The Impacts of Cation Stoichiometry and Substrate Surface Quality on Nucleation,Structure, Defect Formation,and Intermixing in Complex Oxide Heteroepitaxy–LaCrO3 on SrTiO3(001)

The ability to design and fabricate electronic devices with reproducible properties using complex oxides is critically dependent on our ability to controllably synthesize these materials in thin‐film form. Structure‐property relationships are intimately tied to film and interface composition. Here the effect of cation stoichiometry on structural quality and defect formation in LaCrO₃ heteroepitaxial films prepared using molecular beam epitaxy is reported. From first principles the regions of stability of various candidate defects, along with the predicted effects of these defects on structural parameters, are calculated as a function of Cr and O chemical potential. Epitaxial LaCrO₃ films readily nucleate and remain coherently strained on SrTiO₃(001) over a wide range of La‐to‐Cr atom ratios, but La‐rich films are of considerably lower structural quality than stoichiometric and Cr‐rich films. Cation imbalances are accompanied by anti‐site defect formation. Cation mixing occurs at the interface for all La‐to‐Cr ratios investigated and is not quenched by deposition on SrTiO₃(001) at ambient temperature. Indiffused La atoms occupy Sr sites. Intermixing is effectively quenched by using molecular beam epitaxy to deposit LaCrO₃ at ambient temperature on defect free Si(001). However, analogous pulsed laser deposition on Si is accompanied by cation mixing.     

Tuning of a Vertical Spin Valve with a Monolayer of Single Molecule Magnets

The synthesis and the chemisorption from solution of a terbium bis‐phthalocyaninato complex suitable for the functionalization of lanthanum strontium manganite (LSMO) are reported. Two phosphonate groups are introduced in the double decker structure in order to allow the grafting to the ferromagnetic substrate actively used as injection electrode in organic spin valve devices. The covalent bonding of functionalized terbium bis‐phthalocyaninato system on LSMO surface preserves its molecular properties at the nanoscale. X‐ray photoelectron spectroscopy confirms the integrity of the molecules on the LSMO surface and a small magnetic hysteresis reminiscent of the typical single molecule magnet behavior of this system is detected on surface by X‐ray magnetic circular dichroism experiments. The effect of the hybrid magnetic electrode on spin polarized injection is investigated in vertical organic spin valve devices and compared to the behavior of similar spin valves embedding a single diamagnetic layer of alkyl phosphonate molecules analogously chemisorbed on LSMO. Magnetoresistance experiments have evidenced significant alterations of the magneto‐transport by the terbium bis‐phthalocyaninato complex characterized by two distinct temperature regimes, below and above 50 K, respectively.     

有机分子纳米线的合成及其导体性质

分子导线是分子电子学研究的重要内容,是分子电子器件的基础。结合分子导线的电子传导性对获取有机线性分子的方法进行了论述。指出共价合成法具有形貌和电子传递的可控性,但合成及纯化的困难限制了其进一步发展。自组装法尽管存在着结构缺陷,而且目标线性分子的直径和维度也难以控制,但其制备方法简单、灵活,所以是有机分子导线的主要研究方向。对一些研究者以自组装法为基础,通过分子间弱的相互作用、定向原子堆积、配位键等研发出的新的有机线性分子建构方法进行了介绍。目前,有机线性材料尽管在微纳电子器件的应用中很难与无机材料竞争,但其作为无机材料的补充,展示了良好的应用前景。     

Low‐Cost Post‐Growth Treatments of Crystalline Silicon Nanoparticles Improving Surface and Electronic Properties

Freestanding silicon nanocrystals (Si‐ncs) offer unique optical and electronic properties for new photovoltaic, thermoelectric, and other electronic devices. A method to fabricate Si‐ncs which is scalable to industrial usage has been developed in recent years. However, barriers to the widespread utilization of these nanocrystals are the presence of charge‐trapping defects and an oxide shell formed upon ambient atmosphere exposure hindering the charge transport. Here, we exploit low‐cost post‐growth treatment routes based on wet‐etching in hydrofluoric acid plus surface hydrosilylation or annealing enabling a complete native oxide removal and a reduction of the defect density by up to two orders of magnitude. Moreover, when compared with only H‐terminated Si‐ncs we report an enhancement of the conductivity by up to a factor of 400 for films of HF etched and annealed Si‐ncs, which retain a defect density below that of untreated Si‐ncs even after several months of air exposure. Further, we demonstrate that HF etched and hydrosilylated Si‐ncs are extremely stable against oxidation and maintain a very low defect density after a long‐term storage in air, opening the possibility of device processing in ambient atmosphere.     

Redox‐Induced Asymmetric Electrical Characteristics of Ferrocene‐Alkanethiolate Molecular Devices on Rigid and Flexible Substrates

The electrical properties of ferrocene‐alkanethiolate self‐assembled monolayers (SAMs) on a high yield solid‐state device structure are investigated. The devices are fabricated using a conductive polymer interlayer between the top electrode and the SAM on both silicon‐based rigid substrates and plastic‐based flexible substrates. Asymmetric electrical transport characteristics that originate from the ferrocene moieties are observed. In particular, a distinctive temperature dependence of the current (i.e., a decrease in current density as temperature increases) at a large reverse bias, which is associated with the redox reaction of ferrocene groups in the molecular junction, is found. It is further demonstrated that the molecular devices can function on flexible substrates under various mechanical stress configurations with consistent electrical characteristics. This study enhances the understanding of asymmetric molecules and may lead to the development of functional molecular electronic devices on both rigid and flexible substrates.     

Towards Integrated Molecular Electronic Devices: Characterization of Molecular Layer Integrity During Fabrication Processes

Reproducible carbon/molecule/Cu molecular junctions are made with high yield using diazonium reduction of aromatic molecules on carbon with direct evaporation of Cu as a top contact. This report investigates the stability of these devices in response to fabrication steps. Raman spectroscopy through a transparent support shows that direct deposition of Au or Cu causes little change in molecular layer structure, while Ti and Pt deposition cause significant damage to the molecules. AFM, Raman, and XPS examination of Au, Cu, and Ti devices after removal of deposited metal confirm that Cu and Au have minimal effects on molecular structure. However, the molecular layer is rougher after Au deposition, probably due to partial penetration of Au atoms into the molecular layer. Completed carbon/molecule/Cu devices can be heated to 250 °C without significant changes in electronic behaviour while nitroazobenzene molecular layers on carbon were unaffected by photolithography or by 5 min at 400 °C in vacuum. Completed devices could be sealed with parylene‐N, stabilizing them to aqueous etching solution. The stability of carbon/molecule/Cu junctions is due, in part, to the strong carbon–carbon bonding and aggressive nature of diazonium surface modification. The results significantly expand the range of processing variables compatible with molecular electronic junctions.     

The metal/organic monolayer interface in molecular electronic devices

The metal/molecules/metal is the basic device used to measure the electronic properties of organic molecules envisioned as the key components in molecular-scale devices (molecular diode, molecular wire, molecular memory, etc.). This review paper describes the main techniques used to fabricate a metal/molecules/metal device (or more generally electrode/molecules/electrode junctions, with electrodes made of metal or semiconductor). We discuss several problems encountered for the metallization of organic monolayers. The organic/electrode interface plays a strong role in the electronic properties of these molecular devices. We review some results on the relationships between the nature of the electrode/molecule interface (physisorbed or chemisorbed, evaporated metal electrode, mechanical contact, etc.) and the electronic transport properties of these molecular-scale devices. We also discuss the effects of symmetric versus asymmetric coupling of the two ends of the molecules with the electrodes.     

Surface‐Dominated Transport Properties of Silicon Nanowires

Surface effects are widely recognized to significantly influence the properties of nanostructures, although the detailed mechanisms are rarely studied and unclear. Herein we report for the first time a quantitative evaluation of the surface‐related contributions to transport properties in nanostructures by using Si nanowires (NWs) as a paradigm. Critical to this study is the capability of synthesizing SiNWs with predetermined conduction type and carrier concentration from Si wafer of known properties using the recently developed metal‐catalyzed chemical etching method. Strikingly, the conductance of p‐type SiNWs is substantively larger in air than that of the original wafer, is sensitive to humidity and volatile gases, and thinner wires show higher conductivity. Further, SiNW‐based field‐effect transistors (FETs) show NWs to have a hole concentration two orders of magnitude higher than the original wafer. In vacuum, the conductivity of SiNWs dramatically decreases, whereas hole mobility increases. The device performances are further improved by embedding SiNW FETs in 250 nm SiO₂, which insulates the devices from atmosphere and passivates the surface defects of NWs. Owing to the strong surface effects, n‐type SiNWs even change to exhibit p‐type characteristics. The totality of the results provides definitive confirmation that the electrical characteristics of SiNWs are dominated by surface states. A model based on surface band bending and carrier scattering caused by surface states is proposed to interpret experimental results. The phenomenon of surface‐dependent transport properties should be generic to all nanoscale structures, and is significant for nanodevice design for sensor and electronic applications.     

Si基纳米结构的电子性质

各种Si基纳米发光材料在Si基光电子器件及其全Si光电子集成技术中具有潜在的应用前景,从理论和实验上对其电子结构进行研究,有助于我们深化对其发光机制的认识与理解。本文主要从量子限制效应发光这一角度,着重介绍了Si纳米晶粒、Ge/Si量子点,SiO2/Si超晶格和超小尺寸Si纳米团簇等不同Si基纳米结构的电子性质以及它们与发光特性之间的关系。还讨论了介质镶嵌和表面钝化对其电子结构的影响。     

Implementing Thermometry on Silicon Surfaces Functionalized by Lanthanide‐Doped Self‐Assembled Polymer Monolayers

The thermal gradients generated at submicrometer scale by the millions of transistors contained in integrated circuits are becoming the key limiting factor for device integration in micro‐ and nanoelectronics. Noncontact thermometric techniques with high‐spatial resolution are, thus, essential for noninvasive off‐chip characterization and heat management on Si surfaces. Here, the first ratiometric luminescent molecular thermometer implemented in a self‐assembled polymer monolayer functionalized Si surface is reported. The functionalization of Si surfaces with luminescent thermometers constitutes a proof‐of‐concept that foretells a wide range of applications in Si‐based micro‐ and nanostructures. The thermometric functionalization of the Si surface with Tb³⁺ and Eu³⁺ complexes leads to a thermal sensitivity up to 1.43% K^?1, a cycle–recycle reliability of 98.6%, and a temperature uncertainty of less than 0.3 K. The functionalized surface presents reversible bistability that can be used as an optically active molecular demultiplexer.     

Electronic Contact Deposition onto Organic Molecular Monolayers: Can We Detect Metal Penetration?

Using a semiconductor as the substrate to a molecular organic layer, penetration of metal contacts can be clearly identified by the study of electronic charge transport through the layer. A series of monolayers of saturated hydrocarbon molecules with varying lengths is assembled on Si or GaAs and the junctions resulting after further electronic contact is made by liquid Hg, indirect metal evaporation, and a “ready‐made” metal pad are measured. In contrast to tunneling characteristics, which are ambiguous regarding contact penetration, the semiconductor surface barrier is very sensitive to any direct contact with a metal. With the organic monolayer intact, a metal–insulator–semiconductor (MIS) structure results. If metal penetrated the monolayer, the junction behaves as a metal–semiconductor (MS) structure. By comparing a molecule‐free interface (MS junction) with a molecularly modified one (presumably MIS), possible metal penetration is identified. The major indicators are the semiconductor electronic transport barrier height, extracted from the junction transport characteristics, and the photovoltage. The approach does not require a series of different monolayers and data analysis is quite straightforward, helping to identify non‐invasive ways to make electronic contact to soft matter.     

Dynamic Photoswitching of Electron Energy Levels at Hybrid ZnO/Organic Photochromic Molecule Junctions

The functionality of interfaces in hybrid inorganic/organic (opto)electronic devices is determined by the alignment of the respective frontier energy levels at both sides of the heterojunctions. Controlling the interface electronic landscape is a key element for achieving favourable level alignment for energy and charge transfer processes. Here, it is shown that the electronic properties of polar ZnO surfaces can be reversibly modified using organic photochromic switches. By employing a range of surface characterization techniques combined with density functional theory calculations, it is demonstrated that self‐assembled monolayers (SAMs) of photochromic phosphonic acid diarylethenes (PA‐DAEs) can be employed to reversibly change the electronic properties of polar ZnO/SAM structures by light stimuli. The highest occupied molecular orbital level of PA‐DAE is raised by 0.7 eV and the lowest unoccupied one lowered by 0.9 eV, respectively, upon illumination by ultraviolet light and the levels shift back to their original position upon illumination by green light. The results thus provide a pathway to tailor hybrid interface electronic properties in a dynamic manner upon simple light illumination, which can be exploited to reversibly tune the electrical properties of photoswitchable (opto)electronic devices.     

Reversible Switching Phenomenon in Diarylethene Molecular Devices with Reduced Graphene Oxide Electrodes on Flexible Substrates

Photoswitching molecular electronic devices with reduced graphene oxide (rGO) top electrodes on flexible substrates are fabricated and characterized. It has been reported previously that diarylethene molecular devices with poly‐(3,4‐ethylenedioxythiophene) stabilized with poly‐(4‐styrenesulfonic acid)/Au top electrodes can hold two stable electrical conductance states when the devices are exposed to UV or visible light during device fabrication. However, those devices fail to show the reversible switching phenomenon in response to illumination after device fabrication. By employing conducting and transparent rGO top electrodes, it is demonstrated that the diarylethene molecular devices show a reversible switching phenomenon, i.e., the fabricated devices change their conductance state in response to the alternating illumination with UV and visible light. Furthermore, the molecular devices with rGO top electrodes also exhibit good longtime stability and reliable electrical characteristics when subjected to various mechanical stresses (bending radius down to 5 mm and bending cycle over 10⁴).     

The Growth of Transition Metals on H‐Passivated Si(111) Substrates

The growth of Co and Ag layers on wet‐processed H‐passivated Si(111) substrates by molecular beam epitaxy (MBE) has been studied using high resolution scanning tunneling microscopy (STM) with regard to possible applications of the layers in magnetoelectronic devices. Roughness and intermixing at interfaces as functions of deposition temperature and layer thickness are key parameters for the performance of such devices. The initial growth of Co and Ag and the influence of Ag atoms on the Si(111) surface reconstructions provide insight into adatom–substrate interactions.     

Novel Heterogeneous Integration Technology of III–V Layers and InGaAs FinFETs to Silicon

Heterogeneous integration of III–V compound semiconductors to Si substrates is regarded as a necessary step for advancing high‐speed electronics and hybrid optoelectronic systems for data processing and communications, and is extensively being pursued by the semiconductor industry. Here, an innovative fab‐compatible, hybrid integration process of III–V materials to Si, namely InGaAs thin films to insulator‐on‐Si, is reported, and the first III–V FinFET devices on Si are demonstrated. Transfer of crystalline InGaAs layers with high quality to SiO2/Si is accomplished by the formation of a robust interfacial nickel‐silicide (NiSi) bonding interface, marking the first report for using silicides in III–V hybrid integration technology. The performance of optimally fabricated InGaAs FinFETs on insulator on Si is systematically investigated for a broad range of channel lengths and Fin perimeters with excellent switching characteristics. This demonstrates a viable approach to large‐scale hybrid integration of active III‐V devices to mainstream Si CMOS technology, enabling low‐power electronic and fully‐integrated optoelectronic applications.     

Synthesis and Local Probe Gating of a Monolayer Metal-Organic Framework

Achieving large-area uniform 2D metal-organic frameworks (MOFs) and controlling their electronic properties on inert surfaces is a big step toward future applications in electronic devices. Here a 2D monolayer Cu-dicyanoanthracene MOF with long-range order is successfully fabricated on an epitaxial graphene surface. Its structural and electronic properties are studied by low-temperature scanning tunneling microscopy and spectroscopy complemented by density-functional theory calculations. Access to multiple molecular charge states in the 2D MOF is demonstrated using tip-induced local electric fields. It is expected that a similar strategy could be applied to fabricate and characterize 2D MOFs with exotic, engineered electronic states.     

Effective Controlling of Film Texture and Carrier Transport of a High‐Performance Polymeric Semiconductor by Magnetic Alignment

The controlling of molecular orientation and structural ordering of organic semiconductors is crucial to achieve high performance electronic devices. In this work, large‐area highly oriented and ordered films of an excellent electron transporter Poly{[N,N′‐bis(2‐octyldodecyl)‐1,4,5,8‐naphthalenedicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)) are achieved by improved solution‐cast in high magnetic field. Microstructural characterizations reveal that the chain backbones of P(NDI2OD‐T2) are highly aligned along the applied magnetic field in the films. Based on the synchrotron‐based X‐ray diffraction analysis of the polymer films cast from different solvents, a mechanism which controls the alignment process is proposed, which emphasizes that molecular aggregates of P(NDI2OD‐T2) preformed in the solution initiate magnetic alignment and finally determine the degree of film texture. Furthermore, the time‐modulated magnetic field technique is utilized to effectively control the orientation of π‐conjugated plane of the backbones, thus the degree of face‐on molecular packing of P(NDI2OD‐T2) is enhanced significantly. Thin film transistors based on the magnetic‐aligned P(NDI2OD‐T2) films exhibit an enhancement of electron mobility by a factor of four compared to the unaligned devices, as well as a large mobility anisotropy of seven.     

Investigation on copper diffusion depth in copper wire bonding

Due to the dramatic price increase of precious metals, the replacement of Au with Cu in wire bonding has become an emerging trend for IC packaging nowadays. Similar to the Pb-free soldering transition, such a replacement is not just a simple drop-in material change. Comprehensive processing and reliability investigations are required before a mass production of electronic devices with Cu wire bonding can be implemented. However, among the existing studies on Cu wire bonding, it appears that most researchers just focused on issues above the wire bond pads. In fact, the Cu in the wire bonds may diffuse into the Si chip and impose reliability threats to the electronic devices. So far there was no research on the Cu-to-Si diffusion issue in Cu wire bonding. In this paper, an experimental study on the Cu-to-Si diffusion in Cu wire bond is reported. The Cu diffusion depth was characterized with the secondary ion mass spectrometry (SIMS) technique. Specimens with various configurations were designed and fabricated to investigate the effects of several parameters on the Cu-to-Si diffusion depth. The issues of concern include the amount of Cu supply, the bond pad deformation, and the barrier layer under the bond pad. In addition, some samples with conventional Au wire bonding were fabricated and tested in parallel for comparison.     

Electronic Structure of Self‐Assembled Monolayers on Au(111) Surfaces: The Impact of Backbone Polarizability

Modifying metal electrodes with self‐assembled monolayers (SAMs) has promising applications in organic and molecular electronics. The two key electronic parameters are the modification of the electrode work function because of SAM adsorption and the alignment of the SAM conducting states relative to the metal Fermi level. Through a comprehensive density‐functional‐theory study on a series of organic thiols self‐assembled on Au(111), relationships between the electronic structure of the individual molecules (especially the backbone polarizability and its response to donor/acceptor substitutions) and the properties of the corresponding SAMs are described. The molecular backbone is found to significantly impacts the level alignment; for molecules with small ionization potentials, even Fermi‐level pinning is observed. Nevertheless, independent of the backbone, polar head‐group substitutions have no effect on the level alignment. For the work‐function modification, the larger molecular dipole moments achieved when attaching donor/acceptor substituents to more polarizable backbones are largely compensated by increased depolarization in the SAMs. The main impact of the backbone on the work‐function modification thus arises from its influence on the molecular orientation on the surface. This study provides a solid theoretical basis for the fundamental understanding of SAMs and significantly advances the understanding of structure–property relationships needed for the future development of functional organic interfaces.     

Interface Modification to Improve Hole‐Injection Properties in Organic Electronic Devices

The performance of organic electronic devices is often limited by injection. In this paper, improvement of hole injection in organic electronic devices by conditioning of the interface between the hole‐conducting layer (buffer layer) and the active organic semiconductor layer is demonstrated. The conditioning is performed by spin‐coating poly(9,9‐dioctyl‐fluorene‐co‐N‐ (4‐butylphenyl)‐diphenylamine) (TFB) on top of the poly(3,4‐ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) buffer layer, followed by an organic solvent wash, which results in a TFB residue on the surface of the PEDOT:PSS. Changes in the hole‐injection energy barriers, bulk charge‐transport properties, and current–voltage characteristics observed in a representative PFO‐based (PFO: poly(9,9‐dioctylfluorene)) diode suggest that conditioning of PEDOT:PSS surface with TFB creates a stepped electronic profile that dramatically improves the hole‐injection properties of organic electronic devices.