Schottky Thermionic Emission Research Articles

Temperature dependence of carrier injection in small molecular organic light emitting device with a mixed single layer

We investigated the carrier injection properties in small molecular organic light emitting devices with a mixed single layer by measuring current-voltage characteristics using Au/MoO 3 as hole and Al alloy as electron injection electrodes. The carrier injection mechanism at the interface was primarily ascribed to the Schottky thermionic emission with the barrier height of 0.25 eV for holes and 0.67 eV for electrons. It was found that the barrier height of the electron was dependent on LUMO level of electron transport material. In addition, the interfacial state analysis demonstrated that the electron barrier height was also dependent on the interfacial conditions of the devices.

Investigation of carrier injection mechanism in small molecular organic light emitting device with a mixed single organic layer

Injection properties of electrons and holes in a mixed single layer organic light emitting device with mixed small molecules tris-(8-hydroxy-quinoline) aluminum (Alq3), 2,5-bis(6′-(2′,2″-bipyridyl))-1,1-dimethyl-3,4-diphenylsilole (PyPySPyPy), 4′-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl (α-NPD), and 5,6,11,12-tetraphenylnaphthacene (rubrene) were investigated using Au/MoO3 as hole and Al alloy as electron injection electrodes. On the basis of measuring the temperature dependence of currents through the interface between the electrodes and the mixed single organic layer, the carrier injection mechanism was primarily ascribed to the Schottky thermionic emission with the barrier height of 0.25 eV for holes and 0.67 eV for electrons. By adding the dopant material rubrene and the electron transport material PyPySPyPy into the mixed single layer, the barrier height of electrons could be reduced. The interfacial state analysis demonstrated that the electron barrier height was also dependent on the interfacial conditions of the device.

P‐162: Carrier Injection Mechanism in a Mixed Single Layer Organic Light Emitting Device

AbstractInjection properties of electrons and holes in a small organic light emitting device with a mixed single organic layer were investigated by current vs voltage measurements using Au/MoO3 and Al alloy as an electron and hole injection electrodes, respectively. On the basis of the temperature dependence of currents through the interface between electrodes and mixed single organic layer, the carrier injection mechanism at the interface was primarily ascribed to the Schottky thermionic emission with the barrier height of 0.25 eV and 0.67 eV for holes and electrons, respectively. It was found that the barrier height of the electron could be controlled by changing LUMO level of mixed organic materials. The interfacial state analysis demonstrated that the electron barrier height was also dependent on the interfacial conditions of the devices.

Analysis of current-voltage characteristics of Fe/MgO/GaAs junctions using self-consistent field modeling

Current-voltage data in metal-insulator-semiconductor devices are usually interpreted by a model of the tunneling current or the Schottky thermionic emission current. In these models, the barrier through which the electrical current flows is normally assumed to be rectangular or at best trapezoidal. For metal-insulator-metal junctions, this is a reasonable assumption. However, when one electrode is a doped semiconductor, the parameters of the current-voltage models require a self-consistent field description and the bias-dependent band bending within the semiconductor must be taken into account. These bias-dependent energy-band profiles are modeled with a Schr\"odinger-Poisson solver and incorporated into the fitting procedure for the current-voltage data. We find that the ratio of the Schottky thermionic emission to the tunneling current, as well as the tunnel barrier heights can be determined using this approach. With this approach, one can quantitatively distinguish between tunneling and thermionic transport regimes and this is particularly applicable to the interpretation of spin-transport experiments in metal-insulator-semiconductor devices.

Investigation on internal electric field distribution of organic light‐emitting diodes (OLEDs) with Eu2O3 buffer layer

AbstractWe have found that organic light‐emitting diode (OLED) performance was highly improved by using europium oxide (Eu2O3) as a buffer layer on indium tin oxide (ITO) in OLEDs based on tris‐(8‐hydroxyquinoline) aluminium (Alq3), which showed low turn‐on voltage, high luminance, and high electroluminescent (EL) efficiency. The thickness of Eu2O3 generally was 0.5–1.5 nm. We investigated the effects of Eu2O3 on internal electric field distributions in the device through the analysis of current–voltage characteristics, and found that the introduction of the buffer layer balanced the internal electric field distributions in hole transport layer (HTL) and electron transport layer (ETL), which should fully explain the role of the buffer layer in improving device performance. Our investigation demonstrates that the hole injection is Fowler–Nordheim (FN) tunnelling and the electron injection is Richardson–Schottky (RS) thermionic emission, which are very significant in understanding the operational mechanism and improving the performance of OLEDs.

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.

Properties of the transport layer formed at the interface between two polymer films

Electronic transport properties of a 2D boundary layers formed by two films of an organic polymer material are investigated. It is established that the conductivity of such a layer is several orders of magnitude higher than the surface conductivity of individual films forming the experimental structure. Temperature measurements show that transport of charge carriers over this layer can be described using the models of Schottky thermionic emission and hopping transport over trap levels.

Mechanisms of Conductivity in Metal-Polymer-Si Thin Film Structures

Thin film structures of the metal-polymer-Si type have been fabricated. The current-voltage characteristics of these structures versus temperature have been measured and analyzed. The results obtained are treated within such models of charge transport as the Schottky thermionic emission, Pool-Frenkel and hopping mechanisms. The preliminary conclusions about the basic mechanisms of charge transport have been made. The charge carriers transport is explained by the Schottky thermionic emission in the case of the metal-polymer interface and the hopping mechanism in a polymer.

Transition from dispersive to non-dispersive transport of holes in poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) light-emitting diodes investigated by time of flight measurements

The transport properties (conductivity and mobility) of holes and electrons in poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) light-emitting diodes were investigated using direct current–voltage curves ( I– V) and time of flight measurements (TOF) as a function of electric field and temperature. TOF results revealed that the transport of holes in the bulk follows a non-dispersive behavior at room temperature (300 K), exhibiting a progressive transition to a dispersive behavior as the temperature decreases down to 220 K. The dispersive transport characteristics were interpreted in the framework of carrier hopping in an exponential density of states. On the other hand, the analysis of the negative photocurrent transients indicated that the transport of negative charge carriers (electrons) is strongly dominated by trapping in the entire range of temperature studied. The I– V curves presented remarkable temperature dependence, being analyzed in terms of the classic Fowler–Nordheim tunneling, Richardson–Schottky thermionic emission and trap-controlled transport.

Electrical characterization of PbZr0.4Ti0.6O3 capacitors

We have conducted a careful study of current-voltage (I-V) characteristics in fully integrated commercial PbZr0.4Ti0.6O3 thin-film capacitors with Pt bottom and Ir∕IrO2 top electrodes. Highly reproducible steady-state I-V were obtained at various temperatures over two decades in voltage from current-time data and analyzed in terms of several common transport models including space charge limited conduction, Schottky thermionic emission under full and partial depletion, and Poole-Frenkel conduction, showing that the latter is the most plausible leakage mechanism in these high-quality films. In addition, ferroelectric hysteresis loops and capacitance-voltage data were obtained over a large range of temperatures and discussed in terms of a modified Landau-Ginzburg-Devonshire theory accounting for space charge effects.

Investigation of charge-carrier injection characteristics in NPB/Alq 3 heterojunction devices

The effect of copper phthalocyanine (CuPc) and LiF interfacial layers on the charge-carrier injection in N, N′-di(naphthalene-1-yl)- N, N′-diphenyl-benzidine (NPB)/tris(8-hydroxyquinoline) aluminium (Alq 3) organic heterojunction devices have been studied through the analysis of current–voltage characteristics. The investigation clearly demonstrated that the hole injection into NPB from anode is Fowler–Nordheim (FN) tunneling and the electron injection into Alq 3 from cathode is Richardson–Schottky (RS) thermionic emission. The barrier heights obtained from the FN and RS models proved that the band alignments for charge-carrier injection are greatly improved by the CuPc and LiF interfacial layers, which should fully clarify the role of the interfacial layer on the improvement of device performance.

Dielectric Characteristics of Sol-Gel-Derived BST/BSLaT/BST Multilayer

BST/BSLaT/BST multiplayer (ML) films were prepared by sol-gel method on the Pt/Ti/SiO2/Si substrate. X-ray diffraction analysis and atomic force microscopy showed that dopant La causes decreased grain size of BST thin films obviously, and ML films were similar to BST films in size. The dielectric constant versus temperature curve of ML films becomes sharper comparing with that of BST films near the phase transition point, which indicated that the pyroelectric coefficient is propitious to be enhanced. The ML films also improve the leakage current characteristics of BST thin films and the linear relationship of the log J versus E1/2 curves shows in a good agreement with the Schottky thermionic emission model. This work clearly reveals the highly promising potential of BST/BSLaT/BST multiplayer films compared with BST films for application in uncooled infrared focal plane arrays system.

Current limiting mechanisms in indium-tin-oxide/poly3-hexylthiophene/aluminum thin film devices

We studied the temperature dependent current-voltage characteristics of regioregular poly (3-hexylthiophene 2.5-diyl) (P3HT) thin films sandwiched between indium tin oxide (ITO) and aluminum (Al) electrodes (ITO/P3HT/Al devices), with the aim of determining the current limiting mechanism(s) in these devices, and the temperature and/or applied electric field range(s) in which these mechanisms are valid. The current-voltage characteristics of the ITO/P3HT/Al devices showed that current flow across the device is limited by hole injection at the Al/P3HT interfaces at temperatures below 240 K, when the device is biased with high potential on Al. Above this temperature, the bulk transport properties control the characteristics. For the reverse bias, the ITO/P3HT contact does not limit the current; instead it is controlled by a space charge that accumulates due to the low charge carrier mobility in the polymer. An expression that provides a criterion to determine the validity of applying either the Richardson–Schottky thermionic emission model or the Fowler–Nordheim field emission model was deduced. It can be employed to determine the electrical field at which the transition from charge injection by thermionic emission to that by field emission for a given temperature and interface potential barrier height takes place. Our experimental data fit to the deduced expression. Theoretical limits of the model are also discussed. By considering the regions of the current-voltage curves where field emission or thermionic emission was applicable, the interface potential barriers were estimated, respectively. Hence, conclusions on whether the current-voltage behavior of the devices was contact limited or bulk limited could be drawn.

Effect of Fe2O3 doping on the electrical properties of a SnO2 based varistor

The effect of Fe2O3 addition on the densification and electrical properties of the (0.9895 − x) SnO2 + 0.01CoO + 0.005Nb2O5 + xFe2O3 system, where x = 0.005 or 0.01, was considered in this study. The samples were sintered at 1300°C for 2 h. Microstructure analysis by scanning electron microscopy showed that the effect of Fe2O3 addition is to decrease the SnO2 grain size. J × E curves indicated that the system exhibit a varistor behavior and the effect of Fe2O3 is to increase both, the non-linear coefficient (α) and the breakdown voltage (Er). Considering the Schottky thermionic emission model the potential height and width were estimated. Small amount addition of Fe2O3 to the basic system increases both the potential barrier height and width.

Leakage Current and Fatigue Properties of Pb(Zr, Ti)O3 Ferroelectric Films Prepared by RF-Magnetron Sputtering on Textured LaNiO3 Electrode

The leakage current and the fatigue properties of Pb(Zr, Ti)O3 (PZT) films prepared by rf-magnetron sputtering on (100)-textured LaNiO3 electrode were investigated. Sputtering parameters such as excess PbO content in the target, sputtering power, Ar/O2 ratio, and working pressure were studied. PZT films of the perovskite phase with (100)-preferred orientation can be easily obtained under our sputtering conditions. The optimal sputtering conditions are: 30% excess PbO, sputtering power 40 W, working pressure 5 mTorr, and Ar/O2=95/5. The effect of these sputtering parameters on leakage current and fatigue properties can be explained by the variation of the concentrations of lead vacancies and holes produced by the sputtering conditions. PZT films deposited under these optimal conditions can endure fatigue up to more than 1011 cycles and possess a low leakage current density (10-8–10-7 A/cm2) at an electric field strength lower than 100 kV/cm. The conduction mechanism of the leakage is proposed to be a Schottky thermionic emission.

Effect of La2O3 doping on the microstructure and electrical properties of a SnO2-based varistor

The effect of La2O3 addition on the densification and electrical properties of the (0.9895-xSnO2+0.01CoO+0.0005Nb2O5+x La2O5 system, where x=0.0005 or 0.00075, was considered in this study. The samples were sintered at 1300 °C for 2 and 4 h and a single SnO2 phase was identified by X-ray diffraction. Microstructure analysis by scanning electron microscopy showed that the affect of La2O3 addition is to decrease the SnO2 grain size. J versus E curves indicated that the system exhibits a varistor behavior and the effect of La2O3 is to increase both the non-linear coefficient (α) and the breakdown voltage (E2). Considering the Schottky thermionic emission model the potential height and the width were estimated. The addition of small amounts of La2O3 to the basic system increases the potential barrier height and decreases both grain size and potential barrier width. © 2001 Kluwer Academic Publishers

Light-emitting devices based on evans blue under alternating-current and direct-current modes: different charge injection mechanisms

Light-emitting devices (LEDs) have been fabricated with a blend of evans blue dye and poly(methylmethacrylate) (PMMA) insulating material. Luminance has been observed under both direct-current (DC) and alternating-current (AC) modes. Different charge injection and operation mechanisms have been shown to be applicable under DC and AC biases. From the current–voltage characteristics under DC voltage, Fowler–Nordheim (FN) tunnelling mechanism and Richardson–Schottky thermionic emission have been found to be applicable in forward and reverse bias directions, respectively. In general, luminance has been observed only in the forward bias direction. Under AC voltage, there has been accumulated hole-assisted electron injection, which has resulted in luminance in both bias directions for all molar concentrations of evans blue, and particularly higher luminance level in the reverse-biased half-cycle. Moderately high-frequency (40 kHz) electroluminescence (EL) has been obtained from these devices.

Poly(3-hexylthiophene)-zinc oxide rectifying junctions

The rectifying junction formed at the interface of solvent-cast, as-prepared poly(3-hexylthiophene) (P3HT) and conductive ZnO-coated glass was investigated by recording the l-V and C-V characteristics. The data have been discussed in terms of the Schottky thermionic-emission current transport model. The diode “quality factor” determined for these devices was 3.6, and indicated imperfect model fitting. The Schottky barrier was estimated as 0.93 eV. A negative differential resistance regime was observed for P3HT/ZnO devices under reverse-bias high-field conditions. This effect was concluded to be a filament-formation burnout failure process.

Electron transport mechanism in Al/Al2O3/n-InTe/Bi thin-film structures

Thin-film metal-insulator-semiconductor structures have been prepared by deposition of n-type indium telluride films on partially oxidized aluminum layers. An analysis of the current-voltage characteristics indicates the current transfer mechanism in these structures to be of the Schottky thermionic emission type in low field and space-charge limited at high fields. The barrier height presented to the conduction electrons in such structures has also been calculated from the temperature dependence of the I-V characteristics.

Dependence of negative resistance characteristics of avalanching MIM diodes on current injection mechanisms

Current-controlled negative resistance characteristics are computed for avalanche injection metal/insulator/metal diodes. Current injection is taken to occur by either Schottky thermionic emission or the Fowler-Nordheim tunneling mechanism. The results presented show that the diode negative resistance characteristics depend critically on both the magnitude and field dependance of injected currents.