Thermal Activation Conduction Research Articles

Carrier transport and photoconductivity properties of BN50/NiO50 nanocomposite films

BN50/NiO50 and Au-loaded BN50/NiO50 nanocomposite films were separately fabricated on the glass substrates for carrier transport and photoconductivity properties. X-ray diffraction pattern of the films show the hexagonal structure of BN and presence of defect states by Nelson Riley factor analysis. Morphological images show spherical shaped particles with highly porous structure. The incorporation of NiO may hindered growth of BN layers and resulted in spherical particles. Temperature-dependent conductivity describes semiconductor transport behaviour for deposited nanocomposite films. Thermal activation conduction with low activation energy (∼0.308 eV) may be responsible for the resulting conductivity. Further, the light intensity dependent photoelectrical properties of BN50/NiO50 and Au-loaded BN50/NiO50 nanocomposites have been explored. The effect of Au nanoparticles loading on enhanced photo-conductivities (∼22% increase) than bare nanocomposite film has been elaborated by proposed mechanism. This study provided the insightful information for carrier transport and photoconductivity of BN-based nanocomposites.

Giant switchable non thermally-activated conduction in 180° domain walls in tetragonal Pb(Zr,Ti)O3

Conductive domain walls in ferroelectrics offer a promising concept of nanoelectronic circuits with 2D domain-wall channels playing roles of memristors or synoptic interconnections. However, domain wall conduction remains challenging to control and pA-range currents typically measured on individual walls are too low for single-channel devices. Charged domain walls show higher conductivity, but are generally unstable and difficult to create. Here, we show highly conductive and stable channels on ubiquitous 180° domain walls in the archetypical ferroelectric, tetragonal Pb(Zr,Ti)O3. These electrically erasable/rewritable channels show currents of tens of nanoamperes (200 to 400 nA/μm) at voltages ≤2 V and metallic-like non thermally-activated transport properties down to 4 K, as confirmed by nanoscopic mapping. The domain structure analysis and phase-field simulations reveal complex switching dynamics, in which the extraordinary conductivity in strained Pb(Zr,Ti)O3 films is explained by an interplay between ferroelastic a- and c-domains. This work demonstrates the potential of accessible and stable arrangements of nominally uncharged and electrically switchable domain walls for nanoelectronics.

Determination of the electron trap level in Fe-doped GaN by phonon-assisted conduction phenomenon

We acoustically measured the energy level for thermally activated conduction (TAC) in high-resistivity Fe-doped GaN using the non-contacting antenna-transmission acoustic-resonance method. The acoustic attenuation takes a maximum at a specific temperature, where the TAC is accelerated with the help of phonon energy. The Debye-type relaxation is thus observed for acoustic attenuation, and its activation energy (0.54 0.04 eV) was determined with attenuation measurements at various frequencies and temperatures. This value agrees with the E3 level in GaN, indicating that thermally associated conduction originates from the E3 trap level. We also measured the five independent elastic constants at high temperatures.

Nonlinear electrical behaviors and conductivity mechanisms in Ca1‐xYbxCeNbWO8 ceramics

AbstractIn this study, we investigated the nonlinear electrical behaviors and conductivity mechanisms of Ca1‐xYbxCeNbWO8 ceramics, which are prepared by the solid‐state sintering method. CeO2 and CeNbO4 phases occur when the Yb3+ doping amount is greater than 0.1 in the CaCeNbWO8‐based ceramics. With the continuous incorporation of Yb3+, the grain size of the ceramics first decreases and then increases. The Ce3+ concentration in the ceramics decreases with increasing Yb3+ content. In the temperature range of 298.15 to 1073.15K, the relationship between ln ρ and 1000/T shows nonlinear electrical behaviors, which is attributed to the different conductivity mechanisms at high and low temperatures (ρ and T are resistivity and temperature, respectively). In the temperature range of 298.15 to 773.15K, the electrical conduction in Ca1‐xYbxCeNbWO8 ceramics follows the Efros‐Shklovskii variable‐range hopping (ES‐VRH) conduction mechanism, thermal activation conduction (x < 0.2), and two‐dimensional Mott VRH conduction mechanism (x = 0.2). Furthermore, the electrical conduction obeys the thermal activation conduction mechanism in the temperature range of 773.15 to 1073.15K. The activation energy for the electrical conduction is calculated based on the different hopping conduction mechanisms.

Temperature dependence of the electrical characteristics of ZnO thin film transistor with high-k NbLaO gate dielectric

ZnO thin film transistor with high-k NbLaO/SiO2 bilayer gate dielectric was fabricated by sputtering, and the temperature dependence of the electrical properties of the device was investigated in the temperature range of 293–353 K for clarifying thermally activated carrier generation and carrier transport mechanisms in the conducting channel. With the increase in the temperature, the transfer curve shifts toward the negative gate voltage direction with a negative shift of the threshold voltage, an increase in the off-state current and the subthreshold slope, and a significant increase in carrier mobility. The decrease in the threshold voltage is originated from the formation of oxygen vacancy and the release of free electrons in the ZnO channel, and the formation energy can be estimated to be approximately 0.3 eV. In both subthreshold and above-threshold regimes, the temperature dependence of the drain current shows Arrhenius-type dependence, and the activation energy is around 0.94 eV for a gate voltage of 2 V, reducing with the increase in the gate voltage. The temperature dependence of the ZnO film resistance also exhibits an Arrhenius-type behavior, indicating that the thermal activation conduction process is the dominant conduction mechanism in the ZnO film. Two types of thermal activation conduction processes are observed in the 303–373 K temperature range. This is explained in terms of the existence of two types of deep donors that are consecutively excited to the conduction band as the temperature increases.

Investigation of transport phenomenon and magnetic behavior of Fe doped In[formula omitted]O[formula omitted

In this report the pursuance of Fe concentration on the structural, microstructural, optical, transport and magnetic properties of Indium Oxide (In2O3) have been systematically investigated. Rietveld refined X-ray diffraction, high resolution transmittance electron microscope (HRTEM), selected area electron diffraction (SAED) and Raman patterns substantiated the pure single phase of In2−xFexO3 (x = 0.0-0.4) nanocrystalline cubic bixbyite structure. The average crystallite (grain) size is found to be decreased from ∼ 38.3 to ∼ 27.2 nm with x = 0.0 to 0.4 Fe doping and average particle size of the samples for x = 0.0 and x = 0.2 are measured ∼ 49 nm and ∼ 41 nm respectively from TEM, while the optical band gap stays around ∼ 3.7 eV for each sample. The electrical resistivity is observed to increase with increase in Fe doping which is concomitant with the decrease in oxygen vacancy observed from X-ray photoelectron spectroscopy (XPS) study and photoluminescence (PL). The samples with x = 0.0, 0.01, and 0.2 showed the normal semiconducting behavior and well fitted with thermal activation conduction band (TACB), nearest neighbor hopping (NNH) and Mott-variable range hopping (Mott-VRH) models. The magnetic behavior has a concurrence with the bound magnetic polaron (BMP) model in the vicinity of oxygen vacancies.

Study of Microstructural, Electrical and Dielectric Properties of La0.9Pb0.1MnO3 and La0.8Y0.1Pb0.1MnO3 Ceramics

The present work studies the microstructural and electrical properties of La0.9Pb0.1MnO3 and La0.8Y0.1Pb0.1MnO3 ceramics synthesized by solid-state route method. Microstructure and elemental analysis of both samples were carried out by field emission scanning electron microscope (FESEM) and energy dispersive spectroscopy (EDS) method, respectively. Phase analysis by X-ray diffraction (XRD) indicated formation of single phase distorted structure. The XRD data were further analyzed by Rietveld refinement technique. Raman analysis reveals that Y atom substitutes La site into the LPMO with shifting of phonon modes. The temperature variation of resistivity of undoped and Y-doped La0.9Pb0.1MnO3 samples have been investigated. The electrical resistivity as a function of temperature showed that all samples undergo an metal-insulator (M-I) transition having a peak at transition temperature TMI. Y-doping increases the resistivity and the metal-insulator transition temperature (TMI) shifts to lower temperature. The temperature-dependent resistivity for temperatures less than metal-insulator transition is explained in terms the quadratic temperature dependence and for T > TMI, thermally activated conduction (TAC) is appropriate. Variation of frequency dispersion in permittivity and loss pattern due to La-site substitution in LPMO was observed in the dielectric response curve.

Electrical Transport Mechanisms and Photoconduction in Undoped Crystalline Flash-Evaporated Lead Iodide Thin Films

The flash-evaporation technique was utilized to fabricate undoped 1.35-μm and 1.2-μm thick lead iodide films at substrate temperatures \( T_{\rm{s}} = 150 \)°C and 200°C, respectively. The films were deposited onto a coplanar comb-like copper (Cu-) electrode pattern, previously coated on glass substrates to form lateral metal–semiconductor–metal (MSM-) structures. The as-measured constant-temperature direct-current (dc)-voltage (\( I\left( {V;T} \right) - V \)) curves of the obtained lateral coplanar Cu-PbI2-Cu samples (film plus electrode) displayed remarkable ohmic behavior at all temperatures (\( T = 18 - 90\,^\circ {\hbox{C}} \)). Their dc electrical resistance \( R_{\rm{dc}} (T \)) revealed a single thermally-activated conduction mechanism over the temperature range with activation energy \( E_{\rm{act}} \approx 0.90 - 0.98 \,{\hbox{eV}} \), slightly less than half of room-temperature bandgap energy \( E_{\rm{g}} \) (\( \approx \,2.3\, {\hbox{eV}} \)) of undoped 2H-polytype PbI2 single crystals. The undoped flash-evaporated \( {\hbox{PbI}}_{\rm{x}} \) thin films were homogeneous and almost stoichiometric (\( x \approx 1.87 \)), in contrast to findings on lead iodide films prepared by other methods, and were highly crystalline hexagonal 2H-polytypic structure with c-axis perpendicular to the surface of substrates maintained at \( T_{\rm{s}} { \gtrsim }150^\circ {\hbox{C}} \). Photoconductivity measurements made on these lateral Cu-PbI2-Cu-structures under on–off visible-light illumination reveal a feeble photoresponse for long wavelengths (\( \lambda > 570\,{\hbox{nm}} \)), but a strong response to blue light of photon energy \( E_{\rm{ph}} \) \( \approx \,2.73 \, {\hbox{eV}} \) (\( > E_{\rm{g}} \)), due to photogenerated electron–hole (e–h) pairs via direct band-to-band electronic transitions. The constant-temperature/dc voltage current–time \( I\left( {T,V} \right) - t \) curves of the studied lateral PbI2 MSM-structures at low ambient temperatures (\( T < 50^\circ {\hbox{C}} \)), after cutting off the blue-light illumination, exhibit two trapping mechanisms with different relaxation times. These strongly depend on \( V \) and \( T \), with thermally generated charge carriers in the PbI2 mask photogenerated (e–h) pairs at higher temperatures.

Synthesis of various morphological Ca0.75Er0.25MnO3 powders by hydrothermal method and their electrical properties

The Ca0.75Er0.25MnO3 powders with better conductive properties and various morphologies have been prepared by using hydrothermal method with different concentrations of KOH and polyethylene glycol 400 (PEG-400). The resistivity of Ca0.75Er0.25MnO3 powders was decreased gradually with increase in KOH concentration. When it was 1000 g L−1, the resistivity obtained the smallest value and in this case, the electrical conduction mechanism was followed thermal activation conduction mechanism. Furthermore, the effect of PEG-400 concentration on the morphology and the mechanism for forming different morphologies were elaborated. When PEG-400 concentration was 1 g L−1, the particles were presented cube structure. And when it was 2 g L−1, the particles were presented rod structure. While the concentration was more than 2 g L−1, the powders were all presented planchet structure, but the particles were enlarged with increase in PEG-400. The resistivity of the powders with the rod structure reached the smallest value of 0.2705 kΩ m, because the rod structure is conducive to the formation of the conductivity pathway.

Veritable electronic characteristics in ZnO nanowire circuits uncovered by the four-terminal method at a low temperature

Understanding the natural electrical properties in semiconductor channels and the carrier transport across the metal-semiconductor contact is essential to improve the performance of nanowire devices. This work presents the true electronic characteristics of ZnO nanowire devices measured by a four-electrode method at a low-temperature environment. The temperature rise leads to the decrease in near-band-gap emission, which is attributed to two non-radiative recombination processes. For ZnO circuits, thermionic emission carrier transport mechanism plays a dominant role at Ti-Au/ZnO interface and the transport mechanism in ZnO nanowires is governed by two competitive thermal activation conduction processes: optical or acoustic phonons assisting hopping.

Impedance spectroscopy of nanocrystalline MgFe2O4 and MnFe2O4 ferrite ceramics: Effect of grain boundaries on the electrical properties

Two ferrite ceramic materials, MgFe2O4 and MnFe2O4, were successfully fabricated by a conventional sintering of nanosized powders (at 1373 K for 2 h) synthesized by soft mechanochemical route. The particle size and morphology of powders were studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD analysis was carried out for the determination of phase purity, crystal structure and average crystallite size of sintered ferrites. Both mechanosynthesized ferrite samples show mean crystallite sizes in the nm-range. Over the frequency range of 100 Hz to 1 MHz, impedance spectra of prepared ferrite ceramics are investigated at and above room temperature. Changes in the impedance plane plots with temperature have been discussed and correlated to the microstructure of materials. An equivalent circuit model is applied to explore the electrical parameters (resistance and capacitance) associated with grains and grain boundaries. Complex impedance analysis indicates the dominance of grain boundary effects which control the overall electrical behaviour of studied ferrites. The decrease in grain boundary resistance with temperature suggests a thermally activated conduction mechanism.

Electrical transport property of zirconium oxynitride thin film deposited by magnetron sputtering process

Zirconium oxynitride thin films were deposited on the sapphire substrate by DC magnetron sputtering process, which was performed in argon/oxygen/nitrogen mixture gases with various flow rates of reactive gases (N2+O2). It was observed from XRD, Raman and XPS results that oxygen had a great effect not only on the composition but also on the structure of zirconium oxynitride films. Moreover, the electrical behavior of zirconium oxynitride thin film was investigated and correlated with their compositional and structural properties. For samples deposited at relatively low flow rate, the 2D–3D process dominates the electrical transport mechanism in a weakly localized regime. When the samples deposited at a relatively high-flow rate, the electronic transport is governed by thermal activation at the high temperatures. While a crossover from thermal activation conduction to Mott’s variable hopping conduction (VRH) was observed at the lower temperature range. The experimental results demonstrate that the addition of a small amount of oxygen into the zirconium nitride films can be an effective way to optimize the compositional, structural as well as electrical conduction properties of zirconium oxynitride thin films.

Effect of SnO2 concentration on the tuning of optical and electrical properties of ZnO-SnO2 composite thin films

ZnO-SnO2 composite thin films have been deposited at 400 °C on glass substrates using targets of different SnO2 content (1 to 40 wt. %) by pulsed laser deposition technique. The structural, optical, and electrical properties of the composite films have been studied as a function of SnO2 content. It is revealed from X-ray diffraction analysis that films are crystalline in nature and the crystallite size decreases from 20–23 nm to 5–7 nm with increase of SnO2 content. X-ray photoelectron spectroscopy analysis indicates that Sn is predominantly doped into the ZnO lattice upto a SnO2 content of 15 wt. % in the composite. For higher concentration, a separate SnO2 phase is segregated in the composite. The band gap energy as well as the electrical conductivity can be tuned by varying the SnO2 content in the composite. Low temperature electrical conductivity measurements show three dominant conduction mechanisms in the temperature range of 20–300 K. At high temperature range of 200–300 K, thermal activation conduction process is dominant. Nearest neighbor hopping conduction mechanism, which occurs in the shallow impurity bands, is dominant in the temperature range of 90–200 K. In the low temperature range of 20–90 K, the electronic transport occurs through Mott's variable range hopping conduction process.

Variable-Range Hopping and Thermal Activation Conduction of Y-Doped ZnO Nanocrystalline Films

ZnO and Y-doped ZnO nanocrystalline films were separately fabricated on the glass substrates by sol-gel spin-coating method. X-ray diffraction patterns of the films show the same wurtzite hexagonal structure and (0 0 2) preferential orientation. Scanning electron microscope images show that grain size and thickness of the nanocrystalline films decrease with increasing doping concentration. The decrease of optical bandgap with the increase of Y doping is deduced from the transmittance spectra. Temperature-dependent resistivity reveals a semiconductor transport behavior for all ZnO and Y-doped ZnO nanocrystalline films. The resulting conductivity originates from the combination of thermal activation conduction and Mott variable-range hopping (VRH) conduction. In the high-temperature range, the temperature-dependent resistivity can be described by the Arrhenius equation, σ (T) = σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> exp[ -(E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> /kT)], which shows the thermal activation conduction. The activation energy Ea increases from 0.47 meV for ZnO film to 0.83 meV for Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.98</sub> Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.02</sub> O film. On the contrary, in the low-temperature range, the temperature-dependent resistivity can be fitted well by the relationship, σ(T) = σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h 0</sub> exp[-(T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /T) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/4</sup> ], which indicates the behavior of Mott VRH. The results demonstrate that the crystallization and the corresponding carrier transport behavior of the ZnO and Y-doped ZnO nanocrystalline films are affected by Y doping.

High-temperature electronic transport properties of (YCa)BaCo4O7 compounds

Y1−xCaxBaCo4O7 (0.0≤x≤1.0) samples were prepared by the solid-state reaction method and their high-temperature electronic transport properties were investigated in nitrogen and oxygen respectively. Phase structure of Y1−xCaxBaCo4O7 transforms from hexagonal symmetry for x ≤0.6 samples to orthorhombic symmetry for x≥0.8 samples. In nitrogen, Y1−xCaxBaCo4O7 samples evolve three kinds of electronic transport behaviors with the increase of Ca content: thermal activation conduction, small polaron hopping conduction, and a possible mixed conduction. Ca doping increases the hole concentration and thus decreases Seebeck coefficients. In oxygen, the temperature dependence of electrical resistivity and Seebeck coefficients of Y1−xCaxBaCo4O7 samples displays similar change to their respective thermogravimetric curve, showing their electronic transport behavior under the control of their oxygen adsoption/desorption process.

Correlation between intrinsic defects and electrical properties in the high-quality Cu2ZnSnS4 single crystal

Temperature dependent Hall effect measurements from 20 to 300 K have been performed on the quaternary compounds Cu2ZnSnS4 (CZTS) single crystals. The conductivity mechanisms can be described by a two-path system using Mott variable range hopping and typical thermal activation conduction. The center level of the acceptor band is 132 meV above the valence band maximum and is of width 40 meV. A correlation between the activation energy and acceptor concentration in CZTS is observed.

Electrical transport, microstructure and optical properties of Cr-doped In2O3 thin film prepared by sol–gel method

High transparent In2O3 and Cr-doped In2O3 (In2−xCrxO3) nanocrystalline thin films were prepared using a simple sol–gel method followed by a spin coating technique. The effect of Cr concentration on the structural, microstructure, electrical and optical properties of In2−xCrxO3 were systematically investigated using X-ray diffractometer (XRD), atomic force microscopy (AFM), UV–vis spectroscopy, field emission scanning electron microscopy (FESEM) and Hall effect technique. The films have good crystallization with preferred orientation to (2 2 2) direction. The lattice parameters, a, of In2O3 system increased at lowest dopants (x = 0.025) and decreased as the dopant was further increased. The optical transmittance of films increased up to 98% for x = 0.05 and decreased for further Cr concentrations. From AFM measurement the films nanocrystals morphology was depending on Cr concentrations. The band gap was around 3.76 eV for pure and with x ⩽ 0.075 however it increased. The effect of Cr concentrations on conducting mechanisms of In2O3 film has been investigated from 80 to 300 K using thermal activated conduction band and hopping models. The films, at x = 0.0–0.075, have typical semiconductor behaviour. Three different conducting mechanisms have been estimated. All thermal activation energies and conduction hopping parameters have been determined and analysed in details.

Ferromagnetism, variable range hopping, and the anomalous Hall effect in epitaxial Co:ZnO thin film

A series of high quality single crystalline epitaxial Zn0.95Co0.05O thin films is prepared by molecular beam epitaxy. Superparamagnetism and ferromagnetism are observed when the donor density is manipulated in a range of 1018 cm−3−1020 cm−3 by changing the oxygen partial pressure during film growth. The conduction shows variable range hopping at low temperature and thermal activation conduction at high temperature. The ferromagnetism can be maintained up to room temperature. However, the anomalous Hall effect is observed only at low temperature and disappears above 160 K. This phenomenon can be attributed to the local ferromagnetism and the decreased optimal hopping distance at high temperatures.

Electrical conduction processes in ZnO in a wide temperature range 20–500 K

We have investigated the electrical conduction processes in as-grown and thermally cycled ZnO single crystal as well as as-grown ZnO polycrystalline films over the wide temperature range 20–500 K. In the case of ZnO single crystal between 110 and 500 K, two types of thermal activation conduction processes are observed. This is explained in terms of the existence of both shallow donors and intermediately deep donors that are consecutively excited to the conduction band as the temperature increases. By measuring the resistivity ρ(T) of a given single crystal after repeated thermal cycling in vacuum, we demonstrate that oxygen vacancies play an important role in governing the shallow donor concentrations but leave the activation energy (≈ 27±2 meV) largely intact. In the case of polycrystalline films, two types of thermal activation conduction processes are also observed between ~150 and 500 K. Below ~150 K, we found an additional conductionprocess due to the nearest-neighbor-hopping conduction mechanism, which takes place in the shallow impurity band. As the temperature further decreases below ~80 K, a crossover to the Mott variable-range-hopping conduction process is observed. Taken together with our previous measurements on ρ(T) of ZnO polycrystalline films in the temperature range 2–100 K [Y. L. Huang et al., J. Appl. Phys. 107, 063715 (2010)], this work establishes a quite complete picture of the overall electrical conduction mechanisms in the ZnO material from liquid-helium temperatures up to 500 K.

Electrical conduction mechanisms in natively doped ZnO nanowires (II)

We have measured the intrinsic electrical resistivities,ρ(T), of three individual single-crystalline ZnO nanowires (NWs) from 320 downto 1.3 K. The NWs were synthesized via carbon thermal chemical vapordeposition and the four-probe Pt contacting electrodes were made by thefocused-ion-beam technique. Analysis of the overall temperature behavior ofρ(T) confirms that the charge transport processes in natively doped ZnO NWs are due to acombination of the thermal activation conduction and the nearest-neighbor hoppingconduction processes, as proposed and explained in a recent work (Chiu et al 2009Nanotechnology 20 015203) where the ZnO NWs were grown by a different thermalevaporation method and the four-probe electrodes were made by the electron-beamlithography technique. Taken together, the observations of these two complementarystudies firmly establish that the electrical conduction mechanisms in natively doped ZnONWs are unique and now satisfactorily understood.