Session Th-C3

a-Si: Electronic Structure, Defects, Metastability III

Chair: Robert Johanson, University of Saskatchewan

Th-C3.1 14:00–14:20

Metastability in Hydrogenated Amorphous Silicon Revisited

A. R. Middya

Silicon Solar, Inc., Fremont, CA

Metastability in hydrogenated amorphous silicon, known as Staeble-Wronski effect has been a major obstacle for large-scale commercialization of a-Si solar cells. In this paper, author will discuss on recent developments on advancement of understanding of this interesting effect in a-Si:H thin-film. Mobility of atomic hydrogen (H) is considered to be the main reason for metastability in a-Si:H and its alloys (a-SiC:H and a-SiGe:H). However, author found next-generation of amorphous silicon thin-film that shows higher stability under intense light (>100 mW/cm2), however, no correlation is found between total bonded hydrogen content and saturated defect density. Author found if a-Si:H thin-film is deposited by plasma-enhanced chemical vapor (PECVD) technique by decomposing silane (SiH4) and helium (He) or argon (Ar), we have a-Si:H having bonded H-content 5 at.% and 20 at.% respectively. It has been also observed that in both types of a-Si:H thin-film, nanovoid fraction is much lower than that of standard a-Si:H [1]. The saturated defect density in case of a-Si:H having low (5 at.%) and high hydrogen content (20 at.%) are lower than that of a-Si:H having hydrogen content 10 at.%. We also found that if a-Si:H i-layer is deposited under argon diluted silane plasma, we found better stability in solar cells compared to H-diluted a-Si:H solar cells fabricated under similar plasma environment. If hydrogen diffusion is the reason for light-induced defect formation, we would not see a-Si:H having high hydrogen (20 at.%) shows higher stability. It has been observed that the saturated defect density of helium diluted a-Si:H is even lower than that of a-Si:H deposited by hot wire chemical vapor deposition (hot-wire CVD) technique at 450°C [1]. The hydrogen content of these hot-wire CVD a-Si:H is only 1 at.% vs. 5 at.% in case of helium diluted a-Si:H, clearly, contradicting atomic hydrogen mediated defect formation process in Staebler-Wronski effect. Is more hydrogenation to a-Si:H matrix is the way to make more relaxed Si-Si lattice, consequently, more stable a-Si:H, instead of reducing hydrogen content in a-Si:H? We found some early evidence of this hypothesis in argon diluted a-Si:H thin-film. Argon diluted silane plasma technology in PECVD, at present, is found to be very promising, for example, nanocrystalline Si developed under such plasma environment is found to be the best nanocrystalline silicon, obtained so far [1]. Author will present materials and solar cells data to discuss the physics in Staebler-Wronski effect in a-Si:H that mobility of H may not be the reason for light-induced degradation.

[1] U. K. Das, A. R. Middy, J. K. Rath, C. Longeaud, D. L. Williamson, and P. Chaudhuri, J. Non-Crystalline Solids 276 (2000) 46

[2] A. H. Mahan, J. Carapella, B. P. Nelson, R. S. Crandall, and I. Ballbergs, J. Appl. Phys. 69 (1991) 6728

Th-C3.2 14:20–14:40

Influence of Hydrogen Concentration on Void-related Microstructure in Low Hydrogen Amorphous and Crystalline Silicon Materials

W. Beyer (1,2,3), U. Breuer (4), R. Carius (2), D. Lennartz (2), F.C. Maier (2,3), N. H. Nickel (1), F. Pennartz (2), P. Prunici (3), and U. Zastrow (2)

1. Institut für Silizium-Photovoltaik, HZB, Kekuléstrasse 5, D-12489 Berlin, Germany

2. IEK5-Photovoltaik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany

3. Malibu GmbH & Co.KG, Böttcherstrasse 7, D-33609, Bielefeld, Germany

4. ZEA-3, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany

The influence of hydrogen concentration CH on void-related microstructure in amorphous silicon films is still poorly understood. One reason is that for changing CH in plasma-deposited hydrogenated amorphous silicon (a-Si:H) usually the deposition conditions are varied which may cause changes in microstructure as well. A more direct way to vary CH is hydrogen implantation. As starting substances we use plasma-grown a-Si of high substrate temperature, unhydrogenated evaporated a-Si as well as c-Si wafer material. Ten hydrogen distributions were implanted at energies varying between 20 keV and 110 keV at increments of 10 keV giving rise to a nearly constant CH in a depth between about 0.25 μm and 0.9 μm, as measured by SIMS. By changing the dose of the individual H implantations from 3x1014 cm–2 to 1016 cm–2, the implanted H concentration was varied from about 0.1 at.% to about 3 at.%. Infrared (IR) absorption and effusion of hydrogen and of implanted He were measured. Significant differences between amorphous and crystalline material are observed in infrared absorption. While IR absorption of the implanted hydrogen in the investigated H concentration range shows in amorphous Si almost entirely the 2000 cm–1 Si-H stretching mode, in crystalline silicon Si-H stretching modes in the range 2000 to 2200 cm–1 are observed. Hydrogen effusion measurements showing for all three materials a peak near 600–700°C indicate the presence of dense Si:H with no diffusion of H2. Effusion of implanted He in hydrogenated c-Si shows a very pronounced high temperature (HT) peak which has been associated with He trapped in isolated voids [1]. The evaporated films show isolated voids in the as-deposited material but these voids disappear at CH ≈ 3 at.%. Almost no HT He peak is observed for plasma-deposited a-Si:H in the whole H concentration range. These results demonstrate that there is no direct correlation between the concentration of voids (accessible to helium penetration) and ( low) H concentrations in a-Si:H. The results are compared with plasma-grown a-Si:H data. Microstructure changes upon annealing and H out-diffusion are also studied. Again, no direct relation between CH and the density of isolated voids is found. An interesting result is that the annealing temperature range where isolated voids in unhydrogenated material disappear is similar to the temperature where hydrogen starts to effuse from the H-implanted material. This result supports the concept that for the onset of hydrogen diffusion in a-Si:H some temperature induced flexibility of the silicon network is required [2].

[1] W. Beyer, Physica Status Solidi (c) 1 (2004) 1144

[2] W. Beyer, in: Semiconductors and Semimetals 61, N. H. Nickel, ed. (Academic Press, San Diego, 1999) p. 165

Keywords: amorphous silicon, hydrogen, microstructure, voids

Th-C3.3 14:40–15:00

Atmospheric Aging and Light-induced Degradation of Amorphous and Nanostructured Silicon Using Photoconductivity and Electron Spin Resonance

Zaki M. Saleh (1,2), Gizem Nogay (1,3), Engin Ozkol (1,4), Gokhan Yilmaz (5), Mehmet Güneş (5), and Rasit Turan (1,3)

1. Center for Solar Energy Research and Applications (GüNAM), Middle East Technical University, Ankara, Turkey

2. Department of Physics, Arab American University-Jenin, Jenin, Palestine

3. Department of Physics, Middle East Technical University, Ankara, Turkey

4. Department of Chemical Engineering, Middle East Technical University, Ankara, Turkey

5. Department of Physics, Mugla University, Mugla, Turkey

Previous studies indicate that the dark conductivity of amorphous silicon increases with accelerated aging in de-ionized water (DIW) at elevated temperatures while some reports show a reduced or even opposite trend in samples with a microcrystalline phase. However, dark conductivity always decreases with light-induced degradation (LID). Aging can have long-term effects that negatively impact the performance of solar cells as does the light-induced effect although its origin is probably different. Even though accelerated aging may influence the accurate evaluation of LID, the two effects are rarely evaluated independently. While the light-induced effect is linked to the formation of paramagnetic dangling bonds (D0), the origin of accelerated aging remains unclear. The effect of both treatments on the nanostructured phase of silicon is generally smaller, but the origins of both effects in this material remain controversial. Electron Spin Resonance (ESR) may be used to differentiate defects based on their paramagnetic properties. In this study, we use ESR, steady-state photoconductivity (SSPC), and dual-beam photoconductivity (DBPC) to study and compare the behaviors of both structured phases of silicon under the two degradation conditions: (a) accelerated aging in DI water at 80°C and (b) light-soaking at room temperature to compare the relative effects on defect density, dark and photo-conductivities. Two sets of hydrogenated amorphous silicon (a-Si:H) and nanostructured silicon (nc-Si) samples were deposited in a dedicated, intrinsic silicon reactor of a Capacitively-Coupled Plasma-Enhanced Chemical Vapor (CC-PECVD) cluster tool. The nanostructured phase was achieved by heavy dilution of silane with hydrogen during deposition and was confirmed and estimated by Raman scattering measurements. To reduce the stress and prevent delamination of thick films, the nanostructured samples were deposited in layers with H2 plasma in between. Since the ESR signal in these samples is very weak, films deposited over large substrate areas were collected as powdered samples into quartz tubes for the ESR measurements. In spite of the changes in dark and photo-conductivities due to aging in DI water, no increase in the dark ESR signal is observed for either sample. Since the light-induced effect is strongly linked to an increase in the dark ESR signal, and therefore to the density of dangling bonds, these results suggest that accelerated aging in DI water, and probably slow aging in humid atmosphere, do not alter the native or light-induced dangling bonds. Although both effects have somewhat similar reversible behaviors, they exhibit different annealing behaviors. The larger change in SSPG and DBPH for the amorphous samples suggests that both aging and light-induced effects are probably associated with the amorphous phase in the nanostructured material. The degree of degradation in SSPC and DBPC for the fresh, annealed, aged, and light-soaked samples of both amorphous and nanostructured phases is presented. The temperature dependence of photoconductivity was also measured in the temperature range of 100–340K and the calculated activation energies are used to propose a model for the defects formed during aging and compare that with the light-induced defects.

Keywords: nanostructured silicon, atmospheric aging, light-induced degradation, ESR, photoconductivity

Th-C3.4 15:00–15:20 cancelled

Ultrafast Dispersive Transport in a-Si1–xGex:H Investigated by Time-Resolved Near-Infrared and Terahertz Spectroscopy

J. J. Felver, C. R. Hamner, and S. L. Dexheimer

Department of Physics and Astronomy, Washington State University, Pullman, WA 99164-2814

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