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Electronic transport in amorphous carbon nitride (a-CNx:H) and carbon oxide (a-COx:H) films
Solid State Communications 130 (2004) 331–334 www.elsevier.com/locate/ssc
Electronic transport in amorphous carbon nitride (a-CNx:H) and carbon oxide (a-COx:H) films Sushil Kumar1, C. Godet* Laboratoire de Physique des Interfaces et des Couches Minces, UMR 7647 CNRS—Ecole Polytechnique, 91128 Palaiseau Cedex, France Received 28 May 2003; received in revised form 31 December 2003; accepted 6 February 2004 by A.K. Sood
Abstract The dielectric properties of a-CNx:H and a-COx:H thin films have been investigated using ohmic sðTÞ and field-dependent sðFÞ electrical conductivity measurements. A lower density of electronic states in carbon oxides has been inferred from the LnðsÞ < T 21=4 behavior, consistent with a hopping mechanism. The dielectric constant 1 values deduced from the Poole– Frenkel field-dependence are found to decrease (increase) with increasing oxygen (nitrogen) content. Using refractive index values obtained from ellipsometry, a good agreement between 1 and n2 is found for the oxygen-rich (18 – 20%) alloys, having values of 1 , 1:9 – 2:3; which could be considered as a new low dielectric constant material. q 2004 Elsevier Ltd. All rights reserved. PACS: 72.20.Ee; 77.22.Ch; 73.61.Jc Keywords: A. Thin films; D. Dielectric response; D. Electronic transport; D. Electronic states (localized); D. Tunneling
The electronic properties of hydrogenated amorphous carbon (a-C:H) films are governed by the p and pp electronic states distributions which form the band edges near the pseudo-gap and which lie largely within the s – sp states. The high localization of p and pp states is related to the dihedral angle distribution [1]. In dense low band gap carbon films, a Poole – Frenkel type defect conduction mechanism has been observed at high electric fields [2,3] while hopping between sp2 clusters has been proposed as a possible mechanism for room temperature conduction [4]. Carbon films behavior varies between semiconductor and insulator depending upon the growth conditions and doping concentration. Various kinds of devices using (doped and undoped) a-C:H films have been realized, such as Schottky diodes [5], metal – insulator – semiconductor (MIS) diodes [6,7], heterojuction devices with silicon [8,9] and field emission devices [10,11]. Fluorinated carbon [12] and 1 Permanent address: Thin Film Technology Group, National Physical Laboratory, Dr K.S. Krishnan Road, New Delhi 110 012, India. Tel.: þ 91-1125742610; fax: þ91-1125726938 * Corresponding author. Fax: þ 33-1-69-33-30-06. E-mail address: [email protected] (C. Godet)
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.02.012
carbon nitride [13] films have also been studied as low-k material interesting for ULSI applications. The electronic properties of a-CNx:H films have been investigated for potential electronic applications, such as electron field emission devices [11] and electrodes inert to the electrochemical decomposition of water solvent [14]. In contrast, very few studies have been devoted to the properties of a-COx:H films [15,16], although oxygen etching has been frequently used to enhance the diamond growth. The aim of the present paper is to show a comparative study of electronic properties of ECR-deposited a-CNx:H and a-COx:H films, in particular the relationship between refractive index n (estimated from spectroscopic ellipsometry) and dielectric permittivity 1 (deduced from the Poole– Frenkel behavior of the field dependent conductivity) and to emphasize oxygen rich a-COx:H films as a new low-k material. The other properties, such as stoichiometry, hydrogen content, structure and optical absorption of our a-CNx:H and a-COx:H films are presented elsewhere [17,18]. The a-CNx:H and a-COx:H films have been deposited near room-temperature (no deliberate heating) on glass and p-type crystalline silicon substrates by using the
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S. Kumar, C. Godet / Solid State Communications 130 (2004) 331–334
decomposition of acetylene mixed with either nitrogen (for a-CNx:H) or oxygen (for a-COx:H) in a high density Integrated Distributed Electron Cyclotron Resonance (IDECR) plasma system [19]. As the flow rate ratio N2/ C2H2 or O2/C2H2 increases from 0 to 1, nitrogen (N/ N þ C) and oxygen (O/O þ C) content values estimated using nuclear reaction analysis vary from 0.025 to 0.22 in a-CNx:H and from 0.02 to 0.20 in a-COx:H films (Table 1). Electrical measurements were carried out both in transverse and coplanar structures. The current density has been measured in 0.1 cm gap-cell coplanar (Al/a-CNx:H or a-COx:H/Al) structures on glass substrates and sandwich (Al/a-CNx:H or a-COx:H/p-Si/Al) structures with 0.0314 cm2 contact area. In transverse structures, electric fields up to 5 £ 105 V cm21 have been applied, as deduced from the applied voltages V and the film thickness values d determined by ellipsometry. In coplanar structures, the lowfield (F , 103 V cm21) ohmic conductivity has been measured in the temperature range 250– 430 K. Fig. 1 shows the ohmic electrical conductivity as a function of temperature for a-CNx:H and a-COx:H films deposited with various gas mixtures. Opposite trends are observed in the magnitude of conductivity which decreases with increasing oxygen content in a-COx:H films (from 1 £ 1028 to 2 £ 10210 V21 cm21 with oxygen incorporation from 2 to 20 at.%) while nitrogen incorporation increases strongly the conductivity in a-CNx:H films. The linear dependence of logðsÞ vs. T 21=4 ; is an evidence for a hopping conduction mechanism in both a-CNx:H and a-COx:H films, which is described by Mott’s law: s ¼ s00 exp½2ðT0 =TÞ1=4 : In the ohmic regime, T0 is given by the expression kT0 NðEF Þg23 ¼ ½Constant; where the value of the constant depends upon the shape of density of states [20], k is the Boltzmann’s constant, g21 is the decay length of the wave
Fig. 1. Dependence of conductivity ðsÞ vs. T 21=4 for a-CNx:H and aCOx:H films.
function associated with the localized states and NðEF Þ is the density of states at the Fermi level. In this work, we have used the expression kT0 NðEF Þg23 ¼ Constant in order to estimate the variations in the value of the density of states for a-COx:H and a-CNx:H films. At this point, we have no independent estimate of NðEF Þ and g21 values, hence we can only assume that g21 is not strongly dependent on the alloy concentration. To support this assumption, a preliminary tendency has been derived from the electric field dependence of the apparent conductivity with a comparison of two samples (pure a-C:H and 20% N content a-CNx:H
Table 1 Gas flow ratio and stoichiometry for: (a) a-COx:H films and (b) a-CNx:H films. Typical error bars in refractive index values ðn ^ 0:03Þ and dielectric constant values ð1 ^ 0:1Þ have been estimated Sample no.
Gas flow (O2:C2H2) or (N2:C2H2)
Stoichiometry O/(O þ C) or (N/N þ C)
Refractive index
Dielectric constant
NCO13 NCO14 NCO15 NCO16 NCO17 NCO18 NCO19 NCO20 NCO21 NCO22 ACN03 ACN25 ACN50 ACN100
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.03 0.25 0.5 1.00
0.027 0.070 0.081 0.118 0.122 0.152 0.181 0.168 0.189 0.201 0.025 0.65 0.10 0.22
2.185 2.175 2.15 2.11 2.12 2.06 2.05 2.025 2.025 2.01 2.12 2.14 2.15 2.14
17.4 7.08 13.4 3.34 8.57 4.37 3.81 1.94 2.37 2.14 4.37 7.08 8.57 17.5
S. Kumar, C. Godet / Solid State Communications 130 (2004) 331–334
333
alloy); the results indicate a moderate decrease of about 15% of the localization radius g21 ; with increasing N content [21]. Fig. 2 shows that the prefactor s00 and slope T01=4 are systematically higher for a-COx:H films, as compared to a-CNx:H films. Higher values of T01=4 corresponding to lower density of states near the Fermi level [20], it is concluded that a-COx:H films are generally less defective than a-CNx:H films. Fig. 3 shows the current density ðJÞ vs. square root of the applied electric field ðF 1=2 Þ for reverse biased a-CNx:H/p-Si and a-COx:H/p-Si heterojuctions. The linear characteristics at high fields reveal either Poole –Frenkel or Schottky effects of the form: J ¼ J0 exp½eðbF 1=2 2 Vg Þ=kT where bPF ¼ ½e=p110 1=2 for Poole– Frenkel conduction and bS ¼ ½e=4p110 1=2 for a Schottky barrier emission [22], F is the electric field applied across the film, eVg is the depth of the trap level below the conduction energy level in the case of the Poole – Frenkel effect or the work function difference in the case of the Schottky effect. Since 1 values estimated assuming a Schottky behavior are unphysical (in some cases even less than one) Schottky behavior is ruled out in our films. Similarly, none of the J – V characteristics has been reasonably fitted with a Space-Charge Limited Current behaviour: ðJÞ < ðV 2 =d 3 Þ or ðJÞ < ðV Lþ1 =d 2Lþ1 Þ; or alternatively there is no field independent region of dðlogðJ=FÞÞ= dðlogFÞ: Hence, in a-CNx:H and a-COx:H films, the main transport mechanism observed at high fields is consistent with a Poole – Frenkel behavior. Fig. 4 shows the values of 1 extracted from the Poole – Frenkel slopes, as a function of n2 (at 2 eV photon energy)
obtained by spectroscopic ellipsometry for a-CNx:H and a-COx:H films. For a-COx:H, the values of 1 are rather linear with n2 ; but these values of 1 can be much higher than the values ðn2 ¼ 4 – 5Þ expected from the refractive index values. Very high values of 1 , 20 have also been reported for carbon nitride films grown using the layer by layer technique [13]. The dielectric permittivity is described as a sum of three
Fig. 2. Relationship between the parameters s00 and T01=4 in the ohmic regime.
Fig. 4. Dielectric permittivity 1 vs. square of the refractive index ðn2 Þ for a-CNx:H and a-COx:H films.
Fig. 3. Current density ðJÞ vs. square root of field ðFÞ1=2 characteristics of a-CNx:H/p-Si and a-COx:H/p-Si heterojuctions for reverse bias.
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S. Kumar, C. Godet / Solid State Communications 130 (2004) 331–334
contributions [23]: (i) a purely electronic high frequency component equal to n2 ; (ii) a ionic displacement component, rather unlikely in covalently bonded carbon films, and (iii) a low-frequency polarization component due to the electric field orientation of permanent dipoles attributed to bonds between atoms with different electronegativities (C – H, C– O, C– N bonds). The polarization component effect appears to be significant in most IDECR deposited carbonbased films, with the exception of oxygen-rich alloys which show a good agreement between 1 and n2 values. Finally, as (O/O þ C) increases, the observed decrease in the a-COx:H film density, refractive index and dielectric permittivity can be tentatively attributed to some structural modifications such as a reduction in the average coordination number, due to the replacement of CyC and C– H bonds by CyO and C– OH bonds which make the films less dense and more polymer like [24]. In conclusion, the simultaneous increase of resistivity (up to 5 £ 109 V cm), decrease of the defect density and decrease of dielectric permittivity (down to 1.9 – 2.3) with the increase of oxygen content in a-COx:H films, could make IDECR-deposited oxygen-rich (O/O þ C , 0.18 – 0.20) alloys useful as a low-stress interlayer insulator in ultra large scale integration (ULSI) devices. Thermal stability characterizations are currently performed.
Acknowledgements The authors wish to thank Dr G. Adamopoulos for the deposition of the films. One of the authors (SK) is grateful to the Department of Science and Technology (Government of India) for providing BOYSCAST fellowship during the present work.
References [1] C.W. Chen, J. Robertson, J. Non-Cryst. Solids 227–230 (1998) 602.
[2] H. Hofsa¨ss, Amorphous Carbon: State of the Art, World Scientific, Singapore, 1998, p. 296. [3] R.U.A. Khan, S.R.P. Silva, Int. J. Mod. Phys. B 14 (2000) 195. [4] D. Dasgupta, F. Demichelis, A. Tagliaferro, Philos. Mag. B 63 (1991) 1255. [5] G.A. Amaratunga, D.E. Segal, D.R. McKenzie, Appl. Phys. Lett. 59 (1991) 69. [6] N. Konofaos, C.B. Thomas, Appl. Phys. Lett. 61 (1992) 2895. [7] A.A. Khan, J.A. Woolam, Y. Chung, J. Appl. Phys. 55 (1984) 4299. [8] N. Konofaos, C.B. Thomas, J. Appl. Phys. 81 (1997) 6238. [9] N.A. Hastas, C.A. Dimitriadis, P. Patsalas, Y. Panayiotatos, D.H. Tassis, S. Logothetidis, J. Appl. Phys. 89 (2001) 2832. [10] G.A.J. Amaratunga, S.R.P. Silva, Appl. Phys. Lett. 68 (1996) 2529. [11] B.S. Satyanarayana, A. Hart, W.I. Milne, J. Robertson, Appl. Phys. Lett. 71 (1997) 1430. [12] A. Grill, in: S.R.P. Silva (Ed.), Properties of Amorphous Carbon, EMIS Datareviews Series no 29, INSPEC, London, 2003, p. 324. [13] M. Aono, S. Nitta, Diamond Relat. Mater. 11 (2002) 1219. [14] H. Cachet, C. Deslouis, M. Chouiki, B. Saidani, N. Conway, C. Godet, J. Electrochem. Soc. 149 (2002) E233. [15] N. Nakamura, T. Itani, H. Chiba, K. Watanabe, K. Kuhikara, Mater. Res. Soc. Symp. Proc. 593 (2000) 415. [16] F. Thiery, C. Valle´e, Y. Pauleau, F. Gaboriau, A. Lacoste, Y. Arnal, J. Pelletier, Surf. Coat. Technol. 151-152 (2002) 165. [17] C. Godet, N.M.J. Conway, J.E. Boure´e, K. Bouamra, A. Grosman, C. Ortega, J. Appl. Phys. 91 (2002) 4154. [18] G. Adamopoulos, C. Godet, B. Dre´villon, Y. Saito, D.N. Batchelder, A. Grosman, C. Ortega, Diamond Relat. Mater. 12 (2003) 983. [19] P. Bulkin, N. Bertrand, B. Dre´villon, Thin Solid Films 296 (1997) 66. [20] C. Godet, J. Non-Cryst. Solids 299-302 (2002) 333. [21] S. Kumar, C. Godet, A. Goudovskikh, J.P. Kleider, G. Adamopoulos, V. Chu, J. Non-Cryst. Solids (2004) in press. [22] M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981, p. 402. [23] C. Kittel, Introduction to Solid State Physics, third ed., Wiley, New York, 1966, p. 373. [24] G. Adamopoulos, S. Kumar, C. Godet, unpublished.
Electronic transport in amorphous carbon nitride (a-CNx:H) and carbon oxide (a-COx:H) films Sushil Kumar1, C. Godet* Laboratoire de Physique des Interfaces et des Couches Minces, UMR 7647 CNRS—Ecole Polytechnique, 91128 Palaiseau Cedex, France Received 28 May 2003; received in revised form 31 December 2003; accepted 6 February 2004 by A.K. Sood
Abstract The dielectric properties of a-CNx:H and a-COx:H thin films have been investigated using ohmic sðTÞ and field-dependent sðFÞ electrical conductivity measurements. A lower density of electronic states in carbon oxides has been inferred from the LnðsÞ < T 21=4 behavior, consistent with a hopping mechanism. The dielectric constant 1 values deduced from the Poole– Frenkel field-dependence are found to decrease (increase) with increasing oxygen (nitrogen) content. Using refractive index values obtained from ellipsometry, a good agreement between 1 and n2 is found for the oxygen-rich (18 – 20%) alloys, having values of 1 , 1:9 – 2:3; which could be considered as a new low dielectric constant material. q 2004 Elsevier Ltd. All rights reserved. PACS: 72.20.Ee; 77.22.Ch; 73.61.Jc Keywords: A. Thin films; D. Dielectric response; D. Electronic transport; D. Electronic states (localized); D. Tunneling
The electronic properties of hydrogenated amorphous carbon (a-C:H) films are governed by the p and pp electronic states distributions which form the band edges near the pseudo-gap and which lie largely within the s – sp states. The high localization of p and pp states is related to the dihedral angle distribution [1]. In dense low band gap carbon films, a Poole – Frenkel type defect conduction mechanism has been observed at high electric fields [2,3] while hopping between sp2 clusters has been proposed as a possible mechanism for room temperature conduction [4]. Carbon films behavior varies between semiconductor and insulator depending upon the growth conditions and doping concentration. Various kinds of devices using (doped and undoped) a-C:H films have been realized, such as Schottky diodes [5], metal – insulator – semiconductor (MIS) diodes [6,7], heterojuction devices with silicon [8,9] and field emission devices [10,11]. Fluorinated carbon [12] and 1 Permanent address: Thin Film Technology Group, National Physical Laboratory, Dr K.S. Krishnan Road, New Delhi 110 012, India. Tel.: þ 91-1125742610; fax: þ91-1125726938 * Corresponding author. Fax: þ 33-1-69-33-30-06. E-mail address: [email protected] (C. Godet)
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.02.012
carbon nitride [13] films have also been studied as low-k material interesting for ULSI applications. The electronic properties of a-CNx:H films have been investigated for potential electronic applications, such as electron field emission devices [11] and electrodes inert to the electrochemical decomposition of water solvent [14]. In contrast, very few studies have been devoted to the properties of a-COx:H films [15,16], although oxygen etching has been frequently used to enhance the diamond growth. The aim of the present paper is to show a comparative study of electronic properties of ECR-deposited a-CNx:H and a-COx:H films, in particular the relationship between refractive index n (estimated from spectroscopic ellipsometry) and dielectric permittivity 1 (deduced from the Poole– Frenkel behavior of the field dependent conductivity) and to emphasize oxygen rich a-COx:H films as a new low-k material. The other properties, such as stoichiometry, hydrogen content, structure and optical absorption of our a-CNx:H and a-COx:H films are presented elsewhere [17,18]. The a-CNx:H and a-COx:H films have been deposited near room-temperature (no deliberate heating) on glass and p-type crystalline silicon substrates by using the
332
S. Kumar, C. Godet / Solid State Communications 130 (2004) 331–334
decomposition of acetylene mixed with either nitrogen (for a-CNx:H) or oxygen (for a-COx:H) in a high density Integrated Distributed Electron Cyclotron Resonance (IDECR) plasma system [19]. As the flow rate ratio N2/ C2H2 or O2/C2H2 increases from 0 to 1, nitrogen (N/ N þ C) and oxygen (O/O þ C) content values estimated using nuclear reaction analysis vary from 0.025 to 0.22 in a-CNx:H and from 0.02 to 0.20 in a-COx:H films (Table 1). Electrical measurements were carried out both in transverse and coplanar structures. The current density has been measured in 0.1 cm gap-cell coplanar (Al/a-CNx:H or a-COx:H/Al) structures on glass substrates and sandwich (Al/a-CNx:H or a-COx:H/p-Si/Al) structures with 0.0314 cm2 contact area. In transverse structures, electric fields up to 5 £ 105 V cm21 have been applied, as deduced from the applied voltages V and the film thickness values d determined by ellipsometry. In coplanar structures, the lowfield (F , 103 V cm21) ohmic conductivity has been measured in the temperature range 250– 430 K. Fig. 1 shows the ohmic electrical conductivity as a function of temperature for a-CNx:H and a-COx:H films deposited with various gas mixtures. Opposite trends are observed in the magnitude of conductivity which decreases with increasing oxygen content in a-COx:H films (from 1 £ 1028 to 2 £ 10210 V21 cm21 with oxygen incorporation from 2 to 20 at.%) while nitrogen incorporation increases strongly the conductivity in a-CNx:H films. The linear dependence of logðsÞ vs. T 21=4 ; is an evidence for a hopping conduction mechanism in both a-CNx:H and a-COx:H films, which is described by Mott’s law: s ¼ s00 exp½2ðT0 =TÞ1=4 : In the ohmic regime, T0 is given by the expression kT0 NðEF Þg23 ¼ ½Constant; where the value of the constant depends upon the shape of density of states [20], k is the Boltzmann’s constant, g21 is the decay length of the wave
Fig. 1. Dependence of conductivity ðsÞ vs. T 21=4 for a-CNx:H and aCOx:H films.
function associated with the localized states and NðEF Þ is the density of states at the Fermi level. In this work, we have used the expression kT0 NðEF Þg23 ¼ Constant in order to estimate the variations in the value of the density of states for a-COx:H and a-CNx:H films. At this point, we have no independent estimate of NðEF Þ and g21 values, hence we can only assume that g21 is not strongly dependent on the alloy concentration. To support this assumption, a preliminary tendency has been derived from the electric field dependence of the apparent conductivity with a comparison of two samples (pure a-C:H and 20% N content a-CNx:H
Table 1 Gas flow ratio and stoichiometry for: (a) a-COx:H films and (b) a-CNx:H films. Typical error bars in refractive index values ðn ^ 0:03Þ and dielectric constant values ð1 ^ 0:1Þ have been estimated Sample no.
Gas flow (O2:C2H2) or (N2:C2H2)
Stoichiometry O/(O þ C) or (N/N þ C)
Refractive index
Dielectric constant
NCO13 NCO14 NCO15 NCO16 NCO17 NCO18 NCO19 NCO20 NCO21 NCO22 ACN03 ACN25 ACN50 ACN100
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.03 0.25 0.5 1.00
0.027 0.070 0.081 0.118 0.122 0.152 0.181 0.168 0.189 0.201 0.025 0.65 0.10 0.22
2.185 2.175 2.15 2.11 2.12 2.06 2.05 2.025 2.025 2.01 2.12 2.14 2.15 2.14
17.4 7.08 13.4 3.34 8.57 4.37 3.81 1.94 2.37 2.14 4.37 7.08 8.57 17.5
S. Kumar, C. Godet / Solid State Communications 130 (2004) 331–334
333
alloy); the results indicate a moderate decrease of about 15% of the localization radius g21 ; with increasing N content [21]. Fig. 2 shows that the prefactor s00 and slope T01=4 are systematically higher for a-COx:H films, as compared to a-CNx:H films. Higher values of T01=4 corresponding to lower density of states near the Fermi level [20], it is concluded that a-COx:H films are generally less defective than a-CNx:H films. Fig. 3 shows the current density ðJÞ vs. square root of the applied electric field ðF 1=2 Þ for reverse biased a-CNx:H/p-Si and a-COx:H/p-Si heterojuctions. The linear characteristics at high fields reveal either Poole –Frenkel or Schottky effects of the form: J ¼ J0 exp½eðbF 1=2 2 Vg Þ=kT where bPF ¼ ½e=p110 1=2 for Poole– Frenkel conduction and bS ¼ ½e=4p110 1=2 for a Schottky barrier emission [22], F is the electric field applied across the film, eVg is the depth of the trap level below the conduction energy level in the case of the Poole – Frenkel effect or the work function difference in the case of the Schottky effect. Since 1 values estimated assuming a Schottky behavior are unphysical (in some cases even less than one) Schottky behavior is ruled out in our films. Similarly, none of the J – V characteristics has been reasonably fitted with a Space-Charge Limited Current behaviour: ðJÞ < ðV 2 =d 3 Þ or ðJÞ < ðV Lþ1 =d 2Lþ1 Þ; or alternatively there is no field independent region of dðlogðJ=FÞÞ= dðlogFÞ: Hence, in a-CNx:H and a-COx:H films, the main transport mechanism observed at high fields is consistent with a Poole – Frenkel behavior. Fig. 4 shows the values of 1 extracted from the Poole – Frenkel slopes, as a function of n2 (at 2 eV photon energy)
obtained by spectroscopic ellipsometry for a-CNx:H and a-COx:H films. For a-COx:H, the values of 1 are rather linear with n2 ; but these values of 1 can be much higher than the values ðn2 ¼ 4 – 5Þ expected from the refractive index values. Very high values of 1 , 20 have also been reported for carbon nitride films grown using the layer by layer technique [13]. The dielectric permittivity is described as a sum of three
Fig. 2. Relationship between the parameters s00 and T01=4 in the ohmic regime.
Fig. 4. Dielectric permittivity 1 vs. square of the refractive index ðn2 Þ for a-CNx:H and a-COx:H films.
Fig. 3. Current density ðJÞ vs. square root of field ðFÞ1=2 characteristics of a-CNx:H/p-Si and a-COx:H/p-Si heterojuctions for reverse bias.
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S. Kumar, C. Godet / Solid State Communications 130 (2004) 331–334
contributions [23]: (i) a purely electronic high frequency component equal to n2 ; (ii) a ionic displacement component, rather unlikely in covalently bonded carbon films, and (iii) a low-frequency polarization component due to the electric field orientation of permanent dipoles attributed to bonds between atoms with different electronegativities (C – H, C– O, C– N bonds). The polarization component effect appears to be significant in most IDECR deposited carbonbased films, with the exception of oxygen-rich alloys which show a good agreement between 1 and n2 values. Finally, as (O/O þ C) increases, the observed decrease in the a-COx:H film density, refractive index and dielectric permittivity can be tentatively attributed to some structural modifications such as a reduction in the average coordination number, due to the replacement of CyC and C– H bonds by CyO and C– OH bonds which make the films less dense and more polymer like [24]. In conclusion, the simultaneous increase of resistivity (up to 5 £ 109 V cm), decrease of the defect density and decrease of dielectric permittivity (down to 1.9 – 2.3) with the increase of oxygen content in a-COx:H films, could make IDECR-deposited oxygen-rich (O/O þ C , 0.18 – 0.20) alloys useful as a low-stress interlayer insulator in ultra large scale integration (ULSI) devices. Thermal stability characterizations are currently performed.
Acknowledgements The authors wish to thank Dr G. Adamopoulos for the deposition of the films. One of the authors (SK) is grateful to the Department of Science and Technology (Government of India) for providing BOYSCAST fellowship during the present work.
References [1] C.W. Chen, J. Robertson, J. Non-Cryst. Solids 227–230 (1998) 602.
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