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Electron spin resonance in thin film silicon after low temperature electron irradiation
Thin Solid Films 515 (2007) 7513 – 7516 www.elsevier.com/locate/tsf
Electron spin resonance in thin film silicon after low temperature electron irradiation O. Astakhov a,b,⁎, F. Finger a , R. Carius a , A. Lambertz a , Yu. Petrusenko b , V. Borysenko b , D. Barankov b b
a Forschungszentrum Jülich, Institute of Photovoltaics, 52425 Jülich, Germany National Science Center-Kharkov Institute of Physics & Technology, Institute of Materials Science & Technology, 61108, Kharkov, Ukraine
Available online 16 January 2007
Abstract Paramagnetic defects in amorphous and microcrystalline silicon (a-Si:H and μc-Si:H) with various structure compositions and doping levels were investigated by electron spin resonance (ESR). Samples were prepared by PECVD. The defect density was varied with 2 MeV electron bombardment at 100 K and stepwise annealing in the range of 80 K–433 K. In intrinsic material the spin density of the dominant ESR signal, presumably originating from dangling bonds (db), increases by up to 3 orders of magnitude after irradiation. In doped μc-Si:H material the pronounced conduction electron (CE) resonance disappears after irradiation and is replaced by the db resonance like in the irradiated intrinsic material. Generally the initial spin density and the line shape can be restored upon annealing at 433 K. Additional features at g-values of g ≈ 2.010 and g ≈ 2.000 in the ESR spectra are observed after irradiation together with the strongly enhanced Si db line at about g = 2.004–2.005. These features decrease rapidly on the first annealing steps and cannot be observed after the final annealing stage. © 2006 Elsevier B.V. All rights reserved. Keywords: Amorphous and microcrystalline silicon; Electron irradiation; ESR; Defects
1. Introduction Structural defects which result in deep-gap or tail states in disordered silicon films like amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon are of greatest importance for the electronic quality of these materials. The influence of these states on transport and recombination determine the performance in thin film devices such as solar cells. However, investigation and understanding of such defects, in particular in the mixed phase material μc-Si:H, is still a challenge. A successful method to study defects in a-Si:H and μc-Si:H is electron spin resonance (ESR). It was shown that both in a-Si:H and in μc-Si:H the paramagnetic defects, which are detected by ESR, represent the majority of deep defects in the material [1,2]. In addition it is possible to investigate paramagnetic tail states when by doping the Fermi level is shifted closer to the mobility or band edge. To investigate the effect of defects on the ⁎ Corresponding author. Forschungszentrum Jülich, Institute of Photovoltaics, 52425 Jülich, Germany. Tel.: +49 2461 61 39 23; fax: +49 2461 61 37 35. E-mail address: [email protected] (O. Astakhov). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.115
electronic properties it is desirable to vary the defect density in one and the same sample. Similarly it is desirable to probe a wide range of gap energies. Both requests are successfully approached by variation of the defect density by electron irradiation and stepwise annealing [3–8]. In intrinsic material this will vary the density of dominating dangling bond type defect [3–7]. In doped material variation of the db density will lead to a Fermi level shift first deep into the gap (after irradiation) and then back into the tail states towards the mobility edges (after annealing) [9]. In general such defect creation is reversible by annealing close to the material preparation temperature [4–8]. Recently we have established a sophisticated irradiation and annealing experiment, which allows a sample treatment cycle from irradiation to ESR measurement where the sample is kept at 100 K [8]. This guarantees creation and preservation of a high defect density, which would otherwise be already partly annealed by sample treatment and storage at room temperature. Based on these earlier experiments we continue our study on high quality μc-Si:H and a-Si:H material. A strong interest is for a mixed phase material with up to 50% amorphous structure,
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3. Results and discussion
Fig. 1. ESR spectra of the samples with different silane concentration normalized on sample mass. Arrows indicate evolution of the lines with SC. (a) — before irradiation. (b) — after irradiation. Note the as-deposited spectra are scaled by x60. (c) — Spin densities as a function of SC before and after irradiation. Also note that for clarity some spectra are omitted from Fig. 1a and b.
which shows optimum solar cell performance. In the latter material low n-type doping levels are used to vary the Fermi level position from deep states into the tail states.
We investigated material prepared with silane concentrations SC between 3% and 100%. The material with SC = 3–6% shows clear crystalline peaks in the Raman spectra. The material with SC = 9–100% shows pure amorphous Raman spectra. We shall call this material “Raman-amorphous” to emphasize that despite of the absence of any crystalline feature in the Raman spectra, the ESR spectra of this material shows distinct differences between low (9%) and the highest SC values both before and after irradiation. Fig. 1(a) shows the ESR spectra of these μc-Si: H and a-Si:H samples prepared at different silane concentration. The spectra are normalized to the sample mass so that the signal amplitude is proportional to spin density (Ns). Spectra of μc-Si: H material are shown in grey, of a-Si:H material in black. Ns shows a minimum for material at the transition from microcrystalline to amorphous material with the lowest values for Raman-amorphous material prepared at SC = 10% (Fig. 1(a) and (c)). Within the microcrystalline phase highest spin densities are found for material with the highest crystalline volume fraction in agreement with earlier investigations [12]. Note that the material prepared at SC = 100% was grown under non-optimized high deposition rate condition which could be
2. Experiment The material was prepared with VHF-PECVD (95 MHz) at a substrate temperature of 200 °C in a deposition system warranting high purity standards. By variation of the silane concentration SC = [SiH4] / [SiH4] + [H2] material with different microstructure from highly crystalline to amorphous was prepared. bnN-type doping was achieved by adding PH3 in the gas phase [10]. Films of about 5 μm thickness were deposited onto thin molybdenum foil and on quartz substrates. To peel off the material the foil is bent and after weighing the resulting powder is filled into quartz tubes at 0.5 bar helium atmosphere. ESR measurements were performed in the X-band (9.5 GHz) at 40 K using lock-in technique. The microstructure of the samples was determined by Raman spectroscopy measured using the 647 nm line of a Krypton laser for excitation. The crystalline volume fraction, ICRS, was semiquantitatively determined using the integrated intensities of the Raman signal at 520 cm− 1 and 500 cm− 1 (attributed to the crystalline phase) and 480 cm− 1 (attributed to the disordered phase), i.e. ICRS = (I500 + I520) / (I480 + I500 + I520) [11]. The samples were irradiated with 2 MeV electrons up to dose about 1018 e⁎cm− 2 with flux of 5 μA⁎cm− 2. During irradiation the samples were kept in the flow of evaporating liquid nitrogen (LN2) that ensured temperature around 100 K. Prior to the measurements the samples were kept at LN2 temperature. From 300 K to 433 K isochronal annealing with time step of 30 min was applied. The temperature sequence was the following: 323 K, 353 K, 393 K, and 433 K. Thereafter, isothermal annealing at 433 K was used with increasing time intervals from 30 min to 180 min. This temperature was chosen well below deposition temperature (473 K) to avoid changes of the material structure during annealing.
Fig. 2. ESR lines of the μc-Si:H and a-Si:H prepared at different SC. (a) — asdeposited. (b) — after irradiation. Note, all spectra normalized to the same amplitude in order to compare the lineshapes.
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the reason for the fairly high Ns. After irradiation the spin density increases by up to 3 orders of magnitude (Fig. 1(b) and (c)). Note the scale difference of a factor 60 between Fig. 1(a) and (b). The increase in signal intensity occurs predominantly at the resonance position, which was also observed in the asdeposited material. However, there are also very distinct additional features which appear as shoulders to the dominant central line in all samples but most clearly pronounced in the Raman-amorphous material prepared between SC = 9% and 25%. Remarkably the dependence of the spin density on SC after irradiation follows nicely that of the spin density in the asdeposited state, i.e. a minimum at SC = 10% and highest values for highly crystalline and SC = 100% material. The difference in lines shape before and after irradiation is seen in Fig. 2 where the corresponding spectra are shown normalized to the same peak-to-peak height. It appears that in addition to the increase in the central line, two satellites at gvalues around 2.000 and 2.010 are created with irradiation. The position and shape of the satellite at g ≈ 2.000 seems independent from SC. The satellite on the low field side (g ≈ 2.010) on the other hand is nearly absent at SC = 100% while it looks more like a line broadening at SC = 3&6%. In order to estimate the contribution of the additional features in the spectra of the irradiated samples we applied a simple line de-convolution using as-deposited spectra and simple Gaussian lines in addition. Estimated Ns of the satellites and total Ns after irradiation are shown in Fig. 3 together with crystalline volume fraction (ICRS). The ratio of the satellites depends on the material structural composition. The origin of the new features is not clear. Since the positions of the satellites are nearly symmetric in respect to the db line, the hyperfine splitting of the db state is the first idea to consider. A pair due to Si29 nuclei can be ruled out by the larger fraction of the signal of the satellites with respect to the dangling bonds compared to the natural abundance of Si29 of 4.7%. The splitting due to hydrogen nuclear magnetic moment could be considered as an origin of the given satellites. Other possible
origin is a powder pattern of the anisotropic state like for instance vacancy or vacancy complex in hydrogen implanted cSi [13], but care has to be taken here since no data on such centers were reported for a-Si:H or μc-Si:H before. Finally, the creation of two or more separate centers after irradiation could not be ruled out definitely. After first measurements of the irradiated samples, all samples were stepwise annealed. It was found that in particular the satellite features anneal rapidly and nearly synchronously in the first annealing steps. On Fig. 4(a) the line shape evolution during annealing indicates the fast annealing of the satellites in comparison to the central line. The spin densities of the different samples vs. annealing steps are shown in Fig. 4(b). Ns recovers upon annealing close to the value measured in the as-deposited state. In most cases there is complete recovery of the line shape and Ns after annealing.
Fig. 3. Spin density in irradiated μc-Si:H and a-Si:H vs. silane concentration. Black points — total Ns in the material. Full triangles — Ns of the additional line on the low field side of the spectrum (g ≈ 2.00). Empty triangles — Ns of the additional line on the high field side of the spectrum (g ≈ 2.01). X — crystalline volume fraction estimated from Raman spectra.
Fig. 5. Comparison of ESR lines of intrinsic and bnN doped μc-Si:H, both prepared at SC= 6%. (a) — as-deposited resonance. (b) — resonance after irradiation. Spectra are normalized to the same amplitude for lineshapes comparison.
Fig. 4. (a) — ESR spectra of the a-Si:H sample prepared at SC = 9%. Spectra correspond to the treatment steps (irradiation and annealing) and are scaled for lineshape analysis. Arrows indicates drop of the additional features in the resonance upon annealing. (b) — spin density in the material prepared with different SC versus treatment.
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To get information on the density of gap states in μc-Si:H material used in high quality solar cells, i.e. material prepared at SC = 6% close to the microcrystalline-to-amorphous transition [14], we investigated such material with low n-type doping levels. For comparison doped (1 ppm to 13 ppm PH3) samples and an undoped μc-Si:H sample were irradiated and annealed in parallel. The spectra before and after irradiation are shown in Fig. 5 again normalized to the same peak-to-peak height. The ESR signal of the as-deposited doped sample shows the pronounced conduction electron (CE) resonance [9,14] at g ≈ 1.998 with the remaining db resonance only observable as a small shoulder. This together with the conductivity results confirms the effective doping process and shift of the Fermi level even with small doping concentrations. While the ESR signals of the as-deposited intrinsic and doped samples are clearly different, after irradiation there is a striking similarity between both materials. The prominent increase of the db resonance and even occurrence of the satellites is clearly seen at g ≈ 2.000. 4. Conclusions Low temperature electron irradiation is a successful approach for generation of high defect density in μc-Si:H and a-Si:H. The defect density can be increased by up to 3 orders of magnitude and can be annealed out at temperatures below the substrate temperature during deposition, i.e. the effect is reversible. In undoped material the ESR lines after irradiation were found at nearly the same position as the initial resonance for a wide range of microstructure from highly microcrystalline to fully amorphous. After irradiation two satellites at g ≈ 2.000 and g ≈ 2.010 appear in the spectra superimposed on the well-known dangling bond resonance. The satellites, to our knowledge have, not been reported before likely because of their rapid annealing. This confirms importance of the low temperature procedure of irradiation and storage of the samples. In addition similar research in the past was mainly concentrated either on the a-Si: H far from transition [3–5] either on the μc-Si:H with high
crystallinity [6,7] i.e. in the region where the effect is less pronounced. Intrinsic behavior of the doped samples after irradiation indicates the shift of the Fermi level towards midgap by the strong enhancement of the midgap defect density. There is a significant difference in the ESR spectra of amorphous material prepared with different SC. Acknowledgements This work was in part be supported by STCU project #655 A. We are grateful to M. Hülsbeck for Raman measurements and Prof. I. Nekliudov for the comprehensive support of the irradiation experiments in Ukraine. References [1] R.A. Street, Hydrogenated Amorphous Silicon, Cambridge University Press, 1991, p. 108. [2] T. Dylla, Electron Spin Resonance and Transient Photocurrent Measurements on Microcrystalline Silicon, Forshungszentrum Jülich GmbH389336-410-2, 2005. [3] R. Street, D. Biegelsen, J. Stuke, Philos. Mag., B 40 (6) (1979) 451. [4] H. Dersch, L. Shweitzer, J. Stuke, Phys. Rev., B 28 (1983) 8. [5] H. Dersch, A. Skumanich, N.M. Amer, Phys. Rev., B 31 (1985) 10. [6] W. Bronner, M. Mehring, R. Brüggemann, Phys. Rev., B 65 (2002) 165212. [7] W. Bronner, J.P. Kleider, R. Brüggemann, M. Mehring, Thin Solid Films 427 (2003) 51. [8] O. Astakhov, F. Finger, R. Carius, A. Lambertz, Yu. Petrusenko, V. Borysenko, D. Barankov, J. Non-Cryst. Solids 352 (9–20) (2006) 1020. [9] C. Malten, F. Finger, P. Hapke, T. Kulessa, C. Walker, R. Carius, R. Flückiger, H. Wagner, Mater. Res. Soc. Symp. Proc. 385 (1995) 757. [10] F. Finger, J. Müller, C. Malten, R. Carius, H. Wagner, J. Non-Cryst. Solids 266–269 (2004) 511. [11] L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, H. Wagner, Philos. Mag., A 77 (1998) 1447. [12] A.L.B. Neto, T. Dylla, S. Klein, T. Repmann, A. Lambertz, R. Carius, F. Finger, J. Non-Cryst. Solids 338–340 (2004) 168. [13] Yu.V. Gorelkinskii, Kh.A. Abdullin, B.N. Mukashev, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 19 (2002) 397. [14] O. Vetterl, R. Carius, L. Houben, C. Scholten, M. Luysberg, A. Lambertz, F. Finger, H. Wagner, Mater. Res. Soc. Symp. Proc. 609 (2000) A1521.
Electron spin resonance in thin film silicon after low temperature electron irradiation O. Astakhov a,b,⁎, F. Finger a , R. Carius a , A. Lambertz a , Yu. Petrusenko b , V. Borysenko b , D. Barankov b b
a Forschungszentrum Jülich, Institute of Photovoltaics, 52425 Jülich, Germany National Science Center-Kharkov Institute of Physics & Technology, Institute of Materials Science & Technology, 61108, Kharkov, Ukraine
Available online 16 January 2007
Abstract Paramagnetic defects in amorphous and microcrystalline silicon (a-Si:H and μc-Si:H) with various structure compositions and doping levels were investigated by electron spin resonance (ESR). Samples were prepared by PECVD. The defect density was varied with 2 MeV electron bombardment at 100 K and stepwise annealing in the range of 80 K–433 K. In intrinsic material the spin density of the dominant ESR signal, presumably originating from dangling bonds (db), increases by up to 3 orders of magnitude after irradiation. In doped μc-Si:H material the pronounced conduction electron (CE) resonance disappears after irradiation and is replaced by the db resonance like in the irradiated intrinsic material. Generally the initial spin density and the line shape can be restored upon annealing at 433 K. Additional features at g-values of g ≈ 2.010 and g ≈ 2.000 in the ESR spectra are observed after irradiation together with the strongly enhanced Si db line at about g = 2.004–2.005. These features decrease rapidly on the first annealing steps and cannot be observed after the final annealing stage. © 2006 Elsevier B.V. All rights reserved. Keywords: Amorphous and microcrystalline silicon; Electron irradiation; ESR; Defects
1. Introduction Structural defects which result in deep-gap or tail states in disordered silicon films like amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon are of greatest importance for the electronic quality of these materials. The influence of these states on transport and recombination determine the performance in thin film devices such as solar cells. However, investigation and understanding of such defects, in particular in the mixed phase material μc-Si:H, is still a challenge. A successful method to study defects in a-Si:H and μc-Si:H is electron spin resonance (ESR). It was shown that both in a-Si:H and in μc-Si:H the paramagnetic defects, which are detected by ESR, represent the majority of deep defects in the material [1,2]. In addition it is possible to investigate paramagnetic tail states when by doping the Fermi level is shifted closer to the mobility or band edge. To investigate the effect of defects on the ⁎ Corresponding author. Forschungszentrum Jülich, Institute of Photovoltaics, 52425 Jülich, Germany. Tel.: +49 2461 61 39 23; fax: +49 2461 61 37 35. E-mail address: [email protected] (O. Astakhov). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.115
electronic properties it is desirable to vary the defect density in one and the same sample. Similarly it is desirable to probe a wide range of gap energies. Both requests are successfully approached by variation of the defect density by electron irradiation and stepwise annealing [3–8]. In intrinsic material this will vary the density of dominating dangling bond type defect [3–7]. In doped material variation of the db density will lead to a Fermi level shift first deep into the gap (after irradiation) and then back into the tail states towards the mobility edges (after annealing) [9]. In general such defect creation is reversible by annealing close to the material preparation temperature [4–8]. Recently we have established a sophisticated irradiation and annealing experiment, which allows a sample treatment cycle from irradiation to ESR measurement where the sample is kept at 100 K [8]. This guarantees creation and preservation of a high defect density, which would otherwise be already partly annealed by sample treatment and storage at room temperature. Based on these earlier experiments we continue our study on high quality μc-Si:H and a-Si:H material. A strong interest is for a mixed phase material with up to 50% amorphous structure,
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3. Results and discussion
Fig. 1. ESR spectra of the samples with different silane concentration normalized on sample mass. Arrows indicate evolution of the lines with SC. (a) — before irradiation. (b) — after irradiation. Note the as-deposited spectra are scaled by x60. (c) — Spin densities as a function of SC before and after irradiation. Also note that for clarity some spectra are omitted from Fig. 1a and b.
which shows optimum solar cell performance. In the latter material low n-type doping levels are used to vary the Fermi level position from deep states into the tail states.
We investigated material prepared with silane concentrations SC between 3% and 100%. The material with SC = 3–6% shows clear crystalline peaks in the Raman spectra. The material with SC = 9–100% shows pure amorphous Raman spectra. We shall call this material “Raman-amorphous” to emphasize that despite of the absence of any crystalline feature in the Raman spectra, the ESR spectra of this material shows distinct differences between low (9%) and the highest SC values both before and after irradiation. Fig. 1(a) shows the ESR spectra of these μc-Si: H and a-Si:H samples prepared at different silane concentration. The spectra are normalized to the sample mass so that the signal amplitude is proportional to spin density (Ns). Spectra of μc-Si: H material are shown in grey, of a-Si:H material in black. Ns shows a minimum for material at the transition from microcrystalline to amorphous material with the lowest values for Raman-amorphous material prepared at SC = 10% (Fig. 1(a) and (c)). Within the microcrystalline phase highest spin densities are found for material with the highest crystalline volume fraction in agreement with earlier investigations [12]. Note that the material prepared at SC = 100% was grown under non-optimized high deposition rate condition which could be
2. Experiment The material was prepared with VHF-PECVD (95 MHz) at a substrate temperature of 200 °C in a deposition system warranting high purity standards. By variation of the silane concentration SC = [SiH4] / [SiH4] + [H2] material with different microstructure from highly crystalline to amorphous was prepared. bnN-type doping was achieved by adding PH3 in the gas phase [10]. Films of about 5 μm thickness were deposited onto thin molybdenum foil and on quartz substrates. To peel off the material the foil is bent and after weighing the resulting powder is filled into quartz tubes at 0.5 bar helium atmosphere. ESR measurements were performed in the X-band (9.5 GHz) at 40 K using lock-in technique. The microstructure of the samples was determined by Raman spectroscopy measured using the 647 nm line of a Krypton laser for excitation. The crystalline volume fraction, ICRS, was semiquantitatively determined using the integrated intensities of the Raman signal at 520 cm− 1 and 500 cm− 1 (attributed to the crystalline phase) and 480 cm− 1 (attributed to the disordered phase), i.e. ICRS = (I500 + I520) / (I480 + I500 + I520) [11]. The samples were irradiated with 2 MeV electrons up to dose about 1018 e⁎cm− 2 with flux of 5 μA⁎cm− 2. During irradiation the samples were kept in the flow of evaporating liquid nitrogen (LN2) that ensured temperature around 100 K. Prior to the measurements the samples were kept at LN2 temperature. From 300 K to 433 K isochronal annealing with time step of 30 min was applied. The temperature sequence was the following: 323 K, 353 K, 393 K, and 433 K. Thereafter, isothermal annealing at 433 K was used with increasing time intervals from 30 min to 180 min. This temperature was chosen well below deposition temperature (473 K) to avoid changes of the material structure during annealing.
Fig. 2. ESR lines of the μc-Si:H and a-Si:H prepared at different SC. (a) — asdeposited. (b) — after irradiation. Note, all spectra normalized to the same amplitude in order to compare the lineshapes.
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the reason for the fairly high Ns. After irradiation the spin density increases by up to 3 orders of magnitude (Fig. 1(b) and (c)). Note the scale difference of a factor 60 between Fig. 1(a) and (b). The increase in signal intensity occurs predominantly at the resonance position, which was also observed in the asdeposited material. However, there are also very distinct additional features which appear as shoulders to the dominant central line in all samples but most clearly pronounced in the Raman-amorphous material prepared between SC = 9% and 25%. Remarkably the dependence of the spin density on SC after irradiation follows nicely that of the spin density in the asdeposited state, i.e. a minimum at SC = 10% and highest values for highly crystalline and SC = 100% material. The difference in lines shape before and after irradiation is seen in Fig. 2 where the corresponding spectra are shown normalized to the same peak-to-peak height. It appears that in addition to the increase in the central line, two satellites at gvalues around 2.000 and 2.010 are created with irradiation. The position and shape of the satellite at g ≈ 2.000 seems independent from SC. The satellite on the low field side (g ≈ 2.010) on the other hand is nearly absent at SC = 100% while it looks more like a line broadening at SC = 3&6%. In order to estimate the contribution of the additional features in the spectra of the irradiated samples we applied a simple line de-convolution using as-deposited spectra and simple Gaussian lines in addition. Estimated Ns of the satellites and total Ns after irradiation are shown in Fig. 3 together with crystalline volume fraction (ICRS). The ratio of the satellites depends on the material structural composition. The origin of the new features is not clear. Since the positions of the satellites are nearly symmetric in respect to the db line, the hyperfine splitting of the db state is the first idea to consider. A pair due to Si29 nuclei can be ruled out by the larger fraction of the signal of the satellites with respect to the dangling bonds compared to the natural abundance of Si29 of 4.7%. The splitting due to hydrogen nuclear magnetic moment could be considered as an origin of the given satellites. Other possible
origin is a powder pattern of the anisotropic state like for instance vacancy or vacancy complex in hydrogen implanted cSi [13], but care has to be taken here since no data on such centers were reported for a-Si:H or μc-Si:H before. Finally, the creation of two or more separate centers after irradiation could not be ruled out definitely. After first measurements of the irradiated samples, all samples were stepwise annealed. It was found that in particular the satellite features anneal rapidly and nearly synchronously in the first annealing steps. On Fig. 4(a) the line shape evolution during annealing indicates the fast annealing of the satellites in comparison to the central line. The spin densities of the different samples vs. annealing steps are shown in Fig. 4(b). Ns recovers upon annealing close to the value measured in the as-deposited state. In most cases there is complete recovery of the line shape and Ns after annealing.
Fig. 3. Spin density in irradiated μc-Si:H and a-Si:H vs. silane concentration. Black points — total Ns in the material. Full triangles — Ns of the additional line on the low field side of the spectrum (g ≈ 2.00). Empty triangles — Ns of the additional line on the high field side of the spectrum (g ≈ 2.01). X — crystalline volume fraction estimated from Raman spectra.
Fig. 5. Comparison of ESR lines of intrinsic and bnN doped μc-Si:H, both prepared at SC= 6%. (a) — as-deposited resonance. (b) — resonance after irradiation. Spectra are normalized to the same amplitude for lineshapes comparison.
Fig. 4. (a) — ESR spectra of the a-Si:H sample prepared at SC = 9%. Spectra correspond to the treatment steps (irradiation and annealing) and are scaled for lineshape analysis. Arrows indicates drop of the additional features in the resonance upon annealing. (b) — spin density in the material prepared with different SC versus treatment.
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To get information on the density of gap states in μc-Si:H material used in high quality solar cells, i.e. material prepared at SC = 6% close to the microcrystalline-to-amorphous transition [14], we investigated such material with low n-type doping levels. For comparison doped (1 ppm to 13 ppm PH3) samples and an undoped μc-Si:H sample were irradiated and annealed in parallel. The spectra before and after irradiation are shown in Fig. 5 again normalized to the same peak-to-peak height. The ESR signal of the as-deposited doped sample shows the pronounced conduction electron (CE) resonance [9,14] at g ≈ 1.998 with the remaining db resonance only observable as a small shoulder. This together with the conductivity results confirms the effective doping process and shift of the Fermi level even with small doping concentrations. While the ESR signals of the as-deposited intrinsic and doped samples are clearly different, after irradiation there is a striking similarity between both materials. The prominent increase of the db resonance and even occurrence of the satellites is clearly seen at g ≈ 2.000. 4. Conclusions Low temperature electron irradiation is a successful approach for generation of high defect density in μc-Si:H and a-Si:H. The defect density can be increased by up to 3 orders of magnitude and can be annealed out at temperatures below the substrate temperature during deposition, i.e. the effect is reversible. In undoped material the ESR lines after irradiation were found at nearly the same position as the initial resonance for a wide range of microstructure from highly microcrystalline to fully amorphous. After irradiation two satellites at g ≈ 2.000 and g ≈ 2.010 appear in the spectra superimposed on the well-known dangling bond resonance. The satellites, to our knowledge have, not been reported before likely because of their rapid annealing. This confirms importance of the low temperature procedure of irradiation and storage of the samples. In addition similar research in the past was mainly concentrated either on the a-Si: H far from transition [3–5] either on the μc-Si:H with high
crystallinity [6,7] i.e. in the region where the effect is less pronounced. Intrinsic behavior of the doped samples after irradiation indicates the shift of the Fermi level towards midgap by the strong enhancement of the midgap defect density. There is a significant difference in the ESR spectra of amorphous material prepared with different SC. Acknowledgements This work was in part be supported by STCU project #655 A. We are grateful to M. Hülsbeck for Raman measurements and Prof. I. Nekliudov for the comprehensive support of the irradiation experiments in Ukraine. References [1] R.A. Street, Hydrogenated Amorphous Silicon, Cambridge University Press, 1991, p. 108. [2] T. Dylla, Electron Spin Resonance and Transient Photocurrent Measurements on Microcrystalline Silicon, Forshungszentrum Jülich GmbH389336-410-2, 2005. [3] R. Street, D. Biegelsen, J. Stuke, Philos. Mag., B 40 (6) (1979) 451. [4] H. Dersch, L. Shweitzer, J. Stuke, Phys. Rev., B 28 (1983) 8. [5] H. Dersch, A. Skumanich, N.M. Amer, Phys. Rev., B 31 (1985) 10. [6] W. Bronner, M. Mehring, R. Brüggemann, Phys. Rev., B 65 (2002) 165212. [7] W. Bronner, J.P. Kleider, R. Brüggemann, M. Mehring, Thin Solid Films 427 (2003) 51. [8] O. Astakhov, F. Finger, R. Carius, A. Lambertz, Yu. Petrusenko, V. Borysenko, D. Barankov, J. Non-Cryst. Solids 352 (9–20) (2006) 1020. [9] C. Malten, F. Finger, P. Hapke, T. Kulessa, C. Walker, R. Carius, R. Flückiger, H. Wagner, Mater. Res. Soc. Symp. Proc. 385 (1995) 757. [10] F. Finger, J. Müller, C. Malten, R. Carius, H. Wagner, J. Non-Cryst. Solids 266–269 (2004) 511. [11] L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, H. Wagner, Philos. Mag., A 77 (1998) 1447. [12] A.L.B. Neto, T. Dylla, S. Klein, T. Repmann, A. Lambertz, R. Carius, F. Finger, J. Non-Cryst. Solids 338–340 (2004) 168. [13] Yu.V. Gorelkinskii, Kh.A. Abdullin, B.N. Mukashev, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 19 (2002) 397. [14] O. Vetterl, R. Carius, L. Houben, C. Scholten, M. Luysberg, A. Lambertz, F. Finger, H. Wagner, Mater. Res. Soc. Symp. Proc. 609 (2000) A1521.