Quantum oscillations in surface properties

Many properties of metallic thin films have been shown to oscillate with film thickness due to quantum size effects, i.e. the confinement of electrons inside epitaxial metal overlayers which causes quantization of the electronic states. This is a very general phenomenon and it affects both bulk properties of the films, such as resistivity or superconducting transition temperatures, and surface properties, such as chemical reactivity, diffusivity, thermal stability, i.e surface roughening transitions. In this paper we describe some of these thickness-dependent properties which affect the stability of nanostructures and allow us to tailor their properties. We shall concentrate in the paradigmatic example of thin films of Pb grown on metallic and semiconducting substrates and how one can achieve the growth of highly perfect, atomically flat, epitaxial films on different substrates, due to the kinetic constrains imposed by the presence of QSE, a topic increasingly important in the production of nanoscale quantum devices.     

Interface of transition metal oxides at the atomic scale

Remarkable phenomena arise at well-defined heterostructures, composed of transition metal oxides, which is absent in the bulk counterpart, providing us a paradigm for exploring the various electron correlation effects. The functional properties of such heterostructures have attracted much attention in the microelectronic and renewable energy fields. Exotic and unexpected states of matter could arise from the reconstruction and coupling among lattice, charge, orbital and spin at the interfaces. Aberration-corrected scanning transmission electron microscopy (STEM) is a powerful tool to visualize the lattice structure and electronic structure at the atomic scale. In the present study some novel phenomena of oxide heterostructures at the atomic scale are summarized and pointed out from the perspective of electron microscopy.     

Pulsed laser deposition of lead-zirconate-titanate thin films and multilayered heterostructures

There is an increasing interest in lead-zirconate-titanate (PZT) based ferroelectric thin film and devices in recent years. Pulsed laser deposition (PLD) technique has been demonstrated to be a versatile and successful tool for the deposition of epitaxial multi-component metal oxide films and heterostructures. This review presents a reasonable understanding of the relationship between PLD processing and composition, crystal structure and orientation of PZT ferroelectric thin films, and heterostructures. Processing-related issues from PLD of PZT thin films and material-integration strategies developed to fabrication of highly oriented or epitaxial PZT thin film based capacitors with excellent ferroelectric properties are discussed in detail. PACS 81.15.Fg; 68.55.Jk; 77.22.Ej     

Heteroepitaxial film growth of layered compounds with the ZrCuSiAs-type and ThCr2Si2-type structures: From Cu-based semiconductors to Fe-based superconductors

FeAs-based layered superconductors such as F-doped LaFeAsO have recently been investigated intensively because of their high superconducting transition temperatures. Epitaxial films of these compounds are important to examine their intrinsic materials properties as well as to transfer them to device applications. In this review, we first present our research route from transparent p-type oxides semiconductors to the Fe-based superconductors. Then we review growth of epitaxial thin films for the layered oxychalcogenides and oxypnictides. Reactive solid-phase epitaxy technique was inevitable to prepare epitaxial thin films of the oxychalcogenides and Zn-based oxypnictides. On the other hand, epitaxial thin films of Mn-based oxypnictides were grown by standard pulsed laser deposition. These techniques, however, did not grow epitaxial thin films for LaFeAsO. Thus, we developed a modified pulsed laser deposition process and succeeded in obtaining epitaxial thin films of FeAs-based superconductors, LaFeAsO and cobalt-doped SrFe₂As₂.     

Molecular beam epitaxy

Molecular beam epitaxy (MBE) is a process for growing thin, epitaxial films of a wide variety of materials, ranging from oxides to semiconductors to metals. It was first applied to the growth of compound semiconductors. That is still the most common usage, in large part because of the high technological value of such materials to the electronics industry. In this process beams of atoms or molecules in an ultra-high vacuum environment are incident upon a heated crystal that has previously been processed to produce a nearly atomically clean surface. The arriving constituent atoms form a crystalline layer in registry with the substrate, i.e., an epitaxial film. These films are remarkable because the composition can be rapidly changed, producing crystalline interfaces that are almost atomically abrupt. Thus, it has been possible to produce a large range of unique structures, including quantum well devices, superlattices, lasers, etc., all of which benefit from the precise control of composition during growth. Because of the cleanliness of the growth environment and because of the precise control over composition, MBE structures closely approximate the idealized models used in solid state theory.

This discussion is intended as an introduction to the concept and the experimental procedures used in MBE growth. The refinement of experimental procedures has been the key to the successful fabrication of electronically significant devices, which in turn has generated the widespread interest in the MBE as a research tool. MBE experiments have provided a wealth of new information bearing on the general mechanisms involved in epitaxial growth, since many of the phenomena initially observed during MBE have since been repeated using other crystal growth processes. We also summarize the general types of layered structures that have contributed to the rapid expansion of interest in MBE and its various offshoots. Finally we consider some of the problems that remain in the growth of heteroepitaxial structures, specifically, the problem of mismatch in lattice constant between layers and between layer and substrate. The discussion is phenomenological, not theoretical; MBE has been primarily an experimental approach based on simple concepts.     

Strain-mediated insulator-metal transition in topotactically hydro-reduced SrFeO_2

Controlling oxygen redox reactions in transition metal oxides offers an attractive route to tune their physical properties; a topotactic structural transformation from their parent phases effectively modifies the electronic state. In this work, infinitelayered SrFeO_2 thin films were produced from brownmillerite SrFeO_(2.5) via low-temperature hydro-reduction. After the structural transition, their out-of-plane lattice constants dramatically shrank by ~12%; tensilely strained samples exhibited metallic character, whereas the compressively strained ones maintained the insulating behavior of their bulk form. According to X-ray linear dichroism results, this strain-mediated electronic anisotropy may be attributed to electron redistribution within degenerated orbitals. This suggests a possible mechanism for the metallic conductivity of infinite-layered SrFeO_2, giving a hint for understanding emergent quantum phenomena, such as the recently discovered superconductivity in nickelates, and stimulating various applications, including in ionic conductivity and oxygen catalysis.     

Preparation of smooth Fe (001) on MgO (001)

The high mobility of metal adsorbates on perfect oxide surfaces favours an island-growth mode, which results in a granular morphology for metallic thin films. At low temperature, a fast substrate coverage is achieved due to the reduced mobility which, on the other hand, inhibits the development of an atomically smooth surface. In this study we used a controlled procedure to grow smooth epitaxial films of a few nanometres of Fe on MgO (001). The development of the dynamic conductivity as a measure of morphological roughness was observed during metal deposition by means of in-situ infrared spectroscopy. At various steps of thin-film preparation we used low-energy electron diffraction (LEED) for a characterisation of the crystalline surface quality. With ex situ atomic force microscopy (AFM) we investigated the surface topology of the prepared films with respect to smoothness. For example, for a controlled-growth 2-nm film, we observed a sharp LEED pattern and a metallic dynamic conductivity, and we did not find the distinct grooves which are characteristic of a granular structure. Received: 23 June 2000 / Accepted: 19 December 2000 / Published online: 21 March 2001     

Growth of SrTiO3 on Si(001) by hybrid molecular beam epitaxy

We report the heteroepitaxial growth of SrTiO₃ thin films on Si(001) by hybrid molecular beam epitaxy (hMBE). Here, elemental strontium and the metal‐organic precursor titanium tetraisopropoxide (TTIP) were co‐supplied in the absence of additional oxygen. The carbonization of pristine Si surfaces during native oxide removal was avoided by freshly evaporating Sr into the hMBE reactor prior to loading samples. Nucleation, growth and crystallization behavior as well a structural properties and film surfaces were characterized for a series of 46‐nm‐thick SrTiO₃ films grown with varying Sr to TTIP fluxes to study the effect of non‐stoichiometric growth conditions on film lattice parameter and surface morphology. High quality SrTiO₃ thin films with epitaxial relationship (001)SrTiO₃ || (001)Si and [110]SrTiO₃ || [100]Si were demonstrated with an amorphous layer of around 4 nm thickness formed at the SrTiO₃/Si interface. The successful growth of high quality SrTiO₃ thin films with atomically smooth surfaces using a thin film technique with scalable growth rates provides a promising route towards heterogeneous integration of functional oxides on Si. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Giant flexoelectric effect in ferroelectric epitaxial thin films

We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.     

The Atomic Structure of Oxide/Oxide Interface

The physical and chemical properties of thin or ultrathin oxide film deposited on another oxide bulk or thin film usually differ strongly from the bulk. The properties of the heterostructures ultimately rely on the structure and the chemistry of the oxide/oxide interface. Data in the literature indicated that atomically abrupt interfaces between oxides show abnormal electronic and magnetic properties. This article reviews the interfacial structures of oxide/oxide interfaces in an atomic scale. The origins of the unique physical and chemical properties at the oxide/oxide interfaces are also discussed.     

基于PbMg_(1/3)Nb_(2/3)O_3-PbTiO_3压电单晶的磁电复合薄膜材料研究进展

电子信息技术的迅速发展对磁电功能器件的微型化、智能化、多功能化以及灵敏度、可靠性、低功耗等都提出了更高的需求,传统的块体磁电功能材料已日渐不能满足上述需求,而层状磁电复合薄膜材料同时具有铁电性、铁磁性和磁电耦合等多种特性,因此能满足上述需求且有望应用于新一代磁电功能器件.层状磁电复合材料不仅具有非常丰富的物理现象和效应,而且在弱磁探测器、多态存储器、电写磁读存储器、电场可调低功耗滤波器、移相器、天线等微波器件中也具有广阔的应用前景,因而受到材料科学家和物理学家广泛的关注和研究.在层状磁电复合材料中,功能薄膜/铁电单晶异质结因其制备简单、结构设计和材料选择灵活以及电场调控方便和有效,最近十余年引起了越来越多的研究人员的兴趣.目前,以具有优异铁电和压电性能的(1-x)PbMg_(1/3)Nb_(2/3)O_3-PbTiO_3(PMN-PT)单晶作为衬底,构建功能薄膜/PMN-PT异质结已成为国内外多铁性复合薄膜材料研究领域的重要方向之一.相比于其他国家,我国科学家无论在发表的文章数量还是在文章被引用次数方面都处于领先地位,表明我国在功能薄膜/PMN-PT单晶异质结方面的研究卓有成效.迄今为止,研究人员已构建了锰氧化合物/PMN-PT、铁氧体/PMN-PT、铁磁金属/PMN-PT、稀磁半导体/PMN-PT、发光材料/PMN-PT、二维材料/PMN-PT、多层薄膜/PMN-PT、超导薄膜/PMN-PT等多种类型的异质结,在理论研究和实验方面都取得了丰富的研究成果.本文对基于PMN-PT压电单晶的磁电复合薄膜材料的研究进展进行了总结:简要介绍了与功能薄膜/PMN-PT异质结相关的研究论文发表现状;介绍了PMN-PT单晶在准同型相界附近的相图和应变特性;按照功能薄膜材料所属的体系对异质结进行了分类,并选取部分代表性的研究成果,介绍了材料的磁电性能和内涵的物理机制;最后就目前有待解决的问题和未来可能的应用方向进行了总结和展望.     

Pulsed laser deposition of complex oxide heteroepitaxy

The modern studies of complex oxides have been mainly driven by the development of advanced growth and characterization techniques, which provide researchers unprecedented access to new insights and functionalities of these materials. Epitaxial growth of thin films and related architectures offers a pathway to the discovery and stabilization of a wide spectrum of new possibilities in conjunction with the availability of high quality materials that produced with larger lateral sizes and being grown constrainedly. Compared with conventional growth techniques, such as sputtering, spin coating, sol–gel processes, metal-organic chemical vapor deposition, molecular beam epitaxy and so on, no other single advance in the creation of oxide materials has had as pronounced an impact as pulsed laser deposition. In pursuit of the fruitful functionalities and exciting physical phenomena among complex oxides, pulsed laser deposition technique has played an important role to fulfill the flurry of complex oxides in recent decades. In this article, we focus on the details of the growth of epitaxial oxide thin films and the related polymorphs, as well as recent advances in control of the oxide heteroepitaxy via pulsed laser deposition.     

在预刻蚀的衬底上通过分子束外延直接生长出拓扑绝缘体薄膜的微器件

在利用光刻将拓扑绝缘体外延薄膜加工成微米尺寸结构的过程中,所用的各种化学物质会导致薄膜质量的下降.在实验中,通过在钛酸锶衬底上预先光刻出Hall bar形状的凸平台并以此为模板进行拓扑绝缘体(Bi x Sb1-x)2Te3薄膜的分子束外延生长,直接获得了薄膜的Hall bar微器件,从而避免了光刻过程对材料质量的影响.原子力显微镜和输运测量结果均显示该微器件保持了(Bi x Sb1-x)2Te3外延薄膜原有的性质.这种新的微器件制备方法有助于在拓扑绝缘体中实现各种新奇的量子效应,并可推广于其他外延生长的低维系统.     

Epitaxial growth of bcc Fe films on Al(1 0 0) surfaces using Ti as an interface stabilizer

An approach is described to promote epitaxial growth of thin metal films on single-crystal metal substrates by stabilizing the interface with an extremely thin metallic interlayer. A single atomic layer of a metal is deposited at the interface, Ti on Al(1 0 0) in this case, prior to the growth of the metal film of interest to produce an epitaxial interface in a system that is otherwise characterized by interdiffusion and disorder. The stabilized interface reduces interdiffusion and serves as a template for ordered film growth. Using Rutherford backscattering and channeling techniques along with low-energy electron diffraction and low-energy He⁺ scattering, it is demonstrated that an atomically thin layer of Ti metal deposited at the Fe-Al interface, a system well known for considerable intermixing at room temperature, reduces interdiffusion and promotes the epitaxial growth of Fe films on the Al(1 0 0) surface. The decrease in ion scattering yield for Al atoms, Fe-Fe shadowing and long-range order of the surface suggest that the epitaxial growth of Fe is greatly improved when the Ti interlayer is introduced prior to Fe deposition. Off-normal ion channeling experiments provide clear evidence for the bcc structure of Fe on the Ti/Al(1 0 0) template with the measured average (1 0 0) interplanar distance of 1.44 Å in the Fe overlayer.     

Quantum phase transitions and vortex dynamics in superconducting networks

Josephson-junction arrays are ideal model systems to study a variety of phenomena such as phase transitions, frustration effects, vortex dynamics and chaos. In this review, we focus on the quantum dynamical properties of low-capacitance Josephson-junction arrays. The two characteristic energy scales in these systems are the Josephson energy, associated with the tunneling of Cooper pairs between neighboring islands, and the charging energy, which is the energy needed to add an extra electron charge to a neutral island. The phenomena described in this review stem from the competition between single-electron effects with the Josephson effect. They give rise to (quantum) superconductor–insulator phase transitions that occur when the ratio between the coupling constants is varied or when the external fields are varied. We describe the dependence of the various control parameters on the phase diagram and the transport properties close to the quantum critical points. On the superconducting side of the transition, vortices are the topological excitations. In low-capacitance junction arrays these vortices behave as massive particles that exhibit quantum behavior. We review the various quantum–vortex experiments and theoretical treatments of their quantum dynamics.     

Ultrafast strain engineering in complex oxide heterostructures

We report on ultrafast optical experiments in which femtosecond midinfrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, a long-lived five-order-of-magnitude increase of the electrical conductivity of NdNiO(3) epitaxial thin films is observed as a structural distortion propagates across the interface. Vibrational excitation, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates.     

Interfacial effects in electron transmission through Ag films on Cu(111)

The transmission coefficient for very low energy electrons ($\text{? 10}\text{eV}$) normally incident on thin epitaxial films of Ag on Cu(111) is modulated by reflection of the electronic wavefunction at the interfaces. The observation of such quantum size oscillations in this epitaxial system demonstrates that this effect is truly interfacial in origin.     

Electronic Transport and Thermopower in 2D and 3D Heterostructures—A Theory Perspective

The impact of interfaces and heterojuctions on the electronic and thermoelectric transport properties of materials is discussed herein. Recent progress in understanding electronic transport in heterostructures of 2D materials ranging from graphene to transition metal dichalcogenides, their homojunctions (grain boundaries), lateral heterojunctions (such as graphene/MoS₂ lateral interfaces), and vertical van der Waals heterostructures is reviewed. Work on thermopower in 2D heterojunctions, as well as their applications in creating devices such as resonant tunneling diodes (RTDs), is also discussed. Last, the focus turns to work in 3D heterostructures. While transport in 3D heterostructures has been researched for several decades, here recent progress in theory and simulation of quantum effects on transport via the Wigner and non‐equilibrium Green's functions approaches is reviewed. These simulation techniques have been successfully applied toward understanding the impact of heterojunctions on transport properties and thermopower, which finds applications in energy harvesting, and electron resonant tunneling, with applications in RTDs. In conclusion, tremendous progress has been made in both simulation and experiments toward the goal of understanding transport in heterostructures and this progress will soon be parlayed into improved energy converters and quantum nanoelectronic devices.     

Quantum oscillations in Pb/Si (111) heterostructure system

This paper summarizes our recent work on the study of quantum size effects (QSE) and novel physical properties of the Pb/Si (111) heterostructure. Two different types of samples were investigated. One is wedge-shaped Pb islands, and the other is atomically flat Pb thin films. With scanning tunneling microscopy (STM) manipulation, we observed an intriguing morphology dynamics of the islands that swings between two extreme energy states, like that in a classical pendulum. We show that the dynamics is a result of the competition between the QSE and the classical step free energy minimizing effect. For the second type of the samples, the QSE is studied in terms of thickness-dependent film stability, electronic structure and physical properties by using STM, angle-resolved photoemission spectroscopy (ARPES) and transport measurement. The results consistently reveal the formation of quantum well states (QWS) due to electron confinement in the films. This size effect could greatly modify the electronic structure near the Fermi level and lead to quantum oscillations in superconductivity, electron-phonon coupling and thermal expansion. The work unambiguously demonstrates the possibility of quantum engineering of physical properties of thin films by exploiting well-controlled and thickness-dependent QSE.     

Stability diagram for elastic domains in epitaxial ferroelectric thin films

The statics of isolated elastic domains (twins) in epitaxial thin tetragonal films grown on a cubic substrate is investigated theoretically. Different possible variants of the geometric shape of a domain are studied: plate, trapezoidal, and triangular. The nonuniform internal stresses, which also exist in polydomain epitaxial systems, are calculated by the effective-dislocation method. Hence the elastic energies stored in heterostructures with different domains are determined. The equilibrium width of a domain is calculated by minimizing the total internal energy of the heterostructure. Next, the stability diagram for isolated domains in epitaxial films is constructed from energy considerations. It is shown that in a large part of this diagram trapezoidal domains are energetically more advantageous than plate-shaped domains. The effect of an external electric field on the stability of 90° domains in epitaxial ferroelectric films is investigated. Fiz. Tverd. Tela (St. Petersburg) 39, 127–134 (January 1997)