Emission Theory Research Articles

High Light Outcoupling Efficiency from Periodically Corrugated OLEDs.

Organic light-emitting diodes (OLEDs) suffer from poor light outcoupling efficiency (ηout < 20%) due to large internal waveguiding in the high-index layers/substrate, and plasmonic losses at the metal cathode interface. A promising approach to enhance light outcoupling is to utilize internal periodic corrugations that can diffract waveguided and plasmonic modes back to the air cone. Although corrugations can strongly diffract trapped modes, the optimal geometry of corrugations and limits to ηout are not well-established. We develop a general rigorous scattering matrix theory for light emission from corrugated OLEDs, by solving Maxwell’s equations in Fourier space, incorporating the environment-induced modification of the optical emission rate (Purcell effect). We computationally obtain the spectrally emissive power inside and outside the OLED. We find conformally corrugated OLEDs, where all OLED interfaces are conformal with a photonic crystal substrate, having triangular lattice symmetry, exhibit high light outcoupling ηout ∼60–65%, and an enhancement factor exceeding 3 for optimal pitch values between 1 and 2.5 μm. Waveguided and surface plasmon modes are strongly diffracted to the air cone through first-order diffraction. ηout is insensitive to corrugation heights larger than 100 nm. There is a gradual roll-off in ηout for a larger pitch and sharper decreases for small pitch values. Plasmonic losses remain below 10% for all corrugation pitch values. Our predicted OLED designs provide a pathway for achieving very high light outcoupling over the full optical spectrum that can advance organic optoelectronic science and solid-state lighting.

Slurs as ballistic speech

Slurs are words with a well-known tendency to conjure up painful memories and experiences in members of their target communities. Owing to this tendency, it’s widely agreed that one ought to exercise considerable care when even mentioning a slur, so as to avoid needlessly inflicting distressing associations on members of the relevant group. This paper argues that this tendency to evoke distressing associations is precisely what makes slurs impactful verbal weapons. According to the ballistic theory, slurs make such potent insults because they enable their users to maliciously inflict noxious associations on members of their target groups. To motivate this theory, I demonstrate its ability to explain a number of facts about slurs’ offensiveness that pose formidable difficulties for competing theories of slurs. I argue that the ballistic theory not only explains why slurs make such explosively impactful terms of abuse, but also why: (i) uses of slurs can provoke offense even when they aren’t interpreted as expressions of the speaker’s racist (or otherwise bigoted) attitudes; and (ii) why mentions of slurs are inoffensive in some contexts, but in other contexts, warrant a level of offense every bit as severe as that warranted by weaponized uses. I conclude with a brief discussion of a practical application of the ballistic theory. I demonstrate that the theory has an important consequence for legislative efforts to regulate and punish weaponized uses of slurs.

Rational Design of Dual-Emission Lanthanide Metal-Organic Framework for Visual Alkaline Phosphatase Activity Assay.

The alkaline phosphatase (ALP) activity assay is very significant for disease diagnosis and biomedical research. Lanthanide metal-organic framework (Ln-MOF) based fluorescence sensors have great application potential in ALP activity assays. However, it is critical but challenging to investigate the emission law of Ln-MOFs for revealing rational design principles and selecting an appropriate MOF. Here, we describe a reasonable design strategy for dual-emission Ln-MOFs based on theoretical calculations. This strategy combines Reinhoudt empirical rule, intramolecular charge transfer theory, and aggregation/coordination-induced emission theory; reveals the luminescence law of Ln-MOFs; and provides theoretical guidance for the rational design of dual-emission Ln-MOFs. On the basis of this strategy, we create a dual-emission Tb-MOF fluorescent probe used for ALP activity assay and investigate the detection mechanism. The probe shows ultrasensitive (limit of detection 0.002 mU mL-1) and selective response to ALP, and it suits for point-of-care visual detection coupled with a self-designed portable enzyme activity assay kit and smartphone-assisted visual device. The kit-based visual assay method can accurately quantify the activity of ALP in real serum samples (recovery >93%, and relative error is less than 6.8% compared with the results of fluorescence spectrometer-based method) and consumes only 25 μL of serum. In addition, a logical decoder based on the "dual-key unlocking strategy" is designed, providing a feasible solution for the development of intelligent ALP activity detection equipment. As far as we know, this is the first report of a theoretical calculation-guided versatile design strategy for dual-emission Ln-MOFs and a portable enzyme activity assay kit for visual detection.

Analytic theory for field emission driven microscale gas breakdown for a pin-to-plate geometry

Decreasing electronics size necessitates better characterization of electron emission at the micro- and nanoscales for applications including microplasmas, micro- and nanoelectromechanical systems, and directed energy. While Paschen's law (PL) has historically predicted breakdown voltage based on the Townsend avalanche, field emission must be incorporated for gap sizes below ∼15 μm. Extensive studies have modified PL to explicitly include field emission for planar geometries; however, many practical experiments use pin-to-plate geometries. We modify a previous theory coupling PL and field emission to account for pin-to-plate geometries by replacing the field enhancement factor, which has been used primarily as a fitting parameter, with the appropriate vacuum electric field. This requires explicitly accounting for the spatial dependence of ionization and non-uniform space charge in Poisson's equation. We derive a breakdown equation of the form previously obtained for planar geometry [Venkattraman and Alexeenko, Phys. Plasmas 19, 123515 (2012)] that agrees well with experimental data with the work function as the fitting parameter. The work function was consistently lower (∼2 eV) than anticipated (∼4.5 eV) but was generally fairly consistent (∼ ± 7%). We then derived closed form solutions in the limit of low ionization, corresponding to the field emission regime, and recovered an analytic solution for a parallel plate geometry in the limit of small gap distance that differed from prior analytic results because of the explicit consideration of spatial dependence in charge density. This theory may ultimately be applied to other nonplanar geometries by applying the appropriate equation for the vacuum electric field.

Evaluation of gamma-irradiation effects on the electrical properties of Al/(ZnO-PVA)/p-Si type Schottky diodes using current-voltage measurements

In this study, Al/(ZnO-PVA)/p-Si (MPS) type Schottky diodes (SDs) were produced and the radiation effects on their electrical properties were investigated using the current-voltage (I–V) measurements. The I–V measurements were performed before irradiation and after various irradiation doses in the wide voltage range (±4 V) at room temperature. To determine gamma-irradiation effects on the MPS-type SDs accurately, one SD was preferred as a sample, and its significant electrical parameters such as zero-bias barrier height (ΦB0), ideality factor (n), and reverse-saturation-current (I0) were calculated using the linear parts of the ln(I)–V characteristics. Besides, to observe the effects of gamma-rays on MPS-type SDs in different voltage regions, some diode parameters were obtained by different calculation methods such as Cheung and Norde functions as well as Thermionic Emission (TE) theory. The calculations showed that high doses of gamma-irradiation (>5 kGy) caused the annealing effect, which leads to an improvement in some electrical parameters of SD, especially in the high electric field region. On the other hand, the energy distribution of the surface states (Nss) was obtained by utilizing the voltage-dependent ideality factor and the effective barrier height, with and without considering the series resistance (Rs) effect. It was observed that Nss values decreased almost as exponentially from the mid-band gap of the semiconductor towards the upper edge of the valance-band. Also, the density of surface states decreased with increasing radiation doses. As a result, almost all diode parameters are affected by irradiation. However, no significant defect has been detected that would affect the stable operation of the diode. Hence, Al/(ZnO-PVA)/p-Si type SD can be used as an MPS-type detector instead of MIS/MOS-type detectors due to some advantages of the organic/polymer interlayer such as being cheap, light per molecule, flexible and requiring low energy consumption.

Porous-shaped n-CdZnO/p-Si heterojunctions for UV photodetectors

In this study, n-type CdyZn1−yO (y = 0, 0.5, 1, 1.5 at%) thin films were deposited on p-Si and soda lime glass substrates using sol–gel spin coating technique. From the X-ray diffraction patterns, it was observed that all thin films had wurtzite structure with (002) orientation. Scanning electron micrograph results revealed that the morphology varied with higher Cd doping in ZnO. The bandgap was decreased from 3.29 eV (undoped ZnO) to 3.26 eV (y = 1.5 at%) as Cd content increased. The electronic parameters of the CdyZn1−yO (y = 1 at%) thin film heterojunction such as ideality factor (η) and barrier height (ΦB) were obtained using the thermionic emission theory as 2.53 and 0.70 eV, respectively. CdyZn1−yO (y = 1 at%) showed responsivity 0.01 A/W on 3.2 mW/cm2 exposure of UV light (365 nm) at 4 V bias voltage. The heterojunction so formed seems to be an excellent candidate to be used as an optical sensor (UV photodetector) in optoelectronics.

Investigation on a novel SiC Schottky barrier diode hydrogen sensor with trench-insulator structure

A novel SiC Schottky barrier diode (SBD) hydrogen gas sensor with trench-insulator structure was proposed in this paper. A physical model is built for this hydrogen sensor based on 4H-SiC SBD thermionic emission theory, tunneling effect of carriers, adsorption/desorption principle of hydrogen and modulation effects of Schottky barrier height. Use Silvaco TCAD, the semiconductor simulation software, to analyze SBDs with trench-insulator layer and to compute current-voltage characteristics at different temperature, hydrogen concentration and trench width under forward bias. The temperature and hydrogen concentration affect the I–V characteristics of the devices by changing the Schottky barrier height. Compared with normal metal-silicon carbide and metal-insulator-silicon carbide devices at 573 K in terms of on-off voltage, current resolution, response speed, and stability, the trench-insulator hydrogen sensor showed good performance. Relationship between device characteristics and trench width was researched using the above model. Trench width has an opposite effect on sensor resolution and sensitivity.

The pre-exponential voltage-exponent as a sensitive test parameter for field emission theories.

For field electron emission (FE), an empirical equation for measured current Im as a function of measured voltage Vm has the form Im = CVmk exp[–B/Vm], where B is a constant and C and k are constants or vary weakly with Vm. Values for k can be extracted (i) from simulations based on some specific FE theory, and in principle (ii) from current–voltage measurements of sufficiently high quality. This paper shows that a comparison of theoretically derived and experimentally derived k-values could provide a sensitive and useful tool for comparing FE theory and experiment, and for choosing between alternative theories. Existing methods of extracting k-values from experimental or simulated current–voltage data are discussed, including a modernized ‘least residual’ method, and existing knowledge concerning k-values is summarized. Exploratory simulations are reported. Where an analytical result for k is independently known, this value is reliably extracted. More generally, extracted k-values are sensitive to details of the emission theory used, but also depend on assumed emitter shape; these two influences will need to be disentangled by future research, and a range of emitter shapes will need examination. Other procedural conclusions are reported. Some scientific issues that this new tool may eventually be able to help investigate are indicated.

State-switching Mechanism of Intermittent Pulsars

Abstract The emission feature of intermittent pulsars is significant for understanding the pulsar emission mechanism. Using the observational evidence of radio emissions turning on and off and the corresponding spin-down rates in these two states of an intermittent pulsar, we will examine the polar-cap potential drop, gap height, and curvature radii of a few intermittent pulsars within the regime of the pulsar polar-cap emission theory by applying the current loss and energy flux of particle flow to pulsar braking, which are generally associated with radio emission from the polar cap. It is seen that the polar-cap parameters of the intermittent pulsars are almost equal to their maximum values, which is the main prediction of the pulsar polar-cap theory with respect to the breaking of the radio emission. It is also noticed that the intermittent pulsars are distributed near the dipole death line in the diagram, which is consistent with their emission features and the calculated polar-cap parameters. To further confirm the state switching of the intermittent pulsar, the relationships among spin-down rate, gap height, potential drop, and activity duty cycles of PSR B1931+24 are discussed. It is found that the gap height has an anticorrelation with the activity duty cycle, which indicates that the intermittency of the radio emission has a close connection to the gap height, as indicated by the pulsar polar-cap emission theory.

Quick thinking: how Einstein did (and did not) refute the ether frame of reference

This paper addresses and proposes to resolve a longstanding problem in the philosophy of physics: whether and in what sense Albert Einstein’s Chasing the Light thought experiment was significant in the development of the theory of special relativity. Although Einstein granted this thought experiment pride of place in his 1949 Autobiographical Notes, philosophers and physicists continue to debate about what, if anything, the experiment establishes. I claim that we ought to think of Chasing the Light as Einstein’s first attempt to problematize the very idea of the electromagnetic ether frame, and that it thereby contributed to his eventual adoption of one of special relativity’s two foundational axioms: the “light postulate”. This interpretation requires the assumption that Einstein had presupposed special relativity’s other axiom, the “principle of relativity”, when initially considering Chasing the Light. This argument is novel insofar as it provides evidence that such a presupposition by Einstein is both conceptually and historically plausible. Moreover, this paper directly challenges John D. Norton’s compelling claim that Chasing the Light is best understood as a refutation of emission theories of light propagation; while both interpretations of the experiment are conceptually coherent, I argue that the interpretation found in this paper is supported more straightforwardly by historical evidence.

Barrier height nature and photovoltaic properties of (2-(2-(2-nitrophenyl)diazenyl)malononitrile)/p-Si heterojunction

Small molecule, (2-(2-(2-nitrophenyl)diazenyl)malononitrile) (NPDAM), is synthesized and its chemical structure is characterized by different analyses. NPDAM powder is identified as polycrystalline monoclinic structure, whereas the thermally deposited films have nanostructure character with average particle size of 39 nm. NPDAM films are utilized for manufacturing hybrid solar cells in the structure of Au/NPDAM/p-Si/Al. The dark current-voltage (I-V) curves of the diode are measured in the temperature range 303-383 K and the thermionic emission theory is employed to extract the ideality factor, n, and potential barrier height, ∅ at forward applied voltages of V≤0.20 volt. The n value increases, whereas the ∅ value decreases with decreasing temperature. This behavior is analyzed in terms of the lateral inhomogeneous barrier following Gaussian distribution at the NPDAM/p-Si interface. Cheung and Cheung functions are used for extracting the values of series resistance, Rs, n and ∅. The behavior of these parameters as a function of temperature is discussed. At forward applied voltages 0.20<V≤2, the I-V relation reveals crossover from thermionic emission to square power law. This behavior is discussed by means of the space charge limited current. Under illumination, the device exhibits a response to light exposure with a photoelectrical conversion efficiency of 2.91 %.

Mechanical Manipulation of Silicon-based Schottky Diodes via Flexoelectricity

Silicon-based Schottky barrier diodes (SBDs), as a two-terminal circuit element, are widely used in modern industries due to its low cost and better compatibility with the complementary metal oxide semiconductor (CMOS) technology. Flexoelectricity resulting from strain gradient is a distinctive electromechanical coupling phenomenon compared to piezoelectricity resulting from strain. Flexoelectricity can exist in any dielectric due to the non-uniform strain breaking symmetry of materials. In this paper, we utilize the induced flexoelectric polarization to tune the behavior of a SBD made of metal and p-type Silicon. Making use of conductive atomic force microscopy (C-AFM), we measured current-voltage of the fabricated Si-based SBD under different tip forces. In order to conduct a quantitative study on the tuning effect of flexoelectricity on the performance of the fabricated Si-based SBD, we introduce an effective barrier height and provide a modified current equation according to the classical thermionic emission theory. The results show that flexoelectricity plays a significant role in dictating the Schottky barrier height, maximum rectified current (density), reverse saturation current (density), rectification ratio, and open voltage of the fabricated SBD. In addition, based on the Hertzian contact model and the measured data, the nonzero flexoelectric coefficients of the used p-type silicon are obtained.

Electromagnetic emission due to nonlinear interaction of laser wakefields colliding in plasma at an oblique angle

Head-on collision of laser-induced plasma wakefields with differing profiles of electrostatic potential has been recently found to be an efficient mechanism for generating high-power electromagnetic emission at the second harmonic of the plasma frequency (Timofeev et al 2017 Phys. Plasmas 24 103106). This mechanism is attractive for creating a source of tunable narrow-band coherent radiation in the terahertz frequency range. In this paper, we generalize the theory of electromagnetic emission produced by non-linear interaction of two plasma wakes to the case of an arbitrary collision angle. Such a theory is used to evaluate the angular distribution of the second harmonic radiation as well as its total generation efficiency for parameters of the proof-of-principle experiment in which laser axes will be aligned with a small finite angle. Theoretical predictions are qualitatively confirmed by particle-in-cell simulations.

Impact of doping on the performance of p-type Be-doped Al0.29 Ga0.71As Schottky diodes

The effects of changing the acceptors concentration on the electrical characteristics of Au/Ti on Be-doped Al0.29Ga0.71As Schottky contact have been investigated in the temperature range of 100—400 K. Using three devices with three different doping levels, the barrier height (ΦB), ideality factor (n) and series resistance (RS) for each diode were evaluated using both thermionic emission (TE) theory and Cheung's method. Our experimental results showed that the sample with a moderate doping concentration of 3 · 1016 cm-3 has the best performance, including ideality factor of 1.25 and rectification ratio of 2.24 · 103 at room temperature. All samples showed an abnormal behavior of reducing ΦB and increasing n with increase of temperature. This behavior was attributed, in case of low concentration samples, to barrier inhomogeneity and was explained by assuming a Gaussian distribution of barrier heights at the interface. While for the heavily doped sample, such non-ideal manner was ascribed with tunneling through the field emission (FE) mechanism.

Effect of Sn doping concentration on structural, optical and electrical properties of ZnS/p-Si (111) diodes fabricated by sol-gel dip-coating method

Tin doped zinc sulfide (ZnS:Sn) thin films have been fabricated by sol-gel dip-coating method on glass and p-Si substrates by varying Sn concentration between 0 wt % and 10 wt %. The structural and optical properties of ZnS:Sn thin films have been studied by X-ray diffraction and spectrophotometer measurements. X-ray results show the films grown on glass and p-Si substrate are cubic structure. Optical band gap values found from the transmittance spectra have decreased from 3.58 eV to 3.42 eV with the addition of Sn. The effect of tin concentration on the device performance of ZnS:Sn/p-Si has been investigated through the analysis of the I–V characteristics under dark and illuminated conditions. Diode parameters such as ideality factor n, barrier height Φb, saturation current Io, series resistance value RS have been calculated by means of thermionic emission theory, and Norde method. Photosensitivity PS, photoresponsivity R, and detectivity D values of the diodes have been determined from I–V measurements under 100 mW/cm2 illumination intensity. It has been determined that 3% Sn doped ZnS/p-Si diode showed the best diode and photodiode properties among Sn doped ZnS/p-Si diodes.

Modification of Schottky barrier height using an inorganic compound interface layer for various contact metals in the metal/p-Si device structure

In this study, lithium fluoride (LiF) was employed as an interface material by thermal evaporation technique and its effect of on basic device parameters such as Schottky barrier height and ideality factor was examined for different contact metals. In the device fabrication process, the aluminum (Al) metal was used only as the ohmic contact for all devices while Al, gold (Au), chromium (Cr) and nickel (Ni) were employed as Schottky contacts. The reference device structures were also fabricated without the LiF interfacial layers to better understand the effect of the interfacial layer. Thus, the metal/p-Si/Al and metal/LiF/p-Si/Al Schottky type device structures were obtained by using p type silicon (Si) wafer and Al, Au, Cr, Ni for top metal contacts to investigate the effect of these metals on the device parameters. The electrical measurements of the reference samples and Au/LiF/p-Si/Al, Al/LiF/p-Si/Al, Cr/LiF/p-Si/Al, Ni/LiF/p-Si/Al Schottky type diode structures were characterized by current–voltage (I–V) measurements at the room temperature and dark. Basic diode parameters such as ideality factor (n) and barrier height (Φb) of eight Schottky devices were calculated by the thermionic emission (TE) theory, Cheung and Norde functions at room temperature. The results revealed that the LiF interface material caused an increase in the barrier height (BH) compared to the reference device. This increase in the BH values of the devices in accordance with their reference samples was different for Al, Au, Cr, and Ni metals. In the calculation made by TE theory, the LiF interface increased the BH of the Al/LiF/p-Si device from 0.69 to 0.78, while increasing the BH of the Ni/ LiF/p-Si device from 0.57 to 0.65. The obtained results highlighted that the work function of Schottky metals and interface material are effective in modulating the BH of the device. In addition, I–V measurements of the AI/LiF/p-Si/Al device were taken under light and the results obtained showed that this device can be used in photodiode applications.

Vertical GaN Schottky barrier diodes with area-selectively deposited p-NiO guard ring termination structure

In this study, the effect of p-NiO guard ring length on the properties of vertical GaN Schottky barrier diode was investigated extensively. The forward biased current-voltage characteristics of all diodes with different guard ring length follow the thermionic emission theory, showing nearly the same ideality factor and Schottky barrier height value. While the series resistance increases by nearly two times with the increasing guard ring lengths because the increasing depletion region at the p-NiO/n-GaN junction compresses the conduction path. Under reverse bias, the breakdown voltage of the SBD without any termination is relatively low due to the electric field crowding. The p-NiO guard ring is effective to enhance the breakdown voltage although the leakage current is comparable. However, the breakdown voltage decreases with the increasing guard ring length, which may be ascribed to the tunneling leakage caused by the imperfect p-NiO/n-GaN interface.

Discrete Rising Tone Elements of Whistler‐Mode Waves in the Vicinity of the Moon: ARTEMIS Observations

AbstractWe report on discrete rising‐tone elements of whistler‐mode waves observed by Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) in the vicinity of the Moon. The two‐probe ARTEMIS observations suggest that a free energy source for the wave generation is provided by electron anisotropy resulting from lunar surface absorption and magnetic reflection. High time resolution dynamic spectra reveal that the waves consist of multiple rising tone elements, exhibiting striking similarities to the well‐known whistler‐mode chorus in planetary magnetospheres. The observed frequency sweep rates are generally consistent with those predicted by the nonlinear growth theory of chorus emissions by Omura et al. (2008, https://doi.org/10.1029/2007JA012622). These results imply that whistler‐mode waves can grow nonlinearly into chorus‐like emissions even around airless bodies without magnetospheres and that a well‐defined dipole field is not a prerequisite for the chorus generation.

Enhanced in-plane uniformity and breakdown strength of diamond Schottky barrier diodes fabricated on heteroepitaxial substrates

In this study, pseudo-vertical diamond Schottky barrier diodes (SBDs) were fabricated on heteroepitaxial substrates and a metal impurity-incorporated buffer layer to suppress killer defects was inserted. All SBDs exhibited excellent rectifying actions with suppressed leakage current. The in-plane uniformity was improved after the insertion of the buffer layer. Forward characteristics were fitted by thermionic emission theory and Tung’s model in the temperature range from 300 to 480 K. The perfection of the Schottky-diamond interface is discussed. Moreover, the SBDs exhibited a high breakdown voltage with a sudden increase in current at 375 V, which is the highest value reported for heteroepitaxial diamond. These results indicate that heteroepitaxial substrates are a promising alternative for large-area low-cost diamond electronics.

Weak antilocalization in partially relaxed 200-nm HgTe films

The anomalous magnetoresistance caused by the weak antilocalization (WAL) effects in 200-nm HgTe films is experimentally studied. The film is a high quality 3D topological insulator with much stronger spatial separation of surface states than in previously studied thinner HgTe structures. However, in contrast to that films, the system under study is characterized by a reduced partial strain resulting in an almost zero bulk energy gap. It has been shown that at all positions of the Fermi level the system exhibits a WAL conductivity correction superimposed on classical parabolic magnetoresistance. Since high mobility of carriers, the analysis of the obtained results was performed using a ballistic WAL theory. The maximum of the WAL conductivity correction amplitude was found at a Fermi level position near the bulk energy gap indicating to full decoupling of the surface carriers in these conditions. The WAL amplitude monotonously decreases when the density of either bulk electrons or holes increases that results from the increasing coupling between surface and bulk carriers.