Doping silicon material with gadolinium to increase radiation resistance of solar cell

Mandelkorn, J.

English

NASA Technical Reports Server

e c 
the material is bombarded with high energy atomic par- 
ticles, such as 1 m.e.v. electrons or 10 m.e.v. protons. 
The theory for the limited lifetime of minority carriers, 
of the order of microseconds, in semiconductor material 
3 akmor to 5 postulates the existence of configurations of atoms within 
the material which act to attract and combine minority 
3,3PP,510 
the united States Of *lnerica as represe5ted ’3’ the 
Qf *he Aeronautics and majority carriers, which are alwavs of oaoosite elec- Admidstration 
No Drawing. Filled Mar. 16, 1964, Ser. No. 352,400 
4 Claisi~lls. (C1. 148-1.5) 
The invention described herein may be manufactured 
and used by or for the Government of the United States of 
America for governmental purposes without the payment 
of any royalties thereon or therefor. 
The present invention relates to a semiconductor mate- 
rial having improved properties and greater resistance to 
radiation damage. More particularly, the invention is 
concerned with an improved silicon material for semi- 
conductor devices having a small quantity of a rare earth 
trical sign. Such configurations are-referrid to as re- 
combination centers, and these centers expedite the neu- 
0 tralization of minority carriers thereby decreasing the life- 
time of the minority carriers. 
The increase in resistivity of material which occurs 
when the material is bombarded is attributed to con- 
figurations of atoms in the material called trapping centers 
5 which attract and then immobilize majority carriers for 
a period of time. This effectively increases the resistivity 
because, at any given moment, a quantity of majority 
carriers are immobilized. The quantity of immobilized 
carriers at any instant depends upon the density of the 
element or compound in the silicon. 20 trapping centers. The mechanisms whereby bombard- 
The power output of a silicon solar cell exhibits con- ment increases the density of recombination centers there- 
siderable degradation when it is subjected to high energy by decreasing minority carrier lifetime, and increases the 
atomic particle bombardment, and this affect is detri- density of trapping centers thereby increasing resistivity 
mental to the useful life of space vehicles which utilize were previously presumed to be attributable to vacancies 
such semiconductor devices as sources of power. This 25 created in the lattice by bombardment which form recom- 
radiation damage results from an undesirable loss in the bination or trapping center configurations. 
lifetime of minority carriers within the silicon material It was also previously believed that the concentration of 
from which the solar cells are made. the electrically active impurities in the semiconductor 
It has been ascertained that minority carrier lifetime material was related to the lifetime of minority carriers 
in bombarded silicon is affected by the impurities which 30 only because each impurity atom contributed a majority 
have been added to obtain the desired resistivity of the carrier, and the greater the number of majority carriers, 
material. More particularly, for a particular bombard- the greater the probability of a minority carrier combining 
ment dose the value of minority carrier lifetime in silicon with a majority carrier. The relationship between con- 
material containing a small added quantity of the rare centrations Of these impurity atoms and minority carrier 
earth element gadolinium is higher than that in silicon 35 lifetime is based on considerations of the statistical prob- 
containing certain other impurities added to reduce the ability Of a minority carrier recombining with a majority 
resistivity of the material to equivalent values. carrier through the media of a fixed density of recombi- 
It is, therefore, an object of the present invention to nation centers. 
provide a ‘‘P” type material having improved character- The present invention is based on the discovery that the 
istics for making semiconductor devices. 40 atoms of various electrical active impurity elements them- 
Another object of the invention is to provide an im- selves affect the formation of recombination centers in 
proved silicon solar cell material containing controlled the semiconductor material. It has also been discovered 
amounts of rare earth elements which result in slower that certain of these impurity elements are more desirable 
degradation of the power output of the cell when sub- than others because the presence of atoms of the desirable 
jected to high energy particle bombardment. 45 impurity results in the formation of either less recombi- 
A further object of the invention is to provide a silicon nation centers or less trapping centers than the presence of 
material containing a small quantity of gadolinium to atoms of other electrically active impurity elements in 
improve the propkrties and radiation damage resistance equal concentration. 
of the material. According to the present invention, small amounts of 
Other objects and advantages of the invention will be 50 a rare earth element or compound are added to a semi- 
apparent from the specification which follows. conductor material to reduce its resistivity and preserve 
Certain electrically active impurities are normally added good material Properties. BY way of example, gadolini- 
in controlled quantities to materials, such as silicon, that um is added to silicon, and solar cells made from this ma- 
are to be used in semiconductor devices in order to lower terial are superior to those made from silicon doped with 
the resistivity to a desired value. Boron is an electrically 
of the silicon are altered in this manner because such active impurity which has been added to silicon in the 
impurities ionize in the silicon and each ionized impurity Past to reduce the resistivity. 
atom contributes one mobile charge, called a majority To illustrate the beneficial technical effect of the pres- 
carrier, to the material. ent invention, groups of solar cells were fabricated from 
Other mobile charges, called minority carriers, result 60 several impurity-doped silicon ingots, each having an im- 
from the breaking of covalent bonds between silicon Purity concentration sufficient to produce a resistivity in 
atoms. The operation of many semiconductor devices, the ten to twenty ohm-centimeter range. The first group 
such as transistors, diodes and solar cells, depends upon of cells was fabricated from an ingot of boron-doped si& 
the currents arising from the movement of these minority con grown from a melt in a quartz crucible, and these 
carriers. Therefore, the mobility and lifetime of minority 65 cells are identified as C-B in the Table 1 below. 
carriers in semiconductor materials are extremely signifi- A second group of cells was similarly fabricated from 
cant in determining device performance. However, the a gadolinium-doped ingot, and the cells made from this 
presence of the large quantities of majority carriers in ingot are identified as C-Gd. The gadolinium can be 
silicon required to reduce the resistivity of the material added to silicon by direct addition of pure gadolinium to 
to a desirable value decreases the mobility and lifetime 70 silicon in a crucible or by zone melting the pure gadolini- 
of the minority carriers in the material. A further de- um into the silicon. The addition of this element can 
crease in the lifetime of minority carriers occurs when also be accomplished by adding gadolinium compounds in 
The electrical properties 55 an eqUal concentration Of boron. 
https://ntrs.nasa.gov/search.jsp?R=19710001133 2020-03-17T02:27:34+00:00Z
3,311,510 
4.1X101S 
L CPF, 
12x1015 1.5~1018 
L (microns) L 
percent 
.______________ 
36 13 10 65 
44 16.5 12 55 
3 
the manner described or by the preparation of a master 
alloy of silicon and gadolinium which is added to the 
silicon. 
Both groups of cells were subjected simultaneously to 
a series of 1-m.e.v. electron bombardments as illustrated 
in Table 1. The diffusion length (L) was measured for 
each cell after each bombardment, and the curve power 
factor (CPF) was measured for each cell after the last 
bombardment. Table 1 presents average characteristics 
of the two groups of cells after these bombardments. Be- 
cause the cells within each group had uniform character- 
istics after bombardment, valid differentiation in terms of 
average diffusion length preserved was possible. 
TABLE 1 
Cell Group 
I. 
C-B -. _ _ _ _ _ _ _  
C-Gd ..._._._. 
Number 
Of 
Samples 
2 
2 
Dose (e/sq. em.) I- I I 
The diffusion length (L) in Table 1 is defined by the 
equation L = ~ D x T ,  where D is the diffusion constant 
and r is the minority carrier lifetime. Therefore, the 
diffusion length is directly proportional to  the minority 
carrier lifetime, and a longer diffusion length after bomb- 
ardment indicates a larger preserved value of minority 
carrier lifetime in the material. 
The curve power factor (CPF) is defined as the ratio, 
expressed in percent, of the maximum power output of 
the cell to the product of its open-circuit voltage and 
short-circuit current. It is evident from Table 1 that the 
curve power factor of the C-Gd group is the most ad- 
versely affected by the bombardment while the C-B is 
the least affected. This indicates that gadolinium in sili- 
con acts to increase the majority carrier removal rate. 
It can also be seen in Table 1 that the C-Gd group pre- 
serves longer diffusion lengths under bombardment than 
the C-B group. This shows that gadolinium acts to pre- 
serve longer minority carrier lifetimes under bombard- 
ment. 
It is evident from Table 1 that the properties of gado- 
iinium are superior to the properties of boron insofar as 
the influence of such properties on the formation of re- 
combination centers is concerned. More particularly, the 
boron atom is involved in the formation of bombardment 
introduced recombination centers, and the presence of 
boron in the silicon lattice in high concentration around 
the region of the junction creates strains and distortions 
of the lattice structure and results in the formation of re- 
combination centers in the junction region. The replace- 
ment of boron by gadolinium improves junction character- 
istics as well as radiation damage resistance, and the con- 
centration of gadolinium may be controlled to produce 
desired resistivities. The improved properties of gado- 
linium-doped material can be attributed to the atomic 
structure of the gadolinium atom. 
Because the properties of gadolinium are similar to 
those of other rare earth elements, the addition of other 
rare earth elements to silicon will also reduce the resis- 
tivity of the material and produce improved characteris- 
tics. From experiments with other rare earth elements 
it is found that some of them, such as samarium, ionize 
more readily in silicon than gadolinium, and only small 
quantities of these rare earth elements need be used to 
decrease the resistivity of silicon to the desired values. 
For  example, two grams of gadolinium added to 250 
grams of silicon reduced the resistivity of the silicon to 
50 ohm-centimeters, However, only 0.05 gram of samar- 
-
4 
ium added to the same quantity of silicon produced the 
same resistivity. It is further apparent that other rare 
earth elements will behave in the superior fashion of 
gadolinium in regard to radiation damage because of the 
5 similarity of their atomic structures. 
The higher carrier removal damage rate of gadolinium 
under bombardment is .aq.&ect associated with the su- 
periority of this element & diffusion length preservation 
under bombardment. This effect holds true for a num- 
ber of other impurities which may be added to silicon. 
A small number of the impurity atoms associate with lat- 
tice defects forming both trapping centers and recom- 
bination centers. The%@z@ic structure of the impurity 
element determines whether an atom of the specific im- 
15 purity in association with a Bttice defect will be most 
likely to form either a recombination center or a trapping 
center. In the case where the trapping center configura- 
tion is most favored, the impurity element will induce un- 
desirable majority carrier removal radiation damage in 
20 the material and will induce correspondingly less diffusion 
length radiation damage. Gadolinium is an example of 
this case. 
Since lattice defects are also present in unbombarded 
silicon, the presence of impurity atoms in silicon material 
25 will, in general, affect the characteristics of the material 
and of devices made from the material. In this respect, 
the addition of gadolinium or other rare earth elements 
to silicon to reduce its resistivity will result in the mate- 
rial having longer minority carrier lifetime than it would 
30 have had if an equal concentration of a less desirable im- 
purity element such as boron had been added to the 
material. 
While several examples of semiconductors having im- 
proved radiation damage resistance have been described 
35 along with several methods of making these semiconduc- 
tors, it will be appreciated that various modifications can 
be made to the disclosed material and method without 
departing from the spirit of the invention or the scope of 
the subjoined claims. 
40 What is claimed is: 
1. A method of improving the resistance to radiation 
damage of a silicon semiconductor material comprising 
the steps of 
reducing the number of recombination centers and 
trapping centers by adding an electrically active im- 
purity selected from the rare earth elements to the 
silicon semiconductor material and 
increasing the conductivity of the silicon within said 
''. semiconductor material by controlling the amount of 
the addition of said electrically active impurity from 
@e rare earth elements. 
2. A. method of improving the properties of a semi- 
conductor material as claimed in claim 1 wherein the 
electrically active impurity is gadolinium. 
3. In a method of making a silicon semiconductor de- 
vice wherein a quantity of an impurity is added to the 
silicon to lower the resistivity to a predetermined value 
prior to subjecting the device to high energy atomic par- 
ticle bombardment, the improvement comprising select- 
60 ing impurity from the rare earth elements, and adding 
said rare earth element to the silicon to lower the resist- 
ance thereof to a range between 10 to 20 ohm centimeters 
thereby preserving longer minority carrier lifetimes under 
bombardment. 
4. The method claimed in claim 3 wherein gadolinium 
is added to the silicon. 
45 
50 
55 
65 
References Cited by the Examiner 
UNITED STATES PATENTS 
3,141,848 711964 Enk _ _ _ _ _ _ _ _ _ _ _ _ _  148-1.6 X 
70 2,916,810 12/1959 Smith et al. 
HYLAND BJZOT, Primary Examiner. 


Method of making a silicon semiconductor device Patent

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