Local Current-voltage Characteristics Research Articles

Resistive switching characteristics of methyl-ammonium lead iodide perovskite during atmosphere degradation

Methylammonium lead iodide (MAPbI3) with extraordinary optoelectronic properties has recently been explored for memristors. Despite significant studies on its susceptibility to air, there has been limited attention towards the impact of ambiance on resistive switching (RS), specifically, the nanoscale electrical properties of interest. We used scanning probe microscopy (SPM) to study local current-voltage (I-V) characteristics, time-scale evolution of hysteresis, current maps, and topography to profoundly understand the ambient degradation. The local and macroscopic current-voltage characteristics are studied by interface modulation with (3-Aminopropyl)trimethoxysilane (APTMS) in devices, i.e., ITO/APTMS/MAPbI3 and ITO/ MAPbI3/APTMS. We report three stages of degradation based on quantitative nano-mechanical (QNM) characterizations and local I-V properties: stage 1 comprising of increased Young’s modulus accompanied by the appearance of hysteresis; stage 2 with decreased Young’s modulus when RS and negative differential resistance (NDR) happen with the evolution of lead iodide (PbI2); stage 3 when RS disappears with enhanced Young’s modulus and densification of PbI2. The enhanced nanoscale and macroscale RS characteristics of ITO/MAPBI3 and ITO/APTMS/MAPbI3 could be attributed to the humidity-induced degradation of MAPbI3 into PbI2 indicated by XRD. These characteristics at the nanoscale however diminish on Day 7 showing the nano-scale RS devices requires the prompt attention of researchers.

Local electrical conductivity of carbon black/PDMS nanocomposites subjected to large deformations

The full-field microscale electrical behavior of polydimethylsiloxane (PDMS) matrix composites with 12 wt% (below the electrical percolation threshold) and 20 wt% (above the electrical percolation threshold) carbon black (CB) nanoparticles was obtained with the aid of Conductive Atomic Force Microscopy (C-AFM) as a function of uniaxial tensile strain. A comparison between the macroscale and the microscale 2D strain fields showed that 25 × 25 μm2 specimen domains can serve as a representative surface element (RSE) to describe the macroscale mechanical response under finite deformations. The same surface domain size was shown to serve as RSE for the local electrical behavior of CB/PDMS nanocomposites at small and large strains. The transverse specimen contraction due to the Poisson’s effect increased the through-thickness conductivity of 12 wt% CB specimens, with a rapid increase in the total electric current measured at the scale of the RSE taking place above 23% uniaxial true strain. The locally dominant tunneling conductance in the undeformed specimen state transitioned to a combination of Ohmic (linear local current-voltage (I–V) characteristics) and tunneling (parabolic local I–V characteristics) at 35% uniaxial true strain. In comparison, the local I–V characteristics of undeformed PDMS composites with 20 wt% CB were a combination of Ohmic (contributing the majority of the total current) and tunneling. Upon application of 35% uniaxial true strain the vast majority of local conduction sites became Ohmic, while the total electric current at the scale of the RSE increased relatively linearly with applied strain.

Size Effect of Local Current-Voltage Characteristics of MX 2 Nanoflakes: Local Density of States Reconstruction from Scanning Tunneling Microscopy Experiments

Local current-voltage characteristics for low-dimensional transition-metal dichalcogenides (LDTMD), as well as the reconstruction of their local density of states (LDOS) from scanning tunneling microscopy (STM) experiments, are of fundamental interest and can be useful for advanced applications. Most of the existing models either have limited applicability for complex-shaped LDTMDs (e.g., those based on the Simmons approach) or require solving of an ill-defined integral equation to deconvolute the unknown LDOS (e.g., those based on the Tersoff approach). Using a serial expansion of the Tersoff formulas, we propose a flexible method to reconstruct the LDOS from local current-voltage characteristics measured in STM experiments. We establish a set of key physical parameters, which characterize the tunneling current of a STM-probe--sample contact and the sample LDOS expanded in Gaussian functions. Using a direct variational method coupled with probabilistic analysis, we determine these parameters from the STM experiments for ${\mathrm{MoS}}_{2}$ nanoflakes with different numbers of layers. The main result is the reconstruction of the LDOS in a relatively wide energy range around the Fermi level, which allows us to gain insight into the local band structure of LDTMDs. The reconstructed LDOS reveal pronounced size effects for the single-layer, two-layer, and three-layer ${\mathrm{MoS}}_{2}$ nanoflakes, which we relate to the low dimensionality and strong bending or corrugation of the nanoflakes. We hope that the proposed elaboration of the Tersoff approach, allowing LDOS reconstruction, will be of critical interest for the quantitative description of STM experiments and also of use to better understand the microscopic physical aspects of the surface, strain, and bending contributions to the LDTMDs' electronic properties.

The Nature of CVC Nonlinearity in Low-Voltage Scanning Tunneling Spectroscopy of Semiconductors

A new model of field emission in a scanning tunnelling microscope was developed. The model describes the tunnelling current from a surface of semiconductor (semimetal) and allows estimating the preexponential factor in the expression for the tunneling probability. It is shown that this factor is directly related to the degree of localization of the electron density and determines the shape of the local tunnel current-voltage characteristics (LTCVCs) at low voltages. The model allows separating the contributions of surface electronic states of different symmetry (dimension) of the tunnelling current. The practical application of the model is demonstrated by the example of mathematical processing of the LTCVCs of HOPG surface containing different structural defects.

Current-voltage characteristics of phase boundaries PVDF-TrFE(70/30)/PANI nanocomposite

In this work, the local current-voltage characteristics of nanocomposite phase boundaries in the form of a poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) (70/30) matrix filled with conductive polyaniline (PANI) particles are studied by conductive atomic force microscopy. At negative external bias voltages, these dependencies are shown to have an interval with a negative differential resistance. This fact is caused by the switching of ferroelectric polarization of individual nanocrystallites PVDF-TrFE located near the phase boundaries in the amorphous matrix volume. The obtained results confirm assumptions about the memristive properties of the investigated nanocomposites.

Scanning tunneling microscopy of the tetraphenylporphyrin thin films on a graphite single-crystal substrate

Features of the surface topography of a tetraphenylporphyrin film on a graphite single crystal (HOPG) substrate and the local current-voltage characteristics were studied using a scanning tunneling microscope under ambient conditions. STM images of surface were acquired on a scale from a few nanometers to 4 micrometers. Local tunnelling current-voltage characteristics in different ranges of bias voltage and in different directions of voltage scanning can have hysteresis due to the hopping electron conductivity in porphyrin.

Novel electrical transport properties of native Fe-Nb oxide layers leading to unilateral conductivity of a refractory metallic glass

Fe-based metallic glasses (also called amorphous alloys) are known to have high hardness and high wear resistance. Here we study and present a Fe-Nb amorphous material with an unusual type of electrical conductivity behavior. The electrical transport properties of Fe-Nb oxide layers were studied by measuring local current-voltage characteristics by the atomic-force microscopy technique. At certain voltage levels the samples containing native oxides showed clearly asymmetrical conductivity relative to polarity of the applied potential. Fe-Nb metallic glassy surface oxide film growth process was monitored at ambient conditions. The growth rate keeps constant during the initial 2.5 hours. After that the growth rate drastically decreases and becomes almost zero while the final oxide thickness is 1.0–1.5 nm. The Fe-Nb film sample annealed for 15 minutes at 300 °C demonstrates several times larger oxide thickness and becomes an insulator. X-ray photoelectron spectroscopy was used to characterize the oxidation states in the surface amorphous oxides. This material can be readily applied as inexpensive nanoscale tunnel diode operating at the commonly utilized voltage of ±5 V.

BaTiO3/LaSrMnO3 Heterostructure Grown on Sapphire for Ferroelectric Tunneling Junctions

BaTiO3/LaSrMnO3 (BTO/LSMO) structures with a ferroelectric layer thickness of 10 nm have been formed on r-cut sapphire substrate by means of high-frequency magnetron sputtering. Investigations of the sample structure showed the presence of a crystalline phase, while measurements of electrical properties revealed piezoelectric response of the BTO films. The study of local current–voltage characteristics showed dependence of the sample resistance on the prehistory of voltage application as determined by ferroelectric hysteresis of the BTO layer.

Гетероструктура BaTiO-=SUB=-3-=/SUB=-/LaSrMnO-=SUB=-3-=/SUB=- на сапфире для сегнетоэлектрических туннельных переходов

AbstractBaTiO_3/LaSrMnO_3 (BTO/LSMO) structures with a ferroelectric layer thickness of 10 nm have been formed on r -cut sapphire substrate by means of high-frequency magnetron sputtering. Investigations of the sample structure showed the presence of a crystalline phase, while measurements of electrical properties revealed piezoelectric response of the BTO films. The study of local current–voltage characteristics showed dependence of the sample resistance on the prehistory of voltage application as determined by ferroelectric hysteresis of the BTO layer.

Leakage current analysis for dislocations in Na-flux GaN bulk single crystals by conductive atomic force microscopy

The mechanisms associated with electrical conduction through individual threading dislocations (TDs) in a Na-flux GaN crystal grown with a multipoint-seed-GaN technique were investigated by conductive atomic force microscopy (C-AFM). To focus on individual TDs, dislocation-related etch pits (DREPs) were formed on the Na-flux GaN surface by wet chemical etching, after which microscopic Pt electrodes were locally fabricated on the DREPs to form conformal contacts to the Na-flux GaN crystal, using electron beam assisted deposition. The C-AFM data clearly demonstrate that the leakage current flows through the individual TD sites. It is also evident that the leakage current and the electrical conduction mechanism vary significantly based on the area within the Na-flux GaN crystal where the TDs are formed. These regions include the c-growth sector (cGS) in which the GaN grows in the [0001] direction on top of the point-seed with a c-plane growth front, the facet-growth sector (FGS) in which the GaN grows with {101¯1} facets on the side of the cGS, the boundary region between the cGS and FGS (BR), and the coalescence boundary region between FGSs (CBR). The local current-voltage (I–V) characteristics of the specimen demonstrate space charge limited current conduction and conduction related to band-like trap states associated with TDs in the FGS, BR, and CBR. A detailed analysis of the I–V data indicates that the electrical conduction through TDs in the cGS may proceed via the Poole-Frenkel emission mechanism.

Spatially resolved photoresponse on individual ZnO nanorods: correlating morphology, defects and conductivity.

Electrically active native point defects have a significant impact on the optical and electrical properties of ZnO nanostructures. Control of defect distribution and a detailed understanding of their physical properties are central to designing ZnO in novel functional forms and architecture, which ultimately decides device performance. Defect control is primarily achieved by either engineering nanostructure morphology by tailoring growth techniques or doping. Here, we report conducting atomic force microscopy studies of spatially resolved photoresponse properties on ZnO nanorod surfaces. The photoresponse for super-band gap, ultraviolet excitations show a direct correlation between surface morphology and photoactivity localization. Additionally, the system exhibits significant photoresponse with sub-bandgap, green illumination; the signature energy associated with the deep level oxygen vacancy states. While the local current-voltage characteristics provide evidence of multiple transport processes and quantifies the photoresponse, the local time-resolved photoresponse data evidences large variations in response times (90 ms–50 s), across the surface of a nanorod. The spatially varied photoconductance and the range in temporal response display a complex interplay of morphology, defects and connectivity that brings about the true colour of these ZnO nanostructures.

Nanoscale inhomogeneity of the Schottky barrier and resistivity inMoS2multilayers

Conductive atomic force microscopy (CAFM) is employed to investigate the current injection from a nanometric contact (a Pt coated tip) to the surface of ${\text{MoS}}_{2}$ thin films. The analysis of local current-voltage characteristics on a large array of tip positions provides high spatial resolution information on the lateral homogeneity of the $\mathrm{tip}/{\text{MoS}}_{2}$ Schottky barrier ${\mathrm{\ensuremath{\Phi}}}_{B}$ and ideality factor $n$, and on the local resistivity ${\ensuremath{\rho}}_{\text{loc}}$ of the ${\text{MoS}}_{2}$ region under the tip. Here, ${\mathrm{\ensuremath{\Phi}}}_{B}=300\ifmmode\pm\else\textpm\fi{}24\phantom{\rule{0.28em}{0ex}}\text{meV}, n=1.60\ifmmode\pm\else\textpm\fi{}0.23$, and ${\ensuremath{\rho}}_{\text{loc}}=2.99\ifmmode\pm\else\textpm\fi{}0.68\phantom{\rule{0.28em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\text{cm}$ are calculated from the distributions of locally measured values. A linear correlation is found between the ${\ensuremath{\rho}}_{\text{loc}}$ and ${\mathrm{\ensuremath{\Phi}}}_{B}$ values at each tip position, indicating a similar origin of the ${\ensuremath{\rho}}_{\text{loc}}$ and ${\mathrm{\ensuremath{\Phi}}}_{B}$ inhomogeneities. These findings are compared with recent literature results on the role of sulfur vacancy clusters on the ${\text{MoS}}_{2}$ surface as preferential paths for current injection from metal contacts. Furthermore, their implications on the behavior of ${\text{MoS}}_{2}$ based transistors are discussed.

Nanoscale Probing of Local Electrical Characteristics on MBE-Grown Bi₂Te₃ Surfaces under Ambient Conditions.

The local electrical characteristics on the surface of MBE-grown Bi2Te3 are probed under ambient conditions by conductive atomic force microscopy. Nanoscale mapping reveals a 10-100× enhancement in current at step-edges compared to that on terraces. Analysis of the local current-voltage characteristics indicates that the transport mechanism is similar for step-edges and terraces. Comparison of the results with those for control samples shows that the current enhancement is not a measurement artifact but instead is due to local differences in electronic properties. The likelihood of various possible mechanisms is discussed. The absence of enhancement at the step-edges for graphite terraces is consistent with the intriguing possibility that spin-orbit coupling and topological effects play a significant role in the step-edge current enhancement in Bi2Te3.

Difference in charge transport properties of Ni-Nb thin films with native and artificial oxide

Here, we report on the properties of native and artificial oxide amorphous thin film on a surface of an amorphous Ni-Nb sample. Careful measurements of local current-voltage characteristics of the system Ni-Nb / NiNb oxide/Pt, were carried out in contact mode of an atomic force microscope. Native oxide showed n-type conductivity, while in the artificial one exhibited p-type one. The shape of current-voltage characteristic curves is unique in both cases and no analogical behavior is found in the literature. X-ray photoelectron spectroscopy (XPS) measurements were used to detect chemical composition of the oxide films and the oxidation state of the alloy components. Detailed analysis of the XPS data revealed that the structure of natural Ni-Nb oxide film consists of Ni-NbOx top layer and nickel enriched bottom layer which provides n-type conductivity. In contrast, in the artificial oxide film Nb is oxidized completely to Nb2O5, Ni atoms migrate into bulk Ni-Nb matrix. Electron depletion layer is formed at the Ni-Nb/Nb2O5 interface providing p-type conductivity.

Influence of cracks on the local current–voltage parameters of silicon solar cells

AbstractThe existence of cracks in silicon solar cells can drastically reduce the electrical performance of an individual cell and even of an entire photovoltaic module. An in‐depth understanding of the influence of cracks on solar cells enables therefore calculations of the crack impact and other following effects on module level. This paper shows a detailed analysis of the electrical influence of cracks with two different spatially resolved methods including global and local current–voltage characteristics. The main influence of cracks is an increased recombination current density in the depletion region, which is clearly shown by spatially resolved dark lock‐in thermography measurements with local current–voltage investigation. This increased recombination current density affects further cell parameters such as the efficiency, which is confirmed also by the global current–voltage characteristics. The additionally used ratio image technique based on electroluminescence measurements is in comparison with the local current–voltage method, the more reliable and faster method for the crack detection itself, and allows on cell‐level and module‐level a continuous inspection of cracks. Copyright © 2013 John Wiley & Sons, Ltd.

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.

Synthesis and local electrical characterization of ZnO microwalls grown on cracked GaN/Si(111) by aqueous method

ZnO microwalls have been grown on cracked GaN/Si(111) using aqueous method. The cracks of GaN direct ZnO grow and form two-dimensional microwall networks. Electrical characterization of individual upright standing ZnO microwall was performed by using conductive atomic force microscopy. Enhanced conductivity for the off-axis facet plane present on as-grown ZnO microwalls has been detected. Meanwhile, the local current-voltage characteristics shows that the sidewall has lower Schottky barrier height, it indicated that the off-axis sidewalls planes are more electrically active than c-plane ZnO. Furthermore, it will also provide a unique structure for surface-related applications, such as sensing, catalyzing, energy harvesting, etc.

Structure and Electroluminescent Properties of Thin Tetracene Layers on Gold Island Films

The molecular structure, morphology, and electrical conductivity of thin tetracene layers obtained by the thermal vacuum deposition have been studied with the help of FTIR spectroscopy and STM/STS techniques as well. It has been shown that this preparation method does not change the tetracene molecular structure. The tetracene thin layers on the atomically smooth pyrolytic graphite surface demonstrate an amorphous structure. The local current-voltagecharacteristics (CVC) measured for tetracene films ~10 monolayers thick show a well-defined asymmetry relative to a tunnel voltage. A study of the electrical conductivity and the electroluminescence (EL) of a planar composite consisting of gold island films and tetracene layers has revealed that EL from submicron luminescent centers is observed on a superlinear section of conduction current I–V curves. The EL intensity grows with increasing voltage and becomes maximal at the CVC saturation. It is found that the EL from the nanocomposite has a 560-nm wavelength emission maximum.

Electrical properties of ZnO nanorods studied by conductive atomic force microscopy

ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, we present a complementary electrical characterization of individual upright standing and lying ZnO nanorods using conductive atomic force microscopy (C-AFM). Initially, the electrical properties of the arrays of upright standing ZnO NRs were characterized using two-dimensional current maps. The current maps were recorded simultaneously with the topography acquired by contact mode AFM. Further, C-AFM was utilized to determine the local current-voltage (I-V) characteristics of the top and side facets of individual upright standing NRs. Current-voltage characterization revealed a characteristic similar to that of a Schottky diode. Detailed discussion of the electrical properties is based on local I-V curves, as well as on the 2D current maps recorded from specific areas.

Imaging of nanoscale charge transport in bulk heterojunction solar cells

We have studied the local charge transport properties of organic bulk heterojunction solar cells based on the blends of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester with a photoconductive atomic force microscope (PCAFM). We explore the role of morphology on transport of photogenerated electrons or holes by careful consideration of the sample geometry and the choice of the atomic force microscope (AFM) tip. We then consider the role of the film/tip contact on the local current-voltage characteristics of these structures and present a model based on a drift and diffusion description of transport. We find that our simple 1D model can only reproduce qualitative features of the data using unphysical parameters, indicating that more sophisticated modeling is required to capture all the nonideal characteristics of the AFM transport measurements. Our results show that interpretation of PCAFM contrast and its relation to material morphology or charge transport is not very straightforward.