D. Deresmes

ARE ELECTRICAL PROPERTIES OF AN ALUMINUM-POROUS SILICON JUNCTION GOVERNED BY DANGLING BONDS ?

Using an aluminum–porous p+ silicon junction, we have realized a sensor which dc current increases up to two orders of magnitude in the presence of ammonia, as for a series of various gases. To interpret quantitatively this phenomenon, we assume that the conductivity is governed by the width of a channel resulting from the partial depletion of silicon located between two pores. This depleted region is due to the charges trapped on surface states associated with the Si–SiO2 interface where SiO2 is the native silicon oxide. When some gas is adsorbed, mainly on Si–H bonds, we propose there is an electrical screening of the interface states (mainly dangling bonds located in the neighborhood of the Si–H bonds), leading to a decrease of the depleted region, i.e., an increase of the width of the channel and thus an increase of the current.

Octadecyltrichlorosilane monolayers as ultrathin gate insulating films in metal-insulator-semiconductor devices

In order to fabricate metal‐insulator‐semiconductor (MIS) devices with gate insulating films thinner than 5.0 nm, organic monolayers have been grafted on the native oxide layer of silicon wafers. We demonstrate that a single monolayer of octadecyltrichlorosilane with a 2.8 nm thickness allows to fabricate a silicon based MIS device with gate current density as low as 10−8 A/cm2 at 5.8 MV/cm, insulator charge density lower than 1010 cm−2, fast interface state density of the order of 1011 cm−2 eV−1, and dielectric breakdown field as high as 12 MV/cm. Moreover, this insulating film is thermally stable up to 450 °C.

Surface potential of n- and p-type GaN measured by Kelvin force microscopy

n- and p-type GaN epitaxial layers grown by metal-organic chemical vapor deposition with different doping levels have been characterized by Kelvin probe force microscopy (KFM). To investigate the surface states of GaN beyond instrumental and environmental fluctuations, a KFM calibration procedure using a gold-plated Ohmic contact as a reference has been introduced, and the reproducibility of the KFM measurements has been evaluated. Results show that the Fermi level is pinned for n- and p-type GaN over the available doping ranges, and found 1.34±0.15eV below the conduction band and 1.59±0.18eV above the valence band, respectively.

Charge injection in individual silicon nanoparticles deposited on a conductive substrate

We report on charge injection in individual silicon nanoparticles deposited on conductive substrates. Charges are injected using a metal-plated atomic force microscope tip, and detected by electric force microscopy (EFM). Due to the screening efficiency of the conductive substrate, up to ∼200 positive or negative charges can be stored at moderate (<10 V) tip–substrate injection voltage in ∼40 nm high nanoparticles, with discharging time constants of a few minutes. We propose an analytical model in the plane-capacitor approximation to estimate the nanoparticle charge from EFM data. It falls in quantitative agreement with numerical calculations using realistic tip/nanoparticle/substrate geometries.

Influence of surface defects on the electrical behavior of aluminum‐porous silicon junctions

Using transient‐current measurements on porous silicon layers made on p+ silicon substrate, we characterize the surface defects of the porous silicon material, i.e., the defects located at the interface between porous silicon and a thin layer of native oxide. An energy location near midgap (these defects can be efficient radiative lifetime killers) and a trap concentration in close agreement with the number of trivalent silicon defects—as measured by electron spin resonance—are deduced.

Toward the understanding of the interfacial dairy fouling deposition and growth mechanisms at a stainless steel surface: a multiscale approach.

The microstructures of two dairy fouling deposits obtained at a stainless steel surface after different processing times in a pilot plate heat exchanger were investigated at different scales. Electron-Probe Micro Analysis, Time-of-Flight Secondary Ion Mass Spectrometry, Atomic Force Microscopy, and X-Ray Photo-electron Spectroscopy techniques were used for this purpose. The two model fouling solutions were made by rehydrating whey protein in water containing calcium or not. Results on samples collected after 2h processing show that the microstructure of the fouling layers is completely different depending on calcium content: the layer is thin, smooth, and homogeneous in absence of calcium and on the contrary very thick and rough in presence of calcium. Analyses on substrates submitted to 1 min fouling reveal that fouling mechanisms are initiated by the deposit of unfolded proteins on the substrate and start immediately till the first seconds of exposure with no lag time. In presence of calcium, amorphous calcium carbonate nuclei are detected in addition to unfolded proteins at the interface, and it is shown that the protein precedes the deposit of calcium on the substrate. Moreover, it is evidenced that amorphous calcium carbonate particles are stabilized by the unfolded protein. They are thus more easily trapped in the steel roughnesses and contribute to accelerate the deposit buildup, offering due to their larger characteristic dimension more roughness and favorable conditions for the subsequent unfolded protein to depose.

Conductivity of DNA probed by conducting–atomic force microscopy: Effects of contact electrode, DNA structure, and surface interactions

We studied the electrical conductivity of DNA molecules with conducting–atomic force microscopy as a function of the chemical nature of the substrate surfaces, the nature of the electrical contact, and the number of DNA molecules (from a few molecules to ropes and large fibers containing up to ∼106 molecules). Independent of the chemical nature of the surface (hydrophobic or hydrophilic, electrically neutral or charged), we find that DNA is highly resistive. From a large number of current-voltage curves measured at several distances along the DNA, we estimate a conductivity of about 10−6–10−5Scm−1 per DNA molecule. For single DNA molecules, this highly resistive behavior is correlated with its flattened conformation on the surface (reduced thickness, ∼0.5–1.5nm, compared to its nominal value, ∼2.4nm). We find that intercalating an organic semiconductor buffer film between the DNA and the metal electrode improves the reliability of the contact, while direct metal evaporation usually destroys the DNA and pr...

Note: Quantitative (artifact-free) surface potential measurements using Kelvin force microscopy

The measurement of local surface potentials by Kelvin force microscopy (KFM) can be sensitive to external perturbations which lead to artifacts such as strong dependences of experimental results (typically in a ∼1 V range) with KFM internal parameters (cantilever excitation frequency and/or the projection phase of the KFM feedback-loop). We analyze and demonstrate a correction of such effects on a KFM implementation in ambient air. Artifact-free KFM measurements, i.e., truly quantitative surface potential measurements, are obtained with a ∼30 mV accuracy.

Kelvin force microscopy at the second cantilever resonance: An out-of-vacuum crosstalk compensation setup

We investigate the gap-voltage control loop in a Kelvin force microscopy setup with simultaneous non-contact topography imaging. The Kelvin controller electrostatically excites the second resonance of the cantilever at about 6.3 times the first resonance frequency and adjusts the DC component of the gap voltage to cancel the oscillation amplitude at this frequency, while the non-contact topography imaging is based on a frequency control loop that maintains a constant frequency of the mechanically excited first resonance of the cantilever by adjusting the tip-sample separation. Due to the self-excitation of the first resonance in our setup, it has to be considered that the electrostatic excitation at the second resonance frequency is applied to a closed feedback loop and cannot be considered as a simple superposition to the oscillation at the first resonance frequency. In particular, special care has to be taken about internal capacitive crosstalk between the tip bias and the cantilever deflection output signal. It is shown that such a coupling cannot be corrected by subtraction of a constant offset at the demodulator output since the crosstalk is sent into the self-excitation loop and is multiplied by the closed loop transfer function. We present a circuit that actively compensates, outside the vacuum environment, the internal crosstalk by adding to the deflection output a dephased fraction of the electrostatic excitation signal.

Cross-talk artefacts in Kelvin probe force microscopy imaging: A comprehensive study

We provide in this article a comprehensive study of the role of ac cross-talk effects in Kelvin Probe Force Microscopy (KPFM), and their consequences onto KPFM imaging. The dependence of KPFM signals upon internal parameters such as the cantilever excitation frequency and the projection angle of the KPFM feedback loop is reviewed, and compared with an analytical model. We show that ac cross-talks affect the measured KPFM signals as a function of the tip-substrate distance, and thus hamper the measurement of three-dimensional KPFM signals. The influence of ac cross-talks is also demonstrated onto KPFM images, in the form of topography footprints onto KPFM images, especially in the constant distance (lift) imaging mode. Our analysis is applied to unambiguously probe charging effects in tobacco mosaic viruses (TMVs) in ambient air. TMVs are demonstrated to be electrically neutral when deposited on silicon dioxide surfaces, but inhomogeneously negatively charged when deposited on a gold surface.