Field Emitter Arrays Research Articles

Cold Cathode Flat Panel X-ray Source for Talbot–Lau Grating Interferometer using Zinc Oxide Nanowire Field Emitter Arrays and Periodic Microstructured Anode

Cold Cathode Flat Panel X-ray Source for Talbot–Lau Grating Interferometer using Zinc Oxide Nanowire Field Emitter Arrays and Periodic Microstructured Anode

An In Situ Automated System for Real-Time Monitoring of Failures in Large-Scale Field Emitter Arrays

Nano-scale vacuum transistors (NVCTs) based on field emission have the potential to operate at high frequencies and withstand harsh environments, such as radiation, high temperatures, and high power. However, they have demonstrated instability and failures over time. To achieve high currents from NVCTs, these devices are typically fabricated in large-scale arrays known as field emitter arrays (FEAs), which share a common gate, cathode, and anode. Consequently, the measured currents come from the entire array, providing limited information about the emission characteristics of individual tips. Arrays can exhibit nonuniform emission behavior across the emitting area. A phosphor screen can be used to monitor the emission pattern of the array. Additionally, visible damage can occur on the surface of the FEAs, potentially leading to the destruction of the gate and emitters, causing catastrophic failure of the FEAs. To monitor damage while operating the device, an ITO-coated glass anode, which is electrically conductive and visible-light-transparent, can be used. In this work, a method was developed to automatically monitor the emission pattern of the emitters and the changes in surface morphology while operating the devices and collecting electrical data, providing real-time information on the failure sequence of the FEAs.

Comprehensive analysis of charge carriers dynamics through the honeycomb structure of graphite thin films and polymer graphite with applications in cold field emission and scanning tunneling microscopy

Polymer graphite electron sources have performed satisfactorily as field emission emitters and scanning tunneling microscopy probes in the past few years. However, the emission process was characterized by limited total emission currents. This paper introduces the elemental, vibrational, electronic structure, and optical analysis of polymer graphite and glass-graphite composite field emission cathodes to study these limitations. Moreover, the field emission characteristics are studied including the changes in the potential energy barrier of the used materials and structures. Among the studied structures, the cathodes prepared from graphite thin films deposited on a micropointed glass substrate (film-GMF) showed superior performance as random field emission arrays. This includes obtaining much higher emission current values ≈ 20 times) and lower threshold voltages ≈ 1/2) compared to the results obtained from polymer graphite samples. The enhancement factor in such emitters is believed to be the three-dimensional honeycomb structure of graphite. Moreover, the study includes applying graphite coatings to tungsten nano-field emission cathodes and scanning tunneling microscopy probes, which improves the performance of such cathodes/probes in both microscopic techniques.

Emission enhancement of GaN field emitter arrays in an N2 environment

Field emitter arrays (FEAs) have the potential to operate at high frequencies and in harsh environments. However, the vacuum packaging of these devices poses a challenge due to the sensitivity of the emission phenomena to the surface properties of the cathode. Studying the effect of different residual gases on FEAs can help to understand the interaction of the emission surface with the environment and identify the feasibility and requirements for vacuum packaging. In this work, the effect of N2 exposure on 150 × 150 gallium-nitride-field emitter arrays (GaN-FEAs) was studied. The GaN-FEA was first operated at 10−9 Torr with a 1000 V DC anode voltage and a 50 V DC gate voltage, where the anode current was 6 μA. The device was then exposed to 10 000 l N2 at 10−7 Torr, and the anode current increased by 2.7 times during N2 exposure. The increase in the current was not permanent, and the current gradually decreased to its pre-exposure level after the N2 source was cut off. The results of N2 exposure were compared to Ar and O2.

Characteristics of a Silicon Carbide Field Emission Array under Pre-Breakdown Conditions

Characteristics of a Silicon Carbide Field Emission Array under Pre-Breakdown Conditions

Effect of emitter tip spacing on Si vacuum field emitter arrays

This paper presents a qualitative and quantitative analysis of close-packed vacuum field emitter arrays. Through a combination of computational simulations and experimental verifications, the relationship between emitter spacing and electric field screening is studied. In order to obtain a uniform electron emission and large current densities from a field emitter array, it is essential to minimize the screening effect between the adjacent field emitters. As previously demonstrated, sharpening the emitter tips, reducing the background doping concentration of the emitters, and reducing the anode–cathode distance can suppress the screening effect. Field emitter arrays with different sizes and emitter spacings are fabricated and characterized. Decreasing the array size and increasing emitter spacing lead to a larger average current per emitter (19 nA/tip at an anode voltage of 250 V). However, larger current densities (up to 3.3 A/cm2) are observed in close-packed arrays. In very large arrays with a small emitter spacing, the current is scaled with the emitter area (∼4.2 nA/emitter at an anode voltage of 250 V), indicating a small current contribution from the sides of the array.

Modeling the Temporal Response of Gated ZnO Nanowire Field Emitter by Considering the Charging and Self-Heating Effect for Improving the Response Speed

ZnO nanowire is a promising candidate for large-area gated field emitter arrays. How to improve its temporal response is one of the key problems to be solved for applications. In this work, a device model for a gated ZnO nanowire field emitter with consideration of charging and self-heating effect has been established to investigate its temporal response. It is found that while the charging effect is responsible for the delay at the beginning of the pulse, the self-heating effect which induces delay due to the thermal conduction process can shorten the charging time because of its lowering of nanowire resistance. The response time can be minimized when these two effects are balanced at an optimal field which is below the critical field for thermal runaway. We further investigate the optimal response time of a nanowire with the same resistance but a different length, radius, and electrical properties. The results imply that a lower heat capacity and higher critical temperature for thermal runaway are in favor of a shorter response time, which must be taken into account in the reduction in nanowire resistance for improving response speed. All the above should be useful for the device design of a fast-response gated ZnO nanowire field emitter array.

Investigation of silicon-on-insulator back-gate nano vacuum channel transistor array

Recent advances in nanofabrication have made it possible to combine planar solid-state devices with vacuum electronics to create planar nano vacuum channel transistors that offer the advantages of cold-field emission and ballistic transmission. However, the current research is mainly limited to the study of a single field emission transistor, which has problems such as low current and poor gate control capability. To solve the above problems, a multitip field emission array is used in this work, and gate modulation is performed by a back-gate structure to fabricate and process a back-gate nano vacuum transistor array. First, we conducted simulation modeling of the back-gate nano vacuum transistor, investigated the impact of its structural parameters on its performance, and obtained the optimal simulation results. Then, structural parameters of the back-gate nano vacuum channel transistor array (BG-NVCTA) are selected based on the simulation results and fabricated by electron beam lithography on the silicon wafer. The experimental results, agreed well with the simulation results, show that the BG-NVCTA device has excellent gate control characteristics and a high current density. Its anode current is greater than 5 μA, and the transconductance is 1.05 μS when the anode voltage is 5 V.

Degradation of GaN field emitter arrays induced by O2 exposure

Field emitter arrays (FEAs) have the potential to operate at high frequencies and in harsh environments. However, they have been shown to degrade under oxidizing environments. Studying the effect of O2 on FEAs can help to understand the degradation mechanisms, identify the requirements for vacuum packaging, and estimate the lifetime of the device. In this work, the effect of O2 exposure on 100 × 100 gallium-nitride-field emitter arrays (GaN-FEAs) was studied. The GaN-FEAs were operated at 6 × 10−10 Torr with a 1000 V DC anode voltage and a 50 V DC gate voltage, where the anode current was 1 μA and the gate current was ≤4 nA. The devices were exposed to 10−7, 10−6, and 10−5 Torr of O2 for 100 000 L. The anode current dropped by 50% after 300 L and 98% after 100 000 L. It was observed that the degradation depends on the exposure dose, rather than pressure. The devices mostly degrade when they are ON, confirmed by exposing the device to O2 when the gate voltage was off, and also by the relation between the degradation and duty cycle when pulsing the gate. The results of O2 exposure were compared to Ar exposure to determine whether sputtering and changes in the surface geometry were the primary cause of degradation. The results suggest that changes in the work function and surface chemistry are the cause of emission degradation of GaN-FEA induced by O2.

Effect of water vapor desorption on the performance of gallium nitride field emitter array

We are exploring the potential of gallium nitride (GaN) field emitter arrays in vacuum channel transistors. This study investigated the impact of ultraviolet (UV) light on the emission properties of large arrays of GaN field emitters. Arrays of 150 × 150 emitters were analyzed before and after UV exposure. With a constant collector voltage of 200 V DC, gate voltage sweeps from 0 to 60 V were applied. The initial I–V measurements showed a rapid increase in emission current, indicating a conditioning effect, settling at a stable value of 1.25 μA after three to five sweeps. Remarkably, exposure to UV light resulted in a fivefold increase in the maximum field emission current, reaching an impressive 6 μA. This significant enhancement highlights the potential of UV treatment for improving the performance of GaN-based field emitters. This surge in current can be attributed to the desorption of water vapor caused by the UV light. To compare with the heating-based water desorption technique, another array of 150 × 150 emitters was characterized before and after heating at 400 °C. While the collector voltage remained constant at 200 V DC, the gate voltage was systematically increased from 0 to 75 V in this experiment. This controlled sweeping of the gate voltage provided a precise method for characterizing the field emission properties of the GaN emitters. The I–V measurements revealed that, similar to the UV exposure case, collector current increased by approximately four times after heat treatment at 400 °C for 10 min. This resulted in a maximum field emission current of around 10 μA at 75 V. As with the UV case, this increase can also be attributed to surface desorption, primarily of water.

Simulation Modelling of Silicon Gated Field Emitter Based Electronic Circuits

Vacuum transistors (VTs) are promising candidates in electronics due to their fast response and ability to function in harsh environments. In this study, several oscillator and logic gate circuit simulations using VTs are demonstrated. Silicon-gated field emitter arrays (Si-GFEAs) with 1000 × 1000 arrays were used experimentally to create a VT model. First, transfer and output characteristics sweeps were measured, and based on those data, an LTspice vacuum transistor (VT) model was developed. Then, the model was used to develop Wein and Ring oscillator circuits. The circuits were analytically simulated using LTspice, where the collector bias voltage was 200 V DC, and the gate bias voltage was 30–40 V DC. The Wein oscillator circuit produced a frequency of 102 kHz with a magnitude of 26 Vpp. The Ring oscillator produced a frequency of 1.14 MHz with a magnitude of 4 Vpp. Furthermore, two logic circuits, NOR and NAND gates, were also demonstrated using LTspice modeling. These simulation results illustrate the feasibility of integrating VTs into functional integrated circuits and provide a design approach for future on-chip vacuum transistors applied in logic or radio-frequency (RF) devices.

Five-fold twin structures in sputter-deposited nickel alloy films

The deposition of a Ni-1at. % P film resulted in the facilitation of several quintuple twin junctions that give rise to a five-fold twin structure along the film's growth direction. This type of grain growth promoted a pentagonal pyramid surface topology. Upon increasing the deposition rate or the addition of cobalt to reduce the stacking fault energy, an increase in the areal density of such features was observed. Correlated atomic contrast imaging by electron microscopy with atom probe tomography revealed the preference of phosphorus and residual oxygen Cottrell atmospheres around a dislocation network that would be required to accommodate the misorientation strain from a five-fold twin structure filling space. The ability to tune these structures, over a large area of material, offers grain boundary engineering opportunities to readily create novel surface topologies that can act, for example, as an array of field emitters or textured surfaces.

Anode-Integrated GaN Field Emitter Arrays for Compact Vacuum Transistors

Anode-Integrated GaN Field Emitter Arrays for Compact Vacuum Transistors

Quantitative Field Emission Imaging for Studying the Doping-Dependent Emission Behavior of Silicon Field Emitter Arrays.

Field emitter arrays (FEAs) are a promising component for novel vacuum micro- and nanoelectronic devices, such as microwave power amplifiers or fast-switching X-ray sources. However, the interrelated mechanisms responsible for FEA degradation and failure are not fully understood. Therefore, we present a measurement method for quantitative observation of individual emission sites during integral operation using a low-cost, commercially available CMOS imaging sensor. The emission and degradation behavior of three differently doped FEAs is investigated in current-regulated operation. The measurements reveal that the limited current of the p-doped emitters leads to an activation of up to 55% of the individual tips in the array, while the activation of the n-type FEA stopped at around 30%. This enhanced activation results in a more continuous and uniform current distribution for the p-type FEA. An analysis of the individual emitter characteristics before and after a constant current measurement provides novel perspectives on degradation behavior. A burn-in process that trims the emitting tips to an integral current-specific ideal field enhancement factor is observed. In this process, blunt tips are sharpened while sharp tips are dulled, resulting in homogenization within the FEA. The methodology is described in detail, making it easily adaptable for other groups to apply in the further development of promising FEAs.

Corrections to “Reducing Screening Effect in Close-Packed Si Field Emitter Arrays” [Aug 23 4387-4393

Corrections to “Reducing Screening Effect in Close-Packed Si Field Emitter Arrays” [Aug 23 4387-4393

Reducing Screening Effect in Close-Packed Si Field Emitter Arrays

Electric field screening effect in close-packed semiconductor field emitter arrays (FEAs) changes the current distribution among emitter tips, such that those on the edges of the array emit much higher current than those in the middle, leading to poor reliability characteristics. Using analysis, numerical simulations, and experimental verification, three different techniques to suppress electric field screening and achieve uniform current distribution are proposed. These techniques are: 1) utilizing very sharp emitter tips; 2) reducing the anode–cathode distance (ACD) to be in the order of emitter pitch; and 3) using extremely low-doped but long field emitter tips. By adopting these techniques together, a more uniform emission from the array and a high emission current density can be achieved. FEAs with 160 K emitter tips with different ACDs and emitter sharpness were fabricated using electron beam lithography. In an array with sharp emitter tips, reliable emission with a maximum current of 0.7 mA (4.4 nA/tip) and a maximum current density of 1.75 A/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{2}}$</tex-math> </inline-formula> was achieved.

Effects of gases on the field emission performance of silicon gated field emitter array

Effects of gases on field emission performance were measured using silicon-gated field emitter arrays. Gas was injected into a vacuum chamber with a 1000 × 1000 tip array, which was driven by a DC gate and collector voltages. The collector voltage was fixed at 200 V while the gate voltage was swept to 40 V. For the gas exposure study, N2, He, and Ar were used. The sets of partial pressures, 5 × 10−6, 5 × 10−5, and 5 × 10−4 Torr, were used for the experiment. It was observed that N2 had the least effect and Ar had the worst effect on emission current performance. The degradation of collector current at 5 × 10−4 Torr pressure for Ar was ≈65% where for the N2, at the same level of pressure, the degradation was ≈41%. However, further experiments with high purity Ar gas showed that it was the water vapor present in the gas itself that was the primary cause of reduction in emission current and not the gas itself. The results expressed in reduction in emission current versus Langmuir exposure versus the current clearly showed the effect of water vapor. After the vacuum was recovered, the work function again restored partially to its original value. After ultraviolet light cleaning, the emission current was restored completely to the original state.

Lateral nanoscale field emission comb for frequency mixing

Nanoscale field emission devices are promising candidates to design high-frequency electronics due to the lack of scattering in the vacuum channel that enables ballistic transport. In-plane devices are relatively easy to fabricate with current fabrication techniques and offer sub-fF capacitance. In this work, the characteristics of lateral gold multi-tip field emission arrays are studied. Vacuum gaps between the electrodes of 30 nm are fabricated, which allow &amp;lt; 10 V operation. The effect of number of emitting tips on measured current is investigated. By taking advantage of the strong non-linearity in the emission characteristic, frequency mixing in the MHz range is also demonstrated.

Demonstration of gallium oxide nano-pillar field emitter arrays

We demonstrate field emission characteristics of β-Ga2O3 nano-pillar arrays fabricated using a damage-free etching technique. The technique utilizes Ga flux in an ultra-high vacuum environment (molecular beam epitaxy) to form high aspect ratio Ga2O3 nano-pillars with atomic-scale etching precision. Electrically conductive Ga2O3 nano-pillars with uniform widths of ∼200–300 nm were realized without the use of e-beam lithography. Furthermore, field emission characteristics on the nano-pillars displayed an emission current of 19 nA per tip at an electric field of 385 kV/cm. The field emission characteristics were modeled using the Murphy–Good model, and the measurements were validated with a field emission orthodoxy test.

Effect of praseodymium coating on electron emission from a nanoscale gold field emitter array

Although field emission devices are inherently robust to high temperature and radiation environments as well as have high switching speeds, their development has been hindered by high voltages that are typically required for their operation. In this work, we investigate the effect of thin-film praseodymium (Pr) coating on the emission characteristics of a lateral gold (Au) field emitter array. Because Pr has a significantly lower work function than Au, it is expected to increase the field emission measured current. Pr is deposited onto the device via thermal angled evaporation in a custom-built vacuum chamber with in situ electrical characterization capability. Our experiments demonstrate that a 10 nm-thick Pr layer reduces the turn-on voltage by almost half compared to the noncoated Au structure. These results are promising for the development of power-efficient, low voltage field emission electronics.