Cedric Troadec

Multiple atomic scale solid surface interconnects for atom circuits and molecule logic gates

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.

Bilayer gate dielectric study by scanning tunneling microscopy

An advanced bilayer gate dielectric stack consisting of Sc2O3∕La2O3∕SiOx annealed in nitrogen at 300°C was studied by scanning tunneling microscopy using bias dependent imaging. By changing the sample bias, electrical properties of different layers of the dielectric stack can be studied. At a sample bias of +3.5V, the conduction band of the La2O3 layer is probed revealing a polycrystalline film with an average grain size of about 27nm, in good agreement with that determined from planar transmission electron microscopy. High conductivity at grain boundaries, due possibly to dangling bonds, can be observed in this layer, as also observed in grain boundary assisted current conduction in metal-oxide-silicon structures. Imaging at a sample bias of −4V probes the interfacial SiOx layer and an amorphouslike image of the interfacial layer is obtained.

Ballistic emission spectroscopy and imaging of a buried metal∕organic interface

The silver∕polyparaphenylene 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.

Stable Organic Monolayers on Oxide-Free Silicon/Germanium in a Supercritical Medium: A New Route to Molecular Electronics

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.

Supramolecular Structure of Self-Assembled Monolayers of Ferrocenyl Terminated n-Alkanethiolates on Gold Surfaces

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).

Temperature-dependent transition from injection-limited to space-charge-limited current in metal-organic diodes

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 MO diodes is a 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.

Hot electron transport in Au–HfO2–SiO2–Si structures studied by ballistic electron emission spectroscopy

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.

Electronically transparent graphene barriers against unwanted doping of silicon.

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.

Low temperature nanoscale electronic transport on the MoS2 surface

Two-probe electronic transport measurements on a Molybdenum Disulphide (MoS2) surface were performed at low temperature (30 K) 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 MoS2 surface shows a surface electronic gap (ES) of 1.4 eV measured at a probe separation of 50 nm. Furthermore, the two- probe resistance measured outside the electronic gap shows 2D-like behavior with the two-probe separation.

BEEM studies on metal high K-dielectric HfO2 interfaces

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.