Thermionic Emission Mechanism Research Articles

Low work function thin film growth for high efficiency thermionic energy converter: Coupled Kelvin probe and photoemission study of potassium oxide

Recent researches in thermal energy harvesting have revealed the remarkable efficiency of thermionic energy converters comprising very low work function electrodes. From room temperature and above, this kind of converter could supply low power devices such as autonomous sensor networks. In this type of thermoelectric converters, current injection is mainly governed by a mechanism of thermionic emission at the hot electrode which explains the interest for low work function coating materials. In particular, alkali metal oxides have been identified as excellent candidates for coating converter electrodes. This paper is devoted to the synthesis and characterization of potassium peroxide K2O2 onto silicon surfaces. To determine optimal synthesis conditions of K2O2, we present diagrams showing the different oxides as a function of temperature and oxygen pressure from which phase stability characteristics can be determined. From the experimental standpoint, we present results on the synthesis of potassium oxide under ultra high vacuum and controlled temperature. The resulting surface is characterized in situ by means of photoemission spectroscopy (PES) and contact potential difference (CPD) measurements. A work function of 1.35 eV is measured which and the expected efficiency of the corresponding converter is discussed. It is generally assumed that the decrease of the work function in the alkali/oxygen/silicon system, is attributed to the creation of a surface dipole resulting from a charge transfer between the alkali metal and oxygen.

Investigation of temperature dependent electrical properties of Ni/Al0.26Ga0.74N Schottky barrier diodes

The current–voltage (I–V) characteristics of the Ni/Al0.26Ga0.74N Schottky barrier diodes (SBDs) were measured in the temperature range of 100–310K by the step of 10K. The forward I–V characteristics were analyzed on the basis of the thermionic emission theory. The characteristics of diode parameters such as the Schottky barrier height (SBH) and the ideality factor were investigated as a function of temperature. An experimental SBH value of about 1.021eV was obtained for the Ni/Al0.26Ga0.74N SBD at 300K. The experimental results show that the values of the ideality factor decrease while the values of the SBH increase with increasing temperature. The temperature dependence of the SBH was explained on the basis of a thermionic emission mechanism with the Gaussian distribution of the SBHs due to the SBH inhomogeneities at the metal–semiconductor interface. The values of the mean barrier height Φ¯b0 and the standard deviation σs0 were 1.362eV and 133meV in the temperature range of 210–300K, 1.204eV and 111meV in the temperature range of 100–210K, respectively. The modified Richardson plots according to inhomogeneity of the SBHs have a good linearity in the corresponding temperature range. The values of Richardson constant A* were found to be 31.46Acm−2K−2 and 33.36Acm−2K−2 in the temperature ranges of 210–310K and 100–210K, respectively. The obtained Richardson constant values are in good agreement with the theoretical value of 34.56 Acm−2K−2 known for n-type Al0.26Ga0.74N.

On the electrical characteristics of Au/n-type GaAs Schottky diode

The temperature dependence of the electrical properties of Au/n-type GaAs Schottky contacts have been studied using current–voltage (I–V) and capacitance–voltage (C–V) over a wide temperature range 100–300K. In the low temperature range 100–140K, the absence of temperature dependent tunneling parameters has been explained in terms of thermionic field emission. In the high temperature range 140–300K, the zero-bias barrier height (Φ0bn) was found to decrease and the ideality factor (n) to increase with decreasing temperature. This abnormal temperature dependence of Φ0bn and n is interpreted on the basis of a thermionic emission mechanism by considering the existence of the barrier height inhomogeneities (BHi) at the metal/GaAs interface. From the linear plot of the experimental Schottky barrier height (SBH) vs. 1/T based on the BHi model, the value of the homogeneous SBH (Φ‾0bn) of 1.03eV and a zero-bias standard deviation (σ0s) of 89meV were computed. Furthermore the modified Richardson plot according to the Gaussian distribution model resulted in a homogeneous SBH (Φ‾0bn) of 1.02eV and a Richardson constant (A*) of 7.97A/cm2K2, respectively. The value of A* obtained from this plot is in very close agreement with the theoretical reported value of 8.16A/cm2K2 for n-type GaAs.

Electrical Conduction Mechanisms in n-CdS/p-CuFeO<sub>2</sub> Heterojunction Diode

In this work, n-CdS/p-CuFeO2 heterojunction diode was fabricated by thermal evaporating CdS thin films on 1 mm thick-CuFeO2 ceramic substrate with substrate temperature kept at 373 K during evaporation process. The forward current-voltage characteristics of n-CdS/p-CuFeO2 heterojunction in a temperature range of 100-300 K were investigated to determine the electrical parameters and conduction mechanism. It was found that, at forward bias below 0.5 V, the conduction mechanism of the diode is dominated by thermionic emission (TE) mechanism. At bias voltage above 0.5 V, the current transport is due to space charge limited current (SCLC) controlled by an exponential trap distribution in the band gap of CdS. The temperature dependence of the saturation current and ideality factor are well described by tunneling enhanced recombination at junction interface with activation and characteristic tunneling energy values as about 1.79 eV and E00 = 86 meV, respectively. The value of interface state density (Nss) evaluated from capacitance spectroscopy increases from 2.09x1011 eV-1cm-2 (at 300 K) to 2.70x1011 eV-1cm-2 (at 363 K). Free carrier concentration of 5.80x1013 cm-3 at room temperature was estimated from capacitance-voltage measurements at 50 kHz.

Enhancement of memory margins for stable organic bistable devices based on graphene-oxide layers due to embedded CuInS2 quantum dots

Current–voltage (I–V) curves for Al/CuInS2 (CIS) quantum dots (QDs) embedded in graphene-oxide layer/indium-tin-oxide devices at 300K showed a current bistability with a maximum high conductivity (ON)/low conductivity (OFF) ratio of 1×104, which was 100 times larger than the ON/OFF ratio of the device without CIS QDs. I–V curves and write-read-erase-read voltage cycles demonstrated the rewritable nonvolatile memory properties of the organic bistable devices (OBDs) with ON and OFF current states at the same voltage. The retention time was above 1×105s, indicative of the memory stability of the OBDs. I–V curve at lower voltages up to 0.05V was attributed to the thermionic emission mechanism, and the curve in the applied voltage range from 0.06 to 0.17V was related to an ohmic mechanism. The I–V characteristics in the applied voltage above 0.18V dominantly followed the space-charge-limited-current behaviors.

Investigations of the electrical conduction mechanisms of polyaniline‐DBSA/poly(acrylonitrile‐butadiene styrene) blends

ABSTRACTThe electrical properties of Al/PANI‐DBSA/ABS/Au blend with PANI (5%) w/w have been investigated by using of current‐voltage (I‐V) measurements, in a temperature range of 100–313 K. The analysis of I‐V characteristics in the forward direction was based on thermionic emission mechanism for applied electrical field till ∼3 × 102 V/cm. The thickness dependence of the current‐voltage relationship, clearly demonstrates that the electrical current for larger fields is space charge limited current (SCLC). Temperature dependences of the ideality factor, barrier height, and series resistance have been calculated. The mobility of carriers which is temperature dependent was calculated using the trap free SCLC as 1.53 × 10−4 cm2 V−1 s−1 at room temperature. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40688.

Comparative Study of the Current Transport Mechanisms in Ni<sub>2</sub>Si Ohmic Contacts on n- and p-Type Implanted 4H-SiC

The knowledge of the temperature behavior of Ohmic contacts is an important issue to understand the device operation. This work reports an electrical characterization as a function of the temperature carried out on nickel silicide (Ni2Si) Ohmic contacts, used both for n-type and p-type implanted 4H-SiC layers. The temperature dependence of the specific contact resistance suggested that a thermionic field emission mechanism dominates the current transport for contacts on p-type material, whereas a current transport by tunneling is likely occurring in the contacts on n-type implanted SiC. Furthermore, from the temperature dependence of the electrical characteristics, the activation energies for Al and P dopants were determined, resulting of 145 meV and 35 meV, respectively. The thermal stability of the electrical parameters has been demonstrated upon a long-term (up to ~100 hours) cycling in the temperature range 200-400°C.

Electron/transport in (Mo/Au)/AlGaN/GaN Schottky diode

The knowledge of the conduction mechanisms in a Schottky barrier is essential to calculate the Schottky barrier parameters and to explain the observed effects. In the present work, we report temperature- dependent current-voltage characteristics of (Mo/Au)/Al0.26Ga0.74N/GaN/Si/ Schottky barrier diodes. Measurements were performed in the temperature range of 80 -300 K.Results have been explained based on the thermionic emission mechanism with lateral inhomogeneity at the (Mo/Au/AlGaN/GaN/Si) interface. As is shown, the barrier height ΦB0 as well as the ideality factor n exhibit an important temperature dependence and the anomaly resulting from this dependence has been explained by invoking two sets of Gaussian distributions at the metal/semiconductor interface for temperature ranging from 80 K to 160 K and from 160K to300 K, respectively. It is also found that the values of Rs obtained from Cheung's method strongly depend on temperature and decrease with decreasing temperature. Keywords: Schottky barrier, Current-voltage characteristics, Ideality factor, Inhomogeneity, Thermionic emission.

Electrical properties of Ir/n-InGaN/Ti/Al Schottky barrier diode in a wide temperature range

The temperature dependent current-voltage (I-V) and capacitance-voltage (C-V) characteristics of Ir/n-InGaN Schottky contacts have been investigated and analysed in the temperature range of 100-400 K. The estimated barrier heights and ideality factor of Ir/n-InGaN Schottky diode are 0.30 eV (I-V), 1.15 eV (C-V) and 3.05 at 100 K, and 0.94 eV (I-V), 0.97 eV (C-V) and 1.20 at 400 K respectively. The barrier height (Φb), ideality factor (n) and series resistance (RS) of Ir/n-InGaN Schottky diode are also evaluated using Cheung’s and Norde methods. Results show that the barrier heights (I-V) increase while ideality factor and series resistance decease with increasing temperature. Further, the discrepancy between Schottky barrier heights estimated from I-V and C-V measurements is also explained. It is observed that the interface state density Nss decreases with an increasing temperature. Experimental results showed that the conduction current is dominated by Poole-Frenkel emission in the temperature range from 100 K to 340 K and by Schottky emission above 340 K. The dominate conduction mechanism changed from Poole-Frenkel to Schottky emission in the temperature range from 340 K to 370 K. Finally, it is concluded that the temperature-dependent I–V characteristics of the Ir/n-InGaN Schottky diode can be successfully explained on the basis of thermionic emission (TE) mechanism.

Correlation between microstructure and temperature dependent electrical behavior of annealed Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures

This letter reports on the temperature behavior of the structural and electrical properties of Ti/Al/Ni/Au contacts to AlGaN/GaN heterostructures. While Ohmic contacts formed at 750 °C showed a decreasing temperature behavior of the specific contact resistance ρC, which was explained by a thermionic field emission mechanism, an increasing trend is observed in the contacts formed at 850 °C. In this case, ρC exhibits a “metal-like” behavior, i.e., describable by a T1.8 dependence. The microstructural analysis of the interfacial region allowed to explain the results with the formation of metallic intrusions contacting directly the two dimensional electron gas.

AlGaN/GaN Schottky Diode Fabricated by Au Free Process

An AlGaN/GaN Schottky diode is fabricated using an Au free process. Both ohmic and Schottky contact are studied. The ohmic contact results show low contact resistance and good surface morphology. The specific resistance (ρc) and contact resistance (Rc) of the ohmic contact are ~ 3.5×10-5 Ω×cm2 and 2.1 Ω·mm, respectively. Several current transport mechanisms are employed to analyze the measured I- V curves of the diode. The data have demonstrated that at a lower forward bias the combination of generation recombination and tunneling current mechanisms are dominant, whereas the thermionic emission mechanism becomes more significant at a higher forward bias. The effective Schottky barrier height is estimated to be 0.99 eV. The breakdown voltage of AlGaN/GaN Schottky diode is ~ 125 V.

On the electrical behavior of V2O5/4H-SiC Schottky diodes

A complete analysis of the rectifying behavior of V2O5/4H-SiC (divanadium pentoxide/4H polytype of silicon carbide) junction is reported. The analysis of forward and reverse JD−VD curves of samples fabricated with 5 nm-thick V2O5 films shows that the carrier transport across junction is dominated by the field enhanced thermionic emission mechanism. All the physical and electrical parameters, such as Schottky barrier height, ideality factor, and series resistance, have been evaluated from temperature behavior of JD−VD curves in the range 100–425 K and from CD−VD measurements. It is shown that the barrier height extracted from measurements can be justified in terms of inhomogeneities localized at the interface.

The interface states and series resistance analyzing of Au/SiO2/n-GaAs at high temperatures

Abstract The current–voltage ( I – V ) characteristics of Au/SiO 2 /n-GaAs metal-oxide-semiconductor (MOS) type Schottky barrier diodes (SBDs) have been measured in the temperature range of 300–400 K with 25 K steps. From the I – V characteristics of SBDs, the zero-bias barrier height ϕ Bo and ideality factor ( n ) assuming the thermionic emission (TE) mechanism show strong temperature dependence. While n decreases, ϕ Bo increases with increasing temperature. The obtained values of ϕ Bo and n varied from 0.81 eV and 1.33 at 300 K and 0.93 eV and 1.12 at 400 K, respectively. In addition, the interface states distribution profile ( N ss ) as a function of temperature was extracted from the forward-bias I – V measurements by taking into account the bias dependence of the effective barrier height ϕ e and series resistance ( R s ) for the SBDs. The values of R s were performed using the Cheung’s method. Thus, important electrical parameters as a function of temperature were analyzed by using the I – V measurements.

Electrical properties of Au/perylene-monoimide/p-Si Schottky diode

Abstract In this work, we have fabricated an Au/perylene-monoimide (PMI)/p-Si Schottky barrier diode. We have investigated how electrical and interface characteristics like current–voltage characteristics (I–V), ideality factor (n), barrier height (ΦB) and series resistance (Rs) of diode change with temperature over a wide range of 100–300 K. Detailed analysis on the electrical properties of structure is performed by assuming the standard thermionic emission (TE) model. Possible mechanisms such as image force lowering, generation–recombination processes and interface states which cause deviations of n values from the unity have been discussed. Cheung–Cheung method is also employed to analysis the current–voltage characteristics and a good agreement is observed between the results. It is shown that the electronic properties of Schottky diode are very sensitive to the modification of perylene-monoimide (PMI) interlayer organic material and also to the temperature. The ideality factor was found to decrease and the barrier height to increase with increasing temperature. The temperature dependence of barrier height shows that the Schottky barrier height is inhomogeneous in nature at the interface. Such inhomogeneous behavior was explained on the basis of thermionic emission mechanism by assuming the existence of a Gaussian distribution of barrier heights.

Modelling of infrared optical constants for polycrystalline low pressure chemical vapour deposition ZnO:B films

Doped zinc oxide films are of high interest in thin film solar cell technology for application as transparent conducting oxide. Rapid and detailed characterisation of ZnO thin film properties is required for quality control and optimisation of the deposited films. In the present work, a new model of dielectric functions based on the effective medium approximation (EMA) is developed and is applied for characterisation of polycrystalline boron doped zinc oxide (ZnO:B) films, deposited by low pressure chemical vapour deposition (LPCVD) technique onto glass substrates. The model takes into account that polycrystalline ZnO is considered to consist of crystal grains surrounded by depletion layers. Using this model and Fourier Transform Infrared Spectroscopy (FTIR) performed in reflection configuration over a wide mid-infrared spectral region (from 2 μm up to 25 μm), the properties of depletion layer and the bulk of the grains in ZnO can be rapidly characterised in detail, and the volume fraction of the depletion layer can be extracted. The results are in good agreement with previously presented theories of electron transport in polycrystalline materials. Using electrical measurements like conductivity and Hall techniques in addition to the optically determined parameters, predominant electron scattering mechanisms in polycrystalline films for different doping levels are identified. The measurements show the impact of the doping level on depletion layer of the crystallites. It is shown, furthermore, that under a water vapour rich environment the volume fraction of the depletion layer may increase up to 5 times and more, while the mobility of the charge carriers in the depletion layer drops drastically from about 31 cm2V−1s−1 to about 8 cm2V−1s−1. This indicates that water vapour exposure causes an increase of the potential barrier in the grain boundary depletion layer, limiting the electron transport across the grain boundaries to a classical thermionic emission mechanism.

On the significance of thermoelectric and thermionic emission currents induced by chemical reactions catalyzed on nanofilm metal–semiconductor heterostructures

The possible origins and the magnitude of an electric current arising in nanofilm metal–semiconductor heterostructures when the metal surface is used to catalyze an exothermic chemical reaction are discussed. Two key mechanisms are considered that are responsible for the current generation: electron motion due to a temperature drop across the metal–semiconductor interface (thermionic emission mechanism) and the Seebeck effect in the two layers (thermoelectric mechanism). It is predicted that (i) current up to 10−3 A·cm−2 can arise, (ii) thermoelectric mechanism due to the Seebeck effect in the semiconductor layer plays a dominant role for the current generated under stationary chemical reaction conditions, and (iii) thermoelectric current strongly depends on the temperature. The carrier transport through the metal–semiconductor interface is described by the thermionic emission theory. The obtained results are discussed in view of the experimental data reported earlier for the chemical reaction-induced currents in metal/n-Si structures.

Current-Voltage and Capacitance-Voltage Characteristics of Pd Schottky Diodes Fabricated on ZnO Grown along Zn- and O-Faces

Temperature dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements were utilized to understand the transport mechanism of Pd Schottky diodes fabricated on Zn- and O-faces of ZnO. From I-V measurements, in accordance with the thermionic emission mechanism theory, it was found that the series resistance Rsand the ideality factor n were strongly temperature dependent that decreased with increasing temperature for both the faces (Zn and O-face) of ZnO revealing that the thermionic emission is not the dominant process. The barrier height øB(I-V)increased with increasing temperature for both faces. The measured values of ideality factor, barrier height and series resistance for Zn- and O-faces at room temperature were 4.4, 0.60 eV, 217 Ω and 2.8, 0.49 eV, 251 Ω respectively. The capacitance-voltage (C–V) measurements were used to determine the doping concentration Nd, the built-in-potential Vbi, and the barrier height øB(C-V). The doping concentration was found to be decreased with increasing depth. The barrier height øB(C-V)calculated for O-polar and Zn-polar faces decreases with increasing temperature. The values of barrier height øB(C-V)determined from C-V measurements were found higher than the values of barrier height øB(I-V). Keeping in view the calculated values of ideality factor, barrier height, and series resistance shows that O-polar face is qualitatively better than Zn-polar face.

Analysis of temperature dependent electrical properties of Au/perylene-diimide/n-Si Schottky diodes

The electrical properties of Au/perylene-diimide/n-Si Schottky diode have been determined by means of current–voltage measurements in the temperature range of 75–300K. These devices showed good rectifying behavior and the temperature dependence of the current–voltage characteristics could be explained by thermionic emission mechanism. The experimental values of barrier height and ideality factor for device have been calculated as 0.168eV and 7.63eV at 75K and 0.690eV and 1.57eV at 300K, respectively. The fabricated Schottky diode shows non-ideal current–voltage behavior and so it is thought that the device have a metal-interface layer-semiconductor configuration. In addition to current–voltage measurements, the room temperature capacitance–voltage characteristics of Au/perylene-diimide/n-Si devices were also investigated. The barrier height value of 1.051eV obtained from the capacitance–voltage measurements was found to be higher than that of 0.690eV obtained from the current–voltage measurements at room temperature. Furthermore, the energy distribution of the interface state density determined from current–voltage characteristics increases exponentially with bias from 8.01×1012eV−1cm−2 at (Ec−0.666)eV to 5.86×1013eV−1cm−2 at (Ec−0.575)eV.

A detailed study on current–voltage characteristics of Au/n-GaAs in wide temperature range

In order to obtain the detailed information on the conduction mechanisms of the Au/n-GaAs Schottky barrier diode (SBD), the current–voltage (I–V) characteristics were carried out in the temperature range of 80–340K by the steps of 20K. The ideality factor (n) decreases, while the effective barrier height (ΦBo) increases with increasing temperature (80–340K). The semilogarithmic I–V curve is almost parallel for each temperature. Therefore, its slope and tunneling parameter (Eoo) remained almost constant as independent of temperature with an average of 27.47V−1 and 28.04meV, respectively. Furthermore, the reverse saturation current (Is) ranges, indication that the charge transport mechanism in the Au/n-GaAs SBD is tunneling due to the weak temperature dependence of the Is. Thus, it can be said that the experimental I–V data quite well obey the thermionic field emission (TFE) model especially at low temperatures rather than the other transport mechanisms. On the other hand, an abnormal decrease in the ΦBo and increase in the n with decreasing temperature have been observed which leads to non-linearity in the activation energy plots and a linear relationship between the barrier heights (BHs) and the ideality factors of the SBD. The high value of n especially at low temperatures cannot be also explained only TFE theory. Therefore, we tried to explain the non-ideal behavior of the forward-bias I–V characteristics in Au/n-GaAs SBD with the basis of a thermionic emission (TE) mechanism with a Gaussian distribution (GD) of the BHs. The obtained results show that the temperature dependence of forward bias I–V characteristics of the Au/n-GaAs SBD can be successfully explained in terms of the TE mechanism with a double GD of BHs for low bias region (LBR) and moderate bias region (MBR).

Characterization of Al/p-Si/n-AgGaSe2/Au thin films heterojunction device

Crystalline Al/p-Si/n-AgGaSe2/Au heterojunction device was fabricated by depositing AgGaSe2 thin films at temperature 473 K, onto p-type single crystal silicon wafers with surface orientation (111) and glass substrates. The crystalline nature and the chemical composition of the deposited AgGaSe2 films have been confirmed using both X-ray diffraction (XRD) and energy dispersive X-ray spectrometer (EDX) techniques. The dark current–voltage characteristics of the heterojunction diode have been investigated at various temperatures to elucidate the electrical conduction mechanisms. It was found that at forward bias voltages ≤100 mV, the current is controlled by the thermionic emission mechanism, whereas, at bias voltages higher than 100 mV, the current is controlled by the space charge limited current mechanism. The reverse current mechanism of the diode is controlled by the carrier generation–recombination process in the depletion region. The photovoltaic parameters were determined from the current–voltage characteristics under illumination. The built-in potential, effective carrier concentration and barrier height were also evaluated from the C–V measurement.