Effects of oxygen stoichiometry on resistive switching properties in amorphous tungsten oxide films

The bipolar resistance switching in WO_3 + δ films sandwiched by Al and Pt electrodes was investigated by changing additional oxygen content (δ). Reliable switching voltages and retention were observed for all samples. As δ increases the bi-stable current-voltage characteristics fluctuate leading to unstable switching power consumption. An analysis of the temperature dependence of the bi-stable resistance states revealed additional features that thermionic emission and metallic conduction co-contribute to the electrical transport of the resistance states. The authors propose that the observed resistance switching is due to the combined effects of potential modification near the interface and the formation of a metallic channel.     

Electrical conduction in amorphous As2S3 films

Current-voltage characteristics of As₂S₃ films sandwiched between two aluminium electrodes were measured at low and high voltages in the off state between 290 and 430 K. The thickness of the As₂S₃ films was between about 100 Å and about 1000 Å. The electrical conduction through these films was found to obey Poole's law. We propose that it is necessary to assume a tilting of the conduction band edge to account for the electrical conduction and breakdown in these films.     

Electrical resistivity and Hall effect of K-FeCl3 graphite intercalation compounds

Both in-plane as well as c-axis electrical resistivity of Stage 2 and Stage 5 K-FeCl₃ graphite intercalation compounds were measured from room temperature down to 12 K by using a standard four-probe method. The in-plane electrical resistivity exhibited a metallic behaviour. The c-axis resistivity exhibited an activated behaviour and the data were fitted to a model which assumes a variable-range hopping conduction in parallel with band conduction. The Hall effect was measured in a low magnetic field up to 5 kG at temperatures 300, 77 and 12 K. The Hall coefficient was found to be positive, indicating that the conduction takes place by holes. The carrier concentrations were calculated using a one-carrier model.     

Electrical conduction mechanisms of metal nanoclusters embedded in an amorphous Al2O3 matrix

We present the synthesis and electrical characterization of amorphous nanocomposite layers made of metallic nanoclusters embedded in an alumina matrix (nc-Co:Al₂O₃). The nanostructured materials were fabricated using a pulsed laser deposition (PLD)-derived method based on a nano-cluster generator coupled with a conventional PLD system for host medium co-deposition. The films were subjected to a detailed structural study carried out using high-resolution transmission electron microscopy and atomic force microscopy. The clusters inserted in the alumina matrix are metallic, well crystallized and possess an fcc structure with an average diameter centered at ∼ 2 nm. Dielectric constant and electrical conduction mechanisms of nc-Co:Al₂O₃ layers integrated in metal-insulator-metal capacitive structures were studied for different doping levels and for a broad temperature range (303-473 K). It was concluded that the dielectric constant in the films depends on the doping levels while the major electrical conduction mechanisms are best described by the space charge limited currents formalism, in which the current density J on an applied voltage V follow a power-law dependence (J ∼ Vⁿ) at applied voltages higher than ∼ 2 V. Such composite may find immediate applications as dielectric layers with controlled discharging conduction paths in Radio Frequency-Micro-Electro-Mechanical Systems capacitive structures.     

Surface electrical conductivity in ZnS(Cu,Cl, Mn) thin films

The surface electrical conduction in an In₂O₃(Sn)/ZnS(Cu, Cl, Mn)/Al thin film system was investigated by measuring the dependence of the current through the ZnS(Cu, Cl, Mn) film on the application of a sinusoidal voltage to two electrodes on the surface of the film. The measurements were carried out in an arrangement with an additional d.c. voltage applied between one of the upper electrodes and the lower electrode. The results obtained are discussed on the basis of a proposed theoretical model.     

Electrical conduction in thin films of CeO2/GeO2

A.c. and d.c. conduction in MIM sandwich structures based on CeO₂/GeO₂ as a dielectric prepared by the co-evaporation technique has been investigated for samples having different compositions. A transition of electrical conductivity from amorphous semiconductor to metallic behaviour as the temperature increased is reported. The electroformed samples show a voltage-controlled negative resistance (VCNR) at high values of bias voltage.     

Electrical transport properties of Nd0.67−x Eu x Sr0.33MnO3 (0 ≤ x ≤ 0.67) manganites

A systematic investigation of electrical transport properties viz., electrical conductivity and thermopower of Eu-doped Neodymium-based colossal magnetoresistive manganites with compositional formula, Nd_0.67−x Eu _x Sr_0.33MnO₃ (x = 0–0.67) has been undertaken. These materials were prepared by citrate gel route and characterized by X-ray diffraction, scanning electron microscopy, AC susceptibility, and electrical resistivity measurements. With a view to understand the complex conduction mechanism of these materials, electrical resistivity and thermoelectric power (TEP) data have been analyzed using various theoretical models. It has been concluded that the ferromagnetic metallic part of the conduction mechanism is explained by grain/domain boundary, electron–electron, and magnon scattering mechanisms, while that of high temperature paramagnetic insulating region might be due to small polaron hopping mechanism. The sign change of charge carriers observed in TEP measurements is attributed to the oxygen deficiency of the samples.     

Interstitial Cd doping CdTe films by co-sputtering

A series of CdTe thin films were grown in a Cd enriched ambient by co-sputtering a Cd–CdTe target. Different concentrations were obtained by changing the relative area occupied by the metallic Cd pieces placed onto the CdTe target. It was observed that the process of electrical conduction is not the same for all the samples. For samples with Cd < 50 at% the electrical conductivity is due to the direct activation of charge carriers from the valence band or from impurity states to the conduction band. For samples with Cd > 50 at% electrical conduction is due to the hopping of carriers from state to state by optimizing the distance of tunneling. Conductivity (σ) vs temperature (T ) measurements indicate that electrical conductivity behaves as ln σ α (T₀/T )1, 4, the Mott law for variable range hopping (VRH) in disordered systems. We think that this change in conductivity occurs because Cd atoms place in interstitial sites, doping the CdTe lattice, after the Cd vacancy (V_cd) are filled.     

Magnetism and electrode dependant resistive switching in Ca-doped ceramic bismuth ferrite

Here we report on the preparation and structural, magnetic and electrical characterization of BiFeO₃ and Bi_0.9Ca_0.1FeO₃ ceramic multiferroic samples. We suggest that Ca-doping creates oxygen vacancies and destabilizes the BiFeO₃ spiral magnetic structure. We also study resistive switching effects in Bi_0.9Ca_0.1FeO₃ with metallic electrodes, finding that the appearance of the effect is dependant on the fabrication procedure of the metallic electrode. On the basis of these observations, we critically revise some assumptions in currently available models of resistive switching of complex oxides.     

Electrical and magnetic properties of rare-earth sulfides, (Sm1−xGdx)3S4

Electrical and magnetic properties of mixed-valence rare-earth sulfides, (Sm_1?xGd_x)₃S₄, were investigated. The valence transition from trivalent to divalent samarium ions takes place gradually with increasing gadolinium concentration. In the range between x=0.80 and x=0.85, a drastic change from semiconducting to metallic conduction was observed. This phenomenon is explained by the valence change of the samarium ion in the solid solution. A mechanism of the conduction in the semiconducting sulfide is an electron hopping between Sm²⁺ and Sm³⁺ sites, and that in the metallic sulfide is a band conduction through conduction electrons. A maximum in the resistivity vs temperature curve for a metallic sample appeared near its Curie temperature. This behavior was found to result from the formation of magnetic polarons.     

Electrical conductivity of CuSn5Br11 in solid and molten states

The electrical conductivity of solid and molten CuSn₅Br₁₁ and SnBr₂ compounds was determined using classical a.c. bridge techniques and sealed capillary-type cells with platinum electrodes. The experiments allow us to suspect the solid compound CuSn₅Br₁₁ to be a fast Cu⁺ ion conductor. The specific conductance of the molten compound is nearly the same as that of the molten stannous bromide, so its conduction can be regarded as being mostly due to motion of the anions.     

Electrical conductivity,Seebeck coefficient,and defect structure of oxygen nonstoichiometric Nd2−xSrxNiO4+δ

To elucidate the electronic state and the conduction mechanism of Nd₂NiO_4+δ series oxides at high temperatures, the electrical conductivity, Seebeck coefficient, and nonstoichiometric oxygen content of Nd_2−xSr_xNiO_4+δ (x = 0, 0.2, 0.4) were measured as a function of the Sr content, temperature, and oxygen partial pressure. The hole mobility is estimated from the electrical conductivity and the hole concentration which is defect chemically determined. The mobility slightly decreases as temperature increases as in metals at high temperatures. The relationships between the Seebeck coefficient, electrical conductivity, and hole concentration can be explained by Mott's equation, which expresses the Seebeck coefficient for metals. Semi-quantitative analyses strongly indicate that the electron or hole is itinerant in Nd_2−xSr_xNiO_4+δ, and the conduction mechanism is metal-like band conduction at high temperatures. Based on the experimental results, schematics for energy level and band structure are proposed. At high temperatures, free holes in the σ_x2−y2 band composed of d_x2−y2 orbitals contribute to metallic conduction.     

Selective Growth of Metal‐Free Metallic and Semiconducting Single‐Wall Carbon Nanotubes

A major obstacle for the use of single‐wall carbon nanotubes (SWCNTs) in electronic devices is their mixture of different types of electrical conductivity that strongly depends on their helical structure. The existence of metal impurities as a residue of a metallic growth catalyst may also lower the performance of SWCNT‐based devices. Here, it is shown that by using silicon oxide (SiO_x) nanoparticles as a catalyst, metal‐free semiconducting and metallic SWCNTs can be selectively synthesized by the chemical vapor deposition of ethanol. It is found that control over the nanoparticle size and the content of oxygen in the SiO_x catalyst plays a key role in the selective growth of SWCNTs. Furthermore, by using the as‐grown semiconducting and metallic SWCNTs as the channel material and source/drain electrodes, respectively, all‐SWCNT thin‐film transistors are fabricated to demonstrate the remarkable potential of these SWCNTs for electronic devices.     

Conductivity of reduced solid solutions in the system Na2OAl2O3TiO2

Solid solutions of the “titanium bronze” phase found in the system Na₂OAl₂O₃TiO₂ were made conductive either by H₂ reduction or the use of metallic Ti. Conductivity measurements by pulse and D.C. methods (blocking electrodes) show semiconducting behavior with activation energies decreasing with the extent of reduction, from 101 kJ/mole in “unreduced” samples to 39.8 kJ/mole in samples reduced with Ti metal. At about 600°C, all samples have conductivities about 7×10^?3 ohm^?1cm^?1, indicating that reduction occurs by oxygen loss in the Ar atmosphere. The occurrence of semiconductivity rather than metallic conductivity is explained by the blocking of conduction paths by the Al ions. Absorption spectra show no discrete spectrum for Ti³⁺, but a broad absorption band indicates the presence of delocalized electrons.     

Dc electrical properties and conduction mechanisms of Al-clay based composite resistors

We studied the electrical properties of composite resistors fabricated from materials of natural origin based on Al-kaolinite clay mixture. Our electrical measurements and microstructural studies reveal the effect of annealing schedule, insulating particle size and composition on the resistance of the cermet. We measured small values of activation energy of conduction from temperature dependence of resistance and compared these values of activation energy with those obtained from microstructural examinations using electron microscopy. The small activation energy in the range of 0.53–1.24 μeV obtained suggests conduction by direct or quantum mechanical tunneling between conducting grains. All the resistors were observed to show a negative TCR with some possessing TCR as low as 50 ppm/°C in magnitude, which further confirms the tunneling conduction process. The variation of resistivity with the volume fraction of the metallic phase was found to follow a percolative conduction expression of the form R(X) = R_M(x − x_c)^−t, with the critical exponent t and critical concentration x_c showing a significant dependence on firing temperature. We found that the critical exponent assumed a value of t = 2.0 ± 0.8, which is different from a universal value of 2.0. This was shown to result from tunneling- percolating nature of the cermet.     

CaCu3Ti4O12 thin film capacitors: Evidence of the presence of a Schottky type barrier at the bottom electrode

This study aims to distinguish between the contributions of bottom and top electrodes to the dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) based parallel plate thin film capacitors. For this purpose, Au, Pt, and La_0.9Sr_1.1NiO₄ as electrode materials were compared. Epitaxial and polycrystalline CCTO films were pulsed laser deposited. The nature of electrodes played a major role in altering the dielectric characteristics of the thin films. Existence of one or two Schottky barriers at either or both of the CCTO/electrode interfaces was observed. A careful comparison of the electrical characteristics allowed us to discriminate between the interfaces hosting the Schottky barrier without assuming the conduction type. In return, this knowledge of the Schottky barrier location allowed us to unambiguously establish the carrier's nature. Results point toward n-type carriers in CCTO thin films, in contradiction with previous reports.     

Impact of H‐Doping on n‐Type TMD Channels for Low‐Temperature Band‐Like Transport

Band‐like transport behavior of H‐doped transition metal dichalcogenide (TMD) channels in field effect transistors (FET) is studied by conducting low‐temperature electrical measurements, where MoTe₂, WSe₂, and MoS₂ are chosen for channels. Doped with H atoms through atomic layer deposition, those channels show strong n‐type conduction and their mobility increases without losing on‐state current as the measurement temperature decreases. In contrast, the mobility of unintentionally (naturally) doped TMD FETs always drops at low temperatures whether they are p‐ or n‐type. Density functional theory calculations show that H‐doped MoTe₂, WSe₂, and MoS₂ have Fermi levels above conduction band edge. It is thus concluded that the charge transport behavior in H‐doped TMD channels is metallic showing band‐like transport rather than thermal hopping. These results indicate that H‐doped TMD FETs are practically useful even at low‐temperature ranges.     

Synthesis and electrical properties of cubic NaxWO3 thin films across the metal-insulator transition

Highly oriented (1 0 0) Na_xWO₃ thin films were fabricated in the composition range 0.1 ≤ x ≤ 0.46 by pulsed laser deposition technique. The films showed transition from metallic to insulating behaviour at a critical composition between x = 0.15 and 0.2. The pseudo-cubic symmetry of Na_xWO₃ thin films across the transition region is desirable for understanding the composition controlled metal-insulator transition in the absence of any structural phase transformation. The electrical transport properties exhibited by these films across the transition regime were investigated. While the resistivity varied as T² at low temperatures in the metallic regime, a variable range hopping conduction was observed for the insulating samples. For metallic compositions, a non-linear dependence of resistivity in temperature was also observed from 300 to 7 K, whose exponent varied with the composition of the film.     

Effect of lattice matching on microstructure and electrical conductivity of epitaxial ARuO3 (A = Sr, Ca and Ba) thin films prepared on (001) LaAlO3 substrates by laser ablation

(001) SrRuO₃ (SRO), (001) CaRuO₃ (CRO) and (205) BaRuO₃ (BRO) thin films were epitaxially grown on (001) LaAlO₃ substrates by laser ablation, and the effect of lattice matching on the microstructure and electrical conductivity was investigated. (001) SRO and (001) CRO thin films had a terrace with orthogonal step structure, whereas (205) BRO thin film had an orthogonal structure with tetragonal grains. Epitaxial thin films showed metallic conduction, and the (001) CRO thin films exhibited the highest electrical conductivity, i.e. 1.5 × 10⁵ S m^− 1, among the (001) SRO, (001) CRO and (205) BRO thin films. The smaller misfit between thin film and substrate could be associated with the higher electrical conductivity.     

Thermal and Electrical Conduction in Ultrathin Metallic Films: 7 nm down to Sub‐Nanometer Thickness

For ultrathin metallic films (e.g., less than 5 nm), no knowledge is yet available on how electron scattering at surface and grain boundaries reduces the electrical and thermal transport. The thermal and electrical conduction of metallic films is characterized down to 0.6 nm average thickness. The electrical and thermal conductivities of 0.6 nm Ir film are reduced by 82% and 50% from the respective bulk values. The Lorenz number is measured as 7.08 × 10^?8 W Ω K^?2, almost a twofold increase of the bulk value. The Mayadas‐Shatzkes model is used to interpret the experimental results and reveals very strong electron reflection (>90%) at grain boundaries.