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Nanoscale electrodeposition of Al on n-Si(1 1 1) : H from an ionic liquid
Chemical Physics Letters 434 (2007) 271–275 www.elsevier.com/locate/cplett
Nanoscale electrodeposition of Al on n-Si(1 1 1) : H from an ionic liquid C.L. Aravinda, B. Burger, W. Freyland
*
Institute of Physical Chemistry, University of Karlsruhe (TH), Kaiserstrasse 12, D-76128 Karlsruhe, Germany Received 18 September 2006; in final form 6 November 2006 Available online 8 December 2006
Abstract The H-terminated Si(1 1 1)/ionic liquid interface has been imaged by scanning tunneling microscopy (STM) for the first time. Employing the ionic liquid AlCl3 –½C4 mimþ nanoscale electrodeposition of Al on Si(1 1 1) : H substrates has been investigated by in situ electrochemical scanning probe techniques at room temperature. No underpotential deposition of Al is found. Nucleation of Al begins at the Nernst potential with the formation of large islands spread all over the substrate. Under the influence of the scanning STM tip, these islands are easily disturbed which makes it difficult to image the initial stages of electrochemical phase formation. We explain this by a relatively high mobility of the islands due to the poor wetting of Al on the Si(1 1 1) : H substrate. The 3D growth of Al on Si(1 1 1) : H follows a Volmer–Weber growth mode. Scanning tunneling spectra of larger Al clusters show clearly metallic characteristics. 2006 Elsevier B.V. All rights reserved.
1. Introduction Due to increasing complexity involved in miniaturization of today’s semiconductor devices the study of nanostructures has become more and more crucial. Scanning probe microscopy (SPM) techniques appear to be promising for this purpose [1,2]. Deposition of metals on semiconductor substrate surfaces is an important process in the integrated circuit technology. Metal nanostructures and thin films deposited on top of semiconductor substrate surfaces are used for various electronic device applications such as Schottky barriers, optical applications, and as buffer layers on substrates to deposit functional thin films with improved properties [3–7]. Aluminum metallization is one such example often fabricated by vacuum based deposition techniques such as evaporation [8]. Because of the low lattice mismatch Al is known to grow epitaxially on Si(1 1 1)7 · 7, forming (1 1 1) oriented domains with a few atomic layer height variation [9]. In situ SPM studies on electrodeposition of several metals such as Ag [3], Au [5], and Pb [10] on Si surfaces have been reported.
As far as we know such studies are limited to electrodeposition from aqueous or organic electrolytes. Ionic liquids offer several advantages such as low vapor pressure, high conductivity and more importantly a wide electrochemical window almost thrice the magnitude in comparison with aqueous electrolytes depending on the substrate used [11]. The latter property allows to electrodeposit a wide variety of metals including transition, rare earth and refractory metals which is impossible to achieve with aqueous electrolytes. Previously, nanoscale electrodeposition of metals, alloys and compound semiconductors from ionic liquids on Au(1 1 1) substrates has been reported by us [12–14]. However, for practical electronic device applications metal nanostructures have to be deposited on to semiconducting substrates. In this Letter we report, for the first time, the nanoscale imaging of the Si(1 1 1) : H/ionic liquid interface. In particular, nanoscale electrodeposition of Al on þ Si(1 1 1) : H from AlCl3 –½C4 mim Cl has been studied by in situ scanning tunneling microscopy (STM) and spectroscopy (STS). 2. Experimental
*
Corresponding author. Fax: +49 721 6086662. E-mail address: [email protected] (W. Freyland). 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.12.012
½C4 mimþ Cl was synthesized by slowly adding freshly distilled 1-methylimidazole to 1-chlorobutane (for synthesis,
272
C.L. Aravinda et al. / Chemical Physics Letters 434 (2007) 271–275
both from Merck) and stirring the mixture at 50 C for several days. Anhydrous AlCl3 (Fluka, purity >99.99%) was sublimed twice under vacuum, leading to white crystals. þ An acidic AlCl3 –½C4 mim Cl (58 : 42) melt was obtained by mixing slowly appropriate moles of AlCl3 crystals and þ ½C4 mim Cl . Samples were cut (11 · 11 mm2) from one side polished Phosphorus doped n-Si wafers (Siltronix, France) of 1– 10 X cm resistivity with 0.2 ± 0.1 miscut angle. Si(1 1 1) substrates were subsequently decreased by ultrasonic agitation in acetone, methanol and Milli-Q water, each for 15 min. Then they were cleaned in a hot 1 : 1 mixture of H2 SO4 (98%) and H2 O2 (30%) for 20 min. Prior to etching the samples were also cleaned by a modified RCA procedure [15]. Hydrogen terminated Si(1 1 1) substrates were obtained by immersing the cleaned Si(1 1 1) samples with the polished side facing down in deoxygenated 40% NH4F solution for 20 min following the method developed by Allongue et al. [16]. Finally, the n-Si(1 1 1) : H platelets were rinsed in N2 purged Milli-Q water and dried under N2 stream. A pre-cleaned Teflon electrochemical cell (effective area of 0.36 cm2 with three electrode configuration was used throughout this investigation. An n-Si(1 1 1) : H substrate was sealed on to the electrochemical cell using a Teflon coated O-ring. An Al ring and Al wire (>99.99%, Alfa Acer, Germany) dipped into the melt served as the counter and reference electrode, respectively. Contact with Si(1 1 1) : H working electrode was made with a Ga–In eutectic alloy (Alfa Acer, Germany). The STM tips were freshly prepared by etching W and Pt–Ir (90 : 10, wt.%) wires of 0.25 mm diameter followed by an electrochemical etching process in 2 M NaOH solution and 4 M NaCN, respectively. To avoid the effect of faradaic currents, the tips were coated via electrophoresis with an epoxide electropaint (BASF ZQ 84-3225 0201, Germany). Including the preparation of ionic liquid the assembling of the whole setup for the EC-STM studies was carried out in an argon filled glove box (O2 and H2O < 1 ppm) and later transferred into a clean and Ar-filled leak proof stainless steel vessel to prevent contamination and to ensure relatively long measurement times. In situ STM experiments were performed in feedback control mode with a Molecular Imaging Picoscan STM controller under potentiostatic conditions with a specially designed in house built electrochemical STM setup. STS plots were recorded at a given tip to sample separation defined by the tunneling conditions prior to scanning the tip voltage. The I–U curves were taken in the current imaging tunneling spectroscopy (CITS) mode whereby STM pictures and corresponding STS spectra can be taken simultaneously. The I–U curves are acquired rapidly (200 ls) in comparison with the scan speed of the tip (5 lines s1). Since the tip bias voltage was scanned in a wide range from 1.2 to 1.2 V during the acquisition of I–U curves, the quality of the tip coating was carefully monitored before and after the spectra acquisition which showed Faraday currents of <20 pA.
3. Results and discussion þ
A typical cyclic voltammogram of AlCl3 –½C4 mim Cl (58 : 42) on a Si(1 1 1) : H substrate recorded in the dark is shown in Fig. 1. Classical behaviour of electrodeposition and stripping of Al on Si(1 1 1) : H substrate is seen. There is no distinct reduction peak in the UPD region, but only a slowly rising current prior to bulk deposition is indicated. Similar observations have been reported by Robinson and Osteryoung in their aluminium deposition study from an ionic liquid at different metal electrodes [17]. They ascribe this to a slow UPD process like surface alloying. The current begins to increase rapidly beyond the Nernst potential due to the over potential deposition of Al. A large nucleation loop associated with electrodeposition of metal on a foreign substrate and an onset of an anodic process at +100 mV resulting from the stripping of deposited Al, were observed. There is a slight difference between the cathodic and anodic charges in the CV of 10 lC cm2 which is equivalent to 0.02 ML coverage taking for the ML coverage a charge of 720 lC cm2 – see also [17]. This and the slowly decaying stripping peak indicate a slow stripping reaction. A large number of monoatomically deep holes were seen in STM images (not presented) acquired for the Al dissolution process during anodic polarization at positive potential values of the Si(1 1 1) substrate covered initially with Al deposit. This indicates the surface alloying of Al with the Si(1 1 1) substrate, which is also observed on Au(1 1 1) [14]. STS studies on the substrate after complete dissolution of the Al deposit show a semiconductor behavior expected for n-Si. Large scale in situ STM images (Fig. 2a) shows the topography of the Si(1 1 1) : H surface imaged for the first þ time under the ionic liquid AlCl3 –½C4 mim Cl . At an open circuit potential of +100 mV, atomically flat regularly spaced terraces with few etch pits were seen. As it is indicated in the cyclic voltammogram shown in Fig. 1, no substantial modification in the surface topography is expected
Fig. 1. Cyclic voltammogram of AlCl3 –½C4 mimþ Cl (58 : 42) on Si(1 1 1) : H substrate acquired in the dark at a scan rate of 100 mV s1 and at a temperature of 18 C.
C.L. Aravinda et al. / Chemical Physics Letters 434 (2007) 271–275
273
Fig. 2. (a) Large scale (900 · 900 nm2) in situ STM image showing the surface topography of an n-Si(1 1 1) : H substrate in contact with AlCl3 –½C4 mimþ Cl (58 : 42). (b) STM image (160 · 160 nm2) showing atomically flat, typically bilayer high Si(1 1 1) terraces (inset shows the height profile). In both the cases Esample ¼ 0:2 V, U bias ¼ 1:5 V and I tunn ¼ 0:5 nA.
between the potential limits of +1.5 and 0 V. The height variation (see inset Fig. 2b) of the Si(1 1 1) : H surface ˚ as expected reveals that these steps are of a height of 3 A for Si bilayers. The lateral mean width of the terraces is 65 nm. Fig. 3a–c show a few selected STM images illustrating the characteristic changes of the topography at the Si(1 1 1) : H electrode/ionic liquid interface during the Al electrodeposition process at potentials between 0 and 400 mV. If the potential is decreased to the Nernst potential, nucleation of Al begins with the formation of 2D islands (Fig. 3a). In accordance with previous studies [17], we think that these deposits correspond to a UPD pro-
Fig. 3. In situ STM images showing the stages of Al-deposition on Si(1 1 1) from an AlCl3 –½C4 mimþ Cl (58 : 42) ionic liquid. (a) Initial stages of deposition at Esample ¼ 0:0 V and U bias ¼ 1:6 V (300 · 300 nm2). (b) A large stable cluster formed after about 10 min at Esample ¼ 0:20 V and U bias ¼ 1:5 V (300 · 300 nm2, image tilted by 20). (c) Bulk Al deposit, after 20 min at E ¼ 0:40 V and U bias ¼ 1 V (300 · 300 nm2). In all the cases I tunn ¼ 0:5 nA.
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C.L. Aravinda et al. / Chemical Physics Letters 434 (2007) 271–275
Fig. 4. I–V tunneling spectra for: (a) Si(1 1 1) : H and AlCl3 –½C4 mimþ Cl (58 : 42) ionic liquid interface. (b) An Al cluster (at E ¼ 0:20 V vs. Al/Al3+) and Al bulk deposit (after 20 min at E ¼ 0:40 V vs. Al/Al3+) obtained with I tun ¼ 20 nA. Arrows in Fig. 3 show the positions of the tip where spectra were taken.
cess. No preferential nucleation sites for Al on Si(1 1 1) : H were seen in the STM images. During the scan the 2D islands are swiped off from the scanning area. This effect seems particularly pronounced in the initial stages of growth of 2D islands which are not yet metallic. Under these conditions a number of instabilities appear in the STM images. At a potential of 200 mV, after about 10 min, formation of a large 3D Al cluster of about 140 nm in size and 1 nm high is seen on the surface (Fig. 3b). At potentials well below 300 mV, 3D growth of metallic Al sets in following the Volmer–Weber growth mode. After a prolonged deposition, the surface is completely covered with Al clusters of sizes between 5 and 25 nm (Fig. 3c). When this is completed the interface is no longer semiconducting, and concurrently no imaging problems were faced. Obviously, the free energy at the Al deposit/Si(1 1 1) : H interface changes significantly when the electronic structure of the deposits varies from that of 3D metallic clusters to 2D nonmetallic islands. Similar to this, difficulties during in situ imaging of the Pb deposition process on Si(1 1 1) : H surfaces have been reported [10]. This was explained by the weak interaction between the metal deposit and the H-terminated Si(1 1 1). Scanning tunneling spectra, I vs. V curves, corresponding to the STM tip positions indicated in Fig. 3 are presented in Fig. 4. Clear cut differences in the electronic structure of the surfaces are evident. First, the I vs. V curve (Fig. 4a) recorded prior to deposition on the bare Si(1 1 1) : H surface clearly exhibits a semiconductor behaviour. A band gap of 1.1 eV is estimated – see also the derivative spectrum in the inset – which is in accord with the well known bulk Si value. Distinctly, metallic characteristics are observed from the I–V curves (Fig. 4b) obtained for the Al cluster and bulk deposit, respectively. In summary we have shown that H-terminated Si(1 1 1) in contact with an ionic liquid can be imaged by in situ STM with a resolution comparable to that achieved with other electrolytes like aqueous solutions. Obviously, the
Si(1 1 1) : H surface is not attacked by the acidic þ AlCl3 –½C4 mim Cl melt. Electrocrystallization at the Si(1 1 1) : H/ionic liquid interface has been probed for the example of Al electrodeposition. No underpotential deposition processes have been observed. Bulk Al-deposition as characterized by in situ STM and STS measurements exhibits features similar to Al deposits on metal substrates like Au(1 1 1). Acknowledgements Financial support of this work by the CFN (DFG), University of Karlsruhe, Germany is gratefully acknowledged. References [1] J.W. Wu, X.M. Yang, W.M. Zhu, F.S. Zhu, Z.H. Lu, Y. Wei, Scanning probe microscopy applications to characterization in microelectronics, in: G.L. Baldwin (Ed.), Proceedings of 4th International Conference on Solid-State and Integrated Circuit Technology, October 24–28, Beijing, Institute of Electrical and Electronics Engineers, New York, 1995, p. 754. [2] R.T. Po¨tzschke, G. Staikov, W.J. Lorenz, W. Wiesbeck, J. Electrochem. Soc. 146 (1999) 141. [3] R.M. Stiger, S. Gorer, B. Craft, R.M. Penner, Langmuir 15 (1999) 790. [4] M. Hugelmann, W. Schindler, Appl. Phys. Lett. 85 (2004) 3608. [5] M.L. Munford, F. Maroun, R. Corte`s, P. Allongue, A.A. Pasa, Suf. Sci. 537 (2003) 95. [6] A.A. Pasa, W. Schwarzacher, Phys. Status Solidi A 173 (1999) 73. [7] P. Castrucci, R. Gunnella, R. Bernardini, P. Falcioni, M. De Crescenzi, Phys. Rev. B 65 (2002) 235435. [8] P.A. Baeza, K. Pedersen, J. Rafaelsen, T.G. Pedersen, P. Morgen, Z. Li, Surf. Sci. 600 (2006) 610. [9] H.J. Wen, M. Da¨hne-Prietsch, A. Bauer, M.T. Cuberes, I. Manke, G. Kaindl, J. Vac. Sci. Technol. A 13 (1995) 2399. [10] J.C. Ziegler, A. Reitzle, O. Bunk, J. Zegenhagen, D.M. Kolb, Electrochim. Acta 45 (2000) 4599. [11] P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, WileyVCH, Weinheim, 2003. [12] W. Freyland, in: G. Staikov (Ed.), Electrocrystallization and Nanotechnology, Wiley-VCH, Weinheim, 2006.
C.L. Aravinda et al. / Chemical Physics Letters 434 (2007) 271–275 [13] C.L. Aravinda, W. Freyland, Chem. Commun. 16 (2006) 1703. [14] C.L. Aravinda, I. Mukhopadhyay, W. Freyland, Phys. Chem. Chem. Phys. 6 (2004) 5225. [15] W. Kern, D. Puotinen, RCA Rev. 31 (1970) 187.
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[16] P. Allongue, C.H. De Villeneuve, S. Morin, R. Boukherroub, D.D.M. Wayner, Electrochim. Acta 45 (2000) 4591. [17] J. Robinson, R.A. Osteryoung, J. Electrochem. Soc. 127 (1980) 122.
Nanoscale electrodeposition of Al on n-Si(1 1 1) : H from an ionic liquid C.L. Aravinda, B. Burger, W. Freyland
*
Institute of Physical Chemistry, University of Karlsruhe (TH), Kaiserstrasse 12, D-76128 Karlsruhe, Germany Received 18 September 2006; in final form 6 November 2006 Available online 8 December 2006
Abstract The H-terminated Si(1 1 1)/ionic liquid interface has been imaged by scanning tunneling microscopy (STM) for the first time. Employing the ionic liquid AlCl3 –½C4 mimþ nanoscale electrodeposition of Al on Si(1 1 1) : H substrates has been investigated by in situ electrochemical scanning probe techniques at room temperature. No underpotential deposition of Al is found. Nucleation of Al begins at the Nernst potential with the formation of large islands spread all over the substrate. Under the influence of the scanning STM tip, these islands are easily disturbed which makes it difficult to image the initial stages of electrochemical phase formation. We explain this by a relatively high mobility of the islands due to the poor wetting of Al on the Si(1 1 1) : H substrate. The 3D growth of Al on Si(1 1 1) : H follows a Volmer–Weber growth mode. Scanning tunneling spectra of larger Al clusters show clearly metallic characteristics. 2006 Elsevier B.V. All rights reserved.
1. Introduction Due to increasing complexity involved in miniaturization of today’s semiconductor devices the study of nanostructures has become more and more crucial. Scanning probe microscopy (SPM) techniques appear to be promising for this purpose [1,2]. Deposition of metals on semiconductor substrate surfaces is an important process in the integrated circuit technology. Metal nanostructures and thin films deposited on top of semiconductor substrate surfaces are used for various electronic device applications such as Schottky barriers, optical applications, and as buffer layers on substrates to deposit functional thin films with improved properties [3–7]. Aluminum metallization is one such example often fabricated by vacuum based deposition techniques such as evaporation [8]. Because of the low lattice mismatch Al is known to grow epitaxially on Si(1 1 1)7 · 7, forming (1 1 1) oriented domains with a few atomic layer height variation [9]. In situ SPM studies on electrodeposition of several metals such as Ag [3], Au [5], and Pb [10] on Si surfaces have been reported.
As far as we know such studies are limited to electrodeposition from aqueous or organic electrolytes. Ionic liquids offer several advantages such as low vapor pressure, high conductivity and more importantly a wide electrochemical window almost thrice the magnitude in comparison with aqueous electrolytes depending on the substrate used [11]. The latter property allows to electrodeposit a wide variety of metals including transition, rare earth and refractory metals which is impossible to achieve with aqueous electrolytes. Previously, nanoscale electrodeposition of metals, alloys and compound semiconductors from ionic liquids on Au(1 1 1) substrates has been reported by us [12–14]. However, for practical electronic device applications metal nanostructures have to be deposited on to semiconducting substrates. In this Letter we report, for the first time, the nanoscale imaging of the Si(1 1 1) : H/ionic liquid interface. In particular, nanoscale electrodeposition of Al on þ Si(1 1 1) : H from AlCl3 –½C4 mim Cl has been studied by in situ scanning tunneling microscopy (STM) and spectroscopy (STS). 2. Experimental
*
Corresponding author. Fax: +49 721 6086662. E-mail address: [email protected] (W. Freyland). 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.12.012
½C4 mimþ Cl was synthesized by slowly adding freshly distilled 1-methylimidazole to 1-chlorobutane (for synthesis,
272
C.L. Aravinda et al. / Chemical Physics Letters 434 (2007) 271–275
both from Merck) and stirring the mixture at 50 C for several days. Anhydrous AlCl3 (Fluka, purity >99.99%) was sublimed twice under vacuum, leading to white crystals. þ An acidic AlCl3 –½C4 mim Cl (58 : 42) melt was obtained by mixing slowly appropriate moles of AlCl3 crystals and þ ½C4 mim Cl . Samples were cut (11 · 11 mm2) from one side polished Phosphorus doped n-Si wafers (Siltronix, France) of 1– 10 X cm resistivity with 0.2 ± 0.1 miscut angle. Si(1 1 1) substrates were subsequently decreased by ultrasonic agitation in acetone, methanol and Milli-Q water, each for 15 min. Then they were cleaned in a hot 1 : 1 mixture of H2 SO4 (98%) and H2 O2 (30%) for 20 min. Prior to etching the samples were also cleaned by a modified RCA procedure [15]. Hydrogen terminated Si(1 1 1) substrates were obtained by immersing the cleaned Si(1 1 1) samples with the polished side facing down in deoxygenated 40% NH4F solution for 20 min following the method developed by Allongue et al. [16]. Finally, the n-Si(1 1 1) : H platelets were rinsed in N2 purged Milli-Q water and dried under N2 stream. A pre-cleaned Teflon electrochemical cell (effective area of 0.36 cm2 with three electrode configuration was used throughout this investigation. An n-Si(1 1 1) : H substrate was sealed on to the electrochemical cell using a Teflon coated O-ring. An Al ring and Al wire (>99.99%, Alfa Acer, Germany) dipped into the melt served as the counter and reference electrode, respectively. Contact with Si(1 1 1) : H working electrode was made with a Ga–In eutectic alloy (Alfa Acer, Germany). The STM tips were freshly prepared by etching W and Pt–Ir (90 : 10, wt.%) wires of 0.25 mm diameter followed by an electrochemical etching process in 2 M NaOH solution and 4 M NaCN, respectively. To avoid the effect of faradaic currents, the tips were coated via electrophoresis with an epoxide electropaint (BASF ZQ 84-3225 0201, Germany). Including the preparation of ionic liquid the assembling of the whole setup for the EC-STM studies was carried out in an argon filled glove box (O2 and H2O < 1 ppm) and later transferred into a clean and Ar-filled leak proof stainless steel vessel to prevent contamination and to ensure relatively long measurement times. In situ STM experiments were performed in feedback control mode with a Molecular Imaging Picoscan STM controller under potentiostatic conditions with a specially designed in house built electrochemical STM setup. STS plots were recorded at a given tip to sample separation defined by the tunneling conditions prior to scanning the tip voltage. The I–U curves were taken in the current imaging tunneling spectroscopy (CITS) mode whereby STM pictures and corresponding STS spectra can be taken simultaneously. The I–U curves are acquired rapidly (200 ls) in comparison with the scan speed of the tip (5 lines s1). Since the tip bias voltage was scanned in a wide range from 1.2 to 1.2 V during the acquisition of I–U curves, the quality of the tip coating was carefully monitored before and after the spectra acquisition which showed Faraday currents of <20 pA.
3. Results and discussion þ
A typical cyclic voltammogram of AlCl3 –½C4 mim Cl (58 : 42) on a Si(1 1 1) : H substrate recorded in the dark is shown in Fig. 1. Classical behaviour of electrodeposition and stripping of Al on Si(1 1 1) : H substrate is seen. There is no distinct reduction peak in the UPD region, but only a slowly rising current prior to bulk deposition is indicated. Similar observations have been reported by Robinson and Osteryoung in their aluminium deposition study from an ionic liquid at different metal electrodes [17]. They ascribe this to a slow UPD process like surface alloying. The current begins to increase rapidly beyond the Nernst potential due to the over potential deposition of Al. A large nucleation loop associated with electrodeposition of metal on a foreign substrate and an onset of an anodic process at +100 mV resulting from the stripping of deposited Al, were observed. There is a slight difference between the cathodic and anodic charges in the CV of 10 lC cm2 which is equivalent to 0.02 ML coverage taking for the ML coverage a charge of 720 lC cm2 – see also [17]. This and the slowly decaying stripping peak indicate a slow stripping reaction. A large number of monoatomically deep holes were seen in STM images (not presented) acquired for the Al dissolution process during anodic polarization at positive potential values of the Si(1 1 1) substrate covered initially with Al deposit. This indicates the surface alloying of Al with the Si(1 1 1) substrate, which is also observed on Au(1 1 1) [14]. STS studies on the substrate after complete dissolution of the Al deposit show a semiconductor behavior expected for n-Si. Large scale in situ STM images (Fig. 2a) shows the topography of the Si(1 1 1) : H surface imaged for the first þ time under the ionic liquid AlCl3 –½C4 mim Cl . At an open circuit potential of +100 mV, atomically flat regularly spaced terraces with few etch pits were seen. As it is indicated in the cyclic voltammogram shown in Fig. 1, no substantial modification in the surface topography is expected
Fig. 1. Cyclic voltammogram of AlCl3 –½C4 mimþ Cl (58 : 42) on Si(1 1 1) : H substrate acquired in the dark at a scan rate of 100 mV s1 and at a temperature of 18 C.
C.L. Aravinda et al. / Chemical Physics Letters 434 (2007) 271–275
273
Fig. 2. (a) Large scale (900 · 900 nm2) in situ STM image showing the surface topography of an n-Si(1 1 1) : H substrate in contact with AlCl3 –½C4 mimþ Cl (58 : 42). (b) STM image (160 · 160 nm2) showing atomically flat, typically bilayer high Si(1 1 1) terraces (inset shows the height profile). In both the cases Esample ¼ 0:2 V, U bias ¼ 1:5 V and I tunn ¼ 0:5 nA.
between the potential limits of +1.5 and 0 V. The height variation (see inset Fig. 2b) of the Si(1 1 1) : H surface ˚ as expected reveals that these steps are of a height of 3 A for Si bilayers. The lateral mean width of the terraces is 65 nm. Fig. 3a–c show a few selected STM images illustrating the characteristic changes of the topography at the Si(1 1 1) : H electrode/ionic liquid interface during the Al electrodeposition process at potentials between 0 and 400 mV. If the potential is decreased to the Nernst potential, nucleation of Al begins with the formation of 2D islands (Fig. 3a). In accordance with previous studies [17], we think that these deposits correspond to a UPD pro-
Fig. 3. In situ STM images showing the stages of Al-deposition on Si(1 1 1) from an AlCl3 –½C4 mimþ Cl (58 : 42) ionic liquid. (a) Initial stages of deposition at Esample ¼ 0:0 V and U bias ¼ 1:6 V (300 · 300 nm2). (b) A large stable cluster formed after about 10 min at Esample ¼ 0:20 V and U bias ¼ 1:5 V (300 · 300 nm2, image tilted by 20). (c) Bulk Al deposit, after 20 min at E ¼ 0:40 V and U bias ¼ 1 V (300 · 300 nm2). In all the cases I tunn ¼ 0:5 nA.
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C.L. Aravinda et al. / Chemical Physics Letters 434 (2007) 271–275
Fig. 4. I–V tunneling spectra for: (a) Si(1 1 1) : H and AlCl3 –½C4 mimþ Cl (58 : 42) ionic liquid interface. (b) An Al cluster (at E ¼ 0:20 V vs. Al/Al3+) and Al bulk deposit (after 20 min at E ¼ 0:40 V vs. Al/Al3+) obtained with I tun ¼ 20 nA. Arrows in Fig. 3 show the positions of the tip where spectra were taken.
cess. No preferential nucleation sites for Al on Si(1 1 1) : H were seen in the STM images. During the scan the 2D islands are swiped off from the scanning area. This effect seems particularly pronounced in the initial stages of growth of 2D islands which are not yet metallic. Under these conditions a number of instabilities appear in the STM images. At a potential of 200 mV, after about 10 min, formation of a large 3D Al cluster of about 140 nm in size and 1 nm high is seen on the surface (Fig. 3b). At potentials well below 300 mV, 3D growth of metallic Al sets in following the Volmer–Weber growth mode. After a prolonged deposition, the surface is completely covered with Al clusters of sizes between 5 and 25 nm (Fig. 3c). When this is completed the interface is no longer semiconducting, and concurrently no imaging problems were faced. Obviously, the free energy at the Al deposit/Si(1 1 1) : H interface changes significantly when the electronic structure of the deposits varies from that of 3D metallic clusters to 2D nonmetallic islands. Similar to this, difficulties during in situ imaging of the Pb deposition process on Si(1 1 1) : H surfaces have been reported [10]. This was explained by the weak interaction between the metal deposit and the H-terminated Si(1 1 1). Scanning tunneling spectra, I vs. V curves, corresponding to the STM tip positions indicated in Fig. 3 are presented in Fig. 4. Clear cut differences in the electronic structure of the surfaces are evident. First, the I vs. V curve (Fig. 4a) recorded prior to deposition on the bare Si(1 1 1) : H surface clearly exhibits a semiconductor behaviour. A band gap of 1.1 eV is estimated – see also the derivative spectrum in the inset – which is in accord with the well known bulk Si value. Distinctly, metallic characteristics are observed from the I–V curves (Fig. 4b) obtained for the Al cluster and bulk deposit, respectively. In summary we have shown that H-terminated Si(1 1 1) in contact with an ionic liquid can be imaged by in situ STM with a resolution comparable to that achieved with other electrolytes like aqueous solutions. Obviously, the
Si(1 1 1) : H surface is not attacked by the acidic þ AlCl3 –½C4 mim Cl melt. Electrocrystallization at the Si(1 1 1) : H/ionic liquid interface has been probed for the example of Al electrodeposition. No underpotential deposition processes have been observed. Bulk Al-deposition as characterized by in situ STM and STS measurements exhibits features similar to Al deposits on metal substrates like Au(1 1 1). Acknowledgements Financial support of this work by the CFN (DFG), University of Karlsruhe, Germany is gratefully acknowledged. References [1] J.W. Wu, X.M. Yang, W.M. Zhu, F.S. Zhu, Z.H. Lu, Y. Wei, Scanning probe microscopy applications to characterization in microelectronics, in: G.L. Baldwin (Ed.), Proceedings of 4th International Conference on Solid-State and Integrated Circuit Technology, October 24–28, Beijing, Institute of Electrical and Electronics Engineers, New York, 1995, p. 754. [2] R.T. Po¨tzschke, G. Staikov, W.J. Lorenz, W. Wiesbeck, J. Electrochem. Soc. 146 (1999) 141. [3] R.M. Stiger, S. Gorer, B. Craft, R.M. Penner, Langmuir 15 (1999) 790. [4] M. Hugelmann, W. Schindler, Appl. Phys. Lett. 85 (2004) 3608. [5] M.L. Munford, F. Maroun, R. Corte`s, P. Allongue, A.A. Pasa, Suf. Sci. 537 (2003) 95. [6] A.A. Pasa, W. Schwarzacher, Phys. Status Solidi A 173 (1999) 73. [7] P. Castrucci, R. Gunnella, R. Bernardini, P. Falcioni, M. De Crescenzi, Phys. Rev. B 65 (2002) 235435. [8] P.A. Baeza, K. Pedersen, J. Rafaelsen, T.G. Pedersen, P. Morgen, Z. Li, Surf. Sci. 600 (2006) 610. [9] H.J. Wen, M. Da¨hne-Prietsch, A. Bauer, M.T. Cuberes, I. Manke, G. Kaindl, J. Vac. Sci. Technol. A 13 (1995) 2399. [10] J.C. Ziegler, A. Reitzle, O. Bunk, J. Zegenhagen, D.M. Kolb, Electrochim. Acta 45 (2000) 4599. [11] P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, WileyVCH, Weinheim, 2003. [12] W. Freyland, in: G. Staikov (Ed.), Electrocrystallization and Nanotechnology, Wiley-VCH, Weinheim, 2006.
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