IEEE ΤΜΛ&αοΑίοη* on UucLicui Science, t/o£#WS-2/# Pecembe* 1974

EFFECTS OF ION BOKBARIKENT ON Na AND Ct MOTION IN S i 0 2 THIN FILMS*

Wendland Beezhold Sandia Laboratories

Albuquerque, New Mexico 87115

Abst rac t

The effects of l i g h t and heavy ion bombardment on Na and Ot motion in Si02ïSi t h i n films have heen s tud­ied using proton-induced cha rac t e r i s t i c x-ray (PIX) and Rutherford backscat ter ing (RBS) techniques. Re­s u l t s show tha t both Na and C£ atoms may be trapped in the oxides by implantation damage. Release does not occur under ionizing i r r a d i a t i o n (proton bombardment). P a r t i a l release or motion does occur after annealing to 700°C or by further heavy ion bombardments. Na motion i s consistent with t he movement of s ingle Na+

ions in the oxide. In con t r a s t , the movement of Ci. atoms appears t o be dominated by enhanced Ot diffusion or "by iiiotion of posi t ive Ot-defect complexes whenever heavy ion bombardment damage i s introduced.

I . In t roduct ion

Impurity Na and Ot atoms in the Si02 of metal-oxide-semiconductor (MOS) devices strongly influence device performance.·*·>2 Sodium atoms also play an im­por tan t role in radia t ion e f fec t s tudies of i n su la to r s and MOS devices. 3>^ For example, ion-bombardment i n ­duced Na movement and/or t rapping in SiOp may occur as a r e s u l t of ion-neut ra l iza t ion processes? o r , in c e r ­t a i n cases , may be due t o the formation of g lasses by g l a s s-forming ions bombarding the oxide. " Na atoms t h a t become charged may move to the SiOgïSi i n t e r f ace , become trapped, and s h i f t the C-V e l e c t r i c a l proper­t i e s of the MOS device. This paper presents exper i ­ments designed to t e s t these concepts by providing d i r e c t measurements of impurity motion and bombardment e f fec t s in SiOg. The experiments described here u t i ­l i zed the in s i t u ion beam analys is techniques of proton-induced c h a r a c t e r i s t i c X rays (PIX) '>" and Rutherford backscattering (RBS).9>1° Although simul­taneous recordings of e l e c t r i c a l measurements are com­pa t i b l e with the PIX and RBS methods, none were made in t h i s study. PIX techniques were used to iden t i fy Na and Ot motion before and a f t e r annealing and a f t e r ion bombardment of Si02:Si and Au:SiOg:Si t h i n film s t r u c t u r e s .

X RAY

PATH 1

PROTON

PATH 2

PROTON

PHOTONS AND/OR PARTICLES EMITTED

1. PROTON-INDUCED CHARACTERISTIC X-RAYS

2. RUTHERFORD BACKSCATTERED PROTONS

3. NUCLEAR REACTION PRODUCTS

4. AUGER ELECTRONS

Figure 1. Bombarded atom schematic. Path 1 shows an incoming proton c rea t ing a K-shell vacancy and subse­quent emission of a K X ray (proton-induced charac te r ­i s t i c x-ray emission or PIX). Path 2 shows an incoming proton backscat tered by the nucleus (Rutherford back-sca t te r ing or RBS).

s h e l l s . The e l ec t ron t r ans i t i on may be accompanied by emission of a K X ray with energy c h a r a c t e r i s t i c of the pa r t i cu la r t a r g e t atom. Path 2 shows an incoming proton backscat tered by the target atom's nucleus. This RBS process gives the approximate mass of the t a rge t atom and, i f the mass is known prec i se ly , the approximate depth of the target atom in the t a rge t subs t ra te .

I I . Experimental

Samples of 1000 À th i ck Si02 dry oxides on S i 1 1

were ion implanted a t room temperature, some with h-5 keV" Na+ atoms and others with 75 keV Gt+ atoms t o f lu -ences of 3 X lO1^ atoms/cm2. For PIX analys is , a 250 or 275 keV H+ beam was used t o exci te cha rac t e r i s t i c X rays from the target atoms. The s e n s i t i v i t y was R ~ 1CA5 Na or Ct/cm2 with k x 10 1 2 incident H+ i o n s . 0

Additional 5000 A thick oxides were implanted to 10 1 6

and 2 X 101 δ ions/cm2 with 70 keV Na and 120 keV Cl atoms. Several unimplanted oxide samples were covered with 500 A th ick Au e lec t rodes to form MOS s t r u c t u r e s . Using PIX analysis , these samples were found to be contaminated with 0t ; t h i s contamination presumably occurred during fabr ica t ion .

Target atoms were probed using an energetic beam of protons. The processes of PIX and RBS are sche­mat ica l ly i l l u s t r a t e d in F ig . 1. For PIX ana lys is an incoming proton (path 1) c rea tes an inner s h e l l vacan­cy which i s f i l l e d by an e lec t ron from the L, M, or N

This work was supported by the TJ. Energy Commission.

S. Atomic

The experimental arrangement for recording the PIX and RBS data of t h i s paper has been.described e l s e ­where . 1 2 General fea tures of the system include: ( l ) an ul trahigh vacuum 10V Torr or b e t t e r to minimize beam-induced contamination 3 0 f the oxide samples, (2) a windowless S i (L i ) x-ray detector with r e so lu t ion of 125 eV on the 1.7^ keV Si(K) l ine , (3) an x-ray f i l t e r wheel with f i l t e r s such as 500 A C, Ο.75 μ A£, and 8 Λ χ 10"° m Be to allow detect ion of low Z elements down to carbon or a t t enua t ion of X rays through Si(K),, and (h) a surface b a r r i e r pa r t i c l e detector for RBS with resolut ion of ~ 9 keV.

The effects of annealing and ion bombardment on Na+- and Ot+-implanted Si02îSi were invest igated using PIX and RBS a n a l y s i s . Annealing was performed a t 700°C for 20 min in an N2 atmosphere. AU sample types were bombarded with IT**, Ne+, or A+ ions a t room temperature for flue nee s ranging from 101 2 to lO1? ions/cm2. After each fluence i n t e r v a l , PIX and/or RBS data were recorded.

The H* bombardments mainly produced ioniza t ion events within the oxide with very l i t t l e displacement damage. By c o n t r a s t , the heavy ions pr imari ly l o s t

62

energy in the SiC>2 v ia damage or atonic displacement processes . Both l i g h t and heavy ion bombardments, fts wel l as PIX and RBS a n a l y s i s , v?re «pawned to s l i g h t l y charge the oxide curface p o s i t i v e l y during bombardment.

I I I . Ion Beam Analysis in S i 0 2 : S i Thin Films

PIX analys is i d e n t i f i e d the element of in teres t end provided Na and Cl depth information r e l a t i v e to the unannealed, prebombarded-implanted oxides with r e ­so lut ion of 500-1000 A. A decrease in x-ray y i e l d corresponded to a movement of Na or Cl inward; an i n ­crease corresponded t o Na or Cl movement toward the surface. Conversion from changes in x-ray y i e l d to changes in depth were estimated from thick target x -ray y i e l d s for Si02· Additional experimental c a l i ­brations were provided by implanting A+ ions at various depths in the oxide and, assuming stat ionary implanted d i s t r i b u t i o n s , comparing the PIX A ( K ) x-ray y i e l d s .

BBS analys i s was not able to detect Na but was used to detect Cl, check the oxide thickness , oxide stoichioraetry, and, with PIX data, determine the change in Cl d i s tr ibut ion a f t er ?00°C annealing.

A. PIX Analysis υ+ Implanted Na and Ct Atoms in S i 0 2 : S i

Figure 2 g ives an example of PIX analys i s of a Na+-implanted sample on the l e f t and a OC+-implanted sample on the r i g h t . Fluences were 2 .0 X 1 0 1 6 NaVcm2

and 1.0 X 101" CM+/cm2. The lower spectrum in each case displays the strong SIÎKQ,) x-ray output at 1.7^ keV. The low-energy portion on the l e f t , magnified 10 times and displaced upwards, shows the NafKoJ x-ray y i e l d at 1.04 keV. The higher energy portion on the r ight , again sh i f ted upward and magnified by 10, shows the Cl(K) X rays a t 2 .62 keV. The presence of Cl was f i r s t observed with larger Ot(K) x-ray y i e l d s from higher proton f luences . The small y i e l d shown in Fig . 2 i s t y p i c a l of a low fluence PIX analys i s for Cl used in the Ne"** and A"*" bombardment é tud ie s .

I t was poss ib le , in the case of Na and 0 X raye, to s i g n i f i c a n t l y improve the s igna l to background ra t io by the appropriate use of x-ray f i l t e r s . Na and u r-ray y i e l d enhancement i s shown in F ig . 3> where Fig . Jt i s a repeat of Fig . 2a . Here, an At f i l t e r (0 .75 um thick) was used to absorb the higher energy Si(K) X rays but more e a s i l y transmit the lower energy Na(K) and 0(K) X rays . Beducing the Si(K) y i e l d a l so reduced the low energy background because fewer Si(K) X rays

«t»> M»* ΙΜΡΙΑΝΠ0 ' P I * f . TOkeV « B O * SiO Si

• 2 . 1 0 » * ♦ ■ · € » * a î m i . B e f l l T

8 °l·

* < * « >

(al UNIMPLANTIO

«Jt Na* IMPlAMTtO PI Y WOO A ατ5Μ AI m u »

MKKA>

0<Κα> I

<c> UNIMPLANTED

3.0 0 1.0

X-RAY ENERGY (keV)

Figure 3 . Enhancement of Na x-ray y i e l d by use of Ο.75 Mm At f i l t e r . Part (b) shows PIX y i e l d for nor­mally used 0.3 mil Be window. Part (d) shows the im­provement using At f i l t e r . Parts ( a ) and ( c ) show un-implanted portions of the S i 0 2 i S i .

were able to contribute to stray charge c o l l e c t i o n in the x-ray detector . The Be f i l t e r shown on the l e f t , and a l so used i n Fig . 2 , i s a standard x-ray spec­trometer detector window.

Spectra on the r ight of Fig . 3 are for the 0.75 urn kl f i l t e r and those on the l e f t are for the 8 Λ x 10""̂ m Be f i l t e r . In each case an uninplanted region of S iO^Si i e shown in the lower portions ( a ) and ( c ) .

B. RBS Analysis of Implanted SiOp'Si

Figure U shows RBS spectra from several 5000 A thick Si02-'Si samples. The leading edges for back-scat ter ing from Si and 0 atoms in the oxide are shown by the arrows. Backscattering from substrate Si was sh i f t ed to lower energies due to energy lo s se s of the incoming protons penetrating the oxide to reach the substrate .

Reference unimplanted SiC^.'SI i e shown in the lower l e f t . The upper l e f t i s annealed, Na*-implanted SlOg'.Si; RBS was unable to detect implanted Na atoms and the spectrum was similar to unimplanted SiOgrSi. (M+-implanted Si02*Si i s shown in the lower right of

Ni -IMPLANTED

Silfce»

I 2 rf.

y=

SMKo» Skiv [ f

Si (U> X [r-V DfTECTOt M x-nAY KTECTO*

-i\->

PROTON-INDUCED CHARACTERISTIC X-RAY SPECTRA

IMPLANTED

to 1.0

f ENERGY (IctV)

Figure NaVcm2-

2. PIX analysis of 2 X IO16 and 1 X 1016 Ot+/cm2-implanted

Si02^Si. Na(K) X rays are shown on the left in the expanded scale. Cl(K) X rays are shown on the right. The x-ray yields are taken from the total area In a given x-ray peak.

63

Na -IMPLANTED 700°C ANNEAL

·· *··̂ ·'.

Si IN Si

Si IN SiO,

~Γ π 1 1 r .PARTICLE DETECTOR JK

CI-IMPLANTED \ ^ 700'CAKNIAL „ ^ ^ ^

CI -IMPLANTED UNANNEALED

RUTHERFORD BACKSCATTERING

a2 0.4 0.6 0.8 1.0 0 a2

PROTON ENERGY (x 275 keV) a6 <u 1.0

Figure **. RBS analysis of several 5000 Â thick oxide samples. The lover l e f t i s unimplanted SiOgrSi. The lower right shows a trace of backscattering from im­planted Cl in unannealed Si02ïSi . The arrow points to the weak raised portion near 0.83. This small raised portion disappeared after 700°C annealing. An­nealed RBS spectra were similar to spec­tra from unimplanted SiC^Si .

F ig . k. A very weak but reproducible backscattering y i e l d from Cl atoms appears at ~ Ο.83, f i l l in g in part of the Si-0 step at 0 .8 . This location of Cl in the RBS spectrum i s consistent with the projected range of implanted Cl being ~ 1000 Â into the oxide.

C- Annealing at 700° C for 20 Minutes

Recent work has shown that for high fluence im­plants in Si02> most of the damage anneals by 700°C.1^ Therefore, half the Na+- and Ot+-implanted samples were annealed at 700°C for 20 min. PIX and RBS data were recorded in order to check for redistribution of the implanted Na and Ct atoms. Figure k displays the RBS resul t s for Cl in the upper r ight . The f i l l - i n por­t i on to the Si-0 step has disappeared and the annealed spectrum looks much l ike the unimplanted spectrum. However, PIX spectra indicated that the Cl(K) x-ray y ie ld did not change after the 7O0cC anneal. There­fore , i t i s concluded that the buried implanted Cl layer spread out in the oxide during annealing about the implant depth, but without any substantial move­ment to the Si02:Si interface or to the Si02 surface.

In contrast, PIX analysis on the lO^/cm2 Na+-implanted Si02:Si indicated that annealing at 700° C caused most of the Na atoms to move toward the Si02:Si interface region. These resul t s imply that annealing at 700°C removes most of the Na+-implant damage and allows Na atoms to move toward the Si02:Si interface. Na movement could have occurred during the f i r s t post-annealing PIX run, but i t i s more l ike ly that Na atoms moved toward the interface during or after annealing as a result of Na+ atoms being attracted to their image charge in the n-type Si substrate.

IV. Movement of Na and Cl Atoms in Ion Bombarded SiOgiSi

In order to study the e f f ec t s of ion bombardment on impurity motion in both annealed and unannealed Na+-and C£+-implanted samples, three different ions were chosen so as t. span a fu l l range of ionization deposi­t ion and atomic displacement density.-^ Figure 5 shows various profi les for the three ions H*1", Ne, and A+. H* ions were chosen because, as shown in the top of Fig. 5> energy into atomic displacement or damage processes within the oxide was small compared with the energy

into ionization or electronic processes; a l so , low fluences of 275 H+ ions were used for the PIX and RBS analys is . Displacement processes or damage were intro­duced by Ne+ and A+ bombardments. The lower two por­tions of F ig . 5 show the ionization and damage prof i l es for Ne+ and A+. The dotted regions correspond to the f ina l distributions of the bombarding ions Hf*-, Ne+, and A+. The vert ical dotted l i n e represents the Si02:Si interface for the 5000 1 the annealing studies.

thick oxides used in

Figure 5· Energy deposition prof i l e s for three bom­barding ions , H"1", Ne+, and A+. These curves were generated from a program developed by D. K. Brice of Sandia Laboratories.

6<*

UJ

s ce

§ 1.5

1.0 h^r-fr 0.5h

0h

η « i | i i j i r PIX ANALYSIS OF

H+AND Ne+INDUCED Na MOVEMENT IN SiO^Si

Ί - --i-H-

E(H+) - 275keV-^-H^SI02:Si

E (Ne+) -100 keV-J-Ne"^Si02:Si

-l I I I L J I I I I L

Ko

T—r—r"*—m—■—i—i—r-PIX ANALYSIS OF

H+AND Ne+-INDUCED hi .5 CI MOVEMENT IN Au:Si02:SI

10 »12 10 ,13 101 10 ,15 10161014

-u\ \\\\7 ■ \ 24 HR. WAIT

K. M--Î ■ - £ - H^AurSiO -Si

f -0 - Ϊ - Ne^Au:Si02:Si .

E(H+) · 250 keV

E(Ne+) - 220 keV

l i l i L_L 10J ,15 10"

FLUENCE <H+ or Ne+/cm2) 1011 10 ,18

Figure 6 . PIX y ie lds of H+ and Ne+ bombarded unannealed Si02(Na):Si and Au:Si02:Si, respect ive ly .

A. Unannealed-Implanted Si0 2 :Si

Results of the H* and Ne+ bombardments of unan­nealed Na+-implanted lOOO Â thick SiC^rSi are shown on the l e f t s ide of Fig . 6. No changes in the Na d i s t r i ­bution were observed for the proton bombardments over the fluence range 2 X 101 2 to 101 6 H+/cm2. However, Ne+ bombardment a t 100 keV caused a reduct ion in the PIX Na(K) y i e l d . This reduction i s cons i s ten t with the Na atoms having moved— 500 Â in to the S i0 2 :S i interface reg ion . Heavy ion 200 keV A+ bombardment caused s imilar Na motion, but by a fluence of 2 X 10^3 A+/cm^ ΐ η β motion ceased indicat ing t h a t the Na atoms were trapped before ful ly reaching the in ter face region.

These r e s u l t s suggest f i r s t of a l l t ha t Na atoms are trapped in S i0 2 because of implant damage i n t r o ­duced during the shallow Na+-implantât ion . lon iza t ion processes in the oxide and posi t ive charging of the oxide surface together (prctcn bombardment) w i l l not induce Na motion in unannealed implanted S i 0 2 . Second­ly, heavy ion bombardment w i l l cause movement of im­planted Na within the unannealed oxide even though the Na movement i s not quite the same for Ne+ and A+ bom­bardments. I t i s suggested tha t the Na movement in unannealed S i0 2 :S i i s caused by displacement damage from the heavy ion bombardments. Poss ib ly the addi­t iona l bombardment damage de traps Na atoms and they diffuse as Na+ towards the S i0 2 :S i in te r face being trapped and detrapped severa l times on t h e i r way.

The r i g h t side of Fig. 6 shows the r e s u l t s of PIX analysis of contaminant CM atoms in zero biased Au:Si02:Si. As with Na, CM motion occurred a f t e r Ne+

bombardment but not a f te r H4" bombardment. However, un­l ike the Na, the o r ig ina l Cl d i s t r i bu t ion reappeared af ter a 2k-hr wai t . Presumably, CM atoms were able to diffuse back in the Si02 region e i t h e r from the i n t e r ­face or from nearby l a t e r a l regions which were not damaged by Ne+-bombardment.

B. Annealed-Implanted S i0 o :S i

The ion energies shown in Fig. 5 were used t o in­duce impurity motion in annealed 5000 A S i 0 2 : S i . As mentioned in Section I I I . C , annealing a t 700° C caused the implanted Na atoms to move toward the S i0 2 :S i

in t e r face . Further ion bombardments of H* and Ne+ to lO1^ ions/cm2 did not great ly change the Na d i s t r i bu ­t i o n . Therefore, the r e s t of t h i s sec t ion i s devoted to the study of C£ motion in ion bombarded, annealed S i0 2 :S i . Also, as mentioned i n Section I I I . C , anneal­ing the CM+-implanted Si02:Si presumably l e f t the sam­ple r e l a t i v e l y damage f ree . The CM atoms were mainly s t i l l a t the implanted depth of ~ 1000 Â but smeared out in a d i s t r i b u t i o n wider than t h a t before annealing.

Results of these bombardments are shown in Fig. 7· The number of PIX detected Ql atoms i s p lo t ted versus H+, Ne+, or A+ f luence. The reference standard was taken to be the unannealed PIX yie ld , which was con­s i s t en t with the expected implanted d i s t r i bu t i on of 120 keV CM+ in SiOa· The top port ion of Fig. 7 gives the r e s u l t s for H4"-bombardments. No C-t movement was detected u n t i l ~ 1 0 1 6 H4*/™2. Between 101 6 and lO1? H"Vcm2 the PIX Cl y i e ld increased, ind ica t ing Cl mo­t ion toward the sur face . Movement toward the surface i s consistent with negatively charged CM" ions in the oxide moving toward the posi t ively charged bombarded surface.

Ne+ and A+ bombardments cons i s t en t ly produced Cl movement toward the Si02 :Si interface -as shown in the lower two port ions of Fig. 7· I t i s d i f f i c u l t to ex­p la in CM motion toward the interface and away from a pos i t ive ly charged oxide surface because ionized CM in the oxide should be negative. Since motion toward the surface was observed for H4* bombardment, where very l i t t l e displacement damage was produced, motion to the interface during Ne+ and A+ bombardment must be r e l a t ed to the damage introduced during bombardment. One pos s ib i l i t y i s t h a t CM atoms combine with defects to form a pos i t i ve ly charged Ct-defect c l u s t e r . This c lus ter then diffuses toward the i n t e r f ace . Another p o s s i b i l i t y i s t h a t CM atoms are a t t r a c t e d to the r e ­gions of highest damage, through a process of enhanced diffusion, and migrate toward the i n t e r f a c e . Better understanding of t he CM motion "requires further study and possibly post-bombardment p r o f i l i n g with spu t te r ­ing or layer removal to obtain a more accurate f inal C£ d i s t r i b u t i o n .

65

1.5h Si02(CI):Si PIX ANALYSIS OF H+. Net

AND A r + - INDUCEO Cl MOVEMENT

S 1.0 LU

0.5

S o en LU oc o. O

OJr

1.0

0.5

>-2

t X g 1.0

0.5h

X PRE-ANNEAL

700 C ANNEAL

275 keV H +

700% ANNEAL

275 keV Ne+

700% ANNEAL

275 keV A r +

10 ,14 10 15 101 il6 J_ 10J ,17

FLUENCE (No. /cm2)

Figure nealed

T. PIX y i e l d s of H*, N e + , and A+ bombarded an-S i 0 2 ( C £ ) : S i .

V. Summary

The e f f e c t s of l i g h t and heavy ion bombardment on Na and Cl motion i n S i 0 2 : S i t h i n f i l m s have been s tud ied us ing proton-induced c h a r a c t e r i s t i c x-ray (PIX) and Rutherford backsca t t er ing (RBS) t echn iques . D i s t r i b u ­t i o n s o f implanted Na and Cl atoms in unannealed oxides were found t o be s t a b l e dur ing PIX and RBS a n a l y s i s and high f luence proton bombardments. However, annealing a t 700°C for 20 min produced Na movement toward the SiOptSi i n t e r f a c e and a spread ing of the implanted Cl d i s t r i b u t i o n .

Subsequent h igh f luence proton bombardment caused measurable movement o f Cl toward the Si02 s u r f a c e . In contras t , heavy ion Ne+ and A+ bombardments produced movement of Na and 06 atoms i n both unannealed and annealed samples toward the S i 0 2 : S i i n t e r f a c e .

V I . Conc lus ions

The fo l lowing c o n c l u s i o n s are c o n s i s t e n t w i t h the reproducible data o f these exper iments . However, addi­t i o n a l post-bombardment s p u t t e r i n g or l a y e r removal p r o f i l i n g experiments should be performed t o further v e r i f y the f i n a l Na and Ct d i s t r i b u t i o n s and support the fo l lowing s ta t ement s :

1 . Both Na and Cl atoms may be trapped i n S i 0 2 by implantation damage.

2 . When Na and Cl atoms are trapped i n S i 0 2 by implantation damage, i o n i z a t i o n i r r a d i a t i o n (proton bombardment) w i l l not produce Na or Cl r e l e a s e or motion.

3 · Na and Cl r e l e a s e or motion may occur during annea l ing t o 700° C or by f u r t h e r heavy i o n bombardments.

4 . Na motion during anneal ing or under p o s i t i v e ion bombardment i s pr imar i ly t o the S i0 2 . 'S i i n t e r f a c e . During annea l ing the Na motion i s c o n s i s t e n t w i t h p o s i ­t i v e l y charged Na+ moving t o the i n t e r f a c e under image charge a t t r a c t i o n from the n-type Si s u b s t r a t e . Dur­ing bombardment t h e motion i s c o n s i s t e n t with p o s i t i v e ­l y charged Na+ atoms moving away from a p o s i t i v e l y charged oxide surface and toward the s u b s t r a t e image charge.

5- I n annealed OC/^-implanted samples, Ot motion i s toward the S i 0 2 surface during i o n i z i n g proton bombardment. This motion i s c o n s i s t e n t with n e g a t i v e ­l y i o n i z e d Cl" atoms moving toward the p o s i t i v e l y charged surface during proton bombardment.

6 . Motion of .C£ i n annealed S i 0 2 during Ne + and A+ bombardments i s toward t h e S i 0 2 : S i i n t e r f a c e . I t i s s u g g e s t e d t h a t damage i n the oxide i s r e s p o n s i b l e for c a u s i n g motion o f Cl atoms toward t h e i n t e r f a c e through a proces s o f enhanced d i f f u s i o n or by c r e a t i n g p o s i t i v e l y charged mobile C l - d e f e c t complexes .

Acknowledgments

The author thanks D. Weaver and R. Burghard o f Sandia Laborator ies for provid ing the SiCU samples , N. Wing f o r a s s i s t i n g wi th the experiments , and H. J . S t e i n of Sandia Labs for h e l p f u l comments on the paper .

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2 1 , 592 ( 1 9 7 2 ) .

15. E . P . EerNisse and C B. Norr i s , J . Appl. Phys . ( t o be p u b l i s h e d ) .

16. D. K. B r i c e , Sandia Laborator ies . For the method used s e e , R a d i â t . Ef f . 6 , 77 (1970) and Ion Implantat ion i n Semiconductors and Other M a t e r i a l s , e d . B . L . Crowder (Plenum P r e s s , N. Y . , 1 9 7 3 ) , p . 1 7 1 .

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