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The effect of disturbed PECVD electrode surfaces on the homogeneity of microcrystalline silicon films
Surface & Coatings Technology 205 (2011) S415–S418
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
The effect of disturbed PECVD electrode surfaces on the homogeneity of microcrystalline silicon films S. Muthmann ⁎, M. Meier, R. Schmitz, W. Appenzeller, A. Mück, A. Gordijn IEF5-Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany
a r t i c l e
i n f o
Available online 18 February 2011 Keywords: Solar cells Microcrystalline silicon PECVD Process control
a b s t r a c t In this work we present a novel electrode design for the plasma enhanced chemical vapor deposition of microcrystalline silicon thin films that enables optical access to the growing layer under normal incidence. The optical access is realized by piercing the electrode with a conical feed through of 10 mm diameter at the electrode side facing the plasma. The influence of the feed through on deposition homogeneity is studied in different pressure regimes from 8 mbar to 24 mbar on intrinsic layers optimized for state of the art thin-film silicon solar cells. The homogeneity of the layers was determined by spatially resolved thickness measurements and evaluation of the crystalline volume fraction by Raman spectroscopy. With the aim to minimize the influence of the disturbance of the electrode surface the effect of different insets in the feed through on the homogeneity is studied. To find a maximum in optical transmission of the insets at optimal film homogeneity different designs of metallic grids were tested. We show that using the modified electrode it is possible to deposit microcrystalline silicon layers which are comparable in homogeneity to those fabricated with an unchanged standard electrode. This was achieved by covering only 19% of the area of the feed through by a metallic inset. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Plasma enhanced chemical vapor deposition (PECVD) of thin-film silicon for the use in solar cells and thin-film transistors is a well studied process [1–4]. Often multi-junction cells are applied to increase the total efficiency of thin-film silicon solar-cells. A typical application of hydrogenated microcrystalline silicon (μc-Si:H) is the inclusion as bottom cell in a tandem device with an amorphous silicon top cell. With this structure the solar spectrum can be utilized more efficiently due to the different band gaps of amorphous and microcrystalline silicon [2,5]. Apart from solar cells μc-Si:H is also used in thin-film transistors because of its higher carrier mobility compared to amorphous silicon [6]. Microcrystalline silicon solar-cells show the best performance when the intrinsic absorber material is deposited close to the phase transition to amorphous growth. The process window to grow this type of material is rather small [7]. Furthermore deposition parameters like film temperature, deposition pressure or atomic hydrogen concentration [8] have a great impact on the properties of the growing material. To study the interdependence of deposition conditions and material growth an insitu study of the growing material is of utmost interest. Some of the important material characteristics like film crystallinity cannot be
⁎ Corresponding author. Tel.: +49 2461 61 3242; fax: +49 2461 61 3735. E-mail address: [email protected] (S. Muthmann). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.02.037
measured during growth using the presently existing deposition systems due to the lack of optical access to the growing film. Thus, our aim is to introduce a PECVD system that allows optical access to the growing film under small angles to enable the in-situ use of material characterization techniques. 2. Materials and method Microcrystalline silicon layers were deposited in a conventional UHV system by radio frequency PECVD at 13.56 MHz at an electrode distance of 10 mm. Silane and hydrogen were used as growth precursors. The substrate temperature was controlled by a heater which was held at a temperature of 200 °C. The silicon layers were deposited on 10 cm × 10 cm zinc oxide covered glass substrates at pressures varying from 8 mbar to 24 mbar and power densities of 0.4– 1 W/cm2. The deposition conditions were adjusted to achieve a growth rate of about 5 Å/s for all samples. To increase the adhesion of the microcrystalline layers on the substrate and guarantee for the same incubation conditions as in a solar-cell a thin microcrystalline ptype layer was deposited prior to the intrinsic layer. The p-type layers were deposited using tri-methyl-boron for doping. A showerhead electrode was used in the PECVD process to realize a homogeneous distribution of the source gases in the plasma volume. To determine the layer thickness of the deposited films the silicon was removed on various positions by laser scribing. Afterwards the thickness at these positions was determined by scanning with a DEKTAK profilometer. The crystallinity of the layers at the positions
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S. Muthmann et al. / Surface & Coatings Technology 205 (2011) S415–S418
marked via laser scribing was measured using a micro-Raman setup at an excitation wavelength of 488 nm [9].
3. Results 3.1. Modification of the electrode The methods to study the deposition of thin-film silicon in-situ that are presently available rely on either optical viewports from the side of the electrode or through the substrate [10–12]. To realize optical access under normal incidence angle to the growing film and the volume close to it a circular opening with a diameter of 10 mm was pierced through the electrode as shown in Fig. 1. The feed through has a conical shape to increase the amount of collected light with a given opening at the side of the electrode facing the plasma volume. The electrode is mounted horizontally in the plasma chamber with the substrate above it. The gas distribution through the electrode by the showerhead principle was not altered by the modification. Additionally, it was possible to shield the opening of the electrode by integrating different types of insets as it can be seen in Fig. 1.
3.2. Film homogeneity at different deposition pressures Fig. 2 shows thickness measurements of films deposited with the modified electrode at chamber pressures of 8, 13 and 24 mbar. The edge of the substrate is situated at the top of the figures at a y-position of 22 mm. During these depositions no inset was placed in the feed through. The scanned area was 20 mm × 20 mm. It can be seen that the film thickness is larger in the area opposite to the feed through in the electrode. This effect is more pronounced at lower deposition pressures. All the films were deposited with relevant parameters meaning that gas flows and rf-power were optimized for solar-cell performance in the given regime. The maximum thickness difference for the film deposited at 8 mbar is more than 50%. This means that the part of the layer facing the feed through is twice as thick as the rest of the layer on the sample. In Fig. 3 the Raman crystallinities of the films deposited at different pressures are compared. Again, the inhomogeneity is most prominent for low deposition pressures. A lower crystalline volume fraction is measured in the area opposite to the feed through. The layer deposited at 24 mbar shows an overall lower crystalline volume fraction than the other two. This was a result of solar cell optimization in this pressure regime. The crystallinity below 25% at the edge of the deposition zone is due to boundary effects determined by the deposition system and not attributed to effects caused by the feed through. Hence, at 24 mbar no inhomogeneity due to effects caused by the optical access can be determined from Fig. 3.
Fig. 1. Sketched side view of the opening in the electrode. The conical shape was chosen to enable a larger angle of acceptance for optical measurements. Below the electrode different types of insets are shown that can be used to cover the feed through partially.
Fig. 2. Thickness profiles for deposition pressures of 8, 13, and 24 mbar. The viewgraphs show the deviation to the maximum thickness. The electrode feed through is located opposite the area indicated by a deviation of 0%. The layers were deposited with an unshielded feed through.
3.3. Reducing the influence of the optical feed through To minimize the influence of the optical port and to get a better understanding of the origin of the inhomogeneity different insets were soldered into the feed through using silver paste. To simulate an unchanged electrode surface a metal disc was used to cover the feed through completely. A glass disc was applied to restrict the plasma volume to the area between the electrodes without shielding the feed through electrically. For practical applications the glass disc is of minor relevance since it is covered with the growing film at approximately the same deposition rate as the substrate. This prevents optical measurements through the glass disc after short deposition times. Different types of grids were applied to reduce the effect of the feed through with still allowing optical transmission. In Fig. 1 the applied grids are sketched. Grid 1 consists of four bars with a width of 0.5 mm each. The construction of grid 2 is more complex with increased density of bars towards the edges and a reduced width of the individual bars. For optical measurements the area covered by the
S. Muthmann et al. / Surface & Coatings Technology 205 (2011) S415–S418
S417
with the metal plate covering the feed through (simulating a standard electrode) show a thickness deviation of 8% at most. This can be considered as the maximum accomplishable film homogeneity with the given electrode. The glass disc also leads to a great improvement of the film homogeneity especially at low pressures without improving the film as much as the metal disc. Depositions without any kind of passivation of the electrode feed through lead to poor results in the 13 mbar and 8 mbar regimes demonstrated by the largest thickness deviations. For the two grids an improvement can be observed for all studied deposition pressures. In the 8 mbar regime the maximum thickness deviation is still larger than 25% for both grids. In the 13 mbar regime the application of both grids results in a deviation close to the one with the metal disc. For a deposition at 24 mbar no difference can be observed between the layer deposited with the metal disc and the one deposited with grid 2. However grid 1 leads to the same thickness deviation as found with the unshielded feed through which is at a low level anyway. 4. Discussion
Fig. 3. Raman crystallinity profiles for deposition pressures of 8, 13, and 24 mbar. The viewgraphs show the absolute Raman crystallinity measured with an excitation wavelength of 488 nm using a micro-Raman setup.
grids Ac is important. For grid 1 Ac amounts to roughly 12% and for grid 2 Ac amounts to roughly 19%. In Fig. 4 the influence of different insets on layer homogeneity is compared to depositions without an inset as a function of deposition pressure. It can be seen that films deposited
The two effects that could be responsible for the inhomogeneities close to the electrode feed through are i) the disturbed electrical field by the pierced electrode surface and ii) an extension of the plasma into the conical cavity. The increase of the plasma volume is expected to be more pronounced at lower pressures. This corresponds to the observed effects in film homogeneity. Since the layer is thicker in the area facing the feed through, an increased electrical field at the sharp rim of the feed through resulting in an increase in gas dissipation could also explain the increased layer thickness in this area. With the inserted glass disc the electrical field should not be altered too much from the conditions without an inset. The homogeneity is still increased drastically especially under low pressure conditions without removing the effect of the feed through completely. This means that a combination of the above mentioned reasons (the enhanced electric field and the enlarged plasma volume) are likely to be the origin of the inhomogeneity caused by the feed through. For in-situ optical characterization it is necessary that the area of the film facing the feed through experiences the same deposition conditions as the rest of the film. Only then measurements through the feed through allow drawing conclusions about the whole deposition area. For the 13 mbar and the 24 mbar regimes the metal grids lead to films with an adequate homogeneity to fulfill this requirement. 5. Conclusion We presented a novel electrode design for the PECVD of thin-film silicon solar cells which enables in-situ optical access to the growing film and the volume close to it under normal incidence. Without shielding the optical access the conditions in the volume facing the optical access are considerably different from those in the bulk of the plasma zone as indicated by the reduced homogeneity of the deposited films. By increasing the deposition pressures to 13 mbar and 24 mbar it was possible to reduce the influence of the optical port. This effect was greatly enhanced by the use of metal grids. Hence the deposition of homogenous μc-Si:H tin-films with the novel electrode was possible with a covered area of the feed through of only 19%. Acknowledgements
Fig. 4. Maximum thickness deviation for different insets at deposition pressures of 8, 13, and 24 mbar compared to measurements on an unshielded electrode.
The authors would like to thank Reinhard Carius, Florian Köhler and Dzmitry Hrunsky for the fruitful discussions. We are grateful to Gunnar Schoepe for the technical assistance. We would like to thank Sandra Schicho for carefully reading the manuscript.
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S. Muthmann et al. / Surface & Coatings Technology 205 (2011) S415–S418
References [1] L.H. Guo, M. Kondo, M. Fukawa, K. Saitoh, A. Matsuda, Jpn. J. Appl. Phys. 2 37 (10A) (1998) L1116. [2] B. Rech, T. Roschek, T. Repmann, J. Muller, R. Schmitz, W. Appenzeller, Thin Solid Films 427(1–2) p.157 (2003). [3] A. Sah, J. Meier, E. Vallat-Sauvain, C. Droz, U. Kroll, N. Wyrsch, J. Guillet, U. Graf, Thin Solid Films 403 (2002) 179. [4] J. Perrin, J. Schmitt, C. Hollenstein, A. Howling, L. Sansonnens, Plasma Phys. Controlled Fusion 42 (2000) B353. [5] J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A.A. Selvan, N.P. Vaucher, C. Hof, D. Fischer, H. Keppner, A. Shah, K.D. Ufert, P. Giannoules, J. Koehler, Mater. Res. Soc. Symp. Proc. 420 (1996) 3.
[6] C.H. Lee, A. Sazonov, A. Nathan, J. Robertson, Appl. Phys. Lett. 89 (25) (2006) 252101. [7] M.N. v. d. Donker, T. Kilper, D. Grunsky, B. Rech, L. Houben, W.M.M. Kessels, M.C. M. v. d. Sanden, Thin Solid Films 515 (19) (2007) 7455. [8] M.N. v. d. Donker, B. Rech, R. Schmitz, J. Klomfass, G. Dingemans, F. Finger, L. Houben, W.M.M. Kessels, M.C.M. v. d. Sanden, J. Mater. Res. 22 (7) (2007) 1767. [9] L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, H. Wagner, Philos. Mag. A 77 (6) (1998) 1447. [10] G. Dingemans, M. van den Donker, D. Hrunski, A. Gordijn, Proceedings of the european photovoltaic solar energy conference, 20078, Milan Italy. [11] M. Meier, S. Muthmann, G. Dingemans, M.C.M. van de Sanden, M. Schulte, A. Gordijn, Conference Proceedings 25nd EU PVSEC Valencia, 2010. [12] Y.M. Li, A.N. Ilsin, H.V. Nguyen, C.R. Wronski, R.W. Collins, J. Non-Cryst. Solids 137 (1991) 787.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
The effect of disturbed PECVD electrode surfaces on the homogeneity of microcrystalline silicon films S. Muthmann ⁎, M. Meier, R. Schmitz, W. Appenzeller, A. Mück, A. Gordijn IEF5-Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany
a r t i c l e
i n f o
Available online 18 February 2011 Keywords: Solar cells Microcrystalline silicon PECVD Process control
a b s t r a c t In this work we present a novel electrode design for the plasma enhanced chemical vapor deposition of microcrystalline silicon thin films that enables optical access to the growing layer under normal incidence. The optical access is realized by piercing the electrode with a conical feed through of 10 mm diameter at the electrode side facing the plasma. The influence of the feed through on deposition homogeneity is studied in different pressure regimes from 8 mbar to 24 mbar on intrinsic layers optimized for state of the art thin-film silicon solar cells. The homogeneity of the layers was determined by spatially resolved thickness measurements and evaluation of the crystalline volume fraction by Raman spectroscopy. With the aim to minimize the influence of the disturbance of the electrode surface the effect of different insets in the feed through on the homogeneity is studied. To find a maximum in optical transmission of the insets at optimal film homogeneity different designs of metallic grids were tested. We show that using the modified electrode it is possible to deposit microcrystalline silicon layers which are comparable in homogeneity to those fabricated with an unchanged standard electrode. This was achieved by covering only 19% of the area of the feed through by a metallic inset. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Plasma enhanced chemical vapor deposition (PECVD) of thin-film silicon for the use in solar cells and thin-film transistors is a well studied process [1–4]. Often multi-junction cells are applied to increase the total efficiency of thin-film silicon solar-cells. A typical application of hydrogenated microcrystalline silicon (μc-Si:H) is the inclusion as bottom cell in a tandem device with an amorphous silicon top cell. With this structure the solar spectrum can be utilized more efficiently due to the different band gaps of amorphous and microcrystalline silicon [2,5]. Apart from solar cells μc-Si:H is also used in thin-film transistors because of its higher carrier mobility compared to amorphous silicon [6]. Microcrystalline silicon solar-cells show the best performance when the intrinsic absorber material is deposited close to the phase transition to amorphous growth. The process window to grow this type of material is rather small [7]. Furthermore deposition parameters like film temperature, deposition pressure or atomic hydrogen concentration [8] have a great impact on the properties of the growing material. To study the interdependence of deposition conditions and material growth an insitu study of the growing material is of utmost interest. Some of the important material characteristics like film crystallinity cannot be
⁎ Corresponding author. Tel.: +49 2461 61 3242; fax: +49 2461 61 3735. E-mail address: [email protected] (S. Muthmann). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.02.037
measured during growth using the presently existing deposition systems due to the lack of optical access to the growing film. Thus, our aim is to introduce a PECVD system that allows optical access to the growing film under small angles to enable the in-situ use of material characterization techniques. 2. Materials and method Microcrystalline silicon layers were deposited in a conventional UHV system by radio frequency PECVD at 13.56 MHz at an electrode distance of 10 mm. Silane and hydrogen were used as growth precursors. The substrate temperature was controlled by a heater which was held at a temperature of 200 °C. The silicon layers were deposited on 10 cm × 10 cm zinc oxide covered glass substrates at pressures varying from 8 mbar to 24 mbar and power densities of 0.4– 1 W/cm2. The deposition conditions were adjusted to achieve a growth rate of about 5 Å/s for all samples. To increase the adhesion of the microcrystalline layers on the substrate and guarantee for the same incubation conditions as in a solar-cell a thin microcrystalline ptype layer was deposited prior to the intrinsic layer. The p-type layers were deposited using tri-methyl-boron for doping. A showerhead electrode was used in the PECVD process to realize a homogeneous distribution of the source gases in the plasma volume. To determine the layer thickness of the deposited films the silicon was removed on various positions by laser scribing. Afterwards the thickness at these positions was determined by scanning with a DEKTAK profilometer. The crystallinity of the layers at the positions
S416
S. Muthmann et al. / Surface & Coatings Technology 205 (2011) S415–S418
marked via laser scribing was measured using a micro-Raman setup at an excitation wavelength of 488 nm [9].
3. Results 3.1. Modification of the electrode The methods to study the deposition of thin-film silicon in-situ that are presently available rely on either optical viewports from the side of the electrode or through the substrate [10–12]. To realize optical access under normal incidence angle to the growing film and the volume close to it a circular opening with a diameter of 10 mm was pierced through the electrode as shown in Fig. 1. The feed through has a conical shape to increase the amount of collected light with a given opening at the side of the electrode facing the plasma volume. The electrode is mounted horizontally in the plasma chamber with the substrate above it. The gas distribution through the electrode by the showerhead principle was not altered by the modification. Additionally, it was possible to shield the opening of the electrode by integrating different types of insets as it can be seen in Fig. 1.
3.2. Film homogeneity at different deposition pressures Fig. 2 shows thickness measurements of films deposited with the modified electrode at chamber pressures of 8, 13 and 24 mbar. The edge of the substrate is situated at the top of the figures at a y-position of 22 mm. During these depositions no inset was placed in the feed through. The scanned area was 20 mm × 20 mm. It can be seen that the film thickness is larger in the area opposite to the feed through in the electrode. This effect is more pronounced at lower deposition pressures. All the films were deposited with relevant parameters meaning that gas flows and rf-power were optimized for solar-cell performance in the given regime. The maximum thickness difference for the film deposited at 8 mbar is more than 50%. This means that the part of the layer facing the feed through is twice as thick as the rest of the layer on the sample. In Fig. 3 the Raman crystallinities of the films deposited at different pressures are compared. Again, the inhomogeneity is most prominent for low deposition pressures. A lower crystalline volume fraction is measured in the area opposite to the feed through. The layer deposited at 24 mbar shows an overall lower crystalline volume fraction than the other two. This was a result of solar cell optimization in this pressure regime. The crystallinity below 25% at the edge of the deposition zone is due to boundary effects determined by the deposition system and not attributed to effects caused by the feed through. Hence, at 24 mbar no inhomogeneity due to effects caused by the optical access can be determined from Fig. 3.
Fig. 1. Sketched side view of the opening in the electrode. The conical shape was chosen to enable a larger angle of acceptance for optical measurements. Below the electrode different types of insets are shown that can be used to cover the feed through partially.
Fig. 2. Thickness profiles for deposition pressures of 8, 13, and 24 mbar. The viewgraphs show the deviation to the maximum thickness. The electrode feed through is located opposite the area indicated by a deviation of 0%. The layers were deposited with an unshielded feed through.
3.3. Reducing the influence of the optical feed through To minimize the influence of the optical port and to get a better understanding of the origin of the inhomogeneity different insets were soldered into the feed through using silver paste. To simulate an unchanged electrode surface a metal disc was used to cover the feed through completely. A glass disc was applied to restrict the plasma volume to the area between the electrodes without shielding the feed through electrically. For practical applications the glass disc is of minor relevance since it is covered with the growing film at approximately the same deposition rate as the substrate. This prevents optical measurements through the glass disc after short deposition times. Different types of grids were applied to reduce the effect of the feed through with still allowing optical transmission. In Fig. 1 the applied grids are sketched. Grid 1 consists of four bars with a width of 0.5 mm each. The construction of grid 2 is more complex with increased density of bars towards the edges and a reduced width of the individual bars. For optical measurements the area covered by the
S. Muthmann et al. / Surface & Coatings Technology 205 (2011) S415–S418
S417
with the metal plate covering the feed through (simulating a standard electrode) show a thickness deviation of 8% at most. This can be considered as the maximum accomplishable film homogeneity with the given electrode. The glass disc also leads to a great improvement of the film homogeneity especially at low pressures without improving the film as much as the metal disc. Depositions without any kind of passivation of the electrode feed through lead to poor results in the 13 mbar and 8 mbar regimes demonstrated by the largest thickness deviations. For the two grids an improvement can be observed for all studied deposition pressures. In the 8 mbar regime the maximum thickness deviation is still larger than 25% for both grids. In the 13 mbar regime the application of both grids results in a deviation close to the one with the metal disc. For a deposition at 24 mbar no difference can be observed between the layer deposited with the metal disc and the one deposited with grid 2. However grid 1 leads to the same thickness deviation as found with the unshielded feed through which is at a low level anyway. 4. Discussion
Fig. 3. Raman crystallinity profiles for deposition pressures of 8, 13, and 24 mbar. The viewgraphs show the absolute Raman crystallinity measured with an excitation wavelength of 488 nm using a micro-Raman setup.
grids Ac is important. For grid 1 Ac amounts to roughly 12% and for grid 2 Ac amounts to roughly 19%. In Fig. 4 the influence of different insets on layer homogeneity is compared to depositions without an inset as a function of deposition pressure. It can be seen that films deposited
The two effects that could be responsible for the inhomogeneities close to the electrode feed through are i) the disturbed electrical field by the pierced electrode surface and ii) an extension of the plasma into the conical cavity. The increase of the plasma volume is expected to be more pronounced at lower pressures. This corresponds to the observed effects in film homogeneity. Since the layer is thicker in the area facing the feed through, an increased electrical field at the sharp rim of the feed through resulting in an increase in gas dissipation could also explain the increased layer thickness in this area. With the inserted glass disc the electrical field should not be altered too much from the conditions without an inset. The homogeneity is still increased drastically especially under low pressure conditions without removing the effect of the feed through completely. This means that a combination of the above mentioned reasons (the enhanced electric field and the enlarged plasma volume) are likely to be the origin of the inhomogeneity caused by the feed through. For in-situ optical characterization it is necessary that the area of the film facing the feed through experiences the same deposition conditions as the rest of the film. Only then measurements through the feed through allow drawing conclusions about the whole deposition area. For the 13 mbar and the 24 mbar regimes the metal grids lead to films with an adequate homogeneity to fulfill this requirement. 5. Conclusion We presented a novel electrode design for the PECVD of thin-film silicon solar cells which enables in-situ optical access to the growing film and the volume close to it under normal incidence. Without shielding the optical access the conditions in the volume facing the optical access are considerably different from those in the bulk of the plasma zone as indicated by the reduced homogeneity of the deposited films. By increasing the deposition pressures to 13 mbar and 24 mbar it was possible to reduce the influence of the optical port. This effect was greatly enhanced by the use of metal grids. Hence the deposition of homogenous μc-Si:H tin-films with the novel electrode was possible with a covered area of the feed through of only 19%. Acknowledgements
Fig. 4. Maximum thickness deviation for different insets at deposition pressures of 8, 13, and 24 mbar compared to measurements on an unshielded electrode.
The authors would like to thank Reinhard Carius, Florian Köhler and Dzmitry Hrunsky for the fruitful discussions. We are grateful to Gunnar Schoepe for the technical assistance. We would like to thank Sandra Schicho for carefully reading the manuscript.
S418
S. Muthmann et al. / Surface & Coatings Technology 205 (2011) S415–S418
References [1] L.H. Guo, M. Kondo, M. Fukawa, K. Saitoh, A. Matsuda, Jpn. J. Appl. Phys. 2 37 (10A) (1998) L1116. [2] B. Rech, T. Roschek, T. Repmann, J. Muller, R. Schmitz, W. Appenzeller, Thin Solid Films 427(1–2) p.157 (2003). [3] A. Sah, J. Meier, E. Vallat-Sauvain, C. Droz, U. Kroll, N. Wyrsch, J. Guillet, U. Graf, Thin Solid Films 403 (2002) 179. [4] J. Perrin, J. Schmitt, C. Hollenstein, A. Howling, L. Sansonnens, Plasma Phys. Controlled Fusion 42 (2000) B353. [5] J. Meier, P. Torres, R. Platz, S. Dubail, U. Kroll, J.A.A. Selvan, N.P. Vaucher, C. Hof, D. Fischer, H. Keppner, A. Shah, K.D. Ufert, P. Giannoules, J. Koehler, Mater. Res. Soc. Symp. Proc. 420 (1996) 3.
[6] C.H. Lee, A. Sazonov, A. Nathan, J. Robertson, Appl. Phys. Lett. 89 (25) (2006) 252101. [7] M.N. v. d. Donker, T. Kilper, D. Grunsky, B. Rech, L. Houben, W.M.M. Kessels, M.C. M. v. d. Sanden, Thin Solid Films 515 (19) (2007) 7455. [8] M.N. v. d. Donker, B. Rech, R. Schmitz, J. Klomfass, G. Dingemans, F. Finger, L. Houben, W.M.M. Kessels, M.C.M. v. d. Sanden, J. Mater. Res. 22 (7) (2007) 1767. [9] L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, H. Wagner, Philos. Mag. A 77 (6) (1998) 1447. [10] G. Dingemans, M. van den Donker, D. Hrunski, A. Gordijn, Proceedings of the european photovoltaic solar energy conference, 20078, Milan Italy. [11] M. Meier, S. Muthmann, G. Dingemans, M.C.M. van de Sanden, M. Schulte, A. Gordijn, Conference Proceedings 25nd EU PVSEC Valencia, 2010. [12] Y.M. Li, A.N. Ilsin, H.V. Nguyen, C.R. Wronski, R.W. Collins, J. Non-Cryst. Solids 137 (1991) 787.