We report the effect of the hydrogen (H) content (CH) on the crystallization kinetics, surface morphology and grain growth for Hot Wire CVD a-Si:H films containing 12.5 and 2.7 at.% H which are crystallized by rapid thermal anneal (RTA). For the high CH film we observe explosive H evolution, with a resultant destruction of the film for RTA temperatures >750 deg C. At RTA temperatures ~600 deg C, both films remain intact with similar morphologies. At this same lower RTA, the incubation and crystallization times decrease, and the grain size as measured by X-Ray Diffraction increases with decreasing film CH. SIMS measurements indicate that a similar film CH (<0.5 at.%) exists in both films when crystallization commences. The benefits of a two-step annealing process for the high CH film are documented

Ginley, D. S.

Mahan, A. H.

Readey, D. W.

Reedy, R. C. Jr.

Roy, B.

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 A national laboratory of the U.S. Department of EnergyOffice of Energy Efficiency & Renewable EnergyNational Renewable Energy Laboratory Innovation for Our Energy Future Rapid Thermal Annealing of HWCVD a-Si:H Films: The Effect of the Film Hydrogen Content on the Crystallization Kinetics, Surface Morphology, and Grain Growth A.H. Mahan, R.C. Reedy Jr., and D.S. Ginley National Renewable Energy Laboratory B. Roy and D.W. Readey Colorado School of Mines Presented at the 2005 DOE Solar Energy Technologies  Program Review Meeting November 7–10, 2005 Denver, Colorado Conference Paper NREL/CP-520-38953 November 2005 NREL is operated by Midwest Research Institute ● Battelle     Contract No. DE-AC36-99-GO10337  NOTICE The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes. This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available electronically at http://www.osti.gov/bridgeAvailable for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone:  865.576.8401 fax: 865.576.5728 email:  mailto:reports@adonis.osti.govAvailable for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone:  800.553.6847 fax:  703.605.6900 email: orders@ntis.fedworld.gov online ordering:  http://www.ntis.gov/ordering.htmPrinted on paper containing at least 50% wastepaper, including 20% postconsumer waste   Rapid Thermal Annealing of HWCVD a-Si:H Films: The Effect of the Film Hydrogen Content on the Crystallization Kinetics, Surface Morphology, and Grain Growth A.H. Mahan,1 B. Roy,2 R.C. Reedy Jr.,1 D.W. Readey,2 and D.S. Ginley11National Renewable Energy Laboratory, Golden, Colorado, harv_mahan@nrel.gov2Colorado School of Mines, Golden, ColoradoABSTRACT We report the effect of the hydrogen (H) content (CH) on the crystallization kinetics, surface morphology and grain growth for Hot Wire CVD a-Si:H films containing 12.5 and 2.7 at.% H which are crystallized by rapid thermal anneal (RTA). For the high CH film we observe explosive H evolution, with a resultant destruction of the film for RTA temperatures >750C. At RTA temperatures ~600C, both films remain intact with similar morphologies. At this same lower RTA, the incubation and crystallization times decrease, and the grain size as measured by X-Ray Diffraction increases with decreasing film CH. SIMS measurements indicate that a similar film CH (<0.5 at.%) exists in both films when crystallization commences. The benefits of a two-step annealing process for the high CH film are documented.  1. Objectives The ability to crystallize thin a-Si:H into large grain      Si can lead to significant improvements in Si solar cells.  We are examining what factors play a role in determining crystallite grain size and/or defect density when a-Si:H films undergo RTA to induce crystallization. Such factors include film H content, crystallization anneal temperature, higher temperature post anneals to possibly reduce the film defect density, and how to rehydrogenate the grain boundaries.  This work addresses these first two factors. 2. Technical Approach 0.8-1.0µm thick a-Si:H films with different initial hydrogen content (CH) were deposited on 1737 corning glass by the HWCVD method.1 Films were annealed at temperatures (TA’s) between 600°C to 800°C in a custom built RTA system for times ranging from 6 sec to 400 min.2 The shift of the reflection fringes in the visible region and the saturation of the c-Si reflectance UV peak intensities were used as indictors of the completeness of crystallization.3  XRD spectra of all films were measured for 2θ between 20­60° to define the amorphous incubation time, confirm completeness of crystallization and determine the grain size from the (111) diffraction peak FWHM using Sherrer’s formula. The experimental procedure was to anneal the same piece of each film consecutively at each TA, and measure the reflectance and XRD after every step. SIMS was also employed to measure the hydrogen profiles. Finally, optical micrographs of the Films were taken to examine changes in the surface morphology of the films due to different TA’s. 3. Results and Accomplishments While the incubation (TI) and crystallization (TC) times are similar for films of different CH for a TA >750C, the significant H evolution has a dramatic effect on the surface morphology for the high CH film at this TA. In particular, for the high CH film we observe explosive H evolution, with a resultant destruction of the film. This does not occur for the low CH film, as the surface morphology remains unaffected by the much smaller amount of H evolved.  Optical photographs of these films are seen in Figure 1.  On the other hand, the TI and Tc Fig. 1. Surface morphology of the high CH (top) and low CH (bottom) films annealed at 800C for 6 s. times are markedly affected by the CH at a lower TA of 600C.2  In particular, TI and TC decrease from 11,400s and 24,000s to 6,000s and 13,500s respectively as the film CH is lowered from 12.5 to 2.7 at. %. At the same time, the XRD grain size is also quite different at low TA. As seen in Figure 2, the maximum grain size for the 12.5 at.% CH film is ~500Å, while that for the 2.7 at.% CH film is ~800Å. We note that XRD linewidth broadening relates to the development of structural order on a scale longer than that existing in amorphous films. We are currently examining identically deposited and RTA annealed films by TEM to determine whether this measure of long-range order extends entirely across  a  ‘grain’, as defined in the 1800 Low C H High C H when nucleation first occurs.  Further, since most of the initial CH is lost very early in the incubation period,2 we suggest that nucleation may be initiated not be evolution of the more loosely bound, or ‘mobile’ H, proposed to assist in a-Si:H crystallization,4 but by evolution of the more tightly bonded H which generates a larger number of dangling bonds upon evolution.5 Second, the high CH film retains most of this ~0.1 at.% CH as it is annealed to complete crystallization, while the low CH film looses all its remaining CH following a similar 600C anneal. Since the XRD grain sizes are roughly similar when nucleation commences,2 the data of Fig. 1 suggest 700 600 500 400 800 750 700 650 600 Annealing temperature (C) Final grain size (�) Fig. 2 . XRD grain size versus film CH for different TA. usual sense of a ‘crystallo graphic region separated by high angle grain boundaries’.  However, in the present work we use the term ‘grain size’ to indicate the spatial context of the long range order as measured by XRD line broadening. Figure 3 shows SIMS H profiles for the two films annealed at 600C.  Shown are the as grown CH, the film CH when nucleation is first observed by XRD, and also the film CH at complete crystallization.  Two points of interest can be noted.  First, irrespective of the initial film CH, both films exhibit a similar CH (~0.1-0.2 at.%)   23 10 that the CH retained by the high CH film may act to retard grain growth and/or grain coalescence as the film is annealed to complete crystallization. Finally, we investigate whether a two–step anneal of the high CH film might result in a final grain size similar to that of the low CH film. Accordingly, we pre­annealed our high CH film for 50 hours in a conventional furnace at 400C to evolve a significant fraction of the 12.5 at.% H.  This film was then RTA annealed at 650C in a fashion identical to the previous experiment, and the grain size was examined. We observed that the grain size increased using this two­step procedure (from ~475Å to ~625Å), to a value more comparable to the results for the low CH film. This promising preliminary result suggests an avenue to further improve the crystallization of our high CH HWCVD films. As Grown Start Crystallization /175m Complete Crystallization /400m High C H  /600C/RTA ACKNOWLEDGEMENTS 2110 We thank Q. Wang, P. Stradins, and H.M. Branz for extensive discussions and exchange of ideas, and one 22 10 of us (B. Roy) thanks Q. Wang and P. Stradins for 20 10 mentoring in the early stages of this research. The NREL work was performed under DOE contract DE­19 10 AC36-99-GO10337. 1810 0.0 0.2 0.4 0.6 0.8 1.0 REFERENCES 1A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall, Depth (µm) and I. Balberg, J. Appl. Phys. 69, 6728 (1991). 23 2A.H. Mahan, B. Roy, R.C. Reedy Jr., D.W. Readey, 10 As Grown Start Crystallization /100m Complete crystallization /225m Low C H /600C/RTA Phys. Lett. 66, 3146 (1995)20 5S. Zafar and E.A. Schiff, Phys. Rev. Lett. 66, 1493 and D.S. Ginley, J. Appl. Phys. (2005) in press. 3P. Stradins, D. Young, H.M. Branz, M. Page, and Q. 22 10 Wang, Proc. Spring 2005 MRS Conf. (Symposium A), in press.4C. Godet, N. Layadi, and P. Roca i Cabarrocas, Appl. 21Atoms/cc 10 10 (1991). 19 10 MAJOR FY 2005 PUBLICATIONS 18 10 See reference 2 above. 0.0 0.2 0.4 0.6 0.8 1.0 Depth (µm) Fig. 3. SIMS H depth profiles for the 12.5 (top) and 2.7 (bottom) at.% CH films. 2 REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188 The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION. 1. REPORT DATE (DD-MM-YYYY) November 2005 2. REPORT TYPE Conference Paper 3. DATES COVERED (From - To)       5a. CONTRACT NUMBER DE-AC36-99-GO10337 5b. GRANT NUMBER  4. TITLE AND SUBTITLE Rapid Thermal Annealing of HWCVD a-Si:H Films: The Effect of the Film Hydrogen Content on the Crystallization Kinetics, Surface Morphology, and Grain Growth 5c. PROGRAM ELEMENT NUMBER  5d. PROJECT NUMBER NREL/CP-520-38953 5e. TASK NUMBER PVA6.4201 6. AUTHOR(S) A.H. Mahan, B. Roy, R.C. Reedy Jr., D.W. Readey, and D.S. Ginley5f. WORK UNIT NUMBER  7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO 80401-3393 8. PERFORMING ORGANIZATION REPORT NUMBER NREL/CP-520-38953 10. SPONSOR/MONITOR'S ACRONYM(S) NREL 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)  11. SPONSORING/MONITORING AGENCY REPORT NUMBER  12. DISTRIBUTION AVAILABILITY STATEMENT National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 13. SUPPLEMENTARY NOTES  14. ABSTRACT (Maximum 200 Words) We report the effect of the hydrogen (H) content (CH) on the crystallization kinetics, surface morphology and grain growth for Hot Wire CVD a-Si:H films containing 12.5 and 2.7 at.% H which are crystallized by rapid thermal anneal (RTA).  For the high CH film we observe explosive H evolution, with a resultant destruction of the film for RTA temperatures >750C.  At RTA temperatures ~600C, both films remain intact with similar morphologies.  At this same lower RTA, the incubation and crystallization times decrease, and the grain size as measured by X-Ray Diffraction increases with decreasing film CH.  SIMS measurements indicate that a similar film CH (<0.5 at.%) exists in both films when crystallization commences. The benefits of a two-step annealing process for the high CH film are documented. 15. SUBJECT TERMS Photovoltaics; solar; film hydrogen content; rapid thermal annealing; HWCVD a-Si:H films; PV; NREL 16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON  a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACTUL 18. NUMBER OF PAGES  19b. TELEPONE NUMBER (Include area code)  Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 

Rapid Thermal Annealing of HWCVD a-Si:H Films: The Effect of the Film Hydrogen Content on the Crystallization Kinetics, Surface Morphology, and Grain Growth

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Rapid Thermal Annealing of HWCVD a-Si:H Films: The Effect of the Film Hydrogen Content on the Crystallization Kinetics, Surface Morphology, and Grain Growth

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