Low-temperature Conductivity Measurements Research Articles

Low-temperature electrical conductivity of composite film formed by carbon nanotubes with MoS2 flakes

Multifunctional composite nanosystems containing both one-dimensional and two-dimensional nanostructures possess improved electrical, mechanical, and thermal properties which offer a wide range of applications. In this work, the composite films formed by single-walled carbon nanotubes and MoS2 flakes (MoS2-SWNTs) are studied exploiting Raman spectroscopy, scanning electron microscopy, and low-temperature conductivity measurements (5–312 K). The MoS2-SWNTs and SWNTs films demonstrate the semiconductor behavior with negative temperature coefficient of resistance. The temperature dependence of the composite film resistance in the range of 5–204 K is considered whithin the framework of Mott model that describes the motion of electrons with variable range hopping due to thermally activated tunneling (3D Mott VRH model). At T > 204 K, the temperature dependence of composite film resistance was fitted by the Arrhenius-like equation. The empirical parameters included in two transport models were evaluated. The comparison between the composite and SWNTs films shows that the conductivity of the composite is mostly determined by nanotubes.

Estimation of density of charged defect states in some glasses of SeTeSnPb system using low-temperature d.c. conductivity measurements

The charged defect states in chalcogenide glasses (ChGs) are responsible for the hopping conduction. The estimation of the density of defect states (i.e., DDS) is a crucial task when we choose a specific ChG as electronic material for its utilization in industry-level applications. ChGs are also famous as electronic materials since they possess exceptional electrical properties (e.g., thermally governed ac/dc conduction, resistive switching, dielectric dispersion). The present study reports the results of the low-temperature dc conductivity measurements in some multicomponent glasses of SeTeSnPb. For this, the dc conductivity has been measured in the low-temperature range. The detailed analysis of obtained data indicates that the dc conductivity obeys Mott’s T−1/4 law in this region. The DDS has been determined and its composition dependence is also discussed.

Metal-insulator transition in a semiconductor nanocrystal network.

Many envisioned applications of semiconductor nanocrystals (NCs), such as thermoelectric generators and transparent conductors, require metallic (nonactivated) charge transport across an NC network. Although encouraging signs of metallic or near-metallic transport have been reported, a thorough demonstration of nonzero conductivity, σ, in the 0 K limit has been elusive. Here, we examine the temperature dependence of σ of ZnO NC networks. Attaining both higher σ and lower temperature than in previous studies of ZnO NCs (T as low as 50 mK), we observe a clear transition from the variable-range hopping regime to the metallic regime. The critical point of the transition is distinctly marked by an unusual power law close to σ ∝ T 1/5. We analyze the critical conductivity data within a quantum critical scaling framework and estimate the metal-insulator transition (MIT) criterion in terms of the free electron density, n, and interparticle contact radius, ρ.

Conduction properties of thin films from a water soluble carbon nanotube/hemicellulose complex

We have examined the conductive properties of carbon nanotube based thin films, which were prepared via dispersion in water by non-covalent functionalization of the nanotubes with xylan, a type of hemicellulose. Measurements of low temperature conductivity, Kelvin probe force microscopy, and high frequency (THz) conductivity elucidated the intra-tube and inter-tube charge transport processes in this material. The measurements show excellent conductive properties of the as prepared thin films, with bulk conductivity up to 2000 S cm−1. The transport results demonstrate that the hemicellulose does not seriously interfere with the inter-tube conductance.

Investigation of the Interface Quality and Reliability of 4H-SiC MOS Structure with NO and Forming Gas Annealing Treatment

Effects of NO and forming gas annealing treatment on the interface quality and reliability of 4H-SiC MOS were systematically studied by low temperature conductance measurements in combination with time-zero dielectric breakdown and time-dependent dielectric breakdown methods. The interface trap density (Dit) showed no obvious reduction after forming gas annealing, and the values of Dit decreased significantly after combined NO and forming gas annealing treatment. The F-N barrier height, electric field to breakdown (Ebd) and charge to breakdown (Qbd) of the MOS structure increased from 2.42 eV, 10 MV/cm, 1mC/cm2 to 2.62 eV, 10.7 MV/cm, 78mC/cm2 after forming gas annealing. The values of F-N barrier height, Ebd and Qbd for MOS capacitors with combined NO and forming gas annealing treatment are 2.69 eV, 10.2 MV/cm, and 24mC/cm2. These results suggest that forming gas annealing is more effective in reliability improvement. While when considering the interface trap density, it seems that combined NO and forming gas annealing treatment is a better choice.

Rethinking Coal: Thin Films of Solution Processed Natural Carbon Nanoparticles for Electronic Devices.

Disordered carbon materials, both amorphous and with long-range order, have been used in a variety of applications, from conductive additives and contact materials to transistors and photovoltaics. Here we show a flexible solution-based method of preparing thin films with tunable electrical properties from suspensions of ball-milled coals following centrifugation. The as-prepared films retain the rich carbon chemistry of the starting coals with conductivities ranging over orders of magnitude, and thermal treatment of the resulting films further tunes the electrical conductivity in excess of 7 orders of magnitude. Optical absorption measurements demonstrate tunable optical gaps from 0 to 1.8 eV. Through low-temperature conductivity measurements and Raman spectroscopy, we demonstrate that variable range hopping controls the electrical properties in as-prepared and thermally treated films and that annealing increases the sp(2) content, localization length, and disorder. The measured hopping energies demonstrate electronic properties similar to amorphous carbon materials and reduced graphene oxide. Finally, Joule heating devices were fabricated from coal-based films, and temperatures as high as 285 °C with excellent stability were achieved.

Characterization of the effects of nitrogen and hydrogen passivation on SiO2/4H-SiC interface by low temperature conductance measurements**Project supported by the National Natural Science Foundation of China (Nos. 61106080, 61275042) and the National Science and Technology Major Project of the Ministry of Science and Technology of China (No. 2013ZX02305).

We investigate the effects of NO annealing and forming gas (FG) annealing on the electrical properties of a SiO2/SiC interface by low-temperature conductance measurements. With nitrogen passivation, the density of interface states (DIT) is significantly reduced in the entire energy range, and the shift of flatband voltage, ΔVFB, is effectively suppressed to less than 0.4 V. However, very fast states are observed after NO annealing and the response frequencies are higher than 1 MHz at room temperature. After additional FG annealing, the DIT and ΔVFB are further reduced. The values of the DIT decrease to less than 1011 cm−2 eV−1 for the energy range of EC − ET > 0.4 eV. It is suggested that the fast states in shallow energy levels originated from the N atoms accumulating at the interface by NO annealing. Though FG annealing has a limited effect on these shallow traps, hydrogen can terminate the residual Si and C dangling bonds corresponding to traps at deep energy levels and improve the interface quality further. It is indicated that NO annealing in conjunction with FG annealing will be a better post-oxidation process method for high performance SiC MOSFETs.

Influence of Annealing, Oxidation and Doping on Conduction-Band near Interface Traps in 4H-SiC Characterized by Low Temperature Conductance Measurements

Current power MOSFET devices on Silicon Carbide show a limited inversion channel mobility, which can be a result of the expected very high density of interface states near the conduction band . In the current work, the effect of the post implantation annealing temperature, the thermal oxidation and the nitrogen doping of the n-epi layer on the density of these interface traps is investigated using capacity-conductance measurements. Instead of the usage of very high frequencies as used in , in this investigation the measurements were performed in liquid nitrogen to decrease the recharging times of the interface traps.Due to the different processing the samples showed a wide spreading of the inversion channel mobility. The conductance measurements show a characteristic peak caused by the conduction band near interface traps especially for the low temperature measurements. But these traps could not be correlated to the mobility. Instead, a correlation to the nitrogen doping of the epi layer could be observed.

Characterization of very fast states in the vicinity of the conduction band edge at the SiO2/SiC interface by low temperature conductance measurements

We have investigated the unique interface states (NI) generated by NO annealing at the SiO2/SiC interfaces by low-temperature conductance measurements, which is more suitable for characterization of very fast interface states than high-frequency conductance measurements at room temperature. Although only a part of the NI states can be evaluated by measurements at room temperature, the whole picture of the NI states, especially near the conduction band edge (0.07 eV ≤ EC−ET), has been revealed by the low temperature measurements. The NI peak was present at the interface even without NO annealing. The NI density increased with NO annealing temperature. The NI density at the energy levels shallower than 0.2 eV exceeded 1012 cm−2eV−1 after NO annealing. The capture cross section of the NI states is uniquely larger than that of conventional interface states.

Crystal Structure and Transport in Merged InAs Nanowires MBE Grown on (001) InAs

Molecular beam epitaxy growth of merging InAs nanowire intersections, that is, a first step toward the realization of a network of such nanowires, is reported. While InAs nanowires play already a leading role in the search for Majorana fermions, a network of these nanowires is expected to promote their exchange and allow for further development of this field. The structural properties of merged InAs nanowire intersections have been investigated using scanning and transmission electron microscope imaging. At the heart of the intersection, a sharp change of the crystal structure from wurtzite to perfect zinc blende is observed. The performed low-temperature conductance measurements demonstrate that the intersection does not impose an obstacle to current transport.

Thermoelectric properties of magnesium silicide fabricated using vacuum plasma thermal spray

The thermoelectric properties of magnesium silicide samples prepared by Vacuum Plasma Spray (VPS) are compared with those made from the conventional hot press method using the same feedstock powder. Thermal conductivity, electrical conductivity, Seebeck coefficient, and figure of merit are characterized from room temperature to 700 K. X-ray diffraction and scanning electron microscopy of the samples are obtained to assess how phase and microstructure influence the thermoelectric properties. Carrier concentration and Hall mobility are obtained from Hall Effect measurements, which provide further insight into the electrical conductivity and Seebeck coefficient mechanisms. Low-temperature electrical conductivity measurements suggest a 3D variable range hopping effect in the samples. VPS samples achieved a maximum ZT = 0.16 at 700 K, which is around 30% of the hot press sample ZT = 0.55 at 700 K using the same raw powder. The results suggest that thermal spray is a potential deposition technique for thermoelectric materials.

Absence of phase transitions in an oxygen stoichiometric cobaltite, YBaCo4O7

Polycrystalline YBaCo4O7 compounds are synthesized by solid state reaction in air atmosphere. X-ray powder diffraction at room temperature indicates its single phase nature. Rietveld refinement of X-ray diffraction data shows distortions in bond lengths in the cobalt-oxygen tetrahedra. Electrical resistivity of YBaCo4O7 suggests a variable range hopping type of conductivity from 80 K up to room temperature. Raman scattering spectra exhibit several broad phonon modes at ambient temperature. Raman spectroscopic measurements are carried out as a function of temperature to look for any structural phase transitions from 80 to 823 K. Raman scattering results show no phase transitions, supporting low temperature conductivity measurements. From temperature variation of Raman spectra, it is found that the structure is stable in the temperature range of present study.

Silicon Nanowire Esaki Diodes

We report on the fabrication and characterization of silicon nanowire tunnel diodes. The silicon nanowires were grown on p-type Si substrates using Au-catalyzed vapor-liquid-solid growth and in situ n-type doping. Electrical measurements reveal Esaki diode characteristics with peak current densities of 3.6 kA/cm(2), peak-to-valley current ratios of up to 4.3, and reverse current densities of up to 300 kA/cm(2) at 0.5 V reverse bias. Strain-dependent current-voltage (I-V) measurements exhibit a decrease of the peak tunnel current with uniaxial tensile stress and an increase of 48% for 1.3 GPa compressive stress along the <111> growth direction, revealing the strain dependence of the Si band structure and thus the tunnel barrier. The contributions of phonons to the indirect tunneling process were probed by conductance measurements at 4.2 K. These measurements show phonon peaks at energies corresponding to the transverse acoustical and transverse optical phonons. In addition, the low-temperature conductance measurements were extended to higher biases to identify potential impurity states in the band gap. The results demonstrate that the most likely impurity, namely, Au from the catalyst particle, is not detectable, a finding that is also supported by the excellent device properties of the Esaki diodes reported here.

Conductance fluctuations in metallic nanogaps made by electromigration

We report on low temperature conductance measurements of gold nanogaps fabricated by controlled electromigration. Fluctuations of the conductance due to quantum interferences and depending both on bias voltage and magnetic field are observed. By analyzing the voltage and magnetoconductance correlation functions, we determine the type of electron trajectories generating the observed quantum interferences and the effective characteristic time of phase coherence in our device.

Structural and electrical properties of Er-doped HfO 2 and of its interface with Ge (0 0 1)

Er-doped HfO 2 thin films with Er content ranging from 0% to 15% are deposited by atomic layer deposition on native oxide free Ge(001). The crystallographic phase is investigated by X-ray diffraction and is found to depend on the Er%. The cubic fluorite structure develops on Ge for Er% as low as 4% and is stable after annealing at 400 °C in N 2. Microstrain increases with increasing the Er content within the fluorite structure. Time of flight secondary ion mass and electron energy loss spectroscopy evidence a Ge diffusion from the substrate that results in the formation of a Ge-rich interfacial region which does not present a structural discontinuity with the oxide. The diffusion of Ge is enhanced by the annealing and causes a reordering of the crystal lattice. In annealed films the interface defect density measured by low temperature conductance measurements is found to decrease with decreasing the Er content.

Atomic structure of conducting nanofilaments in TiO2 resistive switching memory

Resistance switching in metal oxides could form the basis for next-generation non-volatile memory. It has been argued that the current in the high-conductivity state of several technologically relevant oxide materials flows through localized filaments, but these filaments have been characterized only indirectly, limiting our understanding of the switching mechanism. Here, we use high-resolution transmission electron microscopy to probe directly the nanofilaments in a Pt/TiO(2)/Pt system during resistive switching. In situ current-voltage and low-temperature (approximately 130 K) conductivity measurements confirm that switching occurs by the formation and disruption of Ti(n)O(2n-1) (or so-called Magnéli phase) filaments. Knowledge of the composition, structure and dimensions of these filaments will provide a foundation for unravelling the full mechanism of resistance switching in oxide thin films, and help guide research into the stability and scalability of such films for applications.

Low-Temperature Magnetoresistance Studies on Composite Films of Conducting Polymer and Multiwalled Carbon Nanotubes†

We report on low-temperature electrical conductivity and magnetoresistance (MR) measurements on composite films of conducting polyaniline (PANI) and multiwalled carbon nanotubes (CNT). The conductivity below 150 K follows Mott’s variable-range-hopping model in three dimensions. The MR of the CNT/PANI composite films is strongly dependent on temperature, magnetic field, and CNT loading. Evident transition from negative to positive MR was observed with lowering temperature, increasing magnetic field, or decreasing CNT loading. The positive and negative MR is discussed in terms of a wave function-shrinkage effect and a quantum interference effect on hopping conduction.

Vibrational Excitations in Weakly Coupled Single-Molecule Junctions: A Computational Analysis

In bulk systems, molecules are routinely identified by their vibrational spectrum using Raman or infrared spectroscopy. In recent years, vibrational excitation lines have been observed in low-temperature conductance measurements on single-molecule junctions, and they can provide a similar means of identification. We present a method to efficiently calculate these excitation lines in weakly coupled, gateable single-molecule junctions, using a combination of ab initio density functional theory and rate equations. Our method takes transitions from excited to excited vibrational state into account by evaluating the Franck-Condon factors for an arbitrary number of vibrational quanta and is therefore able to predict qualitatively different behavior from calculations limited to transitions from ground state to excited vibrational state. We find that the vibrational spectrum is sensitive to the molecular contact geometry and the charge state, and that it is generally necessary to take more than one vibrational quantum into account. Quantitative comparison to previously reported measurements on pi-conjugated molecules reveals that our method is able to characterize the vibrational excitations and can be used to identify single molecules in a junction. The method is computationally feasible on commodity hardware.

Low-temperature conductance measurements of surface states in HfO 2–Si structures with different gate materials

Metal-oxide-semiconductor capacitors based on HfO 2 gate stack with different metal and metal compound gates (Al, TiN, NiSi and NiAlN) are compared to study the effect of the gate electrode material on the trap density at the insulator–semiconductor interface. C– V and G– ω measurements were made in the frequency range from 1 kHz to 1 MHz in the temperature range 180–300 K. From the maximum of the plot G/ω vs. ln(ω) the density of interface states was calculated, and from its position on the frequency axis the trap cross-section was found. Reducing temperature makes it possible to decrease leakage current through the dielectric and to investigate the states located closer to the band edge. The structures under study were shown to contain significant interface trap densities located near the valence band edge (around 2×10 11 cm −2eV −1 for Al and up to (3.5–5.5)×10 12 cm −2 eV −1 for other gate materials). The peak in the surface state distribution is situated at 0.18 eV above the valence band edge for Al electrode. The capture cross-section is 5.8×10 −17 cm 2 at 200 K for Al–HfO 2–Si structure.

Quantum interference in multiwall carbon nanotubes

Recent low temperature conductance measurements on multiwall carbon nanotubes in perpendicular and parallel magnetic fields are reported. An efficient gating technique allows for considerable tuning of the nanotube doping level. This enables us to study extensively the effect of the nanotube bandstructure on electron quantum interference effects such as weak localization, universal conductance fluctuations and the Aharonov–Bohm effect. We show that the magnetoresistance in the perpendicular magnetic field is strongly suppressed at certain gate voltages Ugate which can be linked with the bottoms of one-dimensional electronic subbands. This assignment allows a detailed comparison of theoretical calculations with the experimental data. In agreement with the theory, a pronounced energy dependence of the elastic mean free path with a strong enhancement close to the charge neutrality point is observed. In the large parallel magnetic field, we observe a superposition of h/2e-periodic Altshuler–Aronov–Spivak oscillations and an additional h/e-periodic contribution to the conductivity. The latter contribution shows a diamond-like pattern in the B − Ugate-plane, which reflects the magnetic field dependence of the density of states of the nanotube's outermost shell.