Field emission properties of zinc oxide nanowires fabricated by thermal evaporation

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Physica E 36 (2007) 86–91 www.elsevier.com/locate/physe

Field emission properties of zinc oxide nanowires fabricated by thermal evaporation Weiwei Wang, Gengmin Zhang, Ligang Yu, Xin Bai, Zhaoxiang Zhang, Xingyu Zhao Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China Received 16 February 2006; received in revised form 6 August 2006; accepted 16 August 2006 Available online 6 October 2006

Abstract Arrays of randomly oriented zinc oxide (ZnO) nanowires were fabricated on silicon wafers via a simple thermal evaporation method. During the fabrication, the temperature around the substrate was below 500 1C. The products were analyzed by conventional means and determined to be single crystals of wurtzite-type ZnO that grew along the c-axis. These nanowires were 10–100 nm in diameter and 10–100 mm in length, suggesting a possible high field enhancement factor. The dependence of the field emission current on the anode–cathode voltage (I–V behavior) of the ZnO nanowire arrays was measured in a lab-built ultrahigh vacuum system with a base pressure of 107 Pa. After surface cleaning by heat treatment, two characteristic electric fields, under which 10 mA/cm2 and 1 mA/cm2 current densities were extracted, were measured to be 4.0 and 4.7 V/mm, respectively. As observed with a transparent anode, emission occurred uniformly over the whole sample surface. A 72 h-long test on emission stability was performed under a constant voltage of 2.75 kV. The current dropped occasionally to approximately 80% of the initial value during the test owing to the poor adherence of the nanowires to the substrate. These preliminary results have shown the perspective of, as well as a major drawback to, a ZnO nanowire array being developed into a cold electron source to be used in future electronic devices. r 2006 Elsevier B.V. All rights reserved. PACS: 79.70.+q; 81.07.b; 81.16.c; 85.45.Db Keywords: Zinc oxide nanowire; Field emission; Thermal evaporation; Silicon substrate; Adherence

1. Introduction Field emitters have advantages over thermionic cathodes such as low energy consumption, instant turn-on capability, small energy spread and, in some cases, no requirement for a focusing system [1]. At present, onedimensional nanometer-scale materials have proven to be promising candidates for cold cathodes that can deliver electrons at low fields. Compared with the traditional Spindt-type field emitter arrays (Spindt FEAs) [2], the new-type field emitters are relatively easy to fabricate, especially over a large area. In this regard, the carbon nanotube (CNT) is the leading material by a long way. A prototype of a CNT-based field emission Corresponding author. Tel.: +86 10 627 51773; fax: +86 10 627 62999.

E-mail address: [email protected] (G. Zhang). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.08.008

display (FED) has already been developed [3]. Moreover, recently some researchers have argued that a CNT field emitter could be used as a good electron source in a portable betatron [4] and even a free electron laser (FEL) [5]. Meanwhile, some one-dimensional semiconductor materials are also drawing the attention of researchers [6]. Among these materials, different nanostructures of zinc oxide (ZnO) are particularly interesting. Field emission has been obtained from well-aligned nanowires, nanotubes and nanopins of ZnO [7–10]. As reported in this paper, we managed to fabricate ZnO nanowire arrays using a simple and inexpensive thermal evaporation method and then systematically studied their field emission properties. Encouraging results were achieved. The major problem to be tackled is also indicated.

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2. Sample preparation and analysis 2.1. Sample preparation Our fabrication approach was based on a simple thermal evaporation method [11] in which the frequently used metallic catalysts and vacuum environment were not required. The synthesis process was carried out in a quartz tube that was inserted into a horizontal furnace. The tube was 28 mm in inner diameter, 3 mm in thickness and 1.2 m in length. A quartz boat that contained the source materials, i.e., powders of ZnO (99.0% purity) and graphite mixed in a molar ratio of 1:2, was pushed to the middle of the tube. The substrate, a silicon wafer, was placed in another quartz boat near the downstream outlet of the tube. The temperature would not be uniform along the tube during heating. It would be highest in the middle of the tube and about 500 1C lower around the substrate. Before heating began, argon was introduced into the tube with a flow of 40 cm3/min for a sufficiently long period, approximately 40 min. Then the furnace was switched on and the argon flow was lowered to 20 cm3/min. It took about 18 min for the temperature in the middle of the tube to reach 960 1C and that around the substrate to 440 1C accordingly. Then the relay started to work and the tube temperature was kept unchanged for 10 min. During this process, the source materials reacted with each other and the evaporated products were transported to the substrate, where the synthesis of ZnO nanostructures occurred. Finally, the furnace was switched off and the sample was allowed to cool down to room temperature in the tube with the gas still flowing. As will be shown in the following section, the above condition was found to yield non-aligned ZnO nanowires. Actually, the configuration of the products depended very sensitively on the fabrication conditions, including the temperatures of the zinc source and the silicon substrate, the flow of argon gas, the time of the temperature increase and the time of the reaction. Generally, the softening point of the display glass is in the range of 500–600 1C [12]. We intentionally controlled the substrate temperature below 500 1C in our sample fabrication. ZnO nanowires were achieved on the substrate regardless of this relatively low temperature. In this sense, it would be relatively easy to incorporate our fabrication approach in a mass production process of FEDs. 2.2. Sample analysis and characterization The products of the above fabrication were then observed with a scanning electron microscope (SEM) and the results are shown in Fig. 1. The silicon wafer was found to be covered with a layer of randomly oriented nanowires, whose diameter and length were in the range of approximately 10–100 nm and 10–100 mm, respectively. Thus, the aspect ratio of these nanowires was of the order of 103, which could result in a large field enhancement factor.

Fig. 1. SEM images of randomly oriented nanowires on a silicon wafer: (a) low magnification; (b) medium magnification; (c) high magnification; and (d) side view. The scale bars in the four images are 50, 20, 6 and 20 mm, respectively.

The chemical composition of the product was first revealed by X-ray photoelectron spectroscopy (XPS) analysis (Fig. 2). Besides a photoelectron line for silicon, which obviously originated from the substrate, only lines for zinc and oxygen can be found in Fig. 2, hence it can be affirmed that the nanowires were only composed of zinc and oxygen.

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Zn2p3/2

160000 140000

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O1s Zn2p1/2

100000 80000 Zn (L3M4, 5M4, 5) 60000 40000

Si2s

20000 0 1200

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0

Binding Energy (eV) Fig. 2. Result of the XPS analysis. (No charging correction has been done. The ordinate is the counts of electrons per unit time.)

Si

12

intensity (cps)

Furthermore, the chemical shift of the zinc element was utilized to determine the chemical state of zinc in our sample. Zinc is an element with a small photoelectron shift and much larger Auger shift in different chemical environments. Hence, the modified Auger parameter, rather than the individual photoelectron lines, is a more effective spectral feature for the investigation of its chemical state. The definition of the Auger parameter, a, is

10 (100)

8

(002)

(101) (102)

6

(110) (103) (004) (202)

4

a ¼ EðAugerÞ2EðPEÞ, 2

where E(Auger) and E(PE) are the kinetic energies of the Auger electron and the photoelectron, respectively [13]. The modified Auger parameter, a*, differs from a by a constant: a ¼ a þ hv, where hn is the energy of the X-ray photon [13]. For zinc, the commonly used Auger electron and photoelectron lines are the L3M45M45 and 2p3/2 lines, respectively. Their binding energies in the nanowires, as shown in Fig. 2, were measured to be 497.3 and 1020.4 eV, respectively. Both values were raw data without charging correction, which simply cancels during the calculation of a* and thus is unnecessary. The separation of the two peaks is identical to the difference between the kinetic energies of the L3M45M45 Auger electron and the 2p3/2 photoelectron. hn was 1486.6 eV. Thus a* is calculated to be 2009.7 eV. The modified Auger parameter of zinc in the pure metal state is in the range of 2013.5–2014.4 eV, while that of zinc in the ZnO state is in the range of 2009.5–2010.2 eV [14]. Consequently, the XPS result suggests that the zinc existed in the nanowires in the ZnO state. As shown in Fig. 3, the overall crystal structure of the products was analyzed by X-ray diffraction (XRD). Except for the strongest peak, which is obviously attributable to the silicon substrate, all of the peaks in Fig. 3 arise from the crystal peaks of hexagonal structured ZnO (a ¼ 0.3243 nm,

0

10

20

30

40

50

60

70

80

90

degrees (2@) Fig. 3. XRD of ZnO nanowires on silicon wafer.

c ¼ 0.5195 nm). That is, our product was wurtzite-type ZnO with a high purity. A high-resolution transmission electron microscope (HRTEM) was employed to investigate some individual ZnO nanowires. Bifurcation structures were often observed and one example is shown in Fig. 4(a). All three branches are uniform in diameter. The left branch appeared to be most transparent to the electrons and was picked up in the high resolution observation. As shown in Fig. 4(b), the spacing between two adjacent lattice planes is 0.26 nm, commensurate with that between the (0 0 0 2) planes in ZnO. This result is in agreement with that of the XRD. Moreover, it shows that the nanowires grew along the c-axis. 3. Field emission performance 3.1. Field emission behavior The dependence of field emission current on the anode–cathode voltage (I–V behavior) of the ZnO nanowire arrays was measured in a lab-built ultrahigh vacuum system with a base pressure of 107 Pa. The sample, 35 mm2 in area, was fixed on a molybdenum holder and

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1800 1600 1400

I (µA)

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V (kV)

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ln (I/V2)

5 4 3 2 1 0 Fig. 4. TEM observation of individual ZnO nanowires: (a) TEM image of three ZnO nanowire branches; and (b) HRTEM image of the left branch.

used as the cathode. In this experiment, we used two kinds of anode. One was a glass screen coated with tin oxide (SnO2). This ‘‘transparent anode technique’’ allowed us to observe the two-dimensional distribution of emission sites on the cathode. The other was a molybdenum anode. It could withstand long-term bombardment of relatively high current density, thus we used it in the stability test. The cathode surface was separated from the transparent anode and the molybdenum anode by 0.5 and 0.8 mm, respectively. A tungsten filament behind the cathode was employed to heat the cathode by thermal radiation when surface cleaning was desired. We define ‘‘the apparent field’’ as the applied voltage divided by the cathode–anode separation: E  ¼ V =d. Moreover, we define here a diode-type field emitter as being turned on when it delivers a 10 mA/cm2 current density [15]. We also define the threshold field as extracting a 1 mA/cm2 current density, which is sometimes considered sufficient for practical application in an FED [7]. Fig. 5 gives the respective I–V behaviors of one sample before and after heat treatment at 400 1C and they are found to approximately follow the Fowler–Nordheim (FN) theory [16]. Non-linearity manifests itself at high voltages in Fig. 5(b). Since the space charge effect usually comes

-1 0.20

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(b)

0.35 0.40 1/V (kV-1)

0.45

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Fig. 5. I–V behavior of a non-aligned ZnO nanowire array. (‘‘1’’ and ‘‘2’’ are the cases before any heat treatment and after heat treatment at 400 1C, respectively). (a) I–V behavior (b) FN plot (I and V in the ordinate are in microamperes and kilovolts, respectively).

into play only at a very high current density [17–19], this non-linearity is believed to be a consequence of a high resistance at the ZnO–silicon interface. Before heating, the turn-on field and the threshold field were 4.6 and 6.4 V/mm, respectively. The heat treatment obviously improved the field emission performance, bringing the turn-on and threshold fields down to 4.0 and 4.7 V/mm, respectively. To date, the FN theory [16] is still the most convenient, although not necessarily the most appropriate, formulation of the I–V behavior of a field emitter. The field emission current density, J, is a function of the actual local field, E, around the nanowires, J ¼ AE 2 exp ðBF3=2 =EÞ.

(1)

Here A and B can be approximated by two constants; F is the work function of the emitter. Due to the large aspect ratio of nanowires, E is usually much stronger than the ‘‘apparent field’’, E*. Thus a field enhancement factor, b, is often introduced to describe this disparity: E ¼ bE*. When we calculate the value of b using the slope of the FN plot in

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Table 1 Field enhancement factor versus work function of ZnO 4.370.1 [20] 1.2–1.1

4.2570.05 [21,22] 1.1

Fig. 5(b), we have to know the value of F a priori. Thus we collected the results on the work function of the (0001) plane of ZnO by previous researchers. Depending on the approaches they used, their results differed from each other. Using these different values for the work function, we obtained different values of the enhancement factor, as listed in Table 1. As shown in Table 1, b is around 103. We doubt that the actual field enhancement factor can be that high. In fact, the validity of the FN theory is dubious as modern field emitters are becoming smaller and smaller [28]. Recent rapid development of nanometer-scale semiconductor field emitters has further complicated the situation. Concerning the mechanism of the field emission from ZnO nanowires, there are also other questions that are still to be answered. For instance, the ZnO nanowires in our samples were all non-aligned. We used to directly observe field emission from flank walls of ZnO nanowires using a field-emission microscope (FEM) [29]. Therefore, we maintain that both the tops and the flank walls of the ZnO nanowires played a key role in field emission.

3.9 [23] 0.95

3.7570.05 [24,25] 0.93–0.87

I

II

3.7–3.2 [26] 0.89–0.70

3.1570.15 [27] 0.75–0.63

III

500 450 current (µA)

Work function, F (eV) Enhancement factor, b (103)

400 350 300 250 200 150 10

20

30

40

50

60

70

80

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test time (hours) Fig. 6. Curve of field emission current versus time (I–T curve) of a silicon based ZnO nanowire array.

3.2. Field emission stability In addition to the I–V behavior, the stability of the field emission from our samples was also tested. Fearing that the SnO2 thin film might be damaged under long-term electron bombardment, we replaced it with the molybdenum anode. The test lasted 72 h under a constant voltage of 2.75 kV. The initial current was about 330 mA, approximately corresponding to the threshold current density. The current vs. time curve (I–T curve) is presented in Fig. 6. It effectively consists of three main segments. In the first 4 h, the current rose steeply. We attribute this phenomenon to the surface cleaning that resulted from the temperature rise in the nanowires under high current density. Then the current began to drop. We observed the distribution of the emission sites using the transparent anode before and after the stability test and a comparison between the results shows that the poor adherence of the nanowires to the substrate was responsible for the current degradation. As demonstrated in Fig. 7(a), the emission site density (ESD) of the cathode was uniform and high on almost the whole surface before the test, while in Fig. 7(b), the illuminated area was much smaller. That is, a large portion of the ZnO nanowires shed from the silicon substrate after the long-term delivery of a high current density. In the third region of the I–T curve in Fig. 6, which lasted approximately 40 h until the artificial conclusion of the test, the remainder of the ZnO nanowires exhibited

Fig. 7. Distribution of emission sites of the ZnO nanowire array on the silicon wafer: (a) before the stability test; and (b) after the stability test.

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fairly satisfactory stability. The degradation became less serious and the fluctuation was also small. Therefore, if the adherence of the ZnO nanowires to the substrate can be improved, a good stability will probably be achievable. In this regard, silicon is not an appropriate substrate material. We are now searching for a substrate material that is more compatible to the ZnO nanowires. 4. Conclusion Randomly oriented ZnO nanowires were fabricated on a silicon substrate by a simple and inexpensive method of thermal evaporation. During the fabrication, the temperature of the substrate was controlled to be lower than the softening point of glass. Field emission was available from the ZnO nanowire array at relatively low fields and the distribution of emission sites was uniform on the whole cathode surface. Our results show that arrays of randomly oriented ZnO nanowires can probably find their application in future electronic devices, for example, FEDs. It has also been found that one of the major drawbacks to further elevating the current density lies in the poor adherence of the ZnO nanowires to the substrate. At the moment, effort is being devoted to searching for a better substrate to overcome this disadvantage. Acknowledgements The sample characterization was supported by the National Center for Nanoscience and Technology of China and the Instrumental Analysis Fund of Peking University. This work was supported by the National Natural Science Foundation of China (Nos. 60471008, 60231010, and 50202002). References [1] X. Li, C. Yang, G. Bai, F. Zhang, F. Liao, J. Feng, M. Ding, Y. Du, Appl. Surf. Sci. 215 (2003) 249. [2] C.A. Spindt, J. Appl. Phys. 39 (1968) 3504.

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