By calibrating X-ray machine with cells of known diffusion lengths, measurements on test cells can be madeate rate of one every two minutes with standard deviation of less than two percent. test cells are compared with calibration cells whose diffusion lengths have been measured by an electron beam method

Lamneck, J. H., Jr.

English

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September 1970	 Brief 70-10150 
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Determination of Diffusion Lengths in Silicon by an X-Ray Method 
The problem: 
The study of radiation damage in silicon solar cells 
has necessitated large numbers of measurements of 
diffusion lengths of the cells. The diffusion length 
is the distance that current carriers—free electrons or 
"holes" (the absence of electrons)—produced by il-
lumination, move in the cell before being stopped by 
recombination. This length is important since, if a 
carrier does not reach the cell junction, it does not 
contribute to current flow in the cell. Measurements 
of diffusion lengths were previously made by a method 
which relied on the measurement of short-circuit cur-
rent generated by electron beam radiation. The elec-
tron beam method required the use of an expensive 
bombardment facility and many man-hours of experi-
mental work. Therefore, the amount of data obtain-
able on diffusion lengths was limited by practical 
considerations. 
The solution: 
The need for a rapid method of measuring diffusion 
lengths of silicon solar cells led to the development 
of a new procedure utilizing a standard X-ray ma-
chine. After the X-ray machine was calibrated with 
cells of known diffusion lengths, measurements on test 
cells could be made at the rate of one every two min-
utes with a standard deviation of less than two per-
cent. 
How it's done: 
This procedure is not an absolute method for 
determining diffusion lengths since it depends on a 
comparison between test cells and calibration cells 
whose diffusion lengths have been measured accurate-
ly by another method. The calibration cells are pro-
duced by electron beam bombardment to various 
dosages, thereby altering their crystal structure and 
resulting in cells with different diffusion lengths cover-
ing a range from 5 to 200 microns. These diffusion 
lengths are determined as accurately as possible by 
a method using an electron beam. The resultant series 
of cells are used to calibrate an industrial-type X-ray 
machine for rapid diffusion length measurements on 
unknown cells. 
A monitor cell is used to set the X-ray output to 
the same radiation value for all tests. Calibrated cells 
are exposed to the constant X-ray radiation and their 
current outputs per unit area are plotted against their 
known diffusion lengths. Current readings are then 
measured for the unknown cells with the same con-
stant that X-ray output and diffusion lengths are read 
from the calibration curve. 
Notes: 
I. This method may be of interest for the determina-
tion of diffusion lengths in other semiconductor 
devices. 
2. The following documentation may be obtained 
from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.65) 
Reference: 
NASA-TM-X-1894 (N69-37827), Diffusion 
Lengths in Silicon Obtained by an X-Ray 
Method
(continued overleaf) 4
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Technical questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B70-10150 
Patent status: 
No patent action is contemplated by NASA. 
Source: J. H. Lamneck, Jr. 
Lewis Research Center 
(LEW- 10984)
is 
Brief 70-10150	 Category 01


Determination of diffusion lengths in silicon by an X-ray method

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