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Infrared absorption strengths of ion-implanted hydrogenated amorphous silicon
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Thin Solid Films 516 (2008) 3383 – 3386 www.elsevier.com/locate/tsf
Infrared absorption strengths of ion-implanted hydrogenated amorphous silicon P. Danesh a,⁎, B. Pantchev a , B. Schmidt b a
b
Institute of Solid State Physics, Bulgarian Academy of Sciences, Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria Research Center Dresden-Rossendorf Inc., Institute of Ion Beam Physics and Materials Research, D-01314 Dresden, Germany Received 7 May 2007; received in revised form 24 August 2007; accepted 11 October 2007 Available online 18 October 2007
Abstract Silicon and hydrogen ion implantations have been used to affect the absorption of the infrared stretching modes in hydrogenated amorphous silicon (a-Si:H). Hydrogen ions have been implanted with ion energy of 16 keV and the doses in the range of 2.2 × 1014–7.2 × 1016 cm− 2. Silicon ion implantation has been carried out with the energy of 160 keV and the doses in the range of 9.5 × 1012 cm− 2–1.7 × 1015 cm− 2. The a-Si:H films have been prepared by plasma-enhanced chemical vapor deposition. Nuclear reaction analysis has been used for the determination of the hydrogen concentration in the as-deposited and ion-implanted samples. It has been established that the values of the absorption strengths of stretching modes of the isolated monohydrides, A2000, and clustered hydrogen forms, A2100, are not equal and remain constant for all ion implantation doses. A2100 has been considered as a weighted average of the absorption strengths of polyhydrides and clustered monohydrides, A2100,SiHx and A2100,(SiH)n. It has been established that the ion implantation does not induce any change in the ratio between polyhydrides and clustered monohydrides. It has been suggested that the absorption strengths do not vary when a post-deposition treatment of samples is associated with the introduction of structural defects in the amorphous silicon network. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous silicon; Infrared spectroscopy; Ion implantation; Absorption strengths
1. Introduction
calculated from the integrated absorbance of the vibrational modes:
The application fields of hydrogenated amorphous silicon (aSi:H) are related to its electrical and optical properties which are to a high degree determined by the amount and the bonding configuration of hydrogen present in the silicon network. Therefore, in order to control the material properties it is important to be able to determine precisely both the concentration and the bonding form of hydrogen. The infrared (IR) spectroscopy is a powerful tool to accomplish this task. A typical IR spectrum of a device grade a-Si:H has two hydrogen related absorption bands — the wagging–rocking mode at 640 cm− 1 and the stretching mode at about 2000–2100 cm− 1. The hydrogen concentration, N, can be
N ¼ A x Ix
⁎ Corresponding author. E-mail address: [email protected] (P. Danesh). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.10.068
where Iω = ∫(α / ω) dω, α is the absorption coefficient, ω is the frequency, and Aω is the absorption strength of the specific absorption mode. It has been shown that the absorption strength A640 of the wagging–rocking mode does not depend on the a-Si:H film preparation conditions and on the hydrogen content [1]. Therefore, although the value of A640 has not unambiguously been established, the total concentration of the silicon-bonded hydrogen is usually determined by taking the integral over this band. The absorption band of the stretching mode at 2000 cm− 1 corresponds to the amount of the isolated monohydrides, SiH. The absorption band at 2100 cm− 1 is associated with the clustered monohydrides, (SiH)n and polyhydrides, SiH2, SiH3 [2]. For these bands it is still under debate whether their absorption strengths are sample dependent and whether they have equal values.
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In the literature the absorption strengths A640, A2000, A2100 and their dependence on the film microstructure and hydrogen content have been evaluated by studying series of a-Si:H films deposited under different conditions or by different techniques [3–5]. However, there is a strong interplay between the microstructure, hydrogen content and hydrogen bonding, so that all three features vary simultaneously with the preparation parameters. Using such an approach it is hardly possible to separate the influences of the hydrogen concentration and the structural defects on the values of A2000 and A2100. Therefore, in the present work we have used samples prepared in one deposition run. Moreover, it has been shown that the quantitative analysis of the IR spectra is rather sensitive to the film thickness, particularly when it is below 1 μm. Even small variations in the film thickness of the different samples may lead to an essential scatter in the values of the absorption coefficient, making the following data processing ambiguous. To overcome such errors the analysis has been carried out using the IR spectra of the same sample before and after treatment. The properties of the samples have been varied using silicon and hydrogen ion implantations with different doses, as the variation in the hydrogen concentration in the samples has been as small, as 3 at.%. The aim of this study is to establish whether and to what extent the values of the absorption strength of the stretching bands are sample dependent, as the emphasis is on the effect of the structural defects in the amorphous silicon network.
Ion implantation experiments were carried out at room temperature. Hydrogen ions were implanted with ion energy of 16 keV and the doses in the range of 2.2 × 1014–7.2 × 1016 cm− 2. Silicon ion implantation was carried out with the energy of 160 keV and the doses between 9.5 × 10 12 cm − 2 and 1.7 × 1015 cm− 2. Some samples were implanted with both hydrogen and silicon ions in different sequences. Note, that the concentration of the implantation-induced defects in a-Si:H is in the range of 1020–1022 cm− 3, which is orders of magnitude higher than 1016–1017 cm− 3—the amount of dangling bonds in the as-deposited material.
2. Experimental procedure
N640 ¼ N2000 þ N2100
The a-Si:H films were prepared by plasma-enhanced chemical vapor deposition with 10% silane diluted in hydrogen. The deposition experiments were carried out at a substrate temperature of 270 °C, an applied power density of 130 mW/ cm2 and a gas pressure of 133 Pa. The film thickness was 380 nm. The films were deposited on the double-side-polished (100) crystalline silicon substrates. Nuclear reaction analysis was used for a direct determination of the hydrogen concentration. The measurements were carried out at the 5 MV tandem accelerator of the Research Center Dresden-Rossendorf using resonant nuclear reaction between the hydrogen atoms and accelerated 15N ions. The 15N ion beam current was 20 nA and the beam spot was with an area of 5 × 5 mm2. The γ-rays were detected by a 2″ × 4″ NaI (Tl) scintillation detector. The analysis was carried out taking 2.1 g cm− 3 for a-Si:H density. Our previous studies have shown that the measurements under these conditions do not lead to any radiation-induced change in the hydrogen concentration and its depth distribution beneath a 30 nm thick surface layer [6,7]. The IR transmission measurements were performed with a Nicolet Fourier transform infrared spectrometer. Plain Si substrates were used as reference. The absorption coefficient has been calculated from the transmission spectra using the method suggested by Brodsky et al. [8]. Special care has been taken to reduce the uncertainties induced by the interference due to the refractive index mismatch between the a-Si:H films and crystalline silicon substrates [3,9].
The values of A2000 and A2100 can be determined using the expression:
3. Experimental results and discussion The hydrogen concentrations established by nuclear reaction analysis and determined from the IR wagging–rocking mode coincide within an accuracy of 5%, when the value of 2.1 × 10 19 cm− 2 is taken for A640 [3–5]. The hydrogen concentration in the as-deposited a-Si:H films has been about 15.5 ± 0.5 at.%. The increase in the hydrogen concentration in the samples due to the hydrogen ion implantation has been detected by both methods, as the equality of the hydrogen concentrations holds for all implantation doses. This implies that the value of A640 has not been affected by the implantations and can be taken as a constant for our samples. The concentrations of the silicon-bonded hydrogen determined from the wagging–rocking band, N640, and the stretching bands, N2000, N2100, is equal:
A640 I640 ¼ A2000 I2000 þ A2100 I2100 ; which can be rewritten as a relationship between the ratios of the integrated absorbances: I2000 =I640 ¼ a b I2100 =I640 ;
ð1Þ
where a = A640/A2000 and b = A2100/A2000. If changing the integrated absorbances, Iω, the absorption strengths, Aω, remain constant, then the plot of Eq. (1) represents a straight line. Fig. 1 shows the effect of ion implantation on the IR spectra of a-Si:H films. The change in the wagging–rocking mode has been performed using hydrogen ion implantation. There is an increase in I640 corresponding to the increase in the hydrogen concentration in the material (Fig. 1a). The silicon ion implantation leads to a change in the areas of the stretching modes, as there is an increase in I2000 and a decrease in I2100 (Fig. 1b). Fig. 2 shows the effect of the silicon ion implantation doses on the variations of I2000 and I2100, relative to the whole area of the stretching band. The rearrangement of the absorption bands proceeds at implantation doses below 1 × 1014 cm− 2 and seems to stop at higher doses. The observed transformation can be caused by the variation in the hydrogen bonding, i.e. the conversion of the polyhydrides and clustered monohydrides
P. Danesh et al. / Thin Solid Films 516 (2008) 3383–3386
3385
Fig. 3. Plot of I2000/I640 versus I2100/I640. Solid line indicates the least square fit.
into isolated monohydrides. Another reason can be the change in the absorption strengths of the bands, which would lead to a non-linear plot of I2000/I640 versus I2100/I640. Fig. 3 shows the plot of I2000/I640 versus I2100/I640. In order to extend the variation range of Iω, various combinations of hydrogen and silicon ion implantations with different doses have been used. Note, that the used doses of hydrogen ions lead
to the variation in the hydrogen concentration in a narrow range of 3 at.%, so that any change in the absorption strengths should be related to the effect of the implantation-induced structural defects in the silicon network. The plot in Fig. 3 is a straight line, which implies that all absorption strengths are constant for the used set of samples. The slope of the line is about 1.61 and the intersection with the y-axis is at about 0.237. The latter gives the value of 8.9 × 1019 cm− 2 for A2000, which is in excellent accordance with (9.0 ± 1) × 1019 cm− 2 published by Langford et al. [3]. The value of slope indicates that the absorption strengths are different for the monohydride and clustered hydrogen absorption bands. The difference between A2000 and A2100 has been established by Langford et al. and Manfredotti et al. [3,4]. In contrast, equal values of A2000 and A2100 have been reported by Beyer and Abo Ghazala [5], Gracin et al. [10], and Daey Ouwens et al. [11,12]. The value of A2100 estimated from the plot in Fig. 3 is about 1.4 × 1020 cm− 2. The mean value of the ratios A640/A2000 and A640/A2100 is about 0.19. It is in agreement
Fig. 2. Effect of the implantation dose of silicon ions on the stretching bands of a-Si:H films. Solid lines are guides to the eye.
Fig. 4. Plot of (N2000 + N2100) / N640 versus displacements per atom. Solid line is guide to the eye.
Fig. 1. Effect of ion implantation on the IR absorption modes of a-Si:H films: (a) Wagging–rocking mode, hydrogen implantation dose — 5 × 1016 cm− 2; (b) Stretching modes, silicon ion implantation dose — 9.5 × 1013 cm− 2.
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with the reported ones, determined under assumption of equal A2000 and A2100 [5,10–12], suggesting that the equal values for A2000 and A2100 taken in those studies are in fact their average. Fig. 4 shows the plot of the ratio of the hydrogen concentrations, (N2000 + N2100) / N640, versus the number of the implantation-induced displacements per atom (dpa). The concentrations of hydrogen, N2000 and N2100, have been obtained, using the determined values of the absorption strengths for the stretching bands. As seen, the ratio is close to unity, i.e. hydrogen concentration estimated from the wagging–rocking mode is equal to the concentration obtained from the two stretching bands. It is apparent from this plot that the ion implantation-induced structural defects in the silicon network do not affect the absorption strengths even for the highest implantation doses, which cause the displacement of nearly all silicon and hydrogen atoms from their original positions. Therefore, the absorption strengths of the stretching modes of the silicon-bonded hydrogen do not depend on the post-preparation history of a-Si:H films. Note, that our results do not exclude the possibility the absorption strengths of the stretching bands to change with a strong variation in the hydrogen concentration. In this study, the change in the hydrogen concentration is in a narrow range of 3 at.%. A2100 is determined by the proportion between the concentrations of polyhydrides and clustered monohydrides and its value can be considered as a weighted average of their absorption strengths, A2100,SiHx and A2100,(SiH)n [3]. Hydrogen in these bonding configurations covers the internal surfaces of voids in the amorphous silicon network. The amount of voids is strongly related to the film deposition conditions and techniques and reflects the growth mechanism of a-Si:H. From this point of view it is plausible that A2100 does not have a unique value for a-Si:H prepared in different laboratories. The constancy of A2100 in our samples can be discussed using the following expression [3]: A2100 ¼ KA2100;ðSiHÞn þ A2100;SiHx =ð K þ 1Þ; where K is the proportionality coefficient between I2100,(SiH)n and I2100,SiHx. If K changes due to the ion implantation, then A2100,(SiH)n and A2100,SiHx should change in a rather sophisticated manner to keep A2100. If K does not change, a coordinated variation in both A2100,(SiH)n and A2100,SiHx must proceed, namely KΔA2100, (SiH)n = − ΔA2100,SiHx. But A2100,(SiH)n and A2100,SiHx are in principle independent physical values. Therefore the constant
value of A2100 suggests that the values of A2100,SiHx and A2100, (SiH)n do not depend on the amount of the structural defects in a-Si:H, i.e. they are constant similarly to A640 and A2000. Moreover, it appears that although the implantation-induced increase in the amount of isolated monohydrides is associated with the decrease in the absorption band at 2100 cm− 1, illustrated in Fig. 1(b), it proceeds without changing the ratio between polyhydrides and clustered monohydrides. 4. Conclusion In the present study, silicon and hydrogen ion implantations have been used to affect the absorption of the stretching modes at 2000 cm− 1 and 2100 cm− 1. It has been shown that the values of their absorption strengths are not equal, being 8.9 × 1019 cm− 2 and 1.4 × 1020 cm− 2 for A2000 and A2100, respectively. The obtained results suggest that the postdeposition treatment of a-Si:H does not affect these values, when it is not associated with essential variation of the hydrogen concentration. Considering A2100 as a weighted average of the absorption strengths of polyhydrides and clustered monohydrides, A2100,SiHx and A2100,(SiH)n, the constant value of A2100 suggests that A2100,SiHx and A2100,(SiH)n are constant, as well. References [1] H. Shanks, C.J. Fang, L. Ley, M. Cardona, F.J. Demond, S. Kalbitzer, Phys. Status Solidi, B Basic Res. 110 (1980) 43. [2] W.B. Pollard, G. Lucovsky, Phys. Rev. Lett. 26 (1982) 3172. [3] A.A. Langford, M.L. Fleet, B.P. Nelson, W.A. Lanford, N. Maley, Phys. Rev., B 45 (1992) 13367. [4] C. Manfredotti, F. Fizzotti, M. Boero, P. Pastorino, P. Polesello, E. Vittone, Phys. Rev., B 50 (1994) 18046. [5] W. Beyer, M.S. Abo Ghazala, Mater. Res. Soc. Symp. Proc. 507 (1998) 601. [6] P. Danesh, B. Pantchev, K. Antonova, E. Liarokapis, B. Schmidt, D. Grambole, J. Baran, J. Phys., D, Appl. Phys. 37 (2004) 249. [7] B. Pantchev, P. Danesh, E. Liarokapis, B. Schmidt, J. Schmidt, D. Grambole, Jpn. J. Appl. Phys. 43 (2004) 454. [8] M.H. Brodsky, M. Cardona, J.J. Cuomo, Phys. Rev., B 16 (1977) 3556. [9] N. Maley, Phys. Rev., B 46 (1992) 2078. [10] D. Gracin, U.V. Desnica, M. Ivanda, J. Non-Cryst. Solids 149 (1992) 257. [11] J. Daey Ouwens, R.E.I. Schropp, W.F. vand der Weg, Appl. Phys. Lett. 65 (1994) 204. [12] J. Daey Ouwens, R.E.I. Schropp, Mater. Res. Soc. Symp. Proc. 377 (1995) 419.
Thin Solid Films 516 (2008) 3383 – 3386 www.elsevier.com/locate/tsf
Infrared absorption strengths of ion-implanted hydrogenated amorphous silicon P. Danesh a,⁎, B. Pantchev a , B. Schmidt b a
b
Institute of Solid State Physics, Bulgarian Academy of Sciences, Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria Research Center Dresden-Rossendorf Inc., Institute of Ion Beam Physics and Materials Research, D-01314 Dresden, Germany Received 7 May 2007; received in revised form 24 August 2007; accepted 11 October 2007 Available online 18 October 2007
Abstract Silicon and hydrogen ion implantations have been used to affect the absorption of the infrared stretching modes in hydrogenated amorphous silicon (a-Si:H). Hydrogen ions have been implanted with ion energy of 16 keV and the doses in the range of 2.2 × 1014–7.2 × 1016 cm− 2. Silicon ion implantation has been carried out with the energy of 160 keV and the doses in the range of 9.5 × 1012 cm− 2–1.7 × 1015 cm− 2. The a-Si:H films have been prepared by plasma-enhanced chemical vapor deposition. Nuclear reaction analysis has been used for the determination of the hydrogen concentration in the as-deposited and ion-implanted samples. It has been established that the values of the absorption strengths of stretching modes of the isolated monohydrides, A2000, and clustered hydrogen forms, A2100, are not equal and remain constant for all ion implantation doses. A2100 has been considered as a weighted average of the absorption strengths of polyhydrides and clustered monohydrides, A2100,SiHx and A2100,(SiH)n. It has been established that the ion implantation does not induce any change in the ratio between polyhydrides and clustered monohydrides. It has been suggested that the absorption strengths do not vary when a post-deposition treatment of samples is associated with the introduction of structural defects in the amorphous silicon network. © 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous silicon; Infrared spectroscopy; Ion implantation; Absorption strengths
1. Introduction
calculated from the integrated absorbance of the vibrational modes:
The application fields of hydrogenated amorphous silicon (aSi:H) are related to its electrical and optical properties which are to a high degree determined by the amount and the bonding configuration of hydrogen present in the silicon network. Therefore, in order to control the material properties it is important to be able to determine precisely both the concentration and the bonding form of hydrogen. The infrared (IR) spectroscopy is a powerful tool to accomplish this task. A typical IR spectrum of a device grade a-Si:H has two hydrogen related absorption bands — the wagging–rocking mode at 640 cm− 1 and the stretching mode at about 2000–2100 cm− 1. The hydrogen concentration, N, can be
N ¼ A x Ix
⁎ Corresponding author. E-mail address: [email protected] (P. Danesh). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.10.068
where Iω = ∫(α / ω) dω, α is the absorption coefficient, ω is the frequency, and Aω is the absorption strength of the specific absorption mode. It has been shown that the absorption strength A640 of the wagging–rocking mode does not depend on the a-Si:H film preparation conditions and on the hydrogen content [1]. Therefore, although the value of A640 has not unambiguously been established, the total concentration of the silicon-bonded hydrogen is usually determined by taking the integral over this band. The absorption band of the stretching mode at 2000 cm− 1 corresponds to the amount of the isolated monohydrides, SiH. The absorption band at 2100 cm− 1 is associated with the clustered monohydrides, (SiH)n and polyhydrides, SiH2, SiH3 [2]. For these bands it is still under debate whether their absorption strengths are sample dependent and whether they have equal values.
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In the literature the absorption strengths A640, A2000, A2100 and their dependence on the film microstructure and hydrogen content have been evaluated by studying series of a-Si:H films deposited under different conditions or by different techniques [3–5]. However, there is a strong interplay between the microstructure, hydrogen content and hydrogen bonding, so that all three features vary simultaneously with the preparation parameters. Using such an approach it is hardly possible to separate the influences of the hydrogen concentration and the structural defects on the values of A2000 and A2100. Therefore, in the present work we have used samples prepared in one deposition run. Moreover, it has been shown that the quantitative analysis of the IR spectra is rather sensitive to the film thickness, particularly when it is below 1 μm. Even small variations in the film thickness of the different samples may lead to an essential scatter in the values of the absorption coefficient, making the following data processing ambiguous. To overcome such errors the analysis has been carried out using the IR spectra of the same sample before and after treatment. The properties of the samples have been varied using silicon and hydrogen ion implantations with different doses, as the variation in the hydrogen concentration in the samples has been as small, as 3 at.%. The aim of this study is to establish whether and to what extent the values of the absorption strength of the stretching bands are sample dependent, as the emphasis is on the effect of the structural defects in the amorphous silicon network.
Ion implantation experiments were carried out at room temperature. Hydrogen ions were implanted with ion energy of 16 keV and the doses in the range of 2.2 × 1014–7.2 × 1016 cm− 2. Silicon ion implantation was carried out with the energy of 160 keV and the doses between 9.5 × 10 12 cm − 2 and 1.7 × 1015 cm− 2. Some samples were implanted with both hydrogen and silicon ions in different sequences. Note, that the concentration of the implantation-induced defects in a-Si:H is in the range of 1020–1022 cm− 3, which is orders of magnitude higher than 1016–1017 cm− 3—the amount of dangling bonds in the as-deposited material.
2. Experimental procedure
N640 ¼ N2000 þ N2100
The a-Si:H films were prepared by plasma-enhanced chemical vapor deposition with 10% silane diluted in hydrogen. The deposition experiments were carried out at a substrate temperature of 270 °C, an applied power density of 130 mW/ cm2 and a gas pressure of 133 Pa. The film thickness was 380 nm. The films were deposited on the double-side-polished (100) crystalline silicon substrates. Nuclear reaction analysis was used for a direct determination of the hydrogen concentration. The measurements were carried out at the 5 MV tandem accelerator of the Research Center Dresden-Rossendorf using resonant nuclear reaction between the hydrogen atoms and accelerated 15N ions. The 15N ion beam current was 20 nA and the beam spot was with an area of 5 × 5 mm2. The γ-rays were detected by a 2″ × 4″ NaI (Tl) scintillation detector. The analysis was carried out taking 2.1 g cm− 3 for a-Si:H density. Our previous studies have shown that the measurements under these conditions do not lead to any radiation-induced change in the hydrogen concentration and its depth distribution beneath a 30 nm thick surface layer [6,7]. The IR transmission measurements were performed with a Nicolet Fourier transform infrared spectrometer. Plain Si substrates were used as reference. The absorption coefficient has been calculated from the transmission spectra using the method suggested by Brodsky et al. [8]. Special care has been taken to reduce the uncertainties induced by the interference due to the refractive index mismatch between the a-Si:H films and crystalline silicon substrates [3,9].
The values of A2000 and A2100 can be determined using the expression:
3. Experimental results and discussion The hydrogen concentrations established by nuclear reaction analysis and determined from the IR wagging–rocking mode coincide within an accuracy of 5%, when the value of 2.1 × 10 19 cm− 2 is taken for A640 [3–5]. The hydrogen concentration in the as-deposited a-Si:H films has been about 15.5 ± 0.5 at.%. The increase in the hydrogen concentration in the samples due to the hydrogen ion implantation has been detected by both methods, as the equality of the hydrogen concentrations holds for all implantation doses. This implies that the value of A640 has not been affected by the implantations and can be taken as a constant for our samples. The concentrations of the silicon-bonded hydrogen determined from the wagging–rocking band, N640, and the stretching bands, N2000, N2100, is equal:
A640 I640 ¼ A2000 I2000 þ A2100 I2100 ; which can be rewritten as a relationship between the ratios of the integrated absorbances: I2000 =I640 ¼ a b I2100 =I640 ;
ð1Þ
where a = A640/A2000 and b = A2100/A2000. If changing the integrated absorbances, Iω, the absorption strengths, Aω, remain constant, then the plot of Eq. (1) represents a straight line. Fig. 1 shows the effect of ion implantation on the IR spectra of a-Si:H films. The change in the wagging–rocking mode has been performed using hydrogen ion implantation. There is an increase in I640 corresponding to the increase in the hydrogen concentration in the material (Fig. 1a). The silicon ion implantation leads to a change in the areas of the stretching modes, as there is an increase in I2000 and a decrease in I2100 (Fig. 1b). Fig. 2 shows the effect of the silicon ion implantation doses on the variations of I2000 and I2100, relative to the whole area of the stretching band. The rearrangement of the absorption bands proceeds at implantation doses below 1 × 1014 cm− 2 and seems to stop at higher doses. The observed transformation can be caused by the variation in the hydrogen bonding, i.e. the conversion of the polyhydrides and clustered monohydrides
P. Danesh et al. / Thin Solid Films 516 (2008) 3383–3386
3385
Fig. 3. Plot of I2000/I640 versus I2100/I640. Solid line indicates the least square fit.
into isolated monohydrides. Another reason can be the change in the absorption strengths of the bands, which would lead to a non-linear plot of I2000/I640 versus I2100/I640. Fig. 3 shows the plot of I2000/I640 versus I2100/I640. In order to extend the variation range of Iω, various combinations of hydrogen and silicon ion implantations with different doses have been used. Note, that the used doses of hydrogen ions lead
to the variation in the hydrogen concentration in a narrow range of 3 at.%, so that any change in the absorption strengths should be related to the effect of the implantation-induced structural defects in the silicon network. The plot in Fig. 3 is a straight line, which implies that all absorption strengths are constant for the used set of samples. The slope of the line is about 1.61 and the intersection with the y-axis is at about 0.237. The latter gives the value of 8.9 × 1019 cm− 2 for A2000, which is in excellent accordance with (9.0 ± 1) × 1019 cm− 2 published by Langford et al. [3]. The value of slope indicates that the absorption strengths are different for the monohydride and clustered hydrogen absorption bands. The difference between A2000 and A2100 has been established by Langford et al. and Manfredotti et al. [3,4]. In contrast, equal values of A2000 and A2100 have been reported by Beyer and Abo Ghazala [5], Gracin et al. [10], and Daey Ouwens et al. [11,12]. The value of A2100 estimated from the plot in Fig. 3 is about 1.4 × 1020 cm− 2. The mean value of the ratios A640/A2000 and A640/A2100 is about 0.19. It is in agreement
Fig. 2. Effect of the implantation dose of silicon ions on the stretching bands of a-Si:H films. Solid lines are guides to the eye.
Fig. 4. Plot of (N2000 + N2100) / N640 versus displacements per atom. Solid line is guide to the eye.
Fig. 1. Effect of ion implantation on the IR absorption modes of a-Si:H films: (a) Wagging–rocking mode, hydrogen implantation dose — 5 × 1016 cm− 2; (b) Stretching modes, silicon ion implantation dose — 9.5 × 1013 cm− 2.
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P. Danesh et al. / Thin Solid Films 516 (2008) 3383–3386
with the reported ones, determined under assumption of equal A2000 and A2100 [5,10–12], suggesting that the equal values for A2000 and A2100 taken in those studies are in fact their average. Fig. 4 shows the plot of the ratio of the hydrogen concentrations, (N2000 + N2100) / N640, versus the number of the implantation-induced displacements per atom (dpa). The concentrations of hydrogen, N2000 and N2100, have been obtained, using the determined values of the absorption strengths for the stretching bands. As seen, the ratio is close to unity, i.e. hydrogen concentration estimated from the wagging–rocking mode is equal to the concentration obtained from the two stretching bands. It is apparent from this plot that the ion implantation-induced structural defects in the silicon network do not affect the absorption strengths even for the highest implantation doses, which cause the displacement of nearly all silicon and hydrogen atoms from their original positions. Therefore, the absorption strengths of the stretching modes of the silicon-bonded hydrogen do not depend on the post-preparation history of a-Si:H films. Note, that our results do not exclude the possibility the absorption strengths of the stretching bands to change with a strong variation in the hydrogen concentration. In this study, the change in the hydrogen concentration is in a narrow range of 3 at.%. A2100 is determined by the proportion between the concentrations of polyhydrides and clustered monohydrides and its value can be considered as a weighted average of their absorption strengths, A2100,SiHx and A2100,(SiH)n [3]. Hydrogen in these bonding configurations covers the internal surfaces of voids in the amorphous silicon network. The amount of voids is strongly related to the film deposition conditions and techniques and reflects the growth mechanism of a-Si:H. From this point of view it is plausible that A2100 does not have a unique value for a-Si:H prepared in different laboratories. The constancy of A2100 in our samples can be discussed using the following expression [3]: A2100 ¼ KA2100;ðSiHÞn þ A2100;SiHx =ð K þ 1Þ; where K is the proportionality coefficient between I2100,(SiH)n and I2100,SiHx. If K changes due to the ion implantation, then A2100,(SiH)n and A2100,SiHx should change in a rather sophisticated manner to keep A2100. If K does not change, a coordinated variation in both A2100,(SiH)n and A2100,SiHx must proceed, namely KΔA2100, (SiH)n = − ΔA2100,SiHx. But A2100,(SiH)n and A2100,SiHx are in principle independent physical values. Therefore the constant
value of A2100 suggests that the values of A2100,SiHx and A2100, (SiH)n do not depend on the amount of the structural defects in a-Si:H, i.e. they are constant similarly to A640 and A2000. Moreover, it appears that although the implantation-induced increase in the amount of isolated monohydrides is associated with the decrease in the absorption band at 2100 cm− 1, illustrated in Fig. 1(b), it proceeds without changing the ratio between polyhydrides and clustered monohydrides. 4. Conclusion In the present study, silicon and hydrogen ion implantations have been used to affect the absorption of the stretching modes at 2000 cm− 1 and 2100 cm− 1. It has been shown that the values of their absorption strengths are not equal, being 8.9 × 1019 cm− 2 and 1.4 × 1020 cm− 2 for A2000 and A2100, respectively. The obtained results suggest that the postdeposition treatment of a-Si:H does not affect these values, when it is not associated with essential variation of the hydrogen concentration. Considering A2100 as a weighted average of the absorption strengths of polyhydrides and clustered monohydrides, A2100,SiHx and A2100,(SiH)n, the constant value of A2100 suggests that A2100,SiHx and A2100,(SiH)n are constant, as well. References [1] H. Shanks, C.J. Fang, L. Ley, M. Cardona, F.J. Demond, S. Kalbitzer, Phys. Status Solidi, B Basic Res. 110 (1980) 43. [2] W.B. Pollard, G. Lucovsky, Phys. Rev. Lett. 26 (1982) 3172. [3] A.A. Langford, M.L. Fleet, B.P. Nelson, W.A. Lanford, N. Maley, Phys. Rev., B 45 (1992) 13367. [4] C. Manfredotti, F. Fizzotti, M. Boero, P. Pastorino, P. Polesello, E. Vittone, Phys. Rev., B 50 (1994) 18046. [5] W. Beyer, M.S. Abo Ghazala, Mater. Res. Soc. Symp. Proc. 507 (1998) 601. [6] P. Danesh, B. Pantchev, K. Antonova, E. Liarokapis, B. Schmidt, D. Grambole, J. Baran, J. Phys., D, Appl. Phys. 37 (2004) 249. [7] B. Pantchev, P. Danesh, E. Liarokapis, B. Schmidt, J. Schmidt, D. Grambole, Jpn. J. Appl. Phys. 43 (2004) 454. [8] M.H. Brodsky, M. Cardona, J.J. Cuomo, Phys. Rev., B 16 (1977) 3556. [9] N. Maley, Phys. Rev., B 46 (1992) 2078. [10] D. Gracin, U.V. Desnica, M. Ivanda, J. Non-Cryst. Solids 149 (1992) 257. [11] J. Daey Ouwens, R.E.I. Schropp, W.F. vand der Weg, Appl. Phys. Lett. 65 (1994) 204. [12] J. Daey Ouwens, R.E.I. Schropp, Mater. Res. Soc. Symp. Proc. 377 (1995) 419.