4
Solid State Communications 151 (2011) 1650–1653 Contents lists available at SciVerse ScienceDirect Solid State Communications journal homepage: www.elsevier.com/locate/ssc 2D planar field emission devices based on individual ZnO nanowires Qing Zhao , Cheng-Kuang Huang, Rui Zhu, Jun Xu, Li Chen, Dapeng Yu State Key Laboratory for Mesoscopic Physics, and Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, China article info Article history: Received 27 June 2011 Accepted 4 August 2011 by E.G. Wang Available online 11 August 2011 Keywords: A. ZnO nanowires D. Field emission E. In situ measurement abstract 2D planar field emission devices based on individual ZnO nanowires were achieved on Si/SiO 2 substrate via a standard e-beam lithography method. The anode, cathode and ZnO nanowires were on the same substrate; so the electron field emission is changed to 2D. Using e-beam lithography, the emitter (cathode) to anode distance could be precisely controlled. Real time, in situ observation of the planar field emission was realized in a scanning electron microscope. For individual ZnO nanowires, an onset voltage of 200 V was obtained at 1 nA. This innovative approach provides a viable and practical methodology to directly implement into the integrated field emission electrical devices for achieving ‘‘on-chip’’ fabrication. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction One-dimensional (1D) nanostructures have been intensively studied as cold cathode field emitters due to their small sharp tips and very large aspect ratio [1–3]. Among them, carbon nan- otubes (CNTs) demonstrated outstanding field emission proper- ties and lots of work has been reported [4–8]. Particularly, 1D ZnO nanostructures have been proved to be promising candidates as field emitters due to its peculiar properties of high melting point and high stability under harsh environment as compared to CNTs [9–11]. So far, a number of studies have been performed on the field emission properties of 1D ZnO nanostructures for their pos- sible applications in vacuum micro/nanoelectronics [9–17]. Most of the measurements were performed on ZnO nanowire films or arrays; thus the results represent a collective and average contri- bution of large number of ZnO field emitters. In order to explore the field emission properties of specific pristine ZnO nanoemitters and exclude the screening effect in ZnO nanowire arrays [11], in situ study of field emission from individual ZnO nanowires is highly de- sired. In-situ study of the field emission performance of individual ZnO nanowires generally needs the probe measurement methods taken in characterization instruments. For example, Huang et al. studied the field emission properties of individual ZnO nanowires by a transmission electron microscope [18]. She et al. studied the correlation between resistance and field emission performance of individual 1D ZnO nanostructures in a modified scanning electron microscope (SEM) system [19]. On the other hand, most of the previous reports on ZnO nanowire field emitters were designed in the framework of Corresponding author. E-mail addresses: [email protected] (Q. Zhao), [email protected] (D. Yu). traditional vacuum tubes, where a three-dimensional (3D) tip to plate setup was adopted. Recently, novel two-dimensional (2D) planar field emission devices based on individual ultrathin CNTs have been reported [20]. It demonstrated great advantage over 3D setup in easy integration of circuits on a chip, lowered turn-on field, helping to overcome the overheating problems, etc. Scheme of such device configurations was demonstrated in Fig. 1. Electron field emission has been reduced from 3D to 2D since the cathode, the anode, and the emitters lay on an insulating substrate at the same time. Fig. 1(a) is the so-called tip to electrode (unilateral) configuration, and Fig. 1(b) is the tip to tip (bilateral) configuration. Difference between the two lies in the different electric field distributions. In the unilateral configuration, the electric field at the cathodic emitter tip is much stronger than that at the smooth anodic electrode; so the field emission of electrons is unilateral from the emitter tip to the opposite smooth electrode. While in the bilateral model, the electric field is enhanced both at the cathodic emitter tip and the anodic emitter tip; so the field emission of electrons is bilateral between two opposite emitter tips. Using standard electron beam lithography (EBL), planar field emission devices are readily realized and the electrodes could be flexibly designed and located. One obvious advantage of such design compared to 3D setup is that the distance between the emitter (cathode) and the electrode (anode) could be made very small (<1 µm); so the electric field exerted on the emitter could be strongly enhanced, leading to lower turn-on field and larger emission current. Additionally, such devices are compatible with integrated circuit technology. Furthermore, the material of the emitter can be chosen so as to suit the requirements of corresponding applications such as gas detection, switches, flexible displays, etc. In this work, 2D planar field emission devices based on individ- ual ZnO nanowires were fabricated in unilateral configuration on SiO 2 /Si substrate via EBL technique. One metal electrode deposited 0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.08.010

2D planar field emission devices based on individual ZnO nanowires

Embed Size (px)

Citation preview

Solid State Communications 151 (2011) 1650–1653

Contents lists available at SciVerse ScienceDirect

Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

2D planar field emission devices based on individual ZnO nanowiresQing Zhao ∗, Cheng-Kuang Huang, Rui Zhu, Jun Xu, Li Chen, Dapeng Yu ∗

State Key Laboratory for Mesoscopic Physics, and Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, China

a r t i c l e i n f o

Article history:Received 27 June 2011Accepted 4 August 2011by E.G. WangAvailable online 11 August 2011

Keywords:A. ZnO nanowiresD. Field emissionE. In situ measurement

a b s t r a c t

2D planar field emission devices based on individual ZnO nanowires were achieved on Si/SiO2 substratevia a standard e-beam lithography method. The anode, cathode and ZnO nanowires were on the samesubstrate; so the electron field emission is changed to 2D. Using e-beam lithography, the emitter (cathode)to anode distance could be precisely controlled. Real time, in situ observation of the planar field emissionwas realized in a scanning electron microscope. For individual ZnO nanowires, an onset voltage of 200 Vwas obtained at 1 nA. This innovative approach provides a viable and practical methodology to directlyimplement into the integrated field emission electrical devices for achieving ‘‘on-chip’’ fabrication.

© 2011 Elsevier Ltd. All rights reserved.

1. Introduction

One-dimensional (1D) nanostructures have been intensivelystudied as cold cathode field emitters due to their small sharptips and very large aspect ratio [1–3]. Among them, carbon nan-otubes (CNTs) demonstrated outstanding field emission proper-ties and lots of work has been reported [4–8]. Particularly, 1D ZnOnanostructures have been proved to be promising candidates asfield emitters due to its peculiar properties of high melting pointand high stability under harsh environment as compared to CNTs[9–11]. So far, a number of studies have been performed on thefield emission properties of 1D ZnO nanostructures for their pos-sible applications in vacuum micro/nanoelectronics [9–17]. Mostof the measurements were performed on ZnO nanowire films orarrays; thus the results represent a collective and average contri-bution of large number of ZnO field emitters. In order to explore thefield emission properties of specific pristine ZnO nanoemitters andexclude the screening effect in ZnO nanowire arrays [11], in situstudy of field emission from individual ZnOnanowires is highly de-sired. In-situ study of the field emission performance of individualZnO nanowires generally needs the probe measurement methodstaken in characterization instruments. For example, Huang et al.studied the field emission properties of individual ZnO nanowiresby a transmission electron microscope [18]. She et al. studied thecorrelation between resistance and field emission performance ofindividual 1D ZnO nanostructures in a modified scanning electronmicroscope (SEM) system [19].

On the other hand, most of the previous reports on ZnOnanowire field emitters were designed in the framework of

∗ Corresponding author.E-mail addresses: [email protected] (Q. Zhao), [email protected] (D. Yu).

0038-1098/$ – see front matter© 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2011.08.010

traditional vacuum tubes, where a three-dimensional (3D) tip toplate setup was adopted. Recently, novel two-dimensional (2D)planar field emission devices based on individual ultrathin CNTshave been reported [20]. It demonstrated great advantage over 3Dsetup in easy integration of circuits on a chip, lowered turn-onfield, helping to overcome the overheating problems, etc. Schemeof such device configurations was demonstrated in Fig. 1. Electronfield emission has been reduced from 3D to 2D since the cathode,the anode, and the emitters lay on an insulating substrate at thesame time. Fig. 1(a) is the so-called tip to electrode (unilateral)configuration, and Fig. 1(b) is the tip to tip (bilateral) configuration.Difference between the two lies in the different electric fielddistributions. In the unilateral configuration, the electric fieldat the cathodic emitter tip is much stronger than that at thesmooth anodic electrode; so the field emission of electrons isunilateral from the emitter tip to the opposite smooth electrode.While in the bilateral model, the electric field is enhanced both atthe cathodic emitter tip and the anodic emitter tip; so the fieldemission of electrons is bilateral between two opposite emittertips. Using standard electron beam lithography (EBL), planar fieldemission devices are readily realized and the electrodes couldbe flexibly designed and located. One obvious advantage of suchdesign compared to 3D setup is that the distance between theemitter (cathode) and the electrode (anode) could be made verysmall (<1 µm); so the electric field exerted on the emittercould be strongly enhanced, leading to lower turn-on field andlarger emission current. Additionally, such devices are compatiblewith integrated circuit technology. Furthermore, the material ofthe emitter can be chosen so as to suit the requirements ofcorresponding applications such as gas detection, switches, flexibledisplays, etc.

In this work, 2D planar field emission devices based on individ-ual ZnO nanowires were fabricated in unilateral configuration onSiO2/Si substrate via EBL technique. Onemetal electrode deposited

Q. Zhao et al. / Solid State Communications 151 (2011) 1650–1653 1651

Cathode

Anode

Cathode/Anode

Anode/Cathode

Fig. 1. Scheme of the unilateral (a) and bilateral (b) planar field emission devices. Nanowires and metal electrodes were deposited on Si/SiO2 substrate. In the asymmetricdesign (a), the field emission of electrons is unilateral from the emitter tip to the opposite electrode. In the symmetric design (b), the field emission of electrons is bilateralbetween the two emitter tips.

on ZnO nanowires served as cathode and the othermetal electrodeserved as anode. Real time observation and in situ measurementof field emission were realized in SEM chamber. By EBL technique,a precision control of the tip to electrode distance was achieved.Using this method, field emission performance of individual ZnOnanowires was investigated. Remarkably, due to its versatility, thisapproach can be directly implemented into the pre-integrated fieldemission electrical devices for achieving ‘‘on-chip’’ fabrication.

2. Experimental

ZnO nanowires in this investigation were synthesized via asimple chemical vapor deposition method. 0.1 g Zn powder and0.1 g ZnO powder were mixed and put into an alumina boat. Apiece of Si wafer was put downstream of the alumina boat ascollecting substrate. Subsequently, the alumina boat was placedat the center of a quartz tube and inserted into a rapid heatingfurnace. Argon was used to clean the furnace for 10 min and thenset to 80 sccm as carrier gas during growth. The furnacewas heatedup to 800 °C in 20 min and held for 15 min and then cooleddown to room temperature naturally. Oxygen (1.0 sccm) wasadded as the reactive gas when the furnace temperature reached800 °C. After growth, the substrate was covered by a layer of ZnOnanowires. Typical SEM (FEI, Quanta 200F) image was shown inFig. 2(a). ZnO nanowires were uniformly distributed on substrate,with length from 10 to 20 µm, and diameter from 50 to 300 nm.After dispersion in ethanol, ZnO nanowires were transferred ontoa pre-marked, highly doped Si substrate with 800 nm SiO2 layerusing glass tips, as reported elsewhere [21]. Standard EBL and lift-off process were subsequently used to deposit metal electrodes(20 nm Ti and 130 nm Au) on the individual ZnO nanowiresprecisely to form a unilateral field emission configuration, asshown in Fig. 1(a). Fig. 2(b)–(d) depict SEM images of the unilateralfield emission devices with different tip to electrode distance. Thedistance between individual ZnO nanowire emitters and anodeelectrode was 400 nm, 3.7 µm, 3 µm in Fig. 2(b)–(d), respectively.Through EBL approach, the distance between emitter and anodecould be precisely pre-defined and controlled, and especially, suchdistance could be reached very small (<1 µm).

Field emission measurements of the planar field emissiondevices of the individual ZnO nanowires were conducted in a SEMchamber (FEI, Quanta 200F) under a vacuum of 2 × 10−6 Torr.The electrodes were ultrasonically bonded to peripheral circuitswith Al wires for electrical measurements. A Keithley-6430 Sub-Femtoamp Remote SourceMeter together with its pre-amplifierwas used to apply the voltage and measure the emission currentwith the voltage ramp steps of 1 V. The peripheral circuit resistanceof our setup was less than 5 �. Fig. 3(a) is a schematic illustrationof the current measurement setup. The leakage current betweena pair of blank electrodes with no ZnO nanowires on the same

Si/SiO2 substrate was measured and the value was less than 30 pAunder 200 V bias. The measured current higher than ∼50 pA wassafely taken as the field emission current. The electron beam ofthe SEM was shut off when the I–V data of the field emission wasacquired.

3. Results and discussion

The real time in situ observation of field emission fromindividual ZnO nanowires is very challenging and has rarely beenreported. Here, Fig. 3(b) and (c) demonstrate the real time, insitu SEM images of unilateral field emission device based onindividual ZnO nanowires, when bias voltage was applied. InFig. 3(b), the contrast of the ZnO nanowires and the cathodeelectrode was brighter, with the darker contrast of the anodeelectrode. The situation in Fig. 3(c) is vice versa, showing a muchbrighter cathode electrode and darker anode with ZnO nanowires.It should be pointed out that field emission from the nanowirescould be obtained only in the configuration in Fig. 3(b), since thenanowires should act as cathode to emit electrons. From Fig. 3(b)and (c), the contrast of the ZnO nanowires and the electrodes wasinverted on the reversal of applied voltage, because of the voltageenhanced/depressed secondary electron emission in SEM system.The positive anode was darker because they drewmore secondaryelectrons back, while the negative cathode was brighter becausethey expelled more secondary electrons out. Therefore, regionswith the same brightness in the SEM images were equipotentialin the field emission circuit. And it was confirmed that the ZnOnanowire acted as real cathode and the ZnO nanowire was ingood contact with the metal electrode. The result was a directexhibition of the unilateral planar field emission from individualZnO nanowire within SEM.

Fig. 3(d) is the obtained emission current-applied voltage curveof the planar field emission device based on individual ZnOnanowires shown in Fig. 3(b). From the inset of Fig. 3(b), thenanowire is ∼9 µm long, ∼100 nm in diameter and the distancebetween the nanowire tip and the anode is ∼675 nm. Such smallnanowire tip to anode distance could not be easily achieved usingother methods in 3D setup, demonstrating the great advantage ofour approach. The expected one-way conductivity was confirmedfor the unilateral configuration. Since leakage current is indepen-dent of the direction of bias voltage, the unilateral I–V curve gavestrong evidence of electron field emission from ZnO nanowires.The onset voltage (defined at 1 nA for individual ZnO nanowires)is 200 V. This result is comparable with the onset voltage (115 V)of a much thinner CNT in 3D field emission setup [22], where thetip to electrode distance is ∼1 µm. Note that the CNT they used isvery thin, only ∼10 nm in diameter, which is an order of magni-tude smaller than that of the ZnO nanowires we used. It is knownthat the local electric field is inversely proportional to the radius

1652 Q. Zhao et al. / Solid State Communications 151 (2011) 1650–1653

a b

c d

Fig. 2. (a) Typical SEM image of the as-grown ZnO nanowires. (b–d) SEM images of planar field emission devices in unilateral configurations based on individual ZnOnanowires with different tip to electrode distances. The distance between individual ZnO nanowire emitters and anode electrode was 400 nm, 3.7 µm, 3 µm in (b), (c), (d),respectively. Inset of (b): Magnified SEM image showing the close distance between nanowire tip to the opposite electrode.

Fig. 3. (a) An illustration of the measurement setup in a SEM with a vacuum environment of 10−6 Torr. A Keithley-6430 Sub-Femtoamp Remote SourceMeter was used tosource the voltage and measure the current. (b, c) Real time, in situ SEM images of a unilateral planar field emission device when the voltage was applied. (b) The electrodewith the individual ZnO nanowire was negatively charged as cathodes and the ZnO nanowire connected to it was emitting electrons. (c) When the applied voltage wasinverted, the electrode with the individual ZnO nanowire was positively charged as anode and there is no field emission. Inset of (b): Magnified SEM image of the nanowiretip and the opposite electrode, revealing the very close distance between them. (d) Field emission I–V curve of the planar field emission device showing in (b). Inset:Corresponding FN plot.

Q. Zhao et al. / Solid State Communications 151 (2011) 1650–1653 1653

curvature of the emitter [19], so the field emission performanceof ZnO nanowires is expected to improve significantly if thinnernanowires are used. Since the field emission performance of in-dividual ZnO nanowires is comparable with that of much thinnerindividual CNTs, the 2D planar field emission setup demonstratessuperior advantage and promising potential in future field emis-sion applications.

The FN field emission theory is basically the rationalization ofthe quantum tunneling phenomenon [23]. The theory has beenproved useful in describing the relationship between the fieldemission current I and the local field F at the emitter surface. Fis conventionally written as F = βV/d, where V is the appliedvoltage, β is the geometric enhancement factor, and d is thetip–electrode distance. The law relating I and F is thus writtenas [24]

I = A1.5 × 10−6

φ

Vd

2

β2 exp10.4√

φ

× exp

−6.44 × 109φ1.5d

βV

(1)

where A has the dimension of an area m2 and φ is the workfunction in eV of the emitting material. From Eq. (1), it can beobserved that if ln(I/V 2) is plotted against 1/V , then, at emissionfield range, one will arrive at a linear function with a slope of−6.44 × 109φ1.5d/β; this is the so called FN plot. By fitting theexperimental data in a FN plot, either φ or the field enhancementfactor β can be determined. The FN plot has also been used overthe past as a supporting evidence for field emission. From the insetof Fig. 3(d), the linear dependence of the corresponding FN plotsreveals that the current we acquired is field emission current fromindividual ZnO nanowires.

In field emission in our configuration, electrons are actuallypulled out from the ZnO nanowire tip by the tip enhanced localelectric field due to its very small curvature, but not the meanelectric field between the anode and cathode. Therefore, fieldemission conditions could be matched before any breakdown ofthe insulating surface. The thickness of SiO2 layer on substratewas set to 800 nm in order to avoid this problem as well.Experimentally, it is easy to distinguish the one-way field emissionfrom the leakage current in a unilateral field emission devices sincethe leakage current between the anode and cathode is independentof the direction of bias voltage. In addition, several methodsare expected to further lower the onset voltage of our device,such as reducing the emitter tip to electrode distance, growth ofthinner and longer nanowires, and decreasing the resistivity of theemitting materials.

4. Conclusions

In conclusion, using a combination of standard EBL techniqueand in-situ measurement of field emission in SEM, we demon-strated the innovative fabrication of 2D planar field emission de-vices based on individual ZnO nanowires on Si/SiO2 substrate. ByEBL process, we were able to control the location and the distancebetween the tip and electrode precisely, which is crucial for field

emission. Real time observation and in situ I–V measurement ver-ified that the current is actually from the field emission of indi-vidual ZnO nanowires. This approach could provide a viable andpractical methodology for the creation of range of complex inte-grated circuits of field emission devices on a chip. In light of theinherently parallel nature of the EBL technique, we expect thatthe present planar field emission device fabrication approach mayopen up a possible new avenue for the development of individual1D nanostructure-based cathode for vacuum nanoelectronics.

Acknowledgments

This project is financially supported by theNational Natural Sci-ence Foundation of China (NSFC 50902004 and 11023003), andNational 973 projects (No. 2007CB936202, 2009CB623703, MOST)from China’s Ministry of Science and Technology and the ResearchFund for the Doctoral Program of Higher Education. We acknowl-edge the International Science and Technology Cooperation Pro-gram of China, Sino–Swiss Science and Technology CooperationProgram (2010DFA01810); and FP7 EU IRSES project (MICROCARE)No. 247641.

References

[1] J.M. Bonard, C. Klinke, K.A. Dean, B.F. Coll, Phys. Rev. B 67 (2003) 11.[2] Q. Zhao, X.Y. Xu, X.F. Song, D.P. Yu, C.P. Li, L. Guo, Nanotechnology 17 (2006)

S351.[3] J.M. Bonard, J.P. Salvetat, T. Stockli, L. Forro, A. Chatelain, Appl. Phys. A 69

(1999) 245.[4] Y. Saito, S. Uemura, Carbon 38 (2000) 169.[5] W.I. Milne, K.B.K. Teo, G.A.J. Amaratunga, P. Legagneux, L. Gangloff, J.P. Schnell,

V. Semet, V.B. Thien, O. Groening, J. Mater. Chem. 14 (2004) 933.[6] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179.[7] Z.W. Pan, C.F. Au, H.L. Lai, W.Y. Zhou, L.F. Sun, Z.Q. Liu, D.S. Tang, C.S. Lee,

S.T. Lee, S.S. Xie, J. Phys. Chem. B 105 (2001) 1519.[8] S.S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J. Dai,

Science 283 (1999) 512.[9] Y.W. Zhu, H.Z. Zhang, X.C. Sun, S.Q. Feng, J. Xu, Q. Zhao, B. Xiang, R.M. Wang,

D.P. Yu, Appl. Phys. Lett. 83 (2003) 144.[10] Q. Zhao, X.Y. Xu, X.F. Song, X.Z. Zhang, D.P. Yu, C.P. Li, L. Guo, Appl. Phys. Lett.

88 (2006) 033102.[11] Q. Zhao, H.Z. Zhang, Y.W. Zhu, S.Q. Feng, X.C. Sun, J. Xu, D.P. Yu, Appl. Phys. Lett.

86 (2005) 203115.[12] G.Z. Shen, Y. Bando, B.D. Liu, D. Golberg, C.J. Lee, Adv. Funct. Mater. 16 (2006)

410.[13] Q. Wan, K. Yu, T.H. Wang, C.L. Lin, Appl. Phys. Lett. 83 (2003) 2253.[14] A. Wei, X.W. Sun, C.X. Xu, Z.L. Dong, M.B. Yu, W. Huang, Appl. Phys. Lett. 88

(2006) 213102.[15] U.K. Gautam, L.S. Panchakarla, B. Dierre, X.S. Fang, Y. Bando, T. Sekiguchi,

A. Govindaraj, D. Golberg, C.N.R. Rao, Adv. Funct. Mater. 19 (2009) 131.[16] Q. Zhao, J.Y. Gao, R. Zhu, T.C. Cai, S. Wang, X.F. Song, Z.M. Liao, X.H. Chen,

D.P. Yu, Nanotechnology 21 (2010) 095701.[17] Q. Zhao, T.C. Cai, S. Wang, R. Zhu, Z.M. Liao, D.P. Yu, Appl. Phys. A 100 (2010)

165.[18] Y. Huang, X. Bai, Y. Zhang, J. Qi, Y. Gu, Q. Liao, J. Phys.: Condens. Matter. 19

(2007) 176001.[19] J.C. She, Z.M. Xiao, Y.H. Yang, S.Z. Deng, J. Chen, G.W. Yang, N.S. Xu, ACS Nano

2 (2008) 2015.[20] X.F. Song, J.Y. Gao, Q. Fu, J. Xu, Q. Zhao, D.P. Yu, Nanotechnology 20 (2009)

405208.[21] X.B. Han, L.Z. Kou, X.L. Lang, J.B. Xia, N. Wang, R. Qin, J. Lu, J. Xu, Z.M. Liao,

X.Z. Zhang, X.D. Shan, X.F. Song, J.Y. Gao, W.L. Guo, D.P. Yu, Adv. Mater. 21(2009) 4937.

[22] J.M. Bonard, K.A. Dean, B.F. Coll, C. Klinke, Phys. Rev. Lett. 89 (2002) 19.[23] R.H. Fowler, L. Nordheim, Proc. R. Soc. Lond. Ser. A 119 (1928) 781.[24] J.W. Gadzuk, E.W. Plummer, Rev. Modern Phys. 45 (1973) 487.