- Email: [email protected]
Electron spin resonance studies of microcrystalline and amorphous silicon irradiated with high energy electrons
Journal of Non-Crystalline Solids 352 (2006) 1020–1023 www.elsevier.com/locate/jnoncrysol
Electron spin resonance studies of microcrystalline and amorphous silicon irradiated with high energy electrons Oleksandr Astakhov *, Friedhelm Finger, Reinhard Carius, Andreas Lambertz, Yuri Petrusenko, Valery Borysenko, Dmitriy Barankov Forschungszentrum Julich, Institute of Photovoltaics, Leo-Brandt-Str., 52425 Julich, Germany Available online 29 March 2006
Abstract Paramagnetic defects in lc-Si:H and a-Si:H with various structure compositions were investigated by electron spin resonance (ESR). The defect density was varied by high energy electron bombardment and subsequent annealing. The spin density increases by up to 3 orders of magnitude. In most cases the initial spin density can be restored upon annealing at 160 C. 2006 Elsevier B.V. All rights reserved. PACS: 61.72.Qq; 61.80.Fe; 61.82.Fk; 76.30. v Keywords: Silicon; Defects; Electron spin resonance
1. Introduction A versatile tool to investigate electronic defects in amorphous and microcrystalline silicon (a-Si:H, lc-Si:H) is electron spin resonance (ESR). In a-Si:H an ESR line is observed with a g-value of 2.0055 and a line width of DHpp = 7 G. Identification of this resonance with Si dangling bonds (db) is widely accepted. ESR studies on lc-Si:H do show signals at g = 2.0040 2.0053, significantly shifted from the db g-value in a-Si:H and which are considerably affected by preparation conditions, structure composition and post-preparation treatment [1–13]. The density of these states is closely related to the electronic quality of the material [3]. A number of experimental results indicate the existence of two resonances in lc-Si:H at around g = 2.0043 and g = 2.0052 with a line width of about 4–8 G, which have been suggested to arise from different defects in different microscopic environments giving the observed asymmetric ESR signal of lc-Si:H. Alternatively it has been proposed that the signal is due to a Pb-like *
Corresponding author. Tel.: +49 2461 613923; fax: +49 2461 613735. E-mail address: [email protected] (O. Astakhov).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.060
defect with a pronounced axial symmetric g-tensor [11–13]. So far no conclusive interpretation exists. We investigated the paramagnetic defects in lc-Si:H with various structure compositions where the defect density was varied by high energy electron bombardment and subsequent annealing [14–21]. The idea is to individually enhance resonances in such a way that a de-convolution of the superposition signal becomes easier with better distinguishable ESR line parameters. 2. Experimental procedure Samples were prepared at the Institute of Photovoltaics (Forschungszentrum Ju¨lich Germany) with VHF (95 MHz)-PECVD at 200 C substrate temperature. Different silane-to-hydrogen mixtures (SC) were used to prepare material with different structure compositions. The films were deposited onto molybdenum foil, which was bent after deposition to peel off the film as a powder. The powder was sealed in ESR quartz tubes in 0.5 bar helium atmosphere. The samples were irradiated at the National Science Center – Kharkov Institute of Physics and Technology,
O. Astakhov et al. / Journal of Non-Crystalline Solids 352 (2006) 1020–1023
1021
Ukraine. The samples were irradiated with 2 MeV electrons at 100 K with three doses: 1.1 · 1017, 3.3 · 1017, and 3.3 · 1018 e cm 2. A van-de-Graaf type electron accelerator was used as an electron beam source with current density of 5 lA cm 2. Irradiation was done in a liquid nitrogen (LN2) cooled cryostat. To avoid annealing of the accumulated defects, all sample handling and shipment was performed in a LN2-cooled environment. ESR was measured at 40 K in X-band. To be independent for the moment from ambiguities of line de-convolution, the zero crossing of the first derivative of the ESR line is used for the calculation of the g-value even in the case of possibly overlapping lines. The spin density and the g-value were obtained from comparison with reference measurements of the sputtered silicon sample. 3. Results In Fig. 1 the g-values of a-Si:H and lc-Si:H material prepared over the whole range of silane concentration are shown. Full symbols indicate the samples which exhibit microcrystalline features in the Raman spectra, and open symbols correspond to the samples which are ‘Ramanamorphous’ i.e., show no microcrystalline peak. The g-value has an evident dependence on silane concentration. Most interesting is a gradual transition of the g-value from the amorphous material prepared at SC = 100%, to material prepared at SC = 9% which shows no traces of crystallinity in Raman spectra. Four samples for irradiation were selected to cover the range from highly crystalline through transition to completely amorphous material. Fig. 2 shows the total spin density versus treatment steps for the samples irradiated with the highest dose of 3.3 · 1018 e cm 2. The spin density increases by up to 3 orders of magnitude. In most cases the initial spin density can be restored upon annealing at 160 C.
Fig. 2. Spin density of samples deposited at different SC versus treatment steps.
In Fig. 3 the ESR spectra of the four samples in the as-deposited state, after irradiation and after the final annealing step are shown. The spectra are normalized to the same amplitude in order to emphasize the changes in the line shape. The spectra are representative and the results are confirmed by the irradiations at lower dose as well as by repeated irradiations. As important results it can be stated: (1) the signal shape after irradiation depends on the material structure, (2) the electron bombardment creates paramagnetic defects with ESR line parameters close to those already present in the material, (3) additional features observed occasionally at the final annealing step in lc-Si:H (Fig. 3(a) and (b)) are in agreement with earlier reports [4,10], (4) the changes upon irradiation in the lc-Si:H material are not compatible with a simple increase of just one resonance, neither at g = 2.0043 nor at g = 2.0052, (5) fully amorphous material shows a distinct line shape variation on the high magnetic field side upon irradiation (Fig. 3(d)). This latter effect leads to increase of the peak-to-peak line width (DHpp) and consequently to negative g-value shift. The dependence of the resonance g-value and DHpp on the spin density for the material prepared with SC = 100% and irradiated with different doses is shown in Fig. 4. The different spin density points of each sample correspond to annealing steps. The g-value, the line width as well as the spin density can be restored to initial values by annealing. Repetition of the irradiation-annealing sequence on the same samples reproduces the results. 4. Discussion
Fig. 1. g-value of ESR resonance line as a function of silane concentration during samples preparation. Note the logarithmic SC axis. Full symbols: crystalline peak in Raman; open symbols: no crystalline Raman peak.
The results show that the approach for a reproducible and reversible increase in the defect density by high electron energy bombardment is very successful. In particular the sample transfer, transport and handling in a LN2
1022
O. Astakhov et al. / Journal of Non-Crystalline Solids 352 (2006) 1020–1023
Fig. 4. g-value and peak-to-peak line width versus spin density of a-Si:H deposited with SC = 100%.
Fig. 3. Line shapes of normalized ESR spectra of lc-Si:H and a-Si:H after different treatment: (a) lc-Si:H, SC = 3%; (b) lc-Si:H, SC = 6%; (c) a-Si:H SC=9%; (d) a-Si:H, SC = 100%. Dose = 3.3 · 1018 e cm 2.
which was observed after treatment in oxygen-containing atmosphere. The latter of course is surprising because samples in the present study were kept in a high purity helium atmosphere. The line broadening which results in an apparent g-value shift in a-Si:H has been, to our knowledge, not been reported before. We propose that it is a result of spin-spin interaction, which will influence the shape of the inhomogenously broadened powder spectrum. Evaluation with signal deconvolution and simulation are under way. 5. Conclusions
atmosphere guarantees to maintain and observe very high defect density after irradiation. The fact that resonances after irradiation appear in very high density only where they were also found in the asdeposited state, makes us confident that the irradiation only creates those ‘intrinsic’ defects, which are relevant for the electronic properties in the material, which by this method can be varied reversibly over 3 orders of magnitude. However, the hope that irradiation will individually enhance resonances in such a way that their line parameters can be obtained from signal deconvolution, was not fulfilled. The signal shape after irradiation is complex and depends on the material structure, crystalline content and silane concentration during deposition. Additional features after high temperature (160 C) annealing have been observed in earlier studies [4]. This includes appearance of the conduction electron (CE) resonance in highly crystalline material and occasionally an irreversible resonance,
An experiment for high energy electron irradiation of a-Si:H and lc-Si:H with sample treatment and transfer in LN2 atmosphere was successfully established between the National Science Center Kharkov Institute of Physics and Technology, Ukraine and the Forschungszentrum Ju¨lich, Germany. Over the whole material composition range the irradiation creates in a reversible and reproducible manner only defects with ESR g-values close to where they were also found in as-deposited material. Acknowledgements We thank Professor I. Neklyudov, director general of NSC KIPT (Kharkov, Ukraine) for his support of irradiation procedure and samples shipping in Ukraine, customs broker M. Aleksankina of STCU (Ukraine) for easy and fast
O. Astakhov et al. / Journal of Non-Crystalline Solids 352 (2006) 1020–1023
customs clearing of samples, and K. Petter from HMI Berlin, Germany for high accuracy ESR sample g-value calibration. The work is supported by European Community via Science and Technology Center in Ukraine (Project #655A). References [1] Mu¨ller, F. Finger, R. Carius, H. Wagner, Phys. Rev. B 60 (1999) 11666. [2] F. Finger, A.L. Baia Neto, R. Carius, T. Dylla, S. Klein, Phys. Status Solidi C 1 (2004) 1248. [3] F. Finger, S. Klein, T. Dylla, A.L. Baia Neto, O. Vetterl, R. Carius, MRS Symp. Proc. 715 (2002) 123. [4] F. Finger, R. Carius, T. Dylla, S. Klein, S. Okur, M. Gu¨nes, IEE Proc. Circ. Dev. Sys. 150 (2003) 300. [5] K. Lips, P. Kanschat, W. Fuhs, Sol. Energy Mater. Sol. Cells 78 (2003) 513. [6] K. Lips, P. Kanschat, D. Will, C. Lerner, W. Fuhs, J. Non-Cryst. Sol. 227–230 (1998) 1021. [7] T. Ehara, Appl. Surf. Sci. 113&114 (1997) 126. [8] T. Ehara, J. Appl. Phys. 88 (2000) 1698.
1023
[9] J. Rath, Sol. Energy Mater. Sol. Cells 76 (2003) 431. [10] D. Will, C. Lerner, W. Fuhs, K. Lips, MRS Symp. Proc. 467 (1997) 361. [11] M. Kondo, S. Yawasaki, A. Matsuda, J. Non-Cryst. Solid 266–269 (2000) 544. [12] M.M. de Lima, P.C. Taylor, S. Morrison, A. LeGeune, F.C. Marques, Phys. Rev. B 65 (2002) 235. [13] K. Morigaki, H. Hikita, M. Yamaguchi, Y. Fujita, Mater. Sci. Eng. B 103 (2003) 37. [14] F. Finger, W. Fuhs, R. Carius, Philos. Mag. Lett. 57 (1988) 235. [15] R.A. Street, D. Biegelsen, J. Stuke, Philos. Mag. B 40 (1979) 451. [16] R.V. Navkhandewala, K.L. Narasimhan, S. Guha, Phys. Rev. B 24 (1981) 7443. [17] H. Derch, A. Skumanich, N.M. Amer, Phys. Rev. B 31 (1985) 6913. [18] U. Voget-Grote, W. Kummerle, R. Fischer, J. Stuke, Philos. Mag. B 41 (2) (1980) 127. [19] C. Malten, F. Finger, P. Hapke, T. Kulessa, C. Walker, R. Carius, R. Flu¨ckiger, H. Wagner, Mat. Res. Soc. Symp. Proc. 358 (1995) 757. [20] W. Bronner, M. Mehring, R. Bru¨ggemann, Phys. Rev. 65 (2002) 165212. [21] R. Bru¨ggemann, W. Bronner, M. Mehring, Solid State Commun. 119 (2001) 23.
Electron spin resonance studies of microcrystalline and amorphous silicon irradiated with high energy electrons Oleksandr Astakhov *, Friedhelm Finger, Reinhard Carius, Andreas Lambertz, Yuri Petrusenko, Valery Borysenko, Dmitriy Barankov Forschungszentrum Julich, Institute of Photovoltaics, Leo-Brandt-Str., 52425 Julich, Germany Available online 29 March 2006
Abstract Paramagnetic defects in lc-Si:H and a-Si:H with various structure compositions were investigated by electron spin resonance (ESR). The defect density was varied by high energy electron bombardment and subsequent annealing. The spin density increases by up to 3 orders of magnitude. In most cases the initial spin density can be restored upon annealing at 160 C. 2006 Elsevier B.V. All rights reserved. PACS: 61.72.Qq; 61.80.Fe; 61.82.Fk; 76.30. v Keywords: Silicon; Defects; Electron spin resonance
1. Introduction A versatile tool to investigate electronic defects in amorphous and microcrystalline silicon (a-Si:H, lc-Si:H) is electron spin resonance (ESR). In a-Si:H an ESR line is observed with a g-value of 2.0055 and a line width of DHpp = 7 G. Identification of this resonance with Si dangling bonds (db) is widely accepted. ESR studies on lc-Si:H do show signals at g = 2.0040 2.0053, significantly shifted from the db g-value in a-Si:H and which are considerably affected by preparation conditions, structure composition and post-preparation treatment [1–13]. The density of these states is closely related to the electronic quality of the material [3]. A number of experimental results indicate the existence of two resonances in lc-Si:H at around g = 2.0043 and g = 2.0052 with a line width of about 4–8 G, which have been suggested to arise from different defects in different microscopic environments giving the observed asymmetric ESR signal of lc-Si:H. Alternatively it has been proposed that the signal is due to a Pb-like *
Corresponding author. Tel.: +49 2461 613923; fax: +49 2461 613735. E-mail address: [email protected] (O. Astakhov).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.01.060
defect with a pronounced axial symmetric g-tensor [11–13]. So far no conclusive interpretation exists. We investigated the paramagnetic defects in lc-Si:H with various structure compositions where the defect density was varied by high energy electron bombardment and subsequent annealing [14–21]. The idea is to individually enhance resonances in such a way that a de-convolution of the superposition signal becomes easier with better distinguishable ESR line parameters. 2. Experimental procedure Samples were prepared at the Institute of Photovoltaics (Forschungszentrum Ju¨lich Germany) with VHF (95 MHz)-PECVD at 200 C substrate temperature. Different silane-to-hydrogen mixtures (SC) were used to prepare material with different structure compositions. The films were deposited onto molybdenum foil, which was bent after deposition to peel off the film as a powder. The powder was sealed in ESR quartz tubes in 0.5 bar helium atmosphere. The samples were irradiated at the National Science Center – Kharkov Institute of Physics and Technology,
O. Astakhov et al. / Journal of Non-Crystalline Solids 352 (2006) 1020–1023
1021
Ukraine. The samples were irradiated with 2 MeV electrons at 100 K with three doses: 1.1 · 1017, 3.3 · 1017, and 3.3 · 1018 e cm 2. A van-de-Graaf type electron accelerator was used as an electron beam source with current density of 5 lA cm 2. Irradiation was done in a liquid nitrogen (LN2) cooled cryostat. To avoid annealing of the accumulated defects, all sample handling and shipment was performed in a LN2-cooled environment. ESR was measured at 40 K in X-band. To be independent for the moment from ambiguities of line de-convolution, the zero crossing of the first derivative of the ESR line is used for the calculation of the g-value even in the case of possibly overlapping lines. The spin density and the g-value were obtained from comparison with reference measurements of the sputtered silicon sample. 3. Results In Fig. 1 the g-values of a-Si:H and lc-Si:H material prepared over the whole range of silane concentration are shown. Full symbols indicate the samples which exhibit microcrystalline features in the Raman spectra, and open symbols correspond to the samples which are ‘Ramanamorphous’ i.e., show no microcrystalline peak. The g-value has an evident dependence on silane concentration. Most interesting is a gradual transition of the g-value from the amorphous material prepared at SC = 100%, to material prepared at SC = 9% which shows no traces of crystallinity in Raman spectra. Four samples for irradiation were selected to cover the range from highly crystalline through transition to completely amorphous material. Fig. 2 shows the total spin density versus treatment steps for the samples irradiated with the highest dose of 3.3 · 1018 e cm 2. The spin density increases by up to 3 orders of magnitude. In most cases the initial spin density can be restored upon annealing at 160 C.
Fig. 2. Spin density of samples deposited at different SC versus treatment steps.
In Fig. 3 the ESR spectra of the four samples in the as-deposited state, after irradiation and after the final annealing step are shown. The spectra are normalized to the same amplitude in order to emphasize the changes in the line shape. The spectra are representative and the results are confirmed by the irradiations at lower dose as well as by repeated irradiations. As important results it can be stated: (1) the signal shape after irradiation depends on the material structure, (2) the electron bombardment creates paramagnetic defects with ESR line parameters close to those already present in the material, (3) additional features observed occasionally at the final annealing step in lc-Si:H (Fig. 3(a) and (b)) are in agreement with earlier reports [4,10], (4) the changes upon irradiation in the lc-Si:H material are not compatible with a simple increase of just one resonance, neither at g = 2.0043 nor at g = 2.0052, (5) fully amorphous material shows a distinct line shape variation on the high magnetic field side upon irradiation (Fig. 3(d)). This latter effect leads to increase of the peak-to-peak line width (DHpp) and consequently to negative g-value shift. The dependence of the resonance g-value and DHpp on the spin density for the material prepared with SC = 100% and irradiated with different doses is shown in Fig. 4. The different spin density points of each sample correspond to annealing steps. The g-value, the line width as well as the spin density can be restored to initial values by annealing. Repetition of the irradiation-annealing sequence on the same samples reproduces the results. 4. Discussion
Fig. 1. g-value of ESR resonance line as a function of silane concentration during samples preparation. Note the logarithmic SC axis. Full symbols: crystalline peak in Raman; open symbols: no crystalline Raman peak.
The results show that the approach for a reproducible and reversible increase in the defect density by high electron energy bombardment is very successful. In particular the sample transfer, transport and handling in a LN2
1022
O. Astakhov et al. / Journal of Non-Crystalline Solids 352 (2006) 1020–1023
Fig. 4. g-value and peak-to-peak line width versus spin density of a-Si:H deposited with SC = 100%.
Fig. 3. Line shapes of normalized ESR spectra of lc-Si:H and a-Si:H after different treatment: (a) lc-Si:H, SC = 3%; (b) lc-Si:H, SC = 6%; (c) a-Si:H SC=9%; (d) a-Si:H, SC = 100%. Dose = 3.3 · 1018 e cm 2.
which was observed after treatment in oxygen-containing atmosphere. The latter of course is surprising because samples in the present study were kept in a high purity helium atmosphere. The line broadening which results in an apparent g-value shift in a-Si:H has been, to our knowledge, not been reported before. We propose that it is a result of spin-spin interaction, which will influence the shape of the inhomogenously broadened powder spectrum. Evaluation with signal deconvolution and simulation are under way. 5. Conclusions
atmosphere guarantees to maintain and observe very high defect density after irradiation. The fact that resonances after irradiation appear in very high density only where they were also found in the asdeposited state, makes us confident that the irradiation only creates those ‘intrinsic’ defects, which are relevant for the electronic properties in the material, which by this method can be varied reversibly over 3 orders of magnitude. However, the hope that irradiation will individually enhance resonances in such a way that their line parameters can be obtained from signal deconvolution, was not fulfilled. The signal shape after irradiation is complex and depends on the material structure, crystalline content and silane concentration during deposition. Additional features after high temperature (160 C) annealing have been observed in earlier studies [4]. This includes appearance of the conduction electron (CE) resonance in highly crystalline material and occasionally an irreversible resonance,
An experiment for high energy electron irradiation of a-Si:H and lc-Si:H with sample treatment and transfer in LN2 atmosphere was successfully established between the National Science Center Kharkov Institute of Physics and Technology, Ukraine and the Forschungszentrum Ju¨lich, Germany. Over the whole material composition range the irradiation creates in a reversible and reproducible manner only defects with ESR g-values close to where they were also found in as-deposited material. Acknowledgements We thank Professor I. Neklyudov, director general of NSC KIPT (Kharkov, Ukraine) for his support of irradiation procedure and samples shipping in Ukraine, customs broker M. Aleksankina of STCU (Ukraine) for easy and fast
O. Astakhov et al. / Journal of Non-Crystalline Solids 352 (2006) 1020–1023
customs clearing of samples, and K. Petter from HMI Berlin, Germany for high accuracy ESR sample g-value calibration. The work is supported by European Community via Science and Technology Center in Ukraine (Project #655A). References [1] Mu¨ller, F. Finger, R. Carius, H. Wagner, Phys. Rev. B 60 (1999) 11666. [2] F. Finger, A.L. Baia Neto, R. Carius, T. Dylla, S. Klein, Phys. Status Solidi C 1 (2004) 1248. [3] F. Finger, S. Klein, T. Dylla, A.L. Baia Neto, O. Vetterl, R. Carius, MRS Symp. Proc. 715 (2002) 123. [4] F. Finger, R. Carius, T. Dylla, S. Klein, S. Okur, M. Gu¨nes, IEE Proc. Circ. Dev. Sys. 150 (2003) 300. [5] K. Lips, P. Kanschat, W. Fuhs, Sol. Energy Mater. Sol. Cells 78 (2003) 513. [6] K. Lips, P. Kanschat, D. Will, C. Lerner, W. Fuhs, J. Non-Cryst. Sol. 227–230 (1998) 1021. [7] T. Ehara, Appl. Surf. Sci. 113&114 (1997) 126. [8] T. Ehara, J. Appl. Phys. 88 (2000) 1698.
1023
[9] J. Rath, Sol. Energy Mater. Sol. Cells 76 (2003) 431. [10] D. Will, C. Lerner, W. Fuhs, K. Lips, MRS Symp. Proc. 467 (1997) 361. [11] M. Kondo, S. Yawasaki, A. Matsuda, J. Non-Cryst. Solid 266–269 (2000) 544. [12] M.M. de Lima, P.C. Taylor, S. Morrison, A. LeGeune, F.C. Marques, Phys. Rev. B 65 (2002) 235. [13] K. Morigaki, H. Hikita, M. Yamaguchi, Y. Fujita, Mater. Sci. Eng. B 103 (2003) 37. [14] F. Finger, W. Fuhs, R. Carius, Philos. Mag. Lett. 57 (1988) 235. [15] R.A. Street, D. Biegelsen, J. Stuke, Philos. Mag. B 40 (1979) 451. [16] R.V. Navkhandewala, K.L. Narasimhan, S. Guha, Phys. Rev. B 24 (1981) 7443. [17] H. Derch, A. Skumanich, N.M. Amer, Phys. Rev. B 31 (1985) 6913. [18] U. Voget-Grote, W. Kummerle, R. Fischer, J. Stuke, Philos. Mag. B 41 (2) (1980) 127. [19] C. Malten, F. Finger, P. Hapke, T. Kulessa, C. Walker, R. Carius, R. Flu¨ckiger, H. Wagner, Mat. Res. Soc. Symp. Proc. 358 (1995) 757. [20] W. Bronner, M. Mehring, R. Bru¨ggemann, Phys. Rev. 65 (2002) 165212. [21] R. Bru¨ggemann, W. Bronner, M. Mehring, Solid State Commun. 119 (2001) 23.