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Quantification and minimization of disorder caused by focused electron beam induced deposition of cobalt on graphene
Microelectronic Engineering 88 (2011) 2063–2065
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Quantification and minimization of disorder caused by focused electron beam induced deposition of cobalt on graphene J.M. Michalik a,b,⇑, S. Roddaro c, L. Casado a, M.R. Ibarra a,b,d, J.M. De Teresa b,d a
Instituto de Nanociencia de Aragón, Universidad de Zaragoza, Zaragoza 50018, Spain Departamento de Física de la Materia Condensada, Universidad de Zaragoza, Facultad de Ciencias, Zaragoza 50009, Spain c NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, I56127 Pisa, Italy d Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, Facultad de Ciencias, Zaragoza 50009, Spain b
a r t i c l e
i n f o
Article history: Available online 7 December 2010 Keywords: Graphene Electron beam induced deposition Raman spectroscopy Contamination Disorder
a b s t r a c t Graphene has attracted a lot of attention due to its unique transport properties, including applications in Spintronics. Therefore, one of the key issues is the investigation of the spin polarized transport phenomena, which requires the use of magnetic contacts for electrical measurements. Here we present results of the micro-Raman spectroscopy and in situ transport measurements during the electron beam irradiation and electron beam induced deposition of metallic Co on graphene. We aim to understand the effects caused by the Co deposition on the graphene in order to minimize undesired deterioration and induced disorder. Annealing procedures have been also carried out to recover the initial graphene properties. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Graphene – a single-layer hexagonal lattice of carbon atoms – has recently attracted a lot of attention in the scientific community, in particular after the observation of exotic transport phenomena such as the anomalous quantum Hall effect [1]. Moreover, graphene displays unique electronic transport properties such as very high [2] and tunable conductivity at room temperature together with a spin polarized transport up to room temperature [3]. This makes graphene a promising material for applications in Spintronics. One of the key issues in the investigation of the spin polarized transport in graphene is the distinction between charge and spin signals. In that sense, a ‘‘non-local’’ technique has been used by Tombros et al. [4] combined with the use of magnetic contacts. The drawback of the commonly-used technique for fabricating electrical contacts (electron beam lithography, EBL) is that for the spin polarized transport measurement one needs an additional tunnel barrier between the contact and the sample in order to adjust their respective spin resistances. This increases the complexity of the lithography process. Here we investigate the adoption of focused-electron-beaminduced deposition (FEBID) technique as an alternative route to grow resistance-matched contacts for the investigation of spin
⇑ Corresponding author at: Instituto de Nanociencia de Aragón, Universidad de Zaragoza, Zaragoza 50018, Spain. Tel.: +34 976762463. E-mail address: [email protected] (J.M. Michalik). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.12.002
transport in graphene. Recent studies [5] show that FEBID-grown cobalt deposits show resistivity values comparable to those of graphene. 2. Experimental The cobalt deposition was done with a field-emission scanning electron microscope (equipment Nova 200 Nanolab by FEI) using Co2(CO)8 precursor gas. The electron gun was operating typically at 10 kV, 1.6 nA beam current and the working distance was kept at 5.2 mm. We have also performed the electrical transport properties measurements of graphene during electron beam irradiation and Co deposition using electrical microprobes by Kleindiek in combination with a Keithley 2821A/6221 setup. Spatially-resolved micro-Raman was performed using a custom system based on a 488 nm laser by ModuLaser, a monocromator HR320 by Jobin Yvon and a peltier-cooled CCD by Roper Scientific. Heating in controlled atmosphere was accomplished using a PECVD chamber by Sistec. The graphene samples used in the present study have been produced using a standard mechanical exfoliation method. Graphene flakes were deposited on oxidized silicon wafers with nominal oxide thickness of 285 nm. This allows inspection of the sample surface with optical microscope and selection of thin graphene flakes. These were then carefully examined with Raman spectroscopy in order to confirm the thickness to be one atomic layer. A fully operative device for ‘‘non-local’’ magnetotransport measurements consists of a graphene flake with four electrical contacts
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J.M. Michalik et al. / Microelectronic Engineering 88 (2011) 2063–2065
Fig. 1. Right: SEM image of the graphene flake together with EBL connections prepared for resistance measurement with the microprobes. Left: complete device for spin dependent transport measurements: graphene flake with EBID prepared connections of different width.
. . . . . .
of different width (see Fig. 1, left photo). The different contact widths produce different coercive fields [6], allowing the investigation of the spin dependent transport. We have selected the contacts to be 1 lm, 500 nm, 350 nm and 150 nm wide and spacing between them of the order of 2.5 lm. Under optimized conditions, a good quality device with Co deposits is obtained with a total deposition time of 20 min. This process time is much shorter than required for damaging graphene, as will be shown later. Despite the clear advantages of FEBID in terms of complexity of the fabrication process, one should be aware of the negative effects of prolonged electron beam irradiation on graphene [7].
.
3. Results
.
3.1. Effects of electron beam irradiation
Fig. 2. Graphene resistance as a function of the 1.6 nA e beam irradiation time. In the inset the reversibility of the resistance change under irradiation with 98 pA beam current – beam is switched off in blue colored zones (accelerating voltage of 5 kV was used in both cases). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
By using two electrical microprobes as well as EBL contact pads (Ti/Au), we have performed in situ resistance measurements of the effect of electron-beam irradiation on graphene sheets (see Fig. 1, right photo). First, we have used low electron beam current (98 pA). Under such conditions a partial reversibility of the effect of resistance increase is observed. The system returns to lower R state after the beam is switched off as can be seen in the inset of
Fig. 3. The induced disorder calculated as micro-Raman I(D)/I(G) peak ratio after FEBID Co deposition (left) and subsequent annealing at 500 °C (right) (red denotes highest disorder, while dark blue a lack of disorder). In the inset: graphene flakes with deposited Co contacts. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
J.M. Michalik et al. / Microelectronic Engineering 88 (2011) 2063–2065
2065
contacts. This indicates a need of performing heat-induced ‘‘cleaning’’ of graphene by a removal of attached precursor gas molecules from the graphene surface. Interestingly, we have observed that graphene flakes are not damaged at all if exposed to the precursor gas without switching on the electron beam. The D peak in Raman spectra is not visible after the exposition of graphene samples to the precursor gas during 20 min. This indicates a crucial role of electrons in activating the long range precursor-induced disorder in graphene. 4. Conclusions
Fig. 4. Raman spectra of graphene taken after FEBID Co contacts growth (red line) and after two consecutive heat treatments (500 °C and 700 °C) in Ar atmosphere. In the inset Raman spectra of graphite after Co deposition and 500 °C heating in Ar. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Second, graphene was irradiated with 1.6 nA beam current at an energy of 5 keV. We observe that an increase of the resistivity of graphene becomes clear after an hour of continuous irradiation (see Fig. 2, main panel). We interpret this effect as a consequence of the irreversible deterioration of the graphene layer. Also, we have observed a drop of resistance during the first few minutes of measurement, which can be easily explained by local heating of the sample by the electron beam and subsequent desorption of gas molecules adsorbed to graphene. 3.2. Growth of cobalt by FEBID on graphene flakes The effects produced by the exposure of graphene to the precursor gas – Co2(CO)8 and growth of Co contacts were investigated by micro-Raman spectroscopy measurements. FEBID damage was quantified by looking at the ratio of the D/G Raman peaks, which is expected to provide a reliable evaluation of the disorder in graphene [8]. For the purpose of investigation of the spatial distribution of such disorder we have performed a total number of 60 Raman scans at thicker (graphite) material. The D and G Raman peaks were fitted with Lorentz-shaped peaks and their intensity was numerically calculated. Graphene is very sensitive to any contaminants on its surface [9], and indeed we have observed a significant FEBID-induced disorder not only at the deposition spot but even up to a distance of 2 mm (see Fig. 3). Moreover the highest D/G peak ratio is not observed at the deposition spot but away of it, i.e., at the point coinciding with the highest flow of the precursor gas. On the other hand, it is crucial to note that such a deterioration of graphene can be almost completely reversed by heating the sample up to 500 °C in Ar atmosphere (see Figs. 3 and 4), suggesting that graphene is not irreversibly damaged by the growth of Co
It can be concluded that graphene deterioration due to irradiation with electron beam is not the most important effect when dealing with Co-contact deposition on top of graphene flakes using focused electron beam. Our results indicate that a main source of disorder in graphene is the precursor gas, whose molecules become adsorbed on the surface of the sample and dissociated by the electron beam. The induced damage can be removed by heating the samples under controlled atmosphere. Our preliminary study also indicates the possibility of recovering graphene electronic properties after the Co deposition process by heating the device directly with applied electrical current (as was suggested by Moser et al. [10]). This seems enough to remove the produced surface contaminants. The charge neutrality point is shifted from high gate voltage to a value comparable with EBL processed device. In such a way, one would be able to obtain a fully operative graphene device in just one step through focused electron beam induced cobalt deposition. Acknowledgements This work was supported by Spanish Ministry of Science (through Project MAT2008-06567-C02, including FEDER funding), and the Aragon Regional Government (Projects E26 and PI046/ 09). We acknowledge experimental help by Rosa Córdoba. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Nat. Lond. 438 (2005) 197. [2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666; A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [3] N. Tombros, C. Jozsa, M. Popinciuc, H.T. Jonkman, B.J. van Wees, Nat. Lond. 448 (2007) 571. [4] N. Tombros, S. Tanabe, A. Veligura, C. Jozsa, M. Popinciuc, H.T. Jonkman, B.J. van Wees, Phys. Rev. Lett. 101 (2008) 046601. [5] A. Fernández-Pacheco, J.M. De Teresa, R. Córdoba, M.R. Ibarra, J. Phys. D: Appl. Phys. 42 (2009) 055005. [6] A. Fernández-Pacheco, J.M. De Teresa, A. Szkudlarek, R. Córdoba, M.R. Ibarra, D. Petit, L. O’Brien, H.T. Zeng, E.R. Lewis, D.E. Read, R.P. Cowburn, Nanotechnology 20 (2009) 475704. [7] D. Teweldebrhan, A.A. Balandin, Appl. Phys. Lett. 94 (2009) 013101. [8] A.C. Ferrari, Solid State Commun. 143 (2007) 47. [9] J.M. Caridad, F. Rossella, V. Bellani, M. Maicas, M. Patrini, E. Díez, J. Appl. Phys. 108 (2010) 084321. [10] J. Moser, A. Barreiro, A. Bachtold, Appl. Phys. Lett. 91 (2007) 163513.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Quantification and minimization of disorder caused by focused electron beam induced deposition of cobalt on graphene J.M. Michalik a,b,⇑, S. Roddaro c, L. Casado a, M.R. Ibarra a,b,d, J.M. De Teresa b,d a
Instituto de Nanociencia de Aragón, Universidad de Zaragoza, Zaragoza 50018, Spain Departamento de Física de la Materia Condensada, Universidad de Zaragoza, Facultad de Ciencias, Zaragoza 50009, Spain c NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, I56127 Pisa, Italy d Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, Facultad de Ciencias, Zaragoza 50009, Spain b
a r t i c l e
i n f o
Article history: Available online 7 December 2010 Keywords: Graphene Electron beam induced deposition Raman spectroscopy Contamination Disorder
a b s t r a c t Graphene has attracted a lot of attention due to its unique transport properties, including applications in Spintronics. Therefore, one of the key issues is the investigation of the spin polarized transport phenomena, which requires the use of magnetic contacts for electrical measurements. Here we present results of the micro-Raman spectroscopy and in situ transport measurements during the electron beam irradiation and electron beam induced deposition of metallic Co on graphene. We aim to understand the effects caused by the Co deposition on the graphene in order to minimize undesired deterioration and induced disorder. Annealing procedures have been also carried out to recover the initial graphene properties. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Graphene – a single-layer hexagonal lattice of carbon atoms – has recently attracted a lot of attention in the scientific community, in particular after the observation of exotic transport phenomena such as the anomalous quantum Hall effect [1]. Moreover, graphene displays unique electronic transport properties such as very high [2] and tunable conductivity at room temperature together with a spin polarized transport up to room temperature [3]. This makes graphene a promising material for applications in Spintronics. One of the key issues in the investigation of the spin polarized transport in graphene is the distinction between charge and spin signals. In that sense, a ‘‘non-local’’ technique has been used by Tombros et al. [4] combined with the use of magnetic contacts. The drawback of the commonly-used technique for fabricating electrical contacts (electron beam lithography, EBL) is that for the spin polarized transport measurement one needs an additional tunnel barrier between the contact and the sample in order to adjust their respective spin resistances. This increases the complexity of the lithography process. Here we investigate the adoption of focused-electron-beaminduced deposition (FEBID) technique as an alternative route to grow resistance-matched contacts for the investigation of spin
⇑ Corresponding author at: Instituto de Nanociencia de Aragón, Universidad de Zaragoza, Zaragoza 50018, Spain. Tel.: +34 976762463. E-mail address: [email protected] (J.M. Michalik). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.12.002
transport in graphene. Recent studies [5] show that FEBID-grown cobalt deposits show resistivity values comparable to those of graphene. 2. Experimental The cobalt deposition was done with a field-emission scanning electron microscope (equipment Nova 200 Nanolab by FEI) using Co2(CO)8 precursor gas. The electron gun was operating typically at 10 kV, 1.6 nA beam current and the working distance was kept at 5.2 mm. We have also performed the electrical transport properties measurements of graphene during electron beam irradiation and Co deposition using electrical microprobes by Kleindiek in combination with a Keithley 2821A/6221 setup. Spatially-resolved micro-Raman was performed using a custom system based on a 488 nm laser by ModuLaser, a monocromator HR320 by Jobin Yvon and a peltier-cooled CCD by Roper Scientific. Heating in controlled atmosphere was accomplished using a PECVD chamber by Sistec. The graphene samples used in the present study have been produced using a standard mechanical exfoliation method. Graphene flakes were deposited on oxidized silicon wafers with nominal oxide thickness of 285 nm. This allows inspection of the sample surface with optical microscope and selection of thin graphene flakes. These were then carefully examined with Raman spectroscopy in order to confirm the thickness to be one atomic layer. A fully operative device for ‘‘non-local’’ magnetotransport measurements consists of a graphene flake with four electrical contacts
2064
J.M. Michalik et al. / Microelectronic Engineering 88 (2011) 2063–2065
Fig. 1. Right: SEM image of the graphene flake together with EBL connections prepared for resistance measurement with the microprobes. Left: complete device for spin dependent transport measurements: graphene flake with EBID prepared connections of different width.
. . . . . .
of different width (see Fig. 1, left photo). The different contact widths produce different coercive fields [6], allowing the investigation of the spin dependent transport. We have selected the contacts to be 1 lm, 500 nm, 350 nm and 150 nm wide and spacing between them of the order of 2.5 lm. Under optimized conditions, a good quality device with Co deposits is obtained with a total deposition time of 20 min. This process time is much shorter than required for damaging graphene, as will be shown later. Despite the clear advantages of FEBID in terms of complexity of the fabrication process, one should be aware of the negative effects of prolonged electron beam irradiation on graphene [7].
.
3. Results
.
3.1. Effects of electron beam irradiation
Fig. 2. Graphene resistance as a function of the 1.6 nA e beam irradiation time. In the inset the reversibility of the resistance change under irradiation with 98 pA beam current – beam is switched off in blue colored zones (accelerating voltage of 5 kV was used in both cases). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
By using two electrical microprobes as well as EBL contact pads (Ti/Au), we have performed in situ resistance measurements of the effect of electron-beam irradiation on graphene sheets (see Fig. 1, right photo). First, we have used low electron beam current (98 pA). Under such conditions a partial reversibility of the effect of resistance increase is observed. The system returns to lower R state after the beam is switched off as can be seen in the inset of
Fig. 3. The induced disorder calculated as micro-Raman I(D)/I(G) peak ratio after FEBID Co deposition (left) and subsequent annealing at 500 °C (right) (red denotes highest disorder, while dark blue a lack of disorder). In the inset: graphene flakes with deposited Co contacts. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
J.M. Michalik et al. / Microelectronic Engineering 88 (2011) 2063–2065
2065
contacts. This indicates a need of performing heat-induced ‘‘cleaning’’ of graphene by a removal of attached precursor gas molecules from the graphene surface. Interestingly, we have observed that graphene flakes are not damaged at all if exposed to the precursor gas without switching on the electron beam. The D peak in Raman spectra is not visible after the exposition of graphene samples to the precursor gas during 20 min. This indicates a crucial role of electrons in activating the long range precursor-induced disorder in graphene. 4. Conclusions
Fig. 4. Raman spectra of graphene taken after FEBID Co contacts growth (red line) and after two consecutive heat treatments (500 °C and 700 °C) in Ar atmosphere. In the inset Raman spectra of graphite after Co deposition and 500 °C heating in Ar. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Second, graphene was irradiated with 1.6 nA beam current at an energy of 5 keV. We observe that an increase of the resistivity of graphene becomes clear after an hour of continuous irradiation (see Fig. 2, main panel). We interpret this effect as a consequence of the irreversible deterioration of the graphene layer. Also, we have observed a drop of resistance during the first few minutes of measurement, which can be easily explained by local heating of the sample by the electron beam and subsequent desorption of gas molecules adsorbed to graphene. 3.2. Growth of cobalt by FEBID on graphene flakes The effects produced by the exposure of graphene to the precursor gas – Co2(CO)8 and growth of Co contacts were investigated by micro-Raman spectroscopy measurements. FEBID damage was quantified by looking at the ratio of the D/G Raman peaks, which is expected to provide a reliable evaluation of the disorder in graphene [8]. For the purpose of investigation of the spatial distribution of such disorder we have performed a total number of 60 Raman scans at thicker (graphite) material. The D and G Raman peaks were fitted with Lorentz-shaped peaks and their intensity was numerically calculated. Graphene is very sensitive to any contaminants on its surface [9], and indeed we have observed a significant FEBID-induced disorder not only at the deposition spot but even up to a distance of 2 mm (see Fig. 3). Moreover the highest D/G peak ratio is not observed at the deposition spot but away of it, i.e., at the point coinciding with the highest flow of the precursor gas. On the other hand, it is crucial to note that such a deterioration of graphene can be almost completely reversed by heating the sample up to 500 °C in Ar atmosphere (see Figs. 3 and 4), suggesting that graphene is not irreversibly damaged by the growth of Co
It can be concluded that graphene deterioration due to irradiation with electron beam is not the most important effect when dealing with Co-contact deposition on top of graphene flakes using focused electron beam. Our results indicate that a main source of disorder in graphene is the precursor gas, whose molecules become adsorbed on the surface of the sample and dissociated by the electron beam. The induced damage can be removed by heating the samples under controlled atmosphere. Our preliminary study also indicates the possibility of recovering graphene electronic properties after the Co deposition process by heating the device directly with applied electrical current (as was suggested by Moser et al. [10]). This seems enough to remove the produced surface contaminants. The charge neutrality point is shifted from high gate voltage to a value comparable with EBL processed device. In such a way, one would be able to obtain a fully operative graphene device in just one step through focused electron beam induced cobalt deposition. Acknowledgements This work was supported by Spanish Ministry of Science (through Project MAT2008-06567-C02, including FEDER funding), and the Aragon Regional Government (Projects E26 and PI046/ 09). We acknowledge experimental help by Rosa Córdoba. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, S.V. Dubonos, A.A. Firsov, Nat. Lond. 438 (2005) 197. [2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666; A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [3] N. Tombros, C. Jozsa, M. Popinciuc, H.T. Jonkman, B.J. van Wees, Nat. Lond. 448 (2007) 571. [4] N. Tombros, S. Tanabe, A. Veligura, C. Jozsa, M. Popinciuc, H.T. Jonkman, B.J. van Wees, Phys. Rev. Lett. 101 (2008) 046601. [5] A. Fernández-Pacheco, J.M. De Teresa, R. Córdoba, M.R. Ibarra, J. Phys. D: Appl. Phys. 42 (2009) 055005. [6] A. Fernández-Pacheco, J.M. De Teresa, A. Szkudlarek, R. Córdoba, M.R. Ibarra, D. Petit, L. O’Brien, H.T. Zeng, E.R. Lewis, D.E. Read, R.P. Cowburn, Nanotechnology 20 (2009) 475704. [7] D. Teweldebrhan, A.A. Balandin, Appl. Phys. Lett. 94 (2009) 013101. [8] A.C. Ferrari, Solid State Commun. 143 (2007) 47. [9] J.M. Caridad, F. Rossella, V. Bellani, M. Maicas, M. Patrini, E. Díez, J. Appl. Phys. 108 (2010) 084321. [10] J. Moser, A. Barreiro, A. Bachtold, Appl. Phys. Lett. 91 (2007) 163513.