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Electrically conductive properties of tungsten-containing diamond-like carbon films
Diamond & Related Materials 15 (2006) 1902 – 1905 www.elsevier.com/locate/diamond
Electrically conductive properties of tungsten-containing diamond-like carbon films Takanori Takeno a , Hiroyuki Miki a , Toshiyuki Takagi a,⁎, Hideya Onodera b a b
Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, 980-8577, Japan Department of Physics, Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan Available online 31 October 2006
Abstract Tungsten-containing diamond-like carbon films with different metal concentrations were investigated. The films of several hundred nanometers in thickness were deposited on the silicon wafer using RF-PECVD (radio frequency plasma enhanced chemical vapor deposition) method. During deposition, metal component was co-sputtered using DC magnetron of tungsten target. The six samples with the concentration of 3.8, 6.1, 8.0, 16.3, 24.3 and 41.4 at.% of tungsten were made. The structural analyses were performed by TEM (transmission electron microscope) and Raman spectroscopy. These results indicated that tungsten clusters were well dispersed in amorphous carbon host matrix in the case of tungsten concentration from 3.8 to 24.2 at.%. However, no such a structure can be observed in the sample with 41.4 at.%. The AC electrical resistance was measured in the temperature range of 2–300 K using four-probe method in vacuum condition. The observed temperature dependence of electrical conductivity can be expressed by r ¼ r0 expt−2ðC0 =kT Þ1=2 b and tungsten concentration from 3.8 at.% to 24.2 at.%. In addition, the sample with 41.4 at.% showed the resistive superconducting transition at Tc of around ∼5.5 K. © 2006 Elsevier B.V. All rights reserved. Keywords: Tungsten; Diamond-like carbon; Electrical conduction; Effective barrier height
1. Introduction Diamond-like carbon films are well-known materials and their properties such as hardness, surface morphology [1], friction coefficient [2] and electrical conduction properties [3] are strongly dependent on the volume fraction of carbon sp2 bond, sp3 bond, and hydrogen concentration. Various deposition techniques have been reported in order to make objective diamond-like carbon films [4]. Recently, elemental addition into diamond-like films is widely tried in order to get further functionality in the films. Both light and heavy elements can be used as a dopant, and chemical compounds are also used at times. In the case of metal element, the original reason to add metals to diamond-like carbon was to reduce the compressive stress [5]. Doped atoms form a cluster or wire like shape, which depends on the element used [6]. In a viewpoint of mechanical character of these films, these properties are affected by the doped metal and carbon concentration. For instance, in the case of tungsten-containing ⁎ Corresponding author. Tel./fax: +81 22 217 5248. E-mail address: [email protected] (T. Takagi). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.06.020
diamond-like carbon films, the hardness and friction coefficient of the films depends on the tungsten and carbon concentration [7,8]. The variation of the hardness and friction coefficient in these films result from crystalline phase consisted of carbon and tungsten. When we focus on electrical conduction properties of metaldoped diamond-like carbon films, electron transport phenomena can be also governed by the film structure and additional elements. The electron transport properties of metal-containing diamond-like carbon–silicon nanocomposite films were previously discussed in terms of the model of inelastic tunneling of electron in amorphous carbon–silicon dielectrics [9–11]. The electrically conductive properties of them were well affected by the metal concentration because the structural modification of them appeared as increase in metal concentration. In the case of molybdenum-containing films, electrically conductive properties depends on the size of the molybdenum and distances between molybdenum clusters [12]. In this report we choose the tungsten as a doping element. The structure consisted of tungsten and carbon can be changed by the concentration of tungsten, which indicates that tungsten carbide phases come to appear in the high metal concentration regime [13]. This paper
T. Takeno et al. / Diamond & Related Materials 15 (2006) 1902–1905
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deposited on the investigated films. The silver paste was used for the contacts. This experiment was performed in vacuum condition. 3. Results and discussion 3.1. Structural analyses
Fig. 1. Plan-view image of tungsten-containing diamond-like carbon films with tungsten concentration of 24.2 at.%. Insert of this figure shows the electron diffraction pattern.
presents electrical conduction properties of tungsten-containing diamond-like carbon films with various tungsten concentrations. The structural modification and electrically conductive properties are investigated. Transmission electron microscope and Raman spectroscopy were performed for the direct observation of the fine structure and carbon bonding structure of them. Temperature dependence of resistivity was measured down to liquid helium temperature regime. Compared to the results obtained by structural analyses and temperature dependence, electrically conductive properties are discussed. 2. Experimental Tungsten-containing diamond-like carbon films with several hundred nanometers in thickness was deposited by the conventional radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique with DC magnetron sputtering of tungsten target. The silicon substrates were cleaned by argon plasma prior to the deposition. The methane and the argon gases were used for deposition. The tungsten concentrations of the films were varied by changing sputtering power from 60 W to 100 W. The working pressure and the substrate bias voltage were fixed at 2.5 · 10− 2 Torr and − 400 V, respectively. The substrate was cooled by water during the deposition process. The concentrations of the film were measured by the electron probe micro analyzer (EPMA). The structural analyses were carried out by the transmission electron microscope (TEM) and the Raman spectroscopy. The plan-view image and diffraction pattern were obtained by 300 kV acceleration. The Raman spectra were taken using a JobinYvon micro-Raman spectroscope with He–Ne laser excitation (λ = 632.8 nm). The input power was reduced from initial to 2 mW by the filter in order not to damage the films. This experiment was performed at room temperature and ambient air condition. The temperature dependence of resistivity was measured by a four-probe method using Quantum Design Physical Properties Measurement System. The gold layer with 20 nm thickness was
From the result of EPMA, tungsten concentration of the investigated films was estimated as 3.8 at.%, 6.1 at.%, 8.0 at.%, 16.3 at.%, 24.3 at.% and 41.4 at.%, respectively. Fig. 1 shows that plan-view image of the sample with 24.3 at.%. Insert of Fig. 1 shows the diffraction pattern. Tungsten clusters, which sizes were about 4 nm, were formed and well dispersed in carbon matrix. In addition, clear diffraction pattern indicated that these clusters were polycrystalline. However, for the sample with 41.4 at.% concentration of tungsten, structural image and diffraction pattern were different from the one with 24.3 at.%. One of the reasons of these results is the formation of tungsten carbide phase. At the low tungsten concentration regions no structural modifications were observed. But with a further increase in tungsten concentration, tungsten carbide phase comes to exist. Although the critical concentration for the transition to tungsten carbide phase is still not determined, both two samples can be considered to be below and above this concentration. To make clear the intermediate carbon bonding state, Raman spectroscopy was carried out. These results were shown in Fig. 2. In parallel, a spectrum of non-doped diamond-like carbon film that was deposited on silicon substrate under the same bias voltage was shown in Fig. 2 as a reference. The broad-shape peaks were observed in the sample with the concentration up to 24.3 at.%. As in the non-doped film, diamond-like carbon structure also existed in tungsten-containing films. But it was not observed in the case of the sample with 41.4 at.%.
Fig. 2. Raman spectra of tungsten-containing diamond-like carbon films with various tungsten concentration. A spectrum of non-doped diamond-like carbon film is also shown as a reference.
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T. Takeno et al. / Diamond & Related Materials 15 (2006) 1902–1905
Fig. 3. The calculated results of Q and I(D)/I(G) ratio obtained by profile-fitting are plotted against the tungsten concentration.
The obtained Raman spectra were explained as the combination of two spectra; the D peak (“disordered peak”) located around 1350 cm− 1 and the G peak (graphitic) located around 1550 cm− 1. The D peak and the G peak represent the breathing vibration mode of carbon aromatic rings and the stretching vibration mode of carbon pairs, respectively [4]. In this study, Lorenz function and Breit–Wigner–Fano (BWF) function [14] were used for the analysis of the D peak and the G peak. Also, the linear function was for background luminescence. There were so many parameters that need to be considered. But we focused on the two parameters for quantitative comparison. One of them is the intensity ratio of peak maximum at D peak and G peak, I(D)/I(G). This peak intensity ratio is correlated to the volume fraction of sp3 carbon bonding [4,15]. The volume fraction of sp3 carbon decrease as increase in peak intensity ratio [4]. Another is coupling coefficient, Q. Prawer et al. made the samples with various sp2 concentration and inves-
Fig. 5. The obtained results of parameter, 2(C0 / k)1 / 2, is plotted against the tungsten concentration.
tigated the relation between the coupling coefficient and volume fraction of sp2 carbon bonding [14]. When a drastic decrease in coupling coefficient as the volume fraction decreases was observed, the decrease in coupling coefficient means decrease in sp2 volume fraction [14]. I(D)/I(G) peak intensity ratio and coupling coefficient, Q, were plotted against the tungsten concentration were shown in Fig. 3. The result in the sample with the concentration of 41.4 at. % was excluded because broaden peak was not as seen in Fig. 2. Both values of fitting parameter increased even in the small amount of tungsten in the film, which generally indicates increase in volume fraction of sp2 and decrease in sp3. Although the values of I(D)/I(G) peak intensity ratio in tungsten-containing films are constant against the tungsten concentrations, the value in non-doped film is smaller. This tendency suggests that the carbon matrix cannot be affected by the tungsten concentration in the range from 3.8 at.% to 24.3 at.%. 3.2. Electrical conduction properties Firstly, we excluded the result on the sample with the concentration of 41.4 at.%. In this sample, superconducting transition was observed at 5.5 K. The structure and electrical conduction properties were different from other samples. In this paper we focus on the electrical conduction properties in granular structure. The structure of granular materials is that well crystallized metal grains are dispersed in the insulating host matrix. Electrical conduction appears as a result from the transport phenomena of electrons and holes by charged metal grains to neutral grains [16]. In order that this process occurs, an energy Ec is required [17]. This energy is expressed as Ec = (e2 /d)F(s / d), where e is the electronic charge, d is the cluster size, s is the separation between the grains, and F is a function that depends on shape and arrangement of the clusters. According to Abele et al. [16], temperature dependence of conductivity was calculated as rffiffiffiffiffiffi C0 r ¼ r0 exp 2 ð1Þ kT
½
Fig. 4. Temperature dependence of resistivity in tungsten-containing diamondlike carbon films with various tungsten concentration. The profile-fitting results for all experimental results are also shown in this figure.
here, σ0 and C0 are constant values and k is Bolzman constant. The value C0 is consisted of effective barrier height, distance
T. Takeno et al. / Diamond & Related Materials 15 (2006) 1902–1905
between the grains and Ec as mentioned above [12], and σ0 is the constant of temperature independent. Since the structure of the investigated sample is granular in the concentration up to 24.3 at.%, temperature dependence is expected to be followed by T−1/2. In Fig. 4 the conductivity was plotted against T− 1 / 2. Taken into account the above discussion, electrons transport from a tungsten cluster to the nearest tungsten cluster through carbon medium by tunneling. In the case of tunneling effect, the potential barrier between the tungsten clusters plays an important role for electrical conduction. The various shape and height of the barrier were proposed and well studied up to now [18,19]. Since the shape and height of the potential barrier depend on host matrix, various potential barriers can be considered. Simmons established the generalized potential barrier whose shape and height were represented by Φ as a function of the distance from the nearest cluster, x [20]. The form of Φ(x) is arbitrary. Although Φ(x) depends on each tunnel junction in inhomogeneous host matrix, a barrier height determined by experimental results represent the average barrier height of all tunnel junctions. This average is defined as effective barrier height, ΦD. Using assumption reported previously [12,17,21], effective barrier height can be calculated from the equation given by; " #1=2 rffiffiffiffiffiffi C0 1 ð2mUD Þ1=2 sEc 2 ð2Þ ¼2 k h=2k k here, m is mass of electron, h is Planck constant. The calculated value, 2(C0 / k)1 / 2, was given by numerically from the results shown in Fig. 4. The results were plotted against the tungsten concentration was shown in Fig. 5. As increase in tungsten concentration in the film, 2(C0 / k)1 / 2 decreases. In Eq. (4), variable is only ΦD because of the assumption [17,21]. Since carbon medium between the clusters is concentration independent, sizes of clusters and separation between them are one of the important parameters for the estimation of ΦD. Generally speaking, if we assume the simple one-dimensional structure like (tungsten cluster)–(insulating carbon medium)– (tungsten cluster), the separation between the clusters and the cluster sizes increase as tungsten concentration increases, which indicates the growth of effective barrier height. But our results are opposite. As a qualitative explanation, the existence of sp2 carbon cluster can be considered. Because of the geometrical reason in separation between the clusters with the increase of tungsten concentration, sp2 carbon cluster whose energy level is lower than insulating diamond-like carbon matrix can exist between the tungsten clusters, which lower the effective barrier between the tungsten clusters.
4. Summary In this paper, electrical conduction properties of tungstencontaining diamond-like carbon films were investigated. Films with various tungsten concentrations were deposited on silicon
1905
substrates by RF-PECVD and co-sputtering of tungsten metal target. In the case of the sample with the concentration up to 24.3 at.%, structure of the films is classified into granular type. Taken into account that cluster sizes and separation between the tungsten clusters increase as concentration increased, lowering of effective barrier height is due to the geometrical effects. In addition, superconducting transition that temperature is ∼ 5.5 K was observed for the sample with the concentration of 41.4 at.%. Acknowledgements This work was partly supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (No. 16-03420). Authors are greatly thankful to Mr. Takeshi Sato for his technical assistance in Institute of Fluid Science in Tohoku University, and to Mr. Takashi Kamaya and Yuko Sato for EPMA measurement in Nanotechnical laboratory in Institute of Multidisciplinary Research for Advanced Materials in Tohoku University. References [1] X.L. Peng, Z.H. Barber, T.W. Clyne, Surface & Coatings Technology 138 (2001) 23. [2] J. Fontaine, T. Le Mogne, J.L. Loubet, M. Belin, Thin Solid Films 482 (2005) 99. [3] K. Takai, M. Oga, H. Sato, T. Enoki, Y. Ohki, A. Taomoto, K. Suenaga, S. Iijima, Physical Review B 67 (2003) 214202. [4] J. Robertson, Materials Science & Engineering. R, Reports 37 (2002) 129. [5] H. Dimigen, C.P. Klages, Surface & Coatings Technology 49 (1991) 543. [6] K.I. Schiffmann, M. Fryda, G. Goerigk, R. Lauer, P. Hinze, A. Bulack, Thin Solid Films 347 (1999) 60. [7] A.A. Voevodin, J.P. O'Neill, J.S. Zabinski, Thin Solid Films 342 (1999) 194. [8] A.A. Voevodin, J.P. O'Neill, S.V. Prasad, J.S. Zabinski, Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films 17 (1999) 986. [9] T. Takeno, T. Takagi, A. Bozhko, M. Shupegin, T. Sato, Materials Science Forum 475–479 (2005) 2079. [10] A. Bozhko, T. Takagi, T. Takeno, M. Shupegin, Journal of Physics. Condensed Matter 16 (2004) 8447. [11] A. Bozhko, T. Takagi, T. Takeno, M. Shupegin, Japanese Journal of Applied Physics. Part 1, Regular Papers Short Notes and Review Papers 43 (2004) 7566. [12] Q.F. Huang, S.F. Yoon, H. Rusli, B. Yang, K. Gan, J. Chew, Journal of Applied Physics 88 (2000) 4191. [13] C. Strondl, N.M. Carvalho, J.T.M. De Hosson, G.J. van der Kolk, Surface & Coatings Technology 162 (2003) 288. [14] S. Prawer, K.W. Nugent, Y. Lifshitz, G.D. Lempert, E. Grossman, J. Kulik, I. Avigal, R. Kalish, Diamond and Related Materials 5 (1996) 433. [15] M.A. Tamor, W.C. Vassell, Journal of Applied Physics 76 (1994) 3823. [16] B. Abeles, P. Sheng, M.D. Coutts, Y. Arie, Advances in Physics 24 (1975) 407. [17] P. Sheng, B. Abeles, Y. Arie, Physical Review Letters 31 (1973) 44. [18] J.C. Fisher, I. Giaever, Journal of Applied Physics 32 (1961) 172. [19] R. Holm, Journal of Applied Physics 22 (1951) 569. [20] J.G. Simmons, Journal of Applied Physics 34 (1963) 1793. [21] P. Sheng, B. Abeles, Physical Review Letters 28 (1972) 34.
Electrically conductive properties of tungsten-containing diamond-like carbon films Takanori Takeno a , Hiroyuki Miki a , Toshiyuki Takagi a,⁎, Hideya Onodera b a b
Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, 980-8577, Japan Department of Physics, Graduate School of Science, Tohoku University, Sendai, 980-8578, Japan Available online 31 October 2006
Abstract Tungsten-containing diamond-like carbon films with different metal concentrations were investigated. The films of several hundred nanometers in thickness were deposited on the silicon wafer using RF-PECVD (radio frequency plasma enhanced chemical vapor deposition) method. During deposition, metal component was co-sputtered using DC magnetron of tungsten target. The six samples with the concentration of 3.8, 6.1, 8.0, 16.3, 24.3 and 41.4 at.% of tungsten were made. The structural analyses were performed by TEM (transmission electron microscope) and Raman spectroscopy. These results indicated that tungsten clusters were well dispersed in amorphous carbon host matrix in the case of tungsten concentration from 3.8 to 24.2 at.%. However, no such a structure can be observed in the sample with 41.4 at.%. The AC electrical resistance was measured in the temperature range of 2–300 K using four-probe method in vacuum condition. The observed temperature dependence of electrical conductivity can be expressed by r ¼ r0 expt−2ðC0 =kT Þ1=2 b and tungsten concentration from 3.8 at.% to 24.2 at.%. In addition, the sample with 41.4 at.% showed the resistive superconducting transition at Tc of around ∼5.5 K. © 2006 Elsevier B.V. All rights reserved. Keywords: Tungsten; Diamond-like carbon; Electrical conduction; Effective barrier height
1. Introduction Diamond-like carbon films are well-known materials and their properties such as hardness, surface morphology [1], friction coefficient [2] and electrical conduction properties [3] are strongly dependent on the volume fraction of carbon sp2 bond, sp3 bond, and hydrogen concentration. Various deposition techniques have been reported in order to make objective diamond-like carbon films [4]. Recently, elemental addition into diamond-like films is widely tried in order to get further functionality in the films. Both light and heavy elements can be used as a dopant, and chemical compounds are also used at times. In the case of metal element, the original reason to add metals to diamond-like carbon was to reduce the compressive stress [5]. Doped atoms form a cluster or wire like shape, which depends on the element used [6]. In a viewpoint of mechanical character of these films, these properties are affected by the doped metal and carbon concentration. For instance, in the case of tungsten-containing ⁎ Corresponding author. Tel./fax: +81 22 217 5248. E-mail address: [email protected] (T. Takagi). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.06.020
diamond-like carbon films, the hardness and friction coefficient of the films depends on the tungsten and carbon concentration [7,8]. The variation of the hardness and friction coefficient in these films result from crystalline phase consisted of carbon and tungsten. When we focus on electrical conduction properties of metaldoped diamond-like carbon films, electron transport phenomena can be also governed by the film structure and additional elements. The electron transport properties of metal-containing diamond-like carbon–silicon nanocomposite films were previously discussed in terms of the model of inelastic tunneling of electron in amorphous carbon–silicon dielectrics [9–11]. The electrically conductive properties of them were well affected by the metal concentration because the structural modification of them appeared as increase in metal concentration. In the case of molybdenum-containing films, electrically conductive properties depends on the size of the molybdenum and distances between molybdenum clusters [12]. In this report we choose the tungsten as a doping element. The structure consisted of tungsten and carbon can be changed by the concentration of tungsten, which indicates that tungsten carbide phases come to appear in the high metal concentration regime [13]. This paper
T. Takeno et al. / Diamond & Related Materials 15 (2006) 1902–1905
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deposited on the investigated films. The silver paste was used for the contacts. This experiment was performed in vacuum condition. 3. Results and discussion 3.1. Structural analyses
Fig. 1. Plan-view image of tungsten-containing diamond-like carbon films with tungsten concentration of 24.2 at.%. Insert of this figure shows the electron diffraction pattern.
presents electrical conduction properties of tungsten-containing diamond-like carbon films with various tungsten concentrations. The structural modification and electrically conductive properties are investigated. Transmission electron microscope and Raman spectroscopy were performed for the direct observation of the fine structure and carbon bonding structure of them. Temperature dependence of resistivity was measured down to liquid helium temperature regime. Compared to the results obtained by structural analyses and temperature dependence, electrically conductive properties are discussed. 2. Experimental Tungsten-containing diamond-like carbon films with several hundred nanometers in thickness was deposited by the conventional radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique with DC magnetron sputtering of tungsten target. The silicon substrates were cleaned by argon plasma prior to the deposition. The methane and the argon gases were used for deposition. The tungsten concentrations of the films were varied by changing sputtering power from 60 W to 100 W. The working pressure and the substrate bias voltage were fixed at 2.5 · 10− 2 Torr and − 400 V, respectively. The substrate was cooled by water during the deposition process. The concentrations of the film were measured by the electron probe micro analyzer (EPMA). The structural analyses were carried out by the transmission electron microscope (TEM) and the Raman spectroscopy. The plan-view image and diffraction pattern were obtained by 300 kV acceleration. The Raman spectra were taken using a JobinYvon micro-Raman spectroscope with He–Ne laser excitation (λ = 632.8 nm). The input power was reduced from initial to 2 mW by the filter in order not to damage the films. This experiment was performed at room temperature and ambient air condition. The temperature dependence of resistivity was measured by a four-probe method using Quantum Design Physical Properties Measurement System. The gold layer with 20 nm thickness was
From the result of EPMA, tungsten concentration of the investigated films was estimated as 3.8 at.%, 6.1 at.%, 8.0 at.%, 16.3 at.%, 24.3 at.% and 41.4 at.%, respectively. Fig. 1 shows that plan-view image of the sample with 24.3 at.%. Insert of Fig. 1 shows the diffraction pattern. Tungsten clusters, which sizes were about 4 nm, were formed and well dispersed in carbon matrix. In addition, clear diffraction pattern indicated that these clusters were polycrystalline. However, for the sample with 41.4 at.% concentration of tungsten, structural image and diffraction pattern were different from the one with 24.3 at.%. One of the reasons of these results is the formation of tungsten carbide phase. At the low tungsten concentration regions no structural modifications were observed. But with a further increase in tungsten concentration, tungsten carbide phase comes to exist. Although the critical concentration for the transition to tungsten carbide phase is still not determined, both two samples can be considered to be below and above this concentration. To make clear the intermediate carbon bonding state, Raman spectroscopy was carried out. These results were shown in Fig. 2. In parallel, a spectrum of non-doped diamond-like carbon film that was deposited on silicon substrate under the same bias voltage was shown in Fig. 2 as a reference. The broad-shape peaks were observed in the sample with the concentration up to 24.3 at.%. As in the non-doped film, diamond-like carbon structure also existed in tungsten-containing films. But it was not observed in the case of the sample with 41.4 at.%.
Fig. 2. Raman spectra of tungsten-containing diamond-like carbon films with various tungsten concentration. A spectrum of non-doped diamond-like carbon film is also shown as a reference.
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T. Takeno et al. / Diamond & Related Materials 15 (2006) 1902–1905
Fig. 3. The calculated results of Q and I(D)/I(G) ratio obtained by profile-fitting are plotted against the tungsten concentration.
The obtained Raman spectra were explained as the combination of two spectra; the D peak (“disordered peak”) located around 1350 cm− 1 and the G peak (graphitic) located around 1550 cm− 1. The D peak and the G peak represent the breathing vibration mode of carbon aromatic rings and the stretching vibration mode of carbon pairs, respectively [4]. In this study, Lorenz function and Breit–Wigner–Fano (BWF) function [14] were used for the analysis of the D peak and the G peak. Also, the linear function was for background luminescence. There were so many parameters that need to be considered. But we focused on the two parameters for quantitative comparison. One of them is the intensity ratio of peak maximum at D peak and G peak, I(D)/I(G). This peak intensity ratio is correlated to the volume fraction of sp3 carbon bonding [4,15]. The volume fraction of sp3 carbon decrease as increase in peak intensity ratio [4]. Another is coupling coefficient, Q. Prawer et al. made the samples with various sp2 concentration and inves-
Fig. 5. The obtained results of parameter, 2(C0 / k)1 / 2, is plotted against the tungsten concentration.
tigated the relation between the coupling coefficient and volume fraction of sp2 carbon bonding [14]. When a drastic decrease in coupling coefficient as the volume fraction decreases was observed, the decrease in coupling coefficient means decrease in sp2 volume fraction [14]. I(D)/I(G) peak intensity ratio and coupling coefficient, Q, were plotted against the tungsten concentration were shown in Fig. 3. The result in the sample with the concentration of 41.4 at. % was excluded because broaden peak was not as seen in Fig. 2. Both values of fitting parameter increased even in the small amount of tungsten in the film, which generally indicates increase in volume fraction of sp2 and decrease in sp3. Although the values of I(D)/I(G) peak intensity ratio in tungsten-containing films are constant against the tungsten concentrations, the value in non-doped film is smaller. This tendency suggests that the carbon matrix cannot be affected by the tungsten concentration in the range from 3.8 at.% to 24.3 at.%. 3.2. Electrical conduction properties Firstly, we excluded the result on the sample with the concentration of 41.4 at.%. In this sample, superconducting transition was observed at 5.5 K. The structure and electrical conduction properties were different from other samples. In this paper we focus on the electrical conduction properties in granular structure. The structure of granular materials is that well crystallized metal grains are dispersed in the insulating host matrix. Electrical conduction appears as a result from the transport phenomena of electrons and holes by charged metal grains to neutral grains [16]. In order that this process occurs, an energy Ec is required [17]. This energy is expressed as Ec = (e2 /d)F(s / d), where e is the electronic charge, d is the cluster size, s is the separation between the grains, and F is a function that depends on shape and arrangement of the clusters. According to Abele et al. [16], temperature dependence of conductivity was calculated as rffiffiffiffiffiffi C0 r ¼ r0 exp 2 ð1Þ kT
½
Fig. 4. Temperature dependence of resistivity in tungsten-containing diamondlike carbon films with various tungsten concentration. The profile-fitting results for all experimental results are also shown in this figure.
here, σ0 and C0 are constant values and k is Bolzman constant. The value C0 is consisted of effective barrier height, distance
T. Takeno et al. / Diamond & Related Materials 15 (2006) 1902–1905
between the grains and Ec as mentioned above [12], and σ0 is the constant of temperature independent. Since the structure of the investigated sample is granular in the concentration up to 24.3 at.%, temperature dependence is expected to be followed by T−1/2. In Fig. 4 the conductivity was plotted against T− 1 / 2. Taken into account the above discussion, electrons transport from a tungsten cluster to the nearest tungsten cluster through carbon medium by tunneling. In the case of tunneling effect, the potential barrier between the tungsten clusters plays an important role for electrical conduction. The various shape and height of the barrier were proposed and well studied up to now [18,19]. Since the shape and height of the potential barrier depend on host matrix, various potential barriers can be considered. Simmons established the generalized potential barrier whose shape and height were represented by Φ as a function of the distance from the nearest cluster, x [20]. The form of Φ(x) is arbitrary. Although Φ(x) depends on each tunnel junction in inhomogeneous host matrix, a barrier height determined by experimental results represent the average barrier height of all tunnel junctions. This average is defined as effective barrier height, ΦD. Using assumption reported previously [12,17,21], effective barrier height can be calculated from the equation given by; " #1=2 rffiffiffiffiffiffi C0 1 ð2mUD Þ1=2 sEc 2 ð2Þ ¼2 k h=2k k here, m is mass of electron, h is Planck constant. The calculated value, 2(C0 / k)1 / 2, was given by numerically from the results shown in Fig. 4. The results were plotted against the tungsten concentration was shown in Fig. 5. As increase in tungsten concentration in the film, 2(C0 / k)1 / 2 decreases. In Eq. (4), variable is only ΦD because of the assumption [17,21]. Since carbon medium between the clusters is concentration independent, sizes of clusters and separation between them are one of the important parameters for the estimation of ΦD. Generally speaking, if we assume the simple one-dimensional structure like (tungsten cluster)–(insulating carbon medium)– (tungsten cluster), the separation between the clusters and the cluster sizes increase as tungsten concentration increases, which indicates the growth of effective barrier height. But our results are opposite. As a qualitative explanation, the existence of sp2 carbon cluster can be considered. Because of the geometrical reason in separation between the clusters with the increase of tungsten concentration, sp2 carbon cluster whose energy level is lower than insulating diamond-like carbon matrix can exist between the tungsten clusters, which lower the effective barrier between the tungsten clusters.
4. Summary In this paper, electrical conduction properties of tungstencontaining diamond-like carbon films were investigated. Films with various tungsten concentrations were deposited on silicon
1905
substrates by RF-PECVD and co-sputtering of tungsten metal target. In the case of the sample with the concentration up to 24.3 at.%, structure of the films is classified into granular type. Taken into account that cluster sizes and separation between the tungsten clusters increase as concentration increased, lowering of effective barrier height is due to the geometrical effects. In addition, superconducting transition that temperature is ∼ 5.5 K was observed for the sample with the concentration of 41.4 at.%. Acknowledgements This work was partly supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (No. 16-03420). Authors are greatly thankful to Mr. Takeshi Sato for his technical assistance in Institute of Fluid Science in Tohoku University, and to Mr. Takashi Kamaya and Yuko Sato for EPMA measurement in Nanotechnical laboratory in Institute of Multidisciplinary Research for Advanced Materials in Tohoku University. References [1] X.L. Peng, Z.H. Barber, T.W. Clyne, Surface & Coatings Technology 138 (2001) 23. [2] J. Fontaine, T. Le Mogne, J.L. Loubet, M. Belin, Thin Solid Films 482 (2005) 99. [3] K. Takai, M. Oga, H. Sato, T. Enoki, Y. Ohki, A. Taomoto, K. Suenaga, S. Iijima, Physical Review B 67 (2003) 214202. [4] J. Robertson, Materials Science & Engineering. R, Reports 37 (2002) 129. [5] H. Dimigen, C.P. Klages, Surface & Coatings Technology 49 (1991) 543. [6] K.I. Schiffmann, M. Fryda, G. Goerigk, R. Lauer, P. Hinze, A. Bulack, Thin Solid Films 347 (1999) 60. [7] A.A. Voevodin, J.P. O'Neill, J.S. Zabinski, Thin Solid Films 342 (1999) 194. [8] A.A. Voevodin, J.P. O'Neill, S.V. Prasad, J.S. Zabinski, Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films 17 (1999) 986. [9] T. Takeno, T. Takagi, A. Bozhko, M. Shupegin, T. Sato, Materials Science Forum 475–479 (2005) 2079. [10] A. Bozhko, T. Takagi, T. Takeno, M. Shupegin, Journal of Physics. Condensed Matter 16 (2004) 8447. [11] A. Bozhko, T. Takagi, T. Takeno, M. Shupegin, Japanese Journal of Applied Physics. Part 1, Regular Papers Short Notes and Review Papers 43 (2004) 7566. [12] Q.F. Huang, S.F. Yoon, H. Rusli, B. Yang, K. Gan, J. Chew, Journal of Applied Physics 88 (2000) 4191. [13] C. Strondl, N.M. Carvalho, J.T.M. De Hosson, G.J. van der Kolk, Surface & Coatings Technology 162 (2003) 288. [14] S. Prawer, K.W. Nugent, Y. Lifshitz, G.D. Lempert, E. Grossman, J. Kulik, I. Avigal, R. Kalish, Diamond and Related Materials 5 (1996) 433. [15] M.A. Tamor, W.C. Vassell, Journal of Applied Physics 76 (1994) 3823. [16] B. Abeles, P. Sheng, M.D. Coutts, Y. Arie, Advances in Physics 24 (1975) 407. [17] P. Sheng, B. Abeles, Y. Arie, Physical Review Letters 31 (1973) 44. [18] J.C. Fisher, I. Giaever, Journal of Applied Physics 32 (1961) 172. [19] R. Holm, Journal of Applied Physics 22 (1951) 569. [20] J.G. Simmons, Journal of Applied Physics 34 (1963) 1793. [21] P. Sheng, B. Abeles, Physical Review Letters 28 (1972) 34.