Technique fabricates hybrid structure utilizing low oxygen silicon as bulk cell material and shallow overlay of silicon with high oxygen concentration

Iles, P. A.

English

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1972	 B72-10085 
•	 NASA TECH BRIEF 
NASA Pasadena Office
	 -4r 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage1 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Oxygen-Layer Structure Improves Lithium-Doped Silicon Solar Cells 
The problem: 
Solar cells made from lithium-doped silicon with 
a high oxygen content recover up to 100 times more 
slowly after irradiation than cells made with silicon 
of low oxygen content; on the other hand, cells made 
with silicon of low oxygen content are considered 
unstable, for after recovery, their electrical charac-
teristics are found to be degraded. It was considered 
desirable to fabricate a cell which combined the ad-
vantage offered by oxygen-poor silicon in aiding bulk 
recovery with the superior stability of oxygen-rich 
silicon. 
The solution: 
Fabricate a hybrid structure which utilizes low-
oxygen-content silicon as the bulk cell material and 
a shallow overlay of silicon with a high oxygen 
concentration. 
How it's done: 
A slice of low-oxygen-content silicon is selected of 
a size, resistivity, and surface finish similar to the 
usual lithium-doped cells. Then a thin oxygen-rich 
layer is formed by heating the slice for several hours 
at 1150° to 1300°C in an oxygen-rich atmosphere. 
Calculations based on published diffusion coefficients 
for oxygen in silicon indicate that a 0.00254-cm oxy-
gen-rich layer will be formed in 20 hours at 1250°C; 
in such a layer, the distribution of oxygen will range 
from a high value of about 1018 atoms per cm3
 at the 
surface of the silicon slice to a low value of less 
than 1015/cm3
 near the oxygen diffusion front within 
the slice.
Surface oxide films are then removed by dipping 
the slice in hydrofluoric acid or other solutions that 
will not attack and remove any of the diffused-oxygen 
layer. At this point, it is useful to establish the depth 
to which the oxygen-rich layer has been formed; 
several methods are available: 1) The slice is sec-
tioned and etched with a solution that exposes edge 
dislocations; since the density ,
 and size of etch pits 
will be different in the diffused-oxygen layer than 
in the matrix,, the depth of oxygen penetration can 
be measured; 2) The slice is heated at 450°C for 
several hours to convert some oxygen atoms to donors 
so that the oxygen-diffused layer becomes N-type 
and can be located and delineated (with difficulty) 
on the N-type slice; however, it is simpler to locate 
the N-type oxygen-diffused layer on a control P-type 
slice which has been treated with oxygen in the same 
way as N-type silicon slices; 3) The absorption of in-
frared radiation at approximately 9 jm that is trans-
mitted through the sample is determined; increased 
absorption, compared with an identical sample that 
is not oxygen-diffused, indicates that an oxygen layer 
has been formed (but does not provide a quantitative 
value unless calibration curves are prepared for each 
batch). 
Subsequently, a P-layer is formed on the slice by 
boron diffusion, using sources such as boron oxide, 
boron nitride, or boron tribromide; the process must 
not remove appreciable amounts of the oxygen-rich 
layer (preferably none). A front-surface grid contact 
and antireflective coating are applied to the cell and 
then the back surface is treated (lapped, etched, etc.) 
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
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https://ntrs.nasa.gov/search.jsp?R=19720000085 2020-03-17T03:25:17+00:00Z
to expose the starting N-silicon; lithium is diffused 
into the cell as usual. Finally, a back contact is ap-
plied and, if necessary, the PN junction is cleared of 
shorting paths. 
Notes: 
1. Tests have shown that the perfection of the start-
ing silicon crystal is not degraded as a result of 
the oxygen-diffusion process. 
2. Requests for further information may be di-
rected to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B72-10085 
Patent status: 
No patent action is contemplated by NASA. 
Source: Peter A. Iles of

Centralab Semiconductor Division 
under contract to

NASA Pasadena Office 
(NPO-11403)
. 
Category 03


Oxygen-layer structure improves lithium-doped silicon solar cells

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