Conductive Atomic Force Microscopy Technique Research Articles

Conductance of Threading Dislocations in InGaAs/Si Stacks by Temperature-CAFM Measurements

The stacks of III-V materials, grown on the Si substrate, that are considered for the fabrication of highly scaled devices tend to develop structural defects, in particular threading dislocations (TDs), which affect device electrical properties. We demonstrate that the characteristics of the TD sites can be analyzed by using the conductive atomic force microscopy technique with nanoscale spatial resolution within a wide temperature range. In the studied InGaAs/Si stacks, electrical conductance through the TD sites was found to be governed by the Poole-Frenkel emission, while the off-TDs conductivity is dominated by the thermionic emission process.

Nanoscale Mapping of Layer-Dependent Surface Potential and Junction Properties of CVD-Grown MoS2 Domains

In the present study, nanoscale variations in the work function values and the resulting changes in junction properties of chemical-vapor-deposited 2D MoS2 domains have been investigated as a function of number of layers using Kelvin probe force microscopy and conductive atomic force microscopy techniques. Raman spectroscopy has been employed to obtain the magnitude of difference between E2g and A1g peaks, which has been used as a signature of the number of layers. Surface potential of MoS2 monolayer sample exhibits a value of −427 mV (∼7.2 mV for bulk) along with a large spread of ∼29 mV (∼3 mV for bulk). The present study shows that the optical and electronic properties of MoS2 one to two layer samples exhibit a large difference from its bulk counterpart. These characteristic features remain intact, even in the presence of adsorbates and defects, which result in spread in surface potential values and corresponding changes in junction characteristics. These results are important for the application of chemical-vapor-deposition (CVD)-grown MoS2 monolayers for semiconductor devices.

Fast Fourier transform scanning spreading resistance microscopy: a novel technique to overcome the limitations of classical conductive AFM techniques

A new atomic force microscopy (AFM)-based technique named fast Fourier transform scanning spreading-resistance microscopy (FFT-SSRM) has been developed. FFT-SSRM offers the ability to isolate the local spreading resistance (Sr) from the parasitic series resistance (probe, bulk, and back contact). The parasitic series resistance limits the use of classical SSRM in confined volumes and on very highly doped materials, two increasingly important situations in nanoelectronic components. This is realized via a force modulation at controlled frequency (affecting the SR component) and the extraction of the resistance amplitude at the modulation frequency, performing an FFT-based lock-in deconvolution. A systematic evaluation of the FFT-SSRM performances (i.e., resolution, dynamic range, sensitivity, and repeatability) is presented. The impact of various parameters (i.e., modulation frequency and amplitude or cutoff frequency of the current amplifier) on the performances of FFT-SSRM has been evaluated. We demonstrate the possibility to overcome sensitivity losses due to tip saturation in highly doped material and the utility of the technique in two different structures, presenting isolated and confined volumes.

Investigating inhomogeneous electronic properties of radial junction solar cells using correlative microscopy

Solar cells with radial junctions based on silicon nanowires were investigated using correlative microscopy in order to determine the nature and origin of previously reported inhomogeneity of their electronic properties. For correlating various microscopy techniques, we have prepared sets of three Vickers type nanoindents arranged in a right triangle with 20 µm sides as marks of local frame of reference. Due to the shape of the indents (squares with 4 µm sides and clear diagonals) the position of the marks can be located with high precision by various microscopes. This makes it possible to correlate the results from scanning electron microscopy, Kelvin probe force microscopy and conductive atomic force microscopy techniques on the same place of the sample, with a precision down to individual nanowires, obtaining new information about the electronic inhomogeneity. Using the conductive AFM we analyzed the growth process of silicon nanowires step by step in order to find possible origins of the local (photo)current variations.

Electrical characterization of grain boundaries of CZTS thin films using conductive atomic force microscopy techniques

Electrical characterization of grain boundaries (GB) of Cu-deficient CZTS (Copper Zinc Tin Sulfide) thin films was done using atomic force microscopic (AFM) techniques like Conductive atomic force microscopy (CAFM), Kelvin probe force microscopy (KPFM) and scanning capacitance microscopy (SCM). Absorbance spectroscopy was done for optical band gap calculations and Raman, XRD and EDS for structural and compositional characterization. Hall measurements were done for estimation of carrier mobility. CAFM and KPFM measurements showed that the currents flow mainly through grain boundaries (GB) rather than grain interiors. SCM results showed that charge separation mainly occurs at the interface of grain and grain boundaries and not all along the grain boundaries.

Application of Electrochemical Impedance for Characterising Arrays of Bi2S3 Nanowires

Electrochemical Impedance Spectroscopy (EIS) was used to characterise the electrical properties of bismuth sulphide (Bi2S3) nanowires (NWs) templated within anodic aluminium oxide (AAO) membranes. A specially engineered cell, with a nominal electrolyte volume of 0.1–0.2ml, was used to hold and measure the electrochemical impedance of the fragile NW/AAO samples. An equivalent circuit model was developed to determine the filling density of nanowires within the porous templates. The EIS method can be utilised to probe the nanowire filling density in porous membranes over large sample areas, which is often unobtainable using electron microscopy and conductive atomic force microscopy techniques.

Microscopic Damage of Tungsten and Molybdenum Exposed to Low-Energy Helium Ions**supported by the National Magnetic Confinement Fusion Program of China (No. 2011GB108011), National Natural Science Foundation of China (No. 11405023) and the Scientific Research Fund of Liaoning Provincial Education Department (No. L2014539) and the Fundamental Research Funds for the Central Universities of China (No. DC201502080410)

Polycrystalline tungsten (W) and molybdenum (Mo) materials both non-annealed and annealed at temperatures of 800-1750°C have been irradiated with low-energy (220 eV), high-flux (∼1021 ions/m2 ·s) He+ at an irradiation temperature of 600°C and at a dose of 1.0×1025 ions/m2. This non-destructive conductive atomic force microscopy technique provides direct observation and comparison of surface swellings with growth of nanoscale defects in the irradiated materials. A coral-like surface structure and nanostructured defects were formed in W when irradiated at a He+ dose of 1.0×1025 ions/m2. Increasing the annealing temperature resulted in an increase in the size of nanostructured defects and serious surface damage of W. Compared to W, Mo suffered much less surface damage after being irradiated at various He+ doses.

Writing and Low-Temperature Characterization of Oxide Nanostructures

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.

Writing and low-temperature characterization of oxide nanostructures.

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.

Transfer of thin, patterned gold layers from poly(methyl methacrylate) stamp onto photoresist surface

A simple technique for the transfer of thin gold layer from poly(methyl methacrylate) (PMMA) stamp onto SU-8 photoresist is described and verified. The procedure comprises sputtering of thin gold layer on flat or laser patterned PMMA, bringing the PMMA stamp into contact with photoresist substrate, transferring of the gold layer onto photoresist and finally dissolution of the PMMA stamp. Surface morphology of as sputtered and transferred gold layers was determined by profilometry, confocal microscopy, and atomic force microscopy (AFM) techniques at three different scales. Electrical properties were examined by conductive AFM technique. The transfer from patterned PMMA stamp leads to simultaneous patterning of the gold layer and creation of a system of ordered conductive gold strips separated by non-conductive areas on photoresist surface.

Analysis of crystal defects on GaN-based semiconductors with advanced scanning probe microscope techniques

The correlation of surface pits with leakage currents in gallium nitride (GaN) films are studied with scanning probe microscopy (SPM) techniques. The analyses were performed at both single n-GaN films grown on free-standing GaN and completely processed GaN light-emitting diode (LED) structures on sapphire substrates both grown by metal organic chemical vapor phase epitaxy. Topographical SPM images acquired with ultra-sharp probes were superimposed with current maps obtained by conductive atomic force microscopy (CAFM). The applicability of two different modifications of CAFM techniques has been studied. For both sample types, CAFM has revealed a clear correlation between forward-bias leakage current and locations of surface pits. In case of the LED structure, additional local current–voltage characteristics show that enhanced current conduction occurs in both forward and reverse bias on surface pit positions.

Redox-Active π-Conjugated Organometallic Monolayers: Pronounced Coulomb Blockade Characteristic at Room Temperature

We report the electrical transport characteristics of a series of molecular wires, fc-C≡C-C6H4-SAc (fc = ferrocenyl; Ac = acetyl) and AcS-C6H4-C≡C-(fc)n-C≡C-C6H4-SAc (n = 2, 3), consisting of multiple redox-active ferrocenyl centers. The self-assembled monolayers of these molecular wires on Au surfaces were comprehensively characterized by electrochemistry and conductive atomic force microscopy techniques. Characterization of the wires revealed that electron transport is made extremely efficient by the organometallic redox states. There is a strong electronic coupling between ferrocenyl moieties, and superior electron-transport ability exists through these semirigid molecular wires. Standard rate constants for the electron transfer between the electrode and the ferrocenyl moieties were measured for the monolayers by a potential-step chronoamperometry technique. The electron conduction through the molecular wires was estimated using the monolayers as a bridge from the Au(111) metal surface to the gold tip of a conductive atomic force microscope (CAFM). Using the CAFM, Coulomb blockade behavior arising from the capacitive charging of the multinuclear redox-active molecules was observed at room temperature. The conductance switching was mediated by the presence of various ferrocenyl redox states and each current step corresponded to a specific redox state.

Resistive switching in hafnium dioxide layers: Local phenomenon at grain boundaries

Overcoming challenges associated with implementation of resistive random access memory technology for non-volatile information storage requires identifying the material characteristics responsible for resistive switching. In order to connect the switching phenomenon to the nano-scale morphological features of the dielectrics employed in memory cells, we applied the enhanced conductive atomic force microscopy technique for in situ analysis of the simultaneously collected electrical and topographical data on HfO2 stacks of various degrees of crystallinity. We demonstrate that the resistive switching is a local phenomenon associated with the formation of a conductive filament with a sufficiently small cross-section, which is determined by the maximum passing current. Switchable filament is found to be formed at the dielectric sites where the forming voltages were sufficiently small, which, in the case of the stoichiometric HfO2, is observed exclusively at the grain boundary regions representing low resistant conductive paths through the dielectric film.

Effects of microstructure on native oxide scale development and electrical characteristics of eutectic Cu–Cu6La alloys

A combination of electron microscopy, focused ion beam and conductive atomic force microscopy techniques have been used to study the microstructure, oxide scale development, and electrical behavior of a Cu–9at.% La alloy. The as-cast alloy exhibits a eutectic microstructure comprising 30vol.% Cu rods in a Cu6La matrix. The eutectic colonies exhibit a singular orientation relationship with [010] Cu6La parallel to 〈011〉 Cu along the rod axis, and it is shown that this corresponds to lattice matching of the two phases along this direction (∼0.02% misfit). Oxidation of the alloy at 100°C to accelerate formation of a native oxide scale led to the development of a Cu2O layer less than 25nm thick on the Cu rods and a La-doped Cu2O scale up to 1μm thick on the Cu6La matrix. The La-doped regions of the scale are more conductive despite being much thicker, which is consistent with previous contact resistance data obtained for this alloy. The mechanisms responsible for the formation of this non-equilibrium oxide scale structure and for the enhanced electrical conductivity of the La-doped regions are discussed.

The current image of single SnO2 nanobelt nanodevice studied by conductive atomic force microscopy.

A single SnO2 nanobelt was assembled on a pair of Au electrodes by electric-field assembly method. The electronic transport property of single SnO2 nanobelt was studied by conductive atomic force microscopy (C-AFM). Back-to-back Schottky barrier-type junctions were created between AFM tip/SnO2 nanobelt/Au electrode which can be concluded from the I-V curve. The current images of single SnO2 nanobelt nanodevices were also studied by C-AFM techniques, which showed stripes patterns on the nanobelt surface. The current images of the nanobelt devices correlate the microscopy with separate transport properties measurement together.

Application of Scanning Probe Microscopy Techniques for Structural and Electrical Characterization of Dielectrics, Carbon Nanotubes and Nanoelectronic Devices

The challenges in the metrology of nanoelectronic structures demand for powerful and cost-efficient diagnostic techniques. In recent years, scanning probe microscopy techniques, like atomic force microscopy (AFM), have become indispensable tools for non-destructive characterization at the nanometer scale. More recently, the topographic imaging capabilities of the AFM have been extended by implementing current sensing techniques so that a direct correlation of a feature location with its electrical signature is possible. Several examples on the application of combined AFM and conductive AFM techniques will be presented in order to illustrate the potential of the SPM technique for structural and electrical characterization of semiconductor surfaces, dielectrics, carbon nanotubes as well as fabrication of nanoelectronic devices.

Structural and Electrical Characterization of Dielectrics, Carbon Nanotubes and Nanoelectronic Devices by Means of Scanning Probe Microscopy

During the past years, atomic force microscopy (AFM) has become an indispensable tool for non-destructive characterization of structures at the nanometer-scale. By implementing current sensing techniques, the topographic imaging capabilities of the AFM have been extended. Conductive atomic force microscopy (C-AFM) allows direct correlations of feature locations with the corresponding electrical properties. In this paper, several examples on the application of combined AFM and C-AFM techniques will be presented in order to illustrate the potential of the scanning probe microscopy (SPM) technique for structural and electrical characterization of gate dielectrics, carbon nanotubes as well as nanoelectronic device structures.

Growth, structure and electrical conduction of WO 3 nanorods

We present a very simple method to obtain tungsten trioxide nanorods. The nanorods are epitaxially grown on a mica substrate in low supersaturation conditions. Investigations of morphology, crystallographic structure and chemical composition of the nanorods allow us to propose a growth model in which the potassium ions of the substrate play a major role inducing the one-dimensional structure. The nanorod growth is initiated by the formation of a hexagonal tungsten bronze (HTB) epitaxially oriented on the mica. By using a conductive atomic force microscopy technique, we characterise the electrical conduction of WO 3 networks.

Spatially resolved electrical properties of InAs/InP quantum dots andwires

In this paper the electrical properties of InAs/InP nanostructures (wires anddots) were investigated using Kelvin probe electrostatic force microscopyand conductive atomic force microscopy techniques. Surface potentialmeasurements were strongly affected by the presence of the thin InAsfilm at the top surface of the undoped InP buffer layer. These resultsand the electrical images suggest the suppression of the surface depletionregion due to electron accumulation in InAs wires and dots. I–Vspectroscopy shows the formation of a Schottky-type junction between themetal-coated Si tip and the semiconductor surface exposed to air. Largerconductances and different threshold voltages for current onset for the two typesof nanostructure analysed here can be related to the particular electronicstructure of InAs wires and dots.

Molecular arrangement and electrical conduction of self-assembled monolayers made from terphenyl thiols

We investigated molecular arrangement change of self-assembled monolayers (SAMs) made from [1,1′:4′,1″-terphenyl]-4-thiol (TP0) on Au(1 1 1) using scanning tunneling microscopy. In an ethanol solvent, lying down phase of TP0 molecules where the conjugated rings are expected to contact directly the Au surface, were observed at the initial stage of TP0 SAM growth (at 1 min immersion). After 24 h immersion, a molecular lattice, which is similar to (√3×√3) R30° structure (perpendicular arrangement) was observed. We further investigated the molecular arrangement dependence of electrical conduction of the TP0 SAMs using conductive atomic force microscopy technique. The resistances of the TP0 SAMs were ≈10 4 Ω (at parallel arrangement) and ≈10 8 Ω (at perpendicular arrangement). The resistance of the lying down TP0 SAM was lower than that of benzylmercaptane SAM where the thickness was identical to lying down TP0 SAM. The result indicated that the direct contact of the conjugated ring to the Au surface could further reduces the tunneling resistance.