Temperature-dependent Electrical Measurements Research Articles

Sn-doped bismuth telluride nanowires with high conductivity

Bismuth telluride (Bi(2)Te(3)) nanowires with sub-100 nm diameters were synthesized by Au-Sn co-catalyzed chemical vapor deposition. These Bi(2)Te(3) nanowires were single crystals with a hexagonal lattice. The Sn catalyst played a key role in achieving the one-dimensional nanowire structures, while the absence of Sn resulted in other morphologies such as nanoplates, nanooctahedrons and nanospheres. Raman spectra revealed that compared to the Bi(2)Te(3) bulk materials, the Bi(2)Te(3) nanowires displayed an A(1u) spectral peak, implying the breaking of symmetry. The temperature-dependent electrical measurement indicated that these Sn-doped Bi(2)Te(3) nanowires were metallic, with a high conductivity of 1.6 × 10(5) S m(-1) at 300 K.

Ferroelectric Domains in PbTiO3/SrTiO3 Superlattices

Polydomain superlattices composed of ferroelectric PbTiO3 and paraelectric SrTiO3 were investigated by means of temperature-dependent X-ray diffraction and electrical measurements. The ferroelectric nanodomains were found to have an almost isotropic distribution of domain wall alignments and to be highly responsive to applied electric fields, giving rise to large enhancements in the overall dielectric response of the structure. Electrical measurements revealed a dramatic dependence of the dielectric permittivity, tunability, and polarization-voltage characteristics on the superlattice period. For short-period superlattices, double hysteresis loops were observed.

Study of the breakdown failure mechanisms for power AlGaN/GaN HEMTs implemented using a RF compatible process

The breakdown failure mechanisms for a family of power AlGaN/GaN HEMTs were studied. These devices were fabricated using a commercially available MMIC/RF technology with a semi-insulating SiC substrate. After a 10min thermal annealing at 425K, the transistors were subjected to temperature dependent electrical characteristics measurement. Breakdown degradation with a negative temperature coefficient of −0.113V/K for the devices without field plate was found. The breakdown voltage is also found to be a decreasing function of the gate length. Gate current increases simultaneously with the drain current during the drain-voltage stress test. This suggests that the probability of a direct leakage current path from gate to the 2-DEG region. The leakage current is attributed by a combination of native and generated traps/defects dominated gate tunneling, and hot electrons injected from the gate to channel. Devices with field plate show an improvement in breakdown voltage from ∼40V (with no field plate) to 138V and with lower negative temperature coefficient. A temperature coefficient of −0.065V/K was observed for devices with a field plate length of 1.6μm.

Unipolar and bipolar operation of InAs/InSb nanowire heterostructure field-effect transistors

We present temperature dependent electrical measurements on n-type InAs/InSb nanowire heterostructure field-effect transistors. The barrier height of the heterostructure junction is determined to be 220 meV, indicating a broken bandgap alignment. A clear asymmetry is observed when applying a bias to either the InAs or the InSb side of the junction. Impact ionization and band-to-band tunneling is more pronounced when the large voltage drop occurs in the narrow bandgap InSb segment. For small negative gate-voltages, the InSb segment can be tuned toward p-type conduction, which induces a strong band-to-band tunneling across the heterostructucture junction.

Vertical exchange bias effects in multilayer thin films based on iron oxide and chromium oxide

Multilayer thin films based on iron oxide and chromium oxide are grown using pulsed laser deposition technique. Structural, electrical, and magnetic properties of the multilayer films are studied in detail. X-ray diffraction analysis shows the polycrystalline nature of chromium oxide films, while iron oxide shows strong orientation along (222) direction. Temperature dependent electrical measurements show semiconducting nature of the film. These multilayer films show magneto-resistance (MR) of about 1.4% at 1 T field. Zero field cool and field cool magnetization (M) vs. magnetic field (H) studies clearly show the presence of vertical exchange bias effect. This vertical exchange bias effect decreases with increase in temperature and vanishes at room temperature.

Fabrication of High-Quality InSb Nanowire Arrays by Chemical Beam Epitaxy

We present investigations on InSb nanowires grown directly on InSb substrates. A chemical beam epitaxy system was used to synthesize InSb nanowires. Growth at low temperatures (300−400 °C) resulted in extensive parasitic InSb thin film deposition and stacking faults within these nanowires. To circumvent both problems, InSb nanowires were synthesized at temperatures above 410 °C. By further optimizing the growth parameters completely stacking fault-free, free-standing InSb nanowires were obtained. By combining chemical beam epitaxy and laser interference lithography, large areas of ordered InSb nanowires were fabricated. Temperature-dependent electrical measurements on these InSb nanowire arrays showed intrinsic bulklike behavior.

Superconductivity of ultra-fine tungsten nanowires grown by focused-ion-beam direct-writing

The electrical properties of lateral ultra-fine tungsten nanowires, which were grown by focused-ion-beam-induced deposition with 1pA ion-beam current, were investigated. Temperature-dependent electrical measurements show that the wires are conducting and have a superconducting transition with a transition temperature (Tc) about 5.1K. Resistance vs. temperature measurements reveal that, with decreasing cross-sectional area, the wires display an increasingly broad superconducting transition. A residual resistive tail extending down to the low-temperature region is found only for the thinnest tungsten nanowire, which is 10nm thick and 19nm wide. The logarithm of the residual resistance of this wire appears as two linear sections as a function of temperature, one within 300mK below Tc and the other extending down to the lowest measuring temperature of 4.26K. Such features have previously been identified with phase slip processes. Our results are suggestive that the focused-ion-beam technique might be a potential approach to fabricate ultra-thin and ultra-narrow nanowires for the study of superconducting suppression in nanoscale materials and for maskless superconducting device fabrication.

Superconductivity of Freestanding Tungsten Nanofeatures Grown by Focused-Ion-Beam

Freestanding tungsten composite nanofeatures were grown by focused-ion-beam-induced deposition with 1 pA ion beam current. The temperature dependent electrical measurements show that the flying nanoscale tungsten is conducting with a room temperature resistivity of 550 micro omega cm. It is also superconducting with a Tc, above 5.1 K and can be repeatedly thermally cycled. Our results suggest that FIB direct-writing of three-dimensional tungsten composites might be a potential approach to fabricate vertical nanodevices and mask-free superconducting devices.

High conductivity carbon nanotube wires from radial densification and ionic doping

Application of drawing dies to radially densify sheets of carbon nanotubes (CNTs) into bulk wires has shown the ability to control electrical conductivity and wire density. Simultaneous use of KAuBr4 doping solution, during wire drawing, has led to an electrical conductivity in the CNT wire of 1.3×106 S/m. Temperature-dependent electrical measurements show that conduction is dominated by fluctuation-assisted tunneling, and introduction of KAuBr4 significantly reduces the tunneling barrier between individual nanotubes. Ultimately, the concomitant doping and densification process leads to closer packed CNTs and a reduced charge transfer barrier, resulting in enhanced bulk electrical conductivity.

Temperature dependent properties of InSb and InAs nanowire field-effect transistors

We present temperature dependent electrical measurements on InSb and InAs nanowire field-effect transistors (FETs). The FETs are fabricated from InAs/InSb heterostructure nanowires, where one complete transistor is defined within each of the two segments. Both the InSb and the InAs FETs are n-type with good current saturation and low voltage operation. The off-current for the InSb FET shows a strong temperature dependence, which we attribute to a barrier lowering due to an increased band-to-band tunneling in the drain part of the channel.

Interplay of bulk and interface effects in the electric-field-driven transition in magnetite

Contact effects in devices incorporating strongly correlated electronic materials are comparatively unexplored. We have investigated the electrically driven phase transition in magnetite (100) thin films by four-terminal methods. In the lateral configuration, the channel length is less than $2\text{ }\ensuremath{\mu}\text{m}$, and voltage-probe wires $\ensuremath{\sim}100\text{ }\text{nm}$ in width are directly patterned within the channel. Multilead measurements quantitatively separate the contributions of each electrode interface and the magnetite channel. We demonstrate that on the onset of the transition contact resistances at both source and drain electrodes and the resistance of magnetite channel decrease abruptly. Temperature-dependent electrical measurements below the Verwey temperature indicate thermally activated transport over the charge gap. The behavior of the magnetite system at a transition point is consistent with a theoretically predicted transition mechanism of charge gap closure by electric field.

Schottky Barrier Characteristics of Cobalt–Nickel Silicide/n-Si Junctions for Scaled-Si CMOS Applications

Ternary cobalt-nickel silicide/n-Si Schottky diodes have been fabricated by sputtering using an equiatomic cobalt- nickel alloy target. A minimum sheet resistivity of the ternary silicide is found to be 5-7 Omega/sq. Grazing-incidence X-ray diffraction shows the formation of ternary silicide phases. Cross-sectional TEM micrograph shows a fairly uniform diffusion of metals into Si with the formation of fully silicided film. Selected-area electron diffraction pattern exhibits the crystalline nature of the silicide layer. Temperature-dependent electrical current-voltage measurements have been used to characterize an optimized Schottky diode formed by annealing at 450degC. The room-temperature barrier height and ideality factor are found to be 0.656 eV and 1.6, respectively, from the I-V characteristics. The series resistance of the diode has been calculated and is found to be 1-11.8 kOmega. The variation of barrier height has been attributed to the inhomogeneity in Schottky junction.

Two-carrier transport and ferromagnetism in FeSe thin films

α - and β-FeSe thin films were grown by metal organic chemical vapor deposition. Compared to the other parameters, the growth temperature shows decisive influence on the phase transition of the FeSe samples. In temperature-dependent electrical measurements, n-type to p-type reversion was observed for both the α- or and β-FeSe samples. Furthermore, the p-type character of the films becomes more and more obvious with increasing the Se∕Fe atomic ratio in the samples. Ferromagnetism was observed in the α-FeSe films although which is not supported by calculation on density of states. The ferromagnetic character shows significant dependence on Se∕Fe atomic ratio in the films and was attributed to the Fe vacancies or Fe clusters in the α-FeSe thin films. The magnetic domain and hysteresis loop of the β-FeSe thin films are also studied.

Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets

Individual graphene oxide sheets subjected to chemical reduction were electrically characterized as a function of temperature and external electric fields. The fully reduced monolayers exhibited conductivities ranging between 0.05 and 2 S/cm and field effect mobilities of 2-200 cm2/Vs at room temperature. Temperature-dependent electrical measurements and Raman spectroscopic investigations suggest that charge transport occurs via variable range hopping between intact graphene islands with sizes on the order of several nanometers. Furthermore, the comparative study of multilayered sheets revealed that the conductivity of the undermost layer is reduced by a factor of more than 2 as a consequence of the interaction with the Si/SiO2 substrate.

Silicon−Molecules−Metal Junctions by Transfer Printing: Chemical Synthesis and Electrical Properties

Gold electrodes have been deposited by transfer printing (TP) on a thiol-functionalized self-assembled monolayer (SAM) on silicon substrate. A sequential chemical route is reported to incorporate thiol groups (−SH) on a preformed long alkyl chain SAM on silicon. The structural characterizations of the functionalized surfaces are described and confirm success of each chemical step (overall yield of 20%). The electrical measurements of the silicon−molecules−gold TP junctions are compared with junctions made by a classical vacuum evaporation of gold. The temperature-dependent electrical measurements show that the silicon/alkyl/Au TP junctions exhibit a purely temperature-independent tunnel behavior, while a slight temperature-dependent behavior is observed at a low bias (|V| < 0.5 V) for the junctions with evaporated Au electrodes. Admittance spectroscopy measurements confirm the better dielectric behavior of TP junctions.

Characterization of near band-edge properties of synthetic p-FeS 2 iron pyrite from electrical and photoconductivity measurements

Single crystals of pyrite FeS 2 were grown by chemical vapor transport (CVT) using ICl 3 as a transport agent. Optimum conditions were given for growing large single crystals. Band-edge properties of the synthetic crystals were characterized by temperature dependent electrical and photoconductivity measurements. The Hall-effect measurement confirmed p-type semiconducting behavior of the crystal at low temperature. Temperature dependent Hall-effect measurements showed that electrical mixed conduction of electrons and holes begins to dominate at temperatures higher than 60 K. The mixed-conduction behavior is due to the ionization effect of p- and n-type imperfections in synthetic FeS 2. The p-type state (acceptor) may come from a chalcogen-deficiency defect in synthetic FeS 2, while the n-type level (donor) is caused by an unwanted nickel impurity involved inside the crystal. Optical properties of the synthetic p-FeS 2 are characterized using photoconductivity (PC) measurements in the temperature range between 10 and 300 K. The PC spectra at different temperatures can be decomposed into two transition features. The possible origins of the transition features A and B are assigned. Temperature dependences of the transition energies of the features are analyzed. The parameters that describe temperature variations of the transition energies in p-FeS 2 are evaluated and discussed. Based on the experimental analyses of the electrical and optical measurements, a probable band-structure scheme for describing the near-band-edge characteristic of synthetic FeS 2 is constructed.

Grain boundary reoxidation of donor-doped barium titanate ceramics

The reoxidation process of donor-doped BaTiO 3 ceramics which is crucial for the development of the potential barrier at the grain boundary was investigated in situ by oxygen coulometry. Samples with nominal composition of 0.98Ba 1− x La x TiO 3 + 0.02TiO 2 (0.0025 ≤ x ≤ 0.05) were prepared by sintering under highly-reducing conditions and subsequently reoxidized by heating up to maximum 1380 °C at an oxygen partial pressure of 260 Pa. During the heating cycle three oxygen uptake peaks were observed. These effects could be attributed (1) to the filling up of oxygen vacancies which were formed during sintering, (2) to the transformation from the reduced to the oxidized phase of BaTiO 3 in the direct vicinity of the grain boundaries, and (3) to the formation of oxidized phase inside the grains which is controlled by oxygen bulk diffusion. Process (3) is accompanied with Ti-rich precipitations which were detected by TEM methods. The microscopic model of the development of the reoxidation process was confirmed by temperature-dependent dc and ac electrical measurements.

Morphology and resistivity of Al thin films grown on Si (1 1 1) by molecular beam epitaxy

Thin films of aluminium metal with varying thickness between 10 and 200 nm were grown on (1 1 1) Si substrates at 250 °C under UHV conditions using molecular beam epitaxy (MBE). Grown thin films were characterized by in situ X-ray photoelectron spectroscopy, and ex situ X-ray diffraction, atomic force microscopy and temperature-dependent electrical resisitivity measurements. The results showed that (i) films grow via 3D-island Volmer–Weber growth mechanism, (ii) with increasing film thickness the average grain size increases and the coalescence takes place for thickness >60 nm, and (iii) independent of the thickness, films grow with (1 1 1) orientation. The room-temperature value of resistivity contrary to the predictions of existing theoretical models is found to increase monotonically up to a thickness of 40 nm. This anomalous feature was understood in terms of the film morphology, whereby charge transport takes place via variable range hopping (VRH). For film thickness ⩽50 nm, the resistivity vs. temperature curves exhibited a metal-to-insulator (M–I) transition at low temperature, and analysis of insulating region supported the 2D-VRH mechanism of charge transport. However, for thickness ⩾60 nm the resistivity decreased sharply and the M-I transition disappeared. The bulk value of resistivity (2.59 μΩ cm) was obtained for thickness >200 nm.

Electrochemical growth of gas-sensitive polyaniline thin films across an insulating gap

We have prepared and characterized conductive layers of emeraldine base polyaniline (PANI) and tested them as a transducer for gas/organic vapor sensors. The sensing layers of polyaniline were prepared in situ by electrochemical radical polymerization from a drop of an aqueous aniline solution in sulfuric acid placed in the gap between two gold electrodes separated 7–8 μm from each other. The layers were characterized by optical absorption and temperature-dependent electrical conductance measurements. The change in conductance of the 2–20 μm thin polymer layers was used as a sensor signal. The signal dependencies on both impurity concentration and temperature were investigated. The mechanism of charge carrier transport in the films and the interaction with different gas molecules are discussed.

Impact ionization measurements and modeling for power PHEMT

A systematic study of impact ionization in pseudomorphic high electron mobility transistors (PHEMTs) has been carried out using temperature-dependent electrical measurements as well as modeling for optimizing the power performance of the devices through the best layout parameters. A measurement procedure makes it possible to define a safe transistor operation region is proposed. Impact ionization in the channel is parameterized by specific gate current and voltage values. Temperature-dependent measurements are shown to provide distinction between the impact ionization current and the thermionic field emission current. A methodology for defining an optimum vertical structure and a lateral layout for a given application and operational conditions is developed. Empirical models for optimum lateral layout for a power application were developed based on a statistical Device Zoo approach. The results point to an optimal gate-to-drain distance for minimum impact ionization current.