- Email: [email protected]
Synthesis of thin carbon films on 4H-SiC by low temperature extraction of Si with HCl
CARBON
4 8 ( 20 1 0 ) 2 6 4 4–26 7 3
PTFE particles hold the water droplets up and keep them from coming into contact with CNTs. More importantly, air in pores and between PTFE particles helps the surface achieve a great contact angle, much higher than that of water on plain PTFE films. When more PTFE particles are deposited onto the sample surface, a denser coating is produced with a reduced volume of air, leading to a decreased contact angle. However, because of these voids and air trapped in them, the contact angle of water on such a surface is always greater than that on plain PTFE films (Fig. 3). The hot-pressed bulk CNT material, like graphite, can be machined into desired shapes with fine and complex channels. These results enable these internal surfaces of the tubes or channels to be super-hydrophobic, and the super-hydrophobility can be restored by repeating the procedure described above when damage occurs during use.
Acknowledgements The author (W.J.) thanks the support from NSFC (No.50625414, 50821004) and Shanghai Municipal (No.08XD14046), and L.W. thanks the support from Shanghai Municipal (No.09QA1406600).
2671
R E F E R E N C E S
[1] Me´ndez-Vilas A, Bele´n Jo´dar-Reyes A, Gonza´lez-Martı´n M. Wetting of solid surfaces by ultrasmall liquid droplets. Small 2009;5:1366–90. [2] Wenzel R. Surface roughness and contact angle. J Phys Chem 1949;53:1466–70. [3] Gu ZZ, Uetsuka H, Fujishima A. Structural color and the lotus effect. Angew Chem Int Ed 2003;42:894–7. [4] Lee W, Jin MK, Yoo WC, Lee JK. Defined nanometer-scale topography and tailored surface wettability. Langmuir 2004;20:7665–9. [5] Li SH, Li H, Wang X, Song Y, Liu Y, Jiang L, et al. Superhydrophobicity of large-area honeycomb-like aligned carbon nanotubes. J Phys Chem B 2002;106:9274–6. [6] Lau K, Bico J, Teo K, Chhowalla M, Amaratunga G, Milne W, et al. Super hydrophobic carbon nanotube forests. Nano Lett 2003;3:1701–5. [7] Marmur A. Underwater super hydrophobicity: theoretical feasibility. Langmuir 2006;22:1400–2. [8] Li J, Wang L, He T, Jiang W. Transport properties of hot-pressed bulk carbon nanotubes compacted by spark plasma sintering. Carbon 2009;47:1135–40. [9] Cassie A. Contact angles. Discuss Faraday Soc 1948;3:11–6.
Synthesis of thin carbon films on 4H-SiC by low temperature extraction of Si with HCl M.A. Fanton a b
a,* ,
J.A. Robinson b, M. Hollander b, B.E. Weiland a, K. Trumbull b, M. LaBella
b
The Pennsylvania State University, Electro-Optics Center, Freeport, PA 16229, USA The Pennsylvania State University, Electro-Optics Center, University Park, PA 16802, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
Carbon films as thin as 5 nm were synthesized by graphitization of 4H-SiC substrates at
Received 22 December 2009
1200–1350 °C, in a dilute HCl/Ar atmosphere at a pressure of 0.8 bar. These films formed
Accepted 31 March 2010
at significantly lower temperatures and higher pressures than conventional synthesis of
Available online 3 April 2010
epitaxial graphene by sublimation of Si. Graphitization rates of 0.08–1.40 nm/min were observed. The activation energy for graphitization was approximately 460 kJ/mol. Raman spectroscopy indicated that the material was highly disordered with D-peak to G-peak ratios ranging from 0.70 to 1.43, compared to high quality graphene which does not exhibit the disorder induced D-peak. Ó 2010 Elsevier Ltd. All rights reserved.
Recent success of graphene transistor operation in the GHz frequency range demonstrated the potential of this material for high speed electronics [1]. The most promising route for large area, electronic grade graphene is sublimation of Si from the surface of single crystal SiC semiconductor
wafers. This is carried out at temperatures >1400 °C, pressures near 10 6 mbar, for 1–2 h [2]. While this technique has achieved high carrier mobilities >18,000 cm2/V s, the high growth temperatures degrades the SiC surface resulting in non-uniform strain and thickness in the graphene, which in
* Corresponding author: Fax: +1 724 295 6617. E-mail address: [email protected] (M.A. Fanton). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.03.072
2672
CARBON
4 8 ( 2 0 1 0 ) 2 6 4 4 –2 6 7 3
turn lowers carrier mobility [3]. The goal of the work reported in this letter is to examine the kinetics of C film formation on the (0 0 0 1) plane of single crystal SiC in the presence of Cl, and understand what is required to create a highly ordered C film approaching a few atomic layers in thickness. We specifically report on the use of HCl, to extract Si from the SiC surface creating 5 nm thick films of C at temperatures as low as 1200 °C and a pressure of 0.8 bar. All experiments took place in a SiC chemical vapor deposition (CVD) system (Structured Materials Industries), and were carried out on the Si-face of n-type 4H-SiC oriented 4° off-axis (II–VI Inc.). Substrates were chemi-mechanically polished and then etched for 30 min at a temperature of 1550 °C and a pressure of 600 Torr in a mixture of 40% H2 in Ar. The combination of the off-axis substrate and H2 etching provided a high density of well defined step edges with the same crystallographic orientation. Ultimately it is desirable to remove the H2 etching step in order to maintain low process temperatures below 1300 °C. After etching, the H2 concentration was reduced to 8% and the temperature was ramped down to the growth temperature at a rate of 10 °C/min. After reaching the growth temperature a mixture of 5% HCl in He was delivered to the process environment via a Piezocon (Lorex Ind.) acoustic gas concentration controller to achieve a precise HCl concentration in the process environment. Fig. 1 shows the Arrhenius plot of the graphitization rate at HCl concentrations of 0.7 and 1.4 vol.%. Activation energies calculated from the Arrhenius plots are essentially the same for both HCl concentrations at 460 kJ/mol. This indicates that graphitization is limited by chemical reactions at the surface rather than by transport of species from the gas phase to or from the surface of the substrate. This activation energy is of the same order of magnitude of other non-transport limited reactions between gas phase species and the SiC surface. Chemical reactions involving growth of SiC from the vapor phase are of a similar magnitude. Matsunami et al. [4] reported an activation energy of 355 kJ/mol for the lateral propagation of a step edge during SiC CVD from a mixture of SiH4 and C3H8. These activation energies are significantly higher than the activation energy reported for graphitization of polycrystalline SiC ceramics. Cambaz et al. [7] reported a value of 50 kJ/mol using an Ar–Cl2 atmosphere at 600–800 °C. This suggests that the crystallographic direction and surface damage play a significant role in the loss of Si from the SiC surface. The mechanism for removal of Si by Cl is clearly different from the mechanism of Si sublimation from SiC. High temperature sublimation growth of graphene on the Si-face of (0 0 0 1) oriented SiC becomes self limiting under some conditions, resulting in the formation of only a few C layers [5]. In contrast, the formation of C ceases to become self limiting using the HCl-assisted process. As can be seen from Fig. 1, increasing the HCl concentration by a factor of 2 increases the reaction rate by almost a factor of 20 suggesting that the reaction rate may be fourth order with respect to the HCl concentration. Thermodynamic equilibrium calculations suggest that SiCl4 is the most likely reaction product formed between SiC and HCl. It is possible that the reaction mechanism involves four separate collisions between a Si atom and four HCl molecules, in order to create a single SiCl4 molecule, which then desorbs from the surface.
Fig. 1 – Arrhenius plot for graphitization of SiC using dilute HCl in Ar.
Although the mechanism of Si extraction involving Cl is not yet clear, it is possible that Cl is able to diffuse through a highly defective C layer to aid in removal of Si at the SiC/C interface. To evaluate this possibility the structure of the C layers was examined using Raman spectroscopy. Fig. 2 shows the Raman spectrum for a 25 nm thick C film processed at 1200 °C using an HCl concentration of 1.4%. The significant D-peak indicates that this material is not high quality graphene. This peak is indicative of disorder in the sp2 bonded structure and does not appear in a perfect sp2 bonded C structure. In contrast, the G-peak is derived from stretching of the C–C bond in sp2 bonded carbon and increases in intensity as the structural quality improves [6]. The ratio of the intensities of the D-peak and the G-peak for this sample is 0.70. This is considerably better than previously published C films processed on SiC using halogenated gases, which range from 1.6 to 4.7 [7,8]. By comparison, high quality graphene has a D/G ratio approaching zero, while graphene damaged by heavy ion bombardment has a ratio of 2–3 [6]. Fig. 3 shows the relationships between the D/G ratios, the film thickness, and the process temperature for 1 h of growth time. The data in Fig. 3 suggests that thinner films tend to be more ordered. Films formed with slower reaction rates (lower HCl concentration) also tend to have lower D/G ratios. There
Fig. 2 – Raman spectra of a 25 nm thick film processed at 1200 °C and an 85 nm thick film processed at 1250 °C, both for 1 h.
CARBON
4 8 ( 20 1 0 ) 2 6 4 4–26 7 3
2673
well controlled reaction rates leading to high quality single or bi-layer graphene.
Acknowledgement This work was supported by a 2009–2010 gift from the II–VI Foundation.
R E F E R E N C E S
Fig. 3 – The D/G Raman peak ratio for films of varying thickness and process temperature.
is no clear trend toward higher structural order with increased temperature. However, the effects of temperature and reaction rate in this data set are heavily interrelated. It is likely that higher structural order would be obtained at higher temperatures if the rate of reaction were slowed considerably. The high D/G ratio also indicates that the crystallite size is very small. Using the method published by Tuinstra and Koenig [9], the average grain size for the C films is on the order of only 5–7 nm. This small crystallite size may aid in diffusion of species to and from the reaction front. In summary, C films as thin as 5 nm, approaching graphene thickness, were processed on single crystal SiC substrates at temperatures 200–300 °C lower than conventional growth of graphene on SiC by sublimation. Graphitization rates of 0.08–1.4 nm/min were observed. The activation energy of the process was found to be approximately 460 kJ/mol. The films are disordered with respect to high quality epitaxial graphene due in part to the high reaction rate. A full understanding of the kinetics of this process is expected to yield
[1] Moon JS, Curtis D, Hu M, Wong D, McGuire C, Campbell PM, et al. Epitaxial-graphene RF field-effect transistors on Si-Face 6H-SiC substrates. IEEE Elec Dev Lett 2009;30:650–2. [2] Berger C, Song Z, Li X, Wu X, Brown N, Naud C, et al. Electronic confinement and coherence in patterned epitaxial graphene. Science 2006;312:1191–6. [3] Robinson JA, Wetherington M, Tedesco JL, Campbell PM, Weng X, Stitt J, et al. Correlating Raman spectral signatures with carrier mobility in epitaxial graphene: a guide to achieving high mobility on the wafer scale. Nano Lett 2009;9:2873–6. [4] Matsunami H, Kimoto T. Step-controlled epitaxial growth of SiC: high quality homoepitaxy. Mater Sci Eng 1997;R20:125–66. [5] de Heer WA, Berger C, Wu X, First PN, Conrad EH, Li X, et al. Epitaxial graphene. Solid State Commun 2007;143:92–111. [6] Dresselhaus MS, Jorio A, Hofmann M, Dresselhaus G, Saito R. Perspectives on carbon nanotubes and graphene Raman spectroscopy. Nano Lett 2010;10:751–8. [7] Cambaz ZG, Yushin GN, Gogotsi Y, Vyshnyakova KL, Pereselentseva LN. Formation of carbide-derived carbon on b-silicon carbide whiskers. J Am Ceram Soc 2006;89:509–14. [8] Suemitsu M, Miyamoto Y, Handa H, Konno A. Graphene formation on a 3C-SiC(1 1 1) thin film grown on Si(1 1 0) substrate. EJ Surf Sci Nanotech 2009;7:311–3. [9] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53:1126–30.
4 8 ( 20 1 0 ) 2 6 4 4–26 7 3
PTFE particles hold the water droplets up and keep them from coming into contact with CNTs. More importantly, air in pores and between PTFE particles helps the surface achieve a great contact angle, much higher than that of water on plain PTFE films. When more PTFE particles are deposited onto the sample surface, a denser coating is produced with a reduced volume of air, leading to a decreased contact angle. However, because of these voids and air trapped in them, the contact angle of water on such a surface is always greater than that on plain PTFE films (Fig. 3). The hot-pressed bulk CNT material, like graphite, can be machined into desired shapes with fine and complex channels. These results enable these internal surfaces of the tubes or channels to be super-hydrophobic, and the super-hydrophobility can be restored by repeating the procedure described above when damage occurs during use.
Acknowledgements The author (W.J.) thanks the support from NSFC (No.50625414, 50821004) and Shanghai Municipal (No.08XD14046), and L.W. thanks the support from Shanghai Municipal (No.09QA1406600).
2671
R E F E R E N C E S
[1] Me´ndez-Vilas A, Bele´n Jo´dar-Reyes A, Gonza´lez-Martı´n M. Wetting of solid surfaces by ultrasmall liquid droplets. Small 2009;5:1366–90. [2] Wenzel R. Surface roughness and contact angle. J Phys Chem 1949;53:1466–70. [3] Gu ZZ, Uetsuka H, Fujishima A. Structural color and the lotus effect. Angew Chem Int Ed 2003;42:894–7. [4] Lee W, Jin MK, Yoo WC, Lee JK. Defined nanometer-scale topography and tailored surface wettability. Langmuir 2004;20:7665–9. [5] Li SH, Li H, Wang X, Song Y, Liu Y, Jiang L, et al. Superhydrophobicity of large-area honeycomb-like aligned carbon nanotubes. J Phys Chem B 2002;106:9274–6. [6] Lau K, Bico J, Teo K, Chhowalla M, Amaratunga G, Milne W, et al. Super hydrophobic carbon nanotube forests. Nano Lett 2003;3:1701–5. [7] Marmur A. Underwater super hydrophobicity: theoretical feasibility. Langmuir 2006;22:1400–2. [8] Li J, Wang L, He T, Jiang W. Transport properties of hot-pressed bulk carbon nanotubes compacted by spark plasma sintering. Carbon 2009;47:1135–40. [9] Cassie A. Contact angles. Discuss Faraday Soc 1948;3:11–6.
Synthesis of thin carbon films on 4H-SiC by low temperature extraction of Si with HCl M.A. Fanton a b
a,* ,
J.A. Robinson b, M. Hollander b, B.E. Weiland a, K. Trumbull b, M. LaBella
b
The Pennsylvania State University, Electro-Optics Center, Freeport, PA 16229, USA The Pennsylvania State University, Electro-Optics Center, University Park, PA 16802, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
Carbon films as thin as 5 nm were synthesized by graphitization of 4H-SiC substrates at
Received 22 December 2009
1200–1350 °C, in a dilute HCl/Ar atmosphere at a pressure of 0.8 bar. These films formed
Accepted 31 March 2010
at significantly lower temperatures and higher pressures than conventional synthesis of
Available online 3 April 2010
epitaxial graphene by sublimation of Si. Graphitization rates of 0.08–1.40 nm/min were observed. The activation energy for graphitization was approximately 460 kJ/mol. Raman spectroscopy indicated that the material was highly disordered with D-peak to G-peak ratios ranging from 0.70 to 1.43, compared to high quality graphene which does not exhibit the disorder induced D-peak. Ó 2010 Elsevier Ltd. All rights reserved.
Recent success of graphene transistor operation in the GHz frequency range demonstrated the potential of this material for high speed electronics [1]. The most promising route for large area, electronic grade graphene is sublimation of Si from the surface of single crystal SiC semiconductor
wafers. This is carried out at temperatures >1400 °C, pressures near 10 6 mbar, for 1–2 h [2]. While this technique has achieved high carrier mobilities >18,000 cm2/V s, the high growth temperatures degrades the SiC surface resulting in non-uniform strain and thickness in the graphene, which in
* Corresponding author: Fax: +1 724 295 6617. E-mail address: [email protected] (M.A. Fanton). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.03.072
2672
CARBON
4 8 ( 2 0 1 0 ) 2 6 4 4 –2 6 7 3
turn lowers carrier mobility [3]. The goal of the work reported in this letter is to examine the kinetics of C film formation on the (0 0 0 1) plane of single crystal SiC in the presence of Cl, and understand what is required to create a highly ordered C film approaching a few atomic layers in thickness. We specifically report on the use of HCl, to extract Si from the SiC surface creating 5 nm thick films of C at temperatures as low as 1200 °C and a pressure of 0.8 bar. All experiments took place in a SiC chemical vapor deposition (CVD) system (Structured Materials Industries), and were carried out on the Si-face of n-type 4H-SiC oriented 4° off-axis (II–VI Inc.). Substrates were chemi-mechanically polished and then etched for 30 min at a temperature of 1550 °C and a pressure of 600 Torr in a mixture of 40% H2 in Ar. The combination of the off-axis substrate and H2 etching provided a high density of well defined step edges with the same crystallographic orientation. Ultimately it is desirable to remove the H2 etching step in order to maintain low process temperatures below 1300 °C. After etching, the H2 concentration was reduced to 8% and the temperature was ramped down to the growth temperature at a rate of 10 °C/min. After reaching the growth temperature a mixture of 5% HCl in He was delivered to the process environment via a Piezocon (Lorex Ind.) acoustic gas concentration controller to achieve a precise HCl concentration in the process environment. Fig. 1 shows the Arrhenius plot of the graphitization rate at HCl concentrations of 0.7 and 1.4 vol.%. Activation energies calculated from the Arrhenius plots are essentially the same for both HCl concentrations at 460 kJ/mol. This indicates that graphitization is limited by chemical reactions at the surface rather than by transport of species from the gas phase to or from the surface of the substrate. This activation energy is of the same order of magnitude of other non-transport limited reactions between gas phase species and the SiC surface. Chemical reactions involving growth of SiC from the vapor phase are of a similar magnitude. Matsunami et al. [4] reported an activation energy of 355 kJ/mol for the lateral propagation of a step edge during SiC CVD from a mixture of SiH4 and C3H8. These activation energies are significantly higher than the activation energy reported for graphitization of polycrystalline SiC ceramics. Cambaz et al. [7] reported a value of 50 kJ/mol using an Ar–Cl2 atmosphere at 600–800 °C. This suggests that the crystallographic direction and surface damage play a significant role in the loss of Si from the SiC surface. The mechanism for removal of Si by Cl is clearly different from the mechanism of Si sublimation from SiC. High temperature sublimation growth of graphene on the Si-face of (0 0 0 1) oriented SiC becomes self limiting under some conditions, resulting in the formation of only a few C layers [5]. In contrast, the formation of C ceases to become self limiting using the HCl-assisted process. As can be seen from Fig. 1, increasing the HCl concentration by a factor of 2 increases the reaction rate by almost a factor of 20 suggesting that the reaction rate may be fourth order with respect to the HCl concentration. Thermodynamic equilibrium calculations suggest that SiCl4 is the most likely reaction product formed between SiC and HCl. It is possible that the reaction mechanism involves four separate collisions between a Si atom and four HCl molecules, in order to create a single SiCl4 molecule, which then desorbs from the surface.
Fig. 1 – Arrhenius plot for graphitization of SiC using dilute HCl in Ar.
Although the mechanism of Si extraction involving Cl is not yet clear, it is possible that Cl is able to diffuse through a highly defective C layer to aid in removal of Si at the SiC/C interface. To evaluate this possibility the structure of the C layers was examined using Raman spectroscopy. Fig. 2 shows the Raman spectrum for a 25 nm thick C film processed at 1200 °C using an HCl concentration of 1.4%. The significant D-peak indicates that this material is not high quality graphene. This peak is indicative of disorder in the sp2 bonded structure and does not appear in a perfect sp2 bonded C structure. In contrast, the G-peak is derived from stretching of the C–C bond in sp2 bonded carbon and increases in intensity as the structural quality improves [6]. The ratio of the intensities of the D-peak and the G-peak for this sample is 0.70. This is considerably better than previously published C films processed on SiC using halogenated gases, which range from 1.6 to 4.7 [7,8]. By comparison, high quality graphene has a D/G ratio approaching zero, while graphene damaged by heavy ion bombardment has a ratio of 2–3 [6]. Fig. 3 shows the relationships between the D/G ratios, the film thickness, and the process temperature for 1 h of growth time. The data in Fig. 3 suggests that thinner films tend to be more ordered. Films formed with slower reaction rates (lower HCl concentration) also tend to have lower D/G ratios. There
Fig. 2 – Raman spectra of a 25 nm thick film processed at 1200 °C and an 85 nm thick film processed at 1250 °C, both for 1 h.
CARBON
4 8 ( 20 1 0 ) 2 6 4 4–26 7 3
2673
well controlled reaction rates leading to high quality single or bi-layer graphene.
Acknowledgement This work was supported by a 2009–2010 gift from the II–VI Foundation.
R E F E R E N C E S
Fig. 3 – The D/G Raman peak ratio for films of varying thickness and process temperature.
is no clear trend toward higher structural order with increased temperature. However, the effects of temperature and reaction rate in this data set are heavily interrelated. It is likely that higher structural order would be obtained at higher temperatures if the rate of reaction were slowed considerably. The high D/G ratio also indicates that the crystallite size is very small. Using the method published by Tuinstra and Koenig [9], the average grain size for the C films is on the order of only 5–7 nm. This small crystallite size may aid in diffusion of species to and from the reaction front. In summary, C films as thin as 5 nm, approaching graphene thickness, were processed on single crystal SiC substrates at temperatures 200–300 °C lower than conventional growth of graphene on SiC by sublimation. Graphitization rates of 0.08–1.4 nm/min were observed. The activation energy of the process was found to be approximately 460 kJ/mol. The films are disordered with respect to high quality epitaxial graphene due in part to the high reaction rate. A full understanding of the kinetics of this process is expected to yield
[1] Moon JS, Curtis D, Hu M, Wong D, McGuire C, Campbell PM, et al. Epitaxial-graphene RF field-effect transistors on Si-Face 6H-SiC substrates. IEEE Elec Dev Lett 2009;30:650–2. [2] Berger C, Song Z, Li X, Wu X, Brown N, Naud C, et al. Electronic confinement and coherence in patterned epitaxial graphene. Science 2006;312:1191–6. [3] Robinson JA, Wetherington M, Tedesco JL, Campbell PM, Weng X, Stitt J, et al. Correlating Raman spectral signatures with carrier mobility in epitaxial graphene: a guide to achieving high mobility on the wafer scale. Nano Lett 2009;9:2873–6. [4] Matsunami H, Kimoto T. Step-controlled epitaxial growth of SiC: high quality homoepitaxy. Mater Sci Eng 1997;R20:125–66. [5] de Heer WA, Berger C, Wu X, First PN, Conrad EH, Li X, et al. Epitaxial graphene. Solid State Commun 2007;143:92–111. [6] Dresselhaus MS, Jorio A, Hofmann M, Dresselhaus G, Saito R. Perspectives on carbon nanotubes and graphene Raman spectroscopy. Nano Lett 2010;10:751–8. [7] Cambaz ZG, Yushin GN, Gogotsi Y, Vyshnyakova KL, Pereselentseva LN. Formation of carbide-derived carbon on b-silicon carbide whiskers. J Am Ceram Soc 2006;89:509–14. [8] Suemitsu M, Miyamoto Y, Handa H, Konno A. Graphene formation on a 3C-SiC(1 1 1) thin film grown on Si(1 1 0) substrate. EJ Surf Sci Nanotech 2009;7:311–3. [9] Tuinstra F, Koenig JL. Raman spectrum of graphite. J Chem Phys 1970;53:1126–30.