Thermionic Emission Model Research Articles

Positive temperature dependence of the electroluminescent performance in a colloidal quantum dot light-emitting diode

In this study, the temperature dependence of the dynamic behavior in colloidal quantum dot light-emitting diodes (QLEDs) in a wide temperature range (120 K–300 K) is investigated. The results demonstrate that the electroluminescent performance of the device is enhanced as the temperature rises. Particularly, the device current efficiency is improved and the turn-on voltage is lowered. In addition, the decline in temperature results in a blue-shift of the electroluminescence spectrum. Consequently, the charge injection barrier is studied using thermionic emission model and impedance spectroscopy, and the results show that this injection barrier increases when the temperature is elevated, leading to more difficult charge injection. Meanwhile, the charge transport study proves that the average charge lifetime in the device is prolonged at higher temperature, which is beneficial for the transport of charges. Furthermore, the analyses of the trap states and their distribution reveal that more trap states exist at low temperature, causing performance degradation in the device. The analysis and discussion in this work provide fundamental insights into the working mechanism of QLEDs and can be used to predict device performance under extreme operating conditions.

Thermionic emission of electrons from metal surfaces in the warm dense matter regime

Thermionic emission of electrons is a process fundamental to our understanding of laser–matter interactions in the ultra-short pulse regime. Charge collected from an emission process, as well the secondary radiation generated by their collective motion, provides avenues for diagnosing and verifying existing laser–solid interaction models. Laser fluences (∼104 J/m2) are of particular interest as they heat the surface electrons to temperatures on the order of a few electron volts (eV), placing it in the warm dense matter regime where much underlying physics is yet to be fully understood. However, even at such moderate fluences the conventional Richardson–Dushman formula for the emission rate becomes invalid. We consider an additional barrier potential on the surface that appears due to space-charge effects, which then limits the thermionic emission. This provides feedback leading to a self-consistent solution with the emission rate. Unlike the work function, this barrier dynamically evolves during the emission process. Here, we present the first calculation of the barrier potential on the surface, along with analytical expression, from a one-dimensional electrostatic model. The result is a generalization of the Richardson–Dushman picture to moderate laser fluences. The potential barrier has been incorporated into a two-temperature model for thermionic emission from an Al target irradiated by a femtosecond laser. The collisional and transport data for Al have been obtained using an average atom model.

Forward and reverse current transport mechanisms in tungsten carbide Schottky contacts on AlGaN/GaN heterostructures

In this paper, the forward and reverse current transport mechanisms in as-deposited and 400 °C annealed tungsten carbide (WC) Schottky contacts on AlGaN/GaN heterostructures have been studied. In particular, under forward bias, the WC/AlGaN Schottky contacts exhibited a deviation from the ideal thermionic emission model due to the occurrence of a tunneling component of the current. From the temperature dependence of the ideality factor, a characteristic tunneling energy E00 in the range of 33–36 meV has been estimated. On the other hand, two different transport mechanisms have been identified under reverse bias. At low reverse bias (VR < 2 V), Poole–Frenkel emission rules the current transport, with an emission barrier ϕt = 0.68 eV in the as-deposited contact, which increases up to ϕt = 0.79 eV upon annealing at 400 °C. This behavior has been correlated with the improvement of the metal/AlGaN electronic properties. At higher reverse bias (VR > 2 V), the leakage current is dominated by a thermally activated process with an activation energy (0.27 eV) that is independent of the Schottky contact fabrication process. In this case, the temperature dependence of the leakage could be well described by a two-dimensional variable range hopping conduction associated with the presence of surface defects in the material.

Vertical β-Ga2O3 Schottky rectifiers with 750 V reverse breakdown voltage at 600 K

Field-plated vertical Ga2O3 rectifiers were operated up to 600 K with reverse breakdown voltage (V B) of 750 V, 950 V at 500 K and 1460 V at 400 K. The barrier height was 1.3 eV at 300 K and reduced to 0.7 eV at 600 K, with ideality factors of 1.05 ± 0.05 and 2 ± 0.1, respectively at these temperatures. On-state resistance, R ON, was 13 mΩ cm2 at 300 K and 41 mΩ cm2 at 600 K, leading to respective Baliga figures of merit of 151 MW cm−2 (300 K) and 13.9 MW cm−2 (600 K). The on–off ratio was >104 for all temperatures measured. The leakage current showed a good fit to the thermionic field emission model when the reverse voltage was less than 80 V, and it was dominated by the tunneling effect at higher voltage. The transition voltage from thermionic emission to tunneling effect decreased as the temperature increased. At high reverse voltage, a large number of electrons are injected into the drift region, and the current shows an I ∝ Vn relationship with voltage, indicating a trap-assisted space-charge-limited conduction (SCLC) mechanism. We observed this SCLC relation when the reverse voltage was larger than 400 V for 500 and 600 K. The associated trap energies for these two regions were extracted as 0.2 and 0.4 eV, consistent with levels in the gap.

Property evaluation of spin coated Al doped ZnO thin films and Au/AZO/FTO Schottky diodes

In the present work, the structural, optical and electrical properties of pristine and aluminium doped zinc oxide (AZO) thin films deposited on fluorine doped tin oxide (FTO) coated glass substrates by sol-gel spin coating technique are explored. Further, the effect of doping on the performance of Au/AZO/FTO Schottky diode were analysed through I–V characterization at room temperature. The c-axis orientation of the deposited films was confirmed by X-Ray diffraction studies. Micrographs from atomic force microscope revealed the wrinkled morphology and columnar structure for undoped and doped zinc oxide thin films respectively. The diode parameters such as Schottky barrier height (ΦB), ideality factor (n) and series resistance (Rs) were determined by using thermionic emission model and Cheung's model. These parameters were found to depend on the doping concentration. Devices with undoped and 6 at% aluminium doped ZnO thin films have shown better rectification.

Effects of surface treatment and annealing for Au/Ni/n-GaN Schottky barrier diodes

We report the basic electrical characteristics and uniformity of 25 Au/Ni Schottky barrier diodes (SBDs) with three different surface treatments ((I) no treatment (as-grown), (II) alkaline solution, and (III) HCl related solution) before and after post-metallization annealing at 400 °C. The SBDs with (II) showed small diode-to-diode variation in the Schottky barrier height and the ideality factor, and good uniformity over the electrode. In addition, after the annealing, the reverse-biased current significantly reduced to the prediction by the thermionic field emission model. Similar characteristics were obtained for the SBDs with (I), however, the uniformity over the electrode became worse after the annealing. For the SBDs with (III), the diode-to-diode variation was originally large, and the reverse-biased current did not significantly reduce. In addition, inhomogeneity with a line-shape pattern was observed in some SBDs with the large reverse-biased current. Residual Cl atoms may be responsible for low-barrier nature.

Numerical analysis of transport properties of ZnO based Schottky diode

Since numerical methods allow us to generate data faster than experimental methods in less time and with less effort, therefore in the presented work two well-known numerical methods known as Newton-Raphson and Gauss-Seidel were employed to compute electrical parameters of electronic devices at highly sensitive temperature from 300 K to 1000 K to discuss the significance of these parameters on the competency of devices. ZnO based Schottky diode has been considered due to the substantial properties of ZnO which are very crucial in the invention of high-quality devices.The investigations were focused on exploring the consequences of highly sensitive temperature on electrical parameters like ideality factor, saturated current, donor concentration, built in potential and height of barrier of Schottky diode. All parameters were computed by applying Newton-Raphson and Gauss-Seidel method on well-known thermionic emission model and capacitance-voltage equations.

The electrical properties of n-CdS/p-CdTe and n-ZnS/p-CdTe heterojunctions fabricated by a combination of SILAR and vacuum deposition techniques

The paper reports successful fabrication of CdS/CdTe and ZnS/CdTe heterojunctions by a combination of two different deposition techniques viz. successive ionic layer adsorption and reaction (SILAR) and conventional vacuum deposition. The SILAR deposited CdS and ZnS layers formed well adherent and stable heterojunctions with the vacuum deposited CdTe. The electrical characteristics of the heterojunctions were analyzed by thermionic emission model and Cheung's model. The ideality factor, series resistance, barrier height, space charge density and thickness of the depletion region were determined by rigorous I–V, J-V and C–V characterization. The barrier heights of the devices were found to be in the range of 0.8 eV–0.9 eV. The thickness of the depletion region was found to be 5.2 nm and 7.1 nm while the space charge density was found to be 3.6 × 1022 m−3 and 1.59 × 1021 m-3, respectively, for CdS/CdTe and ZnS/CdTe heterojunctions.

Determination of Schottky barrier height of graphene electrode on AlGaN/GaN heterostructure

A graphene Schottky contact was fabricated on an AlGaN/GaN heterostructure and subsequently analyzed. The calculated and experimentally measured Schottky barrier heights (SBHs) determined using the theoretical Schottky–Mott model, the thermionic emission model, the temperature-dependent current–voltage measurement, and the capacitance–voltage measurement were observed to be inconsistent, which was attributed to the ambiguities in both the Schottky contact area and the Richardson constant. While excluding the effects of these two factors, an SBH of 0.15 eV was determined by the temperature-dependent Richardson plot. This result was confirmed by calculating the inverse of the square of the measured capacitance with consideration of the threshold voltage required to deplete the two-dimensional electron gas under the Schottky contact.

Effect of various PCBM doping on the interfacial layer of Al/PCBM:ZnO/p-Si photodiodes

The light can be detected by various devices such as photoconductors, photodetectors, phototransistors, solar cells, and photodiodes, and the light can be employed energy harvesting as well as sensing applications. Various materials can be utilized to raising of the efficiency of light-based devices. In this present study, metal–semiconductor (MS) devices with various [6,6]-Phenyl C61-butyric acid methyl ester (PCBM)-doped zinc oxide (ZnO) interfacial layer were fabricated for photodiode applications. Commercially purchased PCBM and ZnO were mixed in order to obtain undoped, 3%, 5%, and 10% PCBM-doped ZnO interfacial layer for Al/PCBM:ZnO/p-Si devices. The obtained solutions were layered on the p-type silicon (p-Si) substrates by spin-coating method. The thermal evaporation technique was utilized to deposit Al metal electrodes both back and front sides. The morphological properties of the fabricated devices were characterized by AFM. The morphological images of the AFM revealed that PCBM doping affected the surface morphology as well as surface roughness of the ZnO layers. I–V measurements were carried out in the dark and under varying illumination conditions for electrical properties. The devices exhibited good rectifying properties at around 103 rates, both dark and various illumination conditions, according to I–V graphs. The junction parameters of the produced devices were determined by using thermionic emission, Norde, and Cheung models from the I–V characteristics of the devices. The devices have high ideality factors, and these values generally fluctuate with varying PCBM-doping amounts. The current transient measurements show that PCBM doping provide increasing in the light response capacity. According to the results, the devices can be employed as photodiode and photodetector applications for the further works in the industry.

Mechanism of Contact Electrification

Contact electrification has been a well-known phenomenon since B.C. 300. However, the origin of triboelectric charge and the charge transfer mechanism are not well understood. To date, the thermionic emission model, Schottky model, flexoelectric model, and intermolecular force model have been proposed for the contact electrification in conductors, semiconductors, and insulators. This article briefly introduces several important research results on the simple-seeming, but baffling, topic of contact electrification.

Nearly Fermi-level-pinning-free interface in metal/heavily-doped SiC Schottky structures

The barrier heights in Ti/ and Ni/n-SiC Schottky barrier diodes (SBDs) in a wide range of the donor density (N d = 2 × 1017–1 × 1019 cm−3) were investigated. The forward current–voltage characteristics in the heavily-doped SBDs (N d > 2 × 1017 cm−3) are described by the thermionic field emission (TFE) model, which includes an electron tunneling induced by the high electric field (> MV cm−1) at the Schottky interface. The high electric field also causes significant image force lowering (Δϕ ∼ 0.2 eV) in the heavily-doped SBDs (N d = 1 × 1019 cm−3). Through the analysis carefully considering such strong image force lowering, the same slope of the barrier height versus the metal work function plot (S ∼ 0.7) is obtained regardless of N d. This indicates that metal/SiC interfaces are nearly free from Fermi-level pinning independent of N d.

High-Voltage 4H-SiC Schottky Diodes with Field-Plate Edge Termination

High-voltage (2000V) 4H-SiC Schottky diodes are fabricated. To suppress the premature breakdown at the edge of diode structures, a field plate is formed as the edge termination. In this plate, an insulator layer is formed via the self-aligned implantation of high-energy (53 MeV) argon ions into 4H-SiC. To mask the active region of the diodes, 10–12-μm-thick nickel columns with vertical walls are grown on Schottky contacts by local electrochemical deposition. A comparison of the current–voltage characteristics of terminated and nonterminated diodes shows that the forward current–voltage characteristics barely degrade after implantation, whereas the reverse current–voltage characteristics are greatly improved. Both the forward and reverse current–voltage characteristics of terminated diodes are well described in terms of the classical thermionic emission model if lowering of the Schottky-barrier height with increasing bending of energy bands is taken into account.

A low-level acetone gas sensor based on n-type ZnO/p-type CuO composite nanostructure for the diagnosis of diabetes in dynamic situations

Today, gas sensors offer a vital method to detect and monitor the level, type, and environmental information of various gases in order to guarantee the safety of life in modern civilization. In this regard, the analysis of human breath components is one of the novel applications of analytic gas sensors to monitor the body’s metabolism function, which has received a lot of attention. The detection of acetone gas concentration in the exhaled breath is a novel promising method for the diagnosis of diabetes patients in the community health improvement plan. In the present work, a gas sensor based on zinc oxide/copper oxides composite nanostructure (ZCS) has been prepared to meet low-level acetone-sensing requirements. The XRD, FESEM, EDS, and BET analysis confirmed the presence of hexagonal wurtzite ZnO and monoclinic CuO in the composite nanostructure with nanorod morphology. The response, response/recovery time, optimum operating temperature, selectivity, and stability of the sensor were examined in dynamic situations. The results show that the ZCS can efficiently detect low-level acetone concentration. The response and response time obtained around 41.57–60.86%, and 12.6–8.1 s, respectively, toward 0.5–2 ppm target gas at the optimum operating temperature of 90 °C. The possible acetone-sensing mechanism of the ZCS through physicochemical reaction, and applying a standard thermionic emission model on I–V characteristics were discussed in detail. Investigations of temperature and frequency-dependent dielectric parameters of the ZCS were also discussed. The outstanding sensing performance of the ZCS demonstrates that it is a favorable and reliable functional acetone sensor for the monitoring of diabetes at dynamic conditions.

Analysis of LiCoO2 electrodes through principal component analysis of current–voltage datacubes measured using atomic force microscopy

This study aimed to reveal the electrical characteristics of the composite electrodes of Li-ion batteries. LiCoO2 electrodes were analyzed using atomic force microscopy, and three-dimensional datasets of current–voltage (I–V) curves (IV-datacubes) were obtained. The IV-datacubes were then analyzed using principal component analysis to determine the typical I–V curve corresponding to each LiCoO2 particle on a statistical basis. Detailed analysis based on the thermionic emission model with series resistance showed that some of the particles exhibited poor electrical contact in the electrode. Overall, the obtained results suggested that our IV-datacube analysis can reveal the characteristics of buried electrical contacts within electrodes, which is otherwise difficult to accomplish using alternative methods. We, therefore, believe that the method presented herein is a promising analytical method for composite electrodes.

Voltage and temperature dependence of reverse leakage current of lattice-matched InAlN/GaN heterostructure Schottky contact

In this paper, the temperature-dependent current-voltage (<i>T-</i><i>I-</i><i>V</i>) characteristics of lattice-matched InAlN/GaN heterostructure Schottky contact in a reverse direction are measured, and the voltage dependence and temperature dependence of the leakage current are studied. The obtained results are as follows.1) The reverse current is a strong function of voltage and temperature, and the saturation current is much larger than the theoretical value, which cannot be explained by the classical thermionic emission (TE) model. 2) In the low-bias region, the <inline-formula><tex-math id="Z-20210326091652-1">\begin{document}$ \ln(I/E)\text{-}E^{1/2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20201355_Z-20210326091652-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20201355_Z-20210326091652-1.png"/></alternatives></inline-formula> data points obey a good linear relationship, whose current slope and corresponding activation energy are close to the values predicted by the Frenkel-Poole (FP) model, indicating the dominant role of the FP emission mechanism. 3) In the high-bias region, the <inline-formula><tex-math id="Z-20210326091730-1">\begin{document}$ \ln(I/E^2)\text{-}E^{-1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20201355_Z-20210326091730-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20201355_Z-20210326091730-1.png"/></alternatives></inline-formula>data points also follow a linear dependence, but the current slope is a weak function of temperature, indicating that the Fowler-Nordheim tunneling mechanism should be mainly responsible for the leakage current. 4) The current barrier height is extracted to be about 0.60 eV, which is much lower than the value of 2.91 eV obtained from the TE model, confirming the primary leakage path of the conductive dislocations, where the localized barrier is significantly reduced due to the ionization of shallow donor-like traps.

Defect-insensitive current–voltage characteristics of Schottky barrier diode formed on heteroepitaxial α-Ga2O3 grown by mist chemical vapor deposition

An n-type α-Ga2O3 layer was grown by mist chemical vapor deposition on a sapphire substrate, and a Ti/α-Ga2O3 Schottky barrier diode was fabricated. Although the α-Ga2O3 layer has a high threading dislocation density (larger than 109 cm−2), the ideality factor of 1.03 was obtained from the forward current–voltage characteristic in the range of 298 K–423 K, indicating the clear thermionic emission transport. The reverse current–voltage characteristic was also investigated, and the leakage current showed good agreement with the theoretical calculation based on the thermionic field emission model without any fitting parameter in the temperature range of 298 K–423 K.

Electrical Interface Characterization of Ultrathin Amorphous Silicon Layers on Crystalline Silicon

Heterojunctions of amorphous (a‐Si) and crystalline (c‐Si) silicon combine the favorable absorption characteristics of a‐Si for the solar spectrum, high energy conversion efficiencies, low processing temperatures, potential for low production costs, and a reduced amount of silicon used due to thin a‐Si films. To investigate a‐Si/c‐Si heterojunctions, amorphous p‐type silicon (a‐Si) with a thickness of 5 nm is deposited via radio frequency‐pulsed magnetron sputtering on p‐doped, (100)‐oriented, crystalline silicon (c‐Si) wafers. During deposition, the crystalline silicon wafers are kept at 430 °C and the hydrogen flow rate is varied from 0 to 50 sccm (standard cubic centimeters per minute). Temperature‐dependent current–voltage measurements are carried out to investigate the dominant transport mechanisms of electrical conduction. Moreover, the influence of hydrogen on physical properties such as barrier height, activation energy, and electrical conductivity is analyzed. A current–voltage dependence as predicted by the thermionic emission model for the low forward‐bias region below 0.25 V is observed. Temperature regimes for nearest neighbor hopping and band conduction are revealed by the Arrhenius plot and are found to depend on the hydrogen flow rate.

I – V characterization of vacuum deposited zinc selenide – silicon hetero junction

Zinc selenide (ZnSe) thin films were grown on silicon (Si) wafer by thermal evaporation and the hetero-structure was subjected to annealing at various temperatures. X-ray diffractogram recorded for various samples were analysed to extract the structural information including crystallite size, strain and dislocation density. ZnSe films exhibited cubic structure with (111) orientation and the crystallite size has increased from about 21 nm to 43 nm upon annealing at 673 K. Annealing at temperature above this has degraded the films. I – V characterization has shown nonlinear relation and affected by post deposition annealing. Thermionic emission and Cheung models were applied to obtain various parameters that assess the performance of hetero-structured devices. Minimum ideality factor was observed (n = 1.75 from Cheung Model) for as deposited system and it increased after annealing. Analysis has proven that series resistance increases after annealing under air ambience

Gaussian Thermionic Emission Model for Analysis of Au/MoS2 Schottky-Barrier Devices

Schottky-barrier inhomogeneities are expected at the metal--transition-metal-dichalcogenide (TMDC) interface and this can impact device performance. However, it is difficult to account for the distribution of interface inhomogeneity as most techniques average over the spot area of the analytical tool (e.g., few hundred micrometers squared for photoelectron-based techniques), or the entire device measured for electrical current-voltage (I-V) measurements. Commonly used models to extract Schottky-barrier heights neglect or fail to account for such inhomogeneities, which can lead to the extraction of incorrect Schottky-barrier heights and Richardson constants that are orders of magnitude away from theoretically expected values. Here, we show that a Gaussian-modified thermionic emission model gives the best fit to experimental temperature-dependent current-voltage (I-V-T) data of van der Waals $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_{2}$ interfaces and allow the deconvolution of the Schottky-barrier heights of the defective regions from the pristine region. By the inclusion of a Gaussian-distributed Schottky-barrier height in the macroscopic I-V-T analysis, we demonstrate that interface inhomogeneities due to defects are deconvoluted and well correlated to the impact on the device behavior across a wide temperature range from a room temperature of 300 K down to 120 K. We verify the Gaussian thermionic model across two different types of p-${\mathrm{Mo}\mathrm{S}}_{2}$ (geological bulk crystals and synthetic flux-grown crystals), and finally compare the macroscopic Schottky-barrier heights with the results of a nanoscopic technique, ballistic hole emission microscopy (BHEM). The results obtained using BHEM are consistent with the pristine $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_{2}$ Schottky-barrier height extracted from the Gaussian-modified thermionic emission model over hundreds of nanometers. Our findings show that the inclusion of Schottky-barrier inhomogeneities in the analysis of I-V-T data is useful to elucidate the impact of defects (e.g., grain boundaries, metallic impurities, etc.) and hence their influence on device behavior. We also find that the effective Richardson constant, a material-specific constant typically treated as merely a fitting constant, is a useful parameter to check for the validity of the transport model.