Thermionic Emission Model Research Articles

Anomalous electrical transport characteristics of orthorhombic HoMnO3/Nb-doped SrTiO3 epitaxial heterojunctions

Orthorhombic HoMnO3 films with c-axis and a-axis orientations were prepared epitaxially on Nb-1.0 wt %-doped SrTiO3 single crystal substrates by using pulsed laser deposition technique to fabricate all-oxide heterojunctions. The temperature dependent current-voltage measurements display diodelike rectifying behaviors, and the forward current was perfectly fitted using the thermionic emission model. An anomalous increase in the reverse-bias current with decreasing temperature was observed in c-axis-oriented HoMnO3/Nb-doped SrTiO3 junctions, which was not found in a-axis-oriented ones. The tunneling through the junction, which depends on the effective barrier height at the interface, was used to explain the results.

Current transport in Pt Schottky contacts to a-plane n-type GaN

The temperature-dependent electrical properties of Pt Schottky contacts to nonpolar a-plane n-type GaN were investigated. Barrier height and ideality factor, estimated from the conventional thermionic emission model, were highly temperature dependent. A notable deviation from the theoretical Richardson constant value was also observed in the conventional Richardson plot. Analyses using the thermionic field emission model showed that consideration of defect-assisted tunnelling was necessary to explain the observed electrical behaviours.

Junction properties of Au/ZnO single nanowire Schottky diode

In this study, we have analyzed the Au/ZnO single nanowire based Schottky diode by investigating temperature dependent current voltage and x-ray photoelectron spectroscopy (XPS) measurements. The calculated barrier height of the Schottky diodes by using the thermionic emission model is in good agreement with the value obtained from the XPS measurements but lower than the theoretically predicted value. The ionization of interface states has been considered for explaining this discrepancy.

Charge Transfer Mechanism in Organic Memory Device

In this paper, the conduction mechanism in organic bistable memory device was investigated by both experimental and theoretical method. The current voltage (J-V) characteristics showed the electrical bistable properties between an initial low-conductivity state and a high-conductivity state upon application of an external electric field at room temperature. The current transition exhibited a very narrow voltage range that causes an abrupt increase of current. The on-state and the off-state were proposed by space-charge-limited current and thermionic emission model, respectively. That supported by the experimental data to explained the charge transfer mechanism in organic memory device.

Fabrication and Characterization of C60-Based Organic Schottky Diodes

We have fabricated organic Schottky barrier diodes with Cu/LiF/C60/Al sandwiched construction. Cu and Al are selected as the cathode and the anode, respectively. C60 is used as the organic layer and LiF as the buffer layer inserted between the cathode and C60. After the annealing process, Schottky contact is well formed at the Al/C60 interface and Ohmic contact is formed at the (Cu/LiF)/C60 interface. The current density-voltage (J – V) measurements of the diodes present nonlinear behavior. As a result, the rectification ratio reaches 1 × 103. The characteristics of the diodes have been analyzed using the energy band diagram. The values of Schottky barrier height ΦB, ideality factor n and reverse saturation current density Js are extracted according to the standard thermionic emission model.

Effect of hydroquinone on the electrical properties of dodecylbenzene sulfonic acid doped polypyrrole/aluminum Schottky junction

Electrical properties of Schottky barrier diode fabricated using Aluminum for Schottky contact and indium tin oxide for ohmic contact and containing polypyrrole doped with dodecylbenzene sulfonic acid in the presence and in the absence of a plasticizing agent hydroquinone were studied. Various parameters, e.g. saturation current, ideality factor, built in voltage; carrier concentration and barrier height have been calculated and found to be affected by the presence of hydroquinone in the doped polymer. The electrical behavior of the systems was found to be in a good agreement with the thermionic emission model for the Schottky barrier devices. The interaction of the doped polypyrrole with hydroquinone was explained in terms of change in the barrier height and in the carrier concentration of the diodes.

Dependence of current bistabilities on trap density and maximum applied voltage in organic bistable devices

The current–voltage ( I– V) curves and the captured electron density with various trap densities and maximum applied voltages of organic bistable devices (OBDs) were calculated by using the Shockley-Reed trapping rate model and taking into account the thermionic emission model. The current bistabilities in the I– V curves for OBDs were attributed to captured electrons in the traps near the heterointerface between the electrode and the organic layer. The on/off ratio and the width of the memory window of the I– V curves for OBDs gradually increased with increasing trap density in the organic layer and maximum applied voltage. The maximum increase of the on/off ratio and the width of the memory window of the I– V curve for OBDs became saturated to the specific values, regardless of the continuous increase of the trap density and the maximum applied voltage.

Scanning tunneling microscope study of nanosized metal–semiconductor contacts between ErSi 2 nanoislands and Si(0 0 1) substrate

The current–voltage ( I– V) characteristics of the nanosized metal–semiconductor contacts formed between the epitaxially grown ErSi 2 islands and p-Si(0 0 1) substrate are measured in situ by the scanning tunneling microscope. Experimental results show that the current densities passing through the nanocontacts are five orders of magnitude larger than that of the macroscopic ones and have an obvious dependence on the contact area. Especially, it is found that I–V characteristics of the contacts are sensitive to the sample surface adsorption. Our investigations indicate that surface conduction plays an important role in the electrical transport process from ErSi 2 islands to the Si(0 0 1) substrate. Furthermore, for the nanocontacts with surface currents suppressed effectively, the ideality factor and the effective Schottky barrier height are estimated by using the standard thermionic emission model. Our analysis suggests that the current through the interface between ErSi 2 nanoislands and the p-Si(0 0 1) substrate is enhanced due to the effects of tunneling and image force lowering.

Oxygen Stability and Leakage Current Properties of La-Modified Bismuth Titanate Ferroelectric Thin Films

The effects of oxygen annealing on the structural properties and leakage current behaviors of sol–gel-derived Bi3.25La0.75Ti3O12-y ferroelectric thin films have been studied. The thin films were pyrolyzed in different atmospheric conditions including a fully oxygenized ambient of 1 bar and an evacuated ambient of less than 10 mbar. X-ray diffraction patterns of the deoxygenated films showed measurable shifts to a higher angle, indicating that each deoxygenated film had a smaller lattice constant than the oxygenated one. Scanning probe analysis showed well-developed grains and inhomogeneous and asymmetric bound charges in both samples. The leakage current behavior of the films was measured as a function of voltage and temperature and was explained on the basis of the Schottky–Simmons thermionic emission model. X-ray photoemission spectra showed that oxygen stability in Bi2O2 layers plays an important role in determining the level of leakage current in the films.

Barrier height temperature coefficient in ideal Ti/ n-GaAs Schottky contacts

We have measured the I– V characteristics of Ti/ n-GaAs Schottky barrier diodes (SBDs) in the temperature range of 60–320 K by the steps of 20 K. The SBDs have been prepared by magnetron DC sputtering. The ideality factor n of the device has remained almost unchanged between 1.02 and 1.04 from 120 to 320 K, and 1.10 at 100 K. Therefore, it has been said that the experimental I– V data are almost independent of the sample temperature and quite well obey the thermionic emission (TE) model at temperatures above 100 K. Furthermore, the barrier height (BH) Φ b 0 slightly increased with a decrease in temperature, 320–120 K. The Φ b 0 versus temperature plot from intercepts of the forward-bias ln I versus V curves has given a BH temperature coefficient of α = −0.090 meV/K. The Norde’s function has been easily carried out to determine the temperature-dependent series resistance values because the TE current dominates in the I– V characteristics. Therefore, the Φ b 0 versus temperature plot from the Norde’s function has also given a BH temperature coefficient value of α = −0.089 meV/K. Thus, the negligible temperature dependence or BH temperature coefficient close to zero has been attributed to interface defects responsible for the pinning of the Fermi level because their ionization entropy is only weakly dependent on the temperature.

Temperature dependence of current-voltage characteristics in highly doped Ag/p-GaN/In Schottky diodes

To obtain detailed information about the conduction process of the Ag/p-GaN Schottky diodes (SDs) fabricated by us, we measured the I-V characteristics over the temperature range of 80–360 K by the steps of 20 K. The slope of the linear portion of the forward bias I-V plot and nkT=E0 of the device remained almost unchanged as independent of temperature with an average of 25.71±0.90 V−1 and 41.44±1.38 meV, respectively. Therefore, it can be said that the experimental I-V data quite well obey the field emission model rather than the thermionic emission or thermionic field emission model. The study is a very good experimental example for the FE model. Furthermore, the reverse bias saturation current ranges from 8.34×10−8 A at 80 K to 2.10×10−7 A at 360 K, indicating that the charge transport mechanism in the Ag/p-GaN SD is tunneling due to the weak temperature dependence of the saturation current. The possible origin of high experimental characteristic tunneling energy of E00=39 meV, which is ten times larger than possible theoretical value of 3.89 meV, is attributed to the accumulation of a large amount of defect states near the GaN surface or to the deep level defect band induced by high doping or to any mechanism which enhances the electric field and the state density at the semiconductor surface.

Stability and phase transition studies of Ga– pWSe 2 Schottky diode by current–voltage–temperature method

Ga– pWSe 2 Schottky diodes have been fabricated by making contact of liquid gallium on pWSe 2 surfaces. The stability and phase transition of the diodes were investigated in terms of barrier height, ideality factor and series resistance using thermionic emission model by current–voltage method. The anomalous behaviour of these parameters during the initial hours of fabrication is interpreted in terms of the phase related transition of gallium from liquid to solid. Beyond this time period, diodes show stable nature in its parameters.

Fabrication of Poly(vinylidene fluoride−trifluoroethylene)/Poly(3,4-ethylenedioxythiophene)−Polystyrene Sulfonate Composite Nanofibers via Electrospinning

Nanofibers of PVF2−TrFE/PEDOT−PSS were fabricated at room temperature using electrospinning with the thinnest fiber having a diameter of ∼15 nm. This process for generating PVF2−TrFE/PEDOT−PSS composite nanofibers is cheap, fast, and reliable. The presence of conducting PEDOT−PSS assisted in the fabrication of PVF2−TrFE nanofibers at low polymer concentrations in N,N,-dimethylformamide and energy dispersive X-ray spectroscopy confirmed the presence of PEDOT−PSS in the nanofibers. As a fiber mat, they were electrically conducting and used in the fabrication of a Schottky diode, and the diode parameters were calculated assuming the standard thermionic emission model of a Schottky junction. Being a composite, these nanofibers are promising candidates for use in a variety of applications that can take advantage of the ferroelectric and/or conducting properties of each individual component. In addition, the large aspect ratio and even larger surface to volume ratio of the fibers makes them ideal candidates in the fabrication of miniaturized, low power consumption devices, and supersensitive sensors.

Electron emission properties of two-phase argon and argon-nitrogen avalanche detectors

Electron emission properties of two-phase Ar avalanche detectors are studied. The detectors investigated comprised a liquid Ar or Ar+N2 layer followed by a multi-GEM multiplier operated in the saturated vapour at 84 K. Two components of the electron emission through the liquid-gas interface were observed: fast and slow. In Ar, the slow emission component dominated even at higher fields, reaching 2 kV/cm. In Ar+N2 on the contrary, the fast emission component dominated at higher fields, the slow component being disappeared. This is explained by the electron backscattering effect in the gas phase. The slow component decay time constant was inversely proportional to the electric field, which is compatible with thermionic emission model. The electron emission efficiencies in two-phase Ar and Ar+N2 were estimated to be close to each other.

Thermionic emission model for contact resistance in organic field-effect transistor

Injection mechanism of top-contact pentacene field-effect transistor (OFET) was investigated in respect to the internal field. The contact resistance was evaluated by the transmission line method for various applied external voltages as well as various pentacene film thicknesses. The behaviour of contact resistance was described in terms of the thermionic emission model (Schottky injection) and internal electric field generated by excess charges accumulated on pentacene–gate insulator interface. It was shown that pentacene film thickness changes the internal electric field affecting the carrier injection barrier. It was concluded that the space-charge field effect made a significant contribution for smaller pentacene film thicknesses and therefore in accordance to the thermionic model was able to decrease contact resistance representing the potential drop.

Effect of Tunneling Current on Schottky Barrier Height in ZnO Varistors at Low Temperature

On the basis of the Schottky barrier and thermionic emission models, the temperature dependence of barrier height in ZnO varistors is investigated by the I – V characteristics in a wide temperature range from 93 K to 373 K. The obtained barrier height decreases with reducing temperature, which is ascribed to the contribution of tunneling current in measured current. From the proposed equivalent circuit, it is suggested that two current components coexist. One is thermionic emission current, which reflects the thermionic emission barrier height. The other is tunneling current, which appears even at low voltage, especially in low temperature ranges, and thus makes the barrier height obtained from measured current vary with temperature.

Effects of ultraviolet treatment on the contact resistivity and electronic transport at the Ti/ZnO interfaces

We report on the effect of ultraviolet (UV) treatment on the specific contact resistance (ρ) and electronic transport at the Ti/ZnO interfaces. The experimental results show the same barrier height of Ti/ZnO samples without UV treatment as Ti/ZnO samples with UV treatment and the higher ρ of Ti/ZnO samples with UV treatment than Ti/ZnO samples without UV treatment, suggesting the barrier-height independence of ρ. Based on the thermionic-emission model and x-ray photoelectron spectroscopy results, we found that the induced decrease in the number of the hydroxides at the surface region of ZnO by UV treatment resulted in decreases in the electron concentration near the surface region and the excess current component related to tunneling, increasing in ρ of Ti/ZnO samples.

Electronic transport and Schottky barrier heights of Pt/n-type GaN Schottky diodes in the extrinsic region

The current-voltage characteristics of n-type GaN Schottky diodes have been measured in the extrinsic region (that is, the temperature range of 100–300 K). The effective density of states in the conduction band decreases with decreasing temperature and is close to the electron concentration at 100 K, leading to a reduction in the energy difference between the conduction band minimum and Fermi level and an increase in the probability of tunneling. Therefore, changes in the effective density of states in the conduction band and the probability of tunneling at low temperature are responsible for the decrease of the barrier height and increase in the ideality factor on the basis of the thermionic emission model. The mechanism of forward current flow has been clearly established in this study, leading to the thermionic emission (thermionic field emission) interpretation of the I-V characteristics at 300 K (100 K).

On the accuracy of thermionic electron emission models. I. Electron detachment from SF6−

Detailed statistical rate calculations combined with electron capture theory and kinetic modeling for the electron attachment to SF(6) and detachment from SF(6)(-) [Troe et al., J. Chem. Phys. 127, 244303 (2007)] are used to test thermionic electron emission models. A new method to calculate the specific detachment rate constants k(det)(E) and the electron energy distributions f(E,epsilon) as functions of the total energy E of the anion and the energy epsilon of the emitted electrons is presented, which is computationally simple but neglects fine structures in the detailed k(det)(E). Reduced electron energy distributions f(E,epsilon/<epsilon>) were found to be of the form (epsilon/<epsilon>)(n) exp(-epsilon/<epsilon>) with n approximately = 0.15, whose shape corresponds to thermal distributions only to a limited extent. In contrast, the average energies <epsilon(E)> can be roughly estimated within thermionic emission and finite heat bath concepts. An effective temperature T(d)(E) is determined from the relation E - EA = <E(SF(6))(T(d))> + kT(d), where <E(SF(6))(T(d))> denotes the thermal internal energy of the detachment product SF(6) at the temperature T(d) and EA is the electron affinity of SF(6). The average electron energy is then approximately given by <epsilon(E)> = kT(d)(E), but dynamical details of the process are not accounted for by this approach. Simplified representations of k(det)(E) in terms of T(d)(E) from the literature are shown to lead to only semiquantitative agreement with the equally simple but more accurate calculations presented here. An effective "isokinetic" electron emission temperature T(e)(E) does not appear to be useful for the electron detachment system considered because it neither provides advantages over a representation of k(det)(E) as a function of T(d)(E), nor are recommended relations between T(e)(E) and T(d)(E) of sufficient accuracy.

Double barrier nature of Au/p-GaTe Schottky contact: Linearization of Richardson plot

The current–voltage ( I– V) characteristics of Au/p-GaTe Schottky contact were characterized at 60–300 K temperature range and compared with those of Al/p-GaTe. The observed anomalies of Schottky barrier were interpreted on the basis of thermionic emission model. The barrier height and ideality factor values at 280 K for Au were 0.513 eV and 1.022, respectively. As temperature lowered down to 140 K, we observed that these parameters gradually decreased. The barrier height dependence on temperature demonstrated a double-Gaussian distribution. The weighting coefficients for each Gaussian distribution and their standard deviations were found to be 0.76–37.7 meV and 0.24–58.2 meV for Au, respectively. A linearization procedure based on the double barrier distribution was carried out, and the Richardson constant for Au/p-GaTe was found to be 148.03 A K −2 cm −2. This value is reasonably close to the theoretical value given in the literature as 119.4 A K −2 cm −2.