Showing papers on "Field effect published in 2011"
TL;DR: In this article, the effects of residues introduced during the transfer of chemical vapor deposited graphene from a Cu substrate to an insulating (SiO2) substrate on the physical and electrical properties of the transferred graphene are studied X-ray photoelectron spectroscopy and atomic force microscopy.
Abstract: The effects of residues introduced during the transfer of chemical vapor deposited graphene from a Cu substrate to an insulating (SiO2) substrate on the physical and electrical of the transferred graphene are studied X-ray photoelectron spectroscopy and atomic force microscopy show that this residue can be substantially reduced by annealing in vacuum The impact of the removal of poly(methyl methacrylate) residue on the electrical properties of graphene field effect devices is demonstrated, including a nearly 2 × increase in average mobility from 1400 to 2700 cm2/Vs The electrical results are compared with graphene doping measurements by Raman spectroscopy
936 citations
TL;DR: In this article, the ternary sesquichalcogenide (Bi(x)Sb(1-x))2Te3 is shown to be a tunable topological insulator system by tuning the ratio of bismuth to antimony.
Abstract: Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping, and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3, has been explored extensively by means of material doping and electrical gating, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (Bi(x)Sb(1-x))2Te3 is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (Bi(x)Sb(1-x))2Te3 nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.
361 citations
TL;DR: Systematic trends not only show how the intrinsic high-density transport properties of graphene can be accessed by field effect, but also demonstrate the robustness of ion-gated graphene, which is crucial for possible future applications.
Abstract: We present a comparative study of high carrier density transport in mono-, bi-, and trilayer graphene using electric double-layer transistors to continuously tune the carrier density up to values exceeding 10(14) cm(-2). Whereas in monolayer the conductivity saturates, in bi- and trilayer filling of the higher-energy bands is observed to cause a nonmonotonic behavior of the conductivity and a large increase in the quantum capacitance. These systematic trends not only show how the intrinsic high-density transport properties of graphene can be accessed by field effect, but also demonstrate the robustness of ion-gated graphene, which is crucial for possible future applications.
322 citations
TL;DR: In this paper, the electron conduction mechanism in the above-threshold regime in amorphous oxide semiconductor thin film transistors is shown to be controlled by percolation and trap-limited conduction.
Abstract: The electron conduction mechanism in the above-threshold regime in amorphous oxide semiconductor thin film transistors is shown to be controlled by percolation and trap-limited conduction. The band tail state slope controls the field effect mobility, while the average spatial coherence length and potential fluctuation control percolation conduction. In these limits, the field effect mobility is found to follow a power law, from which a universal mobility versus carrier concentration dependence is extracted.
251 citations
TL;DR: A field-effect reconfigurable nanofluidic diode, with a single asymmetrically placed gate or dual split-gate on top of the nanochannel, that offers a fully independent and digitally programmable approach for controlling the preferential conduction of ions and molecules in fluids.
Abstract: Nanofluidic diodes are utilized for the rectification of ionic transport, but their rectifying properties cannot be altered after the devices are made. Here, a field-effect reconfigurable nanofluidic diode is reported in which the forward direction and the degree of rectification can be modulated by a gate voltage.
219 citations
TL;DR: The modulation of Raman scattering intensities of molecules in GERS with the EFE suggests that the Raman enhancement for GERS occurs through a chemical enhancement mechanism.
Abstract: We studied the modulation of Raman scattering intensities of molecules on graphene by tuning the graphene Fermi level with electrical field effect (EFE). A series of metal phthalocyanine (M-Pc) molecules (M = Mn, Fe, Co, Ni, Cu, Zn), which have different molecular energy levels, were used as probe molecules. The Raman intensities of all these M-Pc molecules become weaker when the graphene Fermi level is up-shifted by applying a positive gate voltage, while they become stronger when the graphene Fermi level is down-shifted by applying a negative gate voltage. However, this Raman intensity modulation only occurs when applying the gate voltage with a fast sweep rate, while it is nearly absent when applying the gate voltage with a slow sweep rate, which is likely due to the arising of the hysteresis effect in the graphene EFE. In addition, the Raman modulation ability for M-Pc molecules with smaller energy gaps is larger than that with larger energy gaps due to the difference in the energy alignment between g...
179 citations
TL;DR: In this article, the properties of poly(alkylthiophenes) in solution are found to have a profound impact on the self assembly process and thus the microstructural and electrical properties of the resultant thin films.
Abstract: The properties of poly(alkylthiophenes) in solution are found to have a profound impact on the self assembly process and thus the microstructural and electrical properties of the resultant thin films. Ordered supramolecular precursors can be formed in regioregular poly(3-hexylthiophene) (P3HT) solutions through the application of low intensity ultrasound. These precursors survive the casting process, resulting in a dramatic increase in the degree of crystallinity of the thin films obtained by spin coating. The crystallinity of the films is tunable, with a continuous evolution of mesoscale structures observed as a function of ultrasonic irradiation time. The photophysical properties of P3HT in solution as well in the solid state suggest that the application of ultrasound leads to a π stacking induced molecular aggregation resulting in field effect mobilities as high as 0.03 cm2 V−1 s−1. A multiphase morphology, comprising short quasi-ordered and larger, ordered nanofibrils embedded in a disordered amorphous phase is formed as a result of irradiation for at least 1 min. Two distinct regions of charge transport are identified, characterized by an initial sharp increase in the field effect mobility by two orders of magnitude due to an increase in crystallinity up to the percolation limit, followed by a gradual saturation where the mobility becomes independent of the thin film microstructure.
114 citations
TL;DR: In this paper, the electrolytic gating of α-sexithiophene thin film transistors is described, and the response to different ion concentrations is described by the influence of the ions on the mobility and an electrostatic screening effect.
Abstract: We report on the electrolytic gating of α-sexithiophene thin film transistors, in which the organic semiconductor is in direct contact with an electrolyte. Due to the large capacitance of the electrical double layer at the electrolyte/semiconductor interface, modulation of the channel conductivity via an electrical field effect is achieved at low voltages. The transistors are stable for several hours and are sensitive to variations in the pH resulting from a pH-dependent surface charge, which modulates the threshold voltage. The response to different ion concentrations is described by the influence of the ions on the mobility and an electrostatic screening effect.
107 citations
TL;DR: In this paper, the effects of the Mg composition on the electrical characteristics and thermal stability of MgxZn1−xO thin film transistors (TFTs) are investigated.
Abstract: The effects of the Mg composition (x=0, 006, and 010) on the electrical characteristics and thermal stability of MgxZn1−xO thin film transistors (TFTs) are investigated The Mg006Zn094O TFT shows the smallest subthreshold slope and highest field effect mobility The O1s spectra of x-ray photoelectron spectroscopy measurements indicate that the oxygen vacancies are reduced in Mg006Zn094O relative to a pure ZnO channel device Mg006Zn094O TFTs also show higher thermal stability compared to the pure ZnO TFTs, which is mainly attributed to the suppression of oxygen vacancies in the channel
104 citations
TL;DR: The FET characteristics of the sensor when exposed to different gas concentrations indicate that adsorption of NH3 molecules reduces the carrier mobility in the polyaniline nanofiber, which could be promising for environmental and industrial applications.
Abstract: A single polyaniline nanofiber field effect transistor (FET) gas sensor fabricated by means of electrospinning was investigated to understand its sensing mechanisms and optimize its performance. We studied the morphology, field effect characteristics and gas sensitivity of conductive nanofibers. The fibers showed Schottky and Ohmic contacts based on different electrode materials. Higher applied gate voltage contributes to an increase in gas sensitivity. The nanofiber transistor showed a 7% reversible resistance change to 1 ppm NH3 with 10 V gate voltage. The FET characteristics of the sensor when exposed to different gas concentrations indicate that adsorption of NH3 molecules reduces the carrier mobility in the polyaniline nanofiber. As such, nanofiber-based sensors could be promising for environmental and industrial applications.
74 citations
TL;DR: An unambiguous tuning of the LAO/STO interface conductivity via free surface charges written using conducting atomic force microscopy (AFM) is demonstrated, revealing the efficiency of free charges in controlling the conductivity of this oxide interface.
Abstract: Oxide heterointerfaces are emerging as one of the most exciting materials systems in condensed matter science. One remarkable example is the LaAlO{sub 3}/SrTiO{sub 3} (LAO/STO) interface, a model system in which a highly mobile electron gas forms between two band insulators, exhibiting two dimensional superconductivity and unusual magnetotransport properties. An ideal tool to tune such an electron gas is the electrostatic field effect. In principle, the electrostatic field can be generated by bound charges due to polarization (as in the normal and ferroelectric field effects) or by adding excess free charge. In previous studies, a large modulation of the carrier density and mobility of the LAO/STO interface has been achieved using the normal field effect. However, little attention has been paid to the field effect generated by free charges. This issue is scarcely addressed, even in conventional semiconductor devices, since the free charges are typically not stable. Here, we demonstrate an unambiguous tuning of the LAO/STO interface conductivity via free surface charges written using conducting atomic force microscopy (AFM). The modulation of the carrier density was found to be reversible, nonvolatile and surprisingly large, {approx}3 x 10{sup 13} cm{sup -2}, comparable to the maximum modulation by the normal field effect.more » Our finding reveal the efficiency of free charges in controlling the conductivity of this oxide interface, and suggest that this technique may be extended more generally to other oxide systems.« less
TL;DR: In this article, field-effect transistors realized from VO2 nanobeams with HfO2 as the gate dielectric were studied and a phase lag exists between the response of the conductance and the gate voltage.
Abstract: We study field-effect transistors realized from VO2 nanobeams with HfO2 as the gate dielectric. When heated up from low to high temperatures, VO2 undergoes an insulator-to-metal transition. We observe a change in conductance (∼6%) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system and we discuss its possible origins.
TL;DR: In this paper, the authors investigate the electrically triggered metal-insulator transition (E-MIT) in VO2 thin films at temperatures far below the structural phase transition temperature (∼340 K).
Abstract: We investigate the electrically triggered metal-insulator transition (E-MIT) in VO2 thin films at temperatures far below the structural phase transition temperature (∼340 K). At 77 K, the maximum current jump observed across the E-MIT is nearly 300×. The threshold voltage for E-MIT decreases slightly from ∼2.0 V at 77 K to ∼1.1 V at 300 K across ∼200 nm thick films, which scales weakly over the temperature range of 77–300 K with an activation energy of ∼5 meV. The phase transition properties are found to be stable after over one thousand scans, indicating reproducible measurements. Analysis of the scaling behavior suggests that the observed weak temperature-dependence of the threshold voltages for E-MIT is smaller than that predicted for a purely current induced Joule heating effect and may include contribution from field effect or carrier injection under applied bias. The results are of potential relevance to the field of phase transition oxide electronics and further understanding of the transition mech...
TL;DR: Observations suggest that the Raman enhancement in GERS occurs through a charge-transfer (CT) enhancement mechanism and the CT process can be modulated by the graphene EFE.
Abstract: The modulation of charger-transfer (CT) enhancement in graphene-enhanced Raman scattering (GERS) by an electric field under different atmospheres is reported. The GERS spectra of cobalt phthalocyanine (CoPc) molecules were collected by in situ Raman measurements under ambient air, vacuum, NH3 atmosphere, and O2 atmosphere, in which the Fermi level of graphene was modulated by an electrical field effect (EFE). The Raman scattering intensities of adsorbed molecules can be tuned to be stronger or weaker as the graphene Fermi level down-shifts or up-shifts under electrical field modulation. However, the Raman intensity modulation in GERS is seriously influenced by the hysteresis effect in graphene EFE, which makes the modulation ability small and shows strong gate voltage sweep rate dependence in ambient air. Fortunately, the hysteresis effect in graphene EFE can be decreased by performing the measurement under vacuum conditions, and thus the Raman modulation ability in GERS can be increased. Furthermore, compared with the vacuum condition, the Raman modulation ability shows an increase under an NH3 atmosphere, while it shows a decrease under an O2 atmosphere, which is due to the different Fermi level modulation region in different atmospheres. More interestingly, this Raman intensity modulation in GERS shows a hysteresis-like behavior that is the same as the graphene Fermi level modulation under the EFE in a different atmosphere. All these observations suggest that the Raman enhancement in GERS occurs through a charge-transfer (CT) enhancement mechanism and the CT process can be modulated by the graphene EFE. This technique will benefit the study of the basic properties of both graphene and chemical enhancement mechanism in surface-enhanced Raman spectroscopy (SERS).
TL;DR: In this article, the authors evaluated the currentvoltage characteristics of the injection contact, showing that I-V characteristics can be modeled as a reverse biased Schottky diode, including barrier lowering induced by contact effects.
Abstract: Contact effects have been investigated in fully printed p-channel organic thin film transistors with field effect mobility up to 2 cm2/Vs. Electrical characteristics of the organic thin film transistors, with channel length <200 μm, are seriously influenced by contact effects with an anomalous increase of the contact resistance for increasing source-drain voltage. Assuming that contact effects are negligible in long channel transistors and using gradual channel approximation, we evaluated the current-voltage characteristics of the injection contact, showing that I-V characteristics can be modeled as a reverse biased Schottky diode, including barrier lowering induced by the Schottky effect.
TL;DR: With ionic liquid gating in electric-double-layer transistors (EDLTs), this paper reported field effect operation and electronic state modulation in a layered material of SnS2, demonstrating that the EDLT is applicable to modifying the electronic properties of metal dichalcogenides.
Abstract: With ionic liquid (IL) gating in electric-double-layer transistors (EDLTs), we report field effect operation and electronic state modulation in a layered material of SnS2, demonstrating that the EDLT is applicable to modifying the electronic properties of metal dichalcogenides. The IL-gated SnS2 EDLTs allow us to realize high performance transistor operation and to achieve interfacial carrier accumulation to a level as high as 5.4×1014 cm−2, as quantitatively estimated from the Hall effect. A considerable decrease of the activation energy in temperature-dependent sheet resistance implies that liquid gating is an effective way to tune the electronic states of metal dichalcogenides at EDL interfaces.
TL;DR: In this paper, a microscopic model Hamiltonian for the ferroelectric field effect was introduced for the study of oxide heterostructures with Ferroelectric components, where the long-range Coulomb interaction was incorporated as an electrostatic potential, solved self-consistently together with the charge distribution.
Abstract: A microscopic model Hamiltonian for the ferroelectric field effect is introduced for the study of oxide heterostructures with ferroelectric components. The long-range Coulomb interaction is incorporated as an electrostatic potential, solved self-consistently together with the charge distribution. A generic double-exchange system is used as the conducting channel, epitaxially attached to the ferroelectric gate. The observed ferroelectric screening effect, namely, the charge accumulation/depletion near the interface, is shown to drive interfacial phase transitions that give rise to robust magnetoelectric responses and bipolar resistive switching, in qualitative agreement with previous density functional theory calculations. The model can be easily adapted to other materials by modifying the Hamiltonian of the conducting channel, and it is useful in simulating ferroelectric field effect devices particularly those involving strongly correlated electronic components where ab initio techniques are difficult to apply.
TL;DR: In this article, the oscillator strength and the third order nonlinear refractive index changes of a cylindrical quantum well wire under intense non-resonant laser field were investigated within the effective mass-approximation by using a finite element method.
TL;DR: In this article, C60-based organic field effect transistors (OFETs) that are well optimized for low voltage operation have been reported, and the assembly of metaloxide and organic passivation layer combines the properties of the high dielectric constant of the metal oxide and the good organic-organic interface between semiconductor and insulator provided by a thin capping layer on top of the AlOx film.
TL;DR: In this article, the field effect on the ionic current rectification (ICR) in the conical nanofluidic FET was comprehensively investigated using a continuum model, composed of Nernst-Planck equations for ionic concentrations, Poisson equation for the electric potential, and Navier-Stokes equations for the flow field.
Abstract: A conical nanofluidic field effect transistor (FET) refers to a conical nanopore embedded with an electrically controllable gate electrode. The surface potential of the nanopore can be effectively regulated by manipulating the gate potential applied to the gate electrode, which in turn controls the ionic current through the nanopore. The field effect on the ionic current rectification (ICR) in the conical nanofluidic FET is comprehensively investigated using a continuum model, composed of Nernst–Planck equations for the ionic concentrations, Poisson equation for the electric potential, and Navier–Stokes equations for the flow field. Under the conditions of a low ionic concentration, a low surface charge density of the nanopore, and a high permittivity of the dielectric nanopore, regulation of ICR by FET is significant. The field effect on the ICR with the gate electrode located in the middle region is opposite to that with the gate electrode located near the tip of the nanopore.
Abstract: Bottom-gate oxide thin-film-transistors (TFTs) with improved electrical stability were fabricated with Al doped ZnO (AZO) channel layers grown by atomic layer deposition (ALD) at a relatively low temperature. The ALD growth at 110°C and the addition of 1-5 atom % Al dopant provided the thin films with reliable semiconducting characteristics, and the TFT devices fabricated with the 1 and 3 atom % AZO films showed a good field effect mobility and on-off current ratio. The transfer curves for the AZO channel TFTs exhibited improved hysteresis loop and positive gate bias stress results compared to those for the pure ZnO TFTs. The improved electrical stability was attributed to the coarsening of the crystal size and the preferred orientation along the nonpolar direction afforded by the addition of Al.
TL;DR: In this paper, a field effect transistor embedded probe that can directly image surface charges on a length scale of 25 nm and a time scale of less than 125 μs is presented.
Abstract: Nanoscale manipulation of surface charges and their imaging are essential for understanding local electronic behaviors of polar materials and advanced electronic devices. Electrostatic force microscopy and Kelvin probe force microscopy have been extensively used to probe and image local surface charges responsible for electrodynamics and transport phenomena. However, they rely on the weak electric force modulation of cantilever that limits both spatial and temporal resolutions. Here we present a field effect transistor embedded probe that can directly image surface charges on a length scale of 25 nm and a time scale of less than 125 μs. On the basis of the calculation of net surface charges in a 25 nm diameter ferroelectric domain, we could estimate the charge density resolution to be as low as 0.08 μC/cm2, which is equivalent to 1/20 electron per nanometer square at room temperature.
TL;DR: In this paper, new electrochromic conjugated polymers and their corresponding devices based on EDOT (ethylenedioxythiophene) are described, which display response times on the order of 1s and high switchable contrast in the visible and near-infrared (Vis-NIR) spectral regions.
Abstract: New electrochromic conjugated polymers and their corresponding devices based on EDOT (ethylenedioxythiophene) are described. The best of these polymers display response times on the order of 1s and high switchable contrast in the visible and near-infrared (Vis-NIR) spectral regions. Thin films (70 nm) of these new polymers displayed optical band gaps on the order of 1.73 eV (7a) < 2.19 eV (7b) < 2.23 eV (7c) < 2.31 eV (4) < 2.34 eV (2) as calculated from their extrapolation of the absorption edges. Polymers 4 and 7a show field effect hole mobilities of ca. 6.7 × 10−5 cm2 V−1s−1 (on/off ratio 104) and 2.5 × 10−5 cm2 V−1s−1 (on/off ratio103), respectively, related to their highly ordered inter-chain packing as confirmed by XRD analyses of polymer 4. Electrochromic characterizations show that polymers 7a–c exhibit significant absorption changes in the infrared at low voltage. The resulting solid-state devices offer promise for electrochromic shutters and filters in the IR, since their high charge transfer mobility and ion injection efficiency permits relatively rapid switching and good switchable contrast, while their robustness exceeds that of aqueous devices.
TL;DR: In this paper, an analytical model for the field effect mobility in organic thin-film transistors is presented, which is developed rigorously using the variable range hopping theory, the Gaussian density of states distribution function and the transistor model.
Abstract: An analytical model is presented here for the field effect mobility in organic thin-film transistors. It is developed rigorously using the variable range hopping theory, the Gaussian density of states distribution function and the transistor model. Based on the proposed model, a variety of temperature and gate voltage dependencies of the field effect mobility can be well described. Good agreement between the calculation and recent experimental data is also observed.
TL;DR: In this article, the electric field and temperature dependence of the electron mobility is studied comparatively in the bulk of fullerene (C60) diodes and at the interface with dielectric of organic field effect transistors (OFETs).
Abstract: The electric field and temperature dependence of the electron mobility is studied comparatively in the bulk of fullerene (C60) diodes and at the interface with dielectric of organic field effect transistors (OFETs). Electron mobility values follow a Poole–Frenkel-type electric field dependence in both types of devices. The activation energy for electron transport is electric field dependent and follows the square root law of field in both devices as predicted by Gill’s law. The same Gill’s energy EGill=34 meV is measured in diodes and OFETs, which corresponds well to Meyer–Neldel energy (EMN=35 meV). It is shown that both the electric field and charge carrier concentration must be accounted for the description of disordered charge transport.
TL;DR: This paper links the effect of LTFE to the nature of the corresponding wave functions based on the experiment results obtained for DCM by the field effect on electrolytes and proposes a new resonance transmission model for L TFE-phenomenon, which is alternative to most widely discussed modelsbased on the field-enhancing factors or barrier-lowering mechanisms.
TL;DR: In this article, the authors studied the electronic transport properties of DyScO3/SrTiO3 polar heterointerface grown at different oxygen pressures and found that the DySO3 film deposited under 10−4 ǫmbar oxygen pressure presents an interfacial metal-to-semiconductor conducting mechanism transition at 90 K.
Abstract: Electronic transport properties of DyScO3/SrTiO3 polar heterointerface grown at different oxygen pressures are studied. This DyScO3/SrTiO3 polar heterointerface exhibits much higher charge mobility, up to 104 cm2 V−1 s−1, compared to the LaAlO3/SrTiO3 system due to relatively lower lattice mismatch between the film and substrate. More significantly, the DyScO3 film deposited under 10−4 mbar oxygen pressure presents an interfacial metal-to-semiconductor conducting mechanism transition at 90 K. Field effect transport measurement results reveal that this transition can be modulated by field effect through controlling the electron doping level of the interface originated from interfacial electronic reconstruction.
TL;DR: In this article, the authors studied the effects of electric and magnetic fields on the impurity states in GaAs-Ga1−xAlxAs quantum wells under the Floquet method by modifying the confinement potential associated to the heterostructure.
Abstract: In this work are studied the intense laser effects on the impurity states in GaAs-Ga1−xAlxAs quantum wells under applied electric and magnetic fields. The electric field is taken oriented along the growth direction of the quantum well whereas the magnetic field is considered to be in-plane. The calculations are made within the effective mass and parabolic band approximations. The intense laser effects have been included through the Floquet method by modifying the confinement potential associated to the heterostructure. The results are presented for several configurations of the dimensions of the quantum well, the position of the impurity atom, the applied electric and magnetic fields, and the incident intense laser radiation. The results suggest that for fixed geometry setups in the system, the binding energy is a decreasing function of the electric field intensity while a dual monotonic behavior is detected when it varies with the magnitude of an applied magnetic field, according to the intensity of the laser field radiation.
TL;DR: The bulk and grain boundary conductivities of Ca(Ti1−xZnx)O3−x ceramics increase with time under a dc-bias voltage of ≤100Vcm−1 and at temperatures in the range 250 to 600 °C as mentioned in this paper.
Abstract: The bulk and grain boundary conductivities of Ca(Ti1−xZnx)O3−x ceramics increase with time under a dc-bias voltage of ≤100Vcm−1 and at temperatures in the range 250 to 600 °C. This low field effect, which is not observed in undoped CaTiO3, is attributed to the nature of a defect structure that contains Zn located on a Ti site, giving rise to underbonded surrounding oxide ions that are readily ionised on application of a dc bias. The conduction is predominantly p-type since holes, located on oxygen as O− ions, are more mobile than both the ionised electrons which are trapped, probably at the sample surface, and oxygen vacancies which form as charge compensation for the Zn2+ acceptor dopant. The conductivity increase is reversible on removal of the dc bias. The electrical properties of the ceramics were modelled successfully using an equivalent electrical circuit consisting of a parallel combination of a resistor, R, capacitor, C, and constant phase element, CPE, to model the bulk response, in series with a similar circuit to model the grain boundary response. At the highest temperatures, an additional parallel RC element was added in series to model the sample-electrode response.
TL;DR: This paper proposes a method that combines resistivity and photovoltage measurements with a chemical perturbation to the surface to measure carrier concentration and mobility, as a function of wire diameter, and also to measure the surface state density and the surface band bending before and after the chemical treatment.
Abstract: The ability to control conductivity in semiconductor nanostructures is often challenged by surface states trapping the majority of the charge carriers. Addressing this challenge requires a reliable method for assessing electrical properties such as carrier concentration and mobility. Unfortunately, here we are facing another challenge, as the Hall effect is geometrically inapplicable to nanowires while the field effect model is also challenged by the geometry of the common nanowire field effect transistor, and can only yield channel mobility which is very different from Hall mobility. In this paper, we propose a method that combines resistivity and photovoltage measurements with a chemical perturbation to the surface to measure carrier concentration and mobility, as a function of wire diameter, and also to measure the surface state density and the surface band bending before and after the chemical treatment. We apply this method to CVD grown GaN nanowires, before and after a mild HCl etch. Using transmission electron microscope and x-ray photoelectron spectrometry we find that HCl removes the native gallium oxide. The etch is found to reduce the surface state density from 1 × 1012 to 5.3 × 1011 cm2, which is calculated from a reduction of the critical radius for full depletion from 7.6 to 4 nm, and a reduction of the surface band bending from 0.53 to 0.29 eV, observed using surface photovoltage. On the average, the values of carrier concentration we obtain are about ten times smaller, and the mobility about ten times greater, than values obtained using field effect transistors. Interestingly, the weak size dependence of the mobility disappears after etching, suggesting a causal linkage between the as-grown size dependence of the mobility and the density of surface states. The proposed method provides an experimental handle to the study of surface states and their effects on the electrical properties of nanowires.