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Submitted on 1 Jan 1981
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AMORPHOUS SILICON IN PHOTOTHERMAL
CONVERSION
B. Seraphin, D. Booth, D. Allred
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque Cl, suppzdment au nO1, Tome 42, janvier 1981 page Cl-437
SURFACES SR-ECTIVES
-
HAUTE TEMPERATURE.B.O. Seraphin, D.C. Booth and D.D. Allred
O p t i c a l S c i e n c e s C e n t e r U n i v e r s i t y o f A r i z o n a , T u c s o n , A r i z o n a 85 721, U. S.A.
Abstract.-Efficient conversion of solar energy into heat requires a spectrally selective surface to function as a one-way valve be- tween the incident radiation and heat transfer system. The tandem action of a solar absorber overlying an infrared reflector gives this action, provided the absorber is transparent in the thermal infrared /l/. Our group has fabricated such tandem stacks, dura- ble at high temperatures, by depositing both absorber and reflec- tor layers by Chemical Vapor Deposition (CVD), a method of econo- mic promise for large-scale production /2, 3/.
Although the initial work used polycrystalline silicon as the so- lar absorber, it was realized that amorphous silicon could raise the solar absorptance of the CVD tandem stacks / 4 / . However, there existed a major obstacle to its use : amorphous silicon deposited by the two most widely used methods of physical vapor deposition, evaporation and sputtering, crystallized near 550°C /5/. This tem- perature is too low for photothermal energy converter operation. It is also so low that the pyrolytic decomposition of silane, the basic reaction for producing silicon CVD, proceeds at an infinite-
simal rate.
In an attempt to pass through the amorphous-to-crystalline transi- tion temperature of 550°C without reducing the deposition rate too drastically, we learned that silicon fabricated by CVD is amorphous for substrate temperatures, T,, of up to 670°C. The improved ther- mal stability of the amorphous phase may be credited to the pre- sence of hydrogen, incorporated into the film in amounts to less than 1 at. % depending upon Ts, due to the incomplete breakup of the silane molecule /6/.
Although far below the hydrogen content characteristic for material deposited in an RF flow discharge, this fraction of 1 at. % of hy- drogen effectively terminates dangling bonds. Electron spin reso- nance indicates spin densities in the order of 1 0 l ~ c m - ~ in the CVD material /7/.
The considerable thermal stability of CVD amorphous silicon promi- ses satisfactory operation of the absorber over extended periods of time at 450°C. Specific density, absorption coefficient, and refractive index were determined, the optical data confirming the superiority of amorphous over polycrystalline silicon as a solar absorber. Anneal at temperatures below the crystallization tempera- ture Tc of 670°C proved that CVD amorphous silicon, unlike material deposited by other methods, is anneal-stable
/a/.
The hypothesis that the retardation of the crystallization is cau- sed by the presence of hydrogen stimulated a search for an even better stabilizer. Among the various elements introduced from the vapor phase into the growing film the most promise is shown by car-
X
This work is supported by the following contract : U.S. Department of Energy, Division of Materials Sciences, Office of Basic Energy Scion- ces,
#
ER-78-S-02-4899.JOURNAL DE PHYSIQUE
bon / 9 / . Codepositionof 18 at. % carbon from acetylene gives a ma- terial that retains the solar absorptance of the non-intentionally doped amorphous silicon, but retards crystallization to 950°C. Ex- trapolation on the basis of the crystallization kinetics predicts structural stability of this solar absorber over hundreds of years at 700°C operating temperature.
References
Seraphin, B.O., and Meinel, A.B., Properties of Solids. New Developments,(North-Holland Publishing Cc.) 1976 p
.
927-971.Seraphin
,
B.O., Solar Energy Conversion, Solid State Physics Aspects, B.O. Seraphin, ed., Topics in Applied Physics,(Springer-Verlag,
Berlin-Heidelberg-N.Y.)31
(1979) p.5.
Hahn, R.E. and Seraphin, B.O., Physics of Thin Films, G. Hass, ed., (Academic Press, New York)
10
(1978) p. 1. Hahn, R.E. and Seraphin, B.O., J. Vac. Sci. Technol. 12, N o 4 (1975) 905-908.-
Brodski, M.H., Frisch, M.A., Zeigler, J.F., Lanford, W.A., Appl. Phys. Lett.
30
(1977) 561.Booth, D.C., Allred, D.D., and Seraphin, B.O., J.Non-Cryst. Solids 35/36 (1980) 213-218.
Booth, D.C., Allred, D.D., and Seraphin, B.O., Solar Energy Mater.
