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Field emission studies of pulsed laser deposited LaB6 films on W and Re
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Field emission studies of pulsed laser deposited LaB6 films on W and Re Dattatray J. Latea, Mahendra A. Morea, Pankaj Misrab, B.N. Singhb, Lalit M. Kukrejab, Dilip S. Joaga, a
DST Unit on Nanoscience and Center for Advanced Studies in Material Science and Condensed Matter Physics, Department of Physics, University of Pune, Pune 411007, India b Thin Film Laboratory, Raja Ramanna Centre for Advance Technology, Indore 452013, India
Abstract Lanthanum hexaboride films were grown on tungsten and rhenium tips and foils by pulsed laser deposition. The X-ray diffraction spectra of the PLD LaB6 films on both the substrates show crystalline nature with average grain size 125 nm. The field emission studies of pointed and foil specimens were performed in conventional and planar diode configurations, respectively, under ultra-high vacuum condition. An estimated current density of 1:2 104 A=cm2 was drawn at the electric field of 3 103 and 6 103 V=mm from the LaB6 coated tips of tungsten and rhenium, respectively. The Fowler–Nordheim plots were found to be linear showing metallic behavior of the emitters. The field enhancement factors were calculated from the slopes of the Fowler–Nordheim plots, indicating that the field emission is from LaB6 nanoscale protrusions present on emitter surfaces. The emitters were operated for long time current stability (3 h) studies. The post-field emission surface morphology of the emitters showed no significant erosion of LaB6 films during 3 h continuous operation. The observed behavior indicates that it is linked with the growth of LaB6 films on W and Re. These results reveal that the LaB6 films exhibit high resistance to ion bombardment and excellent structural stability and are more promising emitters for practical applications in field emission based devices. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15Aa; 85.45Bz Keywords: Field emission; Pulsed laser deposition and LaB6
1. Introduction Field electron emission involves a quantum mechanical tunneling process under an applied electric field. It has diverse technological applications in flat panel displays, microwave-generation devices and vacuum micro/nanoelectronic devices. A novel and promising approach to fabricate field emission (FE) cathodes is to use anisotropic nanostructured materials such as carbon nanotubes (CNTs) [1–6], AlN [7], NbS2 [8], GaN [9], ZnO [10] and Lanthanum hexaboride ðLaB6 Þ [11–13]. Lanthanum hexaboride has been widely used in modern technology as an excellent thermionic electron emission source [14], which offers high brightness and long service life. The advantages Corresponding author. Tel.: +91 020 25692678; fax: +91 020 25691684. E-mail address: [email protected] (D.S. Joag).
0304-3991/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2007.02.014
are originated from its low work function (2.6 eV), high melting point ð2600 CÞ and low volatility. Pulsed laser deposition (PLD) [15–18] is a method to grow high quality thin films of a variety of metals and semiconductor materials on suitable substrates in a controlled manner. Craciun et al. have reported the synthesis of crystalline LaB6 thin films on silicon substrates by PLD technique [18]. Recently we have reported FE studies on LaB6 films deposited by PLD on a tungsten tip exhibiting striking features [12]. Here we report the synthesis of nanocrystalline LaB6 thin films on tungsten and rhenium pointed and flat substrates by PLD and their FE studies. 2. Experimentation Lanthanum hexaboride films were deposited on tungsten and rhenium tips and flat substrates (foils) in a vacuum
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chamber (base pressure 1 106 mbar) by pulsed laser ablation from Lanthanum hexaboride pellet target. For PLD, pulsed Nd: YAG laser (2nd harmonic l ¼ 355 nm, repetition rate n ¼ 10 Hz, pulse width t ¼ 6 ns, deposition time 60 s and fluence F1 J=cm2 ) was used as an excitation source. The deposition was carried out in helium atmosphere at 1 103 mbar for 60 s duration. Tungsten and rhenium tips were prepared by electrochemical polishing of pieces of their wires using suitable electrolytes. These tips were mounted on the substrate holder in such a manner that the tip apex was normal to the holder base, facing the target. Pre-cleaned pieces of tungsten foil ð1 cm 1 cmÞ and rhenium foil ð1 cm 0:5 cmÞ were mounted close to the tip on the same holder. The sintered LaB6 pellet was mounted on a rotating target holder at a distance of 5 cm from the substrates. During PLD, the substrates were kept at the temperature 700 C. The PLD of the desired LaB6 films on tungsten and rhenium tips and flat substrates was carried out in the very same experiment. The deposited LaB6 films are approximately 1 mm thick. The PLD of LaB6 on Re and W tips and flat substrates was carried out at least on three specimens each. The flat substrates facilitated structural characterization of the deposited LaB6 films by grazing angle X-ray Diffractometer (Model D-8 Advanced, Brucker AXS). Analytical Scanning Electron Microscope JEOL 6360A was used to investigate the surface morphology of the PLD LaB6 films. The FE measurements of LaB6 /W tip and LaB6 /Re tip were carried out, in an all glass conventional FE microscope (FEM) tube, in separate experiments. Also, the FE studies of LaB6 /W and LaB6 /Re flat substrates were performed in planar diode configuration, in separate experiments. The experimental details pertaining to the vacuum processing of the tube have been described elsewhere [1,9]. 3. Results and discussion 3.1. Characterization by XRD and SEM The grazing angle ð2 Þ XRD patterns of the LaB6 films deposited on W and Re flat substrates using PLD are seen
in Fig. 1(a) and (b), respectively. In both the cases, a set of well-defined diffractions peaks are seen, revealing the crystalline nature of LaB6 on these substrates. The phase identification and plane indexing are done by comparing the observed d values with the standard JCPDS data cards for LaB6 , W and Re [19]. A careful observation of the spectra reveals that the LaB6 film on W foil has preferred orientation with major reflections from the (1 1 0) and the (3 1 1) planes and on Re foil from the (2 0 0) plane. The average grain size obtained using Debye–Scherrer formula is observed to be 125 nm on both the W and Re substrates. The SEM images of the PLD LaB6 films on W tip and foil recorded at different magnifications are shown in Fig. 2(a–d). On both, pointed (a and b) and flat (c and d) substrates, the LaB6 crystallites are seen to cover the entire substrate surface. In case of tip substrate, the overall surface morphology is observed to be rough and characterized by the presence of nanoscale protrusions. The SEM observation shows that on the flat substrates, the LaB6 crystallites have average grain size less than 200 nm. The SEM images of PLD LaB6 films on Re tip and flat substrates recorded at different magnifications are shown in Fig. 3(a–d). On Re tip substrate the entire surface is fully covered with LaB6 film and overall morphology is also seen to be rough with the presence of nanoscale protrusions on the tip surface. In case of Re flat substrate the entire substrate surface is fully covered with nearly micron sized LaB6 particles, on top of which a large number of LaB6 nanoparticles are seen. 3.2. Field emission 3.2.1. W tip and flat emitter The FE characteristics of nanocrystalline LaB6 film on W tip, recorded at the base pressure of 1 109 mbar, are depicted in Fig. 4. For LaB6 =W tip emitter, an emission current of 1 nA was reproducibly observed at an applied field 5:5 102 V=mm, and a current 195 mA with an estimated emission current density of 1:2 104 A=cm2 has been drawn at an applied field of 3 103 V=mm. The FE
Fig. 1. XRD spectra of (a) LaB6 =W foil; (b) LaB6 =Re foil.
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Fig. 2. SEM images of LaB6 deposited on a tungsten tip (a and b) and tungsten foil (c and d).
Fig. 3. SEM images of LaB6 deposited on a rhenium tip (a and b) and rhenium foil (c and d).
behavior of the LaB6 =W planar emitter is shown in Fig. 5. The onset field, required to draw an emission current of 1 nA, was found to be 1:2 V=mm for LaB6 =W planar emitter. For the planar diode configuration, the applied field is defined as F ¼ V =d, where V is applied voltage and d is the distance of separation between cathode and anode ðd ¼ 5 mmÞ. In case of LaB6 =W planar emitter, emission current as high as 2:86 mA was obtained at an applied field
of 2:16 V=mm. As revealed from the SEM images, the LaB6 film on tungsten tip (Fig. 2(a, b)) has very sharp nanometric protrusions. The Fowler–Nordheim (F–N) plots [graph of lnðI=V2 Þ versus 104 =V], obtained from the I–V curves, are shown in Figs. 4(b) and 5(b) for the LaB6 film on W tip and flat emitters. The linear nature of the observed F–N plots confirms the metallic character of the emitters, and that the electron emission is according to
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Fig. 4. (a) Field emission current–voltage (I–V) characteristics of LaB6 deposited on tungsten tip. (b) The F–N plot. (c) (I–t) curves from LaB6 deposited on a tungsten tip. (d) Field emission micrograph of LaB6 deposited on a tungsten tip.
the F–N theory. The field enhancement factor b related to the slope (m) of the F–N plot is identified to be directly dependent on the emitter radius and is expressed by [20] b ¼ ð6:83 103 F3=2 Þ=m,
(1)
where f is the work function of the emitter material in eV. In the present case, the work function of LaB6 (fÞ is taken to be 2.6 eV. The effective geometrical field enhancement factor b is calculated to be 5537 and 3036 cm1 for LaB6 coated W tip and flat emitters, respectively, from Eq. (1). The effective emission area a is calculated using the following equation [11,21]: b ¼ lnð1:5 106 ab2 =FÞ þ 10:4=F3=2 ,
(2)
where b is the y-intercept of the F–N plot. The effective emission area is calculated to be 1:4 106 nm2 for LaB6 =W tip. A small emitting area is an indication of nanometric emitting structures on the tip surface. For FE electron sources, along with the emission competence, the current stability is also a decisive and an important parameter. The FE current stability of LaB6 =W tip emitters, observed at the pre-set current values 10 and 50 mA, over a duration of more than 3 h, is depicted in Fig. 4(c) measured at the base pressure 1 109 mbar. For LaB6 =W flat emitters, current stability was observed at the pre-set value of 1 mA and is shown in Fig. 5(c). The I–t plots show emission current dependence behavior for the tip and flat emitters. The observed current fluctuations are due to adsorption,
desorption and diffusion of adsorbates on the emitter surface. The LaB6 covered W tip emitter surface would become cleaner due to the ion bombardment induced desorption of the residual gases from the tip surface as observed in the current stability experiment. The clean surface thus produced leads to stabilization of the emission current at a higher value of 100 mA. At a lower current level, the competition between residual gas adsorption and ion bombardment induced desorption results in slow decrease and increase in the emission current. The observed FE pattern for the pointed emitter comprised of bright and symmetric oval-shaped spots, as seen in Fig. 4(d) for LaB6 =W tip emitter. In case LaB6 =W flat emitter, a stable and bright spot was observed, as seen in Fig. 5(d). The post-FE SEM studies of LaB6 =W tip were carried out in order to see structural stability of the PLD LaB6 film on this substrate. It is interesting to note that the SEM images Fig. 8(a and b) showed no significant change in emitter surface morphology, which indicates that the PLD LaB6 film on tungsten has very good adhesion and remarkable structural stability against ion bombardment and high field induced mechanical stresses. This is especially important for fabricating FE devices with high current density and stability. 3.2.2. Re tip and flat emitter The results of studies on FE from LaB6 films on Re tip, are depicted in Fig. 6. The applied field required to draw an
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Fig. 5. (a) Field emission current–voltage (I–V) characteristics of LaB6 deposited on tungsten foil. (b) The F–N plot. (c) (I–t) curves from LaB6 deposited on a tungsten foil. (d) Field emission micrograph of LaB6 deposited on a tungsten foil.
emission current of 1 nA was observed to be 2:1 103 V=mm, and estimated emission current density of 1:2 104 A=cm2 has been drawn at an applied field of 6 103 V=mm with 199 mA as the emission current. The FE behavior of the LaB6 =Re flat emitter is shown in Fig. 7. The onset field required to draw an emission current of 1 nA was found to be 2:36 V=mm for LaB6 =Re flat emitter. For LaB6 =Re flat emitter, a maximum emission current of 1:37 mA was obtained at an applied field of 2:6 V=mm. The difference in the onset voltage and emission current densities is attributed to the surface roughness of the LaB6 films. A careful observation of the SEM image of LaB6 film on the rhenium tip shows several nanometric protrusions, as compared with the flat substrate. The F–N plots, obtained from the I–V curves are shown in Figs. 6(b) and 7(b), for the PLD LaB6 =Re tip and flat emitters, respectively. The linear nature of the observed F–N plots, again confirms the metallic character of the emitters revealing that the electron emission is according to the F–N theory. The field enhancement factor b is calculated to be 5450 and 3028 cm1 for the LaB6 /Re tip and flat emitters, respectively. The effective emission area is calculated to be 1:65 106 nm2 for LaB6 =Re tip substrate. The FE current stability of LaB6 /Re tip emitter, observed at the pre-set current values 10 and 50 mA, over a duration of more than 3 h, is shown in Fig. 6(c) measured at base pressure 1 109 mbar. For LaB6 /Re flat emitter,
current stability was checked at pre-set value of 0:5 mA and is shown in Fig. 7(c). The observed FE patterns for LaB6 / Re tip emitter comprised of bright and symmetric ovalshaped spots, as seen in Fig. 6(d). In case of planar emitters, a stable and bright spot was observed, as seen in Fig. 7(d). The post-FE SEM studies of LaB6 =Re tip were also carried out in order to see the structural stability of the PLD LaB6 film. It is interesting to note that the SEM images Fig. 8(c and d) showed no significant change in emitter surface morphology. The post-FE SEM images also show that the LaB6 nanometric protrusions present on the Re tip are even sharper than before emission due to ion bombardment. The studies indicate that the PLD LaB6 film has very good adhesion and remarkable structural stability against ion bombardment and high field induced mechanical stress, on rhenium surfaces. The presence of sharper protrusions on the LaB6 =W tip as compared to the LaB6 /Re tip can also be attributed to the difference in the growth of crystallites on the two substrates. The field required to draw 1 nA current from the LaB6 /Re (tip and flat) are considerably high than that of LaB6 /W (tip and flat). This can be attributed to the difference between growth patterns for the two substrates with different crystallographic orientations. In the XRD of LaB6 (1 1 0) and (3 1 1) are prominent peaks along with W (1 1 0), while in case of Re substrate LaB6 (2 0 0) is a prominent peak along with Re (1 0 0). It is also observed
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Fig. 6. (a) Field emission current–voltage (I–V) characteristics of LaB6 deposited on rhenium tip. (b) The F–N plot. (c) (I–t) curves from LaB6 deposited on a rhenium tip. (d) Field emission micrograph of LaB6 deposited on a rhenium tip.
Fig. 7. (a) Field emission current–voltage (I–V) characteristics of LaB6 deposited on rhenium foil. (b) The F–N plot. (c) (I–t) curves from LaB6 deposited on a rhenium foil. (d) Field emission micrograph of LaB6 deposited on a rhenium foil.
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Fig. 8. Post-field emission SEM images of LaB6 =W tip (a and b) and LaB6 =Re tip (c and d).
that grain growth is lateral in case of LaB6 on Re and the same is inhibited in case of W with the experimental conditions in this case. This is also evident from SEM micrographs of W (Fig. 8(a, b)) and Re (Fig. 8(c, d)) tips. The FE patterns of LaB6 /W (Fig. 5(d)) and LaB6 =Re, Fig. 6(d), show two-fold and four-fold symmetric lobes, respectively. In view of the above discussion, these could be attributed to LaB6 (1 1 0) and LaB6 (1 0 0) planes, respectively. Further analysis requires detailed study of the LaB6 films on these two substrates with varying experimental parameters during the PLD. 4. Conclusions The PLD of LaB6 films on W and Re pointed and flat substrates offer good mechanical adhesion, which is required for stable FE. The pointed substrates offer an advantage of drawing large current density as high as 1:2 104 A=cm2 at 3 103 and 3 103 V=mm from both the LaB6 /W and LaB6 /Re tip substrates, respectively. The growth of LaB6 crystallites on W and Re is found to be highly dependent on the substrate crystal structure and is observed to offer emission behavior. Since PLD is a simple and economic methodology for depositing LaB6 nanostructures, it appears to be a promising technique for making FE devices. Acknowledgments DJL would like to thank the Department of Atomic Energy (DAE) and the Bhabha Atomic Research Center
(BARC) for the grant of senior research fellowship. Authors would like to thank Council of Scientific and Industrial Research (CSIR) and Department of Science and Technology (DST) New Delhi, Government of India, for the financial support enabling DJL to attend IVNC and IFES 2006 held in China.
References [1] R.B. Sharma, D.J. Late, D.S. Joag, A. Govindraj, C.N.R. Rao, Chem. Phys. Lett. 428 (2006) 102. [2] N. De Jonge, Y. Iamy, K. Schoots, T.H. Oosterkamp, Nature 420 (2002) 393. [3] N. De Jonge, N.J. van Druten, Ultramicroscopy 95 (2003) 85. [4] S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H. Da, Science 283 (1999) 512. [5] W.A. de Heer, A. Chaˆtelain, D.A. Ugarte, Science 270 (1995) 1179. [6] W. Yi, T. Jeong, S. Yu, J. Heo, C. Lee, J. Lee, W. Kim, J.-B. Yoo, J. Kim, Adv. Mater. 14 (2002) 1464. [7] V.N. Tondare, C. Balasubramanian, S.V. Shende, D.S. Joag, V.P. Godbole, S.V. Bhoraskar, M. Bhadbhade, Appl. Phys. Lett. 80 (2002) 4813. [8] Y.Z. Jin, W.K. Hsu, Y.L. Chueh, L.J. Chou, Y.Q. Zhu, K. Brigatti, H.W. Kroto, D.R.M. Walton, Angew. Chem. Int. Ed. 43 (2004) 5670. [9] D.S. Joag, D.J. Late, U.D. Lanke, Solid State Commun. 130 (2004) 305. [10] N.S. Ramgir, D.J. Late, A.B. Bhise, M.A. More, I.S. Mulla, D.S. Joag, K. Vijaymohanan, Appl. Phys. Lett. 88 (2006) 42107. [11] H. Zhang, J. Tang, Q. Zhang, G. Zhao, G. Yang, J. Zhang, O. Zhou, L. Qin, Adv. Mater. 18 (2006) 87. [12] D.J. Late, M.A. More, D.S. Joag, P. Misra, B.N. Singh, L.M. Kukreja, Appl. Phys. Lett. 89 (2006) 123510. [13] M. Nakamoto, K. Fukuda, Appl. Surf. Sci. 202 (2002) 289. [14] J.M. Lafferty, J. Appl. Phys. 22 (1951) 299.
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[15] E. Cappelli, C. Scilletta, S. Orlando, R. Flammini, S. Iacobucci, P. Ascarelli, Thin Solid Films 482 (2005) 305. [16] X. Ma, Z. Yan, B. Yuan, B. Li, Nanotechnology 16 (2005) 832. [17] J. Yang, J. Lian, Q. Dong, Z. Guo, Appl. Surf. Sci. 229 (2004) 2. [18] V. Craciun, D. Craciun, Appl. Surf. Sci. 247 (2005) 384.
[19] Powder Diffraction File, International Centre for Diffraction Data, Swarthmore, PA, JCPDS File No. 34-427, 4-806 and 5-702. [20] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. London Ser. A 119 (1928) 173. [21] A.G.J. Van Oostrom, Philips Res. Rep. 1 (1966) 11.
Ultramicroscopy 107 (2007) 825–832 www.elsevier.com/locate/ultramic
Field emission studies of pulsed laser deposited LaB6 films on W and Re Dattatray J. Latea, Mahendra A. Morea, Pankaj Misrab, B.N. Singhb, Lalit M. Kukrejab, Dilip S. Joaga, a
DST Unit on Nanoscience and Center for Advanced Studies in Material Science and Condensed Matter Physics, Department of Physics, University of Pune, Pune 411007, India b Thin Film Laboratory, Raja Ramanna Centre for Advance Technology, Indore 452013, India
Abstract Lanthanum hexaboride films were grown on tungsten and rhenium tips and foils by pulsed laser deposition. The X-ray diffraction spectra of the PLD LaB6 films on both the substrates show crystalline nature with average grain size 125 nm. The field emission studies of pointed and foil specimens were performed in conventional and planar diode configurations, respectively, under ultra-high vacuum condition. An estimated current density of 1:2 104 A=cm2 was drawn at the electric field of 3 103 and 6 103 V=mm from the LaB6 coated tips of tungsten and rhenium, respectively. The Fowler–Nordheim plots were found to be linear showing metallic behavior of the emitters. The field enhancement factors were calculated from the slopes of the Fowler–Nordheim plots, indicating that the field emission is from LaB6 nanoscale protrusions present on emitter surfaces. The emitters were operated for long time current stability (3 h) studies. The post-field emission surface morphology of the emitters showed no significant erosion of LaB6 films during 3 h continuous operation. The observed behavior indicates that it is linked with the growth of LaB6 films on W and Re. These results reveal that the LaB6 films exhibit high resistance to ion bombardment and excellent structural stability and are more promising emitters for practical applications in field emission based devices. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15Aa; 85.45Bz Keywords: Field emission; Pulsed laser deposition and LaB6
1. Introduction Field electron emission involves a quantum mechanical tunneling process under an applied electric field. It has diverse technological applications in flat panel displays, microwave-generation devices and vacuum micro/nanoelectronic devices. A novel and promising approach to fabricate field emission (FE) cathodes is to use anisotropic nanostructured materials such as carbon nanotubes (CNTs) [1–6], AlN [7], NbS2 [8], GaN [9], ZnO [10] and Lanthanum hexaboride ðLaB6 Þ [11–13]. Lanthanum hexaboride has been widely used in modern technology as an excellent thermionic electron emission source [14], which offers high brightness and long service life. The advantages Corresponding author. Tel.: +91 020 25692678; fax: +91 020 25691684. E-mail address: [email protected] (D.S. Joag).
0304-3991/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2007.02.014
are originated from its low work function (2.6 eV), high melting point ð2600 CÞ and low volatility. Pulsed laser deposition (PLD) [15–18] is a method to grow high quality thin films of a variety of metals and semiconductor materials on suitable substrates in a controlled manner. Craciun et al. have reported the synthesis of crystalline LaB6 thin films on silicon substrates by PLD technique [18]. Recently we have reported FE studies on LaB6 films deposited by PLD on a tungsten tip exhibiting striking features [12]. Here we report the synthesis of nanocrystalline LaB6 thin films on tungsten and rhenium pointed and flat substrates by PLD and their FE studies. 2. Experimentation Lanthanum hexaboride films were deposited on tungsten and rhenium tips and flat substrates (foils) in a vacuum
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chamber (base pressure 1 106 mbar) by pulsed laser ablation from Lanthanum hexaboride pellet target. For PLD, pulsed Nd: YAG laser (2nd harmonic l ¼ 355 nm, repetition rate n ¼ 10 Hz, pulse width t ¼ 6 ns, deposition time 60 s and fluence F1 J=cm2 ) was used as an excitation source. The deposition was carried out in helium atmosphere at 1 103 mbar for 60 s duration. Tungsten and rhenium tips were prepared by electrochemical polishing of pieces of their wires using suitable electrolytes. These tips were mounted on the substrate holder in such a manner that the tip apex was normal to the holder base, facing the target. Pre-cleaned pieces of tungsten foil ð1 cm 1 cmÞ and rhenium foil ð1 cm 0:5 cmÞ were mounted close to the tip on the same holder. The sintered LaB6 pellet was mounted on a rotating target holder at a distance of 5 cm from the substrates. During PLD, the substrates were kept at the temperature 700 C. The PLD of the desired LaB6 films on tungsten and rhenium tips and flat substrates was carried out in the very same experiment. The deposited LaB6 films are approximately 1 mm thick. The PLD of LaB6 on Re and W tips and flat substrates was carried out at least on three specimens each. The flat substrates facilitated structural characterization of the deposited LaB6 films by grazing angle X-ray Diffractometer (Model D-8 Advanced, Brucker AXS). Analytical Scanning Electron Microscope JEOL 6360A was used to investigate the surface morphology of the PLD LaB6 films. The FE measurements of LaB6 /W tip and LaB6 /Re tip were carried out, in an all glass conventional FE microscope (FEM) tube, in separate experiments. Also, the FE studies of LaB6 /W and LaB6 /Re flat substrates were performed in planar diode configuration, in separate experiments. The experimental details pertaining to the vacuum processing of the tube have been described elsewhere [1,9]. 3. Results and discussion 3.1. Characterization by XRD and SEM The grazing angle ð2 Þ XRD patterns of the LaB6 films deposited on W and Re flat substrates using PLD are seen
in Fig. 1(a) and (b), respectively. In both the cases, a set of well-defined diffractions peaks are seen, revealing the crystalline nature of LaB6 on these substrates. The phase identification and plane indexing are done by comparing the observed d values with the standard JCPDS data cards for LaB6 , W and Re [19]. A careful observation of the spectra reveals that the LaB6 film on W foil has preferred orientation with major reflections from the (1 1 0) and the (3 1 1) planes and on Re foil from the (2 0 0) plane. The average grain size obtained using Debye–Scherrer formula is observed to be 125 nm on both the W and Re substrates. The SEM images of the PLD LaB6 films on W tip and foil recorded at different magnifications are shown in Fig. 2(a–d). On both, pointed (a and b) and flat (c and d) substrates, the LaB6 crystallites are seen to cover the entire substrate surface. In case of tip substrate, the overall surface morphology is observed to be rough and characterized by the presence of nanoscale protrusions. The SEM observation shows that on the flat substrates, the LaB6 crystallites have average grain size less than 200 nm. The SEM images of PLD LaB6 films on Re tip and flat substrates recorded at different magnifications are shown in Fig. 3(a–d). On Re tip substrate the entire surface is fully covered with LaB6 film and overall morphology is also seen to be rough with the presence of nanoscale protrusions on the tip surface. In case of Re flat substrate the entire substrate surface is fully covered with nearly micron sized LaB6 particles, on top of which a large number of LaB6 nanoparticles are seen. 3.2. Field emission 3.2.1. W tip and flat emitter The FE characteristics of nanocrystalline LaB6 film on W tip, recorded at the base pressure of 1 109 mbar, are depicted in Fig. 4. For LaB6 =W tip emitter, an emission current of 1 nA was reproducibly observed at an applied field 5:5 102 V=mm, and a current 195 mA with an estimated emission current density of 1:2 104 A=cm2 has been drawn at an applied field of 3 103 V=mm. The FE
Fig. 1. XRD spectra of (a) LaB6 =W foil; (b) LaB6 =Re foil.
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Fig. 2. SEM images of LaB6 deposited on a tungsten tip (a and b) and tungsten foil (c and d).
Fig. 3. SEM images of LaB6 deposited on a rhenium tip (a and b) and rhenium foil (c and d).
behavior of the LaB6 =W planar emitter is shown in Fig. 5. The onset field, required to draw an emission current of 1 nA, was found to be 1:2 V=mm for LaB6 =W planar emitter. For the planar diode configuration, the applied field is defined as F ¼ V =d, where V is applied voltage and d is the distance of separation between cathode and anode ðd ¼ 5 mmÞ. In case of LaB6 =W planar emitter, emission current as high as 2:86 mA was obtained at an applied field
of 2:16 V=mm. As revealed from the SEM images, the LaB6 film on tungsten tip (Fig. 2(a, b)) has very sharp nanometric protrusions. The Fowler–Nordheim (F–N) plots [graph of lnðI=V2 Þ versus 104 =V], obtained from the I–V curves, are shown in Figs. 4(b) and 5(b) for the LaB6 film on W tip and flat emitters. The linear nature of the observed F–N plots confirms the metallic character of the emitters, and that the electron emission is according to
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Fig. 4. (a) Field emission current–voltage (I–V) characteristics of LaB6 deposited on tungsten tip. (b) The F–N plot. (c) (I–t) curves from LaB6 deposited on a tungsten tip. (d) Field emission micrograph of LaB6 deposited on a tungsten tip.
the F–N theory. The field enhancement factor b related to the slope (m) of the F–N plot is identified to be directly dependent on the emitter radius and is expressed by [20] b ¼ ð6:83 103 F3=2 Þ=m,
(1)
where f is the work function of the emitter material in eV. In the present case, the work function of LaB6 (fÞ is taken to be 2.6 eV. The effective geometrical field enhancement factor b is calculated to be 5537 and 3036 cm1 for LaB6 coated W tip and flat emitters, respectively, from Eq. (1). The effective emission area a is calculated using the following equation [11,21]: b ¼ lnð1:5 106 ab2 =FÞ þ 10:4=F3=2 ,
(2)
where b is the y-intercept of the F–N plot. The effective emission area is calculated to be 1:4 106 nm2 for LaB6 =W tip. A small emitting area is an indication of nanometric emitting structures on the tip surface. For FE electron sources, along with the emission competence, the current stability is also a decisive and an important parameter. The FE current stability of LaB6 =W tip emitters, observed at the pre-set current values 10 and 50 mA, over a duration of more than 3 h, is depicted in Fig. 4(c) measured at the base pressure 1 109 mbar. For LaB6 =W flat emitters, current stability was observed at the pre-set value of 1 mA and is shown in Fig. 5(c). The I–t plots show emission current dependence behavior for the tip and flat emitters. The observed current fluctuations are due to adsorption,
desorption and diffusion of adsorbates on the emitter surface. The LaB6 covered W tip emitter surface would become cleaner due to the ion bombardment induced desorption of the residual gases from the tip surface as observed in the current stability experiment. The clean surface thus produced leads to stabilization of the emission current at a higher value of 100 mA. At a lower current level, the competition between residual gas adsorption and ion bombardment induced desorption results in slow decrease and increase in the emission current. The observed FE pattern for the pointed emitter comprised of bright and symmetric oval-shaped spots, as seen in Fig. 4(d) for LaB6 =W tip emitter. In case LaB6 =W flat emitter, a stable and bright spot was observed, as seen in Fig. 5(d). The post-FE SEM studies of LaB6 =W tip were carried out in order to see structural stability of the PLD LaB6 film on this substrate. It is interesting to note that the SEM images Fig. 8(a and b) showed no significant change in emitter surface morphology, which indicates that the PLD LaB6 film on tungsten has very good adhesion and remarkable structural stability against ion bombardment and high field induced mechanical stresses. This is especially important for fabricating FE devices with high current density and stability. 3.2.2. Re tip and flat emitter The results of studies on FE from LaB6 films on Re tip, are depicted in Fig. 6. The applied field required to draw an
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Fig. 5. (a) Field emission current–voltage (I–V) characteristics of LaB6 deposited on tungsten foil. (b) The F–N plot. (c) (I–t) curves from LaB6 deposited on a tungsten foil. (d) Field emission micrograph of LaB6 deposited on a tungsten foil.
emission current of 1 nA was observed to be 2:1 103 V=mm, and estimated emission current density of 1:2 104 A=cm2 has been drawn at an applied field of 6 103 V=mm with 199 mA as the emission current. The FE behavior of the LaB6 =Re flat emitter is shown in Fig. 7. The onset field required to draw an emission current of 1 nA was found to be 2:36 V=mm for LaB6 =Re flat emitter. For LaB6 =Re flat emitter, a maximum emission current of 1:37 mA was obtained at an applied field of 2:6 V=mm. The difference in the onset voltage and emission current densities is attributed to the surface roughness of the LaB6 films. A careful observation of the SEM image of LaB6 film on the rhenium tip shows several nanometric protrusions, as compared with the flat substrate. The F–N plots, obtained from the I–V curves are shown in Figs. 6(b) and 7(b), for the PLD LaB6 =Re tip and flat emitters, respectively. The linear nature of the observed F–N plots, again confirms the metallic character of the emitters revealing that the electron emission is according to the F–N theory. The field enhancement factor b is calculated to be 5450 and 3028 cm1 for the LaB6 /Re tip and flat emitters, respectively. The effective emission area is calculated to be 1:65 106 nm2 for LaB6 =Re tip substrate. The FE current stability of LaB6 /Re tip emitter, observed at the pre-set current values 10 and 50 mA, over a duration of more than 3 h, is shown in Fig. 6(c) measured at base pressure 1 109 mbar. For LaB6 /Re flat emitter,
current stability was checked at pre-set value of 0:5 mA and is shown in Fig. 7(c). The observed FE patterns for LaB6 / Re tip emitter comprised of bright and symmetric ovalshaped spots, as seen in Fig. 6(d). In case of planar emitters, a stable and bright spot was observed, as seen in Fig. 7(d). The post-FE SEM studies of LaB6 =Re tip were also carried out in order to see the structural stability of the PLD LaB6 film. It is interesting to note that the SEM images Fig. 8(c and d) showed no significant change in emitter surface morphology. The post-FE SEM images also show that the LaB6 nanometric protrusions present on the Re tip are even sharper than before emission due to ion bombardment. The studies indicate that the PLD LaB6 film has very good adhesion and remarkable structural stability against ion bombardment and high field induced mechanical stress, on rhenium surfaces. The presence of sharper protrusions on the LaB6 =W tip as compared to the LaB6 /Re tip can also be attributed to the difference in the growth of crystallites on the two substrates. The field required to draw 1 nA current from the LaB6 /Re (tip and flat) are considerably high than that of LaB6 /W (tip and flat). This can be attributed to the difference between growth patterns for the two substrates with different crystallographic orientations. In the XRD of LaB6 (1 1 0) and (3 1 1) are prominent peaks along with W (1 1 0), while in case of Re substrate LaB6 (2 0 0) is a prominent peak along with Re (1 0 0). It is also observed
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Fig. 6. (a) Field emission current–voltage (I–V) characteristics of LaB6 deposited on rhenium tip. (b) The F–N plot. (c) (I–t) curves from LaB6 deposited on a rhenium tip. (d) Field emission micrograph of LaB6 deposited on a rhenium tip.
Fig. 7. (a) Field emission current–voltage (I–V) characteristics of LaB6 deposited on rhenium foil. (b) The F–N plot. (c) (I–t) curves from LaB6 deposited on a rhenium foil. (d) Field emission micrograph of LaB6 deposited on a rhenium foil.
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Fig. 8. Post-field emission SEM images of LaB6 =W tip (a and b) and LaB6 =Re tip (c and d).
that grain growth is lateral in case of LaB6 on Re and the same is inhibited in case of W with the experimental conditions in this case. This is also evident from SEM micrographs of W (Fig. 8(a, b)) and Re (Fig. 8(c, d)) tips. The FE patterns of LaB6 /W (Fig. 5(d)) and LaB6 =Re, Fig. 6(d), show two-fold and four-fold symmetric lobes, respectively. In view of the above discussion, these could be attributed to LaB6 (1 1 0) and LaB6 (1 0 0) planes, respectively. Further analysis requires detailed study of the LaB6 films on these two substrates with varying experimental parameters during the PLD. 4. Conclusions The PLD of LaB6 films on W and Re pointed and flat substrates offer good mechanical adhesion, which is required for stable FE. The pointed substrates offer an advantage of drawing large current density as high as 1:2 104 A=cm2 at 3 103 and 3 103 V=mm from both the LaB6 /W and LaB6 /Re tip substrates, respectively. The growth of LaB6 crystallites on W and Re is found to be highly dependent on the substrate crystal structure and is observed to offer emission behavior. Since PLD is a simple and economic methodology for depositing LaB6 nanostructures, it appears to be a promising technique for making FE devices. Acknowledgments DJL would like to thank the Department of Atomic Energy (DAE) and the Bhabha Atomic Research Center
(BARC) for the grant of senior research fellowship. Authors would like to thank Council of Scientific and Industrial Research (CSIR) and Department of Science and Technology (DST) New Delhi, Government of India, for the financial support enabling DJL to attend IVNC and IFES 2006 held in China.
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