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Schottky-barrier inhomogeneities are expected at the metal--transition-metal-dichalcogenide (TMDC) interface and this can impact device performance. However, it is difficult to account for the distribution of interface inhomogeneity as most techniques average over the spot area of the analytical tool (e.g., few hundred micrometers squared for photoelectron-based techniques), or the entire device measured for electrical current-voltage (I-V) measurements. Commonly used models to extract Schottky-barrier heights neglect or fail to account for such inhomogeneities, which can lead to the extraction of incorrect Schottky-barrier heights and Richardson constants that are orders of magnitude away from theoretically expected values. Here, we show that a Gaussian-modified thermionic emission model gives the best fit to experimental temperature-dependent current-voltage (I-V-T) data of van der Waals $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_{2}$ interfaces and allow the deconvolution of the Schottky-barrier heights of the defective regions from the pristine region. By the inclusion of a Gaussian-distributed Schottky-barrier height in the macroscopic I-V-T analysis, we demonstrate that interface inhomogeneities due to defects are deconvoluted and well correlated to the impact on the device behavior across a wide temperature range from a room temperature of 300 K down to 120 K. We verify the Gaussian thermionic model across two different types of p-${\mathrm{Mo}\mathrm{S}}_{2}$ (geological bulk crystals and synthetic flux-grown crystals), and finally compare the macroscopic Schottky-barrier heights with the results of a nanoscopic technique, ballistic hole emission microscopy (BHEM). The results obtained using BHEM are consistent with the pristine $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_{2}$ Schottky-barrier height extracted from the Gaussian-modified thermionic emission model over hundreds of nanometers. Our findings show that the inclusion of Schottky-barrier inhomogeneities in the analysis of I-V-T data is useful to elucidate the impact of defects (e.g., grain boundaries, metallic impurities, etc.) and hence their influence on device behavior. We also find that the effective Richardson constant, a material-specific constant typically treated as merely a fitting constant, is a useful parameter to check for the validity of the transport model.

The holy grail in nanoelectronics is the construction of nanodevices with high density, low cost, and high performance per device and per integrated circuit. One approach is the fabrication of surface nanostructures and atomic-scale templates via the autonomous assembly of atoms and/or molecules on well-defined surfaces. To steer the atomic or molecular growth processes and create a wide range of surface nanostructures with desired properties, a comprehensive understanding of the mechanisms that control the surface self-assembly processes is required. The capability to manipulate the nanodevices at the submolecular level with good controllability is also of paramount importance. This review highlights some key recent developments in the fabrication of low-dimensional nanostructures based on supramolecular self-assembly on predefined surfaces, with particular emphasis on the rapidly expanding field of two-dimensional materials. Special attention is also given to the latest progress in single-molecule manipulation for future device applications.

Oxide-free silicon chemistry has been widely studied using wet-chemistry methods, but for emerging applications such as molecular electronics on silicon, nanowire-based sensors, and biochips, these methods may not be suitable as they can give rise to defects due to surface contamination, residual solvents, which in turn can affect the grafted monolayer devices for practical applications. Therefore, there is a need for a cleaner, reproducible, scalable, and environmentally benign monolayer grafting process. In this work, monolayers of alkylthiols were deposited on oxide-free semiconductor surfaces using supercritical carbon dioxide (SCCO2) as a carrier fluid owing to its favorable physical properties. The identity of grafted monolayers was monitored with Fourier transform infrared (FTIR) spectroscopy, high-resolution X-ray photoelectron spectroscopy (HRXPS), XPS, atomic force microscopy (AFM), contact angle measurements, and ellipsometry. Monolayers on oxide-free silicon were able to passivate the surface for more than 50 days (10 times than the conventional methods) without any oxide formation in ambient atmosphere. Application of the SCCO2 process was further extended by depositing alkylthiol monolayers on fragile and brittle 1D silicon nanowires (SiNWs) and 2D germanium substrates. With the recent interest in SiNWs for biological applications, the thiol-passivated oxide-free silicon nanowire surfaces were also studied for their biological response. Alkylthiol-functionalized SiNWs showed a significant decrease in cell proliferation owing to their superhydrophobicity combined with the rough surface morphology. Furthermore, tribological studies showed a sharp decrease in the coefficient of friction, which was found to be dependent on the alkyl chain length and surface bond. These studies can be used for the development of cost-effective and highly stable monolayers for practical applications such as solar cells, biosensors, molecular electronics, micro- and nano- electromechanical systems, antifouling agents, and drug delivery.

Specific surfaces allowing Ultra-High Vacuum (UHV) investigations are required for the successful development of atomic nanostructures. Surface contamination, atomic roughness and defects density must be controlled in order to ensure the reliability of advanced UHV experiments. Surface preparation is a key parameter and is usually conducted in-situ in the UHV chamber. However the surface preparation requires a complex protocol. A microelectronic clean room environment enables the particles density control and enables 200mm wafer scale developments, especially Si(001)-(2x1):H surface reconstruction. However, this passivated surface is reactive and can be easily deteriorated, particularly during transportation. Consequently, a nanopackaging solution is proposed in order to provide a preserving environment for transportation. The nanopackaging process consists in the direct bonding of two passivated silicon surfaces, and is followed by a wafer dicing step into 1cm2 dies. Samples can be stored, shipped and in-situ opened without additional treatment. Furthermore, these samples can provide a reliable surface onto which molecular grafting can be accomplished. The fabrication modules and the associated characterization results will be described as well as atomic nanostructure manipulations and especially the grafting of a self-assembled monolayer using supercritical carbon dioxide as a medium.

Formation of dense monolayers with proven atmospheric stability using simple fabrication conditions remains a major challenge for potential applications such as (bio)sensors, solar cells, surfaces for growth of biological cells, and molecular, organic, and plastic electronics. Here, we demonstrate a single-step modification of organophosphonic acids (OPA) on 1D and 2D structures using supercritical carbon dioxide (SCCO2) as a processing medium, with high stability and significantly shorter processing times than those obtained by the conventional physisorption-chemisorption method (2.5 h vs 48-60 h).The advantages of this approach in terms of stability and atmospheric resistivity are demonstrated on various 2D materials, such as indium-tin-oxide (ITO) and 2D Si surfaces. The advantage of the reported approach on electronic and sensing devices is demonstrated by Si nanowire field effect transistors (SiNW FETs), which have shown a few orders of magnitude higher electrical and sensing performances, compared with devices obtained by conventional approaches. The compatibility of the reported approach with various materials and its simple implementation with a single reactor makes it easily scalable for various applications.

Compact electrical sources of surface plasmon polaritons (SPPs) are promising for integration with high-speed electronics. Being a highly compact source, the point dipole has the ability to directly couple to surface plasmon modes, and be electrically driven through the inelastic tunneling of electrons, for example, at the tip of a scanning tunneling microscope (STM). However, the directional control of electrically excited SPPs from such compact sources has not been demonstrated, despite its importance in controlling the optical energy flow on a chip. In this paper, we present a comprehensive analysis of the directional excitation of SPPs on Au 1D cavity by moving an STM tip relative to the edge of the cavity stripe and analyzing the light collected through an inverted microscope. The directional propagation of the SPP and its far-field emission exhibit a clear cyclic dependence on the relative distance from this edge. These results provide key steps toward realizing compact solid-state devices with the ...

Detailed low-temperature scanning tunneling microscope images of the Si(100)-2$\times$ 1-H and the Ge(100)-2$\times$ 1-H surfaces show a remarkable contrast inversion between filled- and empty-state images where the hydrogen dimer rows appear bright for filled-state images and dark for empty-state images. This contrast inversion originates from the change in the dominant surface states and their coupling to the tip apex and the bulk channels as a function of the bias voltage.

Diffusion barriers prevent materials from intermixing (e.g., undesired doping) in electronic devices. Most diffusion barrier materials are often very specific for a certain combination of materials and/or change the energetics of the interface because they are insulating or add to the contact resistances. This paper presents graphene (Gr) as an electronically transparent, without adding significant resistance to the interface, diffusion barrier in metal/semiconductor devices, where Gr prevents Au and Cu from diffusion into the Si, and unintentionally dope the Si. We studied the electronic properties of the n-Si(111)/Gr/M Schottky barriers (with and without Gr and M=Au or Cu) by I(V) measurements and at the nanoscale by ballistic electron emission spectroscopy (BEEM). The layer of Gr does not change the Schottky barrier of these junctions. The Gr barrier was stable at 300 °C for 1 h and prevented the diffusion of Cu into n-Si(111) and the formation of Cu3Si. Thus, we conclude that the Gr is mechanically and chemically stable enough to withstand the harsh fabrication methods typically encountered in clean room processes (e.g., deposition of metals in high vacuum conditions at high temperatures), it is electronically transparent (it does not change the energetics of the Si/Au or Si/Cu Schottky barriers), and effectively prevented diffusion of the Cu or Au into the Si at elevated temperatures and vice versa.

Oxide-free Si and Ge surfaces have been passivated and modified with organic molecules by forming covalent bonds between the surfaces and reactive end groups of linear alkanes and aromatic species using single-step deposition in supercritical carbon dioxide (SCCO2). The process is suitable for large-scale manufacturing due to short processing times, simplicity, and high resistance to oxidation. It also allows the formation of monolayers with varying reactive terminal groups, thus enabling formation of nanostructures engineered at the molecular level. Ballistic electron emission microscopy (BEEM) spectra performed on the organic monolayer on oxide-free silicon capped by a thin gold layer reveals for the first time an increase in transmission of the ballistic current through the interface of up to three times compared to a control device, in contrast to similar studies reported in the literature suggestive of oxide-free passivation in SCCO2. The SCCO2 process combined with the preliminary BEEM results opens up new avenues for interface engineering, leading to molecular electronic devices.

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cited By 1; International audience; Transferring molecular nanostructures from one surface to another in ultrahigh vacuum (UHV) by mechanical contact might be a possible route to avoid the severe limitations of in situ molecular synthesis on technologically relevant template surfaces. Here, transfer printing in UHV of molecular structures between metal surfaces is investigated by a combination of scanning tunneling microscopy and scanning electron microscopy/energy dispersive x-ray spectroscopy. The authors present the complete procedure of the printing and characterization process. Microstructured Au-coated MoS2 samples exhibiting a periodic pillar structure are used as stamp surfaces with Au(111) single crystals as target surface. Polymers of 1,3,5-tris(4-bromophenyl)benzene molecules and graphene nanoribbons with an armchair edge structure are grown on the pillars of the stamp surface. After bringing the two surfaces in mechanical contact, the transferred material is found on the target while decapping occurs on the stamp surface. Polymer structures are probably buried under the transferred stamp material, and in rare cases, evidence for molecular structures is found in their vicinity.

A printing technique is proposed for the transfer of metallic nanoislands between two semiconductor surfaces in UHV. For the preparation of the stamp, a systematic study of the growth conditions of small, flat triangular shape Au nanoislands at the top surface of microfabricated MoS2 pillars is presented. Those pillars are organized in a stamping matrix to increase the transfer rate. Up to 10% of Au nanoislands can be transferred to a H-Si(100) surface. The atomic scale quality of the interface between the Au nanoislands and the semiconductor surface is characterized by transmission electron microscopy cross-sectional imaging. This MoS2 stamping technique is extended to other surfaces such as mica, SiO2, and graphite. It permits to handle well shaped Au nanoislands on surfaces where a direct growth of flat nanoislands is not possible. This printing of well defined triangular Au nanoislands offers the unique possibility to construct ultraclean interconnecting nanopad systems by scanning tunneling microscop...

In this work, we present an investigation of the Pt and Pd-HfO2-p-Si interfaces using ballistic electron emission microscopy. The band alignment of the Pt-HfO2-p-Si structure is inferred. The potential drop in the oxide has been determined. Oscillations in the collector current with increasing bias enable estimation of the effective mass of electrons in HfO2 in the range of 0.35-0.44 m0. Stressing studies indicate modest resistance to stressing, with a threshold of 0.5 nC for damage to the base/oxide. Our work is the first successful application of the BEEM technique to metal-high K dielectric interfaces.

We use Ballistic Electron Emission Microscopy (BEEM) technique to determine directly the Schottky barrier distribution over silver /H-T3-(CH2)4-HS (abbreviated as T3C4SH) self assembled monolayer interface area with nanometer scale spatial resolution. The selfassembled monolayer is absorbed on template stripped gold. BEEM images show spatially non-uniform carrier injection. A Wentzel Kramel Brillouin (WKB) calculation is performed and compared with BEEM spectra. The results show that the measured currents are four orders of magnitude larger than the direct tunnelling contribution, indicating molecular levels being accessed. To further substantiate the findings, characterization by STM distance versus potential spectroscopy is carried out to determine injection barriers at the interface. The results from these two techniques are compared and the implications of which will be discussed.

The first experimental demonstration of a controllable rotating molecule gear is presented. A scanning tunneling microscope (STM) is used to construct, manipulate, and observe the rotation of the molecule gear. The appropriate combination of molecule design, molecule manipulation protocol, and surface atomic structure selection leads to the functioning of the molecule gear. Rotation of the molecule gear is done step-by-step and totally under control. The fabrication of solid-state SiO2 nanogears with diameters ranging from 30 nm up to 1 μm and their manipulation using an atomic force microscope tip on a graphite surface is also presented. Ranging in sizes from few tens of nanometers up to submicron diameters, they are going to enable the transmission of mechanical motion from functional mechanical molecule machineries to larger submicron or micron-sized devices through series of solid-state gears and mechanical components compatible with the semiconductor and electronics industry technology.

cited By 4; International audience; Detailed low temperature scanning tunneling microscope images of the Si(100)-2 × 1-H surface show a remarkable contrast inversion between filled- and empty-state images where the hydrogen dimer rows appear bright for filled-state images and dark for empty-state images. This contrast inversion originates from the change in the dominant surface states and their coupling to the tip apex and the bulk silicon channels as a function of the bias voltage: dimer SiSi bonding states dominate the filled-state images and valley states associated with SiSi anti-bonding states dominate the empty-state images. Care is required when constructing and interpreting the atomic structure of dangling-bond structures on the Si(100)-2 × 1-H surface.

In a planar atomic-scale device, defined nano-contacts to liaise between the atomic scale and the macroscale via the intermediate micron scale are crucial to ensure the atomic-scale circuit performances by minimizing the leakage current, minimizing cross talks, and minimizing contact resistance. Whether the macroscale contact is via a multi-probe system or via doped back interconnects preparing for a nano-packaging, defined and easily recognizable contact nano-pads are necessary. A new approach is presented here based on the in situ transfer of single Au crystals grown on a MoS2 surface in ultra-high vacuum (UHV) onto another surface where such metallic nano-crystal growth is usually not compatible. The molybdenite surface is particularly well suited to grow single Au nano-crystal triangles, and transfer-printing in ambient environment has already been successfully demonstrated. A dedicated transfer-printing tool was designed and constructed to perform this transfer printing task in a UHV environment. Preliminary results from scanning electron microscopy and low-temperature scanning tunneling microscopy are presented.

It is important to understand the structure of redox-active self-assembled monolayers (SAMs) down to the atomic scale, since these SAMS are widely used as model systems in studies of mechanisms of charge transport or to realize electronic functionality in molecular electronic devices. We studied the supramolecular structure of SAMs of n-alkanethiolates with ferrocenyl (Fc) end groups (S(CH2)nFc, n = 3 or 4) on Au(111) by scanning tunneling microscopy (STM). In this system, the tilt angle of the Fc units with respect to the surface normal (α) depends on the value of n because the Au-S-C bond angle is fixed. The ordered domains of the SAMs were imaged by STM after annealing at 70 °C at ultrahigh vacuum conditions. High resolution electron energy loss spectroscopy (HREELS) and cyclic voltammetry show that this annealing step only removed physisorbed material and did not affect the structure of the SAM. The STM images revealed the presence of row defects at intervals of 4 nm, that is, six molecules. We determined by near edge X-ray absorption fine structure spectroscopy (NEXAFS) that the Fc units of the SAMs of SC3Fc are more parallel to the Au(111) plane with a tilt angle α = 60.2° than the Fc units of SC4Fc SAMs (α = 45.4°). These tilt angles are remarkably close to the tilt angles measured by X-ray diffraction data of bulk crystals (bc-plane). Based on our data, we conclude that the molecules are standing up and the SAMs pack into lattices that are distorted from their bulk crystal structures (because of the build-up stain due to the differences in size between the Fc units and thiolate anchoring groups).

Nanoporous gold with networks of interconnected ligaments and highly porous structure holds stimulating technological implications in fuel cell catalysis. Current syntheses of nanoporous gold mainly revolve around de-alloying approaches that are generally limited by stringent and harsh multistep protocols. Here we develop a one-step solution phase synthesis of zero-dimensional hollow nanoporous gold nanoparticles with tunable particle size (150-1,000 nm) and ligament thickness (21-54 nm). With faster mass diffusivity, excellent specific electroactive surface area and large density of highly active surface sites, our zero-dimensional nanoporous gold nanoparticles exhibit ~1.4 times enhanced catalytic activity and improved tolerance towards carbonaceous species, demonstrating their superiority over conventional nanoporous gold sheets. Detailed mechanistic study also reveals the crucial heteroepitaxial growth of gold on the surface of silver chloride templates, implying that our synthetic protocol is generic and may be extended to the synthesis of other nanoporous metals via different templates.

The proximity effect in mesoscopic ferromagnet/superconductor(FS) Ni/Al structures of various geometrieswas studied experimentally on both F- and S-sides of thestructures. Samples with a wide range of interfacetransparency were fabricated. The dependence of the effect onFS interface transparency was investigated. The amplitude ofthis effect was found to be larger than expected fromclassical theory of proximity effect. Preliminary experimentsshowed no phase-sensitive oscillations in Andreevinterferometer geometry. Various theoretical models arediscussed.

Two-terminal thin films of poly(3-hexylthiophene) (P3HT) with a wide electrode separation (150 μ m) has been studied using current–voltage characteristics at different temperatures. Space-charge-limited conduction (SCLC) with high injection barriers (1.3 eV) has been observed at all temperatures in the low electric field regime. A possible transition from SCLC to injection-limited conduction (ILC) is reported. The experimental results have been compared with the disorder-controlled injection model proposed by Arkhipov et al. [V.I. Arkhipov, H. von Seggern, E.V. Emilianova, Appl. Phys. Lett. 83 (2003) 5074; V.I. Arkhipov, E.V. Emilianova, Y.-H. Tak, H. Bassler, J. Appl. Phys. 84 (1998) 848; V.I. Arkhipov, U. Wolf, H. Bassler, Phys. Rev. B 59 (1999) 7514].

Charge injection from metal electrodes to organics is a subject of intense scientific investigation for organic electronics. Ballistic electron emission microscopy (BEEM) enables spectroscopy and imaging of buried interfaces with nanometer resolution. Spatial non-uniformity of carrier injection is observed for both Ag–PPP (poly-paraphenylene) and Ag–MEHPPV (poly-2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene) interfaces. BEEM current images are found to correlate only marginally with the surface topography of the Ag film.

Two-probe electronic transport measurements on a Molybdenum Disulphide (MoS_2) surface were performed at low temperature (30K) under ultra-high vacuum conditions. Two scanning tunneling microscope tips were precisely positioned in tunneling contact to measure the surface current-voltage characteristics. The separation between the tips is controllably varied and measured using a high resolution scanning electron microscope. The MoS_2 surface shows a surface electronic gap (E_S) of 1.4eV measured at a probe separation of 50nm. Furthermore, the two- probe resistance measured outside the electronic gap shows 2D-like behavior with the two-probe separation., Comment: 16 pages, 3 figures

In anticipation of devices scaling down further to the few nanometer regime, the ability to characterize material localized within the few nm of a critical device region poses a current challenge, particularly when the material is already buried under other material layers such as under a metal contact. Conventional techniques typically provide indirect information of the nanoscale material quality through a surface or volume averaging perspective. Here we present a study of local (nm range) oxidation in few nanometer thick Co-films using Ballistic Electron Emission Microscopy/Spectroscopy (BEEM/BEES). Co films were grown on n-Si(111) substrates, oxidized in ambient atmosphere before capping with a thin Au film to prevent further oxidation and enable BEEM measurements. In addition to BEES, the temporal progression of Co oxidation was also tracked by X-ray Photoelectron Spectroscopy. At room temperature, we report that the electron injection thresholds are sufficiently different for local regions with Co and oxidized-Co enabling their distinction in BEEM measurements. Our results demonstrate the possibility of using BEEM for nanoscale spatial mapping of the oxidized regions in Co-films, and this can provide critical information toward the successful fabrication of next generation Co-based nano-devices.

The effect of surface contamination on the electron tunneling in the high bias range is investigated from the perspective of ballistic electron emission microscopy (BEEM). A comparative BEEM study on the Au/SiO2/Si devices shows that there is a significant difference in the high bias range between the experiments performed with in situ and ex situ deposited Au. Detailed studies show that the difference arises from the contaminations during air exposure. These contaminations significantly accelerated the material transfer between the tip and the sample during tunneling and lead to the unreliability of BEEM studies in the high bias range on the ex situ prepared sample. Published Version

An atomic scale multiprobe interconnection machine is described in the context of building of the molecular devices. It combines various techniques for fabrication and surface analysis. The characterization part consists of low temperature scanning tunneling microscope (LT-STM), four probes variable temperature scanning tunneling microscope (Multiprobe), high resolution scanning electron microscope (SEM). The fabrication part has field ion microscope (FIM), evaporators and nanoimprinter. The characteristics of every part of the instrument and the preliminary experiments on Si(100) surface are discussed.

The detailed fabrication and manipulations of solid state nano gears up to 350 nm in diameter is reported. Atomic force microscopy (AFM) and ultra high vacuum (UHV) scanning tunneling microscopy (STM) are used to maneuver the gears. The aim is to bridge the gap between the current solid state gears and the now available nanoscale gears. As in many technology integrations, miniaturization is a way to boost efficiency and an opening to new applications.

A systematic study on silicon native oxides grown in ambient air at room temperature is carried out using ballistic electron emission microscopy. The electronic barrier height of Au/native oxide was directly measured for native oxides at different oxidation stages. While the ballistic electron transmission decreases with increasing oxidation time, the electronic barrier height remains the same, even after oxidation for 1 week. After oxidation for 26 months, the oxide layer showed the bulk-like SiO2 barrier; however, some local areas still show the same barrier height as that of an Au/n-Si device. This demonstrates the non-uniformity of native oxide growth. Published version

Based on the assumption that the contact barrier height determines the current flow in organic semiconductor-based electronic devices, charge injection at metal-organic (MO) interfaces has been extensively investigated, while space-charge conduction in organic bulk is generally overlooked. Recent theoretical modeling and simulation have pointed out that such a simplification is questionable due to the hopping nature of charge injection and hopping-related space-charge conduction. Here we show experimentally that charge transport in metal-organic diodes is complex interplay between injection-limited current (ILC) and space-charge-limited current (SCLC). We report the experimental observation of ILC-to-SCLC transition in Ag/pentacene/Ag diodes as a function of temperature.

We study the electrostatic effects of thin organic films in modifying the interface physics of metal/semiconductor Schottky contacts. We work out analytically the electrostatic parameter space pointing out where interface state effects exceed space-charge effects and vice versa. This is done by introducing another treatment of the electrostatic problem. We also find that the image force effect on the barrier height due to the insertion of a material with lower dielectric constant than the semiconductor in between the Schottky contact is small but positive. This is in contrast to what might be expected from effective-medium theory. We conclude with an examination of ballistic electron emission microscopy results of pentacene modified $\text{Au}/n\text{-Si}(111)$ Schottky diodes as a case study. Using the tools fore mentioned, we infer the local charge neutrality level and density of interface gap states (to an area of $500\ifmmode\times\else\texttimes\fi{}500\text{ }{\text{nm}}^{2}$) from barrier height statistics and pentacene monolayer heights.

We have identified a possible electronic origin of metal filaments, invoked to explain the switching behavior of organic devices. Interfaces of two representative organics polyparaphenylene (PPP) and poly(2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene) with Ag are investigated using ballistic emission microscopy. Nanometer scale spatial nonuniformity of carrier injection is observed in ballistic electron emission microscopy images of both interfaces. The measured Schottky barrier (SB) appears to be consistent with metal states tailing into the gap of the PPP. We find that the SB values exhibit a distribution, even for the diodes with low ideality factors. The implications of this distribution on the measured physical properties of the diode are discussed, in light of work on devices of similar geometry, published in the literature. We also demonstrate that patches of low SB are likely to nucleate current filaments which can cause local ionization and are reported to be responsible for the switching behavior observed in metal-organic, metal-CuS and Ag–AgSe structures.

In this work, we present an investigation of the Ag-PPP (polyparaphenylene) interface using ballistic electron emission microscopy. Our work is the first successful application of the BEEM technique to metal-organic interfaces. We observe nanometer scale injection inhomogeneities. They have an electronic origin, since we find corresponding Schottky barrier variations. We also determine the transmission function of Ag-PPP interface and find that it agrees qualitatively with the theoretical calculations for a metal-phenyl ring interface. We conclude that charge transport across inhomogeneous barriers needs to be considered for understanding electronic transport across metal-organic interfaces and organic device characteristics.

The silver-p-phenylene (Ag-PPP) interface is investigated using ballistic electron emission microscopy (BEEM). Multiple injection barriers and spatial nonuniformity of carrier injection over nanometer length scales are observed. No unique injection barrier is found. Physical reasons for these features are discussed. BEEM current images and the surface topography of the silver film are uncorrelated., Comment: To appear in Applied Physics Letters

Metal organic interfaces have been under intensive scientific investigation over the past few years with the objective of improving devices based on organic materials. In this study, we report the first results on spectroscopy and imaging of a buried metal-organic interface using ballistic electron emission microscopy (BEEM). Unlike photoexcitation-based experiments, which typically average over large interfacial areas, BEEM enables direct observation of local interface band structure with nanometer resolution. The interface of silver (Ag) - polyparaphenylene (PPP, a blue emitter with high photoluminescence efficiency) is investigated. Multiple injection barriers and spatial non-uniformity of carrier injection are observed. Possible causes for these features will be discussed. The BEEM current images are found to correlate marginally with the surface topography of the silver film.

A strong mutual influence of superconductors (S) and ferromagnetic (F) conductors in hybrid F/S (Ni/Al) nanostructures is observed. The magnitude of a proximity-induced conductance on the F-side is more than two orders larger than that predicted by theory. Re-entrance of the superconductors to the normal state reciprocated by changes on the F-side has been found in low applied magnetic fields with new peaks in the differential resistance as an effect of the saturation magnetisation. An analysis has been developed providing a base for a numerical description of the system. PACS numbers: 74.50.+r, 74.80.Fp, 85.30St, 7 pages in html format, 3 figures in ps format. Submitted to Phys. Rev. Lett

We observe large amplitude changes in the resistance of ferromagnetic (F) wires at the onset of superconductivity of adjacent superconductors (S). New sharp peaks of large amplitude are found in the magnetoresistance of the F-wires. We discuss a new mechanism for the long-range superconducting proximity effect in F/S nanostructures based on the analysis of the topologies of actual Fermi-surfaces in ferromagnetic metals., Comment: 7 pages in LaTeX, 5 eps figures. Submitted to the Proceedings of MS2000

We report upon a comprehensive investigation of the subthreshold characteristics of the ballistic electron emission microscopy (BEEM) current in ballistic electron emission spectroscopy. Starting from the Bell-Kaiser model, we derive an analytical equation to describe the subthreshold behavior of the BEEM current. It is found that the BEEM current in this region should exhibit a subthreshold swing of ∼60 mV/decade at room temperature, which we experimentally verified. This finding provides a rule of thumb for the detectability of the subthreshold behavior in a spectrum. For spectra where the subthreshold behavior is discernible above the signal noise, it is demonstrated that significant deviations in the near-threshold region can occur when fitting with a simple quadratic model that ignores the subthreshold behavior. To take the subthreshold behavior into account, a simple analytical model is proposed. This model not only fits significantly better in the near threshold region than the square model, but also gives a barrier height closer to the one extracted from the Bell-Kaiser model. More significantly, this model provides a quick method to estimate the subthreshold BEEM current amplitude based on the BEEM current above the barrier height. Published version

Ballistic electron emission microscopy was employed in order to investigate the electronic properties of sub-nanometer high-κ dielectrics (CeO2 and La2O3). The authors found that such a thin dielectric sandwiched between Au and n-Si fails to exhibit the same electronic barrier as its bulk counterpart, but it can still significantly attenuate the ballistic electron transport. The authors attribute the observed smaller barrier height to quantum tunneling and/or induced gap states. The results suggest that such ultrathin high-κ dielectrics in a metal-dielectric-semiconductor structure do not show a fully formed electronic barrier.

A process is presented to fabricate solid-state nano-gears down to a 60 nm outer diameter with six teeth, where the 350 nm diameter ones already have 24 teeth. The small gears are free to move on a polycrystalline gold surface. The gears can be manipulated one by one, using an atomic force microscope (AFM) tip, to construct a train of gears where mechanical motion can be transmitted from one gear to another by mastering the surface friction. This is a first step on the way to bridge the fabrication gap between microfabricated and molecule gears.

The scientific and technical challenges involved in building the planar electrical connection of an atomic scale circuit to N electrodes (N > 2) are discussed. The practical, laboratory scale approach explored today to assemble a multi-access atomic scale precision interconnection machine is presented. Depending on the surface electronic properties of the targeted substrates, two types of machines are considered: on moderate surface band gap materials, scanning tunneling microscopy can be combined with scanning electron microscopy to provide an efficient navigation system, while on wide surface band gap materials, atomic force microscopy can be used in conjunction with optical microscopy. The size of the planar part of the circuit should be minimized on moderate band gap surfaces to avoid current leakage, while this requirement does not apply to wide band gap surfaces. These constraints impose different methods of connection, which are thoroughly discussed, in particular regarding the recent progress in single atom and molecule manipulations on a surface.

In a planar configuration, precise positioning of ultra-flat metallic nano-islands on semiconductor surface opens a way to construct nanostructures for atomic scale interconnects. Regular triangular Au nano-islands have been grown on atomically flat MoS2 substrates and manipulated by STM to form nanometer gap metal-pads connector for single molecule electronics study. The direct assembly of regular shaped metal nano-islands on H-Si(100) is not achievable. Here we present how to transfer Au triangle nano-islands from MoS2 onto H-Si(100) in a clean manner. In this experiment, clean MoS2 substrates are patterned as array of MoS2 pillars with height of 8 μm. The Au triangle nano-islands are grown on top of the pillars. Successful printing transfer of these Au nano-islands from the MoS2 pillars to the H-Si(100) is demonstrated.

The well-known Au/n-Si(111) Schottky interface is modified by a discontinuous pentacene film (∼1.5 nm thick) and studied using spatially resolved ballistic electron emission spectroscopy (BEES). The pentacene film introduced subtle changes to the interface which cannot be definitively detected by current-voltage measurements or a standard BEES analysis of the barrier height. In contrast, analyzing the BEES results in a dual-parameter (transmission attenuation and barrier height) space allows the effect of the pentacene film on the Au/n-Si(111) interface to be clearly demonstrated. We found that the pentacene film behaves like a tunneling barrier and increases the distribution of local barrier heights with a tendency toward lower values. Our results highlight the potential of the dual-parameter BEES analysis for understanding local interface modification by molecules.

Au–HfO2–SiOx–Si structures with 4nm HfO2 and 1.5nm SiOx interfacial layer (IL) have been electrically stressed by ballistic electron emission spectroscopy (BEES). The continuous BEES stressing at the same location induced gradual degradations and finally led to breakdowns in the IL. The degradation and breakdown cannot be observed using macroscopic conventional current-voltage (IV) measurements over the same area just before and after the BEES stressing process. The localized degradation and breakdown in the dielectric is masked by the macroscopic gate area. Tunneling calculations can estimate the critical area required for a macroscopic device to be able to measure such microscopic breakdown, a problem that becomes increasingly important for characterizing ultrathin gate dielectrics.

Hot electron transport in Au–HfO2–SiO2–Si structures with 4nm HfO2 and 1.5nm SiO2 interfacial layer have been investigated by ballistic electron emission spectroscopy (BEES). By controlling the hot electron kinetic energy and injection current, distinctly different barrier heights can be measured. BEES sweeping below −5V with 1nA injection current yields high barrier heights (∼3.8eV), attributable to the interfacial SiO2 layer. BEES sweeping from −6V with high injection current (5nA and above) induced localized breakdown of the SiO2 interfacial layer, allowing the barrier height of the HfO2 layer to be measured (∼1.9eV). The energy-dependent effective mass of electrons in HfO2 is also determined by fitting oscillations in the BEES current.

The proximity effect in mesoscopic ferromagnet/superconductor ($FS$) Ni/Al structures of various geometries was studied experimentally on both $F$- and $S$-sides of the structures. Samples with a wide range of interface transparency were fabricated. The dependence of the effect on $FS$ interface transparency was investigated. The amplitude of this effect was found to be larger than expected from classical theory of proximity effect. Preliminary experiments showed no phase-sensitive oscillations in Andreev interferometer geometry. Various theoretical models are discussed., 10 pages, 6 figures, to appear in J. Low-Temp. Phys

Ballistic electron emission microscopy (BEEM) was employed to study metal/dielectric/semiconductor devices with high spatial resolution. A comprehensive study of the subthreshold characteristics of the BEEM spectra shows that the BEEM current in the subthreshold region decreases at a rate of ~60 mV/decade. A comparative BEEM study of Au/SiO2/n-Si devices with in-situ and ex-situ deposited metal suggests that an ultra¬¬clean metal surface is critical for the BEEM studies of the devices with high electron barriers. Investigation of the ultrathin native oxide and high-κ dielectrics shows that an ultrathin dielectric (probably no thicker than 6–8 Å) does not exhibit the same electronic properties as a thicker dielectric. Spatial non-uniformity of these oxides on the nanometer scale was also observed by BEEM. We also determined the electron barrier height for a few important dielectrics (oxidized GaAs, Al2O3 and HfO2) on GaAs. An analytical equation describing the subthreshold BEEM behavior is derived from the basic BEEM model, i.e., the Bell-Kaiser model. This analytical equation shows that the BEEM current in the subthreshold region decreases exponentially with a subthreshold swing of about 60 mV/decade as the tip bias decreases. The equation is further verified by simulated and experimental BEEM spectra. Realizing the subthreshold characteristics, a new simplified model was proposed for the BEEM spectrum fitting. This proposed model takes the subthreshold behavior into account and it is to the best of our knowledge the first simplified analytical BEEM model that considers the temperature dependence. This model is shown to significantly improve the fitting of the BEEM spectrum compared with using the well-known simplified square model. It also provides a quick method to estimate the BEEM current at and below the threshold. It is shown that the BEEM measurement of samples with high barriers (~3.0 eV or above), cannot be performed properly if the metal base electrode is deposited ex-situ, while it can be performed consistently if the metal is deposited in-situ in an ultra-high vacuum. We further show that the main reason is that contamination of the Au surface strongly affects the tunneling between the tip and Au base in the high bias range, thus affecting the BEEM spectroscopy. DOCTOR OF PHILOSOPHY (EEE)