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.     

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.     

Organic molecular rectifiers

In the field of molecular scale electronics the drive is towards the fabrication of self-assembled, organic, nanoscale architectures which will have an active role to play in novel electronic devices. As a formative step towards this goal the creation of an organic analogue to the p–n junction was proposed by Aviram and Ratner in the 1970s. In their proposal a monomolecular layer of a charge transfer species controls current flow between a pair of metal electrodes, allowing easy flow for only one polarity of the applied voltage. Such metal/molecular layer/metal structures have now been fabricated, utilising the self-ordering properties of Langmuir–Blodgett films to form the organic layer, with one dimension of the device being reduced to the molecular scale. The fabrication techniques involved in the generation of these M/LB/M junctions are now described along with the present understanding of conduction mechanisms through such nanoscale thickness junctions. These structures clearly show that the organic molecular layers can control current passage in electronic devices emulating some of the characteristics of an inorganic semiconducting p–n junction.     

Balancing Intermolecular and Molecule–Substrate Interactions in Supramolecular Assemblies

Self‐assembly of functional supra‐molecular nanostructures is among the most promising strategies for further development of organic electronics. However, a poor control of the interactions driving the assembling phenomena still hampers the tailored growth of designed structures. Here exploration of how non‐covalent molecule‐substrate interactions can be modified on a molecular level is described. For that, mixtures of DIP and F16CuPc, two molecules with donor and acceptor character, respectively are investigated. A detailed study of their structural and electronic properties is performed. In reference to the associated single‐component layers, the growth of binary layers results in films with strongly enhanced intermolecular interactions and consequently reduced molecule‐substrate interactions. This new insight into the interplay among the aforementioned interactions provides a novel strategy to balance the critical interactions in the assembly processes by the appropriate choice of molecular species in binary supra‐molecular assemblies, and thereby control the self‐assembly of functional organic nanostructures.     

Controlling Electrochemically Induced Volume Changes in Conjugated Polymers by Chemical Design: from Theory to Devices

Electrochemically induced volume changes in organic mixed ionic-electronic conductors (OMIECs) are particularly important for their use in dynamic microfiltration systems, biomedical machinery, and electronic devices. Although significant advances have been made to maximize the dimensional changes that can be accomplished by OMIECs, there is currently limited understanding of how changes in their molecular structures impact their underpinning fundamental processes and their performance in electronic devices. Herein, a series of ethylene glycol functionalized conjugated polymers is synthesized, and their electromechanical properties are evaluated through a combined approach of experimental measurements and molecular dynamics simulations. As demonstrated, alterations in the molecular structure of OMIECs impact numerous processes occurring during their electrochemical swelling, with sidechain length shortening decreasing the number of incorporated water molecules, reducing the generated void volumes and promoting the OMIECs to undergo different phase transitions. Ultimately, the impact of these combined molecular processes is assessed in organic electrochemical transistors, revealing that careful balancing of these phenomena is required to maximize device performance.     

Photoluminescence-Based Electron and Lattice Temperature Measurements in GaN-Based HEMTs

Nitride-based semiconductors are gaining importance not only for high-power applications but also for high-temperature electronic devices. Using photoluminescence (PL) techniques, it is now possible to simultaneously determine the temperatures of the lattice and hot electrons in these devices. Therefore, it is possible to use PL mapping measurements to derive temperature profiles for electrons and the lattice in the active region of an operating device with a single set of measurements. This work presents an experimental process to construct such spatially resolved temperature maps for a planar semiconductor device under bias and applies this approach to a specific example using the conductive channels of a biased AlGaN/GaN high-electron-mobility transistor. Studying the temperature distribution inside the conductive channels will help understand how electrons flowing in the device interact with the lattice as well as the process of heat generation within the device.     

Metrology Challenges for Emerging Research Devices and Materials

As silicon complimentary metal-oxide-semiconductor (CMOS) technology approaches its limits, new device structures and computational paradigms will be required to replace and augment standard CMOS devices for ULSI circuits. These possible emerging technologies span the realm from transistors made from silicon nanowires to heteroepitaxial layers for spin transistors to devices made from nanoscale molecules. One theme that pervades these seemingly disparate emerging technologies is that the electronic properties of these nanodevices are extremely susceptible to small perturbations in structural and material properties such as dimension, structure, roughness, and defects. The extreme sensitivity of the electronic properties of these devices to their nanoscale physical properties defines a significant need for precise accurate metrology. This paper will describe some of the most critical metrology required to characterize materials and devices in the research and exploratory stage and how these requirements would potentially change if these research devices were to start into a technology development effort     

An On‐Chip Break Junction System for Combined Single‐Molecule Conductance and Raman Spectroscopies

Systems that are capable of robustly reproducing single‐molecule junctions are an essential prerequisite for enabling the wide‐spread testing of molecular electronic properties, the eventual application of molecular electronic devices, and the development of single‐molecule based electrical and optical diagnostics. Here, a new approach is proposed for achieving a reliable single‐molecule break junction system by using a microelectromechanical system device on a chip. It is demonstrated that the platform can (i) provide subnanometer mechanical resolution over a wide temperature range (≈77–300 K), (ii) provide mechanical stability on par with scanning tunneling microscopy and mechanically controllable break junction systems, and (iii) operate in a variety of environmental conditions. Given these fundamental device performance properties, the electrical characteristics of two standard molecules (hexane‐dithiol and biphenyl‐dithiol) at the single‐molecule level, and their stability in the junction at both room and cryogenic temperatures (≈77 K) are studied. One of the possible distinctive applications of the system is demonstrated, i.e., observing real‐time Raman scattering in a single‐molecule junction. This approach may pave a way to achieving high‐throughput electrical characterization of single‐molecule devices and provide a reliable platform for the convenient characterization and practical application of single‐molecule electronic systems in the future.     

Atomistic study of three-leg molecular devices

Molecular electronics is one of the promising technologies for future electronic applications that is currently gaining a lot of interest. This is because if single molecule could be used as active electronic components this would provide an ultimate device miniaturization. Previously studied molecules provide almost exclusively two terminal devices. In this paper, three-leg molecular devices are examined employing a first-principles study based on density functional theory coupled to the non-equilibrium Green’s function formalism. We illustrate the feasibility of building a prototype molecular transistor using three-leg molecules directly contacted to gold electrodes. We discuss the different factors that control the transport through this molecular transistor. Moreover, we show that a functional standalone NAND logic gate can be implemented using a single three-leg molecular device.     

Toward comprehensive reliability assessment of electronics by a combined loading approach

Many electronic applications, such as portable handheld devices or automotive electronics, experience various loadings during their common operation. Recent investigations have shown, however, that the interactions of the different load components can be highly significant. The commonly employed standardized single load tests neglect these interactions and, therefore, do not represent well enough the use environment loading conditions of many electronic devices. Thus, it has become clear that modifications to the reliability evaluation procedures are necessary. But before loading conditions can be combined in a meaningful manner, the failure mechanisms under single load environments and their possible interactions must be clarified. This paper makes a brief review to the reliability of electronic assemblies under different loading conditions from the perspective of failure modes and mechanisms. The failure modes and mechanisms under pure thermal cycling, power cycling, mechanical shock impact, or vibration conditions are discussed first. Thereafter, the interactions of the loading conditions, when they are combined consecutively or concurrently, are discussed.     

Doping of semiconductors by molecular monolayers: monolayer formation,dopant diffusion and applications

The continuous miniaturization in the semiconductor industry brings electronic devices with higher performance at lower cost. The doping of semiconductor materials plays a crucial role in tuning the electrical properties of the materials. Ion implantation is currently widely used. Yet, this technique faces challenges meeting the requirements for smaller devices. Monolayer doping (MLD) has been proposed as one of the alternative techniques for doping semiconductors. It utilizes dopant-containing organic molecules and grafts them onto semiconductor surfaces. The dopant atoms are subsequently driven into the substrate by high temperature annealing. MLD has shown the capability for ultra-shallow doping and the doping of 3-D structures without causing crystal damage. These features make this technique a promising candidate to dope future electronic devices. In this review the processes for monolayer formation and dopant incorporation by annealing will be discussed, as well as the applications of MLD in device fabrication.     

DNA nanorobotics

This paper presents a molecular mechanics study of nanorobotic structures using molecular dynamics (MD) simulations coupled to virtual reality (VR) techniques. The operator can design and characterize through molecular dynamics simulation the behavior of bionanorobotic components and structures through 3D visualization. The main novelty of the proposed simulations is based on the mechanical characterization of passive/active robotic devices based on double stranded DNA molecules. Their use as new DNA-based nanojoint and nanotweezer are simulated and results are discussed.     

Dopant incorporation in GaAs and AlGaAs grown by MOMBE for high speed devices

Continued improvement in GaAs/AlGaAs device technology requires higher doping levels, both to reduce parasitics such as source resistances, and to enhance speed in devices such as the heterostructure bipolar transistor (HBT). In this paper we will discuss doping issues which are critical to high speed performance. In particular, we will focus on doping of GaAs and AIGaAs using carbon as the acceptor and Sn as the donor. Due to the unique growth chemistry of metalorganic molecular beam epitaxy (MOMBE), both of these impurities can be used to achieve high doping levels when introduced from gaseous sources such as trimethylgallium (TMG) or tetraethyltin (TESn). Comparison of SIMS and Hall measurements show that both elements give excellent electrical activation to 1.5 × 10¹⁹ cm³ for Sn and 5 × 10²⁰ cm⁻³ for C. More importantly, we have found that both impurities canbe used to achieve high quality junctions, indicating that little or no diffusion or segregation is occurring during growth. Because of the excellent incorporation behavior of these dopants, we have been able to fabricate a wide range of devices including field effect transistors (FETs), high electron mobility transistors (HEMTs), and Pnp HBTs whose performance equals or exceeds that of similar devices grown by other techniques. In addition to these results, we will briefly discuss the key differences in growth kinetics which allow such abruptness and high doping levels to be achieved more readily in MOMBE than in other growth techniques.     

Enhancing Organic Semiconductor Molecular Packing Using Perovskite Interfaces to Improve Singlet Fission

Singlet fission, a process by which one singlet exciton is converted into two lower energy triplet excitons, is sensitive to the degree of electronic coupling within a molecular packing structure. Variations in molecular packing can be detrimental to triplet formation and triplet–triplet separation, ultimately affecting the harvesting of triplets for electricity in organic photovoltaic devices. Here, six phase-pure molecular packing structures of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) with varying optoelectronic properties are isolated using 2D lead halide perovskites as tunable, crystalline surfaces for crystallization. Transient absorption spectroscopy reveals that while triplet formation is fast (<100 fs) regardless of template structure, the increased ordering in perovskite-templated samples speeds up triplet–triplet separation and recombination, providing evidence that the benefits of phase-purity offset minor variations in molecular packing. Molecular dynamics modeling of the interface reveals that perovskite-templating allows for closer packing of TIPS-pentacene molecules for all perovskite templates. With an extensive number of organic molecule-perovskite pairings, this work provides a methodology to use ordered, periodic surfaces to elucidate structure–property relationships of small organic molecules in order to adjust structural or optoelectronic responses, such as molecular packing and singlet fission.     

Differentiation of source/drain breakdown mechanisms by a singlering transistor measurment

A measurement technique using a single device to differentiate the effects of junction curvature, surface fields, and isolation implants in MOS source/drain breakdown is described. This use of a single device may complement existing measurement techniques, or be useful for design of test structures in the streets (scribe channels), where the number of devices is pad limited     

Fermi Level Pinning and Orbital Polarization Effects in Molecular Junctions: The Role of Metal Induced Gap States

Understanding the alignment of molecular orbitals and corresponding transmission peaks with respect to the Fermi level of the electrodes is a major challenge in the field of molecular electronics. In order to design functional devices, it is of utmost importance to assess whether controlled changes in the electronic structure of isolated compounds are preserved once they are inserted in the molecular junctions. Here, light is shed on this central issue by performing density functional theory calculations on junctions including diarylethene‐based molecules. It is demonstrated that the chemical potential equalization principle allows to rationalize the existence or not of a Fermi level pinning (i.e., same alignment in spite of a varying ionization potential in the isolated compounds), pointing to the essential role played by metal induced gap states (MIGS). It is further evidenced that the degree of level pinning is intimately linked to the degree of orbital polarization when a bias is applied between the two electrodes.     

Facet Junction Engineering for Photocatalysis: A Comprehensive Review on Elementary Knowledge,Facet-Synergistic Mechanisms,Functional Modifications,and Future Perspectives

A facet junction (also called facet heterojunction or surface heterojunction) is defined as a complex polyhedral single crystal exposed to two, three, or four types of crystallographic planes, and is a promising platform for achieving multifunctional photocatalysis. Compared with simple polyhedral counterparts enclosed by the same crystallographic planes or conventional heterogeneous junctions, facet junctions have notable facet-crosslinking effects arising from different electronic structures of heterogeneous facets, which is beneficial for promoting the transfer and separation of photogenerated charge carriers in a single-crystalline semiconductor without the introduction of external species or interfaces. However, there are few specialized review articles on facet junction engineering of photocatalysts. Herein, an overview of facet junction-based photocatalysts is provided based on the following aspects: elementary knowledge, microstructural features, intrinsic facet-synergistic mechanisms, functional modifications, enhanced photocatalytic mechanisms, challenges and issues, and directions for future investigations. This review article will act as a theoretical base for researchers who are focused on facet-dependent effects to design and fabricate new photocatalysts.     

Time-dependent signatures of acoustic wave biosensors

Acoustic wave devices coated with a biolayer represent one biosensor approach for the detection of medically relevant biomolecules. In a typical application, the acoustic wave device is connected in an oscillator circuit, and the frequency shift /spl Delta/f resulting from a biomolecular event is recorded. In this paper, we discuss our recent work in this field, which has included the use of Rayleigh wave surface acoustic wave devices for vapor phase detection as well as quartz crystal microbalance devices for liquid phase measurements. For all of the results reported herein the biofilm on the surface of the acoustic wave device consists of a layer of antibodies raised against a specific target molecule or antigen. We present our results for the vapor phase detection of small molecules such as uranine and cocaine as well as liquid phase detection of small and large molecules. The data we present from these various experiments is the signature associated with the biomolecular recognition events; that is, we record and present /spl Delta/f(t). Finally, we present the recent results of our time-dependent perturbation theory work, which gives a potential method for resolving the acoustic wave biosensor signature into information relating to molecular structure changes during a molecular recognition event.     

Wafer-chip assembly for large-scale integration

A technique has been developed for achieving a very high density interconnection of active silicon devices to permit the fabrication of large electronic subsystems in essentially monolithic form. The technique has been used to assemble a MOS 2000-bit shift register containing 12 000 MOS transistors on a 300 by 600 mils silicon substrate. The register utilizes ten 200-bit shift-register chips, each containing 1200 transistors. Four-phase MOS logic techniques are used to obtain very low power (0.1 mW/bit) and/or high frequency (10 MHZ) operation. In the technique used to assemble the 2000-bit shift register, silicon large-scale array chips are face-down bonded in adjoining positions on a larger silicon wafer section which may contain additional layers of interconnections and/or active devices as required to form a complete system subassembly. Since the same photoengraving technology is used in the substrate as on the chips, very high packing densities can be achieved, with minimum chip area required for interconnections. This approach also minimizes the parasitic capacitance associated with more conventional techniques for encapsulating and interconnecting large-scale arrays. In the case of MOS circuits, large area-buffer devices are not needed due to the small capacitance in the wafer-chip interconnections. Various techniques have been evolved for processing the chips and substrates produce contact regions which permit the required high fabrication yields. The bonding conditions and metallurgical systems used to date in fabricating large shift-register assemblies will be described and compared with other approaches.     

Miniaturisierung — der Motor der Entwicklung in der Elektronik

At the beginning of this millennium the spiral of miniaturization of electronic components and assemblies is turning increasingly faster. Trendsetters for innovations are mainly telecommunications, consumer electronics, and automotive electronics. Key drivers of the fast progress are new generations of semiconductor circuits with incredibly fine structures. The new trends have a strong impact on almost all ranges and processes in electronics manufacturing. Printed circuit board techniques as well as assembling techniques have to be adapted to suit the decreasing pitch of components, the continuously increasing component density and the rising clock frequency of the integrated circuits. Moreover, the cost-push is related to increasing speed and precision requirements on pick-and-place-machines. Their repeating accuracy is more and more characterized only by statistic terms. The growing complexity of electronic devices requires also far-reaching innovations of test methods since due to miniaturization the accessibility to the test points with state-of-the-art methods becomes more difficult and even impossible. Under cost-push and time pressure developers and designers are challenged to create more and more miniaturized new quality products using new software tools. The spiral keeps on turning and is turning faster and faster. Starting with an overview of state-of-the-art production methods and problems related therewith the article wants to provide an understanding of current changes in the electronics manufacturing industry.