Tunneling Atomic Layer-Deposited Aluminum Oxide: a Correlated Structural/Electrical Performance Study for the Surface Passivation of Silicon Junctions

Passivation is a key process for the optimization of silicon p-n junctions. Among the different technologies used to passivate the surface and contact interfaces, alumina is widely used. One key parameter is the thickness of the passivation layer that is commonly deposited using atomic layer deposition (ALD) technique. This paper aims at presenting correlated structural/electrical studies for the passivation effect of alumina on Si junctions to obtain optimal thickness of alumina passivation layer. High-resolution transmission electron microscope (HRTEM) observations coupled with energy dispersive X-ray (EDX) measurements are used to determine the thickness of alumina at atomic scale. The correlated electrical parameters are measured with both solar simulator and Sinton's Suns-Voc measurements. Finally, an optimum alumina thickness of 1.2 nm is thus evidenced.

Triangular nanoperforation and band engineering of InGaAs quantum wells: a lithographic route toward Dirac cones in III-V semiconductors

The design of two-dimensional periodic structures at the nanoscale has renewed attention for band structure engineering. Here, we investigate the nanoperforation of InGaAs quantum wells epitaxially grown on InP substrates using high-resolution e-beam lithography and highly plasma based dry etching. We report on the fabrication of a honeycomb structure with an effective lattice constant down to 23 nm by realising triangular antidot lattice with an ultimate periodicity of 40 nm in a 10 nm thick InGaAs quantum well on a p-type InP. The quality of the honeycomb structures is discussed in detail, and calculations show the possibility to measure Dirac physics in these type of samples. Based on the statistical analysis of the fluctuations in pore size and periodicity, calculations of the band structure are performed to assess the robustness of the Dirac cones with respect to distortions of the honeycomb lattice.

Electrical detection of plasmon-induced isomerization in molecule-nanoparticle network devices

We use a network of molecularly linked gold nanoparticles (NPSAN: nanoparticle self-assembled network) to demonstrate the electrical detection (conductance variation) of plasmon-induced isomerization (PII) of azobenzene derivatives (azobenzene bithiophene: AzBT). We show that PII is more efficient in a 3D-like NPSAN (cluster-NPSAN) than in a purely two-dimensional NPSAN (i.e., a monolayer of AzBT functionalized Au NPs). By comparison with the usual optical (UV-visible light) isomerization of AzBT, PII shows faster (a factor > ∼10) isomerization kinetics. Possible PII mechanisms are discussed: electric field-induced isomerization, two-phonon process, and plasmon-induced resonance energy transfer (PIRET), the latter being the most likely.

Enhanced photovoltaic performance of inverted polymer solar cells through atomic layer deposited Al2O3 passivation of ZnO-nanoparticle buffer layer

In this work, an atomic layer deposited (ALD) Al₂O₃ ultrathin layer was introduced to passivate the ZnO-nanoparticle (NP) buffer layer of inverted polymer solar cells (PSCs) based on P3HT:PCBM. The surface morphology of the ZnO-NP/Al₂O₃ interface was systematically analyzed by using a variety of tools, in particular transmission electron microscopy (TEM), evidencing a conformal ALD-Al₂O₃ deposition. The thickness of the Al₂O₃ layers was optimized at the nanoscale to boost electron transport of the ZnO-NP layer, which can be attributed to the suppression of oxygen vacancy defects in ZnO-NPs confirmed by photoluminescence measurement. The optimal inverted PSCs passivated by ALD-Al₂O₃ exhibited an ∼22% higher power conversion efficiency than the control devices with a pristine ZnO-NP buffer layer. The employment of the ALD-Al₂O₃ passivation layer with precisely controlled thickness provides a promising approach to develop high efficiency PSCs with novel polymer materials.

Local Schottky contacts of embedded Ag nanoparticles in Al2O3/SiN x :H stacks on Si: a design to enhance field effect passivation of Si junctions

This paper describes an original design leading to the field effect passivation of Si n⁺-p junctions. Ordered Ag nanoparticle (Ag-NP) arrays with optimal size and coverage fabricated by means of nanosphere lithography and thermal evaporation, were embedded in ultrathin-Al₂O₃/SiN _x :H stacks on the top of implanted Si n⁺-p junctions, to achieve effective surface passivation. One way to characterize surface passivation is to use photocurrent, sensitive to recombination centers. We evidenced an improvement of photocurrent by a factor of 5 with the presence of Ag NPs. Finite-difference time-domain (FDTD) simulations combining with semi-quantitative calculations demonstrated that such gain was mainly due to the enhanced field effect passivation through the depleted region associated with the Ag-NPs/Si Schottky contacts.

Shallow Heavily Doped n++ Germanium by Organo-Antimony Monolayer Doping

Functionalization of Ge surfaces with the aim of incorporating specific dopant atoms to form high-quality junctions is of particular importance for the development of solid-state devices. In this study, we report the shallow doping of Ge wafers with a monolayer doping strategy that is based on the controlled grafting of Sb precursors and the subsequent diffusion of Sb into the wafer upon annealing. We also highlight the key role of citric acid in passivating the surface before its reaction with the Sb precursors and the benefit of a protective SiO₂ overlayer that enables an efficient incorporation of Sb dopants with a concentration higher than 10²⁰ cm^-3. Microscopic four-point probe measurements and photoconductivity experiments show the full electrical activation of the Sb dopants, giving rise to the formation of an n++ Sb-doped layer and an enhanced local field-effect passivation at the surface of the Ge wafer.

Enhancement of Electrical Properties of Nanostructured Polysilicon Layers Through Hydrogen Passivation

The light absorption of polysilicon planar junctions can be improved using nanostructured top surfaces due to their enhanced light harvesting properties. Nevertheless, associated with the higher surface, the roughness caused by plasma etching and defects located at the grain boundary in polysilicon, the concentration of the recombination centers increases, leading to electrical performance deterioration. In this work, we demonstrate that wet oxidation combined with hydrogen passivation using SiN(x):H are the key technological processes to significantly decrease the surface recombination and improve the electrical properties of nanostructured n(+)-i-p junctions. Nanostructured surface is fabricated by nanosphere lithography in a low-cost and controllable approach. Furthermore, it has been demonstrated that the successive annealing of silicon nitride films has significant effect on the passivation quality, resulting in some improvements on the efficiency of the Si nanostructure-based solar cell device.

Confinement-modulated junctionless nanowire transistors for logic circuits

We report the controlled formation of nanoscale constrictions in junctionless nanowire field-effect transistors that efficiently modulate the flow of the current in the nanowire. The constrictions act as potential barriers and the height of the barriers can be selectively tuned by gates, making the device concept compatible with the crossbar geometry in order to create logic circuits. The functionality of the architecture and the reliability of the fabrication process are demonstrated by designing decoder devices.

Nanoscale carrier multiplication mapping in a Si diode

Carrier multiplication (CM), the creation of electron-hole pairs from an excited electron, has been investigated in a silicon p-n junction by multiple probe scanning tunneling microscopy. The technique enables an unambiguous determination of the quantum yield based on the direct measurement of both electron and hole currents that are generated by hot tunneling electrons. The combined effect of impact ionization, carrier diffusion, and recombination is directly visualized from the spatial mapping of the CM efficiency. Atomically well-ordered areas of the p-n junction surface sustain the highest CM rate, demonstrating the key role of the surface in reaching high yield.

Direct characterization of native chemical ligation of peptides on silicon nanowires

We describe the site-specific and chemoselective immobilization of peptides on hydrogen-terminated silicon nanowires (SiNWs) using native chemical ligation (NCL) (i.e., the reaction of a thioester group with a cysteine moiety to give a stable amide bond). The SiNWs investigated in this work were grown via a vapor-liquid-solid mechanism and functionalized with a thioester moiety. The immobilization of the peptides on the SiNWs was demonstrated by synthesizing peptides with an N-terminal cysteine residue and labeled with tetramethylrhodamine or trifluoromethyl groups that were detected by fluorescence and X-ray photoelectron spectroscopy, respectively. The peptides labeled with tetramethylrhodamine or trifluoromethyl groups for fluorescence or X-ray photoelectron spectroscopy (XPS) detection studies were synthesized with an N-terminal cysteine residue. N-Terminal seryl peptides and carboxy-terminated SiNWs were used as controls to demonstrate the chemoselectivity of the peptide immobilization.

Narrowing the length distribution of Ge nanowires

Synthesis of nanostructures of uniform size is fundamental because the size distribution directly affects their physical properties. We present experimental data demonstrating a narrowing effect on the length distribution of Ge nanowires synthesized by the Au-catalyzed molecular beam epitaxy on Si substrates. A theoretical model is developed that is capable of describing this puzzling behavior. It is demonstrated that the direction of the diffusion flux of sidewall adatoms is size dependent and has a major effect on the growth rate of differently sized nanowires. We also show that there exists a fundamental limitation on the maximum nanowire length that can be achieved by molecular beam epitaxy where the direction of the beam is close to the growth axis.

Efficient photogeneration of charge carriers in silicon nanowires with a radial doping gradient

by performing electrodeless time-resolved microwave conductivity measurements, the efficiency of charge carrier generation, their mobility, and the decay kinetics on photoexcitation were studied in arrays of Si nanowires grown by the vapor-liquid-solid mechanism. Large enhancements in the magnitude of the photoconductance and charge carrier lifetime are found depending on the incorporation of impurities during the growth. They are explained by the internal electric field that builds up, due to higher doped sidewalls, as revealed by detailed analysis of the nanowire morphology and chemical composition.

Atomic scale investigation of silicon nanowires and nanoclusters

In this study, we have performed nanoscale characterization of Si-clusters and Si-nanowires with a laser-assisted tomographic atom probe. Intrinsic and p-type silicon nanowires (SiNWs) are elaborated by chemical vapor deposition method using gold as catalyst, silane as silicon precursor, and diborane as dopant reactant. The concentration and distribution of impurity (gold) and dopant (boron) in SiNW are investigated and discussed. Silicon nanoclusters are produced by thermal annealing of silicon-rich silicon oxide and silica multilayers. In this process, atom probe tomography (APT) provides accurate information on the silicon nanoparticles and the chemistry of the nanolayers.

Synthesis of long group IV semiconductor nanowires by molecular beam epitaxy

We report the growth of Si and Ge nanowires (NWs) on a Si(111) surface by molecular beam epitaxy. While Si NWs grow perpendicular to the surface, two types of growth axes are found for the Ge NWs. Structural studies of both types of NWs performed with electron microscopies reveal a marked difference between the roughnesses of their respective sidewalls. As the investigation of their length dependence on their diameter indicates that the growth of the NWs predominantly proceeds through the diffusion of adatoms from the substrate up along the sidewalls, difference in the sidewall roughness qualitatively explains the length variation measured between both types of NWs. The formation of atomically flat {111} sidewalls on the <110>-oriented Ge NWs accounts for a larger diffusion length.

Coulomb energy determination of a single Si dangling bond

Determination of the Coulomb energy of single point defects is essential because changing their charge state critically affects the properties of materials. Based on a novel approach that allows us to simultaneously identify a point defect and to monitor the occupation probability of its electronic state, we unambiguously measure the charging energy of a single Si dangling bond with tunneling spectroscopy. Comparing the experimental result with tight-binding calculations highlights the importance of the particular surrounding of the localized state on the effective charging energy.

The collagen assisted self-assembly of silicon nanowires

The paper reports on self-assembly of silicon nanowire junctions assisted by protocollagen, a low cost soluble long fiber protein and precursor of collagen fibrils. First, the collagen was combed on an octadecyl-terminated silicon surface with gold electrodes. Then the combed surface was exposed to an aqueous suspension of silicon nanowires. In order to increase electrostatic interactions between the positively charged collagen and the nanowires, the nanowires were chemically modified with negatively charged sulfonate groups. The interaction of collagen with the sulfonated nanowires, which mimics the native collagen/heparin sulfate interaction, induced self-assembly of the nanowires localized between gold electrodes. The proof of concept for the formation of spontaneous electrode-nanowire-electrode junctions using collagen as a template was supported by current-voltage measurements.

Electrical detection of human immunoglobulins G from human serum using a microbiosensor

A biosensor for the electrical detection of human antibodies from serum has been fabricated and experimentally demonstrated. The device is based on the immobilization of proteins used as probes between a set of microelectrodes. Incubation with diluted human serum was followed by incubation with anti-human secondary antibodies labeled with gold nanoparticles (GNPs) and then precipitation of silver on the nanoparticles. The output of the device was defined as the percentage of short-circuited microelectrodes after silver deposition independently of the gap conductance. Two model probes were studied: protein A and goat antibodies. The effects of the microgap spacing (5, 10, 15 or 20 microm) and the duration of the silver treatment were examined. The data obtained showed that a large spacing (20 microm) led to poor sensitivity. Alternately, 5 microm gaps led to high sensitivity and saturation of the signal. Interestingly, 10-15 microm gaps enabled a non-saturated and distinct signal for both probes that was correlated with the GNP density between the microgaps as determined by atomic force microscopy. Different capture efficiencies could be easily distinguished. The biosensor described here is easy to use and thus can be applied to real detection experiments.

Electron transport via local polarons at interface atoms

Electronic transport is profoundly modified in the presence of strong electron-vibration coupling. We show that in certain situations, the electron flow takes place only when vibrations are excited. By controlling the segregation of boron in semiconducting Si(111)-square root 3 x square root 3 R 30 degrees surfaces, we create a type of adatom with a dangling-bond state that is electronically decoupled from any other electronic state. However, probing this state with scanning tunnelling microscopy at 5 K yields high currents. These findings are rationalized by ab-initio calculations that show the formation of a local polaron in the transport process.

Fluidics of a nanogap

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.

Detecting the chemoselective ligation of peptides to silicon with the use of cobalt-carbonyl labels

While fluorescent-based methods are generally used to detect the immobilization and the interactions of biomolecules to solid supports, recent studies have shown their limitations in the case of silicon surfaces. As an alternative, we investigated the synthesis of peptides labeled with a metal transition complex and their subsequent immobilization to the silicon surfaces. The feasibility of using such probes has been explored by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). By starting with hydrogen-terminated or oxidized silicon surfaces, we functionalized those surfaces with semicarbazide groups and showed the site-specific linkage of glyoxylyl peptides labeled with a Co2(CO)6 moiety.

Semicarbazide-functionalized Si(111) surfaces for the site-specific immobilization of peptides

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.

Direct evidence for shallow acceptor states with nonspherical symmetry in GaAs

We investigate the energy and symmetry of Zn and Be dopant-induced acceptor states in GaAs using cross-sectional scanning tunnelling microscopy (STM) and spectroscopy at low temperatures. The ground and first excited states are found to have a nonspherical symmetry. In particular, the first excited acceptor state has a T(d) symmetry. Its major contribution to the STM empty-state images allows us to explain the puzzling triangular shaped contrast observed in the empty-state STM images of acceptor impurities in III-V semiconductors.

Semiconducting surface reconstructions of p-type Si(100) substrates at 5 K

We report scanning tunneling microscopy (STM) studies of the technologically important Si(100) surface that reveal at 5 K the coexistence of stable surface domains consisting of the p(2 x 1) reconstruction along with the c(4 x 2) and p(2 x 2) reconstructions. Using highly resolved tunneling spectroscopic measurements and tight binding calculations, we prove that the p(2 x 1) reconstruction is asymmetric and determine the mechanism that enables the contrast variation observed in the formation of the bias-dependent STM images for this reconstruction.

Probing nanoscale dipole-dipole interactions by electric force microscopy

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.

Polypeptide Semicarbazide Glass Slide Microarrays: Characterization and Comparison with Amine Slides in Serodetection Studies

We have described in the accompanying article the preparation of peptide-protein semicarbazide microarrays and their use for the simultaneous serodetection of antibodies directed against different pathogens. Here, we present a comparative study between semicarbazide and amine glass slides in an immunofluorescent serodetection assay using HIV (Gp120, Gp41), HCV (mix-HCV, core, NS3, and NS4), and HBV (HBs) recombinant antigens. Amine and semicarbazide surfaces displayed the same sensitivity for antibodies detection just after printing. However, the reactivity of protein antigens changed rapidly upon aging on amine slides but not on semicarbazide slides. Peptide or protein semicarbazide microarrays were found to be remarkably stable for months. Additional data concerning the characterization of the semicarbazide surface (homogeneity of the slides, chemical stability, contact angle measurements, atomic force microscopy studies, reproducibility of serodetection results) are also presented and discussed.