The atomic hydrogen flux during microcrystalline silicon solar
cell deposition
Citation for published version (APA):
Sanden, van de, M. C. M., Dingemans, G., van den Donker, M. N., Hrunski, D., Gordijn, A., & Kessels, W. M. M.
(2009). The atomic hydrogen flux during microcrystalline silicon solar cell deposition. In Proceedings of the 33th
IEEE Photovoltaic Specialist Conference (PVSC 2008) 11-16 May 2008, San Diego, CA, USA (pp.
000705-000708). Institute of Electrical and Electronics Engineers. https://doi.org/10.1109/PVSC.2008.4922764
DOI:
10.1109/PVSC.2008.4922764
Document status and date:
Published: 01/01/2009
Document Version:
Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)
Please check the document version of this publication:
• A submitted manuscript is the version of the article upon submission and before peer-review. There can be
important differences between the submitted version and the official published version of record. People
interested in the research are advised to contact the author for the final version of the publication, or visit the
DOI to the publisher's website.
• The final author version and the galley proof are versions of the publication after peer review.
• The final published version features the final layout of the paper including the volume, issue and page
numbers.
Link to publication
General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain
• You may freely distribute the URL identifying the publication in the public portal.
If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement:
www.tue.nl/taverne
Take down policy
If you believe that this document breaches copyright please contact us at:
openaccess@tue.nl
providing details and we will investigate your claim.
The atomic hydrogen flux during microcrystalline
silicon solar cell deposition
M.C.M. van de Sanden 1, G. Dingemans 1, M. N. van den Donker 3, D. Hrunski 2, A. Gordijn2, W.M.M. Kessels 1 1 Department of Applied Physics, Eindhoven University ofTechnology, P.O. Box 513,5600 MB Eindhoven, The
Netherlands, 2IEF5-Photovoltaik, FZ Juelich, Juelich, Germany, 3Solland Solar BV, Heerlen, The Netherlands
ABSTRACT
Etch product detection by in situ optical emission spectroscopy is used to detect the phase transition from amorphous to microcrystalline silicon. In this contribution it is demonstrated that a calibrated version of this technique can be used to determine the absolute hydrogen flux under state-of-the-art silicon thin film deposition conditions.
INTRODUCTION
The phase transition from amorphous- to microcrystalline silicon and associated growth and nucleation processes are key issues of research in the field of microcrystalline silicon. In the past a range of diagnostic techniques has been applied to study the microstructure of microcrystalline silicon as it evolves as a function of process parameters and film thickness. Nonetheless, a straightforward, easily applicable diagnostic for in-situ studies is still lacking.
A widely used, non-invasive, easy-to-apply and in situ plasma diagnostic is optical emission spectroscopy. In this contribution we present a novel approach to optical emission spectroscopy, by which the phase composition of I.lc-Si:H can be studied in-situ.
The etch product detection method is based on the difference in etch probability between disordered (a-Si) and ordered (c-Si) material. The method was successfully correlated with in-situ absorption spectroscopy and Raman spectroscopy and has been cross-checked on various reactors operating in a wide range of process settings [1].
Employing a calibrated version of the technique the atomic hydrogen flux is experimentally determined under state-of-the-art silicon thin film for different process parameter settings in the high pressure depletion regime, such as pressure, silane concentration,
RF
power and excitation frequency.978-1-4244-1641-7/08/$25.00 ©2008 IEEE
> :I:! rJ) c:.fa
.9! .50.8 en wo
.
::c i:i5 0.6 0.8 (b) 0.6 0.4 0.2 0.0 5 4.fa
~_ - substrate center~~
-0-'"M_
ro~,
I~
i~-Q-Q
SiH4 flow (seem)
0.8 0.7 ~ 8 0.6 ~ 0.5 0.4
Fig. 1: (a) SiH* emission intensity as obtained by H2 plasma etching of silicon films deposited under various SiH4 flows. The inset is a schematic of the experimental procedure employed. (b) Raman crystallinity IcRS of the
silicon films as measured in the center and corner of the substrate. The laser wavelength was 680 nm yielding a Raman penetration depth exceeding the film thickness. (c) Solar energy conversion efficiency 11 and open circuit voltage Voc for the corresponding thin film silicon p-i-n solar cells. No ZnO back reflector was applied.
ETCH PRODUCT DETECTION DURING RF-PECVD OF MICROCRYSTALLINE SILICON
The phase composition of microcrystalline silicon (IJC Si:H) varies with process settings such as SiH4 concentration, RF power and deposition pressure. The optimal phase composition of the IJc-Si:H silicon solar cell absorber layer is found in a narrow process window just before entering the amorphous growth regime (ct. Fig. 1).1-3 The presence of atomic hydrogen is
recognized as a key parameter influencing the crystallinity of films 3-6, and in the literature it is often
speculated that the hydrogen to Si growth precursor flux ratio should exceed a critical value for the nucleation of crystalline phase material to occur. The atomic hydrogen flux during deposition can in principle be determined by advanced techniques such as two-~hoton
laser induced fluorescence or mass spectrometry ,8, but
no easy-to-implement diagnostic has been available. Consequently technologically relevant data conceming the abundance of atomic hydrogen during IJc-Si:H deposition remain largely based on correlations with optical emission spectroscopy (OES) signals and modeling work.6,9-13
Preferential insertion of atomic hydrogen in strained Si Si bonds and subsequent silicon etching leads to a higher etch rate of amorphous silicon (a-Si:H) relative to crystalline Si.14-16 In a recent letter this difference in etch
rate was exploited to determine in-situ the phase composition of silicon films in the IJc-Si:H growth regime. Both optical emission spectroscopy (OES) and infrared absorption spectroscopy in the gas exhaust were employed for the detection of (SiH4) etch products during H2 plasma probing of the film surface, and a correlation between the etch product density and the amorphous silicon fraction of a silicon film was established (ct. Fig. 1).17
THE ATOMIC HYDROGEN FLUX DURING MICROCRYSTALLINE SILICON SOLAR CELL DEPOSITION
In this contribution a quantification of the etch product density in terms of the absolute atomic hydrogen flux is introduced and tested under various plasma conditions and on different reactors. The atomic hydrogen flux is given by
(1)
with <1>etch the etch product flow at low SiH4 dilutions (ct.
Fig. 1), Asurface the total exposed film surface area, Vetch
the atomic hydrogen etch probability, The right hand side of Eq. (1), except for Vetch, can be determined
experimentally employing the technique of etch product detection. Under different pressure and power condition we have determined the silicon etch flow <1>etch through a
calibration of <1>< by studying the phase transformation
978-1-4244-1641-7/08/$25.00 ©2008 IEEE
as function of the silane flow. The result is given in Figs.
2.
Using typical values for the etch yield and the surface area of the electrodes the absolute hydrogen flux can be determined. Since the absolute hydrogen flux is thus directly proportional to the silicon etch flow <1>etch it canbe concluded that the hydrogen flux increases with applied plasma power and decreases with increasing pressure. E 0.8
!
jj 0.6 eO PdOP=
10 Torr 0.4 L...L._~---'-_~--'--_~--'----' 60 80 100 120 power (W) 5 10 15 pressure (Torr)Figure 2: The silicon etch flow <1>etch as function of power
and pressure. The atomic hydrogen flux can be determined if the hydrogen etch yield is know (cf. Eq.
(2».
REFERENCES
1. O. Vetterl, F. Finger, R. Carius, P. Hapke, L. Houben, O. Kluth, A. Lambertz, A. MOck, B. Rech, and H. Wagner, Sol. Energy Mater. Sol. Cells 62. 97 (2000).
2.
M.N. van den Donker, B. Rech, R. Schmitz, J. Klomfass, G. Dingemans, F. Finger, L. Houben, W.M.M. Kessels, M.C.M. van de Sanden, J. Mater. Res. 22. 1767 (2007).3. S. Klein, F. Finger, R. Carius, M. Stutzmann, J. Appl. Phys. 98. 024905 (2005).
4. J. Kocka, T. Mates, H. Stuchlikova, J. Stuchlik, A. Fejfar, Thin Solid Films 501. 107 (2006).
5. P. Roca i Cabarrocas and S. Hamma, Thin Solid Films 337. 23 (1999).
6. Matsuda, J. Non-Crys. Solids 338-340. 1 (2004). 7. X. R. Duan, H. Lange, A. Meyer-Plath, Plasma
Sources Sci. Technol. 12.554 (2003).
8. P. Kae-Nune, J. Perrin, J. Jolly, J. Guillon, Surface Science 360. 495 (1996).
9. L. Guo, M. Kondo, M. Fukawa, K. Saitoh, and A. Matsuda, Jpn. J. Appl. Phys. Part 237. 1116 (1998).
10. E. Amanatides, D. Mataras, D. Rapakoulias, M.N. van den Donker, B. Rech, Sol. Energy Mater. Sol. Cells 87.795 (2005).
11. B. Strahm, AA Howling, L. Sansonnens, Ch. Hollenstein, Plasma Sources Sci. Technol. 16.80 (2007).
12. J. K. Rath, R. H. J. Franken, A. Gordijn, R. E. I. Schropp, W. J. Goedheer, J. Non-Cryst. Solids 338-340. 56 (2004).
13. B. Lyka, E. Amanatides, D. Mataras, J. Non-crystal. Solids 352. 1049 (2006).
14. M. Otobe, M. Kimura, S. Oda, Jpn. J. Appl. Phys. 33. 4442 (1994).
15. Solomon, B. Drevillon, H.Shirai, N. Layadi, J. Non Cryst. Solids 164-166.989 (1993).
16. M. Heintze, W. Westlake, P.V. Santos, J. Non Cryst. Solids 164-166.985 (1993).
17. G. Dingemans, M.N. van den Donker, A. Gordijn, W.M.M. Kessels and M.C.M. van de Sanden, Appl. Phys. Lett. 91.161902 (2007).
