Temperature Dependence Of Current Density Research Articles

Electrical transport properties of thermally evaporated phthalocyanine (H 2Pc) thin films

Thin films of H 2Pc of various thicknesses have been deposited onto glass substrates using thermal evaporation technique at room temperature. The dark electrical resistivity measurements were carried out at different temperatures in the range 298–473 K. An estimation of mean free path ( l o) of charge carriers in H 2Pc thin films was attempted. Measurements of thermoelectric power confirm that H 2Pc thin films behave as a p-type semiconductor. The current density–voltage characteristics of Au/H 2Pc/Au at room temperature showed ohmic conduction mechanism at low voltages. At higher voltages the space-charge-limited conduction (SCLC) accompanied by an exponential trap distribution was dominant. The temperature dependence of current density allows the determination of some essential parameters such as the hole mobility ( μ h), the total trap concentration ( N t), the characteristic temperature ( T t) and the trap density P( E).

Electrical Conduction and Dielectric Properties of Biodegradable Plastics

We examined the electrical conduction and dielectric properties as well as thermal analysis of polylactic acid (PLA) that is biodegradable plastics. From the results of thermal analyses, it was found that the glass transition temperature (Tg) of PLA was about 60°C and the melting point (Tm) was about 166°C. For the temperature dependence of current density (J), J of PLA became smaller than LDPE, PP and Polyester in the temperature from room temperature to 90°C. However, when sample became 90°C or the higher, J of PLA became larger than other plastics. On the other hand, as a result of measuring J of the same sample again, it became small. For the relative permittivity (er') indicated intermediate values between that of polyester and those of LDPE and PP.

Electrical conduction processes in neodymium oxide thin films prepared on Si(1 0 0) substrates

Thin Nd oxide films were prepared on Si (P) substrates to form MOS structure. The oxide films were annealed at different conditions and their crystal structures were determined by X-ray diffraction (XRD). The dc electrical transport properties of the devices with amorphous and crystalline Nd oxide were investigated. The current–voltage J( V g) and current–temperature J( T) characteristics suggest that the carrier transport through the device follows the space charge limited current mechanism (SCLC) controlled by exponential or uniform distribution of localised levels in the band gap, depending on the annealing conditions of the oxide. For the crystalline oxide film annealed in air at 800 °C, the current transfer is governed by SCLC mechanism characterised by uniform distribution of traps in the band gap while for crystalline film annealed at 800 °C in vacuum of 10 −4 Pa the current transfer follows the SCLC mechanism characterised by exponential distribution of localised states in the band gap. But, SCLC mechanism governs partially the current transfer in amorphous oxide films. The effect of hygroscopic nature of Nd oxide is observed only in the temperature dependence of current density of amorphous oxide sample.

Structural and transport properties of evaporated iron phthalocyanine (FePc) thin films

The structural and transport properties of thin FePc films of various thickness deposited onto glass substrates have been studied at several temperatures. The structural studies show that the films belong to monoclinic system of β-phase. This structure is confirmed by infrared absorption analysis. The dark electrical resistivity was found to decrease with increasing the film thickness. Graphical representation of log ρ as a function of reciprocal temperature yields two distinct linear parts indicating in turn the existence of two activation energies ΔE 1 and ΔE 2 . Measurement of the thermoelectric power showed that FePc thin films behave as p-type semiconductor over the temperature range 300-450 K. Analysis of thermoelectric power connected with the resistivity results reveals some essential parameters such as: hole mobility μ h 2 x 10 -6 m 2 V -1 s -1 , hole concentration p ≃ 5 x 10 18 m -3 and the ratio c = μ e /μ h ≃ 0.25. Capacitance-voltage data confirm that the Au/FePc interface does not form a Schottky barrier and measurements of the dependence of capacitance on film thickness indicate that the relative permittivity of the films is approximately 3.7. Room temperature current density-voltage characteristics showed ohmic conduction in the lower voltage range and space-charge-limited conductivity (SCLC) in the relatively high voltage. The SCLC controlled by an exponential distribution of traps above the valence band edge. The temperature dependence of current density in accordance with the theory for the exponential trap distributions yielded some essential parameters such as: the hole mobility, the relative permittivity, the trap concentration, the characteristic temperature and the trap density.

Structural and Transport Properties of Copper Phthalocyanine (CuPc) Thin Films

Thin films of CuPc of various thicknesses have been deposited onto clean glass substrates using thermal evaporation technique at room temperature. The structural studies, using X-ray diffractometry and IR spectral analysis, showed that the films are mainly of β- form with preferential orientation of the plane (100). The dark electrical resistivity and the thermoelectric power measurements were carried out at different temperatures in the range 300-450 K. Room temperature current density-voltage characteristics showed ohmic conduction in the lower voltage range and spacecharge-limited – conductivity (SCLC) in the relatively high voltage. The SCLC are controlled by an exponential distribution of traps above the valence band edge. The temperature dependence of current density allows the calculations of some essential parameters such as the hole mobility, the relative permittivity, the trap concentration, the characteristic temperature and the trap density

Charge transport in poly(p-phenylene vinylene) light-emitting diodes

Abstract Since the discovery of electroluminescence in conjugated polymers it has been recognized that charge transport is a key ingredient for the efficiency of the polymer light-emitting diodes (PLEDs). This review focuses on the charge transport properties of these materials. From temperature dependent current density–voltage characteristics it has been obtained that the hole transport in poly(dialkoxy-p-phenylene vinylene) (PPV) is governed by a combination of space-charge effects and a field- and temperature-dependent mobility. The origin of the hole mobility, which seems to be generic for a large class of disordered materials, arises from hopping in a system with both energetic and structural disorder. The response time of PPV-based PLEDs is governed by the dispersive transport of holes towards the cathode. Based on the results of the electron- and hole-transport a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. The unbalanced electron and hole transport gives rise to a bias dependent efficiency. By comparison with experiment it is found that the bimolecular recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency of current into light of a PLED is temperature independent. The understanding of the device operation of PLEDs indicates directions for further improvement of the performance.

Stability considerations of a high-temperature superconductor tape at different operating temperatures

Stability of a Bi-2223/Ag multifilamentary composite conductor against fast transport current ramps was studied by using a numerical model. The model was based on the two-dimensional magnetic diffusion and heat conduction equations. Calculations were carried out both in an adiabatic mode and pool boiling modes in liquid helium, hydrogen and nitrogen. When estimating the heat load (AC losses), real temperature dependent current density–electric field characteristics were used. The results computed by the finite element method are presented and discussed with special emphasis on differences of the stability considerations between high-temperature and low-temperature superconductors.

DC conduction studies in amylase

DC conductivity measurements have been carried out on an enzyme amylase (EC 3.2.2.1) in solid pellet form, provided with silver-coated electrodes. The results are presented in the form of current–voltage characteristics for the temperature range 300–360K. The analysis of the results obtained by plotting the temperature dependence of current density, Poole–Frenkel plots, Schottky plots, Richardson plots and Arrhenius plots shows that the Schottky–Richardson mechanism is responsible for the observed d.c. conduction in amylase. Furthermore, the presence of Schottky barriers at the metal electrode–insulator interface suggests that, by using silver electrodes, satisfactory contacts can be achieved for electrical investigations on biomaterials. © 1998 Society of Chemical Industry

DC conduction studies in amylase

DC conductivity measurements have been carried out on an enzyme amylase (EC 3.2.2.1) in solid pellet form, provided with silver-coated electrodes. The results are presented in the form of current-voltage characteristics for the temperature range 300-360K. The analysis of the results obtained by plotting the temperature dependence of current density, Poole-Frenkel plots, Schottky plots, Richardson plots and Arrhenius plots shows that the Schottky-Richardson mechanism is responsible for the observed d.c. conduction in amylase. Furthermore, the presence of Schottky barriers at the metal electrode-insulator interface suggests that, by using silver electrodes, satisfactory contacts can be achieved for electrical investigations on biomaterials.

In-plane magnetoresistivity anomalies near the average superconducting transition in and crystals with non-uniformly distributed -inhomogeneities

We present detailed measurements of the in-plane magnetoresistivity, , around the average superconducting transition in inhomogeneous (Y-123) and (Bi-2212) crystals with small -inhomogeneities non-uniformly distributed in the sample volume, associated with small oxygen-content inhomogeneities. In zero applied magnetic field, our results confirm the presence in these inhomogeneous Y-123 crystals of sharp resistivity peaks just above the average , and they also show for the first time the presence of similar anomalies in inhomogeneous Bi-2212 crystals. Our measurements for magnetic fields, H, applied perpendicularly or parallel to the (ab-) planes show that in these inhomogeneous superconductors the so-called magnetoresistivity excess, , is negative and very anisotropic near the average . By using two-dimensional electrical resistor networks, this behaviour is explained, simultaneously and consistently for both directions of the applied magnetic field, in terms of temperature dependent current density redistributions associated with the presence of non-uniformly distributed -inhomogeneities in the crystals.

Temperature dependence of the threshold current density of a quantum dot laser

Detailed theoretical analysis of the temperature dependence of threshold current density of a semiconductor quantum dot (QD) laser is given. Temperature dependences of the threshold current density components associated with the radiative recombination in QDs and in the optical confinement layer (OCL) are calculated. Violation of the charge neutrality in QDs is shown to give rise to the slight temperature dependence of the current density component associated with the recombination in QD's. The temperature is calculated (as a function of the parameters of the structure) at which the components of threshold current density become equal to each other. Temperature dependences of the optimum surface density of QD's and the optimum thickness of the OCL, minimizing the threshold current density, are obtained. The characteristic temperature of QD laser T/sub o/ is calculated for the first time considering carrier recombination in the OCL (barrier regions) and violation of the charge neutrality in QDs. The inclusion of violation of the charge neutrality is shown to be critical for the correct calculation of T/sub o/. The characteristic temperature is shown to fall off profoundly with increasing temperature. A drastic decrease in T/sub o/ is shown to occur in passing from temperature conditions wherein the threshold current density is controlled by radiative recombination in QD's to temperature conditions wherein the threshold current density is controlled by radiative recombination in the OCL. The dependences of T/sub o/ on the root mean square of relative QD size fluctuations, total losses, and surface density of QDs are obtained.

Theoretical evaluation of experimental characterization of metal-insulator-metal structures

The fractional change in the temperature dependent current density and the “logarithmic conductivity” as functions of applied bias are examined for a model thin-film metal-insulator-metal heterojunction. We examine the peaks in these curves for correlation between their respective locations and with the metal-insulator work function in order to assess the value of these methods of characterizing a thin-film heterojunction. The insulator model consists of a square barrier in which are inbedded a number of square wells. The metallic electrodes are treated as Bohr-Sommerfeld metals. Within this model we are able to specify all the relevant phenomenological parameters and compare them with the parameters deduced from model calculation of the “logarithimc conductivity” and the fractional change in the temperature dependent current density. We conclude that these methods of experimental characterization of metal-insulator-metal structures may be ambiguous, inaccurate, or both for thin insulating films but may be reasonably useful for thicker (> 10 unit cells) films provided that elastic tunneling is the dominant current carrying mechanism.