Thermionic Emission Model Research Articles

Detection of alpha particles using DNA/Al Schottky junctions

Deoxyribonucleic acid or DNA can be utilized in an organic-metallic rectifying structure to detect radiation, especially alpha particles. This has become much more important in recent years due to crucial environmental detection needs in both peace and war. In this work, we fabricated an aluminum (Al)/DNA/Al structure and generated current–voltage characteristics upon exposure to alpha radiation. Two models were utilized to investigate these current profiles; the standard conventional thermionic emission model and Cheung and Cheung's method. Using these models, the barrier height, Richardson constant, ideality factor and series resistance of the metal-DNA-metal structure were analyzed in real time. The barrier height, Φ value calculated using the conventional method for non-radiated structure was 0.7149 eV, increasing to 0.7367 eV after 4 min of radiation. Barrier height values were observed to increase after 20, 30 and 40 min of radiation, except for 6, 8, and 10 min, which registered a decrease of about 0.67 eV. This was in comparison using Cheung and Cheung's method, which registered 0.6983 eV and 0.7528 eV for the non-radiated and 2 min of radiation, respectively. The barrier height values, meanwhile, were observed to decrease after 4 (0.61 eV) to 40 min (0.6945 eV). The study shows that conventional thermionic emission model could be practically utilized for estimating the diode parameters including the effect of series resistance. These changes in the electronic properties of the Al/DNA/Al junctions could therefore be utilized in the manufacture of sensitive alpha particle sensors.

Effect of Nano Al Interlayer on the Schottky Contact Property Between Metal and Hg3In2Te6 Wafer.

In this paper, the fabrication and characterization of Schottky diodes based on Hg3In2Te6 (short for MIT) wafer are presented. The Al interlayer with 1 nm thickness was grown on the MIT wafer by Vacuum thermal evaporation method. Diodes characterization of metal/Al/MIT contact were performed by I-V measurement. Then the Schottky barrier height and ideality factor were calculated from I-V test results by means of the thermionic emission model. The results showed that the Schottky barrier heights of Ni/Hg3In2Te6, Au/Hg3In2Te6 were reduced from 0.86, 0.85 eV to 0.74, 0.81 eV respectively, and ideality factors decreased from 1.88 and 1.89 to 1.67 and 1.58 accordingly when the Al thin film about 1 nm in thickness was used for the interlayer of the MIT Schottky diode. It was found that the Al interlayer can significantly alleviates the Fermi-level pinning, and the interface defect mechanisms for the depinning were discussed based on interface states model and the metal-induced gap states model.

Rectification and electroluminescence of nanostructured GaN/Si heterojunction based on silicon nanoporous pillar array**Project supported by the National Natural Science Foundation of China (Grant No. 61176044).

A GaN/Si nanoheterojunction is prepared through growing GaN nanocrystallites (nc-GaN) on a silicon nanoporous pillar array (Si-NPA) by a chemical vapor deposition (CVD) technique at a relatively low temperature. The average size of nc-GaN is determined to be ∼10 nm. The spectral measurements disclose that the photoluminescence (PL) from GaN/Si-NPA is composed of an ultraviolet (UV) band and a broad band spanned from UV to red region, with the feature that the latter band is similar to that of electroluminescence (EL). The electron transition from the energy levels of conduction band and, or, shallow donors to that of deep acceptors of GaN is indicated to be responsible for both the broad-band PL and the EL luminescence. A study of the I–V characteristic shows that at a low forward bias, the current across the heterojunction is contact-limited while at a high forward bias it is bulk-limited, which follows the thermionic emission model and space-charge-limited current (SCLC) model, respectively. The bandgap offset analysis indicates that the carrier transport is dominated by electron injection from n-GaN into the p-Si-NPA, and the EL starts to appear only when holes begin to be injected from Si-NPA into GaN with biases higher than a threshold voltage.

Influence of electronic transmission on the electrical transport properties in metal-semiconductor contacts

A Pt/Si Schottky diode was prepared by depositing thin-film Pt on Si (100) via RF magnetron sputtering. The I–V characteristics of the nano-Schottky structure were measured by conductive-AFM (CAFM) and probe-based measurement, where nano- and micro-size tip was used, respectively. Result from both measurements showed the rectifying behaviors, whereas the leakage current detected by CAFM is about six orders of magnitude larger than that detected by the micromeasurement. It was proposed that this electronic transport discrepancy is mainly ascribed to the different electronic transmission. In order to reveal the characteristics of the Schottky junction, a thermionic emission (TE) model and thermionic-field emission (TFE) model were applied for the I–V data fitting. It was proved that the result can be well fitted by the TE model, with an ideality factor (IF) n = 1–2 and barrier height (BH) 0.75–0.85 eV, indicating this Schottky diode approximates to an ideal Schottky diode (n = 1).

Transport properties of solution processed Cu2SnS3/AZnO heterostructure for low cost photovoltaics

Cu2SnS3 thin films were deposited by a facile sol–gel technique followed by annealing. The annealed films were structurally characterized by grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). The crystal structure was found to be tetragonal with crystallite sizes of 2.4–3nm. Texture coefficient calculations from the GIXRD revealed the preferential orientation of the film along the (112) plane. The morphological investigations of the films were carried out using field emission scanning electron microscopy (FESEM) and the composition using electron dispersive spectroscopy (EDS). The temperature dependent current, voltage characteristics of the Cu2SnS3/AZnO heterostructure were studied. The logI–logV plot exhibited three regions of different slopes showing linear ohmic behavior and non-linear behavior following the power law. The temperature dependent current voltage characteristics revealed the variation in ideality factor and barrier height with temperature. The Richardson constant was calculated and its deviation from the theoretical value revealed the inhomogeneity of the barrier heights. Transport characteristics were modeled using the thermionic emission model. The Gaussian distribution of barrier heights was applied and from the modified Richardson plot the value of the Richardson constant was found to be 47.18Acm−2K−2.

Fabrication and characterization of Au/p-ZnO Schottky contacts

This paper reports the electrical characteristics of gold contacts on p-type ZnO thin films synthesized by spin coating the sol containing zinc acetate and bismuth nitrate as main precursors. The structural, morphological and optical properties of the deposited thin film have been investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and Ellipsometry, respectively. Further, hot probe measurement has been used to ascertain the type of deposited film and it was observed that films doped with the concentration of 10mol% Bi shows p-type nature that was found to be stable over the period of five months. Moreover, reflectance of the Bi doped ZnO with varying Bi concentrations, have also been calculated over the wavelength range of 300–800nm. The optical band gap of Bi doped ZnO films have also been determined for different concentrations of Bi using the data taken by ellipsometer. The gold (Au) contacts have been deposited on the p-ZnO thin films using low cost thermal evaporation method. Electrical parameters such as the reverse saturation current, barrier height and ideality factor have also been determined for Au/p-ZnO thin film based Schottky contact using conventional thermionic emission model and Cheung’s method. The conventional thermionic emission model yields barrier height ∼0.681eV and ideality factor ∼2.3 however Cheung method gives barrier height ∼0.556eV, ideality factor ∼2.186 and series resistance ∼923Ω. The present study establishes the fact that Cheung’s method can be the best and most realistic method for approximating the diode parameters including the effect of series resistance of the Au/p-ZnO Schottky diode under consideration.

Leakage current transport mechanisms of La0.67Sr0.33MnO3/BaTiO3 bilayer films grown on Nb:SrTiO3

La0.67Sr0.33MnO3/BaTiO3(LSMO/BTO) bilayer films were epitaxially grown on Nb:SrTiO3 (NSTO) substrates by the pulsed laser deposition technique. Current–voltage (I–V) characteristics of the LSMO/BTO bilayer films were studied. I–V curves were measured at room temperature, which show rectifying behaviour and can be well fitted by the space-charge-limited current mechanism under forward bias while thermionic emission model under reverse bias. Analysis indicates that a modulating Schottky barrier exists at the LSMO/BTO interface, which dominates the leakage current transport properties of LSMO/BTO bilayer films.

Barrier inhomogeneities and electronic transport of Pt contacts to relatively highly doped n-type 4H-SiC

The barrier characteristics of Pt contacts to relatively highly doped (∼1 × 1018 cm−3) 4H-SiC were investigated using current-voltage (I-V) and capacitance-voltage (C-V) measurements in the temperature range of 160–573 K. The barrier height and ideally factor estimated from the I-V characteristics based on the thermionic emission model are abnormally temperature-dependent, which can be explained by assuming the presence of a double Gaussian distribution (GD) of inhomogeneous barrier heights. However, in the low temperature region (160–323 K), the obtained mean barrier height according to GD is lower than the actual mean value from C-V measurement. The values of barrier height determined from the thermionic field emission model are well consistent with those from the C-V measurements, which suggest that the current transport process could be modified by electron tunneling at low temperatures.

Fabrication and conduction mechanism evaluation of polyfluorene polymeric Schottky diode

A thin film of polyflourene polymer was sandwiched between a conductive polymer deposited on silver nanowire and metal electrode to form a multilayer polymer‐based diode device. The electrical properties of fabricated polymeric diode have been studied by current–voltage method. The current–voltage characteristics of the fabricated device exhibited non‐ideal, asymmetrical, and rectifying behavior. Ohmic current conduction mechanism was observed in the device at low voltage. At higher voltage values, the space‐charge‐limited current conduction mechanism was found to be dominated. The values of the Schottky barrier height, ideality factor, and saturation current density were extracted according to the standard thermionic emission model and discussed. The barrier height and ideality factor were calculated as 0.72 eV and 2.53, respectively. Copyright © 2015 John Wiley & Sons, Ltd.

Enhancement in electrical properties of ITO/PEDOT:PSS/PTCDA/Ag by using calcium buffer layer

This paper reports on electrical characterization of ITO/PEDOT:PSS/PTCDA/Ca/Ag device based on 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and calcium (Ca) buffer layer with improved junction properties. The I–V characteristics have been utilized to extract various electrical parameters such as ideality factor (n), barrier height (ϕB) and series resistance Rs, which are found to be 1.9, 0.79eV and 2.5kΩ, respectively. The device shows good rectifying behavior, with a rectification ratio of 528, and also field-lowering mechanism with a linear dependence of log I on V1/2. The device reported in the present work shows 50% improvement in the rectification ratio and ideality factor as compared to our previously fabricated device. It appears from the experimental data that the transport mechanism in the PTCDA thin film is dominated by the Poole–Frenkel model of thermionic emission, which may be associated with high density of structural defects or traps present in the film.

Barrier height inhomogeneity in electrical transport characteristics of InGaN/GaN heterostructure interfaces

We have grown InGaN/GaN heterostructures using plasma-assisted molecular beam epitaxy and studied the temperature dependent electrical transport characteristics. The barrier height (φb) and the ideally factor (η) estimated using thermionic emission model were found to be temperature dependent. The conventional Richardson plot of ln(Js/T2) versus 1/kT showed two temperature regions (region-I: 400–500 K and region-II: 200–350 K) and it provides Richardson constants (A∗) which are much lower than the theoretical value of GaN. The observed variation in the barrier height and the presence of two temperature regions were attributed to spatial barrier inhomogeneities at the heterojunction interface and was explained by assuming a double Gaussian distribution of barrier heights with mean barrier height values 1.61 and 1.21 eV with standard deviation (σs2) of 0.044 and 0.022 V, respectively. The modified Richardson plot of ln(Js/T2) − (q2σs2/2k2T2) versus 1/kT for two temperature regions gave mean barrier height values as 1.61 eV and 1.22 eV with Richardson constants (A∗) values 25.5 Acm−2K−2 and 43.9 Acm−2K−2, respectively, which are very close to the theoretical value. The observed barrier height inhomogeneities were interpreted on the basis of the existence of a double Gaussian distribution of barrier heights at the interface.

A hybrid model describing ion induced kinetic electron emission

We present a model to describe the kinetic internal and external electron emission from an ion bombarded metal target. The model is based upon a molecular dynamics treatment of the nuclear degree of freedom, the electronic system is assumed as a quasi-free electron gas characterized by its Fermi energy, electron temperature and a characteristic attenuation length.In a series of previous works we have employed this model, which includes the local kinetic excitation as well as the rapid spread of the generated excitation energy, in order to calculate internal and external electron emission yields within the framework of a Richardson–Dushman-like thermionic emission model. However, this kind of treatment turned out to fail in the realistic prediction of experimentally measured internal electron yields mainly due to the restriction of the treatment of electronic transport to a diffusive manner. Here, we propose a slightly modified approach additionally incorporating the contribution of hot electrons which are generated in the bulk material and undergo ballistic transport towards the emitting interface.

Study of triisopropylsilyl pentacene thin film and its interfacial properties for device applications

Two-hundred-nanometer-thick 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS-Pentacene) films were formed on Si substrates by spin coating. The rms roughness of the spun dendrite-type films is ~40nm, determined by atomic force microscopy. Ordered crystalline structures were revealed by x-ray diffraction measurement, and the dominated absorption band from the ordered structures was further confirmed by ultraviolet-visible spectroscopy. The current–voltage characteristics of the junctions formed using TIPS-Pentacene on n-type Si substrates show good rectifying behavior with rectification ratios over 100 at 1V. In the heterojunctions, barrier heights of ~0.8eV and ideality factors of ~2 were determined based on thermionic emission model. It shows that TIPS-Pentacene can work well with n-type Si to form Schottky-type rectifying devices. Capacitance–voltage measurement performed on the metal–insulator–semiconductor structure of TIPS-Pentacene on SiO2 shows obvious effects of accumulation and depletion in TIPS-Pentacene with bias. The maximum width of the depletion layer in TIPS-Pentacene is estimated to be 9.4nm.

Organic/inorganic heterojunction based on conducting polymer and pulse-laser-deposited (PLD) ZnO

ABSTRACTOrganic/inorganic heterojunctions have been fabricated by spin coating p-type poly (3, 4 ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) onto n-type zinc oxide (ZnO) films. The ZnO films were deposited onto indium tin oxide (ITO) coated glass by pulse laser deposition (PLD) technique. The current density-voltage (J-V) characteristics of the PLD-ZnO/PEDOT:PSS junction based on as-deposited PLD-ZnO film shows a good rectifying behavior with a rectification ratio of 156 at ±1 V, indicating the formation of a diode between ZnO and PEDOT:PSS. Using thermionic emission model, the ideality factor (n=2.1) and barrier height (0.66 eV) of the heterojunction were obtained. Those diode parameters are better than those of the chemical vapor deposited ZnO/PEDOT:PSS heterojunction reported elsewhere, indicating that PLD may be a promising technique on fabricating high quality ZnO/polymer heterojunctions.

Double Gaussian distribution of barrier height observed in densely packed GaN nanorods over Si (111) heterostructures

GaN nanorods were grown by plasma assisted molecular beam epitaxy on intrinsic Si (111) substrates which were characterized by powder X-ray diffraction, field emission scanning electron microscopy, and photoluminescence. The current–voltage characteristics of the GaN nanorods on Si (111) heterojunction were obtained from 138 to 493 K which showed the inverted rectification behavior. The I-V characteristics were analyzed in terms of thermionic emission model. The temperature variation of the apparent barrier height and ideality factor along with the non-linearity of the activation energy plot indicated the presence of lateral inhomogeneities in the barrier height. The observed two temperature regimes in Richardson's plot could be well explained by assuming two separate Gaussian distribution of the barrier heights.

Graphene-GaN Schottky diodes

The electrical characteristics of graphene Schottky contacts formed on undoped GaN semiconductors were investigated. Excellent rectifying behavior with a rectification ratio of ∼107 at ±2 V and a low reverse leakage current of 1.0 × 10−8 A/cm2 at −5 V were observed. The Schottky barrier heights, as determined by the thermionic emission model, Richardson plots, and barrier inhomogeneity model, were 0.90, 0.72, and 1.24 ± 0.13 eV, respectively. Despite the predicted low barrier height of ∼0.4 eV at the graphene-GaN interface, the formation of excellent rectifying characteristics with much larger barrier heights is attributed to the presence of a large number of surface states (1.2 × 1013 states/cm2/eV) and the internal spontaneous polarization field of GaN, resulted in a significant upward surface band bending or a bare surface barrier height as high as of 2.9 eV. Using the S parameter of 0.48 (measured from the work function dependence of Schottky barrier height) and the mean barrier height of 1.24 eV, the work function of graphene in the Au/graphene/GaN stack could be approximately estimated to be as low as 3.5 eV. The obtained results indicate that graphene is a promising candidate for use as a Schottky rectifier in GaN semiconductors with n-type conductivity.

Photo-electrical properties of single Cu-TCNQ nanowire

In this letter, photo-electrical transport properties of individual Cu-TCNQ nanowire are studied. The electronic transport mechanism for single Cu-TCNQ nanowire well follows one-dimensional (1D) Mott’s model in the temperature range from 80 to 300 K. The Cu-TCNQ nanowire shows photoconductivity at visible illumination, with much faster response time than recovery time. The thermionic emission (TE) and thermionic-field emission (TFE) models are employed to interpret the current versus voltage (I–V) plots, by considering fitted results for the reversely biased contact barriers. Our present work opens up potential applications of single Cu-TCNQ nanowire in organic photo-electric nanodevice.

Extraction of the Schottky parameters in metal-semiconductor-metal diodes from a single current-voltage measurement

In order to develop a method to extract the parameters of the two inherent Schottky contacts from a single current-voltage (I-V) characteristic curve, the I-V characteristics of metal-semiconductor-metal (MSM) diodes with asymmetric Schottky barrier heights are theoretically investigated using the thermionic emission model. The MSM diode structure is commonly used because an additional MS interface is required for the electrical characterization of MS diodes. A finite charge-injection barrier is generally formed at the additional interface. When a local maximum was detected in the first-order derivative of the measured I-V characteristics for a MSM diode, the parameters for the Schottky contacts, the zero-bias barrier heights of both MS interfaces, the series resistance of the MSM diode, and the effective ideality factor for the MS diode with a higher barrier could be extracted using this method.

Investigation of effective base transit time and current gain modulation of light-emitting transistors under different ambient temperatures

In this report, the modulation of current gain of InGaP/GaAs light-emitting transistors under different ambient temperatures are measured and analyzed using thermionic emission model of quantum well embedded in the transistor base region. Minority carriers captured by quantum wells gain more energy at high temperatures and escape from quantum wells resulting in an increase of current gain and lower optical output, resulting in different I-V characteristics from conventional heterojunction bipolar transistors. The effect of the smaller thermionic lifetime thus reduces the effective base transit time of transistors at high temperatures. The unique current gain enhancement of 27.61% is achieved when operation temperature increase from 28 to 85 °C.

Experimental and numerical analysis of channel-length-dependent electrical properties in bottom-gate, bottom-contact organic thin-film transistors with Schottky contact

Channel length dependence of field-effect mobility and source/drain parasitic resistance in pentacene thin-film transistors with a bottom-gate, bottom-contact configuration was investigated. Schottky barrier effect such as nonlinear behaviors in transistor output characteristics appeared and became more prominent for shorter channel length less than 10μm, raising some concerns for a simple utilization of conventional parameter extraction methods. Therefore the gate-voltage-dependent hole mobility and the source/drain parasitic resistance in the pentacene transistors were evaluated with the aid of device simulation accounting for Schottky contact with a thermionic field emission model. The hole mobility in the channel region shows smaller values with shorter channel length even after removing the influence of Schottky barrier, suggesting that some disordered semiconductor layers with low carrier mobility exist near the contact electrode. This experimental data analysis with the simulation enables us to discuss and understand in detail the operation mechanism of bottom-gate, bottom-contact transistors by considering properly each process of charge carrier injection, carrier flow near the contact region, and actual channel transport.