Surface Conductivity of Si(100) and Ge(100) Surfaces Determined from Four-Point Transport Measurements Using an Analytical N-Layer Conductance Model

An analytical N-layer model for charge transport close to a surface is derived from the solution of Poisson's equation and used to describe distance-dependent electrical four-point measurements on the microscale. As the N-layer model comprises a surface channel, multiple intermediate layers and a semi-infinite bulk, it can be applied to semiconductors in combination with a calculation of the near-surface band-bending to model very precisely the measured four-point resistance on the surface of a specific sample and to extract a value for the surface conductivity. For describing four-point measurements on sample geometries with mixed 2D-3D conduction channels often a very simple parallel-circuit model has so far been used in the literature, but the application of this model is limited, as there are already significant deviations, when it is compared to the lowest possible case of the N-layer model, i.e. the 3-layer model. Furthermore, the N-layer model is applied to published distance-dependent four-point resistance measurements obtained with a multi-tip scanning tunneling microscope (STM) on Germanium(100) and Silicon(100) with different bulk doping concentrations resulting in the determination of values for the surface conductivities of these materials.Comment: 11 pages, 6 color figure

Surface and Step Conductivities on Si(111) Surfaces

Four-point measurements using a multi-tip scanning tunneling microscope (STM) are carried out in order to determine surface and step conductivities on Si(111) surfaces. In a first step, distance-dependent four-point measurements in the linear configuration are used in combination with an analytical three-layer model for charge transport to disentangle the 2D surface conductivity from non-surface contributions. A termination of the Si(111) surface with either Bi or H results in the two limiting cases of a pure 2D or 3D conductance, respectively. In order to further disentangle the surface conductivity of the step-free surface from the contribution due to atomic steps, a square four-probe configuration is applied as function of the rotation angle. In total this combined approach leads to an atomic step conductivity of $\sigma_\mathrm{step} = (29 \pm 9)$ $\mathrm{\Omega}^{-1} \mathrm{m}^{-1}$ and to a step-free surface conductivity of $\sigma_\mathrm{surf} = (9 \pm 2) \cdot 10^{-6}\,\mathrm{\Omega}^{-1}/\square$ for the Si(111)-(7$\times$7) surface.Comment: Main paper: 5 pages, 4 figures, Supplemental material: 6 pages, 3 figures. The Supplemental Material contains details on the sample preparation and measurement procedure, additional experimental results for Si(111) samples with different doping levels, and the description of the three-layer conductance mode

Ladungstransport durch Graphenschichtenund GaAs-Nanodrähte untersucht mit einem Multispitzen-Rastertunnelmikroskop

This work describes the use of the combination of a scanning electron microscope (SEM) and a multitip scanning tunneling microscope (STM) with four tips as a nanoprober. Electrical measurements on graphene layers and freestanding gallium arsenide (GaAs) nanowires were conducted. Four-probe-measurements are necessary to measure the resisitvity of such one- and two-dimensional conductors. Due to unknown voltage drops at contacts that carry currents, additional contacts have to be employed for current-free potential measurements. Therefore, the multitip scanning tunneling microscope with its four individually controllable tips has been upgraded with extended electronics, enabling us to use it as a flexible nanoprober. Graphene layers on insulating SiO$_{2}$ and hexagonal boron nitride (h-BN), prepared by mechanical exfoliation, were contacted with the multitip STM. Tunneling current could not be used as feedback when approaching the first tip. Therefore, a contrast change in the SEM image upon contacting a graphene flake with a tip was used. Once contacted, flakes were scanned by RTM and electrical measurements were conducted. Graphene transferred to h-BN showed bubbles, wrinkles and contaminations. Still, STM images of clean areas revealed a moiré pattern, proving that the atomically thin graphene lay flat on the atomically flat h-BN surface. Four point measurements of these samples showed a poor conductivity of 1/$\sigma$ = 16$^{k \Omega} /box$ and a low field effect mobility of $\mu$ = 300$^{cm^{2}}$/Vs. The reason for this might be the contaminations from the transfer process, as well as effects from prolonged irradiation with electrons from the SEM. Freestanding p-doped GaAs nanowires, grown by metal-organic vapor-phase-epitaxy in the vapor-liquid-solid-growth mode, in a process with two temperature steps, were contacted with the multitip STM. Using three tips as well as the substrate as contacts, four point measurements were performed. It showed that elastic deformation of these flexible nanowires has no significant influence on their conductivity. The high spatial resolution of the combination of a SEM with a multitip STM made it possible to record resistance profiles of freestanding nanowires by performing four point measurements along a nanowire. The main segment of the nanowires, grown at 400$^{\circ}$C for better crystal quality exhibits a resisitivity of a few $^{k\Omega}$/$_{\mu m}$, in agreement with literature values. The nanowire base, grown at 450$^{\circ}$C to facilitate better nucleation, shows an increased resisitvity of several $^{M\Omega}$/$_{\mu m}$. The resistance of the nanowire base is relevant especially for future opto-electronical components based on freestanding nanowires and thus has to be understood. Comparing profiles of nanowires grown by an identical process on different substrates showed that the substrate is not the cause of the increased resistance. From the measured resistivities the dopant concentrations, as well as the thickness of the space charge layer at the surface of the GaAs nanowires were calculated. The nanowire segments grown at 400$^{\circ}$C have a dopant concentration of roughly 10$^{19}$cm$^{-3}$, those grown at 450$^{\circ}$C about 2 $\cdot$10$^{17}$ cm$^{-3}$. In the base the space charge layer poses a considerable constriction to the conduction. A qualitative explanation for the temperature dependence of the dopant concentration is given

High-Tc bolometers with silicon-nitride spiderwebsuspension for far-infrared detection

High-Tc GdBa2Cu3O7-δ (GBCO) superconducting transition edge bolometers with operating temperatures near 90 K have been made with both closed silicon-nitride membranes and patterned silicon-nitride (SiN) spiderweb-like suspension structures. As a substrate silicon-on-nitride (SON) wafers are used which are made by fusion bonding of a silicon wafer to a silicon wafer with a silicon-nitride top layer. The resulting monocrystalline silicon top layer on the silicon-nitride membranes enables the epitaxial growth of GBCO. By patterning the silicon-nitride the thermal conductance G is reduced from about 20 to 3 μW/K. The noise of both types of bolometers is dominated by the intrinsic noise from phonon fluctuations in the thermal conductance G. The optical efficiency in the far infrared is about 75% due to a goldblack absorption layer. The noise equivalent power NEP for FIR detection is 1.8 pW/√Hz, and the detectivity D* is 5.4×1010 cm √Hz/W. Time constants are 0.1 and 0.6 s, for the closed membrane and the spiderweb like bolometers respectively. The effective time constant can be reduced with about a factor 3 by using voltage bias. Further reduction necessarily results in an increase of the NEP due to the 1/f noise of the superconductor

Modeling sublimation by computer simulation: morphology dependent effective energies

Solid-On-Solid (SOS) computer simulations are employed to investigate the sublimation of surfaces. We distinguish three sublimation regimes: layer-by-layer sublimation, free step flow and hindered step flow. The sublimation regime is selected by the morphology i.e. the terrace width. To each regime corresponds another effective energy. We propose a systematic way to derive microscopic parameters from effective energies and apply this microscopical analysis to the layer-by-layer and the free step flow regime. We adopt analytical calculations from Pimpinelli and Villain and apply them to our model. Key-Words: Computer simulations; Models of surface kinetics; Evaporation and Sublimation; Growth; Surface Diffusion; Surface structure, morphology, roughness, and topography; Cadmium tellurideComment: 12 pages, 6 Postscript figures, uses psfig.st

Reduction of Two-Dimensional Dilute Ising Spin Glasses

The recently proposed reduction method is applied to the Edwards-Anderson model on bond-diluted square lattices. This allows, in combination with a graph-theoretical matching algorithm, to calculate numerically exact ground states of large systems. Low-temperature domain-wall excitations are studied to determine the stiffness exponent y_2. A value of y_2=-0.281(3) is found, consistent with previous results obtained on undiluted lattices. This comparison demonstrates the validity of the reduction method for bond-diluted spin systems and provides strong support for similar studies proclaiming accurate results for stiffness exponents in dimensions d=3,...,7.Comment: 7 pages, RevTex4, 6 ps-figures included, for related information, see http://www.physics.emory.edu/faculty/boettcher

Low noise far-infrared detection at 90 K using high-T(c) superconducting bolometers with silicon-nitride beam suspension

High-T(c) GdBa2Cu3O7-d (GBCO) superconducting transition edge bolometers with operating temperatures near 90 K and receiving area of 1 mm2 have been made with both closed silicon-nitride membranes and patterned silicon-nitride (Si(x)N(y)) spiderweb-like suspension structures. To enable epitaxial growth of the GBCO layer, a thin monocrystalline Si layer is prepared on the silicon-nitride base, using fusion bonding techniques. By pattering the silicon-nitride supporting membrane the thermal conductance G is reduced from 20 to 3.5 μW/K. The noise of both types of bolometers is fully dominated by the intrinsic noise from phonon fluctuations in the thermal conductance G. The optical efficiency in the far infrared is about 75% due to a gold black absorption layer. The optical noise equivalent power (NEP) is 1.8 pW/√Hz, and the detectivity D* is 5.4x1010 cm√Hz/W. Time constants are 0.1 and 0.6 s, for the closed membrane and the spiderweb like bolometers respectively. We have observed an empirical limit for the NEP for this type of bolometers. The effective timeconstant can be reduced with a factor of 3 by using an electronic feedback system or by using voltage bias. A further reduction necessarily results in an increase of the NEP due to the 1/f noise of the superconductor

A greedy classifier optimization strategy to assess ion channel blocking activity and pro-arrhythmia in hiPSC-cardiomyocytes

International audienceNovel studies conducting cardiac safety assessment using human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are promising but might be limited by their specificity and predictivity. It is often challenging to correctly classify ionchannel blockers or to sufficiently predict the risk forTorsade de Pointes (TdP). In this study, we developed a method combining in vitro and in silico experiments to improve machine learning approaches in delivering fast and reliable prediction of drug-induced ion-channel blockade and proarrhythmic behaviour.The algorithm is based on the construction of a dictionary and a greedy optimization, leading to the definition of optimal classifiers. Finally,we present a numerical tool that can accurately predict compound-induced pro-arrhythmicrisk and involvement of sodium,calcium and potassium channels,based on hiPSC-CM field potentialdata