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IN .. INVESTIGATION OF THE STRUCTURE

OF RADIATION DAMAGE I N

L I THIUM-DIFFUS ED SILICON

F i n a l Report

3 February 1969

Covering Peri od

-- 8 December 1969

J. C. C o r e l l i and T. Mortka

December 1969

JPL Contract No. 952456

For: National Aeronautics and Space Administration J e t Propulsion Laboratory 4800 Oak Grove Drive Pasadena, Cal i forn ia 91103

Nuclear Science and Engineering Division

Rensselaer Polytechnic I n s t i t u t e Troy, New York 1 2 1 8 1

a

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INVESTIGATION OF THE STRUCTURE

OF RADIATION DAMAGE I N

L I THI UM-DI FFUS ED S I LI CON

F ina l Report Covering Period

3 February -- 8 December 1969

J. C . C o r e l l i and T. Mortka

December 1969

JPL Contract No 952456

For : National Aeronautics and Space Admin. J e t Propulsion Laboratory 4800 Oak Grove Drive Pasadena, Cal i forn ia 91103

" T h i s work w a s performed f o r the Je t Propulsion Laboratory, C a l i - f o r n i a In s t i t u t e of Technology, as sponsored by the National Aeronautics and Space AdminLs+ t r a t i o n under Contract NAS7-100"

Nuclear Science and Engineering Divisrion R en s s e l a e r Poly t e czln i c I n s t i t u t e

Troy, New York 12181

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TABLE OF CONTENTS

Abstract

I ntroduc t ion a) Statement of t he research program Table I b) Discussion of t h e theory of

e x t r i n s i c photoconductivity and i t s relevance t o our measurements

Experimental Methods a) Li-diffusion i n S i b) Electron I r r a d i a t i o n c) Photoconductivity and Inf ra red

Spectroscopy Measurements

Resul ts and Discussion a) Photoconductivity Studies b) Inf ra red Spectroscopy Studies c) Consideration of Errors , Uncertain-

t i e s and Limitations o f t he Measurements

Conclusion

Follow on Ef fo r t

Table I1

Table I11

References

page 20 page 20 page 2 1

page 22

page 25 page 25 page 30

page 32

page 34

page 34

page 36

page 38

page 39

Figure Captions page 40

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ABSTRACT

The e f f e c t s of 1.5 and 5 MeV e lec t ron i r r a d i a t i o n (a t 30OoK) on

Li-doped s i l i c o n were s tud ied using in f r a red photoconductivity (PC) (1 t o 8 microns) and in f r a red spectroscopy I R (1 t o 50 microns) t o

probe t h e radiation-produced defects . The l i th ium was diffused i n samples

cu t from oxygen-rich c ruc ib le grown (CG) s i l i c o n ( 3 1017 oxygen atoms/

cm ) ingots o r from oxygen-lean f l o a t i n g zone (FZ) re f ined s i l i c o n

( 4 1 0 l 6 oxygen atoms/cm3).

before L i d i f fus ion had r e s i s t i v i t i e s i n t h e range 1 t o 1 0 ohm-cm phos-

phorus doped. Lithium was d i f fused i n t o concentrations of

and40”

fluences ranging from lo1‘ e/cm up t o e/cm with some samples

receiving 101’e/cm ’. a paste mixture of l i th ium metal i n mineral o i l and alumina painted on t h e

surface. From t h e photoconductivity measurements we f i n d an energy

l e v e l a t E,-0.62 eV which a c t s as an e lec t ron t r a p and i s produced by

r ad ia t ion i n near ly a l l samples i r r e spec t ive of oxygen content. Lack of

time has precluded de ta i l ed experiments on the dependence o f t h e 0.62eV

l e v e l on e l ec t ron energy, f luence and L i impurity concentration. Other

l e v e l s which give r i s e t o defect photoconductivity a re produced with

predominant concentration and importance f o r those l eve l s grea te r than

approximately 0.20eV below the conduction band edge. From t h e defect

photoconductivity spectrum we do not f i n d t h e l e v e l s a t E,-0.54 eV and

E,-0.39eV which a r e c h a r a c t e r i s t i c of -the dominant divacancy defect i n

s i l i con . Additionally we observe s t rong l i th ium e f f e c t s on the 1.8

divacancy associated defec t absorption band i n which l e s s of t he 1,’ band i s produced as the L i concentration is increased. Absence of a

c l e a r divacancy l e v e l was found f o r a l l l i th ium concentrations s tudied

which may be due t o t h e i r low production r a t e a t

due t o attachment of L i t o t he divacancy which might not be photoconductive.

A de t a i l ed study on the dependence of the divacancy l eve l s on L i con-

centrat ion is needed i n the fu tu re a s i s c l ea r ly evident from the data.

Thus, both the.PC and I R measurements show t h a t t h e L i atoms a c t as

e f f i c i e n t s inks f o r vacancies and divacancies by becoming attached t o

these defec ts and producing new complexes. A n absorption band a t 9.9

found only i n oxygen-rich Li-doped S i i s found i n both i r r a d i a t e d and

u n i r r a d i a t e d mater ia l . The response of

3

I n a l l samples s tudied the s t a r t i n g mater ia l ry

3 l i th ium atoms/cm , and the samples were subjected t o e lec t ron 2 2

The l i th ium was diffused i n by heat ing t o 420°C

/u /cc

N 1.5-5MeV and masking

P

-5-

band t o hea t treatment changes markedly i n samples i r r a d i a t e d e/cm2 48 MeV e lec t ron fluence, Fluences of 1016e/cni) 5 MeV a n d 1.5 MeV

e lec t rons do not e f f e c t t h e band a t 9. even up t o hea t treatment of

500?Cc.

no t understood a t present and requi res fu r the r de t a i l ed experiments.

The mechanism responsible f o r t h e behavior o f t he 9 . 9 ~ band is

1) INTRODUCTION

a) Statement of t h e research program

The objec t ive of t h i s study was t o experimentally inves t iga te the

e f f e c t s of e lec t rons on n-type (P-doped) s i l i c o n l i thium-diffused t o

t h e concentrations given i n Table L below. The probes we used t o study

the radiation-induced defec ts were in f r a red photoconductivity (1 t o 1 0

microns) and in f r a red absorption (1 t o 50 microns). I n Table 1 i s given

t h e c h a r a c t e r i s t i c s of t h e s t a r t i n g s i l i c o n mater ia l , the incident e lec t ron

energy and t h e e lec t ron fluences. A l imi t ed number of annealing experi-

ments were run on t h e samples i n order t o determine t h e s t a b i l i t y of

defec ts t o hea t treatment.

TABLE 1-- Matrix giving combination of mater ia ls , and i r r a d i a t i o n s

. t o be s tud ied i n t h i s inves t iga t ion f ++ 2 L i conc. F.Z. Q.C. 1 ,5 , 3 and 5 MeV e lec t ron Fluences e/cm

* 1 O l 4 * io1& *

16 - 3

17 -3 1 0 cm

1 0 cm

= Float ing Zone S i phosphorus doped) C.= Quartz Crucible S i phosphorus doped) The oxygen concentrations* were atoms/cm3 f o r quartz-

* A s determined by the measured i n t e n s i t y of t he 9 micron band (1110 cm-1) which is i d e n t i f i e d with t h e antisymmetric s t r e t ch ing v ibra t ion of the i n t e r - s t i t i a l oxygen atoms i n crucuble grown S i . The technique of assoc ia t ing the oxygen concentration with t h e i n t e n s i t y o f the band was f i rs t reported by W e Kaiser, P. Keck and C, Lange, Phy,Rev 101, 1 (1956). Note; t h i s method may underestimate the oxygen content s ince oxygen atoms may be present _in i n o ther forms which a r e not i n f r a red act ive. Comparison of oXygen conten vacuum fusion ana lys i s by Kaiser e t , a l , shows t h e in f r a red technique under- estimates the oxygen content by a facfor of a 3 t o -10 (10 beinn u m e r l i m i t ) -

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c ruc ib le S i and ,$ cases t h e e lec t ron i r r a d i a t i o n s were performed with t h e snmplp temperahire

near

atoms/cm3 for floating-zone re f ined S i. T n all

300°K and i n no case higher than 32S°K,

I n order f o r us t o perform t h e research it was necessary f o r us t o

develop t h e capab i l i t y f o r d i f fus ing l i th ium i n s i l i c o n and monitoring

t h e l i thium concentration and hornogeneiTy. This capabi l i ty represents an

addi t ion t o our ove ra l l e f f o r t and is described below.

The program w a s constrained by t h e information and r e s t r i c t i o n s O T ~

experiments as given i n Table 1. The na in t h r u s t of the work was t o t a k e

a general approach r a t h e r than an attempt t o study one s p e c i f i c aspect

of the radiation-produced defects . T h i s fac t forces severe l imi t a t ions on

what could be observed using our measurement techniques. The research work

t o be presented i n t h i s r epor t represents a t o t a l efforF of 1 . 2 man years.

The personnel d i r e c t l y involved i n t h e research consisted of one facul ty

member, one graduate s tudent and a research technictan a l l on a p a r t t i m c

basis I

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b) Discussion of t h e theory of e x t r i n s i c photoconductivity and i t s relevance t o our measurements

A de t a i l ed discussion of the theory r e l a t i n g t o photoconductivity from

deep ly ing s t a t e s i n t h e forbidden gap i s extremely complex. We shall

give here a treatment which fo l lows closely t h a t given by Rit tnerCa) and

Putley(b) and has been reported i n p a r t i n t h e doctoral t h e s i s o f a former

RPI student D r . Arne H. Kalma").

t h i s p a r t i c u l a r sect ion.

i n attempting t o unravel t h e complexity o f radiation-induced loca l ized

s t a t e s i n Li-doped S i u t i l i z i n g t h e technique of photoconductivity.

The references a re given a t t he end of

Inclusion of t h i s mater ia l w i l l serve as an a i d

I n order f o r us t o begin t o approach a t h e o r e t i c a l treatment o f t h i s

problem we must s t a r t with an oversimplified model o f the energy s t a t e s

in t h e forbidden gap of S i . For our model we s h a l l assume t h a t the photo-

conductivity r e s u l t s from % tvpe of donor and one type of acceptor whose

energies a r e c lose t o t h e conduction band and valence band respect ively.

Additionally we assume one imperfection center whose energy is not close

t o e i t h e r band. T h i s model is a t b e s t only a rough approximation and

idea l ized r e l a t i v e t o what we observe i n our radiation-induced photocon-

duc t iv i ty measurements i n Li-doped S i .

I t w i l l be i n s t r u c t i v e t o ou t l ine t h e important d e t a i l s o f t he theary

appl icable t o our s tud ies .

govern the behavior of e lec t rons and holes responsible f o r e x t r i n s i c

photoconductivity:

We s t a r t with the following equations which

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and

L e These a r e e f f ec t ive ly four equations i n t h e four unknowns defined below,

n = instantaneous number of e lec t rons i n the conduction band.

p = instantaneous number of holes i n t h e valence band.

n = instantaneous number of imperfection centers f i l l e d .

q = space-charge density. t

Other terms i n t h e equations a r e defined below.

= c a r r i e r s i n t h e bands with no exc i ta t iqn . OYPO

n

rn,r = l i f e t imes of t h e exci ted c a r r i e r s .

gVt,gtc = r a t e of thermal exc i t a t ion of c a r r i e r s captured on the

r t v , r c t = r a t e of capture of f r e e c a r r i e r s by the imperfection

P

imperfect ion centers .

centers.

f = r a t e of c a r r i e r generation by o p t i c a l exci ta t ion. /XP& = mobi l i t i e s of t he f r e e ca r r i e r s .

D ~ , D ~ = di f fus ion coef f ic ien ts of t h e f r e e c a r r i e r s .

5?*= curren t density. -+ E = e l e c t r i c f i e l d .

I n our treatment we neglect generation of c a r r i e r s by phonon absorption

from l a t t i c e v ibra t ions and surrounding black bcdyphot9ns;we a l so neglect

recombination i n which e l ec t ron ' s energy is given t o apothey f r e e electroii

(Auger effect) and we neglect ion iza t ion of impurity centers by impact: of

f r e e ca r r i e r s .

t a l case a s they a r e generally small e f f ec t s . The subscr ipts n and p r e -

f e r t o e lec t rons and holes respec t ive ly and t , v, and c r e f e r t o the imper-

f ec t ion center , t h e valence band, and the conduction band respeeSively.

The first two equations ( 1) and [ 2) a r e t h e continui?y equations €or X r w holes and electrons. I n these equations

These assumptions a r e r e a l i s t i c t o make f o r our experimen-

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R

Terms i n these equations a r e defined a s follows.

N t = t o t a l number of imperfection centers.

Nc,Nv = e f fec t ive dens i t i e s of s t a t e s i n the bands.

Sp = capture CTOSS sec t ion f o r a f r e e hole of t h e imperfection

sn = capture cross sec t ion f o r a f r e e e.lec$ror! qf $ ; r e impci-17ecti.on centers when occupied by an electron.

cen ters when unoccupied.

E t = energy of t he imperfection centers f r o m the i-ondiiction band.

Energies a r e pos i t i ve and measured f rom the bottom o f t he conduction band.

The Third equation ( 3) i s the cont inui ty equation f o r e lectrons on t h e i m -

perfect ion centers neglected. Equations ( 4), ( . 5 ) , and ( 6 are t h e de- f i n i t i o n s of t h e current dens i t ies . The seventh equation ( 7) is P o i s s o n ’ s

equation. I n t h e cur ren t dens i t i e s , t he Einstein re la t ionships

Dp = - “ P P e

and

Bfi =---pa k7- P L

hold f o r t h e d i f fus ion term.

Ta solve these seven equations, some simplifying and reasonable assump-

t i o n s w i l l be made. Space charge e f f e c t s w i l l be neglected, i . e . ; y = 0 ,

b n + 4 n t = d p (An,d p, a n d b n t a r e t h e change i n c a r r i e r dens i t i e s r e s u l t -

i ng from t h e l i g h t exc i t a t ion . ) ; and e lec t ron and hole l i f e t imes a re

equal, i . e . ; = = r. Now equations ( 1) -i:hrough ( 5) combine t o

give P

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where

- u 2 i 17) /us @ a/&o t @ p a

Equations ( 3) , i n th ree unknowns. These a re s t i l l too d i f f i c u l t t o solve i n general. A

f u r t h e r assumption i s t o neglec t t rapping e f f e c t s which s e t s d n t = 0.

t i o n ( 14) new reduces t o

( 14), and the assumption An + A n t = 4 p give t h r e e equatioris

Equa-

A s 2: D , and,& depend on p and n , t h i s equation is non-linear and s t i l l

too d i f f i c u l t t o solve ana ly t ica l ly . Some simpler cases w i l l now be can-

s idered i n our development.

a) Diffusion or end e f f e c t s a r e neglected i n a uniformly exci ted

homogeneous semiconductor. This means V P p = 0 and Eq. ( 18) can h e

wr i t ten

3

In steady s t a t e Eq. 1 9 becomes

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W e now wish t o look a t t h e assumptions t h a t l e d E q . 20.

of neglect ing space charge and d i f fus ion e f f e c t s a r e reasonable ones,

Neglecting t rapping may no t be a good assumption, but t he equation may be

qr r ived a t with a somewhat d i f f e r e n t s e t o f assumptions,

s t rongly n-type ma te r i a l s o t h a t

The assumptions

These a r e (1)

which leads t o

(2) t he energy l e v e l assoc ia ted with t h e imperfection i s f a r from e i t h e r

band o r

and

E G -Et >> k7- and (3) few f r e e c a r r i e r s a r e captured a t t he imperfection l eve l . For

t o hold, t h e densi ty of imperfection recombination centers must be small

compared with t h e thermal equilibrium c a r r i e r d e n s i t i e s so t h a t t he change

i n charge dens i ty produced by changing t h e concentrations in t h e centers

can be neglected.

Next, we must examine t h e l i f e t i m e of t h e sample. I f d i r e c t recom- c

bina t ion of c a r r i e r s determines t h e l i f e t i m e , t he l i f e t i m e can be considered

t o be given by

r= / I

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where C i s t h e product of t h e capture cross sec t ion o f c a r r i e r s by thc

recombination center and t h e thermal ve loc i ty of t h e ca r r i e r .

Put t ing Eq. 26 i n t o Eq. (20) l eads to

Expanding t h i s f o r small f , i. e . , small exc i ta t ions , t h e r e l a t i v e i,ncre.ano

i n c a r r i e r densi ty is

which is proport ional t o f . For l a r g e f , t h e increase becomes

1 which is proport ional t o e.

If t h e recombination ac tua l ly takes place predominantly through an

imperfection center , a fu r the r look a t t h e l i f e t i m e is required and s ingle

l e v e l recombination theory must be employed. If n is assumed t o be small

when compared with no and po, t h e l i f e t ime becomes

(3 0 )

where Go = 1/Cp and Go = l/Cn a re t h s l i f e t imes f o r holes and electrons

in j ec t ed i n t o s t rongly n-type and s t rongly p-type mater ia l respect ively

and t h e primed q u a n t i t i e s a re t h e f r e e c a r r i e r dens i t i e s when EF = E t

where EF i s the Fermi energy.

p-type material

Four regions o f r o c c u r . (1) I n strongly

where

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and

r= 2- '2,

(2) I n l e s s s t rongly p-type mater ia l ,

and

(3) I n weakly p-type material

and

(4) I n s t rongly n-type mater ia l ,

4 >EF > 0

and

i

I n a l l these cases, t h e l i f e t ime is independent o f d n o r d p and thus d p

w i l l again depend on f l i nea r ly .

If it is now assumed t h a t t h e excess c a r r i e r density is not small com-

pared with t h e equilibrium c a r r i e r dens i t i e s , a more complicated r e s u l t

occurs.

e r order i n b n r e s u l t s i n

If the l i f e t i m e of Eq. 30 is ca l l ed c, including terms o f high-

-14-

where

and

The l i f e t i m e now depends on An. Put t ing Eq, 40 in to Eq. 20 leads t o

Here the re a r e th ree va r i a t ions of d p with f .

(2) if a > b, A P - 2'; and (3) i f a ( b , d p a f ". comes t o b, t h e nearer t h e dependence o f 4 p on f i s l inea r .

mater ia l i s being considered where n,))po,

(1) If a = 5 , d p x T; The c loser a

I f n-type

c loser t o t he conduc- - - t i o n band than E t i s t o e i t h e r band) and since' rpo is usual ly greater than

general and thus it can be seen t h a t it is b e t t e r t o keep the excess car-

r i e r density small compared with the thermal equilbrium c a r r i e r dens i t ies .

This is usual ly t h e case i n our experiments.

c n o by an order of magnitude, a = b. These assumptions a re not t o o

Since we have shown t h a t b p depends l i n e a r l y on f under ce r t a in con-

d i t i ons , it is necessary t o examine t h e dependence of f .

can be wr i t t en as

The quant i ty f

-15 -

Definit ions of terms a re

p = quantum eff ic iency of the defect t o band t r a n s i t i o n . = cross-section of i n t e rac t ion between a photon and an e lec t ron C(E)

capable of being promoted t o t h e conduction band, i a e e , t h e

photoionization c ross section.

/m = density of defec ts occupied by an e lec t ron which may be excited.

Np(E) = photon i n t e n s i t y of energy E = hv.

We assume t h a t m t = N t ( a l l imperfection centers f i l l e d ) w i t h no

6 ( E ) i s 0 u n t i l t h e energy ( h d ) of t h e exc i t ing l i g h t on and t h a t

photons is l a r g e r than E t ,

The difference between equations (46) and (45) i s t h e increase i n photo-

conductivity due t o exc i t a t ion of t he imperfection center.

I t i s seen f rom Eq. (47) t h a t t he increase i n photoconductivity due t o

exc i ta t ion of t h e imperfection center is proport ional t o t h e i l luminat ion

in t ens i ty and t o t he number of imperfection centers under the assumptions

made. Since t h e r e s u l t s apply b e s t f o r small f , t h e i l luminat ion inten-

s i t y and the number of imperfection centers should both be kept small a s

they a r e the only th ings f depends on which can be experimentally varied.

A fu r the r advantage of keeping t h e number of imperfection centers low is t h a t they a r e more l i k e l y t o be completely f i l l e d under t h i s condition and t h i s is necessary if t h e photoconductivity is t o be proport ional t o t h e

t o t a l number o f defects and not merely t h e number occupied.

A summary of t h e important conclusions reached i n t h i s s ec t ion as

they a r e re levant t o t he photoconductivity measurements w i l l be given

here.

-16-

(I) The photoconductivity i s proportional t o t h e number of excited

c a r r i e r s , f , if t h e following experimental conditions a re s a t i s f i e d :

(a) t h e densi ty of recombinatian centers i s small compared w i t h t h e thermal equilibrium c a r r i e r dens i t i e s s o t h a t t h e exci ted carriclr

l i f e t imes of holes and e lec t rons a r e equal. This may not always have been s a t i s f i e d f o r our r e s u l t s .

(b) f and thus t h e excess c a r r i e r density, b n o r d p , is small compared with t h e thermal equilibrium c a r r i e r dens i t ies . This mus’i-

be cont ro l led by changing t h e l i g h t i n t ens i ty s o t h a t E<< 1 and

was always s a t i s f i e d i n our case. T

(e) conditions such as no space charge and no end e f f e c t s t h a t a r e usua l ly s a t i s f i e d i n a l l experiments.

(2) The photoconductivity i s proportional t o t he ‘ l i gh t i n t ens i ty f o r

low i n t e n s i t i e s . Light source i n t e n s i t i e s used i n t h i s inves t iga t ion were

low and s a t i s f i e d t h i s condition.

(3) The photoconductivity i s proportional t o t he number of imperfec-

tion centers i f t h a t number i s small compared with t h e thermal equilibrium

c a r r i e r dens i t ies .

The condition t h a t t h e number of imperfection centers be small compxrilcl

t o t h e thermal equilibrium c a r r i e r dens i t i e s lis s t r i c t l y s a t i s f i e d f o r fluences 5 5 x e/cfi,;r (1.5 MeV electrons) and L i concentrations UI

only make q u a l i t a t i v e comparisons.

s t r i c t l y s a t i s f i e d i n our experiments.

3 1016 em-3. I n other cases when t h i s condi-cion is not f u l f i l l a d , we car, A#

The condition d6,,1 is generally P

I n order t o make s t r i c t comparison of t he r e l a t i v e defect centclr

population and t h e photoconductivity s igna l one must know t h e photoioni-

zat ion cross sec t ion &(E) (see Eq. 443. The only known cross sect ion

measurements have been made on group T I 1 and V impuri t ies i n S i . Since

t h e o p t i c a l absorption cross sec t ion o r what we have ca l l ed the pho-izoioiii-

za t ion cross sec t ion depends on

defects) N we can wr i t e

t he concentration of $mperfecti.ons (o r

f 45)

-17-

where ol. is t h e o p t i c a l absorption coef f ic ien t . For shallow donors one

can estimate (E) using the hydrqgenic model and the e f f ec t ive mass ap-

proximation a s was reported by Berstgin, Picus and $ c l a r f o

(using a theory formulated by Laxg) t h e formula They obtained

where E >Ed and

t h e conduction band, E = is t h e energy of incident l i g h t , K i s t h e dicl-

l e c t r i c constant f o r S i , K = 1 2 , m is t h e e lec t ron mass, m* i s the effec-

t i v e mass pf a hydrogen l i k e center (e lectron vnder t h e ac t ion o f a charge

by coulombic forces) .

i s t h e energy l e v e l pos i t ion o f t he impurity below /

Measurements o f t h e capture photoionization cross sect ions f o r deep

radiation-produced l e v e l s i n S i a r e expected t o vary widely s ince the re i s

considerable va r i a t ion found f o r deep l e v e l s of chemical impuri t ies such

a s Au, Zn, and S. Using the approximation inherent i n the der ivat ion o f

Eq. YB we can obtain an order of mqgnitude of t he cross sect ions a t 78 K 0

f o r deep l e v e l s and a r e tabula ted below f o r various energy values below

the conduction band.

1.4 1 0 - 1 7 0 .4

0.62 10-17 0.6

A t present no exact theory has been developed f o r computing capture crc)ss

sect ions f o r deep ly ing l eve l s . However, one cor re la t ion between the size of t h e cross Section and t h e charge s t a t e on the impurity or defect center

can be made. I n p a r t i c u l a r a defect may have up t o t h ree u n i t s o f a t t r a c -

t i v e o r repuls ive charge and t h e c ross sec t ion should be l a r g e r f o r t he

l a rge r a t t r a c t i v e charge,

In t h e photoconductivity r e s u l t s which w e give i n Figs 1-11 we quote

as the energy l e v e l t h e point on t h e wavelength (or energy) sca l e wherc

we first observe ion iza t ion , I n many cases i n photoconductivity spectra

f o r i r r a d i a t e d Li-doped S i one does no t observe a sharp energy demarca-

t i o n , r a t h e r a smeared out energy s t a t e appears thus making it very d i f f i -

c u l t t o c l ea r ly ex t r ac t energy l e v e l posi t ions. I n our other work on

-18-

pfiotoconductivity s tud ie s of e lec t ron- i r rad ia ted S I containing varying con-

cent ra t ions o f P ando impuri t ies we do i n f a c t observe f a i r l y sharp l i n c s

in t h e specTrum. This i s because we i r r a d i a t e t o fluences which makc1 ob-

servat ion most convenient e.g., loca t ion o f Fermi l e v e l etc.

In a l l photoconductivity spec t ra which we s h a l l present we a r e obser-

ving an exc i ted c a r r i e r f rom a quasibound s t a t e t o the conduction band due

t o t h e pos i t ion of t h e Fermi l eve l . We a r e not- exc i t ing f r e e c a r r i e r s i n -

t o t h e conduction band s ince t h i s process required smaller energies and

would occur only f o r longer wavelengths ),lo microns which l i e s beyond t h n

region which we have invest igated, and moreover represents an extremely inT

s i g n i f i c a n t contr ibut ion t o t h e PC spectrum due t o the very low s igna ls

compared t o s t a t e t o band t r ans i t i ons .

I n view of t h e r e s t r i c t e d manner i n which our photocanductivity s p w t r n

were measured t h e data a r e somewhat l imi t ed and do not warrant a more com-

p l e t e ana lys i s beyond our conclusions given a t t h e end o f t h e repor t ,

since t o make i n depth statements a t the time would bo presumptuous.

-19-

References f o r Section b

(a) E. S . R i t t n e r , "Photoconductivity Conferencer', R . G . Breckenridgci, e d i t o r John Wiley (1956) p. 215.

(b) E. H. Putley, Phys. S t a t . So l . 5, 571 (1964)

(c) A. 8H. Kalma FhD t h e s i s , Rensselaer Polytechkic I n s t i t u t e , 1968, ava i l ab le from University Microfilms, Ann Arbor, Michigan).,

5 (d) W. Schockley and W , T. Read, Jr,, Phys. 3?ev,, 87, 837 (1952)

(e) R. N. Hall , Phys. Rev., 87, 387 (1952)

(f) E. Burstein, G .I Picus, and N. Sc la r , "Photoconductivity Conference" R . G. Breckenridge, e d i t o r John Wiley (1956) p. 353.

(g) M. Lax, "Photoconductivity ConfeTence" , R. G . Breckenridge, ed i to r John Wiley (1956), p. 111. '

-20-

2) EXPERIMENTAL METHODS

a] Li-diffusion i n S i

A complete laboratory was s e t up f o r producing samples of l i thium- diffused s i l i c o n from commercially-available ingots of crucible-grown

and floating-zone r e f ined s ing le c r y s t a l s of si l icon. The technique f o r d i f fus ing t h e lithium i n the s i l i c o n involves a "painting" procedure

whereby a pas te composed of dispersed l i thium metal ** ( ( 1 5 micron p a r t i c l c

s ize) and 15 micron alumina mixed i n mineral o i l is applied t o t he surl'ace

O f a s i l i c o n sample. The sample i s then given a heat treatment a t z 4 5 0 ° ( '

i n an environment through which argon o r helium gas flows. Typical gas

flow r a t e s used a r e 1 t o 3 cu f t h r and do not have an important e f f e c t

on t h e d i f fus ion whereas time and temperature a r e the important var iables

i n the diffusion process. The heat treatment time va r i e s .From 12-15

minutes up t o severa l hours depending on the sample thickness and the

l i th ium concentration desired. If higher temperatures a r e used (say

.k

)SSO°C) t he surface i s badly p i t t e d requi r ing r e l a t i v e l y long times f o r

pol ishing and preparing a mirror- l ike surface. Past experience has s h o w

t h a t a mirror- l ike surface ensures t h a t no absorption due t o surface

condition occurs i n t h e 1 t o 3 micron wavelength region and t h e r e l a t i v e

ease i n subsequent pol ishings f o r example a f t e r heat treatment anneals,

We f i n d thatS45O0C is a bes t compromise as regards diffusion times and

sample surLFace condition.

out t he volume of t h e sample we subject t h e diffused sample t o repeated

heat treatments o r "out diffusion" of 1 t o 1 0 hours. We have found by

t r i a l and e r r o r t h a t 45OoC is an optimum red i s t r ibu t ion temperature

because of reasonable times ( 5 1 0 hrs) and t h e surface i s not strongly

attacked and p i t t e d ,

r e l a t i v e l y easy s ince we can use 15 micron alumina followed by polishes

of 2: 1 and 0.3 micron alumina t o produce a mirror- l ike f i n i s h on t h e

sample surface. * We thank M r s . Marla H. Moore of NASA Goddard Space F l ight Center f o r teaching us t h i s technique.

I n order t o uniformly d i s t r i b u t e the L i through-

This sample condition makes the f i n a l pol ish job

**Available from t h e Li.thium Corporation of America. The sodium impurity content i n t h e l i th ium metal dispersion i s < 0.01% a s specif ied by t h e manufacturer.

- 21-

The l i th ium concentration is monitored by measuring the e l e c t r i c a l r e s i s t i -

v i t y a t 300°K.

probe (manufactured by A & M Pel1 Ltd. Sussex, England). A s a cross

check we a l s o use As-doped gold alloyed ohmic contacts t o measure the

e l e c t r i c a l r e s i s t i v i t y . These two independent measurements of r e s i s t i -

v i t y y i e l d d i f fe rences$ 2% which gives a high confidence l e v e l t o the

accuracy of measuring r e s i s t i v i t y by t h e con-kactless method. I n our

measurement of r e s i s t i v i t y by the contac t less method we have been able t o

achieve an accuracy of 3% o r b e t t e r and have rout ine ly produced samples

o f lithiup concentrations of ,v10’~cm-~, - ,=10’~cm-~ ,* 1 0 ~ ~ c m - ~ ~

Most of t h e measurements a r e made using a l i n e a r 4-point

I

$We must emphasize t h a t our method of determining the l i th ium concentra-

t i o n is only sens i t i ve t o e l e c t r i c a l l y ac t ive l i th ium atoms and thus

may underestimate t h e t o t a l l i th ium content, We s h a l l discuss t h i s

point l a t e r i n t h e repor t . We have attempted t o measure surface

uniformity of t h e l i t h ium and f i n d t h a t t he concentration is un i fo rm over

t h e surface t o 5 5 % .

near t h e edges of t h e sample because of edge correct ions and f o r t h i s

reason we attempt t o minimize edge e f f e c t s by masking t h e sample surface

during t h e photoconductivity measurement.

However it i s very d i f f i c u l t t o ge t accurate r e s u l t s

b) Electron I r r a d i a t i o n s

The e lec t ron i r r a d i a t i o n experiments were* performed with the samples

kept a t $300°K during bembardment. Sample cooling was accomplished

using an i c e and water mixture c i r cu la t ed i n t h e sample housing. We have

made i r r a d i a t i o n experiments a t 1.5 MeV and 5 MeV. During t h e performance

of t h e research it bacame obvious t h a t no new addi t iona l information would

be obtained by i r r a d i a t i n g using electrons of 3 MeV incident energy.

The r e s u l t s obtained a t 1 .5 MeV were not s ign i f i can t ly d i f f e r e n t than

t h e r e s u l t s obtained a t 5 MeV t o warrant t h e addi t iona l 3 MeV bombard-

ments. I n our view t h e conclusions reached i n the 1.5 and 5 MeV

bombardments can a l s o be applied t o what one would expect a t 3 MeV.

*The 1.5 MeV e lec t ron i r r a d i a t i o n s were conducted a t t he General E lec t r i c Research and Development Center, Schenectady, N. Y. and the 5 meV e lec t ron i r r a d i a t i o n s a t the U. S . A i r Force Cambridge Res. Lab. Bedford, Massachvsetts and we thank sylr. C. Turner and M r . L. Low of AFCRL for t h e i r ass i s tance .

-22-

It is important t o emphasize t h a t i n t h e r e s u l t s t o be g i v m here we were only concerned with defec ts t h a t had a long time s t a b i l i t y (1- 3 days). SincE our i r r a d i a t i o n s were a l l conducted near ambient

temperature and because of t h e time elapsed a f t e r bombardment a l l

spontaneous annealing and damage e f f e c t s a f t e r cessat ion OC t h e irradi- a t i o n were no t followed i n our experiments. Therefore, our main sear(-h

was t o study t h e permanent e f f e c t s of l i th ium impuri t ies and t h c i r

i n t e rac t ions with radiation-produced defec ts and o ther chemical

impuri t ies such a s 0 and P.

c) Photoconductivity and Inf ra red Spectroscopy Measurements.

The pho-toconductivity measurements (PC) were made using e i t h e r a

Perkin-Elmer model'98 monochromat'or (1 t o ~ 8 micron wavelength range]

or i n some cases a SPEX model 1500 g ra t ing monochromator (1 t o 2.5

micron wavelength range). The monochromatic l i g h t was incident

on t h e sample which was housedin a l i q u i d nitrogen cryostat . The

sample was cooled by immersion i n helium gas kept a t 78 K. 0

I n order t o make t h e source spectrum measurements with s u f f i c i e n t

accuracy s o as t o y i e l d meaningful photoconductivity it was necessary t o

modify t h e c ryos t a t t a i l piece s o t h a t t h e I R l i g h t was ab le t o pass

through a double se* of windows. Much e f f o r t was spent t o succcssf'ully

complete t h i s modification which we considered an important aspect of

our measurement capabi l i ty . The de tec tors used were PbS and a Perkin-

E l m e r thermocouple. The source spectrum w a s measured with a l i g h t

chopping frequency of 1 3 cps while f o r t h e photoconductivity measure-

ments t h e l i g h t was chopped a t frequencies i n t h e range 10 t o 100

cps t o noise r a t i o .

depending u6on which frequency gave t h e m a x i m u m photo currelit signal

The s i g n a l t o noise r a t i o i s frequency dependent i n the following

ways. F i r s t , t h e c a r r i e r l i f e t i m e of trapped e lec t rons on defec ts a t

loca l ized s t a t e s i s s t rongly e f fec ted by t h e l i g h t chopping frequency

second, t he e l e c t r i c a l pickup noise i n the room has a rrequcncy dcy>cndcwcci,

and f i n a l l y the response of t he p a r t i c u l a r preamplifier used in the

HR-8 lock-in amplif i e r has a s i g n i f i c a n t frequency dependence.

mechanical v ibra t ions eg. bo i l ing l i q u i d nitrogen can a l s o cause noisc I

Small

-23-

Optimum performance of the system is determined i n most p a r t fram the sample impedance, t h e qua l i t y Qf t h e e lec tp ica+ aonCapts on the sarnpJc

and t h e degree t o which t h e gystern has been e l e c t r i c a l l y shielded.

The s p e c t r a l sweep r a t e qnd t h e i n t e g r a t i a e t3rnes @re kppt l o w wifh a reasonable compromise i n energy r e sa lu t ipn (sep Figs. 1-11 )

Although our sample was kept a t a cgnst;an-b and upifqrm tempera-

t u r e during measurements of PC by immersion i n fiel5um gas This necess i ta ted t h e use of cold windows capable of transmikting IR l i g h t i n the 1 - 15 micron wavelength rqnge.

technique for making vacuum *i@~t ban$$ of CAgCb) T.0 9 copppr wjndow holder capabyg of operat ian below 100°K,

sealed w i t h ZI epoxy** t o a f l e x i b l e cppper 'lpqsing MO. OJ$"$hiek.

The copper housing was machined from a singJ.e piece of material O.20OT' t h i c k a t t h e mounting flange!, on t o t h e qryos ta t t a i l niece with the V ~ C U P ~ qeal pade w i n g i m l i p gaskets.

We hqve dev,e;loped a

Tp oyr -tre:ahrlique t h e AgCl AS

The capper fAqsgp was bolrtrd

The e l ec t ron ic detect ion systew employed a Princeton Applied

Research Co~p . phase-sensit ive lock-in ampJJi-fier moqlpl m-8" Thg

s igna l s usua l ly encountered i n our samples were from a low impedance

sample (< 50 ohms) and i n order t o achieve a goad s igna l t a slqise

r a t i o a l l leads , ba t t e ry bqx, preamplif iers (PAR model B wgp used),

switches, and c ryos t a t had t o be enclosed i n me-ballic sh ie lds f r e e a f holes ,

The two contacts made OF t h e sample were such That one s ~ p v a d a s a f l o a t i n g ground thus we used tho d i f f e r e n t i a l inppt qq$e t o t h e preap p l i f j e r . takep, namely, t he preamplifier was pbaced p t +he top of the Cryoqtat

by t h e e l e c t r i c a l l ead feed thwugh s9 t h a t oply short s igna l cablee were neeqed between t h e c ryos ta t aqd +'be prepmplifiett?, The exQwne care i n making the PC measuPements became necessq-ry becaupe of fbe

very low radiation-induced PC i n the samples. With t h i s syatern Izhc

only s ign i f i can t l imi t a t ion t o a high s igna l t o noise r a t i o were ?he noir;e

I n order t o keep tbe noise low, ape fuqther precqvtion was

s igna l s generated a t t he con-tac-bs (due mpstly t o ohmic shot noise)

** A one component f i n e epoxy powdw hea t cure@ f o r 1 hcrur a t 198*C, The epoxy is ava i lab le from t h e Dexter Corporation, FIyqol Q i v i s i w , Olean, N. Y. 14760. A f t e r we had developed our technique we Ciiswvered a s i m i l a r method using a d i f f e r e n t adhesive qnd repokged by V , Roberts, J. Se i . I n s t r . - 31, 2 5 1 (1954)).

-24-

and t h e l a t t e r noise s i g n a l var ied sample t o sample within to l e rab le

l i m i t s .

E)ne extremely important aspect of t h e measurements was t q have phqic

and low noise contacts on t h e samples, we used As-doped gold alloyed

t o the sample a t two poin ts for our contacts. The low noiqe on the

contacts i n our case i s due t o very low o r insignif icank $njec$ion

of c a r r i e r s of opposite s ign (pos i t ive) , low r e s i s t i v z t y contacS p f i ~ s

absence of any Schottky b a r r i e r a t low -kernperatwe,

The experimental methods used i n the measurement of the in f r a red

spec t ra u t i l i z e Erkin-Elmer Models 6 2 1 and 2 1 i n f r a red spectrometers.

The c ryos ta t used and other d e t a i l s have been described i n d e t a i l

elsewhere and w i l l no t be repeated here. ( 8 >

BLOCK DIAGRAM OF PC APPARATUS - -- c_ .....- -

II_ - -- - r CHOPPED LIGHT PULSES

-- <reference s i g n a l from l i g h t chopper I - . . - ..,I_. _ _

~ I..̂ -- ~ _ " c - - '?-Metal box (aluminum) with i n t e r connecli;ing copper pipes

t o completely enclose system and sample i n c ryos ta t f o r e l e c t r i c a l

shielding, box is ea r th ground.

R = Load Resis tor

S = Sample ( a t 78' i n cryostat)

-25-

3) RESULTS AND D I S C U S S I O N

a) Photoconductivity Studies

Our experiments were d.esi@;ned within the cons t ra in ts of t h e matr ix

given i n Table I. This method offered some d i s t i n c t disadvantages i n

t h e performance of t h e photoconductivity experiments. Namely, instead.

of i r r a d i a t i n g t h e sample to a suf f ic ' i en t ly high fluence so tha t

and

i r r a d i a t i o n of t h e samples a f t e r they had accumulated e lec t ron fluences

of lo1', l o x 5 , and 1016e/cm (see t a b l e I) . This meant we were forced

t o measure samples which i n many cases had low s i g n a l t o noise r a t i o s .

However, one advantage obtained from these s tud ie s was the f a c t t h a t we

had t o develop our e l ec t ron ic de tec t ion so t h a t we could measure very

weak s igna ls . After much e f f o r t we succeeded using t h e methods described

e a r l i e r .

l a rge easy t o measure changes occured i n t h e Fermi l e v e l we stopped t h e

We have given a summary of t h e samples s-fiudied i n Ttible I1 and

Table 111, i n which a r e given t h e sample ident i fy ing code number (JPL #) , t h e s t a r t i n g mater ia l r e s i s t i v i t y and whether floating-zone o r Crucible

grown CG, t h e l i t h i u y concentration, t h e e lec t ron energy and fluence,

and t h e r e s i s t i v i t y of t h e sample before and a f t e r i r r ad ia t ion . We

a l s o s ta te whether o r no t t h e photoconductivity spectrum w a s measured.

One pa r t i cu la r ly d i f f i c u l t p a r t of t h e experiments was t h e f a c t t h a t

a f t e r i r r a d i a t i o n t h e contacts exhibi ted r e t i f i c a t i o n and noise a t 78'K

i n som,e of t h e samples, and i n these cases it was not possible t o make

meaningful photoconductivity measurements a t low temperature. A t t h i s

time we cannot explain t h e d i f f i c u l t y with our procedure f o r makipg

contacts i n i r r a d i a t e d Li-doped S i which is similar t o t h e method

successful ly employed by Brucker2 i n i r r a d i a t e d Li-doped S i . Additionally,

we d id not encounter d i f f i c u l t y with contacts i n our e a r l i e r photo-

conductivity s tud ie s i n e lec t ron i r r a d i a t e d P- , As- and Sb-doped S i .

Further work on t h i s problem i s needed and w i l l be necessary i n our

fu tu re program. Some of t h e photoconductivity spec t ra were not measured

due t o l ack of time.

-26-

I n f igu re I we show the photoconductivity spectrum measured on a f l o a t i n g zone s i l i c o n sample (JPL #66) Li-doped t o 2.5 x cm

1 4 2 a f t e r i r r a d i a t i o n with 5 MeV e lec t rons t o a fluence oE 1 0 e/cm . As

given i n Table I1 We have shown arrows a t various points on t h e s p e c t ; ~ ~ where breaks

occur and which a r e outs ide of our experimental errop. These arrows

give an ind ica t ion o f t h e loca t ion of energy l e v e l s i n t h e forbidden

gap. For t h i s sample the maximum e r ro r i n t h e r e l a t i v e photoconductivity

i s -4. 5% and occurs a t Zp .

increase i n photoconductivity caused by the l i g h t . The energy rcsolu- t i p n i n the Worse case ( i .e . wide s l i t s ) is shown on Fig 1 a t 1 and

2p. spectrum o f t h e l i g h t source, From Fig. I we quote energy l eve l s a t

Ec-0.62 eV (2p) and at Ec-0.!35eV(2.25 1 measured w i t 3 respect t o t h e bottom of t he conduction band. We also see i n Fig 1 a a l i g h t break i n

-3

t h e sample exhibited only a 30% increase i n r e s i s t i v i t y .

By r e l a t i v e photoconductivity we mean

where b i s the conductivity i n the dark and & is the incremental 6

The spectrum shown i n Fig I has been normalized r e l a t i v e t o t h e

P c t i v i t y spectrum a t 1.75 micrans which may indica te a

possible l e v e l a t E,-0,7leV,

t h i s break a t 0.71eV a l e v e l because our energy reso lu t ion is not high

enough t o c l ea r ly see the outse t of a pronounced ionizat ion.

However, we do no t f e e l j u s t i f i e d i n c a l l i n g

The photoconductivity spectrum of a f l o a t i n g zone S i sample (JPL # 8 3 )

containing l i t h ium PQ B cvncentra%ion of 5 x 1015cm-3 is show i n F i g 2 .

This sample received 1 0

increased by a f a c t o r 11 (see t a b l e IT). Energy reso lu t ions a re shown

on Fig 2 and t h e maximum e r ro r i n d6 t he r e l a t i v e photoconductivity

i s 2 5% below "energy s t a t e " 0.52 eV below the conduction band. In f i g 2 we see an ind ica t ion of two other l e v e l s one a t 2 microns (Ec-0.62eV) and another

a t 2.8 microns (Ec-O. 45 eV)

l e v e l a t 2.25 microns Sn Fig 2.

15 e/cm2 (1.5 MeV) and i ts r e s i s t i v i t y waff

2F 4 . . The dominant peak a t % 2 . 4 ~ is indica t ive of an

Nolze: t he re i s not an indicat ion of a While t h e i n d i c a t i w s f o r l e v e l s a t

and Ec-Q.45eV appear i n Fig 2 we must be cau?'ious i n making

i n i t i v e concZusiQns because of t h e Fe ia t ive ly poor energy resolut ion

and low signal t o noise f o r sample JPL #83, We would n o t be so cauti.ous

if the breaks i n the spectrum were more pronounced at 2 and 2.8 microns,

-27- The d i f fe rences i n t h e spec t ra shown i n Figs 1 and 2 can be accounted f o r

Yn l a r g e measure by t h e d i f fe rence (factor 10) i n fluence. Additionally

t h e r e s i s t i v i t y increase f o r t h e sample exposed t o higher fluence

(Fig 2) is much l a r g e r than sample JET, #66 (Fig 1) which was exposed t o a lower f luence (see Table 11). with resgec t t o t he l i g h t source.

JPL #83

The spectrum shown i n Fig 2 was normalized

The only other completed photoconductivity spectrum whichwas nor-

malized with respec t t o t h e l i g h t source is shown i n Fig 3, The CG sample

contained 0.9 x Li/cm3 and was i r r a d i a t e d by 5 MeV t o a fluence of

1 0 e/cm The r e s i s t i v i t y of t h e sample increased by a f a c t o r 1 .9 ( a t

30OoK) as a r e s u l t o f t he i r r ad ia t ion .

conductivity is - 2% below 2 and * 5% a t W 2 . 6 p We have shown t h e

energy r e so lu t ion f o r t h e worse cases.

l e v e l a t E,-0,62eV(2 ) .

conductivity beyond 2 ~ , bu t no s ing le l e v e l is dominant.

break a t 2.6 microns i s due t o water vapor (pa r t i a l ly ) and s ince we were

not able t o carry out t h e measurements beyond2 2.7 microns (low signal)

we cannot make d e f i n i t i v e energy l e v e l assignments.

1 4 2

The e r ro r i n t h e r e l a t i v e photo-

/u +

We can see one dominant energy

Other shallow l e v e l s a r e responsible f o r t h e photo? /" I n Fig 3 t h e

I n the remaining photoconductivity spec t ra t o be presented we did

not normalize t h e s i g n a l with respec t t o t h e l i g h t source due t o l ack

of time and d i f f i c u l t y encountered w i t h accurate posi t ioning of t h e

wavelength marker on t h e monochromator, However, f o r purposes of

comparison w e show i n Fig 4 a spectrum of t h e globar l i g h t source measured

with a thermocouple detector . The s t ruc tu re a t ~ 1 . 6 , 2.4 and 3.0 microns

is due t o t h e absorption f rom water vapor while t he 4.2 micron peak

is due t o carbon dioxide absorption. I t must be emphasized t h a t even

though we d id not normalize t h e remaining photoconductivity spec t ra they

s t i l l provide very use fu l information regarding t h e presence of radia-

t i o n produced defect energy s t a t e s i n t h e forbidden gap t h a t may be

associated w i t h L i impurit ies. Lack o f accurate posi t ioning of wave-

length marker due t o a mechanical defect ( l a t e r corrected) prevented us

from being ab le t o normalize t h e spectra .

f rom including the r e s u l t s obtained with accurate wavelength marker i n

t h i s repor t ,

Time l imi t a t ion prohibited us

We show i n Figs 5-11 photoconductivity spec t ra (non-normalized

with respec t t o t h e l i g h t source) f o r various samples o f L i doping

i r r a d i a t e d a t 1.5 o r 5 MeV. The e lec t ron energy and fluences a r e given

i n t h e individual f i gu re captions o r Table I1 o r 111. Taking

-28-

i n t o consideration the l i g h t source spectrum shape we f i n d t h a t a t l t l i i s t

aneenergy l e v e l a t Ec-0.62eV appears i n both oxygen-rich CG S i and i l l

oxygen-lean FZ S i . This l e v e l f irst s t a r t s t o ion ize a t a wavelength

close t o 2 /u and can be seen i n t h e r e s u l t s given i n Figs 6 , 8 , and 9 .

The peak shown i n F ig 5 must be re-examined. Although the measured

spectrum f o r t h e sample given i n Fig 5 exhibi ted a peak a s t h e wavelength

04 l i g h t w a s increased from 1 t o 1.5 microns, t he wavelength ca l ibra-

t i o n w i l l need t o be repeated i n order f o r us t o be ce r t a in of this

r a t h e r d i f f e r e n t behavior i n the photoconductivity spectrum a s compared

with those measured f o r o ther samples.

One experimental f ind ing which can be extracted from t h e photo-

conductivity r e s u l t s i s t h a t defec t energy l e v e l s c loser than 0.2 eV t o

t h e conduction band a r e induced i n i r r a d i a t e d Li-doped s i l i c o n i n lower

concentration than deep l eve l s . However, w e do no t f i n d t h a t one s ingle

l e v e l dominates, r a t h e r t h e photoconductivity appears t o a r i s e from a

continuum of l e v e l s below t h e conduction band edge down t o about Ec-0.20eV.

Irl our samples we never had s u f f i c i e n t s igna l t o noise r a t i o beyond 6

microns which would have enabled us t o search f o r t h e A-center defect

which has been found t o be associated with an energy l e v e l near Ec-0.17eV,

It appears reasonable from an extrapolat ion of our data t h a t f o r oxygen-

r i c h S i containing L i t o concentrations grea te r than 10- t h e A-center

production i s much lower than f o r example i n samples doped w i t h P, A s ,

and Sb. I n view of t he r e l a t i v e l y small changes i n r e s i s t i v i t y of many

of our samples (see Tables I1 and 111) t h e Fermi l e v e l has not moved

subs t an t i a l ly i n t o t h e forbidden gap away from the conduction band edge.

T h i s implies we can see a l l l e v e l s up t o within 0.lOeV of t he conduction

band. The f a c t t h a t t h e photoconductivity s igna l w a s very low f o r

energies l e s s than Ec-0.2 eV s i g n i f i e s t h a t shallow l eve l s a r e prodaced

i n low concentration. Hence defec ts which give r i s e t o shallow l e v e l s

i n Li-doped S i i r r a d i a t e d a t ~ , 3 0 0 O I c may not be a s important a s those

defects which give r i s e t o deeper s t a t e s .

7- 9

15 =

I n e a r l i e r work we reported t h a t t h e presence of L i has an important

e f f e c t i n t h e production of radiation-produced divacancy defects as

monitored by the 1.8 micron band.’ Spec i f ica l ly we found’ t h a t t he

production of divacancies was decreased a s the l i th ium concentra-

t i o n was increased i n i r r a d i a t e d Li-doped s i l i c o n samples.

-29-

I t was proposed t h a t t he L i atoms were made more mobile because o f g rea te r

ease i n d i f fus ion which was enhanced by radiation-produced vacancies,

divacancies, e tc . Our recent work leads us t o bel ieve t h a t t he rad ia-

tion-enhanced diffusion i n t e r p r e t a t i o n is not cor rec t since the l i thium

atom d i f fuses through t h e l a t t i c e i n t e r s t i t i a l l y and the d i f fus ion r a t e

should be r e l a t i v e l y in sens i t i ve t o radiation-produced vacancies.

of t he presence of more i n t e r s t i t i a l Li-atoms i n t h e l a t t i c e they can

become at tached t o vacancies and divacancies thus giving rise t o a

d i f f e r e n t species of defec ts whose concentration increases a s the L i

concentration increases. These new species were observable i n the

form of new defec t i n f r a red absorption bands a t 1 . 4 and 1 . 7 microns.

It i s s i g n i f i c a n t t h a t we a r e observing s imi l a r e f f e c t s i n our photo-

conductivity spectra where except f o r one s l i g h t h i n t i n Fig 1 we have

not observed a s i g n i f i c a n t concentration of divacancies as would be

evidenced by energy l e v e l s a t Ec-0.54eV and Ec-0.39eV.

paper3 using stress-induced dichroism e f f e c t s i n the photoconductivity

spec t ra of i r r a d i a t e d S i we i d e n t i f i e d t h e l e v e l s a t Ec-0.39 eV and

Ec-0.54eV with t h e doubly negative and s ingly negative charge s t a t e s o f

the divacancy defect respect ively. Hence, we can conclude from our present

Because

I n a previous

photoconductivity s tud ies t h a t the L i atoms a r e again apparently competing

f o r vacancies and divacancies thus inh ib i t i ng t h e production of diva-

cancy defec ts and ins tead forming a new complex i n which one L i atom and

two vacancies a r e the dominant const i tuents . I t may be t h a t t h e l e v e l

we observe i n many of our samples a t Ec-0.62eV (see Figs 1-3, 6,8, &9)

may be associated with the L i defect. Although i n recent work reported

by Downing, Carter, and Van Atta of t h e TRW Systems Group they suggest

2 l i thium atoms a r e consumed i n the anneal of t h e S i - A center (vacancy-

oxygen pa i r ) they a l s o s t a t e t h a t t h i s conclusion may be misleading. I n

our case we a r e discussing a d i f f e r e n t defect t h e divacancy and a s a f i r s t

conclusion we argue i n favor of t he simpler and more probable case of

s ing le L i atoms ( ra ther than two L i atoms) becoming attached t o t h e diva-

cancy during production of t he defect s ince our r e s u l t s ’ show an approxi-

mate montonic decrease of divacancy production a s t he L i concentration

was increased as determined by a spectroscopic probe i . e . in f ra red s tudies ,

t

+ Second Quarterly Report, 13154-6006-R0-100 15 Jan 1 9 7 0

-30-

b) Inf ra red Spectroscopy Studies

We have completed our study of i n f r a red spectroscopy (From 1 t o 50

m-icrons) on e lec t ron- i r rad ia ted Li-doped S i as prescr ibed by the matrix i n

Table I , The l i th ium concentrations i n t h e sampLes var ied from 1 0 c m

up t o 1017cm-3 and She incident e l ec t ron energy was e i t h e r 1 .5 o r 5 MeV.

The samples were subjected t o f luences of from 10l4e/cm2 up t o 101Ge/cm2

with some of t h e samples receiving w l o x 7 e/cm . The samples of s i l i c o n

were e i t h e r pu l led c ruc ib le grown (oxygen-rich [ O ] a c n r 3 ) o r f loat

zone r e f ined (oxygen-lesn with [O]$

15 -3

2

A defect absorption band is observed a t 9.9 microns ( ) both a t

The in - /L"

300°K and a t 7S°K i n pul led S i and - not i n f l o a t i n g zone S i .

t e n s i t y of t h i s band increases w i t h l i th ium, and w e f i nd t h a t t h e L i concentration must be a1016cm-3 i n order t o observe t h i s band.

can s t a t e t h a t a concentration of - 2 x 1 0 l 6 centers/cm

lower l i m i t of s e n s i t i v i t y f o r detect ion of defec ts by in f r a red spectro-

scopy. The i n t e n s i t y of t h e 9.9 band i s not a f fec ted by 1 t o 5 MeV

e lec t ron i r r a d i a t i o n up t o a fluence of l o x 6 e/cm

experimental e r ror .

cm-1(9.99,&) with a s h i f t t o s l i g h t l y higher energy a t 1014 cm-'(9.9@

when t h e measurement is made on t h e sample kept a t 7S°K.

band t o higher energy a t t h e lower temperature i s what one would expect

We 3 represents 'our

P 2 within our f 5%

The prec ise loca t ion of t he band a t 300'K i s lOOG

The s h i f t of the

from thermal contract ion of the l a t t i c e thereby giving r i s e t o a higher

e f f ec t ive spr ing constant k. The higher e f fec t ive k w i l l l ead t o a higher

frequency ( f o r harmonic o s c i l l a t o r w = E (m= mass o f atom) which means

t h a t t h e pos i t ion of t h e absorption peak a l s o becomes sharper a t low

temperamre because of reduced v ib ra t iona l broadening. The peak a t 9.9

i s observed i n uni r rad ia ted s i l i c o n mater ia l as spec i f i ed above.

The f i rs t I R spectroscopy r e s u l t s on t h e 9.9 band were reported 4 P

by Chrenko, McDonald and P e l l . I n e a r l i e r work, P e l l ' 5 presented a

model i n which t h e l i th ium ion i n t e r a c t s with uncharged i n t e r s t i t i a l oxygen (arepresent i n pul led Si) t o form a charged defect (OLi) . From the in f r a red spec t ra Chrenko e t a1.4 and P e l l

of t h e defect causing t h e 9 .9 /"i band.

t h a t the band was due t o v ibra t ions of t h e (OLi) complex.

+ 5

came up with a model

They 4 y 'gave convincing evidence +

-31-

4 I n their model t h e l i t h ium pos i t i ve ion is bound by ionic forces a t shor t range (2-3A) t o an i n t e r s t i t i a l oxygen atom with t h e l o c a t i o n o f the oxygen

atom r e l a t i v e t o i t s neares t s i l i c o n neighbor being the same withandwith-

out t h e presence of a l i th ium atom. From a compariSon of t he s t rong bind- ing of exygen t o t h e S i l a t t i c e determined by diffusion measurements t o be

2.55eV and t h e r e l a t i v e l y lower binding of L i f t o oxygen which is 0.42 eV,

it seems reasonable t o expect t h e i n s e n s i t i v i t y of t he oxygen atom posi t ion i r r e spec t ive of L i atoms.

0

The annealing behavior of t h e 9 . 9 ~ micron band is remarkably d i f f e r -

en t i f t h e sample has been i r r ad ia t ed t o a r e l a t i v e l y high fluence of

e lec t rons ry 1018e/cm2.

thus f a r .

c i e n t M(T) a f t e r 15 minutes anneal t o t h e Qc, before annealing a s a

funct ion of t h e annealing temperature i n C. One can see t h a t i n t h e

sample i r r a d i a t e d with 48MeV e lec t rons t o a fluence of 2 x lo1' e/crn2 a

I n Fig. 1 2 we have summarized t h e r e s u l t s obtained

I n Fig. 1 2 w e have p lo t t ed t h e r a t i o o f t h e absorption coef f i -

6

dramatic annealing of t h e 9.9,~ band occurs a f t e r %130°C annealing.

sample was Li-doped t o c ~ l O ~ ~ L i / c m ~ .

new band appears a t 13 microns (770 cm-1).

t h e 1 3 micron band a l s o disappears1.

i r r ad ia t ed w i t h 1.5MeV and 5MeV t o fluences of 10l6 e/cm2 exh ib i t no

change i n t h e 9 . 9 ~ band. and a r e i n f a c t s t a b l e up t o 500C (see Fig. 12).

Further s tud ie s on t h e e f f e c t 5MeV e lec t ron i r r a d i a t i o n of various f l u -

ences on t h e 9 . 9 ~ absorption band a r e needed i n order t o make conclusions

regarding t h e e f f e c t s of lower energy on t h i s band.

The A s t h e 9.9,~ band anneals out a

Upon hea t treatment t o 200°C

Unirradiated mater ia l and samples

0

I n view of t h e model of t he (LiO)' given above i n which t h e oxygen atom

i s more t i g h t l y bound. t o t h e S i l a t t i c e than t o t h e L i atom it may be t h a t

r ad ia t ion produced defec ts o r L i atoms made mobile by the 100°C heat t r e a t -

ment become attached t o t h e (LiO)' complex producing a c l u s t e r containing

more than one 0 and one L i atom and t h i s c l u s t e r has an infrared ac t ive

v ib ra t iona l mode a t 13 . 7 No f u r t h e r r e s u l t s were obtained i n our s tud ies of infrared spectro-

scopy t h a t a r e d i f f e r e n t from thosewehave reported elsewhereonelectron

i r r ad ia t ed Li-doped S i (both pul led and ' f loa t ing zone re f ined) . The main new d i r ec t ion o r approach now needed is t o i r r a d i a t e a t cold temperature

80°K and then measure I R spec t ra upon warm-up t o room temperature. e.4

These experiments were not planned t o be performed i n the present study.

-32-

C) Consideration of E r r o r s , Uncertaint ies and Limitations o f t h t Measurements

One major uncertainty i n our measurements a r i s e s from the l ack o f

knowledge of t h e e f f ec t s of other impuri t ies such as Na (present t o

* 40.01% i n t h e l i t h ium metal dispersion) on the proper t ies of i r r a d i a t e d

material .

r e s i s t i v i t y i s only s e n s i t i v e t o e l e c t r i c a l l y a c t i v e impurrities. This

means we may underestimate the L i concentration throughout t h e bulk

of t he sample if the L i i s present i n a form *hat i s e l e c t r i c a l l y

i n e r t . Naber and Passenheim who used neutron ac t iva t ion techniques t o

measure t h e t o t a l L i concentration reported a higher L i content (by a f a c t o r of 8) than measured by e l e c t r i c a l r e s i s t i v i t y .

t ha t the L i is present i n l a r g e clumps a s a p r e c i p i t a t e i n the sample.

However, w e have found tha t even i n hea t treatments tow600 C we never

observe an increase i n c a r r i e r concentration but r a t h e r a decrease

ind ica t ing a l o s s of L i .

is made f ree and e l e c t r i c a l l y ac t ive by t h e heat treatment. Hence, it is possible t h a t t h e p rec ip i t a t ed L i may not be a dominant e f f ec t .

The method w e use t o monitor the Li content namely, e l e c t r i c a l

8

This would imply

0

One might expect tha t L i present i n clumps

The uncertainty i n the homogeneity of t h e diffused l i th ium throughout

t h e bulk of the sample was obtained by f i r s t measuring t h e r e s i s t i v i t y

of a sample i n i t i a l l y 1.63 mm t h i c k and then remeasuring r e s i s t i v i t y

a f te r t h e sample WES ground down t o a thickness of 0.78 mm. The two values

of r e s i s t i v i t y measured on t h i s sample (before and a f t e r grinding) were

within 2% of each other . A s an add i t iona l check w e measure r e s i s t i v i t y

on opposi.te faces of a l l samples before i rpadia t ion and f i n d values which

d i f f e r by a t t h e most f 5% thus, these measurements imply a d i s t r ibu t ion

of e l e c t r i c a l l y ac t ive lithium throughout t h e sample having inhomogeneities

no larger than f 5% of t h e nominal value with the majority of t h e sample

having concentration inhomogeneities less than ~ 3 % .

I n order t o check whether o r no t the o i l and alumina (A1203) mixture

introduces e l e c t r i c a l l y a c t i v e centers i n t h e sample we "painted" a

sample with t h i s mixture (without l i thium) . Measurements of r e s i s t i v i t y

made on opposite s ides of t h e sample before and a f t e r 20 minute heat

treatment t o 4 1 O o C were t h e same within 1%. t h a t t h e o i l and A1203 does not introduce e l e c t r i c a l l y ac t ive centers .

We t ake t h i s as good evidence

-33-

We have inves t iga ted t h e l i m i t a t i o n on our in f r a red spectroscopy

measurement and f i n d that i n order f o r us t o observe a v ibra t iona l

absorption band we must have % i n f r a red ac t ive centers per cm . I n the case of sharp absorption bands due t o e l ec t ron ic exc i ta t ion we

mus t have25 x 1015 in f r a red ac t ive centers per cm3 f o r observation of

t h e peaks. For most normal absorption bands we rout ine ly measure the

absorption coe f f i c i en t t o an accuracy b e t t e r than 5%.

3 nL,

I n t h e photoconductivity spec t ra we generally use slit widths

r e s u l t i n g i n energy reso lu t ions t h a t tend t o be i n the range of 0 . 0 1 t o

0 - 0 9 eV and which a r e dependent on t h e s p e c i f i c wavelength. The energy

reso lu t ion used i s one which gives a compromise on the accuracy of t he

detected s i g n a l and va r i e s f r o m sample t o sample depending on t h e

condition of contacts , e lec t ron fluence, defec t concentration and

impurity concentration. We have quoted e r r o r s f o r each spectra

presented i n t h e photoconductivity r e s u l t s .

-34-

CONCLUSION

We can b r i e f l y summarize our conclusions a s follows f o r electron-

i r r a d i a t e d ( a t 30OoK) Li-doped S i .

1) No simple co r re l a t ion of defects e x i s t s between l i th ium conccntra-

t i o n , e lec t ron fluence and energy, and o ther impurit ies.

2) The presence of l i th ium atoms g rea t ly i n h i b i t t h e production of t h e impurity-independent divacancy defect and the A-center oxygen

vacancy pa i r .

3) A dominant l e v e l a t E,-0.62 eV i s found from photoconductivity

measurements i n near ly a l l samples of i r r a d i a t e d L i doped S i . Deeper

l e v e l s > Ec-0.20 eV are formed i n grea te r concentration than shallow

l eve l s . N

4) The absence of a photoconductivity l e v e l a t eEc-0.4eV indica tes

f e w E-centers (phosphorus-vacancy pa i r ) a r e produced due t o t h e presence

of L i .

5) The 9.9 (LiO)' band i s s t a b l e up t o hea t treatments of 5 O O 0 C , /I" 0 bu t disappears a f t e r x 1 3 0 C annealing i n samples i r rad ia ted . t o an e lec t ron

fluence of 1 0 e/cm . (48 MeV) 18 2

Follow On E f f o r t

Now t h a t a survey of t he e f f e c t s of 1.5 and 5 MeV e lec t rons on

s i l i c o n doped t o various concentrations of l i th ium has been s t a r t e d it

appears t h e s ing le most important new experimental d i rec t ion i s t o perform

t h e i r r a d i a t i o n s a t cold temperatures say below ~ 9 5 K and measure t h e

in f r a red and photoconductivity spec t ra a t low temperature with subsequent

warmup t o room temperature. I n these experiments t h e L i would be mcch

less l i k e l y t o move e a s i l y s ince a t t h e lower temperature the d i f fus ion

r a t e i s much l e s s than a t w300°K.

0

A de ta i l ed study of the e f f ec t s of rad ia t ion on t h e 9.9 band both /" a t i r r a d i a t i o n temperatures of ~ 8 0 - 9Q°K and a t 300°K while a t t he

same time monitoring t h e A-center production ( 1 2 ~ band) and t h e diva-

cancy production (1.8 and 3.3 bands) would y i e l d important informat ion

regarding whether t h e L i moves t o t h e rad ia t ion defec t o r vice versa. /I"

-35-

bands induced A study of t h e s t r e s s response of t he 1-4, 1 , 7 and 9, i n Li-doped s i l i c o n would y i e l d important information on t h e microscopic

s t ruc tu re of t h e defec ts involved. No such information is current ly

ava i lab le f o r Li-doped S i mater ia ls ,

F ina l ly , a comparison of energy l e v e l s ,found from photoconductivity 2 s tudies with t h e de t a i l ed c a r r i e r concentration measurements of Brucker

would be valuable i n understan ding the defects produced t h e i r character-

i s t i c s and t h e i r i n t e rac t ions with impurit ies.

The work oyt l ined above could now be ca r r i ed out i n our laboratory

using methods already developed during the performance of t he research

reported here.

Acknowledgment

We thank Mr. J. W. Wesfhead, Mr. D. Morse, and Mrs, Rosa Young f o r help and a s s i s t ance given t o us during t h e course of'wdrk on t h i s program.

-36-

I

I

I

I

I

I

I

L n I I

I

I I

I

c o c o b d c n z I m a d r r l = f M r - I O o r l m m m 3 0 0 0 0 0 N I N d o o o . . , . . . . . . .

I I

I

L c

N L4

E + 0 rl

r .- L ... L c - c L c c - c L c - c .- c c c - r -

-37-

a -P

U

L

H H

-38-

A 0

I 'v VJ ;L; I

m a, 4 a =I c, ln

I

I

I

I

I

I

1

I

I

I

I

I

I

1

1

I

I

I

I

I

I

I

I

3 l-4 0 d

3. m l-4

$4 0

4-1

m N

0 a, l-4 a E rd Yj

u3 I-I 0 d x m

ca rl 0 rl

c 0

6 03

rcr \

u E ln CI U cd c, E 0 U

x .., e

4-1 0

Ln

N

a $4

l!i I cn

l-4 rd U *rl h CI U a,

r-l a, I

c2 0 tc(

H H H

w IJ m 2

-39-

REFERENCES

1.

2.

3.

4.

5.

6.

7.

8.

R. C. Young, J. W. Westhead, and J. C , Core l l i , J. APpl. PhYS.

40, 2 7 1 (1969) -

G . J. Brucker, Phys. Rev. - 1 8 3 , 712 (1969)

A. H. Kalma, and J. C. Core l l i , Phys. Rev. 173, 734 (1968)

R. M. Chrenko, R. S. McDonald and E. M. P e l l , PhysRev. 138% A 1775 (1965)

E. M. P e l l , Symposium on Sol id S t a t e Physics i n Electronics and

Telecommunications (Academic Press Inc. , N. Y. 1960)

J, W. Corbett, R. S. McDonald, and.G. D. Watkins, 17. Phys. Chem.

Sol ids 25, 873 (1964) -

E. M, P e l l , J. Appl. Phys. 32, 1048 (1961)

J. A. Naber, B. C . Passenheim, "Radiation Effects i n S i l i c o n Solar

Cells,"

952387, 1 0 Apri l , 1969 , GA-9312.

Quarterly Report t o J e t Propulsion Laboratory on Contract

9. G. K. Wertheim, Phys. Rev. 115, 568 (1959); z, 1272 (1958)

10 . L. J. Cheng, J. C. C o r d l i , J. W. Corbett and G. D. Watkins, Phys.

Rev. 152, 7 6 1 (1966)

- 40-

LIST OF FIGURE CAPTIONS --

Fig 1) 46 (normalized with respect t o l i g h t Relat ive photoconductivity

source) vs wavelength f o r FZ s i l i c o n doped t o 2.5 x lOI5 Li/crn3

a f t e r 5 MeV electrron i r r a d i a t i o n t o a fluence of 1014e/cm2 (Sample

JPL# 66)

“s

E- Fig 2) Relat ive photoconductivity 6 (normalized with respect t o l i g h t

source) vs wavelength f o r FZ s i l i c o n doped t o 5 x Li/cm3

a f t e r 1 .5 MeV e lec t ron i r r a d i a t i o n t o a fluence of 1015 e/cm2

(sample J P L #88)

6 Fig 3) Relat ive photoconductivity

source) vs wavelength f o r CG s i l i c o n doped t o 0 .9 x l o x 7 Li/cm3

a f t e r 5 MeV e lec t ron i r r a d i a t i o n t o a fluence of (Sample JPL #15)

(normalized with respect t o l i g h t

e/crn2.

Fig 4) Spectrum of globar l i g h t source measured with a thermocouple detector .

The System was n o t purged with dry air. and 3.0 microns a r i s e s from water and a t 4.2 microns from C02.

The s t ruc tu re a t 1 . 6 , 2.4,

Fig 5) 66“ Relat ive photoconductivity vs wavelength f o r CG s i l i c o n doped --. t o 2.5 x l ox6 Li/crn5 a f t e r 1.5 MeV e lec t ron i r r a d i a t i o n t o a Eluence

of 5 x 10l6 e/cm2 (Sample JPL #3)

+ 2 % a t 2 u and 2 8% a t 2.8 . The e r r o r i n 4 6 is $8% a t 1 8 /u”

/” - /

Fig 6) Relat ive photoconductivity vs wavelength f o r C .G ~ s i l i c o n doped

20 2.5 x loL5 Li/cm3 a f t e r SMeV e lec t ron i r r a d i a t i o n t o a fluence of 101ye/cm2 (sample JPL #21), t o f 3 % a t 2.4uand$10% a t 3.2k.

The e r r o r i n obis 21% a t 1 . 0 CA b / 1

Fig 7) Relat ive photoconductivity v s wavelength f o r CG s i l i c o n doped

t 9 2 . 3 x lo1’ Li/cm3 a f t e r 5 MeV e lec t ron i r r a d i a t i o n t o a fluence

of 1015 e/cm2.

i- 10% a t 2 . (sample JPL #23) The e r ro r i n g i s 21% a t 1 . 2 .u and

6 - P

- 41-

Fig 8) Rela t ive photoconductivity vs wavelength f o r CG s i l i c o n doped t o

2 x Li/cm a f t e r 5 MeV e lec t ron i r r a d i a t i o n t o a fluence of

1015 e/cm2 (sample JPL #25). The e r ro r i n i s f 10% a t 1. and f 25% a t 2 .7 below 1 . 7 the e r r o r i s 2%.

3

a 6

/" /" 6 Fig 9)

Rela t ive photoconductivity e v s wavelength f o r FZ s i l i c o n doped t o - 1 . 0 x 1017 Li/cm3 a f t e r 5 MeV e lec t ron i r r a d i a t i o n t o a fluence of

/" 1016 e/cm2 (sample JPL #58). The e r r q r i n 6 - is 2% below 1 . 3 and

4 * 15% a t 2 . 2 . /L" 6 6 Fig 10)

Relat ive photoconductivity 2 vs wavelength f o r FZ s i l i c o n doped t o

2 x 1 0 Li/cm af-Ler 5 MeV e lec t ron i r r a d i a t i o n t o a fluence of 1 6 3

P 10" ,e/cm2 (sample JPL #62). . The e r r o r i n r i s 6 f 1% below 1 . 2 6 and p e s up t o 15% a t 4.1 P -

6 Fig 11) Rela t ive photoconductivity ~ v s wavelength f o r FZ s i l i c o n doped t o

2.4 1015e/cm2 (sample JPL #64) . is - 3% from 2 . 5 ~ up t o 3

3 x l o L 5 Li/cm a f te r 5 MeV e lec t ron i r r a d i a t i o n t o a fluence of

The e r r o r i n 6 i s f 1% below 2.5

a t 3 . 5 ~ the e r r o r i s * 15%.

and + d /u- P

Fig 12) '

Effect of isochronal annealing (15 minutes a t each temperature)

on t h e 9 .9

The samples were i r r a d i a t e d w i t h 1.5, 5 and 48 MeV e lec t rons t o t h e

f luences shown. Al so shown is an uni r rad ia ted Li-doped sample.

(1014 c m - l ) band i n CG s2l icon doped with L i t o 1017cm-3. r

I

N E D

3 % . - I 2 2 C

.. w * *

Lz

11 f

I L

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N E '\ 0

(II

.. !4 r;i .

r \

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z w 3

t

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t

0 cd

0 Irj

0 ti

0 rcj

0 oj

0

Fig 2

n

c

W N E

3 > c a, m

I c

0 Ln'

0 ti

0 pcj

9 &I

0

Fig 3

N 0 "0

0 0

Fig 4

N E

$

x m L n r l

.. w g 3 w c l z FLlw

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3 4 0 4

.. W U Z w 3 CIj L4

0 0-

0 Tf

0 m-

0 nj

0

n c

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.. w

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Fig7

.rl

C 3 0 I (3

a,

c)

M

'E 0

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S

L

Fi a J

00 Ln 4k I Q, 3

0 t f j

0 ti

0 rcj

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Fig9

0 cd

(\1 u?

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I

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-f 0 cu'

0

b

Fi g C

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a, S O N rn C .- t U 0 LL C

IJ

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c

cu Lo +O - cu Lo

0 cd

0 lcj

0 ti

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0 cu

0 -

0.8

0.6

0.4

0.2

x Y

I

I - 1

I

I I I

I I I I I I I I

0 Li-Doped

A Li-Doped X Li-Doped

a LI-Doped

X

Si (CG) 48MeV 2 x l O i 8 ~ Si (CG) 5 MeV 1OI6 11

Si ( C G ) 1.5MeV IOi6 I I

SI (C G) unirradia ted

9.9,~ (1014 cm-1)

100 200 300 400 500 0

0

Fig I2