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Film stoichiometry and gas dissociation kinetics in hot-wire chemical vapor deposition of a-SiGe:H
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Thin Solid Films 516 (2008) 526 – 528 www.elsevier.com/locate/tsf
Film stoichiometry and gas dissociation kinetics in hot-wire chemical vapor deposition of a-SiGe:H James R. Doyle ⁎, Yueqin Xu, Robert Reedy, Howard M. Branz, A. Harv Mahan National Renewable Energy Laboratory, Golden, CO 80231, United States Available online 10 July 2007
Abstract The gas phase dissociation rates of silane and germane are measured for HWCVD on a tantalum filament and compared to the a-SiGe:H film composition. The Ge from dissociated germane is converted entirely into film on the substrate and chamber walls. Approximately 85% of the Si from the dissociated silane is converted into film, with the rest incorporated into the filament. The dissociation rate per unit partial pressure of germane is 2–3 times that of silane. The pressure dependence of feed gas depletion rates suggests that the dissociation on the filament is rate limited by filament reactive site availability. © 2007 Elsevier B.V. All rights reserved. Keywords: Hot-wire deposition; Amorphous silicon germanium alloys
Hot-wire chemical vapor deposition (HWCVD) of hydrogenated amorphous silicon (a-Si:H) for solar cell applications offers the potential of high deposition rate, low energy bombardment, and simple implementation compared to the more conventional plasma-enhanced (PECVD) approach. Many studies have been devoted to understanding the physics and chemistry of this method in order to further optimize film properties [1]. Deposition of a-Si1−xGex:H alloys by HWCVD has received less attention [2]. We have recently demonstrated device quality a-SiGe:H films using HWCVD, comparable, or perhaps better, in quality to PECVD material [3]. In seeking to further increase the deposition rate, as well as further optimize the material, some knowledge of the deposition kinetics is desirable. Here we present a study of the hot-wire dissociation of silane and germane on a tantalum filament using mass spectrometric measurements of feed gas depletion and compositional analysis of the films by SIMS under conditions used to produce the best quality a-SiGe:H films.
an adjustable leak valve. The details of the deposition system have been reported previously [4]. The RGA was set to measure m/e peaks 29, 30, and 31 for silane and 73 and 74 for germane. We verified that the measured signals were strictly proportional to the pressure measured on the capacitance manometer in the deposition chamber. Furthermore, the long pathway from the filament region to the mass spectrometer ensured that no radical species were sampled; this was confirmed by the observation that the relative peak heights of the silane and germane mass spectra did not differ between filament on and off conditions. To measure depletion during a film deposition, the filament was first turned on to the desired current. After the current stabilized the gases were introduced and the mass spectrometer signals were allowed to stabilize. Finally the current to the filament was abruptly turned off while the computer continued to monitor the mass peaks. Postdeposition film thickness was measured using optical modeling of reflection/transmission spectra in the non-absorbing region of the spectrum. Film stoichiometry was measured using secondary ion mass spectrometry (SIMS).
2. Experimental details
3. Results
The depletion measurements were carried out using a residual gas analyzer (RGA) connected to the deposition chamber through
The fractional depletion, f, of a feed gas is defined as f = (P0 − P)/P0 = (I0 − I)/I0, where P0 and P are the partial pressures of the feed gas with the filament off and on respectively, and I0 and I are the mass spectrometer signals with the filament off and on respectively. To relate f to the net gas dissociation rate, we use the equation expressing the conservation of gas molecules,
1. Introduction
⁎ Corresponding author. Permanent address: Department of Physics and Astronomy, Macalester College, St. Paul, MN, United States 55105. E-mail address: [email protected] (J.R. Doyle). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.189
J.R. Doyle et al. / Thin Solid Films 516 (2008) 526–528
Fin = Fout + RD, where Fin is the feed gas flow rate into the chamber, Fout is the flow rate out of the chamber, and RD is the feed gas dissociation rate. Writing Fin = P0SP and Fout = PSP where SP is the pumping speed, we have RD = fP0SP =fFin. Mass scans during deposition indicated that production of stable higher silanes (SinH2n+2), germanes (GenH2n+2), or silylgermanes (SinGemH2(n+m)+2) can be safely neglected under our conditions. If we assume that all radical species have sufficiently high sticking coefficients so that they would deposit before being pumped away, then by mass conservation all silicon and germanium atoms from the dissociated silane and germane must deposit either on the chamber walls (and substrate) or incorporate into the filament. In Fig. 1 we have plotted the germanium content x as measured by SIMS versus the relative dissociation rate ρ = fGeH4FGeH4 / ( fGeH4FGeH4 + fSiH4FSiH4) = GeH4 GeH4 SiH4 RD / (RD + RD ). If all of the dissociated germane and silane were incorporated into film, the data points should lie on the unity slope line x = ρ (solid line). The data points indicate a clear systematic trend to higher Ge content in the films, which means that the films are somewhat more Ge rich than expected from the relative gas dissociation rates. Furthermore, the film stoichiometry did not vary from point to point in the deposition chamber when we distributed substrates throughout the chamber. This suggests that the Ta filament preferentially absorbs Si over Ge. This hypothesis was verified using electron microprobe analysis of a used filament: we found that while the filament readily alloys silicon, the germanium content of the filament was below the detection limit. The dashed line in Fig. 1 shows the expected relation between the germanium fraction and the depletion data if it is assumed that 15% of the depleted silane is incorporated into the filament. The incorporation of silicon into Ta (and W) filaments is a well known effect in filament deposition, but to our knowledge the present result is the first demonstration that Ge does not alloy with the filament. We note that for a substrate deposition
Fig. 1. Germanium fraction x in the film as measured by SIMS versus the relative germane dissociation rate in the gas ρ. The solid line assumes that the x = ρ, and the dashed line assumes 15% of the depleted silane is incorporated into the filament. All measurements were made at Ts = 200 °C, total pressure 12.5 mT and a filament temperature of 1750 °C. Squares and circles correspond to filament temperatures of 1850 °C and 1750 °C respectively.
527
Fig. 2. Effective dissociation probability αeff of germane versus total pressure. Squares and circles are for filament temperatures of 1850 °C and 1750 °C respectively. The flow ratio was fixed at FGe/FSi = 0.82.
rate of 3.2 Å/s for a film having composition a-Si0.4Ge0.6:H, approximately 20 h is needed to completely convert our filament to TaSi2. This corresponds roughly to the lifetime of our filaments under these conditions, although it is likely that the filaments will fail well before being completely alloyed. On the other hand, filament lifetime is extended using periodic H2 treatment, presumably by etching poorly alloyed Si from the surface of the filament. The pressure dependence of the dissociation rate is of interest, since it can provide insight into fundamental kinetic processes that occur during gas dissociation and film growth. The dissociation rate can be written as RD = αeffbvNnA/4 where αeff is a phenomenological parameter equal to the probability of dissociation per collision with the filament, n is the gas density at the filament, bvN is the average kinetic speed of the gas, and A is the total surface area of the filament. In calculating bvN we used the temperature of the chamber walls (200 °C), since at these low pressure conditions it is unlikely that the gas is heated significantly by the filament [5]. In general αeff = αHW(1 + N),
Fig. 3. Effective dissociation probability αeff of silane versus total pressure. Squares and circles are for filament temperatures of 1850 °C and 1750 °C respectively. The flow ratio was fixed at FGe/FSi = 0.82.
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J.R. Doyle et al. / Thin Solid Films 516 (2008) 526–528
where αHW represents the dissociation probability for molecules incident on the filament itself, and N is the number of secondary gas phase depletion reactions per hot surface dissociation between the filament dissociation products and the feed gases that further deplete the feed gas (e.g. Si + SiH4) [6]. In Figs. 2 and 3 we show αeff for germane and silane respectively as functions of pressure, for an FGe/FSi = 0.82 mixture (which gives approximately x = 0.60). The pressure was changed by varying the flow rate at a fixed pumping speed, and all data was taken with Ta filaments that had previously been exposed to silane so that the filament surface was alloyed with silicon. We first compare the two figures and find that the ratio of αeff for germane to silane is about 2–3 under our typical film growth conditions (P = 12 mTorr). At the lowest pressures where gas phase reactions can be neglected, N = 0 and αeff = αHW and we estimate αHWsilane ≈ 0.25 and 0.45, and αHWgermane ≈ 0.65 and 0.85 at our filament temperatures of 1750 °C and 1850 °C, respectively. The results for αHWsilane are in good agreement with previous estimates in this temperature range [7]. To our knowledge this is the first measurement of αHWgermane. In this range of temperature we see that the probability of germane dissociation on the filament is about twice that of silane, and at 1850 °C the germane filament dissociation probability approaches its limiting value of unity.
decrease in αeff with pressure is unlikely to be due to a gas phase kinetic process. An alternative explanation is that the observed pressure dependence is due to a change in filament dissociation probability αHW with pressure. In this model the dissociation becomes rate-limited by the availability of reactive surface sites on the filament as the pressure is increased. Such behavior, known as Langmuir–Hinshelwood reaction kinetics, is often observed in heterogeneous catalysis [8]. The relative constancy of αeff for germane implies that this molecule is less selective with regard to reactive sites, consistent with its lower thermodynamic stability and higher dissociation probability. However, secondary depletion reactions are expected to be important at our higher pressures [6] and we expect secondary depletion reactions to increase αeff with pressure. The observed constancy of αeff with pressure for germane is most likely due to the cancellation of site selectivity effects with increasing gas phase reactions and may actually imply some wire site selectivity for germane dissociation. Finally we note that Langmuir–Hinshelwood reaction kinetics imply that the filament dissociation of silane and germane depends on the details of chemical interactions between the feed gas and the hot surface, as opposed to a simple (single step) pyrolytic mechanism. Chemically specific interactions are also supported by studies that demonstrate different dissociation activation energies for different wire materials [9].
4. Discussion Acknowledgements A surprising result from Figs. 2 and 3 is the strong decrease of αeff as a function of pressure for silane. The observed decrease for germane is weaker and occurs only at the lowest pressures. At higher pressures secondary reactions should result in an increase of αeff with pressure (due to an increase in N) and thus cannot account for our result. Filament cooling by the gas at higher pressure is likely ruled out by the observation that we observe no resistance change in the filament upon the introduction of the feed gas to a hot filament. Gas heating in the vicinity of the filament would decrease the rate of arrival of the feed gas to the filament. However, Holt et al. [5] have shown using Direct Simulation Monte Carlo calculations that under these low pressure conditions the gas is not significantly heated by the filament. In addition, we found that the addition of argon gas to 1 mTorr of silane up to 25 mTorr total pressure had no effect on αeff for silane. A complete model for αeff will require knowledge of the currently unknown gas phase reaction rate constants for Si + GeH4, Ge + SiH4, and Ge + GeH4 (assuming that Si and Ge are the principal silicon and germanium products of hot filament dissociation). However, although αeff is a phenomenological parameter representing a potentially complex series of elementary reactions, the discussion above implies that the observed
We acknowledge Bobby To for the electron microprobe measurements. J.R.D. was supported in part by the National Science Foundation grant DMR-0513775. NREL authors were supported by the U.S. DOE under Contract DE-AC36-99GO10337. References [1] A.H. Mahan, Sol. Energy Mater. Sol. Cells 78 (2003) 299. [2] Yueqin Xu, A.H. Mahan, L.M. Gedvilas, R.C. Reedy, H.M. Branz, Thin Solid Films 501 (2006) 198. [3] S. Datta, J.D. Cohen, Yueqin Xu, A.H. Mahan, Amorphous and Nanocrystalline Science and Technology, San Francisco, CA, March 28–April 3, Materials Research Society Symposium Proceedings, vol. 862, 2005, p. 49. [4] Y. Xu, B.P. Nelson, L.M. Gedvilas, R.C. Reedy, Thin Solid Films 430 (2003) 197. [5] J.K. Holt, M. Swiatek, D.G. Goodwin, R.P. Muller, W.A. Goddard III, H.A. Atwater, Thin Solid Films 395 (2001) 29. [6] W. Zheng, A. Gallagher, Thin Solid Films 501 (2006) 21. [7] J. Doyle, R. Robertson, G.H. Lin, M. He, A. Gallagher, J. Appl. Phys. 64 (1988) 3215. [8] A.W. Adamson, The Physical Chemistry of Surfaces, 2nd edition, Interscience Publishers, New York NY, 1967. [9] H.L. Duan, S.F. Bent, Thin Solid Films 485 (2005) 126.
Thin Solid Films 516 (2008) 526 – 528 www.elsevier.com/locate/tsf
Film stoichiometry and gas dissociation kinetics in hot-wire chemical vapor deposition of a-SiGe:H James R. Doyle ⁎, Yueqin Xu, Robert Reedy, Howard M. Branz, A. Harv Mahan National Renewable Energy Laboratory, Golden, CO 80231, United States Available online 10 July 2007
Abstract The gas phase dissociation rates of silane and germane are measured for HWCVD on a tantalum filament and compared to the a-SiGe:H film composition. The Ge from dissociated germane is converted entirely into film on the substrate and chamber walls. Approximately 85% of the Si from the dissociated silane is converted into film, with the rest incorporated into the filament. The dissociation rate per unit partial pressure of germane is 2–3 times that of silane. The pressure dependence of feed gas depletion rates suggests that the dissociation on the filament is rate limited by filament reactive site availability. © 2007 Elsevier B.V. All rights reserved. Keywords: Hot-wire deposition; Amorphous silicon germanium alloys
Hot-wire chemical vapor deposition (HWCVD) of hydrogenated amorphous silicon (a-Si:H) for solar cell applications offers the potential of high deposition rate, low energy bombardment, and simple implementation compared to the more conventional plasma-enhanced (PECVD) approach. Many studies have been devoted to understanding the physics and chemistry of this method in order to further optimize film properties [1]. Deposition of a-Si1−xGex:H alloys by HWCVD has received less attention [2]. We have recently demonstrated device quality a-SiGe:H films using HWCVD, comparable, or perhaps better, in quality to PECVD material [3]. In seeking to further increase the deposition rate, as well as further optimize the material, some knowledge of the deposition kinetics is desirable. Here we present a study of the hot-wire dissociation of silane and germane on a tantalum filament using mass spectrometric measurements of feed gas depletion and compositional analysis of the films by SIMS under conditions used to produce the best quality a-SiGe:H films.
an adjustable leak valve. The details of the deposition system have been reported previously [4]. The RGA was set to measure m/e peaks 29, 30, and 31 for silane and 73 and 74 for germane. We verified that the measured signals were strictly proportional to the pressure measured on the capacitance manometer in the deposition chamber. Furthermore, the long pathway from the filament region to the mass spectrometer ensured that no radical species were sampled; this was confirmed by the observation that the relative peak heights of the silane and germane mass spectra did not differ between filament on and off conditions. To measure depletion during a film deposition, the filament was first turned on to the desired current. After the current stabilized the gases were introduced and the mass spectrometer signals were allowed to stabilize. Finally the current to the filament was abruptly turned off while the computer continued to monitor the mass peaks. Postdeposition film thickness was measured using optical modeling of reflection/transmission spectra in the non-absorbing region of the spectrum. Film stoichiometry was measured using secondary ion mass spectrometry (SIMS).
2. Experimental details
3. Results
The depletion measurements were carried out using a residual gas analyzer (RGA) connected to the deposition chamber through
The fractional depletion, f, of a feed gas is defined as f = (P0 − P)/P0 = (I0 − I)/I0, where P0 and P are the partial pressures of the feed gas with the filament off and on respectively, and I0 and I are the mass spectrometer signals with the filament off and on respectively. To relate f to the net gas dissociation rate, we use the equation expressing the conservation of gas molecules,
1. Introduction
⁎ Corresponding author. Permanent address: Department of Physics and Astronomy, Macalester College, St. Paul, MN, United States 55105. E-mail address: [email protected] (J.R. Doyle). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.189
J.R. Doyle et al. / Thin Solid Films 516 (2008) 526–528
Fin = Fout + RD, where Fin is the feed gas flow rate into the chamber, Fout is the flow rate out of the chamber, and RD is the feed gas dissociation rate. Writing Fin = P0SP and Fout = PSP where SP is the pumping speed, we have RD = fP0SP =fFin. Mass scans during deposition indicated that production of stable higher silanes (SinH2n+2), germanes (GenH2n+2), or silylgermanes (SinGemH2(n+m)+2) can be safely neglected under our conditions. If we assume that all radical species have sufficiently high sticking coefficients so that they would deposit before being pumped away, then by mass conservation all silicon and germanium atoms from the dissociated silane and germane must deposit either on the chamber walls (and substrate) or incorporate into the filament. In Fig. 1 we have plotted the germanium content x as measured by SIMS versus the relative dissociation rate ρ = fGeH4FGeH4 / ( fGeH4FGeH4 + fSiH4FSiH4) = GeH4 GeH4 SiH4 RD / (RD + RD ). If all of the dissociated germane and silane were incorporated into film, the data points should lie on the unity slope line x = ρ (solid line). The data points indicate a clear systematic trend to higher Ge content in the films, which means that the films are somewhat more Ge rich than expected from the relative gas dissociation rates. Furthermore, the film stoichiometry did not vary from point to point in the deposition chamber when we distributed substrates throughout the chamber. This suggests that the Ta filament preferentially absorbs Si over Ge. This hypothesis was verified using electron microprobe analysis of a used filament: we found that while the filament readily alloys silicon, the germanium content of the filament was below the detection limit. The dashed line in Fig. 1 shows the expected relation between the germanium fraction and the depletion data if it is assumed that 15% of the depleted silane is incorporated into the filament. The incorporation of silicon into Ta (and W) filaments is a well known effect in filament deposition, but to our knowledge the present result is the first demonstration that Ge does not alloy with the filament. We note that for a substrate deposition
Fig. 1. Germanium fraction x in the film as measured by SIMS versus the relative germane dissociation rate in the gas ρ. The solid line assumes that the x = ρ, and the dashed line assumes 15% of the depleted silane is incorporated into the filament. All measurements were made at Ts = 200 °C, total pressure 12.5 mT and a filament temperature of 1750 °C. Squares and circles correspond to filament temperatures of 1850 °C and 1750 °C respectively.
527
Fig. 2. Effective dissociation probability αeff of germane versus total pressure. Squares and circles are for filament temperatures of 1850 °C and 1750 °C respectively. The flow ratio was fixed at FGe/FSi = 0.82.
rate of 3.2 Å/s for a film having composition a-Si0.4Ge0.6:H, approximately 20 h is needed to completely convert our filament to TaSi2. This corresponds roughly to the lifetime of our filaments under these conditions, although it is likely that the filaments will fail well before being completely alloyed. On the other hand, filament lifetime is extended using periodic H2 treatment, presumably by etching poorly alloyed Si from the surface of the filament. The pressure dependence of the dissociation rate is of interest, since it can provide insight into fundamental kinetic processes that occur during gas dissociation and film growth. The dissociation rate can be written as RD = αeffbvNnA/4 where αeff is a phenomenological parameter equal to the probability of dissociation per collision with the filament, n is the gas density at the filament, bvN is the average kinetic speed of the gas, and A is the total surface area of the filament. In calculating bvN we used the temperature of the chamber walls (200 °C), since at these low pressure conditions it is unlikely that the gas is heated significantly by the filament [5]. In general αeff = αHW(1 + N),
Fig. 3. Effective dissociation probability αeff of silane versus total pressure. Squares and circles are for filament temperatures of 1850 °C and 1750 °C respectively. The flow ratio was fixed at FGe/FSi = 0.82.
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J.R. Doyle et al. / Thin Solid Films 516 (2008) 526–528
where αHW represents the dissociation probability for molecules incident on the filament itself, and N is the number of secondary gas phase depletion reactions per hot surface dissociation between the filament dissociation products and the feed gases that further deplete the feed gas (e.g. Si + SiH4) [6]. In Figs. 2 and 3 we show αeff for germane and silane respectively as functions of pressure, for an FGe/FSi = 0.82 mixture (which gives approximately x = 0.60). The pressure was changed by varying the flow rate at a fixed pumping speed, and all data was taken with Ta filaments that had previously been exposed to silane so that the filament surface was alloyed with silicon. We first compare the two figures and find that the ratio of αeff for germane to silane is about 2–3 under our typical film growth conditions (P = 12 mTorr). At the lowest pressures where gas phase reactions can be neglected, N = 0 and αeff = αHW and we estimate αHWsilane ≈ 0.25 and 0.45, and αHWgermane ≈ 0.65 and 0.85 at our filament temperatures of 1750 °C and 1850 °C, respectively. The results for αHWsilane are in good agreement with previous estimates in this temperature range [7]. To our knowledge this is the first measurement of αHWgermane. In this range of temperature we see that the probability of germane dissociation on the filament is about twice that of silane, and at 1850 °C the germane filament dissociation probability approaches its limiting value of unity.
decrease in αeff with pressure is unlikely to be due to a gas phase kinetic process. An alternative explanation is that the observed pressure dependence is due to a change in filament dissociation probability αHW with pressure. In this model the dissociation becomes rate-limited by the availability of reactive surface sites on the filament as the pressure is increased. Such behavior, known as Langmuir–Hinshelwood reaction kinetics, is often observed in heterogeneous catalysis [8]. The relative constancy of αeff for germane implies that this molecule is less selective with regard to reactive sites, consistent with its lower thermodynamic stability and higher dissociation probability. However, secondary depletion reactions are expected to be important at our higher pressures [6] and we expect secondary depletion reactions to increase αeff with pressure. The observed constancy of αeff with pressure for germane is most likely due to the cancellation of site selectivity effects with increasing gas phase reactions and may actually imply some wire site selectivity for germane dissociation. Finally we note that Langmuir–Hinshelwood reaction kinetics imply that the filament dissociation of silane and germane depends on the details of chemical interactions between the feed gas and the hot surface, as opposed to a simple (single step) pyrolytic mechanism. Chemically specific interactions are also supported by studies that demonstrate different dissociation activation energies for different wire materials [9].
4. Discussion Acknowledgements A surprising result from Figs. 2 and 3 is the strong decrease of αeff as a function of pressure for silane. The observed decrease for germane is weaker and occurs only at the lowest pressures. At higher pressures secondary reactions should result in an increase of αeff with pressure (due to an increase in N) and thus cannot account for our result. Filament cooling by the gas at higher pressure is likely ruled out by the observation that we observe no resistance change in the filament upon the introduction of the feed gas to a hot filament. Gas heating in the vicinity of the filament would decrease the rate of arrival of the feed gas to the filament. However, Holt et al. [5] have shown using Direct Simulation Monte Carlo calculations that under these low pressure conditions the gas is not significantly heated by the filament. In addition, we found that the addition of argon gas to 1 mTorr of silane up to 25 mTorr total pressure had no effect on αeff for silane. A complete model for αeff will require knowledge of the currently unknown gas phase reaction rate constants for Si + GeH4, Ge + SiH4, and Ge + GeH4 (assuming that Si and Ge are the principal silicon and germanium products of hot filament dissociation). However, although αeff is a phenomenological parameter representing a potentially complex series of elementary reactions, the discussion above implies that the observed
We acknowledge Bobby To for the electron microprobe measurements. J.R.D. was supported in part by the National Science Foundation grant DMR-0513775. NREL authors were supported by the U.S. DOE under Contract DE-AC36-99GO10337. References [1] A.H. Mahan, Sol. Energy Mater. Sol. Cells 78 (2003) 299. [2] Yueqin Xu, A.H. Mahan, L.M. Gedvilas, R.C. Reedy, H.M. Branz, Thin Solid Films 501 (2006) 198. [3] S. Datta, J.D. Cohen, Yueqin Xu, A.H. Mahan, Amorphous and Nanocrystalline Science and Technology, San Francisco, CA, March 28–April 3, Materials Research Society Symposium Proceedings, vol. 862, 2005, p. 49. [4] Y. Xu, B.P. Nelson, L.M. Gedvilas, R.C. Reedy, Thin Solid Films 430 (2003) 197. [5] J.K. Holt, M. Swiatek, D.G. Goodwin, R.P. Muller, W.A. Goddard III, H.A. Atwater, Thin Solid Films 395 (2001) 29. [6] W. Zheng, A. Gallagher, Thin Solid Films 501 (2006) 21. [7] J. Doyle, R. Robertson, G.H. Lin, M. He, A. Gallagher, J. Appl. Phys. 64 (1988) 3215. [8] A.W. Adamson, The Physical Chemistry of Surfaces, 2nd edition, Interscience Publishers, New York NY, 1967. [9] H.L. Duan, S.F. Bent, Thin Solid Films 485 (2005) 126.