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UNCD electron emitters using Si nanostructure as a template
Vacuum 84 (2010) 111–114
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
UNCD electron emitters using Si nanostructure as a template Bohr-Ran Huang a, Huang-Chin Chen b, Shyankay Jou c, *, I-Nan Lin b a
Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan Department of Physics, Tamkang University, Tamsui 251, Taiwan c Graduate Institute of Material Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan b
a b s t r a c t Keywords: Electron field emission Ultra-nanocrystalline diamond Silicon nanorods
The electron field emission (EFE) properties of silicon nanostructures (SiNSs) coated with ultra-nanocrystalline diamond (UNCD) were characterized. The SiNS, comprising cauliflower-like grainy structure and nanorods, was generated by reaction of a Si substrate with an Au film at 1000 C, and used as templates to grow UNCD. The UNCD films were deposited by microwave plasma-enhanced chemical vapour deposition (MPECVD) using methane and argon as reaction gases. The UNCD films can be grown on the SiNS with or without ultrasonication pretreatment with diamond particles. The EFE properties of the SiNS were improved by adding an UNCD film. The turn-on field (E0) decreased from 17.6 V/mm for the SiNS to 15.2 V/mm for the UNCD/SiNS, and the emission current density increased from 0.095 to 3.8 mA/cm2 at an electric field of 40 V/mm. Ultrasonication pretreatments of SiNS with diamond particles varied the structure and EFE properties of the UNCD/SiNS. It is shown that the ultrasonication pretreatment degraded the field emission properties of the UNCD/SiNS in this study. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Diamond and related materials have been extensively studied for their novel mechanical, chemical and electrical properties. Particularly because of its low or negative electron affinity (NEA) diamond is an optimum candidate for electron field emission (EFE) that has potential applications in the areas of flat panel displays and vacuum microelectronic devices [1,2]. Cold cathode field emission has been demonstrated in chemical vapour deposited diamond films [3]. However, the EFE properties of these materials are inferior to those of carbon nanotubes (CNTs) [4] due to the large electric field required for turning on the EFE phenomenon [5]. Methods to reduce the turn-on field for diamond films have been extensively studied. Several techniques have been proposed to enhance the EFE properties of the diamond films, which include reducing the grain size of diamond films [6], doping semiconducting species to increase the conductivity of diamond [7], and using high-aspectratio tips as a template for fabricating diamond tips [8]. High EFE has been demonstrated from ultra-nanocrystalline diamond (UNCD)-coated Si microtips which were synthesized from catalyzed vapour deposition [9]. Aligned Si tip arrays generated by an electrochemical etch have also been used as templates for UNCD emitters [10,11]. In our previous study, Si nanowires (SiNWs) could
* Corresponding author. Tel.: þ886 2 2737 6665; fax: þ886 2 2730 1265. E-mail address: [email protected] (S. Jou). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.04.056
be synthesized by reaction of thin metal films with a Si substrate at a high temperature [12]. In this study, UNCD electron emitters with improved EFE properties were investigated by depositing an UNCD film on templates of silicon nanostructures (SiNSs) that were generated by reaction of gold with the Si substrate. 2. Experimental A p-type (100) silicon wafer was used as a substrate to grow SiNS. The silicon substrates were first cleaned ultrasonically in acetone and in ethanol in turn for 10 min each, and then leached in de-ionized water. Then the substrates were dried and transferred into a metal sputter-coater where a thin layer (10 nm) of gold catalyst was deposited. The silicon nanostructure (SiNS) was synthesized on the silicon substrates via a catalytic reaction in an N2 atmosphere at 1000 C for 30 min. UNCD films were deposited in a microwave plasma-enhanced chemical vapour deposition (MPECVD, IPLAS) system. Before the UNCD deposition, the SiNS substrate was treated by ultrasonication in nano-sized diamond/titanium powder in methanol solution for 0, 20 and 45 min. The UNCD coatings were deposited with a microwave power of 1200 W, methane/argon flows of 1/99 sccm, total pressure of 1.6 104 Pa (120 Torr), deposition temperature of 300 C and growth time of 3 h. Topography of the as-grown SiNS was inspected by atomic force microscopy (AFM, Veeco CP-R). The UNCD coatings were characterized by Micro-Raman spectroscopy (Renishaw 2000) and field
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emission scanning electron microscopy (FESEM, JEOL 6010). MicroRaman measurements were performed at room temperature, using a 514.5-nm laser as the excitation source. The field emission properties of the NCD films were measured in a parallel plate set-up under a high vacuum environment of 1.33 104 Pa (1 106 Torr). The distance between the anode (ITO glass) and the cathode (UNCD films) was approximately 30 mm and the emission measurement area of NCD films was 10 2.5 mm2. The current–voltage properties were measured by a Keithley 237 source measurement unit. 3. Results and discussion
Fig. 1. AFM image for the as-grown SiNS.
Silicon nanostructure (SiNS) was produced on the surface of ptype Si substrate after annealing the Au-coated Si substrate. Fig. 1 displays the surface morphology of the annealed Si substrate. The substrate surface is composed of a cauliflower-like grainy structure and some randomly distributed nanorods of 0.3–1.1 mm in length and 90–160 nm in width. FESEM images for the annealed Si substrates in the as-grown state and after ultrasonication treatment for 45 min are shown in Fig. 2(a) and (b). It is indicated that both the cauliflower-like grainy structure and the nanorods were worn away after the ultrasonication treatment. The surface roughnesses for the annealed Si substrates before and after ultrasonication treatment for 45 min were 43.7 and 10.4 nm,
Fig. 2. FESEM images for (a) the as-grown SiNS, (b) the SiNS with ultrasonication treatment of 45 min, (c) UNCD on the as-grown SiNS and (d) UNCD on the SiNS with ultrasonication treatment of 45 min.
B.-R. Huang et al. / Vacuum 84 (2010) 111–114
respectively. UNCD can be deposited directly on the surface of the as-grown SiNS without a pre-seeding treatment. The UNCD coatings grown on the as-grown SiNS and on the SiNS with ultrasonication treatment for 45 min are shown in Fig. 2(c) and (d). The UNCD coatings have cauliflower-like morphology. The Raman spectra of NCD films normally comprise four broad peaks at about 1140 cm1, 1350 cm1, 1480 cm1 and 1550 cm1 [13,14]. An additional peak at about 1190 cm1 has been observed in NCD films that were produced using mixed CH4, H2 and N2 gases [15,16]. Fig. 3 shows the Raman spectrum of UNCD deposited on the surface of the as-grown SiNS. The Raman spectrum of the UNCD obtained here comprises five peaks that are the same as those reported in Refs. [15,16]. Raman spectra are used to characterize the presence of various carbon phases. Crystalline diamond shows a sharp Raman peak at about 1332 cm1, whereas graphite shows a sharp peak at about 1590 cm1 [17]. The peaks at 1140 cm1 and 1480 cm1 are assigned to nanocrystalline phases of diamond, and two peaks approximately at 1350 cm1 and 1550 cm1 are the D-band and G-band, respectively [18,19]. The D-band, which is indicative of disordered sp2-bonded carbon, and the G-band, which is attributed to graphite clusters, have been observed in various carbon films including diamond-like carbon [20,21]. Purified NCD particles have a clear Raman peak at about 1090–1150 cm1 without the presence of the G-band in the region of 1500–1550 cm1 [22]. Mixed sp2- and sp3-bonded carbon structure is identified by the presence of a Raman peak at around 1190 cm1 [15,16,23]. According to its Raman spectrum, the UNCD coating on top of the as-grown SiNS substrate comprises nanocrystalline diamond (1140 cm1 and 1480 cm1), graphite clusters (1350 cm1 and 1550 cm1) and mixed sp2- and sp3-bonded carbon (1190 cm1) structure. The Raman spectra for the UNCD coatings deposited on the surfaces of ultrasonically treated SiNSs are similar to that on the as-grown SiNS. The current density versus electrical field curves for SiNS (A), UNCD/SiNS (:), UNCD/SiNS þ 20 min ultrasonication (C) and UNCD/SiNS þ 45 min ultrasonication (-) are shown in Fig. 4. The inset in Fig. 4 is the F–N plots [24,25] for these emitters. The turn-on field at a current density of 0.1 mA/cm2 for the SiNS emitter is 17.6 V/ mm, and a current density of 0.095 mA/cm2 is obtained at a field of 40 V/mm. The field enhancement factor (b) for the SiNS is 163 according to the analysis of the F–N plot [24,25], assuming the work function of the SiNS is the same as that of bulk Si crystal (FSi ¼ 4.1 eV) [26,27]. The effective work function (Fe ¼ F3/2/b), which is defined as the ratio of the work function to the field enhancement
Fig. 3. Raman spectrum of an UNCD deposited on SiNS.
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Fig. 4. The electric field emission curves for SiNS (A), UNCD/SiNS (:), UNCD/ SiNS þ 20 min ultrasonication (C) and UNCD/SiNS þ 45 min ultrasonication (-). The inset shows the corresponding F–N plots.
factor (b), for the SiNS emitter is 0.079. As a result, the EFE of Si is improved by the mixed cauliflower-like grainy structure and nanorods. The turn-on field for the UNCD/SiNS is 15.2 V/mm, and the current density at a field of 40 V/mm is 3.83 mA/cm2. As a result, the UNCD coatings further improve the EFE properties of SiNS. Enhancement of EFE by coating UNCD on the surface of high-aspectratio Si tip arrays that were generated by galvanic anisotropic etching of Si substrate has also been observed in a previous study. The turn-on field for the Si tip arrays was 8.6 V/mm and reduced to 7.6 V/mm after a layer of UNCD film was coated on the tip arrays [11]. Ultrasonication treatment of SiNS with diamond nanopowders was employed prior to depositing the UNCD film for comparison of EFE efficiency. The turn-on fields for the UNCD/SiNS with 20 min and 45 min ultrasonication pretreatment are 18.4 and 17 V/mm, respectively. The current densities are 0.86 and 0.87 mA/cm2 at a field of 40 V/mm. According to the results of turn-on fields and current densities, ultrasonication treatment of SiNS does not improve EFE efficiency of UNCD-coated SiNS. Ultrasonication using diamond powders has been an efficient process for enhancing the nucleation sites for the syntheses of UNCD on silicon wafers. A previous study indicated that the EFE properties of UNCD-coated Si tip arrays could be improved by pre-seeding using ultrasonication process, suggesting an increase of UNCD-to-Si contact area for electron transport [11]. Ultrasonic treatment could reduce the turn-on field for the UNCD-coated Si tip arrays from 7.6 to 3.75 V/mm [11]. However, the ultrasonication pretreatment reduces the EFE properties of UNCDcoated SiNS in this study. Although the surface morphologies of UNCD coatings on SiNS with and without ultrasonication pretreatment are similar, as shown in Fig. 2(c) and (d), the ultrasonication process damages the SiNS on Si wafer, as shown in Fig. 2(b). The surface roughness of SiNS changed from 43.7 nm to 10.4 nm after ultrasonic treatment for 45 min. The change of EFE properties of UNCD/SiNS may result from different effective contact areas between the UNCD and Si surfaces. A better UNCD-to-Si contact has been suggested to facilitate transport of electrons from Si to emitting sites [11]. UNCD was able to be deposited on the as-grown SiNS in this study. The as-grown SiNS had larger surface area than that with
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ultrasonication treatment. Therefore, the rough topography of mixed nanorods and cauliflower-like Si surface provided abundant UNCD-to-Si contact. Ultrasonication treatment damaged SiNS and resulted in less contact area between Si and UNCD, hence poorer electron transport and emission efficiencies. Curvature of Si nanostructures may also change after ultrasonication treatment of SiNS. Curvature below the Si tip has been found to influence EFE from cap coating of polycrystalline diamond possibly due to the change of tunnelling barrier at the diamond–Si interface, but it had little effect on the EFE of UNCD [9]. Therefore, the change of EFE of UNCD/SiNS with ultrasonication treatment in this study was possibly caused by the change of contact area at the UNCD/SiNS interface. 4. Conclusion UNCD films can be grown on the Si nanostructure with or without the pretreatment process. The electron field emission properties of the UNCD films are improved due to the utilization of SiNS as a template. Ultrasonication pretreatments with diamond particles varied the morphology of the SiNS and EFE properties of the UNCD/SiNS. It is shown that the ultrasonication pretreatment degrades the field emission properties of the UNCD/SiNS possibly by decreasing the contact area between Si and UNCD. Acknowledgements The authors would like to thank the National Science Council, Republic of China, for the support of this research under project NSC95-2221-E-224-069-MY2.
References [1] Xu NS, Huq SE. Mater Sci Eng R 2005;48:47. [2] Satynarayana BS, Hart A, Milne WI, Robertson J. Appl Phys Lett 1997;71:1430. [3] Djubua BC, Chubun NN. IEEE Trans Electron Devices 1991;38:2314. [4] Zhu W, Kochanski GP, Jin S. Science 1998;282:1471. [5] Xu Z, Bai XD, Wang EG, Wang ZL. Appl Phys Lett 2005;87:163106. [6] Lu X, Yang Q, Chen W, Xiao C, Hirose A. J Vac Sci Technol B 2006;24:2575. [7] Okano K, Koizumi S, Silva SRP, Amaratunga GAJ. Nature 1996;381:140. [8] Wang ZL, Luo Q, Li JJ, Wang Q, Xu P, Cui Z, et al. Diamond Relat Mater 2006;15:631. [9] Krauss AR, Auciello O, Ding MQ, Gruen DM, Huang Y, Zhirnov VV, et al. J Appl Phys 2001;89:2958. [10] Tzeng YF, Liu KH, Lee YC, Lin SJ, Lin IN, Lee CY, et al. Nanotechnology 2007;18:435703. [11] Tzeng YF, Lee YC, Chiu HT, Tai NH, Lin IN. Diamond Relat Mater 2008;17:1817. [12] Hsu JF, Huang BR. Thin Solid Films 2006;514:20. [13] Xiao X, Birrell J, Gerbi JE, Auciello O, Carlisle JA. J Appl Phys 2004;96:2232. [14] Hayashi Y, Soga T. Tribol Int 2004;37:965. [15] Zhang Q, Yoon SF, Ahn J, Rusli GYP. Microelectron J 1998;29:875. [16] Ma KL, Zhang WJ, Zou YS, Chong YM, Leung KM, Bello I, et al. Diamond Relat Mater 2006;15:626. [17] Solin SA, Ramdas AK. Phys Rev B 1970;1:1687. [18] Yarbrough WA, Messier R. Science 1990;247:688. [19] Nemanich RJ, Glass JT, Lucovsky G, Shroder RE. J Vac Sci Technol A 1988;6:1783. [20] Prawer S, Nugent KW, Lifshitz Y, Lempert GD, Grossman E, Kulik J, et al. Diamond Relat Mater 1996;5:433. [21] Casiraghi C, Piazza F, Ferrari AC, Grambole D, Robertson J. Diamond Relat Mater 2005;14:1098. [22] Prawer S, Nugent KW, Jamieson DN, Orwa JO, Bursill LA, Peng JL. Chem Phys Lett 2000;332:93. [23] Wang CZ, Ho KM. Phys Rev Lett 1993;71:1184. [24] Fowler RH, Nordheim LW. Proc R Soc London Ser A 1928;119:173. [25] Nordheim LW. Proc R Soc London Ser A 1928;121:626. [26] Ding M, Kim H, Akinwande AI. Appl Phys Lett 1999;75:283. [27] Zorba V, Tzanetakis P, Fotakis DG. Appl Phys Lett 2006;88:081103.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
UNCD electron emitters using Si nanostructure as a template Bohr-Ran Huang a, Huang-Chin Chen b, Shyankay Jou c, *, I-Nan Lin b a
Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan Department of Physics, Tamkang University, Tamsui 251, Taiwan c Graduate Institute of Material Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan b
a b s t r a c t Keywords: Electron field emission Ultra-nanocrystalline diamond Silicon nanorods
The electron field emission (EFE) properties of silicon nanostructures (SiNSs) coated with ultra-nanocrystalline diamond (UNCD) were characterized. The SiNS, comprising cauliflower-like grainy structure and nanorods, was generated by reaction of a Si substrate with an Au film at 1000 C, and used as templates to grow UNCD. The UNCD films were deposited by microwave plasma-enhanced chemical vapour deposition (MPECVD) using methane and argon as reaction gases. The UNCD films can be grown on the SiNS with or without ultrasonication pretreatment with diamond particles. The EFE properties of the SiNS were improved by adding an UNCD film. The turn-on field (E0) decreased from 17.6 V/mm for the SiNS to 15.2 V/mm for the UNCD/SiNS, and the emission current density increased from 0.095 to 3.8 mA/cm2 at an electric field of 40 V/mm. Ultrasonication pretreatments of SiNS with diamond particles varied the structure and EFE properties of the UNCD/SiNS. It is shown that the ultrasonication pretreatment degraded the field emission properties of the UNCD/SiNS in this study. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Diamond and related materials have been extensively studied for their novel mechanical, chemical and electrical properties. Particularly because of its low or negative electron affinity (NEA) diamond is an optimum candidate for electron field emission (EFE) that has potential applications in the areas of flat panel displays and vacuum microelectronic devices [1,2]. Cold cathode field emission has been demonstrated in chemical vapour deposited diamond films [3]. However, the EFE properties of these materials are inferior to those of carbon nanotubes (CNTs) [4] due to the large electric field required for turning on the EFE phenomenon [5]. Methods to reduce the turn-on field for diamond films have been extensively studied. Several techniques have been proposed to enhance the EFE properties of the diamond films, which include reducing the grain size of diamond films [6], doping semiconducting species to increase the conductivity of diamond [7], and using high-aspectratio tips as a template for fabricating diamond tips [8]. High EFE has been demonstrated from ultra-nanocrystalline diamond (UNCD)-coated Si microtips which were synthesized from catalyzed vapour deposition [9]. Aligned Si tip arrays generated by an electrochemical etch have also been used as templates for UNCD emitters [10,11]. In our previous study, Si nanowires (SiNWs) could
* Corresponding author. Tel.: þ886 2 2737 6665; fax: þ886 2 2730 1265. E-mail address: [email protected] (S. Jou). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.04.056
be synthesized by reaction of thin metal films with a Si substrate at a high temperature [12]. In this study, UNCD electron emitters with improved EFE properties were investigated by depositing an UNCD film on templates of silicon nanostructures (SiNSs) that were generated by reaction of gold with the Si substrate. 2. Experimental A p-type (100) silicon wafer was used as a substrate to grow SiNS. The silicon substrates were first cleaned ultrasonically in acetone and in ethanol in turn for 10 min each, and then leached in de-ionized water. Then the substrates were dried and transferred into a metal sputter-coater where a thin layer (10 nm) of gold catalyst was deposited. The silicon nanostructure (SiNS) was synthesized on the silicon substrates via a catalytic reaction in an N2 atmosphere at 1000 C for 30 min. UNCD films were deposited in a microwave plasma-enhanced chemical vapour deposition (MPECVD, IPLAS) system. Before the UNCD deposition, the SiNS substrate was treated by ultrasonication in nano-sized diamond/titanium powder in methanol solution for 0, 20 and 45 min. The UNCD coatings were deposited with a microwave power of 1200 W, methane/argon flows of 1/99 sccm, total pressure of 1.6 104 Pa (120 Torr), deposition temperature of 300 C and growth time of 3 h. Topography of the as-grown SiNS was inspected by atomic force microscopy (AFM, Veeco CP-R). The UNCD coatings were characterized by Micro-Raman spectroscopy (Renishaw 2000) and field
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emission scanning electron microscopy (FESEM, JEOL 6010). MicroRaman measurements were performed at room temperature, using a 514.5-nm laser as the excitation source. The field emission properties of the NCD films were measured in a parallel plate set-up under a high vacuum environment of 1.33 104 Pa (1 106 Torr). The distance between the anode (ITO glass) and the cathode (UNCD films) was approximately 30 mm and the emission measurement area of NCD films was 10 2.5 mm2. The current–voltage properties were measured by a Keithley 237 source measurement unit. 3. Results and discussion
Fig. 1. AFM image for the as-grown SiNS.
Silicon nanostructure (SiNS) was produced on the surface of ptype Si substrate after annealing the Au-coated Si substrate. Fig. 1 displays the surface morphology of the annealed Si substrate. The substrate surface is composed of a cauliflower-like grainy structure and some randomly distributed nanorods of 0.3–1.1 mm in length and 90–160 nm in width. FESEM images for the annealed Si substrates in the as-grown state and after ultrasonication treatment for 45 min are shown in Fig. 2(a) and (b). It is indicated that both the cauliflower-like grainy structure and the nanorods were worn away after the ultrasonication treatment. The surface roughnesses for the annealed Si substrates before and after ultrasonication treatment for 45 min were 43.7 and 10.4 nm,
Fig. 2. FESEM images for (a) the as-grown SiNS, (b) the SiNS with ultrasonication treatment of 45 min, (c) UNCD on the as-grown SiNS and (d) UNCD on the SiNS with ultrasonication treatment of 45 min.
B.-R. Huang et al. / Vacuum 84 (2010) 111–114
respectively. UNCD can be deposited directly on the surface of the as-grown SiNS without a pre-seeding treatment. The UNCD coatings grown on the as-grown SiNS and on the SiNS with ultrasonication treatment for 45 min are shown in Fig. 2(c) and (d). The UNCD coatings have cauliflower-like morphology. The Raman spectra of NCD films normally comprise four broad peaks at about 1140 cm1, 1350 cm1, 1480 cm1 and 1550 cm1 [13,14]. An additional peak at about 1190 cm1 has been observed in NCD films that were produced using mixed CH4, H2 and N2 gases [15,16]. Fig. 3 shows the Raman spectrum of UNCD deposited on the surface of the as-grown SiNS. The Raman spectrum of the UNCD obtained here comprises five peaks that are the same as those reported in Refs. [15,16]. Raman spectra are used to characterize the presence of various carbon phases. Crystalline diamond shows a sharp Raman peak at about 1332 cm1, whereas graphite shows a sharp peak at about 1590 cm1 [17]. The peaks at 1140 cm1 and 1480 cm1 are assigned to nanocrystalline phases of diamond, and two peaks approximately at 1350 cm1 and 1550 cm1 are the D-band and G-band, respectively [18,19]. The D-band, which is indicative of disordered sp2-bonded carbon, and the G-band, which is attributed to graphite clusters, have been observed in various carbon films including diamond-like carbon [20,21]. Purified NCD particles have a clear Raman peak at about 1090–1150 cm1 without the presence of the G-band in the region of 1500–1550 cm1 [22]. Mixed sp2- and sp3-bonded carbon structure is identified by the presence of a Raman peak at around 1190 cm1 [15,16,23]. According to its Raman spectrum, the UNCD coating on top of the as-grown SiNS substrate comprises nanocrystalline diamond (1140 cm1 and 1480 cm1), graphite clusters (1350 cm1 and 1550 cm1) and mixed sp2- and sp3-bonded carbon (1190 cm1) structure. The Raman spectra for the UNCD coatings deposited on the surfaces of ultrasonically treated SiNSs are similar to that on the as-grown SiNS. The current density versus electrical field curves for SiNS (A), UNCD/SiNS (:), UNCD/SiNS þ 20 min ultrasonication (C) and UNCD/SiNS þ 45 min ultrasonication (-) are shown in Fig. 4. The inset in Fig. 4 is the F–N plots [24,25] for these emitters. The turn-on field at a current density of 0.1 mA/cm2 for the SiNS emitter is 17.6 V/ mm, and a current density of 0.095 mA/cm2 is obtained at a field of 40 V/mm. The field enhancement factor (b) for the SiNS is 163 according to the analysis of the F–N plot [24,25], assuming the work function of the SiNS is the same as that of bulk Si crystal (FSi ¼ 4.1 eV) [26,27]. The effective work function (Fe ¼ F3/2/b), which is defined as the ratio of the work function to the field enhancement
Fig. 3. Raman spectrum of an UNCD deposited on SiNS.
113
Fig. 4. The electric field emission curves for SiNS (A), UNCD/SiNS (:), UNCD/ SiNS þ 20 min ultrasonication (C) and UNCD/SiNS þ 45 min ultrasonication (-). The inset shows the corresponding F–N plots.
factor (b), for the SiNS emitter is 0.079. As a result, the EFE of Si is improved by the mixed cauliflower-like grainy structure and nanorods. The turn-on field for the UNCD/SiNS is 15.2 V/mm, and the current density at a field of 40 V/mm is 3.83 mA/cm2. As a result, the UNCD coatings further improve the EFE properties of SiNS. Enhancement of EFE by coating UNCD on the surface of high-aspectratio Si tip arrays that were generated by galvanic anisotropic etching of Si substrate has also been observed in a previous study. The turn-on field for the Si tip arrays was 8.6 V/mm and reduced to 7.6 V/mm after a layer of UNCD film was coated on the tip arrays [11]. Ultrasonication treatment of SiNS with diamond nanopowders was employed prior to depositing the UNCD film for comparison of EFE efficiency. The turn-on fields for the UNCD/SiNS with 20 min and 45 min ultrasonication pretreatment are 18.4 and 17 V/mm, respectively. The current densities are 0.86 and 0.87 mA/cm2 at a field of 40 V/mm. According to the results of turn-on fields and current densities, ultrasonication treatment of SiNS does not improve EFE efficiency of UNCD-coated SiNS. Ultrasonication using diamond powders has been an efficient process for enhancing the nucleation sites for the syntheses of UNCD on silicon wafers. A previous study indicated that the EFE properties of UNCD-coated Si tip arrays could be improved by pre-seeding using ultrasonication process, suggesting an increase of UNCD-to-Si contact area for electron transport [11]. Ultrasonic treatment could reduce the turn-on field for the UNCD-coated Si tip arrays from 7.6 to 3.75 V/mm [11]. However, the ultrasonication pretreatment reduces the EFE properties of UNCDcoated SiNS in this study. Although the surface morphologies of UNCD coatings on SiNS with and without ultrasonication pretreatment are similar, as shown in Fig. 2(c) and (d), the ultrasonication process damages the SiNS on Si wafer, as shown in Fig. 2(b). The surface roughness of SiNS changed from 43.7 nm to 10.4 nm after ultrasonic treatment for 45 min. The change of EFE properties of UNCD/SiNS may result from different effective contact areas between the UNCD and Si surfaces. A better UNCD-to-Si contact has been suggested to facilitate transport of electrons from Si to emitting sites [11]. UNCD was able to be deposited on the as-grown SiNS in this study. The as-grown SiNS had larger surface area than that with
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B.-R. Huang et al. / Vacuum 84 (2010) 111–114
ultrasonication treatment. Therefore, the rough topography of mixed nanorods and cauliflower-like Si surface provided abundant UNCD-to-Si contact. Ultrasonication treatment damaged SiNS and resulted in less contact area between Si and UNCD, hence poorer electron transport and emission efficiencies. Curvature of Si nanostructures may also change after ultrasonication treatment of SiNS. Curvature below the Si tip has been found to influence EFE from cap coating of polycrystalline diamond possibly due to the change of tunnelling barrier at the diamond–Si interface, but it had little effect on the EFE of UNCD [9]. Therefore, the change of EFE of UNCD/SiNS with ultrasonication treatment in this study was possibly caused by the change of contact area at the UNCD/SiNS interface. 4. Conclusion UNCD films can be grown on the Si nanostructure with or without the pretreatment process. The electron field emission properties of the UNCD films are improved due to the utilization of SiNS as a template. Ultrasonication pretreatments with diamond particles varied the morphology of the SiNS and EFE properties of the UNCD/SiNS. It is shown that the ultrasonication pretreatment degrades the field emission properties of the UNCD/SiNS possibly by decreasing the contact area between Si and UNCD. Acknowledgements The authors would like to thank the National Science Council, Republic of China, for the support of this research under project NSC95-2221-E-224-069-MY2.
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