Change In Conduction Mechanism Research Articles

Electronic Transport Transition of Hydrogenated Amorphous Silicon Irradiated With Self Ions

Electric conductivity variations of hydrogenated amorphous silicon due to self-ion irradiation, i.e. irradiation with Si or H ions, are comprehensively investigated. The anomalous enhancement of dark conductivity (DC) and photoconductivity (PC) are firstly observed due to proton irradiation in the cases of undoped and n-type a-Si:Hs, and after that both decrease with increasing the irradiation fluence. However, Si ion irradiation does not induce the anomalous enhancement and induce a monotonic decrease in DC and PC in the low fluence regime. It is shown from the Seebeck effect analysis that the anomalous enhancement is caused by generation of donor-centers which have metastable nature at room temperature. The decrease in DC and PC is ascribed to the carrier removal effect and the carrier lifetime decrease accompanied by accumulation of dangling bonds, respectively. However, further irradiation causes the loss of photoconduction and the drastic increase in DC. This indicates that the dominant conduction mechanism changes from the band transport to the hopping transport due to excessive accumulation of dangling bonds. The change in the dominant conduction mechanism occurs at above about 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> dpa (displacement per atom) and is independent of the majority carrier concentration before irradiation. It is concluded that the conductivity variations caused by self-ion irradiation can be systematically categorized according to the ratio of the nuclear energy deposition to the electronic energy deposition of incident ions.

Adiabatic to non adiabatic change in conduction mechanism of Zn doped La0.67Sr0.33MnO3 perovskite

We reported the results of electrical transport properties of La0.67Sr0.33Mn1−xZnxO3 (x=0.1 and 0.2) bulk samples prepared by solid-state reaction route over a wide temperature range from 5 to 300K in the presence of magnetic fields. Powder X-ray diffraction patterns at room temperature confirmed that both samples were formed in a single phase. Temperature dependent resistivity data were fitted using Mott’s variable-range hopping (VRH) model for a limited range of temperatures (from 235.5 to 300K for 10% Zn doped and from 133 to 300K for 20% Zn doped samples) to calculate the hopping distance (Rh) and the density of states near the Fermi level, [N(EF)]. It was found that all the parameters vary systematically with the increase in Zn concentration. Moreover, the temperature dependent resistivity data were also fitted using the small polaron hopping (SPH) model. The adiabatic SPH conduction is followed for 10% doped sample at all fields while a non-adiabatic SPH conduction is found to be active for 20% doped sample in all fields. This type of change in the conduction mechanism is accompanied by subtle electronically induced structural changes involving Mn–O–Mn and Zn–O–Mn bond angles and bond lengths. Thus, we suggest that the transport properties can be explained according to the additional delocalization of charge carriers induced by Zn doping.

Donor and acceptor doping effects on the electrical conductivity of CaBi2Nb2O9 ceramics

AbstractThe effects of donor dopants and acceptor dopants (A and B sites) on the resistivity of CaBi2Nb2O9 (CBN) ceramics were studied. The results indicate that the dominant conduction mechanism of pure CBN is p‐type. When the donor doping amount x &lt; 0.05 on A site and x &lt; 0.02 on B site, the resistivity increased with increasing donor doping. With a further increase of donor doping, the resistivity decreased since conductivity mechanism changes from p‐type to n‐type. The resistivity with acceptor doping is decreased at low temperature and equal to the pure CBN ceramic at high temperature since the resistivity of the grain boundary decreases. This work provides a way to obtain high‐resistivity CBN ceramics by appropriate donor doping, which is benefit for use in high‐temperature piezoelectrics.

Preparation and characterization of Zr4+-doped BiFeO3ceramics

Zr4+ doped multi-ferroic BiFe1−xZrxO3 (BFZO, x=0, 0.02, 0.04, 0.06, 0.08, 0.10) samples were prepared by a solid state reaction method. X-Ray diffraction results show that it is of rhombohedral structure and the unit cell parameters increase with the increase of Zr4+ doping concentration, which indicated that the Zr4+ doping ions were entered into the crystal lattice. It is also found that the leakage current of BFZO samples was greatly decreased by about 2orders of magnitude comparing to that of the un-doped samples based on analysis of unsaturated ferroelectric hysteresis (P–E) loops and current density–electric field intensity (J–E) plots. According to the analysis of J–E plots and conduction mechanism, it is believed that the decrease of leakage current is due to the change in conduction mechanism and the competition of two different charge compensationways.

Electrical Conductivity of Li&lt;sub&gt;2&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Zn&lt;sub&gt;2-3&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Ti&lt;sub&gt;1+&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; Crystal

Li2xZn2-3xTi1+xO4(x=0.33, 0.50, 0.60) crystals were grown in a double-mirror type optical floating-zone furnace. The electrical conductivity of Li2xZn2-3xTi1+xO4of crystal was measured in a frequency range from 100 Hz to 10 MHz and in a temperature range from 330 to 700 K, in nitrogen gas. It was revealed the electrical conductivity mechanism changes at the temperature region of 480520 K. The electrical conductivity of polycrystalline Li2xZn2-3xTi1+xO4(x=0.6) shows nearly two orders of magnitude higher values compared to other samples.

Change of conduction mechanism in the impedance of grain boundaries in Pr0.4Ca0.6MnO3

Freshly synthesized polycrystalline Pr0.4Ca0.6MnO3 sample has been analyzed by employing X-ray diffraction, magnetization and impedance spectroscopic techniques. Magnetization M(T) showed charge ordering (TCO) around 268K and below this temperature variation of magnetic and electric field effects is discussed. With the decrease of temperature, the change in the size and shape of the impedance plane plot is explored in terms of modulation of different relaxation processes. An equivalent circuit model (RQ) is used to determine the electrical parameters of Pr0.4Ca0.6MnO3 sample at different temperatures. Impedance of grain boundaries derived from the fitting of experimental data using an equivalent circuit model shows a change in conduction mechanism from small polaron hopping (SPH) to the variable range hopping (VRH) model around 148K. Activation energies calculated from the SPH model and relaxation frequencies of imaginary part of impedance are compared and discussed. Average normalized change (ANC) deduced from the impedance data illustrates a change in conduction mechanism around 148K due to the superexchange networks of Mn3+ and Mn4+.

Dielectric relaxation with polaronic and variable range hopping mechanisms of grains and grain boundaries in Pr0.8Ca0.2MnO3

Polycrystalline Pr0.8Ca0.2MnO3 has been synthesized through solid state reaction route and phase purity is analyzed using synchrotron XRD. Magnetization M(T) showed Mn spin alignment due to ferromagnetic ordering around 125 K (TC), whereas M(H) showed an onset of non-linear behavior from 200 K and with temperature re-orientation of magnetic moment is discussed. The modulation of relaxation processes revealed different formations of impedance plane plots with temperatures. A change in equivalent circuit models from (R1C1)(R2Q2) to (R1C1)(R2Q2)(R3Q3) at 100 K is being reported. The analysis of the impedance data is carried out by calculating impedance of grains and grain boundaries. Fitted parameters derived from these equivalent circuit parameters showed a change in conduction mechanism from small polaronic hopping model (SPH) to Mott's variable range hopping (MVRH) model, around 125 K (TC). Carriers hop to larger distance with multiple activation energies and are described by MVRH below TC. Above this temperature, different trap centers start facilitating these carriers through SPH. Dielectric relaxation shows dispersion around 125 K and the origin of this polarization lies close to the relaxation derived from the grains and their interfaces.

Improved leakage property and reduced crystallization temperature by V2O5 seed layer in K0.4Na0.6NbO3 thin films derived from chemical solution deposition

K0.4Na0.6NbO3 (KNN) thin films were prepared by chemical solution deposition. The V2O5 seed layer was introduced to modify the quality and electric properties of the thin films. The results show that the V2O5 seed layer can reduce the crystallization temperature to 425°C and largely improve the leakage property of the KNN thin films. The KNN thin film with 5nm thickness V2O5 seed layer exhibits high ferroelectric and dielectric properties (2Prmax=26μC/cm2, ε=796, tanδ=0.04) and low leakage current density (4.0×10−5A/cm2) at the electric field of 350kV/cm at room temperature. By adding the V2O5 seed layer, the conduction mechanism changes from SCLC mechanism to Ohmic mechanism in the lower electric field (<200kV/cm), and the Schottky emission mechanism is replaced by the Poole–Frenkel emission mechanism in the higher electric field (>200kV/cm).

Real-Time Experiments on the Spontaneous Restructuring of Self-Assemblies of Naked Ag Nanoparticle Anions: How Close Can Two Nanoparticles Get?

Naked (i.e., ligand-free) Ag nanoparticle anions, 6.5 ± 0.5 nm in diameter, have been synthesized in the gas phase and deposited as monodispersed monolayers. By tuning the average interparticle separation in these assemblies, the monodispersed structure was found to be stable only when the average edge-to-edge distance between nanoparticle anions was greater than 3.5 ± 0.5 nm. This separation was found to mark a fundamental limit where the integrity of the monodispersed structure is compromised due to a spontaneous coalescence process. Conductivity measurements taken in real time, as the interparticle separation was steadily decreased, clearly show a dramatic transition at an interparticle separation of 3.5 ± 0.5 nm that manifests itself as a near complete change in conduction mechanism from tunneling to percolation. The observation of a fundamental 3.5 ± 0.5 nm limit, below which monodispersions of nanoparticles are not stable, has significant consequences in the field of nanofabrication. These results suggest that it is not possible to place two small naked metal nanoparticles any closer than this. To provide some context and understanding of the forces controlling this fundamental limit, a diatomic model is helpful. Much of the chemistry and physics of the nanoparticle anion assemblies can be understood in terms of the classical forces acting between two particles (i.e., a pseudodiatomic). In this context, the coalescence limit is the result of a curve-crossing process that occurs at a 3.5 nm edge-to-edge distance.

Change in AC conduction mechanism together with conversion of submicron to nanoparticles in polyaniline and its poly(vinyl pyrrolidone) blends on heating-induced water elimination

The present work is an investigation of AC impedance behaviors of polyaniline (PANI) and its poly(vinyl pyrrolidone) (PVP) blends. The materials were prepared by polymerization of aniline hydrochloride without/with PVP using FeCl 3 oxidant. PANI–PVP blends have greater AC conductivities than that of PANI in proportion to the amount of PVP incorporated. At lower frequencies (10 Hz – critical frequency f c) conductivity remains independent but at frequencies > f c, conductivity raises with frequency. In the entire range, however, the universal power law σ ac( ω) = Aω s holds good. In fresh condition PANI exhibits an increase in bulk resistance ( R b) and frequency exponent s up to 373 K and blends up to 423 K beyond which they show a decrease. The trend in conduction could be explained by the operation of small polaron tunneling up to 373/423 K and afterwards correlated barrier hopping (CBH) conduction mechanisms. Studied PANI, PANI–PVP blend materials and a fresh but annealed (463 K) PANI all display only a decrease in R b and s values in the entire temperature range of study and hence they follow only CBH mechanism. The fresh samples, together with change of conduction mechanism with temperature also undergo submicron to nanoparticles conversion and changes in other physico-chemical properties. All these changes have the origin on elimination of occluded water from the fresh materials on heating.

Grain size and thickness effect on the performance of sol–gel based TiO2 Schottky diodes

This paper reports thickness and grain size effect on the Schottky diodes of Titanium oxide (TiO2) films made by a sol–gel process. The TiO2 sol was prepared using titanium isopropoxide, ethanol and HCl. Fourier transform infrared spectra indicates that the OH and free water molecule absorption intensities diminish as the heating time increases, whereas TiOTi and isolated TiOH absorption peaks slowly increase as the heating time increases. When annealing temperature effect on the phase of TiO2 was investigated, no phase transformation from anatase to rutile was observed until the annealing temperature of 750°C, and only small portion of the phase transformation occurred after annealing at 850°C. Schottky diodes comprised of p-Si/TiO2/Al were fabricated with various thicknesses of TiO2 film made by different number of TiO2 coatings. In the I–V characteristics, the forward current of Schottky diodes increases as the TiO2 film thickness increases. To understand the cause of the forward current enhancement, three different conduction mechanisms are investigated, and there was no conduction mechanism change with the thickness increase. The enhancement of the forward current was possibly due to the grain size effect.

Resistive switching characteristics and mechanism of thermally grown WOx thin films

Resistive switching characteristics of thermally oxidized tungsten thin films and their switching mechanism were investigated, modifying thickness of the active layer (WOx) by varying oxidation conditions. Two types of switching were observed in Pt/WOx/W memory devices. Thinner film (t ≤ 15 nm) exhibits clockwise switching (CWS) with filamentary characteristics, whereas thicker film (t ≥ 25 nm) exhibits counter-clockwise switching (CCWS) with more homogeneous conduction. Both switching modes are highly reliable and show good cycling endurance. The conduction phenomena in two different switching modes were examined. In the case of CWS, the conduction mechanism changes from Schottky emission to ohmic conduction due to the local bypass of Schottky barrier formed at Pt/WOx interface by oxygen vacancies. Contrary to CWS, CCWS showed a completely different conduction mechanism. The high resistance state is dominated by the Schottky emission at low electric field and by Poole–Frenkel emission at high electric field, whereas the low resistance state exhibits the Schottky emission. Different types of switching behavior might be attributed to the non-homogenous defect distribution across the active layer. A possible conduction sketch for two types switching behaviors is also discussed.

Conducting boron-doped single-crystal diamond films for high pressure research

Epitaxial boron-doped diamond films were grown by microwave plasma chemical vapor deposition for application as heating elements in high pressure diamond anvil cell devices. To a mixture of hydrogen, methane and oxygen, diborane concentrations of 240–1200 parts per million were added to prepare five diamond thin-film samples. Surface morphology has been observed to change depending on the amount of diborane added to the feed gas mixture. Single-crystal diamond film with a lowest room temperature resistivity of 18 mΩ cm was fabricated and temperature variation of resistivity was studied to a low temperature of 12 K. The observed minima in resistivity values with temperature for these samples have been attributed to a change in conduction mechanism from band conduction to hopping conduction. We also present a novel fabrication methodology for monocrystalline electrically conducting channels in diamond and present preliminary heating data with a boron-doped designer diamond anvil to 620 K at ambient pressure.

Reduced leakage current in BiFeO3 thin films with rectifying contacts

BiFeO 3 thin films were grown on Pt/c-sapphire substrates by pulsed laser deposition with different growth rates. With increasing growth rate the leakage current is decreased and the conduction mechanism changes from bulk-limited Poole–Frenkel emission to interface-limited Schottky emission. In the present letter, we show that only the growth rate of the BiFeO3 films close to the metal contacts has to be increased in order to reduce the leakage current and to observe saturated polarization-electric field hysteresis loops.

Electrical properties of natural and synthetic nano-crystalline MgTiO3 geikielite at mantle pressure and temperature conditions

Electrical resistivity of nanocrystalline (30–40 nm crystallite size) and a crystalline sample of geikielite MgTiO 3 has been measured at simultaneous high pressure and high temperature up to 6 GPa and 800 K, respectively. The temperature dependence of the electrical resistivity of both the and sample obeys the Arrehenius behavior in the temperature range between 3 and 800 K and pressure range up to 6.0 GPa. The activation volume of the electrical conduction coarse crystalline sample of geikielite is almost twice that of the nanocrystalline geikielite indicating the increase of activation volume with the crystallite size. The activation energy the electronic conduction decreases from eV at room pressure to 0.25 eV at 6.0 GPa geikielite, and to 0.225 eV in the same pressure range geikielite. The pressure dependence of the activation energy of geikielite sample is found to obey the following expressions \[{\Delta}\mathit{E} (eV) = 0.39 {-} 0.026(1) \mathit{P} + 0.0036 \mathit{P}^{2} for natural sample, and\] \[{\Delta}\mathit{E} (eV) = 0.68 {-} 0.080 (2) \mathit{P} + 0.0007 \mathit{P}^{2} for synthetic sample,\] where P is pressure in GPa. We observe a crossover from extended state type conduction to hopping conduction at 4.0 GPa and 350 K nano-crystalline geikielite. However, there is no such change of conduction mechanism observed the geikielite at high pressures and high temperatures. The present study reveals the phase stability of nano-crystalline geikielite and geikielite up to mantle pressure and temperature conditions, viz. 6 GPa and 800 K, and no phase transition or decomposition is observed in the sample.

Dielectric property and electrical conduction mechanism of ZrO2–TiO2 composite thin films

ZrO2–TiO2 composite films were fabricated by radio frequency magnetron sputtering and post annealing in O2. It was found the films remained amorphous below the annealing temperature of 500 °C. The as-deposited ZrO2–TiO2 film has a high dielectric constant of 22, and which increases to 34 after annealing at 400 °C. At low electric field, the dominant conduction mechanisms are Schottky emission for both the as-deposited and the annealed thin films. At high electric field, the conduction mechanism changes to space-charge-limited current and then changes to Poole–Frenkel (PF) emission after annealing at 400 °C.

Vacuum plasma spray deposition of YSZ–NiO–Ni coatings at different Ar and H 2 gas flow rates

The work presents results of the experimental investigation of vacuum sprayed yttria stabilized zirconia, nickel oxide, nickel (YSZ–NiO–Ni) ceramic composite coatings deposited on Al 2O 3 ceramic and stainless steel substrates produced at different Ar and H 2 gas flow rates. The Ar and H 2 gas flow was varied according to the factorial plan design. It is shown that for the used vacuum plasma spray YSZ and NiO powder mixture the produced coatings were composed of three phases mainly: cubic YSZ (c-YSZ), cubic NiO (c-NiO), and cubic Ni (c-Ni). The quantitative X-ray diffraction (XRD) analysis was used to evaluate each phase amount in the coatings. It was found that the vacuum spray technique enables formation of composite layers with a variable composition and that phase content in the coatings can be controlled choosing the Ar and H 2 gas flow rates. The electrical conductivity measurements revealed that a variation of the phase content in the YSZ–NiO–Ni composites is responsible for the existence of different electrical conduction mechanism and rapid change in the conductivity of coatings with the used powder content. The surface morphology and the cross-section analysis by scanning electron microscope (SEM) have shown porous structures of the deposited coatings.

A new highly conductive organic-inorganic solid polymer electrolyte based on a di-ureasil matrix doped with lithium perchlorate

A new hybrid organic-inorganic polymer electrolyte based on poly(propylene glycol) tolylene 2,4-diisocyanate terminated (PPGTDI), poly(propylene glycol)-block–poly(ethylene glycol)-block-poly(propylene glycol) bis(2-aminopropyl ether) (ED2000) and 3-isocyanatepropyltriethoxysilane (ICPTES) has been synthesized and characterized. A maximum ionic conductivity value of 1.0 × 10−4 S cm−1 at 30 °C and 1.1 × 10−3 S cm−1 at 80 °C is achieved for the hybrid electrolyte with a [O]/[Li] ratio of 32. The conductivity mechanism changes from Arrhenius to Vogel-Tamman-Fulcher (VTF) behavior with the increase in temperature from 20 to 80 °C. The present hybrid electrolyte system offers a remarkable improvement in ionic conductivity by at least one order of magnitude higher than the previously reported organic-inorganic electrolytes. The 7Li NMR (nuclear magnetic resonance) results reveal that there exists a strong correlation between the dynamic properties of the charge carriers and the polymer matrix. Two Li+ local environments are identified, for the first time, in such a di-ureasil based polymer electrolyte. The electrochemical stability window is found to be in the range of 4.6–5.0 V, which ensures that the present hybrid electrolyte is a potential polymer electrolyte for solid-state rechargeable lithium ion batteries.

Charge transport and microstructure in PFO:MEH-PPV polymer blend thin films

Current density–voltage characteristics of poly(9,9′-dihexyl fluorenyl-2,7-diyl) (PFO) thin films (∼120 nm) have been studied in hole only device configuration at different temperatures (100–290 K) in pure form and with blending (∼0.25–50 wt%) of poly(2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV). It has been found that in the case of pure PFO the charge transport at low fields show an ohmic region which is followed by space charge limited conduction region. Blending of PFO with MEH-PPV at low ratio (up to 2 wt%) increases the current density. As the ratio is increased further, the current density decreases. Analysis show that there is a change in conduction mechanism up to 6 V from SCLC to thermally activated ohmic conduction upon blending indicating the creation of new energy level near the transport states of PFO. Morphology of the polymer blended thin films was obtained by atomic force microscopy (AFM) technique. It has been found that the surface roughness of the investigated films is significantly increased upon blending indicating aggregation as well as phase separation at high blending ratios. The decrease in conductivity at high blending ratio can be related to the change in morphology of the films.

Conduction and transmission analysis in gold nanolayers embedded in zinc oxide for flexible electronics

Gold-embedded zinc oxide structures are obtained in which the conduction mechanism changes from conduction through the oxide and activated tunneling between discontinuous metal islands to metallic conduction through a near-continuous layer, with increase in gold thickness. These structures can show resistivity as low as 5.2×10−5 Ω cm. Optical transmission is elucidated in terms of gold’s absorption due to interband electronic transitions, and free carrier absorption losses combined with limitation of the mean free path in discontinuous nanoparticles. The structures show transmittance, photopic averaged transmittance, and Haacke figure of merit values of 93%, 84%, and 15.1×10−3 Ω−1, respectively.