Fabrication of ZnO Nanowire Field-Emitter Arrays With ... - IEEE Xplore

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 65, NO. 5, MAY 2018

Fabrication of ZnO Nanowire Field-Emitter Arrays With Focusing Capability Yuanming Liu, Long Zhao, Zhipeng Zhang, Daokun Chen, Guofu Zhang, Juncong She, Shaozhi Deng , Ningsheng Xu, and Jun Chen , Member, IEEE

Abstract — ZnO nanowire field-emitter arrays (FEAs) show great application potentials in large-area vacuum microelectronics devices. However, the electron divergence is an issue which should be addressed. In this paper, ZnO nanowire FEAs with co-planar focus electrode were designed. A four-mask fabrication process and lift-off process were adopted to achieve the integration of focus electrode. The device demonstrated line-addressing and focusing capability with a spacing of 5 mm between anode and cathode, and the linewidth of field-emission images was reduced from 2.25 to 1.35 mm when the focus voltage changed from 50 to −50 V. Besides, line-shaped emission patterns were observed during focusing process, which was attributed to the emission pattern from the ZnO nanowire and the asymmetrical focusing electrical field. Index Terms — Field emission, field-emitter arrays (FEAs), focus electrode, ZnO nanowire.

I. I NTRODUCTION

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ARGE-area field-emitter arrays (FEAs) have important applications such as field-emission display (FED) and liquid crystal display backlight [1]–[3]. Although FED has failed to be commercialized, the corresponding research demonstrates the feasibility of large-area flat-panel electron source. For the new possible application of large-area FEAs, addressable flat-panel X-ray source would be a promising candidate [4]–[7]. Combined with the address ability of gated FEAs device, addressable X-ray emission may be achieved, which would reduce the dosage of X-ray imaging greatly. Besides, it is expected to reduce the footprint of the imaging system significantly, and portable X-ray imaging system would be realized [8].

Manuscript received January 11, 2018; revised March 3, 2018; accepted March 15, 2018. Date of publication April 2, 2018; date of current version April 20, 2018. This work was supported in part by the National Key Research and Development Program of China under Grant 2016YFA0202001, in part by the Science and Technology Department of Guangdong Province, in part by Fundamental Research Funds for the Central Universities, and in part by the Guangzhou Science Technology and Innovation Commission under Grant 201504010012. The review of this paper was arranged by Editor M. M. Cahay. (Corresponding author: Jun Chen.) The authors are with the State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, School of Electronics and Information Engineering, Sun Yat-sen University, Guangzhou 510275, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2018.2817290

The 1-D nanomaterials, such as nanowires and nanotubes, have high aspect ratio and thus low turn-ON voltages for field emission could be achieved [9]. Furthermore, by adopting the in situ growth process [13], field emitter could be fabricated on large area at low cost without using the sophisticated fabrication process which is required for typical Spindt cathode [10], [11]. Therefore, 1-D nanomaterial field emitters were extensively studied recently [9]. Among various 1-D nanomaterials, ZnO nanowires have excellent properties with high stability, high emission current, and good controllability [12]–[17]. It worthy to note that ZnO nanowires could be prepared by a thermal oxidation method, in which ZnO nanowires can be grown directly from the Zn layer without any catalyst [18], [19]. The method enables the easy integration of ZnO nanowires in the FEAs device structure, and favors the large-area preparation. By using this method, large-area ZnO nanowires with excellent field emission have been reported and addressable ZnO nanowire FEAs have been realized [20]. Also large-area diode structure ZnO nanowire cold cathode flat-panel X-ray source has been demonstrated [8]. However, in order to realize the application of ZnO nanowire FEAs, problem of electron beam divergence must be addressed. As there is a distance between cathode and anode, when electrons are emitted from the surface of cold cathode, the gate electric field would drive electrons away from the axis [21], [22]. The divergence of electron beam would reduce the resolution of FEAs device and may lead to the overlapping of field-emission images. This problem become more serious in the application of X-ray source, in which a large spacing between cathode and anode is required for the high anode voltage to ensure sufficient energy of the bombarding electron. In the previous studies of Spindt-type field emitter, focus electrode serving as the electrostatic lenses have been studied to address the divergence problem. In the Spindt-type field-emitter structure, several types of focus electrode have been proposed such as double-gated type [23], [24], in-plane type [25], [26], to solve the divergence problem. However, for the ZnO nanowire FEAs, it remains a challenge. Since ZnO nanowires were prepared using an in situ growth method [18], [19], the interface between the Zn film and the cathode electrode has an important effect on the growth of nanowires. The conventional etching process would induce contamination and damage to the surface of the cathode electrode, which inhibit the growth of nanowires. Exploring

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LIU et al.: FABRICATION OF ZnO NANOWIRE FIELD-EMITTER ARRAYS

Fig. 1. Schematic of ZnO nanowire FEAs with focus electrode. (a) Top view. (b) Isometric view.

the focusing structure compatible with ZnO nanowire FEA fabrication process would be a problem. In this paper, ZnO nanowire FEAs with co-planar focus electrode was designed and fabricated. The structure, fabrication process, and focusing characteristics of the devices will be reported in detail. II. E XPERIMENTAL M ETHODS The structure of the co-planar focus electrode integrated ZnO nanowire FEAs is shown in Fig. 1. Each cathode pattern has a radius of 30 µm and is surrounded by a ring electrode acting as the control gate. For each pixel, the cathode, gate, and focus electrode were placed from center to outer positions in sequence. Both cathode and control gate are connected to the corresponding bottom electrodes through via-holes, respectively, and the bottom electrodes were arranged in a parallel manner. The designed focus electrodes are placed in the same plane with cathode and gate. As the focus electrode structure along with the control gate and cathode electrode is formed in a single-mask process, the self-alignment of the electrodes can be achieved. ZnO nanowires are formed by a simple thermal oxidation process [18], which could ensure large-area uniform preparation. In this paper, 178 × 178 arrays of ZnO nanowire field emitters with co-planar focus electrode were fabricated on glass substrate [27]. The whole arrays occupy an area

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4.5 cm × 4.5 cm on the substrate. A four-mask fabrication process (i.e., four photolithography processes is needed) was proposed to prepare the designed FEAs device. A lift-off process was adopted to prepare the cathode electrode on which the ZnO nanowires grow, in order avoid the contamination on the surface of electrode induced by the conventional etching process. The details of preparation process can be found in [27]. A brief description is given as follows. First, 120-nm-thick chromium layer was deposited on the glass substrate by magnetron sputtering and patterned as bottom electrodes using a lift-off process. Then, 1.5-µm-thick silicon oxide film was deposited using plasma-enhanced chemical vapor deposition to serve as the insulator layer. Via-holes were obtained by the reactive-ion etching process, which is used to connect the control gate and cathode with corresponding bottom electrodes, respectively. Indium tin oxide layer was deposited using magnetron sputtering and the focus electrode structure was patterned by a single-mask process together with the control gate and cathode. Next, 1600-nm-thick zinc thin film was deposited by electron beam evaporation and patterned on the cathode electrode. Finally, ZnO nanowires were grown by a thermal oxidation process. The samples were put in a quartz furnace in ambient atmosphere at the temperature of about 470 °C for 3 h. The feasibility of the designed focus electrode was investigated by a simulation using commercial software COMSOL Multiphysics (version 4.4) [28]. The morphology of the fabricated FEAs device was examined by scanning electron microscopy (SEM) (Zeiss SUPRA-55). In order to evaluate the addressing and focusing performance, the fabricated ZnO nanowire FEAs panel was assembled with a phosphor screen, which served as the anode. In this way, the image of emitting electrons bombarding on the anode screen could be recorded. Experiments with two groups of different parameters were carried out. For the experiment for investigating the effect of gate and focus electrodes on emission current, a 0.5 mm anode-to-cathode distance was adopted and the anode voltage was 2 kV. A high precise ammeter (Keithley 2657A) was used to accurately record the anode current, which also limited the anode voltage. For the experiment for investigating the effect of focusing voltage on emission pattern, a larger anode-tocathode spacing of 5 mm was used and anode voltage was kept at 6 kV. By using a large spacing, more prominent change of the emission pattern could be observed. All the measurements were conducted in a vacuum chamber with a base pressure lower than 5 × 10−5 Pa. III. R ESULTS AND D ISCUSSION A simulation was conducted using the electrostatics module in COMSOL Multiphysics to investigate the focusing capability of the designed structure. A 2-D model was set up and the schematic of the simulation model is shown in Fig. 2(a). The nanowires were symmetrically distributed along the x-axis with the center as the y-axis. The length of the nanowires is assumed to be 2 µm, whereas the outermost nanowires tilt outward 10°. Electrons were emitted from the top surface of

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Fig. 3. SEM images of the fabricated ZnO nanowire FEAs with co-planar focus electrode. (a) Arrays of the ZnO nanowire FEAs with focus electrode. (b) Single pixel of the ZnO nanowire FEAs. (c) Cross-sectional view of the ZnO nanowires.

Fig. 2. (a) Schematic of electron emission trajectory simulation for the designed device. (b) Diameter of beam spot as a function of Vfocus simulated by COMSOL Multiphysics. Inset: simulation results of the electron trajectory with Vfocus = 50 V (bottom left) and Vfocus = −50 V (top right).

the nanowires with an initial velocity of zero. In the model, the distance between the anode and cathode was 5 mm. Anode voltage (Vanode ) and gate voltage (Vgate ) were set as 6 kV and 130 V, respectively, whereas the cathode was connected to the ground. A voltage (Vfocus) was applied to the focus electrodes, in order to change the electron emission trajectory and minimize the divergence. As shown in Fig. 2(b), with the decrease of the focus voltage from 50 to −50 V, the diameter of beam spot constantly decreases from 3684 to 2490 µm. The results showed that the focusing electrode has an effective control on the emission trajectory of electrons. With the designed fabrication process, ZnO nanowire FEAs with co-planar focus electrode was fabricated successfully. The morphology was observed by SEM as shown in Fig. 3. Fig. 3(a) shows the arrays of the ZnO nanowire FEAs with focus electrode. The ZnO nanowire cathodes were shaped as circular pattern arrays with a center distance of 280 µm. The morphology of a single pixel of the ZnO nanowire FEAs is shown in Fig. 3(b), which clearly shows the structure consists of ZnO nanowire cathode, gate electrode, and focus electrode. The two vias located on the gate and cathode area could also

Fig. 4. Emission image of the device driven with different gate voltages.

been seen, by which the electrical connection to the underlying bottom electrode was realized. A cross-sectional view of the ZnO nanowire cathode is shown in Fig. 3(c). Dense ZnO nanowires were grown on the cathode pad and the length of the as-grown nanowires is about 2 µm. The ZnO nanowire FEAs device was driven successfully and its line-addressing capability was verified. The spacing between the anode and the device was set as 0.5 mm, and the anode was biased at 2 kV. In our experiment, under this anode voltage, the electrical field is not high enough to induce recordable emission. When we further applied the extraction gate voltage, an emission could be observed. Therefore, the field emission from our devices was induced by the combination of the electric field from both the anode and gate electrodes. Single column of the FEAs device was driven and gate electrode was biased from 100 to 150 V with focus electrode grounding. The results of the addressing performance of the fabricated ZnO nanowire FEAs are shown in Fig. 4. The field-emission images indicated that the designed FEAs


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Fig. 5. Anode current-gate voltage characteristics of the focus electrode integrated FEAs with focus electrode grounding. Fig. 6. Field-emission images of one column with Vfocus from 50 to −50 V.

have the capability of line addressing. Besides, anode current (Ianode ) versus gate voltage was recorded during the driving process as shown in Fig. 5, and the fitting curve was shown as the red line in Fig. 5. The emission current could be well controlled by the gate electrode, and a 6.13 µA anode current could be obtained at a gate voltage of 150 V. The results indicated that reliable emission could be achieved. To achieve the focusing of electron beam, negative voltage was always applied to focus electrode, which would suppress the field emission and reduce emission current. Anode currents obtained under different focus voltages were measured to assess the influence of focus voltage. The voltage of gate electrode was biased as 140 V, the anode was kept at 2 kV, and the anode-to-cathode distance is 0.5 mm. When the focus electrode was applied with 0 V, the current is 0.342 µA. And when the voltage decreased to −50 V, the current is 0.176 µA. Typically, the gate electrode is positively biased, whereas the focus electrode is applied with negative voltage. Therefore, the focus structures would suppress the emission current during the focusing process [25], [26]. Compared with the conventional noncoplanar focus electrode structure [29], the focus electrode and gate electrode are located on the same plane in our device structure. It helps in reducing the suppression effect from the focus electrode on the field emission. In order to further explore the capability of focusing and investigate the change of emission pattern with focusing voltage, the spacing between the anode and ZnO FEAs was increased to 5 mm and the anode was biased at 6 kV. The extraction voltage of 130 V was chosen for observing the emission image due to the limit of the insulator layer. Fieldemission images of one column driven with different focus voltages were presented in Fig. 6. The width of line was determined according to the reference marks which were made on the anode screen. Under each focusing voltage, more than five images were photographed, and the final linewidth took the average value. Fig. 7 shows that the linewidth of single column field-emission images could be controlled through focus electrode, and deceased from 2.25 mm with Vfocus = 50 V to 1.35 mm with Vfocus = −50 V. The experimental results

Fig. 7. Variation of width of line with Vfocus from 50 V to −50 V.

could not be quantitatively compared with the simulation results. In the simulation, the model is simplified. In the real experiment, the width and shapes of the beam spot were influenced by more complicated parameters such as space charge, surface state, and the shape of the nanowire tips. However, in this paper, both results reflected the controlling effect of the focusing voltage on the beam size. The similar trends in the two results showed that the designed focused electrode is effective. Careful inspection of Fig. 6 shows that the shape of emission pattern also changed with focus voltage. When the voltage of focus electrode was high enough, several moon-shape or ringshape patterns were observed, which is shown in Fig. 8(a). As the voltage of focus electrode decreased, the emission patterns changed and several sharp line-shaped patterns appeared. A detailed image of typical emission pattern recorded at focus voltage of −40 V is shown in Fig. 8(b) and single sharp lineshaped pattern could be observed from the enlarge emission image. We think that the observed moon-shape or ring-shape patterns may relate to the emission pattern of ZnO nanowires.

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Fig. 8. Field-emission images recorded at (a) Vfocus = 40 V and (b) Vfocus = −40 V. Inset: corresponding enlarged image.

Fig. 9. Schematic of the asymmetrical effect. (a) Typical Spindt-type FEAs structure with integrated focus electrode. (b) ZnO nanowire FEAs structure with integrated focus electrodes.

In [30], we have found that a ring-shape emission pattern could be obtained from single ZnO nanowire, which is attributed to a hot-electron emission process. We proposed the change of the emission pattern with focus voltage might be due to the emission image from the single ZnO nanowire and the asymmetrical electrical field induced by the gate and focus electrodes. Fig. 9(a) shows a typical Spindt cathode with focus electrode. The single tip located at the center of the gate and focus electrodes. Therefore, it is a symmetrical static electrostatic lens. As for the ZnO FEAs, a large number of as-grown nanowires distributed on the cathode pattern, and each nanowire would be in an asymmetrical position relative to the control gate electrode and focus electrode as shown in Fig. 9(b). We proposed that the observed pattern might be caused by the effect of asymmetrical electric field induced by the gate electrode and focus electrode. In order to investigate the asymmetrical electric field effect, a simulation was conducted using COMSOL Multiphysics.


Fig. 10. Simulation results of the electron trajectory with (a) Vfocus = 50, (b) 0, and (c) −50 V.

Single vertical nanowire which was placed at the edge of cathode was acted as the electron emission source. The simulation parameters were set according to the parameters adopted in our measurement mentioned above. The distance between the anode and cathode was 5 mm. Vanode and Vgate were set to 6 kV and 130 V, respectively, whereas the cathode was connected to the ground. The initial emission position and direction of electrons emitted from the nanowire are assumed to be symmetrical, and the effect of the asymmetrical electric field could be reflected by the effect of different focusing voltages on electron trajectory. The simulation results of the electron trajectory with different Vfocus were shown in Fig. 10. As the focus electrode was bias at 50 V, a near symmetric trajectory was shown due to the sufficient emission distance. However, when the Vfocus were reduced to a lower value, the asymmetrical effect became more obvious [Fig. 10(b) and (c)]. As the focus electrode was bias at −50 V [Fig. 10(c)], a large shift of the electron trajectory to the left side could be observed obviously. This indicated that electrons emitted from nanowires would be affected by an asymmetrical electric field. This could cause the distortion of the emission pattern when a focusing voltage is applied. Nevertheless, a simple 2-D simulation is not enough to directly simulate the shape of the emission pattern accurately. Further 3-D simulation work should be carried out to pinpoint the mechanism. IV. C ONCLUSION ZnO nanowire FEAs device with focus electrode was designed and fabricated. The ZnO nanowire FEAs device was driven successfully and line addressing was achieved. Simulation and experimental results verified the focusing capability of the structure, which indicated that the designed focus electrode is helpful for solving the electron divergence problem. Sharp line-shaped emission pattern was observed during the focusing, which was tentatively attributed to the asymmetrical electric effect. The results are significant for promoting the application of large-area ZnO nanowire FEAs. R EFERENCES [1] J. Chen et al., “Field emission display device structure based on doublegate driving principle for achieving high brightness using a variety of field emission nanoemitters,” Appl. Phys. Lett., vol. 90, no. 25, pp. 3105-1–3105-3, Jun. 2007, doi: 10.1063/1.2747192. [2] Y. C. Choi et al., “The high contrast ratio and fast response time of a liquid crystal display lit by a carbon nanotube field emission backlight unit,” Nanotechnology, vol. 19, no. 23, p. 235306, May 2008, doi: 10.1088/0957-4484/19/23/235306.


[3] K. A. Dean, “A new era: Nanotube displays,” Nature Photon., vol. 1, no. 5, pp. 273–275, May 2007, doi: 10.1038/nphoton.2007.64. [4] C. M. Posada et al., “Nitrogen incorporated ultrananocrystalline diamond based field emitter array for a flat-panel X-ray source,” J. Appl. Phys., vol. 115, no. 13, pp. 134506-1–134506-9, Apr. 2014, doi: 10.1063/1.4870928. [5] E. J. Grant, C. M. Posada, C. H. Castaño, and H. K. Lee, “Electron field emission particle-in-cell (PIC) coupled with MCNPX simulation of a CNT-based flat-panel X-ray source,” Proc. SPIE, vol. 7961, pp. 796108-1–796108-11, Mar. 2011, doi: 10.1117/12.878292. [6] E. J. Grant, C. M. Posada, C. H. Castaño, and H. K. Lee, “A Monte Carlo simulation study of a flat-panel X-ray source,” Appl. Radiat. Isotopes, vol. 70, no. 8, pp. 1658–1666, Aug. 2012, doi: 10.1016/j.apradiso.2012.04.011. [7] G. Travish, F. J. Rangel, M. A. Evans, B. Hollister, and K. Schmiedehausen, “Addressable flat-panel X-ray sources for medical, security, and industrial applications,” Proc. SPIE, vol. 8502, pp. 58020L-1–58020L-13, Oct. 2012, doi: 10.1117/12.929354. [8] D. Chen et al., “Transmission type flat-panel X-ray source using ZnO nanowire field emitters,” Appl. Phys. Lett., vol. 107, no. 24, pp. 243105-1–243105-5, Dec. 2015, doi: 10.1063/1.4938006. [9] N. S. Xu and S. E. Huq, “Novel cold cathode materials and applications,” Mater. Sci. Eng., R, Rep., vol. 48, nos. 2–5, pp. 47–165, 2005, doi: 10.1016/j.mser.2004.12.001. [10] C. A. Spindt, “A thin-film field-emission cathode,” J. Appl. Phys., vol. 39, no. 7, pp. 3504–3505, Feb. 1968, doi: 10.1063/1.1656810. [11] C. A. Spindt, I. Brodie, L. Humphrey, and E. R. Westerberg, “Physical properties of thin-film field emission cathodes with molybdenum cones,” J. Appl. Phys., vol. 47, no. 12, pp. 5248–5263, Dec. 1976, doi: 10.1063/1.322600. [12] C. J. Lee, T. J. Lee, S. C. Lyu, Y. Zhang, H. Ruh, and H. J. Lee, “Field emission from well-aligned zinc oxide nanowires grown at low temperature,” Appl. Phys. Lett., vol. 81, no. 19, pp. 3648–3650, Oct. 2002, doi: 10.1063/1.1518810. [13] Y. B. Li, Y. Bando, and D. Golberg, “ZnO nanoneedles with tip surface perturbations: Excellent field emitters,” Appl. Phys. Lett., vol. 84, no. 18, pp. 3603–3605, Apr. 2004, doi: 10.1063/1.1738174. [14] Y. Li et al., “Optimizing the field emission properties of ZnO nanowire arrays by precisely tuning the population density and application in large-area gated field emitter arrays,” ACS Appl. Mater. Interfaces, vol. 9, no. 4, pp. 3911–3921, Jan. 2017, doi: 10.1021/acsami.6b13994. [15] J. Song et al., “Epitaxial ZnO nanowire-on-nanoplate structures as efficient and transferable field emitters,” Adv. Mater., vol. 25, no. 40, pp. 5750–5755, Oct. 2013, doi: 10.1002/adma.201302293. [16] Y. F. Liu et al., “Field emission properties of ZnO nanorod arrays by few seed layers assisted growth,” Appl. Surf. Sci., vol. 331, no. 15, pp. 497–503, Mar. 2015, doi: 10.1016/j.apsusc.2015.01.061. [17] D. Shao et al., “Cl-doped ZnO nanowire arrays on 3D graphene foam with highly efficient field emission and photocatalytic properties,” Small, vol. 11, no. 36, pp. 4785–4792, Jul. 2015, doi: 10.1002/smll.201501411. [18] C. X. Zhao et al., “Large-scale synthesis of bicrystalline ZnO nanowire arrays by thermal oxidation of zinc film: Growth mechanism and high-performance field emission,” Cryst. Growth Des., vol. 13, no. 7, pp. 2897–2905, Jun. 2013, doi: 10.1021/cg400318f.

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[19] Z. Zhang et al., “Controllable preparation of 1-D and dendritic ZnO nanowires and their large area field-emission properties,” J. Alloys Compounds, vol. 690, no. 5, pp. 304–314, Jan. 2017, doi: 10.1016/ j.jallcom.2016.08.123. [20] L. Zhao et al., “Fabrication of large-area arrays of coaxial gated ZnO nanowire field emitters for vacuum microelectronics applications,” in Proc. Int. Vac. Nanoelectron. Conf., Regensburg, Germany, Jul. 2017, pp. 118–119, doi: 10.1109/IVNC.2017.8051570. [21] S. Itoh, T. Watanabe, K. Ohtsu, M. Taniguchi, S. Uzawa, and N. Nishimura, “Experimental study of field emission properties of the Spindt-type field emitter,” J. Vac. Sci. Technol. B, Microelectron. Nanometer Struct., vol. 13, no. 2, pp. 487–490, Jun. 1998, doi: 10.1116/ 1.588339. [22] W. Zhu, Ed., “Spindt field emitter arrays,” in Vacuum Microelectronics. New York, NY, USA: Wiley, 2002, pp. 180–181. [Online]. Available: http://onlinelibrary.wiley.com/doi/10.1002/0471224332.ch4/pdf [23] C. M. Tang et al., “Theory and experiment of field-emitter arrays with planar lens focusing,” in Proc. Int. Vac. Nanoelectron. Conf., Portland, OR, USA, Jul. 1995, pp. 77–80, doi: 10.1109/IVMC.1995.486994. [24] C. M. Tang and T. A. Swyden, “Beam collimation from fieldemitter arrays with linear planar lens,” in Proc. IEEE Int. Conf. Plasma Sci., Boston, MA, USA, Jun. 1996, p. 243, doi: 10.1109/ PLASMA.1996.551467. [25] P. Helfenstein et al., “Highly collimated electron beams from doublegate field emitter arrays with large collimation gate apertures,” Appl. Phys. Lett., vol. 98, no. 6, pp. 061502-1–061502-3, Feb. 2011, doi: 10.1063/1.3551541. [26] Y. Honda et al., “Double-gated, spindt-type field emitter with improved electron beam extraction,” IEEE Trans. Electron Devices, vol. 63, no. 5, pp. 2182–2189, May 2016, doi: 10.1109/TED.2016.2545710. [27] Y. M. Liu et al., “Fabrication of ZnO nanowire field emitter arrays with self-aligned focus electrode structure,” in Proc. Int. Vac. Nanoelectron. Conf., Vancouver, BC, Canada, Jul. 2016, pp. 204–205, doi: 10.1109/IVNC.2016.7551525. [28] M. Tabatabaian, “COMSOL—A modeling tool for engineers,” in COMSOL for Engineers. Boston, MA, USA: Mercury Learning & Information, 2014, pp. 31–43. [29] M. A. Guillorn et al., “Vertically aligned carbon nanofiber-based field emission electron sources with an integrated focusing electrode,” J. Vac. Sci. Technology. B, Microelectron. Nanometer Struct., Process., Meas., Phenom., vol. 22, no. 1, pp. 35–39, Dec. 2004, doi: 10.1116/1.1633768. [30] Y. C. Chen, S. Z. Deng, N. S. Xu, and J. Chen, “Origin of the ring-shaped emission pattern observed from the field emission of ZnO nanowire: Role of adsorbates and electron initial velocity,” Mater. Res. Exp., vol. 1, no. 4, pp. 045050-1–045050-15, Dec. 2014, doi: 10.1088/20531591/1/4/045050.

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