Search

The covalent attachment of semicarbazide-functionalized layers to hydrogen-terminated Si(111) surfaces is reported. The surface modification, based on the photoinduced hydrosilylation of a Si(111) surface with protected semicarbazide-functionalized alkenes, was investigated by means of X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). The removal of the protecting group yielded a semicarbazide-terminated monolayer which was reacted with peptides bearing a glyoxylyl group for site-specific alpha-oxo semicarbazone ligation.

Nanoscale desorption of the Si(100)–2×1 hydrogen terminated surface has been achieved using a scanning tunneling microscope (STM) in an ultrahigh vacuum chamber. We have studied the patterned linewidth as a function of the sample bias and the dose, either with the feedback servo loop on or off. We propose a simple analytical model to explain the variation of the linewidth versus the electron dose. Finally, we show that the best resolution is obtained for pulsed voltages with the STM feedback servo loop on.

Aromatic cryptands can be considered as ‘pseudo fullerene molecules’, containing three separated π systems. The studied molecule [iso hexa imino cryptand (IHIC)] is made of 36 atoms of carbon, eight of nitrogen and 42 of hydrogen. Synthesis of the molecules gives a powder. Once the powder dissolved in chloroform, it is easy to deposit it on highly oriented pyrolytic graphite (HOPG) just by dipping the HOPG in the solution. After evaporation of the solvent, one-dimensional (1D) and two-dimensional (2D) structures are observed by using the scanning tunnelling microscopy technique. The 1D structures (chains) are made of nanoclusters containing typically six or sveen molecules. The distances between each molecule in a nanocluster are compared with those calculated on the basis of van der Waals' and electrostatic interactions. The theoretical approach predicts the formation of nanoclusters, in agreement with the experimental observations. Finally, the observation of an hexagonal network is compared and discussed in terms of the Moire effect.

The growth of homoepitaxial GaP layers using Te-doped GaP as source material has been obtained by the so-called closed space vapor transport technique. The photoluminescence study shows that these layers, when grown under optimized thermodynamical conditions, have both a large luminescence efficiency and the same optical quality as the ones obtained by liquid phase epitaxy. The variation of the luminescence properties with the conditions of growth has been investigated. Both electron paramagnetic resonance and deep level transient spectroscopy detect the presence of deep levels that are not observed in liquid phase epitaxy materials.

Charge injection in individual silicon nanoparticles has been investigated by electric force microscopy (EFM). Stored charges injected from the EFM tip have been counted using a quantitative method. Injection kinetics reveals the setting-up of an equilibrium regime. Equilibrium charge–voltage characteristics are analyzed, and display an overall linear behavior corresponding to successive tunneling through the nonequivalent tip–nanoparticle and nanoparticle–substrate oxide barriers. A hysteretic behavior is observed in the equilibrium charge–voltage characteristics, and attributed to a secondary charge injection process associated with the nanoparticle oxide surface.

We investigated the spectroscopic photocurrent response of photovoltaic devices versus an increasing number of drop-casted CdSe nanoparticles onto planar and nanocones silicon p-i-n junctions. For all samples, a strong enhancement of the photocurrent in the UV range was detected as well as a constant increase of the photocurrent up to 20% (16%) for a planar (nanocones) junction in the range 600–800 nm. The analysis of the photocurrent versus the number of drop casted nanoparticles layers allows us to evidence a down-shifting mechanism in the U-V range and an adaptative index effect below the threshold of absorption.

The defect responsible for the minority‐carrier lifetime in p‐type Czochralski silicon introduced by electron irradiation has been detected and characterized by deep‐level transient spectroscopy and spin‐dependent recombination. From the isotropic g value (2.0055), the defect is tentatively identified as a Si dangling bond originating from a vacancy cluster. Its energetic location in the gap is at 630 meV below the conduction band. The electron and hole cross sections and their variation with temperature have been determined, and found to account for the minority‐carrier lifetime of the material.

We investigated the growth of silicon nanowires from Au-rich catalyst droplets by two different methods: chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). The growth rate is found to be diameter-dependent and increases with increasing precursor partial pressures. The comparison of the experimental results with models shows that the contribution of Si atoms that diffuses from the substrate and the NW sidewalls toward the catalyst droplet can be neglected in CVD for the different pressures used in this study, whereas it is the major source of Si supply for the MBE growth. In addition, by decreasing the number density of catalyst droplet prior to the NW growth in CVD, it is also found that this parameter affects the NWs morphology, increasing the tapering effect when the silane partial pressure is small enough to allow gold atom diffusion from the catalyst droplet.

Defects in monocrystalline silicon have been studied in the past, in particular, defects induced by room‐temperature electron and proton irradiations on both n‐ and p‐type materials, and most of the corresponding defects have been tentatively identified. However, there are still several questions which remain to be answered such as the nature and behavior of the defects introduced in the range 4–300 K. In this work Czochralski‐grown p‐type material has been irradiated at three different temperatures (90, 200, and 300 K) and characterized by deep‐level transient spectroscopy (DLTS) and lifetime measurements. The data show that the defects created after irradiations at 90 and 200 K are different from those reported in the literature for irradiations at 4, 77, and 300 K, showing that three annealing steps exist between 4 and 300 K. These defects are characterized and a tentative identification of them is made. Finally, an attempt to detect the defects responsible for the lifetime, i.e., the recombination cen...

Scanning tunneling microscopy (STM) is used to study low temperature grown GaAs layers. Excess As gives rise to a high concentration of As antisites (AsGa). On these point defects, tunneling spectroscopy reveals a band of donor states. In fact, the measured tunneling current results from a pure tunneling current between the energy levels of the STM tip and the AsGa energy level Et followed by an exchange of carriers between Et and the bands or between Et and donor states of neighbor point defects. We determine the influence of both contributions on the current. We show that hopping conduction is required to explain the observation of the midgap states in tunneling spectroscopy.

Using the capacitance‐voltage technique and mainly deep‐level transient spectroscopy (DLTS), we studied silicon‐silicon epitaxial layer structures grown by the limited reaction process using silane diluted in hydrogen. We develop a simple but original model to interpret the DLTS data. The use of the two techniques allows the determination of the physical parameters of the structure, i.e., the doping level, the thickness of the layers, and the density and capture cross section of the states localized at the different interfaces of the structure.

InAs quantum boxes separated by GaAs spacer layers are known to exhibit a vertical self-organization along the growth direction. The alignment probability between two sets of quantum boxes depends strongly on the spacer layer thickness Zs. In this letter, we study samples containing multiple arrays of quantum boxes separated by GaAs spacer layers of various thicknesses, using cross-sectional scanning tunneling microscopy. This work experimentally evidences that the spacer layer characteristic thickness Zs0 below which a vertical self-alignment occurs, depends on the size of the quantum boxes. These results are interpreted using a theoretical two-dimensional model.

Nanogaps are usually combined with synthetic or biological molecules to produce nanodevices having novel properties. This combination is better realized by controlling the chemical properties of the nanogap. We show here that the presence of a specific chemical group inside nanogaps (30-90 nm) can be probed electrically using 10 nm gold nanoparticles derivatized by complementary functional groups. 100-10(4)-fold current increases were observed following the site-specific insertion of gold nanoparticles into the nanogap.

This article reports on the growth of Au islands on the Si(111) surface as a function of the Au evaporation rate and the temperature of the surface in ultrahigh vacuum. By controlling the density of the Au islands and their size, it is possible to subsequently grow single vertically oriented Si nanowires on top of (111)-oriented silicon micropillar and analyze their chemical composition at the atomic scale with the femtosecond laser assisted tomographic atom probe. Three-dimensional images of the atom distribution in the nanowire, in particular, the distribution of boron impurities, are obtained and compared to the intended impurity concentration.

The authors studied the transport in n- and p-type silicon nanowires. When the temperature decreases from 325to75K, they observed a variation of the nanowire resistance, consistent with a transport governed by electron-phonon scattering. The lateral size of the nanowire down to 25nm is not found to cause further surface scattering, due to the presence of interface states which create a depleted region in the nanowires. Such depleted region allows thus to keep a carrier mobility in the nanowires similar to the bulk one.

Using mainly capacitance measurements vs. voltage, time and temperature, we present, for the first time, a characterization of a layer of defects located near the interface of a Schottky diode. A complete model of the electrical behaviour of such a layer is given, and compared with success to experimental data. The model allows the determination of the thickness of the layer, the concentration of the defect, its associated level in the gap of the semiconductor and its capture cross-section vs. the temperature.

The experimental creation and annihilation of defects on single-walled carbon nanotubes (SWCNT) with the tip of a scanning tunneling microscope are reported. The technique used to manipulate the wall structure of a nanotube at the atomic scale consists of a voltage ramp applied at constant tunneling current between the tip and the nanotube adsorbed on a gold substrate. While topographic images show an interference pattern at the defect position, spatially resolved tunneling spectroscopy reveals the presence of localized states in the band gap of the nanotube. Removal of the defect by the same procedure demonstrates the reversibility of the process. Such a precise control in the local modification of the nanotube wall opens up new opportunities to tailor SWCNT electronic properties at will.

Silicon δ-doped layers in GaAs have been studied using scanning tunneling microscopy and scanning tunneling spectroscopy on cleaved (110) surfaces. The samples were grown using molecular beam epitaxy with two growth temperatures: 480 and 580 °C. The concentration of the δ-doped layer was 0.03 monolayer. We show that, as in the case of bulk doping, the silicon has an amphoteric character: both SiGa and SiAs are observed at 480 °C and donors are formed before acceptors. At 580 °C, the spatial repartition of silicon evidences the segregation of silicon.

Scanning tunneling spectroscopy allows the determination of the density of states of the involved electrodes as well as the associated barrier height. We have measured the barrier height between tungsten tip and the GaAs (110) surface with a scanning tunneling microscope (STM) in ultrahigh vacuum. When the tunneling junction exhibited a low barrier height, we observed and explained an apparent widening of the GaAs gap which can mislead the real doping concentration of the sample. A theoretical model based on the self-consistent Keldysh–Green functions formalism is used to calculate the STM current. It shows a possible lowering of the barrier height according to the adsorption of As or Ga atoms on the apex of the tungsten tip.

Despite the importance of the isocyanate group in chemistry, very few examples of isocyanate-modified silicas have been reported, and all of the strategies described so far led to partial or total hydrolysis or condensation of the isocyanate group. By synthesizing trichlorosilane isocyanate as the coupling reagent, we show that oxidized silicon wafers are successfully modified with the isocyanate group. Our method is achieved in mild conditions, at low temperature, without side-reactions and allows the formation of a self-assembled monolayer (SAM) of isocyanates. The isocyanate group then offers a flexible way to further functionalize silica substrates with different nucleophiles, due to its high and specific reactivity.

Electrostatic properties of single-separated multiwalled carbon nanotubes (MWCNTs) deposited on a dielectric layer have been investigated by charge injection and electric force microscopy (EFM) experiments. We found that upon local injection from the biased EFM tip, charges delocalize over the whole nanotube length (i.e., 1-10 μ m), consistent with a capacitive charging of the MWCNT-substrate capacitance. In addition, the insulating layer supporting the nanotubes is shown to act as a charge-sensitive plate for electrons emitted from the MWCNTs at low electric fields, thus allowing the spatial mapping of MWCNT field-emission patterns. © 2005 American Institute of Physics.

Isochronal annealing experiments on defects produced by 1.5 MeV electron irradiation at room temperature have been performed on n‐type SiGe alloys. Two defects have been studied, labeled P1 and P2, with associated energy levels located at EC−0.32 eV and EC−0.49 eV, respectively, and capture cross sections equal to 1×10−15 cm2 and 2×10−15 cm2. P1 anneals out at 560 K and P2 at 470 K. A comparison with the defects known in Si and Ge allows us to propose the following identification: P1 is associated with the 0/− charge state of the divacancy (V–V)Si and P2 is associated with the 0/− charge state of the E center (P–VSi).

We address the issue of dipole-dipole interaction measurements at the nanometer scale. Electric dipoles with tunable effective momentum in the range 10(3)-10(4) D are generated by charge injection in single silicon nanoparticles on a conductive substrate and probed by a spectroscopic electric force microscopy analysis. Weak dipole-dipole force gradients are measured and identified from their quadratic momentum dependence. The results suggest that dipolar interactions associated with atomic-scale charge displacements or molecules can be probed by noncontact atomic force microscopy.

Ga1-xMnxAs layers with Mn composition of up to 6.2% are investigated by cross-sectional scanning tunneling microscopy and spectroscopy. We identify in the tunneling spectra contributions from Mn-Ga(-) acceptor states, compensating As-Ga(2+) donor states, and additional compensating donor states, which we suggest to be Mn-i(2+) interstitials. On basis of the observed Fermi level shift and a charge carrier compensation analysis, we deduce the concentration of Mn-i(2+) interstitials. Furthermore, scanning tunneling microscopy images suggest an inhomogeneous distribution of Mn dopant atoms. (C) 2003 American Institute of Physics.

The authors report on admittance measurements applied to Al/sub 0.48/In/sub 0.52/As/Ga/sub 1-x/In/sub x/As/ Al/sub 0.48/In/sub 0.5/As single quantum wells grown on InP. Evidence is presented for defect assisted tunneling in these structures. The Ga/sub 1-x/In/sub x/As well is either lattice matched (x = 0.53) or lattice mismatched (x = 0.60) to Al/sub 0.48/In/sub 0.52/As. To ensure that the observed conductance peak corresponds to the thermionic emission from the well above the barriers, complementary characterization of the traps lying in AlInAs layers has been performed by means of deep level transient spectroscopy and admittance measurements. >

This paper presents a complex set of electrical measurements used for the characterization of solar cells for spatial application. The goal of this characterization is to predict the active p layer boron optimum concentration and to find the type of defects that are responsible for the decreasing of the minority carrier lifetime both in nonirradiated and irradiated solar cells.

Using a scanning tunneling microscope (STM), it is possible to get microscopic characterization of defects in semiconductors. The atomic resolution allows the study of local atomic configuration, interface roughness or defects migration. Local spectroscopy allows for the determination the local density of state associated with the defect and its charge state. Illustrations of these possibilities are presented concerning the roughness of GaSb–InAs or InP–GaInAs interfaces and InAs quantum boxes buried in GaAs, the diffusion of vacancy in silicon, silicon dopants in GaAs, the charge state of the arsenic vacancy in GaAs and finally the STM and scanning tunneling spectroscopy (STS) of the antisite As Ga in GaAs. Finally, a discussion is proposed about a basic problem: how tunneling current occurs through defects? There is a necessary coupling between the energy level associated with the defect and the conduction or valence bands. This coupling is analysed in terms of emission and capture rates usually used for electrical characterization of the defects. Some new experimental suggestions are also proposed concerning the determination of the emission rate of a defect using the STM technique.

Starting from silicon on insulator substrates, we show that a thermal treatment (in the 600–900 °C range) induces the creation of silicon islands. To characterize the island formation as well as the initial silicon layer thickness, we use in situ Auger electron spectroscopy analysis in an ultrahigh vacuum chamber. The island size and density are studied with an ex situ atomic force microscope. We show that the formation temperature of the islands increases from 575 to 875 °C as the initial silicon layer thickness increases from 1 to 19 nm. For the 1 nm thickness, the minimum island size is reached (semispherical shape with a 16 nm diameter). The phenomena involved in the island formation are discussed and the study of the variations of the calculated stress tensor (IMPACT software) as a function of the thermal treatment explain the behavior of the top silicon layer.

We have studied the defects introduced by electron irradiation in n-doped Si-Ge alloys. Capacitance techniques were used to characterize the defects. Four deep levels are detected, labeled S1 to S4. Only the two main defects, S3 and S4, have been studied, the other ones having low concentration. Their associated energy levels lie at ${\mathit{E}}_{\mathit{c}}$-0.32 and ${\mathit{E}}_{\mathit{c}}$-0.49 eV with capture cross sections equal to 6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ and 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$, respectively. These two defects have a donor character. The study of their introduction rate versus the fluence indicates that they are not primary intrinsic defects but complexes. The variation of the S4 concentration versus the energy of irradiation shows that the threshold energy is 31\ifmmode\pm\else\textpm\fi{}2 eV. Finally, a comparison with electron-irradiation-induced defects in silicon and germanium is made.

We have determined the filling properties of nanogaps with chemically heterogeneous walls. The quantitative criteria we present allow the prediction of the liquid loading of the nanostructure. They can easily be applied in combination with contact-angle measurements on planar substrates of the nanogap materials. We present an application of the theory to a recently developed nanogap biosensor. Chemical force microscopy (CFM) is employed to characterize the initial silanol properties of the gap. The functionality of the complex surface chemistry of the biosensor is demonstrated by the observation of functionalized nanoparticles in the gap with its resulting characteristic current-voltage relationship.

The effect of thermal annealing on the composition and morphology of the surface of strained SiGe layers grown on Si is investigated in the temperature range 400–900 °C. We show that Ge segregation starts at 400 °C and increases with increasing temperature. Above 700 °C, strain relaxation leads to the formation of islands on the surface. By combining atomic force microscopy and Auger electron spectroscopy we demonstrate that these islands are Ge rich and that at 900 °C rather pure Ge islands are formed on a Si rich underlying layer.

A new technique of measuring capture cross sections is presented, based on a simplified analysis of the kinetics of filling of the traps in the slow regime of filling. The main interest of this technique is that it uses large impulsions, easy to apply on any sample, contrary to previous work which needed very short impulsions. A simplified theory is also developed to extract capture cross sections from the experimental data. The results are compared with those obtained by other techniques on the trap E3, created by irradiation of electrons in n‐type GaAs, showing very good agreement.

Bulk Czochralski and VPE layers of n-GaAs have been irradiated with 1 MeV electrons at about 300°C, temperature at which primary defects are not stable. Up to seven electron traps are thus observed to be introduced. The dominant trap (I1) has an activation energy and a capture cross section very close to the so-called EL2 center, which suggests that EL2 could be introduced by a high temperature electron irradiation. The introduction rate of I1 is about 1cm-1, that is the same order of magnitude as those of the traps (E-traps) introduced by a room temperature irradiation, and two orders of magnitude larger than those of the traps observed after the annealing (200–300°C) of the E-traps. With a starting dopant concentration of 2×1016cm-3, we have obtained a quite complete compensation of free carriers after a total fluence of 1017e/cm2.

Photovoltaic parameters, minority carrier lifetime and defect characterization have been used to study the degradation of P/N lithium-doped solar-cells irradiated by 1 MeV electrons. This work points out that A centers ([V-01 complexes) and divancancies play a dominant role in the degradation of the cells. Annealing studies in conditions similar to the space conditions show that an important recovery of the cell parameters can be obtained. A rapid disappearance of both A centers and divancancies is observed and a new defect presumably associated with lithium (ΔE = E c-0.36 eV; σn≈2.10−16 cm2) forms simultaneoulsy.

An experimental study of the capacitance transients observed in 49-40 A GaAsGaAlAs Si doped superlattices has been performed. These transients are not exponential and cannot be ascribed to isolated and localized defects but to a distribution of states presumably located at the interfaces. A model of the response of these states to a filling pulse followed by the emission of the electrons in the conduction miniband, assuming a constant density of states at the interfaces and taking into account they are periodically localized in the space charge region, has been performed. This model is able to reproduce the transient kinetics and it allows in addition to determine the value of the density of these states, which appear to be important. Then we have improved the model, taking into account the charges trapped on the numerous interfaces, located in the space charge region. The main result is the non parabolicity of the potential across the junction. The variations induced in the determination of the concentration of these traps are presented.

Knowledge of the temperature of a junction is very useful for experimenters or technologists. We propose here an easy but accurate method to measure it for any type of junction, isolated or in a device. The method is based on a numerical analysis of a transient signal, provided by the change of capacitance of the space‐charge region of the junction due to the emission of majority carriers from a trap. This method has been tested on different junctions made on n‐GaAs with traps introduced by electron irradiation or originating from the technology. Here we provide, for illustration, results showing the variation of a junction temperature as a function of the injected current density.

We have performed a systematic study of carrier injection annealing of the defects introduced by electron irradiation in n-type GaAs in the temperature range 300--355 K and with injected current densities ranging from 0.5 to 2 A${\mathrm{?}}^{\mathrm{\ensuremath{-}}2}$. We first observed that the various defects do not anneal in the same fashion as stated in a previous study. We show that, because the associated activation energies decrease continuously as the injected current increases, the model previously proposed for the enhanced annealing in which the enhancement is induced by the energy which is released when a minority carrier is trapped on the defect site cannot apply. However, this enhanced annealing can be quantitatively understood if the annealing is induced by successive changes of the defect charge state when it traps alternatively minority and majority carriers.

The lattice site location of Tm and Er implanted in GaAs were studied using the RBS/channeling technique. For implanted doses up to 10 14 cm −2 both elements show nearly 100% substitutionality in the GaAs lattice. Further increasing the Er concentration causes a fraction to occupy an interstitial regular site. Axial channeling measurements along planar directions show that Er forms ErAs precipitates. The angular scans are compared with Monte Carlo simulations and the Er fractions in the different lattice sites are calculated. With the incorporation of oxygen in the GaAs matrix the fraction of Er in substitutional sites increases. Taking into account these results the photoluminescence (PL) behavior of Er in these samples is discussed and results obtained with Er doped GaAs grown by Molecular Beam Epitaxy are understood.

The results of parallel RBS and photoluminescence studies of erbium implanted GaAs are presented. Low dose implantations do not produce any significant PL signals, and the dose must be in the range le14 to le15 /cm2 in order that Er related emission dominates in the PL spectrum. A comprehensive analysis of the effects of coimplantation with oxygen on the Er luminescence is reported and the data are compared to those of GaAs:Er and AIGaAs:Er samples grown by MBE. The evidence indicates that, at high doses, ErAs precipitates are formed unless oxygen is co-implanted, and that the Er atoms which produce the luminescence occupy substitutional Ga sites.