Thermionic Emission Process Research Articles

Ballistic and resonant negative photocurrents in semiconducting carbon nanotubes

Ultrafast photocurrent experiments are performed on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. The photogenerated excitons are dissociated at large electric fields and the resulting transport of the charge carriers turns out to be ballistic. Thermionic emission processes to the contacts dominate the photocurrent. The charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands.

Theoretical Analysis and Experimental Optimization of Graphene/TMD Heterojunction Barristors

We optimized the graphene/MoS2 and graphene/WSe2 heterojunction barristors in terms of on-/off-current densities and on/off-current ratio by inserting an APTES layer which suppresses the charge puddle effect. The on-/off-current densities and on/off-current ratio were improved by a factor of 2.1/15 and 32 in the graphene/MoS2 barristor and 10/25 and 230 in the graphene/WSe2 barristor, respectively. Particularly, we reported an unusual phenomenon that on-state current density increases as the measurement temperature decreases in the graphene/WSe2 barristor. Finally, based on temperature-dependent current-voltage measurements coupled with first-principle density functional theory (DFT) calculations, we confirmed that the main operating mechanisms of graphene/MoS2 and graphene/WSe2 barristors were thermionic emission and trap-assisted tunneling processes, respectively.

Influence of surface emission processes on a fast-pulsed dielectric barrier discharge in air at atmospheric pressure

This paper presents simulations of an atmospheric pressure air discharge in a point-to-plane geometry with a dielectric layer parallel to the cathode plane. Experimentally, a discharge reignition in the air gap below the dielectrics has been observed. With a 2D fluid model, it is shown that due to the fast rise of the high voltage applied and the sharp point used, a first positive spherical discharge forms around the point. Then this discharge propagates axially and impacts the dielectrics. As the first discharge starts spreading on the upper dielectric surface, in the second air gap with a low preionization density of , the 2D fluid model predicts a rapid reignition of a positive discharge. As in experiments, the discharge reignition is much slower, a discussion on physical processes to be considered in the model to increase the reignition delay is presented. The limit case with no initial seed charges in the second air gap has been studied. First, we have calculated the time to release an electron from the cathode surface by thermionic and field emission processes for a work function eV and an amplification factor . Then a 3D Monte Carlo model has been used to follow the dynamics of formation of an avalanche starting from a single electron emitted at the cathode. Due to the high electric field in the second air gap, we have shown that in a few nanoseconds, a Gaussian cloud of seed charges is formed at a small distance from the cathode plane. This Gaussian cloud has been used as the initial condition of the 2D fluid model in the second air gap. In this case, the propagation of a double headed discharge in the second air gap has been observed and the reignition delay is in rather good agreement with experiments.

Conductance of Threading Dislocations in InGaAs/Si Stacks by Temperature-CAFM Measurements

The stacks of III-V materials, grown on the Si substrate, that are considered for the fabrication of highly scaled devices tend to develop structural defects, in particular threading dislocations (TDs), which affect device electrical properties. We demonstrate that the characteristics of the TD sites can be analyzed by using the conductive atomic force microscopy technique with nanoscale spatial resolution within a wide temperature range. In the studied InGaAs/Si stacks, electrical conductance through the TD sites was found to be governed by the Poole-Frenkel emission, while the off-TDs conductivity is dominated by the thermionic emission process.

Investigating the effect of electron emission pattern on RF gun beam quality

Thermionic radio frequency gun is one of the most promising choices to gain a high quality electron beam, used in the infrared free electron lasers and synchrotron radiation injectors. To study the quality of the beam in a compact electron source, the emission pattern effect on the beam dynamics should be investigated. In the presented work, we developed a 3D simulation code to model the real process of thermionic emission and to investigate the effect of emission pattern, by considering geometrical constraints, on the beam dynamics. According to the results, the electron bunch emittance varies considerably with the emission pattern. Simulation results have been validated via comparison with the well-known simulation codes such as ASTRA simulation code and CST microwave studio, as well as other simulation results in the literature. It was also demonstrated that by using a continuous wave laser beam for heating the cathode, the emission pattern full width at half maximum (FWHM) of the transverse emission distribution is proportional to FWHM of the Gaussian profile for the laser beam. Additionally, by using the developed code, the effect of wall structure around the cathode on the back bombardment effect has been studied. According to the results, for a stable operation of the RF gun, one should consider the nose cone in vicinity of the cathode surface to reduce the back-bombardment effect.

A novel electron accelerator for MRI‐Linac radiotherapy

MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work was to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems. Computational simulations were utilized to model the accelerator, including the thermionic emission process, the electromagnetic fields within the accelerating structure, and resulting particle trajectories through these fields. The spatial and energy characteristics of the electron beam were quantified at the accelerator target and compared to published data for conventional accelerators. The model was then coupled to the fields from a simulated 1 T superconducting magnet and solved for cathode to isocenter distances between 1.0 and 2.4 m; the impact on the electron beam was quantified. For the zero field solution, the average current at the target was 146.3 mA, with a median energy of 5.8 MeV (interquartile spread of 0.1 MeV), and a spot size diameter of 1.5 mm full-width-tenth-maximum. Such an electron beam is suitable for therapy, comparing favorably to published data for conventional systems. The simulated accelerator showed increased robustness to operation in in-line magnetic fields, with a maximum current loss of 3% compared to 85% for a conventional system in the same magnetic fields. Computational simulations suggest that replacing conventional DC electron sources with a RF based source could be used to develop medical electron accelerators which are robust to operation in in-line magnetic fields. This would enable the development of MRI-Linac systems with no magnetic shielding around the Linac and reduce the requirements for optimization of magnetic fringe field, simplify design of the high-field magnet, and increase system flexibility.

Scaling effects in Schottky contacts

This article reports on scaling effects in Schottky contacts on various types of semiconductors, including low resistivity, semi-intrinsic, and deep-level compensated. The investigation was performed using a finite element computation and drift-diffusion transport model. In low resistivity semiconductors, the currents scale with contact area as long as thermionic emission process dominates the current transport, with limited impact of velocity saturation effect. In high resistivity semiconductors, the scaling is much more complex due to the considerable impact of minority carrier contribution. In several cases, the currents scale with contact radius, rather than with area, due to corresponding electric field variations. In some compensated materials, the impact of velocity saturation was shown to boost the current, due to carrier accumulation and corresponding space charge variations.

Modified dust-acoustic waves in dusty plasma with Lennard-Jones potential

Dust-acoustic waves in a dusty plasma are investigated by solving the Vlasov equation including the effect of dust-dust interaction modeled by a Lennard-Jones-like potential. The latter contains a potential well and is applicable when thermionic or photo emission processes are important. It is shown that the excitation and linear dispersion of the dust-acoustic waves are strongly modified. In fact, the phase of the dust acoustic waves is shifted and a cut-off for the long-wavelength modes appears, leading to a purely growing instability.

Electrical characterization of 5.4 MeV alpha-particle irradiated 4H-SiC with low doping density

Nickel Schottky diodes were fabricated on 4H-SiC. The diodes had excellent current rectification with about ten orders of magnitude between −50V and +2V. The ideality factor was obtained as 1.05 which signifies the dominance of the thermionic emission process in charge transport across the barrier. Deep level transient spectroscopy revealed the presence of four deep level defects in the 30–350K temperature range. The diodes were then irradiated with 5.4MeV alpha particles up to fluence of 2.6×1010cm−2. Current–voltage and capacitance–voltage measurements revealed degraded diode characteristics after irradiation. DLTS revealed the presence of three more energy levels with activation enthalpies of 0.42eV, 0.62eV and 0.76eV below the conduction band. These levels were however only realized after annealing the irradiated sample at 200°C and they annealed out at 400°C. The defect depth concentration was determined for some of the observed defects.

Experimental observation of voltage amplification using negative capacitance for sub-60 mV/decade CMOS devices

In this study, an experimental study of negative capacitance is performed in order to overcome the physical limit of subthreshold slope (SS), SS ≥ 60mV/decade at 300 K, which is originated from (i) using the thermionic emission process in complementary metal-oxide-semiconductor (CMOS) technology and (ii) non-scalability of the thermal voltage kBT/q (i.e., in order to realize SS lower than 60mV/decade at 300 K). To make the surface potential higher than the gate voltage, a step-up voltage amplifier is included in the CMOS gate stack using a ferroelectric capacitor implemented with ferroelectric material. The measured SS in long-channel CMOS transistors is 13 mV per decade at 300 K. A simple connection of the ferroelectric capacitor to a complementary metal oxide semiconductor (CMOS) gate electrode would provide a new evolutionary pathway for future CMOS scaling.

Carrier extraction behaviour in type II GaSb/GaAs quantum ring solar cells

The introduction of quantum dot (QD) or quantum ring (QR) nanostructures into GaAs single-junction solar cells has shown enhanced photo-response above the GaAs absorption edge, because of sub-bandgap photon absorption. However, to further improve solar cell performance a better understanding of the mechanisms of photogenerated carrier extraction from QDs and QRs is needed. In this work we have used a direct excitation technique to study type II GaSb/GaAs quantum ring solar cells using a 1064 nm infrared laser, which enables us to excite electron–hole pairs directly within the GaSb QRs without exciting the GaAs host material. Temperature and laser intensity dependence of the current–voltage characteristics revealed that the thermionic emission process produced the dominant contribution to the photocurrent and accounts for 98.9% of total photocurrent at 0 V and 300 K. Although the tunnelling process gives only a low contribution to the photocurrent, an enhancement of the tunnelling current was clearly observed when an external electric field was applied.

Back-gate tuning of Schottky barrier height in graphene/zinc-oxide photodiodes

We demonstrate back-gate-tuning of the Schottky barrier height in graphene/zinc oxide photodiodes that are devised by a selective sputter-growth of ZnO on pre-patterned single-layer graphene sheets. The devices show a clear rectifying behavior (e.g., Schottky barrier height ∼0.65 eV and ideality factor ∼1.15) and an improvement in the photo-response via application of a back-gate voltage. The back-gate bias tunes the effective Schottky barrier-height and also promotes the activation of photo-excited carriers, which leads to an enhancement in the thermionic emission process.

Photon-enhanced thermionic emission from heterostructures with low interface recombination

Photon-enhanced thermionic emission is a method of solar-energy conversion that promises to combine photon and thermal processes into a single mechanism, overcoming fundamental limits on the efficiency of photovoltaic cells. Photon-enhanced thermionic emission relies on vacuum emission of photoexcited electrons that are in thermal equilibrium with a semiconductor lattice, avoiding challenging non-equilibrium requirements and exotic material properties. However, although previous work demonstrated the photon-enhanced thermionic emission effect, efficiency has until now remained very low. Here we describe electron-emission measurements on a GaAs/AlGaAs heterostructure that introduces an internal interface, decoupling the basic physics of photon-enhanced thermionic emission from the vacuum emission process. Quantum efficiencies are dramatically higher than in previous experiments because of low interface recombination and are projected to increase another order of magnitude with more stable, low work-function coatings. The results highlight the effectiveness of the photon-enhanced thermionic emission process and demonstrate that efficient photon-enhanced thermionic emission is achievable, a key step towards realistic photon-enhanced thermionic emission based energy conversion.

Electrical performance and interface states studies of undoped and Zn-doped CdO/p-Si heterojunction devices

A transparent semiconducting layer of CdO thin film was grown on p-type Si substrates to fabricate Zn-doped CdO/p-Si heterojunction devices using sol–gel spin coating method. The current density–voltage characteristics (J–V) of the undoped CdO/p-Si and Zn-doped (at 1%, 2%, 3% and 4%) CdO/p-Si diodes were measured at room temperature. The dark J–V characteristics of the diodes show rectification behavior. The rectification ratio of the diodes is found to be dependent on both applied voltage and the doping ratio of Zn. At lower voltages, the current in the forward direction obeys the thermionic emission process. For relatively higher voltages, the current is dominated by a space charge limited conduction mechanism. Under reverse bias conditions, the J–V characteristics of the diodes can be interpreted using Schottky mechanisms. The important junction parameters such as series resistance (Rs), the shunt resistance (Rsh), the ideality factor (n) and the barrier height (Φb) were determined by performing different plots from the forward bias J–V characteristics. The corrected capacitance–voltage (CAdj–V) and corrected conductance–voltage (GAdj–V) characteristics were measured in the frequency range of 10kHz to 1MHz. It is found that the CAdj–V and GAdj–V curves were strongly influenced with both frequency and presence of Zn-dopant content. The interface state density (Dit) is also depend on frequency and Zn-dopant content, and decreases with increasing frequency and Zn-dopant content. The obtained results indicate that the electrical properties of the CdO/p-Si heterojunction diodes are controlled by Zn-dopant content.

Insight into the thermionic emission regimes under gold film thermal relaxation excited by a femtosecond pulse

We theoretically investigated different thermal relaxation participating in the ultrafast thermionic emission processes on gold film surface with a femtosecond pulse excitation. The thermionic emission regimes under the two temperature relaxation and the thermal diffusion relaxation were demonstrated. The simulations showed that the thermionic emission properties can be defined in the regime under two temperature relaxation by reducing the laser fluence, or widening the pulse duration or increasing the laser wavelength. It was also found that there exists a transition between the two distinct thermionic emission regimes under peculiar laser parameters of laser fluence, pulse duration and laser wavelength. The results were explained as significant intervene of laser irradiation parameters into gold film thermal relaxation processes.

Investigation of current‐voltage characteristics of p‐type silicon during electrochemical anodization and application to doping profiling

AbstractThe analysis of anodic current‐potential I‐V characteristics of p‐type silicon in concentrated hydrofluoric acid solution shows that the mechanism at the origin of porous silicon PS formation is determined by the charge exchange at the silicon surface over a Schottky barrier formed at the interface through a thermionic emission process. In addition, a negative potential shift in the I‐V characteristics is observed with increasing doping concentration which can explain the selective formation of PS towards the doping level of silicon. In fact, when a silicon wafer of different regions of doping concentration is anodized at a fixed current or potential, the dissolution current will be more important in regions of higher doping level and PS formation can only take place there. The property of selectivity is used in this work in order to determine the p‐type doping impurity profiles. During anodization under galvanostatic conditions of a specific test sample presenting dopant concentration variations, it is shown that the measured anodization potential varies according to silicon doping concentration. Consequently, by converting potential values to dopant concentration, and the electrolysis time to a depth scale, a doping impurity profile can be extracted. The profile using this technique appears to be in a good agreement with the secondary ion mass spectrometry (SIMS) analysis. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier transport mechanisms of p-SiC/n-Si hetero-junctions

Hetero-junctions have been fabricated on Al-doped silicon carbide thin films onto n-type silicon single crystals using low-pressure chemical vapor deposition method. Temperature-dependent current–voltage measurements and Hall-effect measurements have been performed to determine the electrical properties of the structures. The temperature-dependent I– V characteristics demonstrated that the forward conduction at lower voltages was dominated by thermionic emission process, while, the forward conduction at higher voltages was determined by multi-step tunneling current. The activation energy of saturation current was about 0.32 eV. Hall-effect measurements proved the p-type properties of obtained SiC films. Deduced from the results of Hall measurements, the junction built-in potential was 1.32 V at room temperature. Moreover, using deep level transient spectroscopy measurements, Al Si acceptor ionization energy was estimated.

Local anodization of silicon as a step for electrical isolation process

The selective formation property of porous silicon PS is presented which is a basis step for the local electrical isolation process. In fact, when a silicon wafer of different doping concentration regions is anodized at a fixed potential, the dissolution current will be more important in the higher doping level regions where PS formation can only take place. The analysis of anodic current-voltage I-V characteristics of p-type silicon in the conditions corresponding to porous silicon formation shows a potential shift in the I-V characteristics with the silicon doping concentration which can explain the selectivity of PS formation towards the doping level of silicon. In addition, it is noticed that the mechanism at the origin of PS formation on p-type silicon in concentrated hydrofluoric acid solution is determined by the charge exchange at the silicon surface over a Schottky barrier which is formed at the interface through a thermionic emission process.

Combined Field and Thermionic Emission Process in ZnO Nanostructure Cold Emission Cathode

With recent research, the author intends to outline the framework of the field emission of ZnO nanostructures. However, many groups’ reports ignored the thermionic emission process in the low electric field. A recently published field emission cathode parameter extraction method [X. He et al., J. Appl. Phys. 102, 056107(2007)] provided unambiguous and reliable cathode. The method utilized Richardson-Laue-Dushman law in low electric field and Fowler-Norheim equation in high electric field to solve a one-dimensional model including both thermionic and field emission. The model gave a much better agreement with the experimental data of ZnO cathode under the applied field and acquired a revised surface field enhancement factors and work function of ZnO cathode in the electron emission process.

Photo- and thermionic emission from potassium-intercalated carbon nanotube arrays

Carbon nanotubes (CNTs) are promising candidates to create new thermionic- and photoemission materials. Intercalation of CNTs with alkali metals, such as potassium, greatly reduces their work functions, and the low electron scattering rates of small-diameter CNTs offer the possibility of efficient photoemission. This work uses a Nd:YAG (YAG denotes yttrium aluminum garnet) laser to irradiate single- and multiwalled CNTs intercalated with potassium, and the resultant energy distributions of photo- and thermionic emitted electrons are measured using a hemispherical electron energy analyzer over a wide range of temperatures. For both single- and multiwalled CNTs intercalated with potassium, the authors observe a temperature dependent work function that has a minimum of approximately 2.0 eV at approximately 600 K. At temperatures above 600 K, the measured work function values increase with temperature presumably due to deintercalation of potassium atoms. Laser illumination causes the magnitudes of collected electron energy distributions to increase substantially but in many cases has little effect on their shape. Simple theoretical models are also developed that relate the photo- and thermionic emission processes and indicate that large numbers of photoexcited electrons partially thermalize (i.e., undergo one or more scattering events) before escaping from the emitter surface.