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Effect of H2 dilution on the surface composition of plasma-deposited silicon films from SiH4
Applied Surface Science 133 Ž1998. 148–151
Letter to the Editor
Effect of H 2 dilution on the surface composition of plasma-deposited silicon films from SiH 4 Denise C. Marra a , Erik A. Edelberg a , Ryan L. Naone b, Eray S. Aydil a
a,)
Department of Chemical Engineering, UniÕersity of California Santa Barbara, Santa Barbara, CA 93106, USA b Materials Department, UniÕersity of California Santa Barbara, Santa Barbara, CA 93106, USA Received 10 December 1997; accepted 24 February 1998
Abstract The surface composition of silicon films deposited from SiH 4 , Ar, and H 2 plasmas was studied using in situ attenuated total reflection Fourier transform infrared spectroscopy with emphasis on the effects of H 2 dilution. In the absence of H 2 , the surface is primarily covered with SiH 3 and SiH 2 . With heavy H 2 dilution, the surface is predominantly monohydride terminated with infrared absorption frequencies consistent with the presence of SiH on Si Ž100. and Si Ž111. surfaces. q 1998 Elsevier Science B.V. All rights reserved. PACS: 52.75.Rx; 81.15.Gh; 68.35.Bs Keywords: Attenuated total reflection Fourier transform infrared spectroscopy; Amorphous silicon; Nanocrystalline silicon; Plasma deposition
Hydrogenated amorphous Ža-Si:H. and nanocrystalline Žnc-Si:H. silicon are important materials for manufacturing electronic devices such as thin film transistors and solar cells. Thin films of a-Si:H and nc-Si:H are deposited by plasma enhanced chemical vapor deposition ŽPECVD. from mixtures of SiH 4 , Ar, and H 2 . It has been shown that the hydrogen content of the film determines its electronic properties w1,2x. In fact, addition of H 2 to the SiH 4 discharge generally improves film quality, and if H 2 dilution is sufficient, nanocrystalline films are deposited w2–4x. Despite the large number of studies on
)
Corresponding author. Tel.: q1-805-893-8205; fax: q1-805893-4731; e-mail: [email protected].
the plasma parameter dependence of deposited film properties, details of the deposition mechanism are currently under debate. In particular, the role of H 2 in nc-Si:H deposition is not well understood. A thorough knowledge of the structure and composition of the film surface during deposition is expected to provide insight into the deposition mechanism. Using attenuated total reflection Fourier transform infrared ŽATR-FTIR. spectroscopy w5–8x, we studied changes in the surface composition of plasma-deposited Si films as a function of H 2 dilution. Previ˚. ous studies of the near surface region Žtop 10–50 A of growing a-Si:H films during plasma deposition relied on infrared phase modulated ellipsometry and infrared reflection absorption spectroscopy w9–11x. Investigators collectively concluded that a-Si:H films grow beneath an overlayer composed of SiH x Ž1 F x
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 2 1 4 - 1
D.C. Marra et al.r Applied Surface Science 133 (1998) 148–151
F 3. with a higher H concentration than the bulk. In this letter, we present well-resolved vibrational absorption spectra of surface silicon hydrides on plasma-deposited a-Si:H and nc-Si:H films. The Si films were deposited in an inductively coupled plasma reactor with a base pressure of 4 = 10y7 Torr. During the deposition, the substrate electrode, the pressure, and power dissipated in the plasma were maintained at 2308C, 10 mTorr, and 100 W, respectively. For all depositions, SiH 4 diluted to 1% in Ar was fed into the reactor at 50 sccm. An additional 50 sccm of an ArrH 2 mixture was used to investigate H 2 dilution effects. Herein, we present surface spectra for films prepared with 0.5 sccm of SiH 4 and 99.5 sccm of Ar, and for films prepared with 0.5 sccm of SiH 4 , 10 sccm of H 2 , and 89.5 sccm of Ar. The enhanced sensitivity needed to detect surface constituents is achieved by in situ ATR-FTIR spectroscopy. This technique, which is described in detail elsewhere w5–8x, utilizes multiple total internal reflections of the IR beam to enhance sensitivity to film and surface species and allows detection of submonolayer coverage of silicon hydrides. Si films are deposited on trapezoidal shaped ATR crystals Ž0.71 mm = 10 mm = 50 mm. with 458 bevels on the short ends. The ATR crystals are cut from double side polished GaAs wafers, which are transparent to approximately 800 cmy1 , thereby enabling detection of the deformation modes of silicon di- and trihydrides. The deposition chamber is also equipped with a spectroscopic ellipsometer ŽSE. for real time detection of film thickness during deposition. To enable detection of the low signal from the surface hydrides, we stop the deposition and expose the film to an Ar plasma for 10 s at 10 mTorr. During this time, the surface is bombarded by 15–20 eV Ar ions which desorb H from the surface w12x, and the corresponding reduction in the silicon hydride absorption is detected using ATR-FTIR. The ion bombardment energy is not energetic enough to cause physical sputtering of the film and, as expected, no change in the film thickness is observed by SE during the Ar plasma exposure. Furthermore, ion bombardment did not cause significant structural change in the film as evidenced by Fig. 1, which shows the real and imaginary components of the film’s pseudo dielectric function before and after 10
149
Fig. 1. The real Ž ´ 1 . and imaginary Ž ´ 2 . pseudo dielectric function of a-Si:H film before and after 10 s of Ar ion bombardment. There is no change in the dielectric function upon mild Arq bombardment. The corresponding infrared spectrum is shown in Fig. 2.
s of Ar ion bombardment. The dielectric function did not change indicating that the primary effect of mild Ž15–20 eV, 10 15 ionsrcm2 s. ion bombardment is desorption of surface H. All spectra are collected and presented in the differential mode w5,13x using the film prior to exposure to Ar ions as the reference spectrum. Thus, the decrease in absorbance corresponds to hydrides desorbed from the as-deposited surface by Arq bombardment. Fig. 2 shows the surface of the a-Si:H film prepared in the absence of H 2 . The SiH x stretching vibration region was deconvoluted using three Gaussian peaks centered at 2136, 2112 and 2086
˚ a-Si:H film Fig. 2. Infrared spectrum of the surface of a 109 A deposited without H 2 dilution. Deposition conditions were 100 W, 10 mTorr, 2308C, 0.5 sccm SiH 4 , and 99.5 sccm Ar.
150
D.C. Marra et al.r Applied Surface Science 133 (1998) 148–151
˚ nc-Si:H film Fig. 3. Infrared spectrum of the surface of a 400 A deposited with a H 2 to SiH 4 ratio of 20 : 1 using 0.5 sccm SiH 4 , 10 sccm H 2 , and 89.5 sccm Ar. Other deposition conditions were as in Fig. 2.
cmy1 . The narrow widths and the location of the peak frequencies in the spectrum shown in Fig. 2 are characteristic of surface hydrides. While the peak widths are broader than those observed on well-defined c-Si surfaces, they are still narrower than those that have been observed on a-Si:H by Blayo and Drevillon w9x and Katiyar et al. w11x. Due to interference from the bulk SiH x absorptions, these studies
have had difficulty resolving the weak surface contribution except during the initial stages of the ˚ of growth, Katiyar et al. growth. For less than 30 A were able to observe a band at 2120 cmy1 which they assigned to all surface hydrides. The width of this band is approximately the total width of the three surface peaks shown in Fig. 2 and its location is consistent with our data. Similar absorptions were also observed by Toyoshima et al. w10x on a-Si:H surfaces. The peak assignments are based on those of Chabal et al. w5,6,14x for H on crystalline Si Ž100. and Ž111. surfaces. We find that the surface is primarily composed of SiH 2 and SiH 3 , which appear at 2112 and 2136 cmy1 , respectively. A shoulder at approximately 2086 cmy1 is attributed to surface SiH. Although only the SiH x stretching region is shown here, further evidence for higher hydrides is provided by the presence of lower frequency deformation modes in the range of 860–910 cmy1 . These results will appear in a future publication. Under conditions of heavy H 2 dilution, the surface of the film is very different from that prepared in the absence of H 2 . The SiH x stretching vibrations are shown in Fig. 3 for a H 2 to SiH 4 ratio of 20 : 1.
Fig. 4. Transmission electron micrograph of a nc-Si:H film deposited with a H 2 to SiH 4 ratio of 20 : 1 using 0.5 sccm SiH 4 , 10 sccm H 2 , and 89.5 sccm Ar. Other deposition conditions were as in Fig. 2.
D.C. Marra et al.r Applied Surface Science 133 (1998) 148–151
In the presence of a H 2-rich feed gas, the surface coverage is predominantly monohydride rather than higher hydrides as in the spectra shown in Fig. 2. Some SiH 3 is detected with minimal or no SiH 2 . The well-resolved peaks at 2077 and 2099 cmy1 correspond to SiH on very well-defined crystalline silicon surfaces w6x. The former peak has been assigned to H on Si Ž111.-Ž7 = 7.H, while the 2099 cmy1 mode is characteristic of H on Si Ž100.-Ž2 = 1.H. This result is consistent with the fact that under conditions of heavy H 2 dilution, the films are nanocrystalline. The transmission electron micrograph of the film deposited using a H 2 to SiH 4 ratio of 20 : 1 is shown in Fig. 4. The nc-Si:H film is composed of crystalline regions of various orientations adjacent and interspersed with some a-Si and void spaces w15x. Thus, H on the surface of such a film should give rise to vibrational bands at locations characteristic of H on several orientations of crystalline surfaces. Similar results were observed on film surfaces prepared using a H 2 to SiH 4 ratio of 10 : 1. We conclude that dilution of SiH 4 with H 2 changes the surface composition of deposited films dramatically. However, investigation of the surface alone does not give insight into the reasons for this drastic change. Our results show that H 2 addition to SiH 4 decreases the concentration of higher hydrides ŽSiH 2 and SiH 3 . on the surface. In summary, we have used in situ ATR-FTIR to investigate the film surface during PECVD using SiH 4rAr with and without H 2 dilution. For films deposited with and without H 2 addition to SiH 4 plasma, we observe well-resolved SiH x Ž1 F x F 3. stretching modes at locations of previously identified frequencies for hydrides on c-Si surfaces. In the absence of H 2 , the surface is predominantly covered by SiH 2 and SiH 3 . Monohydrides dominate the surface coverage of films deposited using a feed gas that is heavily diluted with H 2 . Furthermore, the
151
location of the SiH absorptions on film surfaces prepared with heavy H 2 dilution coincide with the characteristic absorption frequencies of monohydrides on H-terminated reconstructed Si Ž100. and Si Ž111. surfaces. Acknowledgements This material is based upon work supported under a National Science Foundation Graduate Fellowship. The authors also acknowledge funding by the National Science Foundation Young Investigator Award ŽECS 9457758.. We thank Professor D. Maroudas and Mr. S. Ramalingam for insightful discussions. References w1x E. Srinivasan, D.A. Lloyd, G.N. Parsons, J. Vac. Sci. Technol. A 15 Ž1997. 77. w2x J.R. Abelson, Appl. Phys. A 56 Ž1993. 493. w3x A. Matsuda, J. Non-Cryst. Solids 59r60 Ž1983. 767. w4x S. Veprek, Chimia 34 Ž1980. 489. w5x Y.J. Chabal, Surf. Sci. Rep. 8 Ž1988. 211. w6x Y.J. Chabal, in: F.M. Mirabella, Jr. ŽEd.., Internal Reflection Spectroscopy: Theory and Applications, Marcel Dekker, New York, 1993, p. 191. w7x N.J. Harrick, Internal Reflection Spectroscopy, Wiley, New York, 1967. w8x E.S. Aydil, R.A. Gottscho, Y.J. Chabal, Pure Appl. Chem. 66 Ž1994. 1381. w9x N. Blayo, B. Drevillon, Appl. Phys. Lett. 59 Ž1991. 950. w10x Y. Toyoshima, A. Matsuda, K. Arai, J. Non-Cryst. Solids 164–166 Ž1993. 103. w11x M. Katiyar, Y.H. Yang, J.R. Abelson, J. Appl. Phys. 77 Ž1995. 6247. w12x G.H. Lin, J.R. Doyle, M. He, A. Gallagher, J. Appl. Phys. 64 Ž1988. 188. w13x E.S. Aydil, R.A. Gottscho, Solid State Technol. 40 Ž1997. 181. w14x P. Jakob, P. Dumas, Y.J. Chabal, Appl. Phys. Lett. 59 Ž1991. 2968. w15x E. Edelberg, S. Bergh, R. Naone, M. Hall, E.S. Aydil, J. Appl. Phys. 81 Ž1997. 2410.
Letter to the Editor
Effect of H 2 dilution on the surface composition of plasma-deposited silicon films from SiH 4 Denise C. Marra a , Erik A. Edelberg a , Ryan L. Naone b, Eray S. Aydil a
a,)
Department of Chemical Engineering, UniÕersity of California Santa Barbara, Santa Barbara, CA 93106, USA b Materials Department, UniÕersity of California Santa Barbara, Santa Barbara, CA 93106, USA Received 10 December 1997; accepted 24 February 1998
Abstract The surface composition of silicon films deposited from SiH 4 , Ar, and H 2 plasmas was studied using in situ attenuated total reflection Fourier transform infrared spectroscopy with emphasis on the effects of H 2 dilution. In the absence of H 2 , the surface is primarily covered with SiH 3 and SiH 2 . With heavy H 2 dilution, the surface is predominantly monohydride terminated with infrared absorption frequencies consistent with the presence of SiH on Si Ž100. and Si Ž111. surfaces. q 1998 Elsevier Science B.V. All rights reserved. PACS: 52.75.Rx; 81.15.Gh; 68.35.Bs Keywords: Attenuated total reflection Fourier transform infrared spectroscopy; Amorphous silicon; Nanocrystalline silicon; Plasma deposition
Hydrogenated amorphous Ža-Si:H. and nanocrystalline Žnc-Si:H. silicon are important materials for manufacturing electronic devices such as thin film transistors and solar cells. Thin films of a-Si:H and nc-Si:H are deposited by plasma enhanced chemical vapor deposition ŽPECVD. from mixtures of SiH 4 , Ar, and H 2 . It has been shown that the hydrogen content of the film determines its electronic properties w1,2x. In fact, addition of H 2 to the SiH 4 discharge generally improves film quality, and if H 2 dilution is sufficient, nanocrystalline films are deposited w2–4x. Despite the large number of studies on
)
Corresponding author. Tel.: q1-805-893-8205; fax: q1-805893-4731; e-mail: [email protected].
the plasma parameter dependence of deposited film properties, details of the deposition mechanism are currently under debate. In particular, the role of H 2 in nc-Si:H deposition is not well understood. A thorough knowledge of the structure and composition of the film surface during deposition is expected to provide insight into the deposition mechanism. Using attenuated total reflection Fourier transform infrared ŽATR-FTIR. spectroscopy w5–8x, we studied changes in the surface composition of plasma-deposited Si films as a function of H 2 dilution. Previ˚. ous studies of the near surface region Žtop 10–50 A of growing a-Si:H films during plasma deposition relied on infrared phase modulated ellipsometry and infrared reflection absorption spectroscopy w9–11x. Investigators collectively concluded that a-Si:H films grow beneath an overlayer composed of SiH x Ž1 F x
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 2 1 4 - 1
D.C. Marra et al.r Applied Surface Science 133 (1998) 148–151
F 3. with a higher H concentration than the bulk. In this letter, we present well-resolved vibrational absorption spectra of surface silicon hydrides on plasma-deposited a-Si:H and nc-Si:H films. The Si films were deposited in an inductively coupled plasma reactor with a base pressure of 4 = 10y7 Torr. During the deposition, the substrate electrode, the pressure, and power dissipated in the plasma were maintained at 2308C, 10 mTorr, and 100 W, respectively. For all depositions, SiH 4 diluted to 1% in Ar was fed into the reactor at 50 sccm. An additional 50 sccm of an ArrH 2 mixture was used to investigate H 2 dilution effects. Herein, we present surface spectra for films prepared with 0.5 sccm of SiH 4 and 99.5 sccm of Ar, and for films prepared with 0.5 sccm of SiH 4 , 10 sccm of H 2 , and 89.5 sccm of Ar. The enhanced sensitivity needed to detect surface constituents is achieved by in situ ATR-FTIR spectroscopy. This technique, which is described in detail elsewhere w5–8x, utilizes multiple total internal reflections of the IR beam to enhance sensitivity to film and surface species and allows detection of submonolayer coverage of silicon hydrides. Si films are deposited on trapezoidal shaped ATR crystals Ž0.71 mm = 10 mm = 50 mm. with 458 bevels on the short ends. The ATR crystals are cut from double side polished GaAs wafers, which are transparent to approximately 800 cmy1 , thereby enabling detection of the deformation modes of silicon di- and trihydrides. The deposition chamber is also equipped with a spectroscopic ellipsometer ŽSE. for real time detection of film thickness during deposition. To enable detection of the low signal from the surface hydrides, we stop the deposition and expose the film to an Ar plasma for 10 s at 10 mTorr. During this time, the surface is bombarded by 15–20 eV Ar ions which desorb H from the surface w12x, and the corresponding reduction in the silicon hydride absorption is detected using ATR-FTIR. The ion bombardment energy is not energetic enough to cause physical sputtering of the film and, as expected, no change in the film thickness is observed by SE during the Ar plasma exposure. Furthermore, ion bombardment did not cause significant structural change in the film as evidenced by Fig. 1, which shows the real and imaginary components of the film’s pseudo dielectric function before and after 10
149
Fig. 1. The real Ž ´ 1 . and imaginary Ž ´ 2 . pseudo dielectric function of a-Si:H film before and after 10 s of Ar ion bombardment. There is no change in the dielectric function upon mild Arq bombardment. The corresponding infrared spectrum is shown in Fig. 2.
s of Ar ion bombardment. The dielectric function did not change indicating that the primary effect of mild Ž15–20 eV, 10 15 ionsrcm2 s. ion bombardment is desorption of surface H. All spectra are collected and presented in the differential mode w5,13x using the film prior to exposure to Ar ions as the reference spectrum. Thus, the decrease in absorbance corresponds to hydrides desorbed from the as-deposited surface by Arq bombardment. Fig. 2 shows the surface of the a-Si:H film prepared in the absence of H 2 . The SiH x stretching vibration region was deconvoluted using three Gaussian peaks centered at 2136, 2112 and 2086
˚ a-Si:H film Fig. 2. Infrared spectrum of the surface of a 109 A deposited without H 2 dilution. Deposition conditions were 100 W, 10 mTorr, 2308C, 0.5 sccm SiH 4 , and 99.5 sccm Ar.
150
D.C. Marra et al.r Applied Surface Science 133 (1998) 148–151
˚ nc-Si:H film Fig. 3. Infrared spectrum of the surface of a 400 A deposited with a H 2 to SiH 4 ratio of 20 : 1 using 0.5 sccm SiH 4 , 10 sccm H 2 , and 89.5 sccm Ar. Other deposition conditions were as in Fig. 2.
cmy1 . The narrow widths and the location of the peak frequencies in the spectrum shown in Fig. 2 are characteristic of surface hydrides. While the peak widths are broader than those observed on well-defined c-Si surfaces, they are still narrower than those that have been observed on a-Si:H by Blayo and Drevillon w9x and Katiyar et al. w11x. Due to interference from the bulk SiH x absorptions, these studies
have had difficulty resolving the weak surface contribution except during the initial stages of the ˚ of growth, Katiyar et al. growth. For less than 30 A were able to observe a band at 2120 cmy1 which they assigned to all surface hydrides. The width of this band is approximately the total width of the three surface peaks shown in Fig. 2 and its location is consistent with our data. Similar absorptions were also observed by Toyoshima et al. w10x on a-Si:H surfaces. The peak assignments are based on those of Chabal et al. w5,6,14x for H on crystalline Si Ž100. and Ž111. surfaces. We find that the surface is primarily composed of SiH 2 and SiH 3 , which appear at 2112 and 2136 cmy1 , respectively. A shoulder at approximately 2086 cmy1 is attributed to surface SiH. Although only the SiH x stretching region is shown here, further evidence for higher hydrides is provided by the presence of lower frequency deformation modes in the range of 860–910 cmy1 . These results will appear in a future publication. Under conditions of heavy H 2 dilution, the surface of the film is very different from that prepared in the absence of H 2 . The SiH x stretching vibrations are shown in Fig. 3 for a H 2 to SiH 4 ratio of 20 : 1.
Fig. 4. Transmission electron micrograph of a nc-Si:H film deposited with a H 2 to SiH 4 ratio of 20 : 1 using 0.5 sccm SiH 4 , 10 sccm H 2 , and 89.5 sccm Ar. Other deposition conditions were as in Fig. 2.
D.C. Marra et al.r Applied Surface Science 133 (1998) 148–151
In the presence of a H 2-rich feed gas, the surface coverage is predominantly monohydride rather than higher hydrides as in the spectra shown in Fig. 2. Some SiH 3 is detected with minimal or no SiH 2 . The well-resolved peaks at 2077 and 2099 cmy1 correspond to SiH on very well-defined crystalline silicon surfaces w6x. The former peak has been assigned to H on Si Ž111.-Ž7 = 7.H, while the 2099 cmy1 mode is characteristic of H on Si Ž100.-Ž2 = 1.H. This result is consistent with the fact that under conditions of heavy H 2 dilution, the films are nanocrystalline. The transmission electron micrograph of the film deposited using a H 2 to SiH 4 ratio of 20 : 1 is shown in Fig. 4. The nc-Si:H film is composed of crystalline regions of various orientations adjacent and interspersed with some a-Si and void spaces w15x. Thus, H on the surface of such a film should give rise to vibrational bands at locations characteristic of H on several orientations of crystalline surfaces. Similar results were observed on film surfaces prepared using a H 2 to SiH 4 ratio of 10 : 1. We conclude that dilution of SiH 4 with H 2 changes the surface composition of deposited films dramatically. However, investigation of the surface alone does not give insight into the reasons for this drastic change. Our results show that H 2 addition to SiH 4 decreases the concentration of higher hydrides ŽSiH 2 and SiH 3 . on the surface. In summary, we have used in situ ATR-FTIR to investigate the film surface during PECVD using SiH 4rAr with and without H 2 dilution. For films deposited with and without H 2 addition to SiH 4 plasma, we observe well-resolved SiH x Ž1 F x F 3. stretching modes at locations of previously identified frequencies for hydrides on c-Si surfaces. In the absence of H 2 , the surface is predominantly covered by SiH 2 and SiH 3 . Monohydrides dominate the surface coverage of films deposited using a feed gas that is heavily diluted with H 2 . Furthermore, the
151
location of the SiH absorptions on film surfaces prepared with heavy H 2 dilution coincide with the characteristic absorption frequencies of monohydrides on H-terminated reconstructed Si Ž100. and Si Ž111. surfaces. Acknowledgements This material is based upon work supported under a National Science Foundation Graduate Fellowship. The authors also acknowledge funding by the National Science Foundation Young Investigator Award ŽECS 9457758.. We thank Professor D. Maroudas and Mr. S. Ramalingam for insightful discussions. References w1x E. Srinivasan, D.A. Lloyd, G.N. Parsons, J. Vac. Sci. Technol. A 15 Ž1997. 77. w2x J.R. Abelson, Appl. Phys. A 56 Ž1993. 493. w3x A. Matsuda, J. Non-Cryst. Solids 59r60 Ž1983. 767. w4x S. Veprek, Chimia 34 Ž1980. 489. w5x Y.J. Chabal, Surf. Sci. Rep. 8 Ž1988. 211. w6x Y.J. Chabal, in: F.M. Mirabella, Jr. ŽEd.., Internal Reflection Spectroscopy: Theory and Applications, Marcel Dekker, New York, 1993, p. 191. w7x N.J. Harrick, Internal Reflection Spectroscopy, Wiley, New York, 1967. w8x E.S. Aydil, R.A. Gottscho, Y.J. Chabal, Pure Appl. Chem. 66 Ž1994. 1381. w9x N. Blayo, B. Drevillon, Appl. Phys. Lett. 59 Ž1991. 950. w10x Y. Toyoshima, A. Matsuda, K. Arai, J. Non-Cryst. Solids 164–166 Ž1993. 103. w11x M. Katiyar, Y.H. Yang, J.R. Abelson, J. Appl. Phys. 77 Ž1995. 6247. w12x G.H. Lin, J.R. Doyle, M. He, A. Gallagher, J. Appl. Phys. 64 Ž1988. 188. w13x E.S. Aydil, R.A. Gottscho, Solid State Technol. 40 Ž1997. 181. w14x P. Jakob, P. Dumas, Y.J. Chabal, Appl. Phys. Lett. 59 Ž1991. 2968. w15x E. Edelberg, S. Bergh, R. Naone, M. Hall, E.S. Aydil, J. Appl. Phys. 81 Ž1997. 2410.