- Email: [email protected]
Deposition of DLC film from adamantane by using pulsed discharge plasma CVD
Available online at www.sciencedirect.com
Diamond & Related Materials 17 (2008) 684 – 687 www.elsevier.com/locate/diamond
Deposition of DLC film from adamantane by using pulsed discharge plasma CVD M. Umeno ⁎, M. Noda, H. Uchida, H. Takeuchi Department of Electronics and Information Engineering, Chubu University, Matsumoto-cho 1200, Kasugai-shi, Aichi-Ken, 487-8501, Japan Available online 15 February 2008
Abstract Diamond like carbon films are deposited on silicon and quartz substrates using adamantane as a sole source of carbon by pulsed discharge plasma chemical vapor deposition. Tauc band gap of such films has been successfully tuned from 1.7eV to 2.9eV. Iodine incorporation is observed to favor the growth of such films and induces disorder in the films. It also brings down in energy the on-set of photon absorption. Such iodine incorporated diamond like carbon films may be interesting candidates for the new coming applications such as for heterojunction photovoltaic devices. © 2008 Elsevier B.V. All rights reserved. Keywords: Diamond like carbon; Plasma CVD; Optical properties
1. Introduction Diamond like carbon (DLC) thin films have attracted considerable academic and industrial interest owing to their outstanding properties such as high hardness, chemical inertness, high thermal conductivity, high electrical resistivity and optical transparency from ultraviolet to infra-red regions [1]. In the recent years, considerable interest is growing for using such films for fabricating heterojunction solar cells [2–8]. Dopants such as nitrogen, phosphorous, iodine, boron etc. are used in attempts to make such films either n- or p-type [2–11]. In all such efforts, carbon source used either is graphite or low molecular weight hydrocarbons such as CH4, C2H2, C2H4 etc. The use of higher molecular weight hydrocarbons apart from C3, C4 and C6 systems, has largely been ignored. Adamantane (C10H16) is an interesting high molecular weight hydrocarbon material for the synthesis of crystalline diamond and DLC films. It has a tricyclic rigid ring structure as shown in Fig. 1 inset at upper right hand side. It is also described as a ten-carbon “Molecular Diamond”. In this molecule, all the carbon–carbon bond angles are tetrahedral, with a corresponding bond length of 1.54A. Adamantane can sublime easily and has a relatively high vapor pressure, even at room temperature. Partial break down of adamantane is known to yield carbon clusters (CnHx) where n = 3, 5, 6, 7, 8 and 9 of ⁎ Corresponding author. Tel.: +81 9099060866; fax: +81 568 51 1478. E-mail address: [email protected] (M. Umeno). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.01.118
significant abundance. An important potential benefit is that carbon clusters are reported to play a significant role in the growth of DLC films or crystalline diamond. Additionally, adamantane has a low carbon/hydrogen ratio of 1/1.6; unlike methane (in which it is 1:4). It is known that the mechanical properties of DLC films are strongly affected by hydrogen content i.e. because hydrogen is monovalent, it acts as terminating potential carbon– carbon bonds. Hence, it can restrict the formation of strongly crosslinked network desired in certain DLC films. Considering these aspects, adamantane is an interesting candidate for DLC films growth [12,13]. In the present attempt, adamantane (C10H16) is used as a sole source of carbon to deposit DLC films by pulsed discharge plasma chemical vapor deposition. Further, iodine is incorporated in such DLC films are the films are studied for their optical and morphological properties. 2. Experimental Schematic of the experimental set up used by authors is shown in Fig. 1. For more details, please refer to reference no [14]. Chamber is pre-evacuated by rotary pump and H2 gas is supplied in it. The deposition pressure in the chamber is kept constant at about 45Torr. H2 gas is supplied in the chamber at a constant rate of 100sccm. Distance between anode and cathode (L), anode and substrate (D1) and cathode and adamantine crucible (D2) are kept constant at about 3cm, 1.5cm and 4.5cm, respectively, in all experiments. Discharge is performed between anode and cathode
M. Umeno et al. / Diamond & Related Materials 17 (2008) 684–687
685
Fig. 1. Schematic of the experimental set up of pulsed discharge plasma chemical vapor deposition system. Inset (upper right side) shows the schematic of the molecular structure of adamantane.
while no bias is applied to the substrates. Pre-cleaned silicon (Si) and quartz (QZ) substrates are used for depositing the diamond like carbon films. Substrates were kept on insulating substrate holder. Typical time of deposition is about 30min. Experiments were performed using 3g of adamantane solid kept in the crucible and 3g (2.8g adamantane + 0.2g Iodine) of adamantane and iodine mixture. Here it should be noted that in the beginning of the deposition, chamber is pre-evacuated and during this process the crucible containing adamantane is covered with an adjustable lid which could be manipulated from outside the chamber. Pulsed discharge is performed between anode and cathode. The repetition frequency and duty ratio of the pulse is set at 400Hz and 20%, respectively. The peak of the discharge current is about 2A and the voltage applied between anode and cathode is about 2200V. The temperature of the sample is continuously monitored with thermocouple. The discharge time and the non-discharge time corresponding to ON and OFF times of intelligent power module, are set to 0.5 and 2ms, respectively. Since anode, cathode, adamantane crucible and substrates are in close placed with each other; the plasma generated between anode and cathode evaporates the adamantane and its pyrolysis occurs depending on the temperature which gives rise to DLC film on the substrates. The evaporation of adamantane can be controlled by the deposition temperature, deposition time and the amount of adamantane taken in the crucible. By controlling the deposition temperature which in turn depends on the voltage applied between cathode and anode, peak discharge current, deposition pressure (keeping other parameters constant), the optical properties of the DLC film formed on the substrates like Tauc band gap etc. can be controlled. The tuning of band gap of DLC films by changing the deposition parameters also reported by some groups [8,15].
The samples thus obtained are studied for their optical absorption using Shimadzu UV–VIS–NIR spectrophotometer, field emission scanning electron microscopy (FE-SEM, Hitachi S-4300), visible Raman spectroscopy (JASCO NRS-300 Laser Raman Spectrophotometer, 432nm green laser excitation). 3. Results and discussion The optical pseudo gap values of the DLC films deposited on QZ were determined from reflection and transmission spectra and using the Tauc relation αE = B (E − Eg)2, conventionally defined for amorphous semiconductors. Basically, this optical gap is a measure of the gap between the extended states in the valance and conduction bands. Fig. 2 shows the Tauc plot for the some of the
Fig. 2. Tauc plot for the DLC films deposited on QZ.
686
M. Umeno et al. / Diamond & Related Materials 17 (2008) 684–687
Fig. 3. Variation of the coefficient of photon absorption (cm− 1) vs. photon energy (eV) for the DLC film deposited without (a) and with (b) iodine incorporation.
DLC films deposited from pure adamantane. Tauc band gap is estimated by extrapolating the linear part of the curve in the higher energy region. Tauc band gap of the DLC films varies between
1.7eV to 2.9eV, depending on the synthesis conductions; more specifically, depending on the plasma temperature. Fig. 3 shows the variation of coefficient of photon absorption vs. photon energy for the DLC films deposited using adamantane (a) and adamantane + iodine mixture (b). On-set of the optical absorption for the DLC film deposited using adamantane only starts at 1.7eV whereas for the film deposited using the mixture of adamantane + iodine starts at 1.1eV. It is evident that the on-set of the optical absorption shifts to lower energy values for the DLC films deposited using mixture of adamantane + iodine as compared with the films deposited using adamantane only. This is due to the fact that iodine gets incorporated into the DLC films formed using the mixture. Fig. 4 shows the morphology of the DLC films deposited using adamantane and adamantane + iodine mixture and their corresponding visible Raman spectra. (a1) and (a2) shows the SEM picture of the DLC film deposited from adamantane and (a3) shows its corresponding visible Raman spectra. Similarly, (b1) and (b2) shows the SEM picture of the DLC film deposited from adamantane + iodine mixture and (b3) shows its corresponding visible Raman spectra. Comparison suggests increase in the particle size for the films deposited using mixture of adamantane + iodine as compared with the film deposited using only adamantane. Further, it is observable that the DLC films deposited by using the mixture are much thicker than the films without using iodine. This may suggest that iodine helps growth of DLC film. DLC film deposited using adamantane only (a3) shows Raman active graphitic (G) and disordered (D) peaks at about 1607 cm− 1 and 1334 cm− 1, respectively. DLC film deposited using the mixture shows G and D peaks at about 1588 cm− 1 and 1344 cm− 1, respectively. It is observable that the D-peak is enhanced in the film deposited using the mixture which may be due to the fact that iodine incorporation in the DLC film increases the disorder in the
Fig. 4. SEM pictures and visible Raman spectra of the DLC film deposited without (a1, a2, a3) and with (b1, b2, b3) iodine incorporation.
M. Umeno et al. / Diamond & Related Materials 17 (2008) 684–687
film. Such iodine incorporated DLC films are interesting for the new coming applications such as heterojunction solar cells [2–11]. Other dopants such as nitrogen, phosphorus etc. can be easily incorporated in the DLC films fabricated by this method starting form the suitable gaseous precursors. In all, we present a novel method for the deposition of DLC films and doped DLC films using adamantane as a carbon source. The method is simple and has prospects for scale up since we can control the deposition conditions precisely. 4. Conclusions We present a novel method for the synthesis of diamond like carbon (DLC) films with and without incorporation of iodine. Optical absorption properties and Tauc band gap of such films can be easily tuned by changing the experimental conditions. Iodine incorporation is observed to increase the particle size and thickness of the DLC films. This may suggest that iodine enhances the growth of DLC films. However, its incorporation also increases the disorder in the film which is evident from the visible Raman spectroscopy. In all, we present a novel method for the deposition of DLC films and doped DLC films using adamantane as a carbon source. Acknowledgement We thank JSPS, NEDO for financial support.
687
References [1] S.M. Mominuzzaman, K.M. Krishna, T. Soga, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 38 (1999) 658. [2] M. Umeno, S. Adhikary, Diam. Relat. Mater. 14 (2005) 1973. [3] S. Adhikari, S. Adhikary, A.M.M. Omer, M. Rusop, H. Uchida, T. Soga, M. Umeno, Diam. Relat. Mater. 14 (2005) 1824. [4] X. Tian, T. Soga, T. Jimbo, M. Umeno, J. Non-Cryst. Solids 336 (2004) 32. [5] M. Rusop, S. Adhikari, A.M.M. Omer, S. Adhikary, H. Uchida, T. Jimbo, T. Soga, M. Umeno, Mod. Phys. Lett. B 18 (2004) 987. [6] X. Tian, D.K. Mishra, T. Soga, T. Jimbo, M. Umeno, IEEJ Trans. SM 124 (2004) 54. [7] W.I. Milne, Semicond. Sci. Technol. 18 (2003) S81. [8] V.G. Litovchenko, N.I. Kluyi, Sol. Energy Mater. Sol. Cells 68 (2001) 55. [9] Papers of Technical Meeting on Optical and Quantum Devices, IEEJ Journal code: Z0922A. OQD-04 (1–10) (2004) 41. [10] A.M.M. Omer, S. Adhikari, S. Adhikary, M. Rusop, H. Uchida, T. Soga, M. Umeno, Diam. Relat. Mater. 15 (2006) 645. [11] A.M.M. Omer, S. Adhikari, S. Adhikary, H. Uchida, M. Umeno, Appl. Phys. Lett. 87 (2005) 161912. [12] D.A. Zeze, E.P. O'Toole, R.I. Crawford, N. Cui, C.A. Anderson, N.M.D. Brown, Surf. Interface Anal. 26 (1998) 896. [13] D.A. Zeze, D.R. North, N.M.D. Brown, C.A. Anderson, Surf. Interface Anal. 29 (2000) 369. [14] M. Noda, M. Umeno, Diam. Relat. Mater. 14 (2005) 1791. [15] P. Koidl, C. Wild, R. Locher, R.E. Sah, in: R.E. Clausing, L.L. horton, J.C. Angus, P. Koidl (Eds.), Diamond and Diamond Like Films Coatings, NATOASI Series B: Physics, Plenum Publishing Co, Newyork, 1991, p. 243.
Diamond & Related Materials 17 (2008) 684 – 687 www.elsevier.com/locate/diamond
Deposition of DLC film from adamantane by using pulsed discharge plasma CVD M. Umeno ⁎, M. Noda, H. Uchida, H. Takeuchi Department of Electronics and Information Engineering, Chubu University, Matsumoto-cho 1200, Kasugai-shi, Aichi-Ken, 487-8501, Japan Available online 15 February 2008
Abstract Diamond like carbon films are deposited on silicon and quartz substrates using adamantane as a sole source of carbon by pulsed discharge plasma chemical vapor deposition. Tauc band gap of such films has been successfully tuned from 1.7eV to 2.9eV. Iodine incorporation is observed to favor the growth of such films and induces disorder in the films. It also brings down in energy the on-set of photon absorption. Such iodine incorporated diamond like carbon films may be interesting candidates for the new coming applications such as for heterojunction photovoltaic devices. © 2008 Elsevier B.V. All rights reserved. Keywords: Diamond like carbon; Plasma CVD; Optical properties
1. Introduction Diamond like carbon (DLC) thin films have attracted considerable academic and industrial interest owing to their outstanding properties such as high hardness, chemical inertness, high thermal conductivity, high electrical resistivity and optical transparency from ultraviolet to infra-red regions [1]. In the recent years, considerable interest is growing for using such films for fabricating heterojunction solar cells [2–8]. Dopants such as nitrogen, phosphorous, iodine, boron etc. are used in attempts to make such films either n- or p-type [2–11]. In all such efforts, carbon source used either is graphite or low molecular weight hydrocarbons such as CH4, C2H2, C2H4 etc. The use of higher molecular weight hydrocarbons apart from C3, C4 and C6 systems, has largely been ignored. Adamantane (C10H16) is an interesting high molecular weight hydrocarbon material for the synthesis of crystalline diamond and DLC films. It has a tricyclic rigid ring structure as shown in Fig. 1 inset at upper right hand side. It is also described as a ten-carbon “Molecular Diamond”. In this molecule, all the carbon–carbon bond angles are tetrahedral, with a corresponding bond length of 1.54A. Adamantane can sublime easily and has a relatively high vapor pressure, even at room temperature. Partial break down of adamantane is known to yield carbon clusters (CnHx) where n = 3, 5, 6, 7, 8 and 9 of ⁎ Corresponding author. Tel.: +81 9099060866; fax: +81 568 51 1478. E-mail address: [email protected] (M. Umeno). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.01.118
significant abundance. An important potential benefit is that carbon clusters are reported to play a significant role in the growth of DLC films or crystalline diamond. Additionally, adamantane has a low carbon/hydrogen ratio of 1/1.6; unlike methane (in which it is 1:4). It is known that the mechanical properties of DLC films are strongly affected by hydrogen content i.e. because hydrogen is monovalent, it acts as terminating potential carbon– carbon bonds. Hence, it can restrict the formation of strongly crosslinked network desired in certain DLC films. Considering these aspects, adamantane is an interesting candidate for DLC films growth [12,13]. In the present attempt, adamantane (C10H16) is used as a sole source of carbon to deposit DLC films by pulsed discharge plasma chemical vapor deposition. Further, iodine is incorporated in such DLC films are the films are studied for their optical and morphological properties. 2. Experimental Schematic of the experimental set up used by authors is shown in Fig. 1. For more details, please refer to reference no [14]. Chamber is pre-evacuated by rotary pump and H2 gas is supplied in it. The deposition pressure in the chamber is kept constant at about 45Torr. H2 gas is supplied in the chamber at a constant rate of 100sccm. Distance between anode and cathode (L), anode and substrate (D1) and cathode and adamantine crucible (D2) are kept constant at about 3cm, 1.5cm and 4.5cm, respectively, in all experiments. Discharge is performed between anode and cathode
M. Umeno et al. / Diamond & Related Materials 17 (2008) 684–687
685
Fig. 1. Schematic of the experimental set up of pulsed discharge plasma chemical vapor deposition system. Inset (upper right side) shows the schematic of the molecular structure of adamantane.
while no bias is applied to the substrates. Pre-cleaned silicon (Si) and quartz (QZ) substrates are used for depositing the diamond like carbon films. Substrates were kept on insulating substrate holder. Typical time of deposition is about 30min. Experiments were performed using 3g of adamantane solid kept in the crucible and 3g (2.8g adamantane + 0.2g Iodine) of adamantane and iodine mixture. Here it should be noted that in the beginning of the deposition, chamber is pre-evacuated and during this process the crucible containing adamantane is covered with an adjustable lid which could be manipulated from outside the chamber. Pulsed discharge is performed between anode and cathode. The repetition frequency and duty ratio of the pulse is set at 400Hz and 20%, respectively. The peak of the discharge current is about 2A and the voltage applied between anode and cathode is about 2200V. The temperature of the sample is continuously monitored with thermocouple. The discharge time and the non-discharge time corresponding to ON and OFF times of intelligent power module, are set to 0.5 and 2ms, respectively. Since anode, cathode, adamantane crucible and substrates are in close placed with each other; the plasma generated between anode and cathode evaporates the adamantane and its pyrolysis occurs depending on the temperature which gives rise to DLC film on the substrates. The evaporation of adamantane can be controlled by the deposition temperature, deposition time and the amount of adamantane taken in the crucible. By controlling the deposition temperature which in turn depends on the voltage applied between cathode and anode, peak discharge current, deposition pressure (keeping other parameters constant), the optical properties of the DLC film formed on the substrates like Tauc band gap etc. can be controlled. The tuning of band gap of DLC films by changing the deposition parameters also reported by some groups [8,15].
The samples thus obtained are studied for their optical absorption using Shimadzu UV–VIS–NIR spectrophotometer, field emission scanning electron microscopy (FE-SEM, Hitachi S-4300), visible Raman spectroscopy (JASCO NRS-300 Laser Raman Spectrophotometer, 432nm green laser excitation). 3. Results and discussion The optical pseudo gap values of the DLC films deposited on QZ were determined from reflection and transmission spectra and using the Tauc relation αE = B (E − Eg)2, conventionally defined for amorphous semiconductors. Basically, this optical gap is a measure of the gap between the extended states in the valance and conduction bands. Fig. 2 shows the Tauc plot for the some of the
Fig. 2. Tauc plot for the DLC films deposited on QZ.
686
M. Umeno et al. / Diamond & Related Materials 17 (2008) 684–687
Fig. 3. Variation of the coefficient of photon absorption (cm− 1) vs. photon energy (eV) for the DLC film deposited without (a) and with (b) iodine incorporation.
DLC films deposited from pure adamantane. Tauc band gap is estimated by extrapolating the linear part of the curve in the higher energy region. Tauc band gap of the DLC films varies between
1.7eV to 2.9eV, depending on the synthesis conductions; more specifically, depending on the plasma temperature. Fig. 3 shows the variation of coefficient of photon absorption vs. photon energy for the DLC films deposited using adamantane (a) and adamantane + iodine mixture (b). On-set of the optical absorption for the DLC film deposited using adamantane only starts at 1.7eV whereas for the film deposited using the mixture of adamantane + iodine starts at 1.1eV. It is evident that the on-set of the optical absorption shifts to lower energy values for the DLC films deposited using mixture of adamantane + iodine as compared with the films deposited using adamantane only. This is due to the fact that iodine gets incorporated into the DLC films formed using the mixture. Fig. 4 shows the morphology of the DLC films deposited using adamantane and adamantane + iodine mixture and their corresponding visible Raman spectra. (a1) and (a2) shows the SEM picture of the DLC film deposited from adamantane and (a3) shows its corresponding visible Raman spectra. Similarly, (b1) and (b2) shows the SEM picture of the DLC film deposited from adamantane + iodine mixture and (b3) shows its corresponding visible Raman spectra. Comparison suggests increase in the particle size for the films deposited using mixture of adamantane + iodine as compared with the film deposited using only adamantane. Further, it is observable that the DLC films deposited by using the mixture are much thicker than the films without using iodine. This may suggest that iodine helps growth of DLC film. DLC film deposited using adamantane only (a3) shows Raman active graphitic (G) and disordered (D) peaks at about 1607 cm− 1 and 1334 cm− 1, respectively. DLC film deposited using the mixture shows G and D peaks at about 1588 cm− 1 and 1344 cm− 1, respectively. It is observable that the D-peak is enhanced in the film deposited using the mixture which may be due to the fact that iodine incorporation in the DLC film increases the disorder in the
Fig. 4. SEM pictures and visible Raman spectra of the DLC film deposited without (a1, a2, a3) and with (b1, b2, b3) iodine incorporation.
M. Umeno et al. / Diamond & Related Materials 17 (2008) 684–687
film. Such iodine incorporated DLC films are interesting for the new coming applications such as heterojunction solar cells [2–11]. Other dopants such as nitrogen, phosphorus etc. can be easily incorporated in the DLC films fabricated by this method starting form the suitable gaseous precursors. In all, we present a novel method for the deposition of DLC films and doped DLC films using adamantane as a carbon source. The method is simple and has prospects for scale up since we can control the deposition conditions precisely. 4. Conclusions We present a novel method for the synthesis of diamond like carbon (DLC) films with and without incorporation of iodine. Optical absorption properties and Tauc band gap of such films can be easily tuned by changing the experimental conditions. Iodine incorporation is observed to increase the particle size and thickness of the DLC films. This may suggest that iodine enhances the growth of DLC films. However, its incorporation also increases the disorder in the film which is evident from the visible Raman spectroscopy. In all, we present a novel method for the deposition of DLC films and doped DLC films using adamantane as a carbon source. Acknowledgement We thank JSPS, NEDO for financial support.
687
References [1] S.M. Mominuzzaman, K.M. Krishna, T. Soga, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 38 (1999) 658. [2] M. Umeno, S. Adhikary, Diam. Relat. Mater. 14 (2005) 1973. [3] S. Adhikari, S. Adhikary, A.M.M. Omer, M. Rusop, H. Uchida, T. Soga, M. Umeno, Diam. Relat. Mater. 14 (2005) 1824. [4] X. Tian, T. Soga, T. Jimbo, M. Umeno, J. Non-Cryst. Solids 336 (2004) 32. [5] M. Rusop, S. Adhikari, A.M.M. Omer, S. Adhikary, H. Uchida, T. Jimbo, T. Soga, M. Umeno, Mod. Phys. Lett. B 18 (2004) 987. [6] X. Tian, D.K. Mishra, T. Soga, T. Jimbo, M. Umeno, IEEJ Trans. SM 124 (2004) 54. [7] W.I. Milne, Semicond. Sci. Technol. 18 (2003) S81. [8] V.G. Litovchenko, N.I. Kluyi, Sol. Energy Mater. Sol. Cells 68 (2001) 55. [9] Papers of Technical Meeting on Optical and Quantum Devices, IEEJ Journal code: Z0922A. OQD-04 (1–10) (2004) 41. [10] A.M.M. Omer, S. Adhikari, S. Adhikary, M. Rusop, H. Uchida, T. Soga, M. Umeno, Diam. Relat. Mater. 15 (2006) 645. [11] A.M.M. Omer, S. Adhikari, S. Adhikary, H. Uchida, M. Umeno, Appl. Phys. Lett. 87 (2005) 161912. [12] D.A. Zeze, E.P. O'Toole, R.I. Crawford, N. Cui, C.A. Anderson, N.M.D. Brown, Surf. Interface Anal. 26 (1998) 896. [13] D.A. Zeze, D.R. North, N.M.D. Brown, C.A. Anderson, Surf. Interface Anal. 29 (2000) 369. [14] M. Noda, M. Umeno, Diam. Relat. Mater. 14 (2005) 1791. [15] P. Koidl, C. Wild, R. Locher, R.E. Sah, in: R.E. Clausing, L.L. horton, J.C. Angus, P. Koidl (Eds.), Diamond and Diamond Like Films Coatings, NATOASI Series B: Physics, Plenum Publishing Co, Newyork, 1991, p. 243.