface DOS, i.e. surface states, for both SrO-terminated surface of SrTiO₃ and iron-doped SrTiO₃. Electron density plots and electron density distribution through the entire slab models were investigated in order to understand the origin of surface electrons that can participate in oxygen reduction reaction. Furthermore, we have compared oxygen reduction reactions at elevated temperatures for SrO surfaces with and without oxygen vacancies. Our calculations demonstrate that the conduction band, which is formed mainly by the d-states of Ti, and Fe-induced states within the band gap of SrTiO₃, are accessible only on TiO₂ terminated SrTiO₃ surface while the SrO-terminated surface introduces a tunneling barrier for the electrons populating the conductance band. First principle molecular dynamics demonstrated that at elevated temperatures the surface oxygen vacancies are essential for the oxygen reduction reaction.     

生长温度对PLD原位生长SrTiO3薄膜结构与非线性介电性能的影响

采用脉冲激光沉积法 ( PLD ) 制备的Y1B2Cu3O7-x薄膜作为叉指底电极,然后生长 SrTiO3介质薄膜,形成叉指型压控电容结构。通过对 SrTiO3薄膜的原位生长温度与薄膜微观结构及非线性介电性能之间的关系研究,发现随生长温度的升高薄膜晶粒逐步增大然后变小,薄膜的介电常数可调率和本征介电损耗随晶粒大小的增大而增加,而非本征损耗则随晶粒取向的增加而减小。     

SrTiO3 glass ceramics

This is the first in a series of two papers describing the crystallization and dielectric properties of glass-ceramics derived from a particular strontium titanium aluminosilicate glass composition. This first paper concerns the development of crystalline phases and microstructure of glass-ceramics prepared under various crystallization conditions. In the following paper, the dielectric properties of these glass-ceramics are described and correlated with the characterization results.Perovskite strontium titanate (SrTiO₃) was the primary crystalline phase in glass-ceramics crystallized over the temperature range of 800 to 1100° C. At crystallization temperatures below 950° C, SrTiO₃ formed with a spherulitic or dendritic growth habit. X-ray diffraction suggested that the SrTiO₃ crystallized in a perovskite-like precursor phase which transformed to perovskite SrTiO₃ with further crystallization time. However, electron diffraction indicated that this precursor phase was cubic perovskite SrTiO₃. At higher crystallization temperatures, perovskite SrTiO₃ was present as individual crystallites without evidence of the spherulitic habit. The crystallization of SrTiO₃ was followed by that of other phases, the hexacelsian and anorthite forms of SrAl₃Si₂O₈, and the rutile and anatase forms of TiO₂. The crystallization sequence and microstructure of the glass-ceramics were determined by the competition for strontium and titanium between the crystallizing phases, SrTiO₃ and SrAl₂Si₂O₈, and TiO₂.     

Oxidation-reduction behaviour of La-doped SrTiO3

Thermogravimetric measurements as a function of oxygen activity were performed in the temperature range of 1200 to 1400° C on a series of lanthanum SrTiO₃ compounds. A model which assumes the absorption of excess oxygen in the structure which compensates the donors by formation of cationic defects is postulated. Comparison of this model with the experimental results show reasonable agreement.On leave from the Faculty of Engineering, Cairo University, Egypt.     

Role of microstructure in electronic transport behavior of highly crystallized undoped microcrystalline Si Films

Significant correlation is established between the observed electrical properties (room temperature value of dark conductivity and its activation energy) of the plasma deposited highly crystallized undoped hydrogenated microcrystalline silicon (μc-Si:H) films with their microstrucural properties (various aspects like fractional composition of constituent grains, morphology, and crystalline orientation). The conductivity shows three distinct trends with increasing film thickness irrespective of the different deposition parameters or conditions, and these three zones are applicable to all the samples, indicating that all the samples fundamentally belong to three different microstructural classes (with distinct microstructural attributes like thickness and features of grains and conglomerates) corroborative with these three zones of electrical behavior.     

Dielectric properties of donor-doped polycrystalline SrTiO3

Various donor dopants such as Y³⁺ and Nb⁵⁺ were incorporated in SrTiO₃ in amounts 2.5 mol % during sintering in air at 1450° C for 15 h. Dense ceramic materials with grains of optimum uniformity and largest size were obtained when the cation stoichiometry was adjusted to allow for charge compensation of the donor ions by strontium vacancies. For donor levels 0.3 mol%, the dielectric constant measured at 25° C and 1 kHz was up to two orders of magnitude higher than that of undoped strontium titanate. The increase in permittivity, however, was dependent on grain size, was influenced by the method of electrode application, and was suppressed by the presence of 0.1 mol% Mn in the ceramic. These observations, together with data obtained from electrical measurements at other temperatures and frequencies, were consistent with interpretation of the anomalously high dielectric constant as a boundary-layer effect resulting from semiconducting grains and weakly insulating grain-surfaces. It is suggested that donor doping influences the electrical properties of SrTiO₃ mainly by increasing the volatility of oxygen from the grains during sintering, and by decreasing the rate of re-oxidation during cooling.     

Surface Pyroelectricity in Cubic SrTiO3

Symmetry‐imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO₃ is not pyroelectric, the (100) surface of TiO₂‐terminated SrTiO₃ is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO₃. The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO₂ layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO₂ capping layer. The observation of pyroelectricity emerging at the SrTiO₃ surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo‐ and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.     

Doping of graphene exfoliated on SrTiO3

We present atomic force microscopy and scanning Kelvin probe data obtained under ultra-high vacuum conditions from graphene exfoliated on crystalline SrTiO(3) substrates. The contact potential difference shows a monotonic increase with the number of graphene layers until after five layers of saturation is reached. By identifying the saturation value with the work function of graphite we determine the work function of single and bilayer graphene to be Φ(SLG) = 4.409 ± 0.039 eV and Φ(BLG) = 4.516 ± 0.035 eV, respectively. In agreement with the higher work function of single-layer graphene with respect to free-standing graphene, our measurements indicate an accumulation of charge carriers corresponding to a doping of the exfoliated graphene layer with electrons.     

BaTiO3/SrTiO3超晶格的微结构研究

本实验研究利用激光分子束外延法（L－MBE）研究在SrTiO3(STO)(001)基片上生长的BaTiO3(BTO)/SrTiO3(STO)超晶格的微结构,利用小角X射线衍射光谱（SAXRD）的计算机模拟来获得BaTiO3/Sr-TiO3超晶格的微结构参数,如：总的膜厚度,超晶格周期,表面和界面的均方根粗糙度等。实验结果表明超晶格的表面和界面非常平整,均方根粗糙度大约为0.2nm,原子力显微镜（AFM）的实验研究已经证明了超晶格结构的平滑程度,超晶格的＜001＞方向存在着微弱的关联现象。