Thermoionic Emission Research Articles

Formation energies of molecules and anions of europium bromides

The content of saturated vapors above europium dibromide and Eu-EuBr2, Eu-Ba-BaBr2, EuBr2-LaBr3 systems is investigated by means of high-temperature mass-spectrometry in the electron ionization and thermoionic emission regimes. On the basis of the measured equilibrium constants for reactions with participation of molecules and negative ions, the enthalpies of formation ΔfH298° (kJ/mol) are determined using the method of the third law of thermodynamics: −59 ± 13 (EuBr), −349 ± 19 (EuBr2), and −861 ± 24 (EuBr3−).

Influence of sputtering power on composition, structure and electrical properties of RF sputtered CuIn1−xGaxSe2 thin films

In this study, Cu(In1−xGax)Se2 (CIGS) thin films were deposited at room temperature by one step radio frequency (RF) magnetron sputtering process. An one-stage vacuum annealing process without selenization was performed to improve properties of the films. Influences of sputtering power on composition, structure and electrical properties of the as-deposited and annealed films were investigated. As the sputtering power not exceeding a proper power of 100W, the as deposited and annealed films show near stoichiometric composition and polycrystalline chalcopyrite structure. The annealed films exhibit almost the same composition as the as-deposited ones. All the sputtered and annealed films exhibit uniform and compact surface morphology without peeling and cracking. The electrical conductivity measured in 50–290K range reveal that the 50W and 100W deposited films exhibit metal and semiconductor character, respectively. The 100W deposited film present data consist with thermoionic emission at high temperatures of 200–290K. However, Mott law with the variable range hopping mechanism is predominant in the low temperature region.

Effect of Different Low Temperatures on Current Transport Mechanisms and Frequency Effect on Capacitance-voltage Curves for MOS-diodes

Thin SiO2 films with thickness 5-10nm were grown at 700°C in dry ambient. The devices with oxide 5nm approach ideality case at room temperature, in this case thermoionic emission over the barrier is dominate mechanism with traps was dominate. Capacitance-voltagedata in the form of 1/C2 verse voltage plot has been used to extract doping on the space charge region. The barrier height from I-V calculated 0.65, 0.64V for devices with oxide 5, 10nm respectively, this value difference from values extract from C-V plot which found 0.57, 0.64V.

Influence of film thickness on the physical properties of manganite heterojunctions

Rectifying and photoelectronic properties of the La0.67Ba0.33MnO3/SrTiO3:Nb junctions with the film thickness from d=0.5 to 30 nm have been systematically studied. It is found that the electronic transport of the junction is dominated by quantum tunneling or thermoionic emission when film thickness is below or above 1 nm. The rectifying ratio and ideality factor, correspondingly, experience a sudden change as film thickness grows from 0.5 to 1 nm and a smooth variation with film thickness above 1 nm. The threshold film thickness for the establishment of a mature depletion layer is therefore 1 nm. The photoemission properties of the junctions also exhibit a strong dependence on film thickness. As experimentally shown, the photocurrent vanishes in the zero thickness limit, and grows rapidly with the increase in film thickness until d=6 nm, where a maximal photocurrent of ∼770 nA/mm2 under the irradiance of the laser of 5 mW and 532 nm is obtained. After this maximum, an increase-to-decrease turning appears with further increasing film thickness. Taking into account the finite diffusion distance of the photocarriers and the strain-enhanced charge trapping in ultrathin film junctions, a theoretical description that well reproduces the experiment results can be obtained, which reveals the severe depression of finite diffusion distance of the extra carriers on photocurrent. The maximal diffusion distance thus obtained is ∼3.5 nm. Similar analyses have been performed for the La0.67Ca0.33MnO3/SrTiO3:Nb junctions, and the corresponding diffusion distance there is ∼1.5 nm.

ChemInform Abstract: The Electrodynamic Chamber: A Tool for Studying High-Temperature Kinetics Involving Liquid and Solid Particles

Abstract This review presents the development and application of a new approach for studying single particle kinetics that can provide new insights into particulate combustion. The new approach uses the capability of suspending a charged particle motionless at a fixed position in an electrodynamic chamber (EDC). The particle is heated by means of laser radiation. The new approach combines the capabilities of classical thermogravimetric and calorimetric approaches with those of drop tube experiments without some of the accompanied limitations. In this approach the particle can be well characterized prior to reaction. If after the reaction the particle is still suspended it can be further characterized in the chamber then it can be captured and off-line characterization can be carried out. The method is presently capable of providing information prior to, during, and past reaction on the mass, size, density, heat capacity, light absorptivity, porosity, phase transition, reaction enthalpies, thermal and inertial characteristic times, ignition temperatures and ignition delay times. The issues connected with heating with laser radiation include (1) the nature of internal electromagnetic field within the particle, (2) transient heat changes, and (3) photophoretic phenomena. In order to reach the desired temperature by radiation a laser beam has to be focused to a size of the particle diameter. Small movement of the particle may upset the heat balance. It is thus of essential importance to develop methods that will control the particle motionless at the center of the chamber in spite of the changes in the force balance it may experience. A feature of this technique is that the surroundings are at ambient temperature (300 K). The lack of external source (other than laser radiation) permits the steady determination of the heat release rates and the measure of heat capacities. Also, secondary reactions in the boundary layer such as CO oxidation are minimized enabling one to concentrate on the surface reactions. Ignition studies in the presence of controlled external heating can provide new insights into particulate ignition. High relative gas velocities surrounding the particle can be achieved thus simulating reactions under high flow fields. The present approach is however not without its drawbacks. Thermoionic emission can lead to problems of particle stability and the small sample size limits gas analysis to major products. The present review provides a discussion of the capabilities and limitations of the electrodynamic chamber for high temperature applications. The review discusses in detail the methodology of the EDC. Theoretical aspects of the important processes and phenomena in conjunction with the operation of the EDC are provided. The motivation of the detailed theoretical presentation is twofold: (1) To provide the user with tools necessary to estimate the operation conditions of the specific system. (2) To evaluate the force balance from phenomena taking place during the heterogeneous reaction not connected with the reaction and correct the interpretation of the results accordingly.

Pd/Au/SiC Nanostructured Diodes for Nanoelectronics: Room Temperature Electrical Properties

Pd/Au/SiC nanostructured Schottky diodes were fabricated embedding Au nanoparticles (NPs) at the metal-semiconductor interface of macroscopic Pd/SiC contacts. The Au NPs mean size was varied controlling the temperature and time of opportune annealing processes. The electrical characteristics of the nanostructured diodes were studied as a function of the NPs mean size. In particular, using the standard theory of thermoionic emission, we obtained the effective Schottky barrier height (SBH) and the effective ideality factor observing their dependence on the annealing time and temperature being the signature of their dependence on the mean NP size. Furthermore, plotting the effective SBH as a function of the effective ideality factor we observe a linear correlation, indicating that the Au NPs act as lateral inhomogeneities in the Schottky diodes according to the Tung's model. Therefore, we can control the size, fraction of covered area, and surface density of such intentionally introduced inhomogeneities. The application of the Tung's model for the electronic transport in inhomogeneous Schottky contacts allow us to obtain, in particular, the homogeneous SBH. These nanostructured diodes are proposed as possible components of integrated complex nanoelectronic devices.

Nanowire-metal heterostructures for high performance MOSFETs

In this work we report on the formation, of copper-germanide/germanium nanowire (NW) heterostructures with atomically sharp interfaces. The copper-germanide (Cu3Ge) formation process is enabled by a chemical reaction between metallic Cu pads and vapor-liquid-solid (VLS) grown Ge-NWs. The atomic scale aligned formation of the Cu3Ge segments is controlled by in situ SEM monitoring at 310 °C thereby enabling length control of the intrinsic Ge-NW down to a few nm. The single crystal Cu3Ge/Ge/Cu3Ge heterostructures were used to fabricate Ω-gated Ge-NW field effect transistors with Schottky Cu3Ge source/drain contacts. Temperature dependent I/V measurements revealed the metallic properties of the Cu3Ge contacts with a maximum current carrier density of 5 × 107 A/cm2. Prior to the gate deposition the intrinsic Ge-NW was modified with a focussed Ga+ ion beam. According to the thermoionic emission theory we determined an effective Schottky barrier height reduction from 218 meV to about 115 meV due to Ga+ implantation.

Optical, mechanical, and electrical studies of polymer composites based on charge transfer complex of phenothiazine–iodine with poly(vinyl chloride)

Charge transfer complex (CTC) of phenothiazine and iodine (1:2 molar ratio) was prepared by solvent evaporation method in diethyl ether and its composite with poly(vinyl chloride) was prepared in benzene by diffusion method. Infra-red spectra showed overlapped peaks for both components and intensity of individual component was proportional to feed ratio. Optical photographs and scanning electron microscopy of composites showed template growth and connectivity in insulator matrix was proportional to wt% of CTC. Mechanical strength of composites was found to increase with wt% of PVC. The current–voltage study showed percolation threshold of 8 wt% of CTC. The temperature dependence of conductivity showed semiconducting nature of the materials. Transport property of charges were explained by regression analysis of σ dc vs. T −1/1+ n data and meets the basis for the Mott's 2D, 3D variable range hopping or thermoionic emission model, depending on temperature and wt% of CTC content. The impedance spectroscopy was performed between 40 Hz–100 kHz range. Circuit elements consists only combination of resistance and capacitance, which showed homogeneous nature of composites. Thermoelectric factor ‘ S’ was also evaluated and has value of <1 at 303 K in all cases.

Atomic Scale Alignment of Copper-Germanide Contacts for Ge Nanowire Metal Oxide Field Effect Transistors

In this letter, we report on the formation, of copper-germanide/germanium nanowire (NW) heterostructures with atomically sharp interfaces. The copper-germanide (Cu3Ge) formation process is enabled by a chemical reaction between metallic Cu pads and vapor-liquid-solid (VLS) grown Ge-NWs. The atomic scale aligned formation of the Cu3Ge segments is controlled by in situ SEM monitoring at 310 degrees C thereby enabling length control of the intrinsic Ge-NW down to a few nanometers. The single crystal Cu3Ge/Ge/Cu3Ge heterostructures were used to fabricate p-type Ge-NW field effect transistors with Schottky Cu3Ge source/drain contacts. Temperature dependent I /V measurements revealed the metallic properties of the Cu3Ge contacts with a maximum current density of 5 x 10(7) A/cm2. According to the thermoionic emission theory, we determined an effective Schottky barrier height of 218 meV.

Electrical characterizations of SnPc/p-GaAs heterojunction

Current voltage and capacitance-voltage characteristics for SnPc thin film with ?105 nm thickness; deposited on p-GaAs single crystals have been investigated. The dark current voltage-characteristics of the prepared junction have been investigated in a temperature range from ∼303 to 393 K. The obtained results showed rectification behaviour. At low forward and reverse bias, the current was found to be limited by the thermoionic emission, while at high forward voltage, space charge limited current dominated by a single trap level of 0.22 eV. The analysis of the dark capacitance voltage characteristics indicated that the carrier concentration is 1.4 × 1014 cm -3 with a built in voltage ∼0.55 eV.© EDP Sciences.

The effects of the temperature on I-V and C-V characteristics of Al/P2ClAn(C2H5COOH)/p-Si/Al structure

The current-voltage ( I - V ) and capacitance-voltage ( C - V ) characteristics of metal-organic compound conductive polymer-semiconductor Al/P2ClAn(C 2 H 5 COOH)/ p -Si structures were measured in the temperature range 80–340 K. P2ClAn: the poly(2-chloroaniline). The P2ClAn emeraldine salt was synthesized chemically by using propionic (C 2 H 5 COOH) acid. The analysis of I - V characteristics based on the thermoionic emission (TE) mechanism has revealed a decrease of zero-bias barrier height and a increase of the ideality factor at lower temperatures. Temperature dependence of the barrier height (BH) has shown a disagreement with those calculated from C - V characteristics. This behaviour is attributed to Schottky barrier inhomogeneities at interface. The temperature coefficient of BH has been found to be -2.9×10 -4 eV/K for Al/P2ClAn(C 2 H 5 COOH)/ p -Si. ln( I 0 / $T^{2})$ versus 1/ T plot gives activation energy and Richardson constant $A^{\ast}$ as 0.12 eV and 3.82×10 -9 A/K 2 cm 2 , respectively.

Electrical characterization of p-ZnO/p-Si heterojunction

Nitrogen doped p-ZnO film, with urea as nitrogen source, is fabricated by pulsed laser deposition on well-cleaned p-type (1 0 0) Si substrates. The structural and electrical properties of the p–p heterojunction are investigated by current–voltage ( I– V) and capacitance–voltage ( C– V) measurements. It shows a diode-like behavior with turn-on voltage of 0.5 V. The ideality factor η determined by applying positive potential in p-ZnO and negative potential along p-Si is found to be 6. Such a high value of η is attributed to lattice mismatch between ZnO and Si. and other factors responsible are thermoionic emission, minority carrier injection and recombination. C– V results indicate an abrupt interface and a band bending of 0.9 V in the silicon. Heterojunction band diagram for p-ZnO/p-Si is proposed.

Formation and Electrical Properties of Metal/Organic Semiconductor/Si Heterostructures Based on Naphthalene Diimide-Based Compounds

We report on the synthesis of two naphthalene diimide-based organic derivatives: POANT containing electronically isolated 1,4,5,8-naphthalenetetracarboxylic diimide units and FCAND – a low-molar-mass compound with two different (fluorenone and 1,4,5,8-naphthalenetetracarboxylic diimide) electron-accepting units. Initial decomposition temperatures of 305°C and 390°C were indicated for FCAND and POANT, respectively. Organic semiconductor/Si heterostructures have been prepared by using spin coating (POANT, PEPK) and thermal evaporation in vacuum (FCAND). All heterojunctions demonstrated nonlinear I-U dependences and defined rectifying properties. The Schottky thermoionic emission model has been applied to explain the carrier transport and to estimate the barrier height and the ideality factor n of heterojunctions.

Rectifying effect of heterojunctions between metals and doped conducting polymer nanostructure pellets

This paper reports that the Schottky junctions between low work function metals (e.g. Al and In) and doped semiconducting polymer pellets (e.g. polyaniline (PANI) microsphere pellet and polypyrrole (PPy) nanotube pellet) have been prepared and studied. Since Ag is a high work function metal which can make an ohmic contact with polymer, silver paste was used to fabricate the electrodes. The Al/PANI/Ag heterojunction shows an obvious rectifying effect as shown in I – V characteristic curves (rectifying ratio γ = 5 at ±6 V bias at room temperature). As compared to the Al/PANI/Ag, the heterojunction between In and PANI (In/PANI/Ag) exhibits a lower rectifying ratio γ = 1.6 at ±2 V bias at room temperature. In addition, rectifying effect was also observed in the heterojunctions Al/PPy/Ag (γ = 3.2 at ±1.6 V bias) and In/PPy/Ag (γ = 1.2 at ±3.0 V bias). The results were discussed in terms of thermoionic emission theory.

Subpicosecond timescale carrier dynamics in GaInAsSb∕AlGaAsSb double quantum wells emitting at 2.3μm

We report the results of an extensive optical investigation by continuous-wave and time resolved photoluminescence experiments on double GaInAsSb∕AlGaAsSb quantum wells emitting at 2.3μm at room temperature. We have found that, at low temperature (&amp;lt;70K), the recombination is dominated by excitons trapped in disorder and interface defects. Whereas, at higher temperature, free-exciton recombination occurs. The observed temperature dependent photoluminescence quenching is ascribed to the ionization of bound exciton at low temperatures, while thermoionic emission of the hole out of the quantum well dominates photoluminescence quenching at high temperatures. The experimental results are supported by theoretical calculations.

Electrical Properties of Self‐Assembled Nano‐Schottky Diodes

A bottom‐up methodology to fabricate a nanostructured material by Au nanoclusters on 6H‐SiC surface is illustrated. Furthermore, a methodology to control its structural properties by thermal‐induced self‐organization of the Au nanoclusters is demonstrated. To this aim, the self‐organization kinetic mechanisms of Au nanoclusters on SiC surface were experimentally studied by scanning electron microscopy, atomic force microscopy, Rutherford backscattering spectrometry and theoretically modelled by a ripening process. The fabricated nanostructured materials were used to probe, by local conductive atomic force microscopy analyses, the electrical properties of nano‐Schottky contact Au nanocluster/SiC. Strong efforts were dedicated to correlate the structural and electrical characteristics: the main observation was the Schottky barrier height dependence of the nano‐Schottky contact on the cluster size. Such behavior was interpreted considering the physics of few electron quantum dots merged with the concepts of ballistic transport and thermoionic emission finding a satisfying agreement between the theoretical prediction and the experimental data. The fabricated Au nanocluster/SiC nanocontact is suggested as a prototype of nano‐Schottky diode integrable in complex nanoelectronic circuits.

Fabrication and effect of the dielectric permittivity on the ideality factor of MEH-PPV Schottky diodes doped with electron acceptor fluorescent dyes

Polymeric Schottky diodes have been prepared using poly[2-methyl-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) conducting polymers. The I–V characteristics of these diodes are interpreted in terms of thermoionic emission (TIE) conduction process. Upon blending the polymer with Nile blue (NB) dye the conduction shows a cross-over from space-charge limited conduction (SCLC) to TIE at biasing voltages larger than 4V. Disappearance of SCLC is seen in Rhodamine B (RhB)-based diodes and the conduction is interpreted in terms of the TIE process. Inclusion of the dielectric permittivity of the polymer/dye blends in interpreting the I–V characteristics of diodes is found to be crucial. A significant reduction of the ideality factor (n) from 6 to 1.8 is determined when MEH-PPV is mixed with NB or Rhodamine fluorescent dyes.

Characterization and modeling of n-n Si∕SiC heterojunction diodes

In this paper we investigate the physical and electrical properties of silicon layers grown by molecular beam epitaxy on 4H-SiC substrates, evaluating the effect of the Si doping, Si temperature deposition, and SiC surface cleaning procedure. Si∕SiC monolithic integration of Si circuits with SiC power devices can be considered as an attractive proposition and has the potential to be applied to a broad range of applications. X-ray diffraction and scanning electron microscopy are used to determine the Si crystal structure (cubic silicon) and morphology. I-V and C-V measurements are performed to evaluate the rectifying diode characteristics along with the Si∕SiC built-in potential and energy band offsets. In the last section, we propose that our Si∕SiC heteojunction diode current characteristics can be explained by an isojunction drift-diffusion and thermoionic emission model where the effect of doping concentration of the silicon layer and its conduction band offset with SiC is analyzed.

Electrical characterization of Pt∕AlGaN∕GaN Schottky diodes grown using AlN template and their application to hydrogen gas sensors

Pt Schottky diodes were fabricated on high-crystal-quality Al0.2Ga0.8N∕GaN epitaxial films grown using epitaxial AlN/sapphire template (AlN template), and their current transport characteristics were measured and analyzed. It was confirmed that reverse leakage currents in Schottky diodes formed on AlN template were drastically reduced compared with samples on sapphire formed with a low-temperature buffer layer. The current transport characteristics in Schottky diodes formed on AlN template were found to be almost explicable using the thermoionic emission and tunneling current components, unlike with samples formed on sapphire. This indicates that the amount of unintentional impurity levels near the surfaces is extremely small for those high-crystal-quality Schottky diodes. Hydrogen-sensing characteristics were also investigated for a Pt∕AlGaN∕GaN Schottky diode formed on AlN template. The diode sensor exhibited sufficient changes in the reverse current even under exposure to extremely slight H2 concentration of 50ppm and exhibited clear reversible responses. These current changes seemed to be due to the reduction of the Schottky barrier height with H2 exposure.

Fabrication and characteristics of bulk semi-insulating 6H-SiC semiconductor detector for an α radiation at room temperature

The fully depleted SiC Schottky barrier detector was fabricated from a semi-insulating (SI) bulk crystal with a thickness of 388 μm and an orientation of the on axis (0 0 0 1). The bulk SI 6H-SiC detector structure consists of a Ni/Au contact at the Si-face and a Ti/Au contact at the C-face. The fabricated detector dimensions are 10×10 mm with a diameter of 6 mm for the contact dimension. The band gap of the 6H-SiC was determined by using an optical photon absorption spectroscopy in the ranges between 200 and 600 nm. The band gap of 6H-SiC is determined as 3.03 eV. The current density response according to the biased voltage was measured. The Schottky barrier height was determined as 0.86 eV on the basis of the thermo-ionic emission theory. The bulk SI 6H-SiC detector with a multi-layer structure, Au/Ni/6H-SiC/Ti/Au showed a good response for the α radiation in air and at room temperature.