We report the deposition of optimum-quality a-SiGe:H and a-Si:H by the hot-wire chemical vapor deposition (HWCVD) technique using a tantalum filament operating at a low temperature. We gauge the material quality of the a-SiGe:H films by comparing infrared, small-angle x-ray scattering, photocapacitance, and conductivity results to those presented elsewhere

Gedvilas, L. M.

Mahan, A. H.

Xu, Y.

UNT Digital Library

 National Renewable Energy Laboratory Innovation for Our Energy Future A national laboratory of the U.S. Department of EnergyOffice of Energy Efficiency & Renewable EnergyNREL is operated by Midwest Research Institute ● Battelle     Contract No. DE-AC36-99-GO10337 Conference Paper NREL/CP-520-37525 January 2005 a-SiGe:H Deposited by Hot-Wire CVD Using a Tantalum Filament Operated at Low Temperature A.H. Mahan, Y. Xu, and L.M. Gedvilas Presented at the 2004 DOE Solar Energy Technologies  Program Review Meeting October 25-28, 2004 Denver, Colorado 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 a-SiGe:H Deposited by Hot-Wire CVD Using a Tantalum Filament Operated at Low Temperature  A.H. Mahan, Y. Xu, and L.M. Gedvilas National Renewable Energy Laboratory (NREL) 1617 Cole Blvd., Golden, CO 80401 (harv_mahan@nrel.gov)   ABSTRACT  We report the deposition of optimum-quality a-SiGe:H and a-Si:H by the hot-wire chemical vapor deposition (HWCVD) technique using a tantalum filament operating at a low temperature.  We gauge the material quality of the a-SiGe:H films by comparing infrared, small-angle x-ray scattering, photocapacitance, and conductivity results to those presented elsewhere.  1. Introduction The incorporation of high-Ge-content a-SiGe:H into a low-bandgap solar cell commonly involves the use of bandgap (Ge) profiling, where the material with the lowest bandgap, or the highest Ge content, is placed near the middle of the i-layer to avoid an abrupt bandgap transition at the n/i and p/i interfaces [1,2].  Since in this approach the material must be profiled over a wide range of Ge content, all i-layer alloy compositions must be of device quality and must be deposited using similar process conditions.  This work reports on deposition of optimum-quality a-SiGe:H by the HWCVD technique using a tantalum (Ta) filament operating at low temperatures [3].  We gauge the material quality by comparing infrared, small-angle x-ray scattering (SAXS), and photocapacitance results to those presented elsewhere, and conclude that the structural heterogeneity of the HWCVD a-SiGe:H films can be significantly reduced compared to those deposited by plasma-enhanced chemical vapor deposition (PECVD) by using low filament temperatures.  We show that this improved structural homogeneity translates into improved electronic properties, as measured by the σl/σd conductivity ratio.  2. Experimental The a-SiGe:H films were grown by HWCVD using a 0.5-mm-diameter Ta filament located 5 cm from the substrate holder, in a tube reactor described elsewhere [4]. Table 1 gives representative deposition parameters for an a-SiGe:H film containing ~65 at.% Ge using a filament temperature of 1750°C and a Tsub of 200°C.  The Ge film content was varied by changing the GeH4/SiH4 gas flow ratio. Each film was deposited simultaneously on 1737F Corning glass and c-Si wafers, with evaporated coplanar  (width to length = 0.05) Cr contacts on the films on the 1737F substrates enabling conductivity measurements.  Optical measurements were performed on these films using an n&k 1280 Analyzer to determine the Tauc’s bandgap, whereas Fourier transform infrared absorption spectra were obtained from transmission measurements for the films deposited on the c-Si wafers using a Nicolet 510 system operating between 400 and 4000 cm-1. Secondary ion mass spectrometry measurements of the Ge film content enabled the preferential attachment ratio (PA) to be calculated.  In a separate series of depositions, 1.5–2.0-µm-thick films covering a wide range of Ge contents were deposited on high-purity Al foil for the SAXS measurements, and on SS/n+ layers for the photocapacitance measurements.  Table 1. Representative Deposition Parameters for an a-SiGe:H Film Containing ~65 at.% Ge.  SiH4 (sccm) GeH4 (sccm) H2 (sccm) Pch (mT) Rd (Å/s) Eg (eV) l/d 16 9 25 12.5 2.5 1.23 140  3. Results and Discussion Previous work has reported the infrared (IR) stretch mode bonding configurations for HWCVD films deposited for Ge film contents ranging between 0 and 100% [5].  Of interest are the virtual lack of SiH2 and/or (SiH2)n bonding appearing in the ‘shifted’ mode at  2090 cm-1, and the relative amount of H bonded to Ge atoms versus Si atoms. In Fig. 1 we show the IR SiH2/(SiH +SiH2) intensity ratio (R) for Ge film contents ranging from 15 to 80 at.%. This ratio for the HWCVD films is at most 0.1 and becomes difficult to detect for Ge film contents exceeding 50%, whereas that for the PECVD films [6,7] is a factor of 2–3 higher and, for one of the studies, shows an opposite trend for higher Ge film contents [7].  0.50.40.30.20.10.0Microstructure Factor (R)100806040200Film Ge Content (at.%) PECVD (Julich) PECVD (Kyoto) HWCVD (NREL)   Fig. 1. R versus film Ge content.  Figure 2 presents SAXS data for low Tfil for film Ge contents ranging from 0 to 80 at.%.   Whereas prior trends in SAXS densities with increasing film Ge content show a  significant increase in SAXS signal for very low Ge incorporations [8], the present curve remains lower than 1the ‘standard’ curve over the whole composition region.  The only films with lower SAXS values are those deposited using high-bias microwave conditions, and these have high defect densities due to the high energy bombardment conditions.  We further suggest that these low SAXS densities for the present HWCVD films, achieved with minimal bombardment energies, have a beneficial effect on the films’ electronic performance.                   Fig. 2. Integrated SAXS intensity versus film Ge content.  The other factor contributing to the improved structure of the present HWCVD a-SiGe:H films is the increase in the amount of GeH bonding.  Assuming that the proportionality constants for the SiH and GeH stretch modes are similar, we determine the amount of SiH and GeH bonding by normalizing these stretch mode areas to the films’ H content.  These results, shown in Fig. 3 along with representative PECVD data [6], translate into lower preferential attachment ratios for the HWCVD films over the whole range of film Ge contents [5].  Fig. 3.  Ge-H bonding (at.%) versus film Ge content.    Because of these improvements in film heterogeneity, we have submitted a series of a-SiGe:H films for photocapacitance measurements of the film bandtails and defect densities.  Preliminary results for the first film, containing ~29% Ge, have been obtained.  In this film, the Urbach edge value of 42 mV is quite narrow and compares favorably with the narrowest Urbach edges measured for any a-SiGe:H film [9].  Measurements are in progress to determine the film defect density for this film and to extend the results to other film Ge contents.   4. Conclusions  We have reported the deposition of optimum-quality a-SiGe:H by the HWCVD technique using a tantalum filament operating at a low temperature.  We have gauged the material quality of the a-SiGe:H films by comparing infrared, SAXS, photocapacitance, and conductivity results to those presented elsewhere, and have concluded that the present films exhibit a significantly improved structural heterogeneity compared to films deposited by the PECVD technique.   ACKNOWLEDGEMENTS The authors thank D.L. Williamson and J.D. Cohen for SAXS and photocapacitance measurements on these films.  This work was supported by DOE Contract No. DE-AC36-99GO10337.   REFERENCES 1. S. Guha, J. Yang, A. Pawlikiew, T. Glatfeilter, R. Ross, and S.R. Ovshinsky, Appl. Phys. Lett. 54 (1989) 2330. 2. J. Fölsch, H. Stiebig, F. Finger, B. Rech, D. Lundszien, A. Lambertz, and H. Wagner, 25th PVSC IEEE Conference, Washington, D.C. (1996) 1133-1136.    PECVDHWCVD3. A.H. Mahan, Y. Xu, and L.M. Gedvilas, MRS Symp. Proc. 808 (2004) A9.48. 4. Y. Xu, B.P. Nelson, L.M. Gedvilas, and R.C. Reedy, Thin Solid Films 430 (2003) 197-201. 5. Y. Xu, L.M. Gedvilas, R.C. Reedy, H.M. Branz, and A.H. Mahan, Proc. 3rd Intl. Conf. on Hot-Wire CVD (Cat-CVD) Process, Utrecht, The Netherlands, 2004, to be published in Thin Solid Films. 6. A. Terakawa, Ph.D. Thesis, Kyoto University (1999). 7. J. Folsch, F. Finger, T. Kulessa, F. Siebke, W. Beyer, and H. Wagner, MRS Symp. Proc. 377 (1995) 517. 8. D.L. Williamson, NREL Report SR-520-31908 (2002) 37. 9. J.D. Cohen, in EMIS Data Review Series 19 (1998) 180.     43210806040200Ge content (%) 2F1147-E(12/2004) 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) January 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 a-SiGe:H Deposited by Hot-Wire CVD Using a Tantalum Filament Operated at Low Temperature 5c. PROGRAM ELEMENT NUMBER  5d. PROJECT NUMBER NREL/CP-520-37525 5e. TASK NUMBER PVA54101 6. AUTHOR(S) A.H. Mahan, Y. Xu, and L.M. Gedvilas 5f. 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-520-37525 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 deposition of optimum-quality a-SiGe:H and a-Si:H by the hot-wire chemical vapor deposition (HWCVD) technique using a tantalum filament operating at a low temperature.  We gauge the material quality of the a-SiGe:H films by comparing infrared, small-angle x-ray scattering, photocapacitance, and conductivity results to those presented elsewhere. 15. SUBJECT TERMS PV; hot-wire chemical vapor deposition (HWCVD); tantalum filament; tantalum filament; small-angle x-ray scattering; photocapacitance;  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. TELEPHONE NUMBER (Include area code)  Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 

a-SiGe:H Deposited by Hot-Wire CVD Using a Tantalum Filament Operated at Low Temperature

https://digital.library.unt.edu/ark:/67531/metadc788704/m2/1/high_res_d/860925.pdf

a-SiGe:H Deposited by Hot-Wire CVD Using a Tantalum Filament Operated at Low Temperature

Abstract

Similar works

Full text

Available Versions

UNT Digital Library