Effective Emission Area Research Articles

A scattered volume emitter micropixel architecture for ultra efficient light extraction from DUV LEDs

Deep ultraviolet light-emitting diodes (DUV LEDs) typically suffer from strong parasitic absorption in the p-epitaxial layer and rear metal contact/mirror. This problem is exacerbated by a substantial portion of the multiple quantum well (MQW) emissions having a strong out-of-plane dipole component, contributing to emission in widely oblique directions outside the exit cone of the front semiconductor emitting surface. To address this, we propose an architecture that leverages such a heavily oblique angular emission profile by utilizing spaced-apart or scattered volume emitter micropixels that are embedded in a low-index dielectric buffer film with a patterned top surface. This approach achieves high light extraction efficiency at the expense of enlarging the effective emission area, however, it does not require a high-index (e.g., sapphire) substrate or a lens or a nanotextured epi for outcoupling purposes. Hybrid wave and ray optical simulations demonstrated a remarkable larger than three to sixfold increase in light extraction efficiency as compared to that of a conventional planar LED design with a sapphire substrate depending on the assumed epi layer absorption, pixel size, and ratio of light emission area to the MQW active area. An extraction efficiency three times greater than that of a recent nanotextured DUV LED design was also demonstrated. This architecture paves the way for DUV LEDs to have a plug efficiency comparable to that of mercury lamps while being significantly smaller.

Investigation on MOS shunt LVTSCR for ESD application

Continuously scaling down ICs result in more stringent electrostatic discharge (ESD) protection design requirements. Compared with other devices, silicon-controlled rectifier (SCR) has become the first choice for its area efficiency and robustness. In order to improve the latch-up issue of SCR, various schemes have been proposed. A simple method is to extend the SCR path length, which will result in the enlarged ON resistance. Segment technology is also used to improve the holding voltage of SCR, but it will shrink the effective emitter area and lead to the serious degradation of ESD robustness. MS-LVTSCR is used to protect CMOS input ports. The circuit operating voltage is 3.3V and the gate oxide DC breakdown voltage is 19V so that considering the safety margin, the ESD window is from 3.63V to 17.1V. This work proposes a novel MOS shunt low-voltage trigger silicon-controlled rectifier (MS-LVTSCR) electrostatic discharge protection device by inserting an embedded PMOS structure. Compared with the conventional LVTSCR, the proposed MS-LVTSCR achieves 53% improvement in the holding voltage and still maintains high ESD robustness with a current level of 31.5 mA/μm without more device area consumption. In addition, both the TCAD simulation and theoretical analysis were carried out to explore the principle of current shunt effect to improve holding voltage. The extra shunt paths will weaken the conductance modulation effect of the main drift region in the main SCR path and its holding voltage can be further raised by reducing the proportion of main drift region current in the total current. We also conducted detailed studies on the mechanisms and geometry effects of this newly proposed structure via experimental validations.

On the evaluation of field emission parameters of V2O5 and TiO2 nanostructure cathodes

The self-assembled V2O5 and TiO2 nanowires are prepared hydrothermally from vanadium (V) hydroxylamido complex and titanium oxide powder, respectively, and studied for their crystalline phase, morphology, and electron emission characteristics. V2O5 is shown to exhibit an orthorhombic phase with preferential growth of the (010) face along the [010] direction; wire size being diameter 100–400 nm, and length several micrometers. TiO2 nanowires depict a monoclinic β-phase with a typical diameter of ∼ 30 nm. Their bundles serve as potential cathodes giving electron emission following the Fowler–Nordheim (F-N) mechanism but from infinitely small areas with large field enhancement factors. In comparison, β-TiO2 provides better emission characteristics at similar operating parameters (e.g., low threshold voltage 250–400 V and current density 109−1013 A m−2). The unique properties (viz., tip geometry, roughness, and local field enhancement) of one-dimension (1D) nanowires make them prospective candidates for high-brightness electron sources and development of the display devices [1–7]. A simple procedure developed by the authors is applied successfully in actual evaluation of the field emission parameters from the current–voltage data. This involves F-N formulation with physical considerations like variation of work function, effective emission area, and field enhancement factor [31].

Mechanism analysis of field electron emission of titanium

Field electron emission (FEE) is generally considered to be closely correlated with radio frequency (RF) breakdown in accelerating structures and high-power microwave (HPM) devices. In this study, the field emission characteristics of titanium are investigated by using a field emission test system. With the increase of the number of field emission tests and stability tests, the repeatability of the field emission characteristic curve of titanium tends to be better, and the amplitude of the current oscillation in the stability tests gradually decreases, indicating that the field emission performance of titanium gradually becomes stable. Subsequently, the field emission characteristic curve of titanium is analyzed by adopting the field emission theory of metal microprotrusions. Combined with the analysis of the electric field enhancement effects caused by the surface morphology, the dominant effect of the metal microprotrusions on the FEE of titanium is excluded from the two aspects of the electric field enhancement factor and local maximum emission current density. At last, the field emission theory of dielectric micropoints is introduced to analyze the field emission characteristics of titanium. The electric field enhancement factor of 102 ∼ 103 are explained theoretically. Simultaneously, the reasonable effective emission area and local emission current density are given, which can better explain the field emission phenomena of titanium, such as the sharp decrease in emission current and repeatability of the field emission curve. Hence, it is revealed that the key factor that dominates the FEE of titanium is the dielectric impurities on the surface, rather than the metal microprotrusions.

FIB fabrication and electron emission characteristics of overhanging tungsten and platinum nanostructures

The recent advancement in nanofabrication technology has enabled realization of vacuum electron devices with nanogap spacing. These devices have advantages over semiconductor counterparts due to possible operation under hard conditions and some unique functionalities. The challenging task, however, lies with the full understanding of their current–voltage (I–V) characteristics, resulting from various electron emission mechanisms. The reliable extraction of device parameters is, therefore, vital for its potential applications. An attempt has, therefore, been made here to fabricate two three-dimensional overhanging electrodes of tungsten and platinum with a nanoscale gap of 70–100 nm on glass substrates using chemical vapor deposition and focused ion beam milling. Their (I–V) characteristics measured in situ at ∼10−6 mbar are shown to follow the Child–Langmuir law and Fowler–Nordheim field emission at low and high-bias conditions, respectively. The extraction of parameters with a simple procedure suggested earlier yields an effective emission area of ∼3510 Å2, work function of ∼2.5 eV, and field enhancement factor (β) of ∼ 1.8 for tungsten; the values for platinum are 12.5 Å2, 3.0 eV, and 5.0, respectively. The higher β in the case of platinum can be attributed to the formation of a comparatively rough emitter surface with some fine protrusions. The nanostructure gives a current spike at high voltages, which marks its transition to an explosive emission state, breakdown, and dispersion of spherical metal particles over the substrate.

Improved extraction efficiency of CsPbBr3 perovskite light-emitting diodes due to anodic aluminum oxide nanopore structure

In this work, we investigate the improvement in the performance of a CsPbBr3 perovskite light-emitting diode (PeLED) due to an anodic aluminum oxide (AAO) nanopore structure. The AAO structure in the CsPbBr3 PeLED structure can improve the light extraction efficiency of CsPbBr3 PeLEDs in two ways: the emission light in the side direction being redirected to the normal direction due to the light scattering effect caused by aluminum oxide nanopores and the effective emission area as a result of the rough surface of the AAO structure. The peak luminance, current efficiency, and external quantum efficiency (EQE) were 11,460 cd/m2, 2.03 cd/A, and 0.69% at a bias of 6.0 V, respectively. For comparison, the luminance, current efficiency, and EQE values of CsPbBr3 PeLEDs with the AAO structure using 50 V of pore-expanding voltage demonstrated improvements of 282%, 190%, and 1280%, respectively, over CsPbBr3 PeLEDs without the AAO structure.

Beam-Membrane Coupled Piezoelectric Micromachined Ultrasonic Transducers with Enhanced Transmitting Sensitivity.

Piezoelectric micromachined ultrasonic transducers (PMUTs) are a promising alternative to conventional bulk piezoelectric ceramic-based ultrasonic transducers. However, the transmitting sensitivity of the reported PMUTs is far from satisfactory. In this paper, we report a beam-membrane coupled PMUT (BM-PMUT), which enhances the transmitting sensitivity via simultaneously increasing the acoustic emission areas and maintaining the comparable vibration amplitude. Experimental results show that the center and edge transmitting sensitivities of the BM-PMUT are 108.1 and 96 nm/V at 370 kHz, which are 109.9 and 49.6 nm/V at 677 kHz for the traditional PMUT (T-PMUT). Thus, the BM-PMUT realizes piston-like mode shapes and achieves around twofold improvement in the effective acoustic emission area relative to the traditional T-PMUT of the same size. Due to the larger acoustic emission areas and comparable vibration amplitudes, the normalized far-field sound pressure level of the BM-PMUT is 8.5 dB higher than that of the T-PMUT.

Enhanced Field Emission of Single-Wall Carbon Nanotube Cathode Prepared by Screen Printing with a Silver Paste Buffer Layer.

A high emission current with relatively low operating voltage is critical for field emission cathodes in vacuum electronic devices (VEDs). This paper studied the field emission performance of single-wall carbon nanotube (SWCNT) cold cathodes prepared by screen printing with a silver paste buffer layer. The buffer layer can both enforce the adhesion between the SWCNTs and substrate, and decrease their contact resistance, so as to increase emission current. Compared with paste mixing CNTs and screen printed cathodes, the buffer layer can avoid excessive wrapping of CNTs in the silver slurry and increase effective emission area to reduce the operating voltage. The experimental results show that the turn-on field of the screen-printed SWCNT cathodes is 0.9 V/μm, which is lower than that of electrophoretic SWCNT cathodes at 2.0 V/μm. Meanwhile, the maximum emission current of the screen-printed SWCNT cathodes reaches 5.55 mA at DC mode and reaches 10.4 mA at pulse mode, which is an order magnitude higher than that of electrophoretic SWCNTs emitters. This study also shows the application insight of small or medium-power VEDs.

Field electron emission from a nanostructured tungsten surface

In this paper, the electron emission from a nanostructured tungsten surface was investigated. A method for measuring an extremely low current (10−12 – 10−14 A) has been tested. It made possible to reduce the effect of the electric field on the sample surface and to minimize the probability of spontaneous breakdowns. For a detailed study of tungsten fuzz, a point tungsten anode (diameter 90 μm) was used. Field enhancement factor (β = 2000 – 3000) and effective emission area were calculated using the Fowler–Nordheim plots. The pre-breakdown current rise was studied. The emission current waveforms suggest the formation of several emission structures before the breakdown.

Numerical simulations of field emission characteristics of open CNT

Numerical simulations on field emission properties of an open single-walled carbon nanotube with radius r = 1 nm have been carried out. Using finite element method, we have calculated the local electric field Floc and field enhancement factor γ distribution over the surface of the open CNT. From these calculations, we plot theoretical current-voltage characteristics and assess effective field characteristics for CNT with wall thickness w = 0.2 nm and height h = {3, 11, 101, 1001} nm. It was revealed that the maximum emission current did not change when applied field at the top of the CNT corresponds to the scaled barrier field f = 0.45. Obtained values of effective emission area are in good agreement with surface area, which has the highest field enhancement factor. In addition, we have plotted a “map” of maximum field enhancement factor γa for open CNTs with various wall thickness - w ϵ [0.1; 0.9] nm and heights h ϵ [11; 1001] nm. It was shown that maximum field enhancement factor is a nonlinear function of h/r and h/w ratios.

Nano-gap planar metal electrodes: fabrication and I-V characteristics

The nanowires/bars and nano-gap electrodes are vital components for emerging electronics and have wide ranging applications in flat-panel displays, sensors, sub-100 nm transistor circuits, and miniaturized computers/devices. Focused ion beam (FIB) has emerged as a powerful and unique tool for nanofabrication. The research work described here is concerned with (a) the FIB fabrication of planar metallic (copper and gold) nanostructures, (b) their current-voltage (I–V) measurements in situ, and (c) a viable method for extracting the realistic values of emission parameters. The planar electrodes with gap of 80-100 nm are realized by FIB milling of thin metal films. The difficulties faced in objective interpretation of their I-V data (based on known mechanisms) are highlighted. For determining the parameters (namely, effective emission area α eff, apparent work function ф, and the field enhancement factor β), Fowler-Nordheim [ln(I/V2) versus 1/V] plots showing a minimum with straight line of negative slope can be used. The striking findings demonstrated are (i) occurrence of emission from a tiny region (<1 nm2) vis-à-vis physical area (400 μm × 200 nm), (ii) significant lowering of barrier height, and (iii) enhancement of local field due to protrusions present. Typical values of α eff, ϕ, and β deduced are 52.3 Å2, 1.62 eV, and 39.3, respectively for copper planar electrodes (gap ∼100 nm); the corresponding data for the case of gold (gap ∼80 nm) are 29.1 Å2, 1.97 eV, and 12.1, respectively. Moreover, β lowering observed with bias is accompanied by increase in the emission area due to progressive smoothening of protrusions at the cathode surface. The electrodes are found rough/rocky at the nanoscale with protrusions and varying separations at places. These features make the electron emissive region small and pointed with an enhanced local electric field and effectively of a lower barrier height. The current discrepancy in the Child-Langmuir’s space charge regime is attributed to the emission occurring from a restricted area only. These findings are important for futuristic nano-devices like thermo-tunnelling refrigerator, energy harvester, etc.

FEbeam: Cavity and electron emission data conversion, processing, and analysis. A freeware toolkit for rf injectors.

FEbeam is an all-in-one field emission data processing interface with the capability to analyze the field emission cathode performance in an rf injector by extracting the field enhancement factor, local field, and effective emission area from the Fowler-Nordheim equations. It also has the capability of processing beam imaging micrographs using its sister software, FEpic. The current version of FEbeam was designed for the Argonne Cathode Test-stand of the Argonne Wakefield Accelerator facility switch yard. With slight modifications, FEbeam could work for many rf field emission injectors. This software is open-source and can be found at GitHub.

Determining the field enhancement factors of various field electron emitters with high numerical accuracy

Theoretical analysis of field electron emission must consider many parameters, one of the most critical being the field enhancement factor (FEF). In a single tip form, the FEF can vary several orders of magnitude and depends only on the system geometry, when the gap length between the emitter and counter-electrode is much greater than the height of the emitter. In this work, we determine very accurate analytical expressions for the FEF of five emitters with various shapes, which are often considered in the literature: Ellipsoidal, Hemisphere-on-Cylindrical post, Hemisphere-on-Orthogonal cone, Paraboloidal, and Hyperboloidal. We map the FEF as a function of the aspect ratio with an error smaller than 2% to serve as a quick reference database. Additionally, we calculate the electric field distribution over the emitters, which can give an insight into the effective notional emission area and the influence of the emitter’s base.

Strongly anisotropic field emission from highly aligned carbon nanotube films

The field electron emission properties of carbon nanotube (CNT) films composed of densely packed and highly aligned CNTs were investigated. The CNT films were produced by a continuous film casting process and are spooled into long lengths with the CNTs aligned lengthwise in the film. The anisotropic nature of the CNT film morphology was confirmed by performing specific conductivity measurements in directions both parallel and perpendicular to the aligned CNT microstructure. Field emission experiments were performed on 5 and 10 mm wide films that were mechanically cut into small samples and then vertically mounted so that the emission occurred from the film edge. The films were mounted with the aligned CNT microstructure oriented either parallel or perpendicular to the direction of the applied electric field. The highest emission currents were produced by films mounted in the parallel alignment configuration. Additional experiments were performed on films that were folded, which eliminated surface irregularities at the film edge due to the cutting process. SEM imaging performed at the ridge of the folded film before and after field emission (FE) experiments showed that films mounted in the parallel alignment configuration had minimal surface damage after FE, while films mounted in the perpendicular alignment configuration showed substantial damage. The effective emission area and field enhancement factor were extracted from the FE data using the orthodox Fowler–Nordheim theory. Folded CNT film cathodes mounted in the parallel alignment configuration produced the highest emission currents, while demonstrating a larger emission area and lower field enhancement factor.

Field emission of GaN nanofilms on Si substrates enhanced by hydrogen plasma treatment

The (002) oriented GaN nanostructured films prepared by pulsed laser deposition (PLD) on n-type Si (100) substrates were treated by H- and O-plasma, finding both treatments do not make the sizable changes in crystalline structure and morphology of GaN nanofilms. However, the field emission (FE) performance of GaN nanofilms is considerably improved by H-plasma treatment (H-GaN) but deteriorated by O-plasma treatment (O-GaN). The turn-on fields, Eon, for H- and O-plasma treated GaN nanofilms are determined to be 0.52 and 1.79 V μm−1, respectively, in contrast with that of pristine GaN nanofilms, 0.95 V μm−1. The improvement of FE performance by the H-plasma treatments could be attributed to the reasons of the reduced surface potential barrier, the increased electron concentration, the increased surface conductivity, and the increased effective emission area. The first-principles calculations represents that the experimental and theoretical work function results have the same trend with the descending sequence of O-plasma treated GaN > pristine GaN > the H-plasma treated GaN.

Field Emission From Nanostructured Tendril Bundles

Field emission from tungsten surfaces with nano-tendril bundles formed after exposure to helium plasma with nitrogen or neon impurities was measured using a vacuum diode device. Field enhancement factor and effective emission area were calculated using the Fowler–Nordheim formula and compared for different types of nanostructures. The field enhancement factor for these structures can reach several thousands and the effective emission area can be more than 1000 nm2. Anode luminescence was detected using long-exposure photography and allowed for estimation of current density near the anode. Unipolar arc tracks were observed on the samples with the highest field emission intensity. We show that field emission from nano-tendril bundles is much higher than that from tungsten fuzz or pure tungsten, and fusion-relevant plasma conditions may lead to arc initiation.

Theoretical methods for definition of the emission area of multi-tip cathodes and their experimental validation

The processing of the current-voltage characteristics (IVC) taken from multi-tip field emitters (LAFEs) requires a streamlined and objective processing method for obtaining the main emission parameters. This paper considers different approaches for determining the effective emission surface. Methods for determining the area are applied to the analysis of experimental IVCs for single-walled (SW) and multi-walled (MW) LAFEs. The dependence of the effective emission area on the applied voltage was confirmed.

Ten Approaches to Define the Field Emission Area

The emission area is one of the main parameters of field emission systems. A variety of formal determination techniques for the emission area is studied. Using the model hemisphere-on-cylindrical-post constructed with the COMSOL Multiphysics software package, the relation between well-known estimates of the emission area is shown. The corresponding fractions of the emission currents are computed. It is demonstrated that the formal emission area (the ratio of the total current to the current density at the vertex) ensures approximately 75% of the emission current. The effect of abnormal behavior on the effective emission area is obtained (cutoff from the trend line of the current–voltage characteristics) for varying voltage level in standard Fowler–Nordheim coordinates. An experimental assessment of the area with application of modified Fowler–Nordheim coordinates ln(I/U2–η/6) vs 1/U is proposed. The method is applied for analysis of field emission data of the multipoint nanocomposite emitter with carbon nanotubes.

Electron emission characteristics of wet spun carbon nanotube fibers

Wet spun carbon nanotube fibers were characterized using both field emission and electron energy distribution measurements. Fowler-Nordheim analysis of the field emission results showed that the carbon fibers demonstrated a large effective emission area, 2 x 10-12 m2, which resulted in a reduced brightness of 1.84 x 1010 A/m2/sr/V. By considering the emission and number of carbon nanotube emitters it can be shown that the brightness is consistent with previous reports for single nanotube emitters. Additionally, using the effective emission area determined from the Fowler-Nordheim analysis an emittance value around 0.70 μm was found. These characteristics are useful metrics in determining the applicability of using wet spun carbon nanotube fibers for field emission devices.

An Enhanced MLSCR Structure Suitable for ESD Protection in Advanced Epitaxial CMOS Technology

In this paper, an enhanced modified lateral silicon-controlled rectifier (EMLSCR) has been proposed and demonstrated. Compared with the traditional MLSCR, the EMLSCR possesses a lower trigger voltage of 7.0 V as well as a higher robustness, making it very suitable for the electrostatic discharge (ESD) protection for 2.5-V/3.3-V/5-V IOs in the advanced 65-nm epitaxial CMOS technology. Additionally, an improved transmission line pulsing (TLP) measurement strategy by adopting TLP pulses with various rise time has been put forward to provide a convenient and economical approach to explore the conduction uniformity of the ESD device for the first time. Furthermore, two optimization methods are proposed and evaluated to overcome the prefailure issue due to the small well resistance inherited in the EPI process as well. The method of well pickup segmentation is demonstrated to be more efficient since it could increase the well resistance and augment the effective emitter area of parasitic bipolar junction transistors (BJTs) under constant device area.