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Properties of a-Si:H films grown using hot wire-ECR plasma techniques
Thin Solid Films 430 (2003) 91–94
Properties of a-Si:H films grown using hot wire-ECR plasma techniques Vikram L. Dalal*, Paul Seberger, Matt Ring, Puneet Sharma Iowa State University, Department of Electrical and Computer Engineering, Ames, IA 50011, USA
Abstract We report on the growth and properties of a-Si:H films and nin layers prepared using combination of hot wire and ECR-plasma growth techniques. The films were prepared using both W and Ta hot wire filaments. A distinguishing feature of the reactor was the large spacing, 11 cm, of the filament from the substrate, thereby avoiding over-heating of the substrate. Films were grown at pressures from 2 to 50 mT, and the corresponding optical and electronic properties of the films were measured. The temperature of the substrate was varied between ;225 and 350 8C. It was found that the growth rates do not follow the maximum at a pressure-distance (pd) product ;15 mTorr cm postulated by Molenbroek et al.’s model. wJ. Appl. Phys. 82, 1909 (1997)x. It was also discovered that the properties of the hot wire films depend upon the pd product, and that the H bonding and electronic properties depend critically upon the growth rate, and on the substrate temperature. The properties of the hot wire films bear a remarkable similarity to the films deposited using expanding thermal plasma (ETP) techniques at similar temperatures. When the films were subjected to low power He plasma, the properties improved dramatically. It was also found that H ions are more efficient at etching a growing film than H radicals alone. The results show that the H bonding and electronic properties of a-Si:H films are determined primarily by the efficiency of H extraction, and that low energy ions have a useful role to play in this process. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Amorphous Si; ECR plasma; Hot wire deposition; Ion bombardment
1. Introduction The growth chemistry of a-Si:H is a complicated process w1–5x. We have shown earlier w2,6x that the fundamental limitation to growth comes from the removal of hydrogen from the lattice, and that this removal needs an energy of ;1.2–1.3 eV. The energy can come from temperature, or from ions. The ions can be reactive, such as H, or inert, such as He. The reactive ions can go deeper into the material and do etching during growth. The He ions remove the excess bonded H primarily from the surface. This model is in contrast to the standard model (the MGP model w3–5x) which states that the fundamental limitation comes from the diffusion of silyl radicals on the surface, and that excess H is abstracted by silyl groups. Clearly, it would be useful to do an experiment where one can independently study the effects of ions on growth, and also study the effects of temperature. In this paper, we report on such *Corresponding author. Present address: 1-1 Asahidai, Tatsunokuchi, Nomi, Ames, IA, USA; Tel.: q1-515-294-1077; fax: q1-515294-9854. E-mail address: [email protected] (V.L. Dalal).
a study, where the decomposition of silane is done by a hot wire system, with ions simply assisting in film formation as opposed to decomposition. 2. Experimental details The growth was done using a combined hot wireECR plasma reactor, shown schematically in Fig. 1. The plasma gas, He or H, was introduced upstream of the ECR cavity, and silane near the substrate. The ECR geometry was chosen because the plasma can be lit at very low pressures, ;1 mTorr, and the ion density can be carefully controlled w7x. The hot wire system used either W or Ta filaments, with most experiments being done with Ta filament. The area of the wire was 6.8 cm2, and the temperature was kept at ;2200 8C. The substrate was far from the filament, ;11 cm, thereby minimizing radiative heating of the substrate. This is an important point, because the substrate temperature plays a major role in the growth of the film. In most other system, the distance is ;1.5– 5 cm, and the substrates are much hotter than the nominal temperature. The silane flow was varied
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00079-8
92
V.L. Dalal et al. / Thin Solid Films 430 (2003) 91–94
Fig. 1. Schematic diagram of ECRqHot wire reactor. The plasma gas (He or H) are introduced upstream, approximately 30 cm from the hot wire, and the silane is introduced near the hot wire. The distance between the hot wire and substrate is 11 cm to reduce radiative heating of the substrate.
between 2 and 12 sccm. The growth was done at pressures between 2 and 40 mTorr, giving pd values between 23 and 450 mTorr cm. The volume of the chamber was ;16 l, giving a residence time of a few seconds at ;2 mTorr. The power in the ECR plasma was either 75 or 100 W. The resonance zone was ;30 cm from the substrate, thus achieving a remote plasma. The film properties measured were: optical absorption, H bonding, AM 1.5 photo and dark conductivity; activation energy and subgap absorption. Subgap absorption was measured using double-beam photo-conductivity techniques, and film thicknesses were all in the 1 mm range. In addition, a few nin samples were deposited on steel substrates, with both n layers deposited in a separate glow discharge reactor. These samples were subjected to space-charge-limited-current (SCLC) measurements to determine defect densities at the Fermi level. 3. Results 3.1. Hot wire growth alone In Fig. 2, we show the data on growth rates, using hot wire only, as a function of pressure for different
Fig. 3. Subgap absorption spectra for two hot wire films deposited at 240 and 325 8C.
silane flows. Clearly, the peak in growth rate occurs at a pressure of ;10 mTorr, and is a function of silane flow rates. The peak for a wire temperature of 2200 8C occurs at a pd product of ;125–150 mTorr cm, not at a pd product in the range of ;15 as postulated from Mollenbroek’s model. What this observation says is that at the high pd products, the film grows from both silyl and from higher silane radicals. In Fig. 3, we plot the subgap absorption spectra for two films, both deposited at a pressure of ;5 mTorr, but one at 240 8C, and one deposited at ;325 8C. Both films had approximately the same growth rate, ;7.5 Ays. The differences between the absorption spectra are dramatic. The film grown at higher T has all the characteristics of a good film, with low Urbach energy (;46 meV) and low subgap alpha (-1ycm). In the Table 1 below, we show the comparison between the electronic properties. Thus, we demonstrate that higher temperatures are necessary for producing films with better quality at higher growth rates. This observation is completely consistent with the data from ETP grown films from Eindhoven w8x. In Fig. 4, we show the subgap absorption of an ETP film, obtained from Delft University deposited at ;250 8C at ;7Ays. The similarity between this film and the hot wire films deposited under similar conditions is striking, with high a values at 1.2 eV. Table 1 Properties of hot wire films grown at two different temperatures
Fig. 2. Growth rate vs. pressure for two filament temperatures. Growth from hot wire only.
Growth temp.
225 8C
325 8C
Urbach energy Subgap alpha (1.2 eV) R ratio (SiH2 ySiH) AM1.5 photo-cond. (Sycm) Defect density at EF y(cm3 eV)
)60 meV ;10ycm 0.23
46 meV -1ycm 0.11
9=10y6
1.2=10y4
5=1016
1=1016
V.L. Dalal et al. / Thin Solid Films 430 (2003) 91–94
93
Fig. 4. Subgap absorption of an ETP film grown at Delft University.
The FTIR spectra corresponding to the two films, one at 225 8C, and one at 325 8C are shown in Figs. 5 and 6. Clearly, there is much more dihydride bond in the low T film compared to the high T film. Thus, at high growth rates, one needs a higher T to produce good films. 3.2. Hot wireqECR plasma In Fig. 7, we show the subgap a data for a film grown at 10 mT pressure using the hot wireqHe plasma at a low temperature of ;225 8C. The flow rates of He and silane were each 12 sccm. The growth rate decreased to 3.5 Ays when a plasma was present. Note that if the hot wire filament was not turned on, the growth rate dropped to 0.2 Ays, thus demonstrating that the combined hot wireqECR film was being grown primarily by a hot wire mechanism, with He simply providing ion bombardment during growth. The energy of the ions is in the 10 eV range. From Fig. 7, even at low temperatures, the subgap absorption is characteristic of a good a-Si film, with excellent Urbach energy (46 meV) and low subgap alpha (;1ycm). The properties remain good up to approximately 350 8C, after which they degrade.
Fig. 5. FTIR spectrum of a hot wire film grown at 225 8C.
Fig. 6. FTIR spectrum of a hot wire film grown at 325 8C.
The photo-conductivity of this film was 1.2=10y4 Sy cm. To compare with the data of Fig. 6, the FTIR spectrum corresponding to a hot wireqECR film also grown at ;325 8C is shown in Fig. 8, and it confirms that there is little dihydride bonding. The dihydride bonding is less than half of the hot wire alone film grown at 325 8C. When the experiments were repeated using hydrogen instead of helium as the plasma gas, the growth rate dropped to ;2 Ays, demonstrating that ionic H is an effective etchant of a-Si during growth. In the absence of the plasma, the growth rate only changed by ;10% when hydrogen dilution was present. 4. Discussion There are three important results arising from this work. 1. All high growth films, whether ETP or hot wire grown, require either higher temperature growth or
Fig. 7. Subgap absorption of a hot wireqECR film grown at 225 8C.
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V.L. Dalal et al. / Thin Solid Films 430 (2003) 91–94
5. Conclusions In conclusion, we have shown that by combining hot wire and low power ECR growth, one can reduce the deleterious dihydride concentration in lower temperature hot wire films. We have also shown that the electronic properties of the films grown at lower temperatures improve when the ion bombardment is present. The current results suggest that the primary limitation on the growth of high quality films is the removal of excess hydrogen, and that ion bombardment by both He and H appears to help in this removal. Acknowledgments Fig. 8. FTIR spectrum of a hot wireqECR film grown at 325 8C.
ion assist to improve quality. Ion assist reduces dihydride bonding, and also leads to better film properties. This observation strongly supports the hypothesis that the fundamental limitation to growth is not the diffusion of radicals, but the elimination of hydrogen from the film. Clearly, ion energies greater than ;3 eV would break most surface H bonds, and such low energy bombardment is shown to be beneficial. This is a new and important result, which shows the importance of ion-assisted H abstraction during the growth of the material. 2. While good quality a-Si films can be grown using hot wire alone by increasing the temperature, that result may not hold when growing a-(Si,Ge) films, since the higher temperatures required for H removal may result in a loss of H from Ge–H bonds and a consequent increase in defect densities. 3. The growth rate does not saturate at pd products of ;15–30 mTorr cm.
We thank Agnes Petit of Delft University for growing the ETP film. We also thank Max Noack for his many technical contributions. The work was partially supported by NREL. Matt Ring was partially supported by a fellowship from Catron Foundation. References w1x E. Molenbroek, A. Mahan, A. Gallagher, J. Appl. Phys. 82 (1909) 1997. w2x V.L. Dalal, Thin Solid Films 395 (2001) 173. w3x A. Gallagher, J. Appl. Phys. 63 (1988) 2406. w4x A. Matsuda, K. Nomoto, Y. Takeuchi, A. Suzuki, J. Perrin, Surf. Sci. 22 (1990) 50. w5x J. Perrin, in: G. Brunno, P. Capezzuto, A. Madan (Eds.), Plasma Deposition of a-Si materials, Academic, San Diego, 1995, p. 216. w6x V.L. Dalal, Curr. Opinion Solid State (2002), submitted for publication. w7x V.L. Dalal, S. Kaushal, R. Girvan, J. Non-Crystalline Solids 198–200 (1996) 1101. w8x W.M.M. Kessels, A.H.M. Smets, B.A. Korevaar, C.J. Andriaenssens, M.C.M. van de Sanden, D.C. Schramm, Proc. Mater. Res. Soc. 557 (1999) 25.
Properties of a-Si:H films grown using hot wire-ECR plasma techniques Vikram L. Dalal*, Paul Seberger, Matt Ring, Puneet Sharma Iowa State University, Department of Electrical and Computer Engineering, Ames, IA 50011, USA
Abstract We report on the growth and properties of a-Si:H films and nin layers prepared using combination of hot wire and ECR-plasma growth techniques. The films were prepared using both W and Ta hot wire filaments. A distinguishing feature of the reactor was the large spacing, 11 cm, of the filament from the substrate, thereby avoiding over-heating of the substrate. Films were grown at pressures from 2 to 50 mT, and the corresponding optical and electronic properties of the films were measured. The temperature of the substrate was varied between ;225 and 350 8C. It was found that the growth rates do not follow the maximum at a pressure-distance (pd) product ;15 mTorr cm postulated by Molenbroek et al.’s model. wJ. Appl. Phys. 82, 1909 (1997)x. It was also discovered that the properties of the hot wire films depend upon the pd product, and that the H bonding and electronic properties depend critically upon the growth rate, and on the substrate temperature. The properties of the hot wire films bear a remarkable similarity to the films deposited using expanding thermal plasma (ETP) techniques at similar temperatures. When the films were subjected to low power He plasma, the properties improved dramatically. It was also found that H ions are more efficient at etching a growing film than H radicals alone. The results show that the H bonding and electronic properties of a-Si:H films are determined primarily by the efficiency of H extraction, and that low energy ions have a useful role to play in this process. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Amorphous Si; ECR plasma; Hot wire deposition; Ion bombardment
1. Introduction The growth chemistry of a-Si:H is a complicated process w1–5x. We have shown earlier w2,6x that the fundamental limitation to growth comes from the removal of hydrogen from the lattice, and that this removal needs an energy of ;1.2–1.3 eV. The energy can come from temperature, or from ions. The ions can be reactive, such as H, or inert, such as He. The reactive ions can go deeper into the material and do etching during growth. The He ions remove the excess bonded H primarily from the surface. This model is in contrast to the standard model (the MGP model w3–5x) which states that the fundamental limitation comes from the diffusion of silyl radicals on the surface, and that excess H is abstracted by silyl groups. Clearly, it would be useful to do an experiment where one can independently study the effects of ions on growth, and also study the effects of temperature. In this paper, we report on such *Corresponding author. Present address: 1-1 Asahidai, Tatsunokuchi, Nomi, Ames, IA, USA; Tel.: q1-515-294-1077; fax: q1-515294-9854. E-mail address: [email protected] (V.L. Dalal).
a study, where the decomposition of silane is done by a hot wire system, with ions simply assisting in film formation as opposed to decomposition. 2. Experimental details The growth was done using a combined hot wireECR plasma reactor, shown schematically in Fig. 1. The plasma gas, He or H, was introduced upstream of the ECR cavity, and silane near the substrate. The ECR geometry was chosen because the plasma can be lit at very low pressures, ;1 mTorr, and the ion density can be carefully controlled w7x. The hot wire system used either W or Ta filaments, with most experiments being done with Ta filament. The area of the wire was 6.8 cm2, and the temperature was kept at ;2200 8C. The substrate was far from the filament, ;11 cm, thereby minimizing radiative heating of the substrate. This is an important point, because the substrate temperature plays a major role in the growth of the film. In most other system, the distance is ;1.5– 5 cm, and the substrates are much hotter than the nominal temperature. The silane flow was varied
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00079-8
92
V.L. Dalal et al. / Thin Solid Films 430 (2003) 91–94
Fig. 1. Schematic diagram of ECRqHot wire reactor. The plasma gas (He or H) are introduced upstream, approximately 30 cm from the hot wire, and the silane is introduced near the hot wire. The distance between the hot wire and substrate is 11 cm to reduce radiative heating of the substrate.
between 2 and 12 sccm. The growth was done at pressures between 2 and 40 mTorr, giving pd values between 23 and 450 mTorr cm. The volume of the chamber was ;16 l, giving a residence time of a few seconds at ;2 mTorr. The power in the ECR plasma was either 75 or 100 W. The resonance zone was ;30 cm from the substrate, thus achieving a remote plasma. The film properties measured were: optical absorption, H bonding, AM 1.5 photo and dark conductivity; activation energy and subgap absorption. Subgap absorption was measured using double-beam photo-conductivity techniques, and film thicknesses were all in the 1 mm range. In addition, a few nin samples were deposited on steel substrates, with both n layers deposited in a separate glow discharge reactor. These samples were subjected to space-charge-limited-current (SCLC) measurements to determine defect densities at the Fermi level. 3. Results 3.1. Hot wire growth alone In Fig. 2, we show the data on growth rates, using hot wire only, as a function of pressure for different
Fig. 3. Subgap absorption spectra for two hot wire films deposited at 240 and 325 8C.
silane flows. Clearly, the peak in growth rate occurs at a pressure of ;10 mTorr, and is a function of silane flow rates. The peak for a wire temperature of 2200 8C occurs at a pd product of ;125–150 mTorr cm, not at a pd product in the range of ;15 as postulated from Mollenbroek’s model. What this observation says is that at the high pd products, the film grows from both silyl and from higher silane radicals. In Fig. 3, we plot the subgap absorption spectra for two films, both deposited at a pressure of ;5 mTorr, but one at 240 8C, and one deposited at ;325 8C. Both films had approximately the same growth rate, ;7.5 Ays. The differences between the absorption spectra are dramatic. The film grown at higher T has all the characteristics of a good film, with low Urbach energy (;46 meV) and low subgap alpha (-1ycm). In the Table 1 below, we show the comparison between the electronic properties. Thus, we demonstrate that higher temperatures are necessary for producing films with better quality at higher growth rates. This observation is completely consistent with the data from ETP grown films from Eindhoven w8x. In Fig. 4, we show the subgap absorption of an ETP film, obtained from Delft University deposited at ;250 8C at ;7Ays. The similarity between this film and the hot wire films deposited under similar conditions is striking, with high a values at 1.2 eV. Table 1 Properties of hot wire films grown at two different temperatures
Fig. 2. Growth rate vs. pressure for two filament temperatures. Growth from hot wire only.
Growth temp.
225 8C
325 8C
Urbach energy Subgap alpha (1.2 eV) R ratio (SiH2 ySiH) AM1.5 photo-cond. (Sycm) Defect density at EF y(cm3 eV)
)60 meV ;10ycm 0.23
46 meV -1ycm 0.11
9=10y6
1.2=10y4
5=1016
1=1016
V.L. Dalal et al. / Thin Solid Films 430 (2003) 91–94
93
Fig. 4. Subgap absorption of an ETP film grown at Delft University.
The FTIR spectra corresponding to the two films, one at 225 8C, and one at 325 8C are shown in Figs. 5 and 6. Clearly, there is much more dihydride bond in the low T film compared to the high T film. Thus, at high growth rates, one needs a higher T to produce good films. 3.2. Hot wireqECR plasma In Fig. 7, we show the subgap a data for a film grown at 10 mT pressure using the hot wireqHe plasma at a low temperature of ;225 8C. The flow rates of He and silane were each 12 sccm. The growth rate decreased to 3.5 Ays when a plasma was present. Note that if the hot wire filament was not turned on, the growth rate dropped to 0.2 Ays, thus demonstrating that the combined hot wireqECR film was being grown primarily by a hot wire mechanism, with He simply providing ion bombardment during growth. The energy of the ions is in the 10 eV range. From Fig. 7, even at low temperatures, the subgap absorption is characteristic of a good a-Si film, with excellent Urbach energy (46 meV) and low subgap alpha (;1ycm). The properties remain good up to approximately 350 8C, after which they degrade.
Fig. 5. FTIR spectrum of a hot wire film grown at 225 8C.
Fig. 6. FTIR spectrum of a hot wire film grown at 325 8C.
The photo-conductivity of this film was 1.2=10y4 Sy cm. To compare with the data of Fig. 6, the FTIR spectrum corresponding to a hot wireqECR film also grown at ;325 8C is shown in Fig. 8, and it confirms that there is little dihydride bonding. The dihydride bonding is less than half of the hot wire alone film grown at 325 8C. When the experiments were repeated using hydrogen instead of helium as the plasma gas, the growth rate dropped to ;2 Ays, demonstrating that ionic H is an effective etchant of a-Si during growth. In the absence of the plasma, the growth rate only changed by ;10% when hydrogen dilution was present. 4. Discussion There are three important results arising from this work. 1. All high growth films, whether ETP or hot wire grown, require either higher temperature growth or
Fig. 7. Subgap absorption of a hot wireqECR film grown at 225 8C.
94
V.L. Dalal et al. / Thin Solid Films 430 (2003) 91–94
5. Conclusions In conclusion, we have shown that by combining hot wire and low power ECR growth, one can reduce the deleterious dihydride concentration in lower temperature hot wire films. We have also shown that the electronic properties of the films grown at lower temperatures improve when the ion bombardment is present. The current results suggest that the primary limitation on the growth of high quality films is the removal of excess hydrogen, and that ion bombardment by both He and H appears to help in this removal. Acknowledgments Fig. 8. FTIR spectrum of a hot wireqECR film grown at 325 8C.
ion assist to improve quality. Ion assist reduces dihydride bonding, and also leads to better film properties. This observation strongly supports the hypothesis that the fundamental limitation to growth is not the diffusion of radicals, but the elimination of hydrogen from the film. Clearly, ion energies greater than ;3 eV would break most surface H bonds, and such low energy bombardment is shown to be beneficial. This is a new and important result, which shows the importance of ion-assisted H abstraction during the growth of the material. 2. While good quality a-Si films can be grown using hot wire alone by increasing the temperature, that result may not hold when growing a-(Si,Ge) films, since the higher temperatures required for H removal may result in a loss of H from Ge–H bonds and a consequent increase in defect densities. 3. The growth rate does not saturate at pd products of ;15–30 mTorr cm.
We thank Agnes Petit of Delft University for growing the ETP film. We also thank Max Noack for his many technical contributions. The work was partially supported by NREL. Matt Ring was partially supported by a fellowship from Catron Foundation. References w1x E. Molenbroek, A. Mahan, A. Gallagher, J. Appl. Phys. 82 (1909) 1997. w2x V.L. Dalal, Thin Solid Films 395 (2001) 173. w3x A. Gallagher, J. Appl. Phys. 63 (1988) 2406. w4x A. Matsuda, K. Nomoto, Y. Takeuchi, A. Suzuki, J. Perrin, Surf. Sci. 22 (1990) 50. w5x J. Perrin, in: G. Brunno, P. Capezzuto, A. Madan (Eds.), Plasma Deposition of a-Si materials, Academic, San Diego, 1995, p. 216. w6x V.L. Dalal, Curr. Opinion Solid State (2002), submitted for publication. w7x V.L. Dalal, S. Kaushal, R. Girvan, J. Non-Crystalline Solids 198–200 (1996) 1101. w8x W.M.M. Kessels, A.H.M. Smets, B.A. Korevaar, C.J. Andriaenssens, M.C.M. van de Sanden, D.C. Schramm, Proc. Mater. Res. Soc. 557 (1999) 25.