Poole-Frenkel Type Research Articles

DC CONDUCTION MECHANISMS IN AMORPHOUS THIN FILMS OF MIXED OXIDES In2O3–SnO2 SYSTEM DEPOSITED BY CO-EVAPORATION

A discussion of dc conduction mechanisms in thermally co-evaporated amorphous thin films of Al – In 2 O 3– SnO 2– Al structure is presented. Composition (in molar %), film thickness, substrate temperature, and post deposition annealing have profound effects on the electrical properties of the films. The effects of temperature on the I – V characteristics and electrical conductivity of Al – In 2 O 3– SnO 2– Al structure are also reported. The values of dielectric constants estimated by capacitance measurements suggest that high-field conduction mechanism is predominantly of Poole–Frenkel type. At low temperature and low field the electron hopping process dominates but at higher temperature the conduction takes place by transport in the extended states (free-band conduction). The transition from hopping to free band conduction is due to overlapping of localized levels and the free band. The increase in the formation of ionized donors with increase in temperature during electrical measurements indicates that electronic part of the conductivity is higher than the ionic part. The initial increase in conductivity with increase in Sn content in In 2 O 3 lattice is caused by the Sn atom substitution of In atom, giving out one extra electron. The decrease in electrical conductivity above the critical Sn content (10 mol % SnO 2) is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in electrical conductivity with film thickness is caused by the increase in free carriers density, which is generated by oxygen vacancy acting as two electrons donor. The increase in conductivity with substrate temperature and annealing is due either to the severe deficiency of oxygen, which deteriorates the film properties and reduces the mobility of the carriers or to the diffusion of Sn atoms from interstitial locations into the In cation sites and formation of indium species of lower valence state so that the In 3+ oxidation state may be changed to the In 2+ oxidation state.

Effect of high electric field on the dc conduction of TeO2-V2O5-MoO3 amorphous bulk material

Effect of high electric field on the dc conductivity of TeO2-V2O5-MoO3 amorphous bulk samples with different molar ratio of each component was investigated with gap-type electrode arrangement. At low electric fields, the current-voltage (I–V) characteristics has a linear shape, while at high electric fields (>103 V/cm), bulk samples show nonlinear behavior (nonohmic conduction) and current-voltage characteristics shows increasing deviation from Ohm’s law with increasing current density. High-field effect of Pool-Frenkel type was observed at electrical fields about 103−104 V/cm. In addition, positive deviation from Pool-Frenkel effect was observed when a field higher than about 104 V/cm was applied.

Photoconductivity of ZnTe thin films at elevated temperatures

Photoconductivity of thermally evaporated ZnTe thin films was studied at different elevated temperatures. A gap type cell configuration with Al electrodes on glass substrates was used. The conductivity was found to obey two distinct conduction mechanisms within the region of applied fields. At low fields the photoconduction is ohmic and at high fields it is of Poole-Frenkel type. With increase of ambient temperatures, the Poole-Frenkel conductivity regions were found to extend to lower fields. The temperature dependence of dark conductivity also was found to be of similar nature.

A porous silicon diode as a source of low-energy free electrons at milli-Kelvin temperatures

We have developed a porous silicon (PS) diode that yields free-electron currents with energies <0.1eV below 77 K. The power dissipated during emission is low so that pulses of electrons can be produced below 100 mK without raising the temperature of the system. Free electrons were generated in liquid He4 and He3 as well. The device was developed as a source of electrons for a quantum computing system using electrons on the surface of a dielectric film. The results suggest that a Poole-Frenkel type of mechanism accounts for the observed electric-field-enhanced conduction but the electron emission mechanism is not well understood in the present models of PS.

Damage effect of fluorine implantation on PECVD α-SiOC barrier dielectric

In this work the dielectric properties of plasma enhanced chemical vapor deposited (PECVD) amorphous SiOC (α-SiOC) films with various concentrations of oxygen are investigated for the barrier dielectric application. Experimental results show after fluorine (F) ion implanted into α-SiOC film, the leakage current in carbide film is increased due to the generation of trap centers. Afterwards, the traps can be effectively repaired after thermal annealing, leading to the decrease of leakage current further. From the extraction of the current–voltage (J–E) characteristics, the conducting mechanism of the leakage current obeys the Poole–Frenkel type behavior for intrinsic, F-implanted and thermally annealed samples. Also, the barrier height of the F-implanted and thermally annealed samples are extracted and exhibits a higher value than that of the intrinsic sample.

Dc conduction properties of TlSbSe 2 crystals

TlSbSe 2 samples used for electrical measurements were cleaved from larger crystals grown by using the Bridgman–Stockbarger method. Electrical properties of TlSbSe 2 have been carried out as a function of temperature (203–258 K) at different dc electric fields. The dc studies revealed the non-ohmic type of conduction (log I versus log V plot). The field lowering coefficient β is evaluated from log I versus E 1/2 plot. It is found that the dominant conduction mechanism in these samples is Poole–Frenkel type. The activation energies are also calculated at different voltages. It is seen that the activation energy decreases with increase of applied voltage.

Impact of microstructure on electrical characteristics of laser ablation grown ZrTiO4 thin films on Si substrate

Zirconium titanate thin films were deposited on a p-type Si substrate using the pulsed excimer laser ablation technique and the thin films were either highly oriented along (020) or were of a polycrystalline nature, depending upon the processing conditions. A detailed structural characterization using atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy (XPS) and x-ray diffraction of the highly oriented and polycrystalline ZrTiO4 thin films were carried out. The XPS of highly oriented ZrTiO4 thin films deposited on Si substrates was characterized. The O 1s spectrum did not show any splitting, indicating there is no possibility of adsorbed oxygens, and the single peak in O 1s is attributed to the oxygen ions in the lattice. Capacitance–voltage (C–V), conductance–voltage (G–V) and dc leakage current studies were done on both types of thin film. Secondary ion mass spectrometer studies revealed a reasonably sharper interface existing at the thin film/substrate for the polycrystalline films, compared with the highly oriented thin films. A higher dielectric constant and higher leakage current were observed for the highly oriented ZrTiO4 thin films compared with polycrystalline thin films. The C–V and G–V studies were carried in the frequency regime of 1–100 kHz on both types of film at higher temperatures (300–500 K) so as to make the minority carriers of the Si substrate respond to the applied ac modulating signal. The activation energies calculated from the Arrhenius plot from C–V and G–V measurements were nearly equal to half of the silicon energy band gap, thereby suggesting a generation–recombination mechanism prevailing in the depletion region of the Si substrate through bulk traps. The interface traps were calculated using the high frequency method and observed to be higher for the polycrystalline films than for the highly oriented thin films. Dc leakage current studies were done in the accumulation region and both the highly oriented and polycrystalline thin films obeyed a bulk limited Poole–Frenkel type conduction mechanism. Finally, the electrical characteristics were correlated to the microstructure of zirconium titanate thin films.

Composition dependence of the electrical conductivity of Se85−xTe15Sbx (x=2,4,6,8 and 10) glass at room temperature

Abstract Measurements of I – V characteristics and DC electrical conductivity of Se 85− x Te 15 Sb x (where x = 2, 4, 6, 8 and 10) glassy thin pellets, prepared by well established melt quenching technique in bulk (pellets of diameter 12 mm and thickness ≈1 mm were prepared under a constant load of 5 tons), have been carried out at room temperature using Keithley High Resistance Meter/Electrometer 6517A. For the recording of I – V characteristics, this equipment was used in FVMI (Force Voltage Measure Current) mode. It is found evident from the I – V characteristics that the glass containing 4 at. wt.% of Sb has the minimum resistance allowing maximum current through the sample as compared to the other at. wt.% of Sb contents e.g. 2, 6, 8 and 10. At 4 at. wt.% of Sb, conductivity rises to 76% of the value obtained for 2 at. wt.% of Sb. Composition dependence of DC conductivity is discussed in terms of the bond formation between Se and Sb at different compositions. The conduction mechanism is discussed qualitatively on the basis of Poole–Frenkel conduction mechanism. Also the linear relation between ln ( I ) and V 1/2 verifies that the conduction mechanism is of Poole–Frenkel type for both low and high voltage range. The deviation of ohmic behaviour at lower voltage range towards the non-ohmic at higher voltage range is due to the high voltage induced temperature effects in the sample.

Electrical properties of polymeric light-emitting diodes

In this work, the electrical properties of polymeric light-emitting diodes (LEDs) based on poly(2-methoxy-5-(2 ′-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, were studied by dc current–voltage ( I– V) curves and impedance/admittance spectroscopy, at different temperatures. ITO/polymer/metal (Al and Au) structures were used to study the effect of the barrier height on the charge injection at the polymer/metal interfaces. For ITO/MEH-PPV/Al devices, strong temperature influence on the dc conductivity was observed in the forward direction whereas, in the reverse direction, the conductivity was almost temperature independent. On the other hand, ITO/MEH-PPV/Au devices presented temperature dependent conductivities for both polarities. This indicates that, for low barrier height at the interfaces (<0.4 eV), the carrier injection is a thermally activated process while, for higher barrier heights, the injection becomes dominated by a tunneling-type process. Application of a dc bias voltage superimposed to the alternating voltage in the impedance measurements also causes a diminution in the activation energy of the conductivity, resembling Schottky barrier lowering at the interfaces or Poole–Frenkel type effects in the bulk.

Electrical characterization of ITO/CuPc/Al diodes using temperature dependent capacitance spectroscopy and I– V measurements

Sandwich structures were prepared by vacuum sublimation of 100 nm thick CuPc thin films between ITO and Al electrodes. The rectifying behavior of these devices was analyzed with dc electrical measurements as a function of temperature in the range 100–310 K. Different regimes were observed with temperature and bias dependent transition. At low bias and high temperature, the results are best explained with Poole–Frenkel type transport model, while at high bias and low temperature, space charge limited current controlled by an exponential distribution of traps dominates the transport. Electrically active defects were investigated with space charge capacitance spectroscopy, as a function of temperature and frequency. The analysis of the capacitance data shows that the CuPc layer exhibits a rather high density of localized states at Fermi level. These results are discussed and compared with those of the temperature dependent J– V measurements.

Conduction studies on electrodeposited indium selenide thin films

Indium Selenide thin films were electrodeposited on Indium Tin Oxide (ITO) coated glass substrates from a mixture of Indium trichloride (InCl3) and selenium dioxide (SeO2) in aqueous solution by the potentiostatic electrodeposition technique. The InSe films showed that Mott’s variable-range hopping conductivity (VRH) is dominant under low field (∼2×105 Vcm−1) condition. At high field the Current-Voltage studies (in the temperature range 300 – 380 K) revealed that the conductivity of Indium selenide thin films is of Poole-Frenkel type. The plot drawn between Ln (I/V) and þV showed a straight line and also the experimental data and the theoretical data closely matches. Hence Poole-Frenkel mechanism is confirmed and discussed in detail.

Poole–Frenkel conduction in polyvinyl chloride stabilized with dibutylin laurate–maleate

Electric conduction in PVC-stabilized with dibutylin laurate–maleate has been investigated. In the low voltage region, the conduction was found to be due to thermally generated carriers. At higher voltages, the conduction mechanism was identified as Poole–Frenkel type. The charge carriers injected on the polymer surface from the electrodes was found to disappear and become the source of thermally stimulated discharge (TSD) current.

Formation and electric property measurement of nanosized patterns of tantalum oxide by current sensing atomic force microscope

Nanosized patterns of tantalum oxide were fabricated on a tantalum substrate by applying a potential pulse utilizing current sensing atomic force microscopy (CSAFM). The dimensions of the dots were strongly dependent on the bias applied, scan rate, and potential pulse duration. By controlling these variables, the minimum size nanodots with full width at half maximum of 35 nm was achieved. Immediately after pattern formation, the electrical properties of the Ta oxide nanodots were measured using CSAFM. The charge transport at the CSAFM tip and the nanosized Ta oxide dot can be described by Poole–Frenkel type conduction. The relative dielectric constant of the nanosized Ta2O5 dots was calculated to be 17.8–24.3, showing that the quality of the oxide was high. In addition, by controlling the substrate bias applied, pulse duration, and tip scan speed, nanosized Ta oxide lines with the desired dimensions were prepared.

Conductivity and distribution of charge on electroluminescent Si/SiO2 structures investigated by electrostatic force microscopy

Electroluminescent Si/SiO2/Au layer structures on a p-Si wafer are investigated with electrostatic force microscopy and atomic force microscopy. The samples comprise either four Si/SiO2 layer pairs prepared by chemical vapor deposition or a SiO2 thermal oxide layer grown at 950°C on the wafer. A 9–13nm thick Au-film electrode is sputtered on top of the samples. A correlation between the density of electroluminescent dots and distribution of charge on the surface of these structures is found. Measurements of the excitation current through the samples show that the four-layer Si/SiO2/Au structure has Poole–Frenkel type conductivity and the thermal oxide sample is excited through Fowler–Nordheim tunneling.

Electrical characterization of Ba(Zr 0.1Ti 0.9)O 3 thin films grown by pulsed laser ablation technique

In situ annealed thin films of ferroelectric Ba(Zr 0.1Ti 0.9)O 3 were deposited on platinum substrates by pulsed laser ablation technique. The as grown films were polycrystalline in nature without the evidence of any secondary phases. The polarization hysteresis loop confirmed the ferroelectricity, which was also cross-checked with the capacitance–voltage characteristics. The remnant polarization was about 5.9 μC cm −2 at room temperature and the coercive field was 45 kV. There was a slight asymmetry in the hysteresis for different polarities, which was thought to be due to the work function differences of different electrodes. The dielectric constant was about 452 and was found to exhibit low frequency dispersion that increased with frequency. This was related to the space-charge polarization. The complex impedance was plotted and this exhibited a semicircular trace, and indicated an equivalent parallel R– C circuit within the sample. This was attributed to the grain response. The DC leakage current–voltage plot was consistent with the space-charge limited conduction theory, but showed some deviation, which was explained by assuming a Poole–Frenkel type conduction to be superimposed on to the usual space-charge controlled current.

Charge transport in π-conjugated polymers from extraction current transients

For the study of the charge transport mechanism of equilibrium charge carriers in π-conjugated polymers we use a novel, still technically simple method, namely charge extraction in a linearly increasing voltage (CELIV). We briefly outline the method and use it to study the temperature and electric field dependence of the mobility in regioregular poly(3-octylthiophene). We find that mobility can be explained using the well-known Poole–Frenkel type of law μ∝μ 0( T)exp[B(T)√E], with an activated zero-field mobility. We also find that the nature of the mobility is determined by the release time from traps in regioregular poly(3-hexylthiophene).

Electrical properties of some chalcogenide glassy alloys of the system Se 100− xIn x

The I– V characteristics of the system Se 100− x In x (where x=5, 10, 15 and 20 at%) are discussed in terms of Jonscher–Ansari modified Poole–Frenkel type of conduction. The jump distance ( d), the charge carrier concentration ( n) and the trap depth ( ϕ) have been deduced for the system. The DC electrical properties of this system are investigated. According to the Arrhenius equation of conductivity, the activation energy of conduction Δ w 1 and Δ w 2 for the two arms of conductivity has been found in the range from 0.18 to 1.2 eV. Both mobility ( μ) and diffusion coefficient ( D) are calculated for different compositions at different ambient temperatures. The activation energy for mobility (Δ U) and diffusion (Δ Q) have been deduced and their values ranged from 0.02 to 0.4 eV. The behavior of Δ W 1, Δ U and Δ Q has the same trend with composition, which indicates that the conduction mechanism is controlled by one type of process. The Meyer–Neldel rule has been applied to the present results and a confirmation of this rule has been found.

( Ta 1−x Nb x ) 2 O 5 films produced by atomic layer deposition: Temperature dependent dielectric spectroscopy and room-temperature I–V characteristics

Temperature dependent ac dielectric spectroscopy and room-temperature I–V characterization were performed on atomic layer deposited (Ta1−xNbx)2O5 films. The high frequency permittivity, as well as the dc conductivity of the films, were found to increase with increasing Nb content. The conduction mechanism in the mixed Ta–Nb oxide films was of the Poole–Frenkel type, while the high field conduction in pure Ta2O5 was space-charge limited. The activation energy for dc conduction was higher in mixed Ta–Nb oxides compared to pure Ta2O5 and Nb2O5 films. Irreversible changes in the conduction mechanism took place upon heat treatment above a certain irreversibility temperature. This temperature was higher for the mixed oxides than for the binary ones.

Charge transport mechanism in a typical Au/CdTe Schottky diode

The charge transport mechanism in a typical Au/CdTe Schottky diode has been investigated. Evidence for different types of charge transport at different temperature regions has been observed. The dominant transport mechanism in the 100–300 K region is identified as the Poole–Frenkel type. The activation energy of the trap level detected in the 100–300 K temperature range shows a voltage dependence. The transport mechanism changes at a characteristic temperature of about 270 K.

Energetic Disorder, Spatial Correlations, and the High-Field Mobility of Injected Charge Carriers in Organic Solids

In a large class of disordered organic solids, the observed field and temperature dependence of the mobility of photoinjected charge carriers arises from specific statistical features of the disordered potential energy landscape through which they move. In materials with polar constituents, energy fluctuations exhibit strong spatial correlations, with deep energetic valleys developing only over large length scales. We present a scaling analysis that shows how the hierarchical field-induced flattening of fluctuations of different magnitudes gives rise to field dependent (e.g., Poole-Frenkel type) mobilities characteristic of the spatial correlations from which they arise.