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STRUCTURAL AND ELECTRICAL EFFECTS OF
DOPANT SEGREGATION TO SILICON GRAIN
BOUNDARIES
C. Grovenor, D. Smith, C. Wong
To cite this version:
C. Grovenor, D. Smith, C. Wong. STRUCTURAL AND ELECTRICAL EFFECTS OF DOPANT
SEGREGATION TO SILICON GRAIN BOUNDARIES. Journal de Physique Colloques, 1985, 46
(C4), pp.C4-411-C4-416. �10.1051/jphyscol:1985444�. �jpa-00224695�
JOURNAL DE PHYSIQUE
Colloque C*, supplement au n ° ^ , Tome 4-6, avril 1985 page CMtll
STRUCTURAL AND ELECTRICAL EFFECTS OF DOPANT SEGREGATION TO SILICON GRAIN BOUNDARIES
C.R.M. Grovenor, D.A. Smith and C.Y. Wong
Dept. of Metallurgy, Oxford University, Oxford 0X1 ZRQ, U.K.
+IBM T.J. Watson Research Centre, Yorktown Heights, NY 10598, U.S.A.
Résumé - L'analyse STEM à haute résolution de ségrégation aux joints de grains d'arsenic dans le silicium polycristallin a démontré qu'il y a forte ségrégation après chauffage entre 700 et 1000°C. La variation de la ségrégation avec la tempé-rature permet le calcul de l'énergie moyenne de ségrégation d'atomes As aux joints de grains du silicium (0,65eV/atome) et la concentration de saturation moyenne (12at%). Nous considérons deux espèces possibles de sites de ségrégation en insistant sur les effets électriques de la ségrégation à ces sites.
Abstract - High resolution STEM analysis of grain boundary segregation of arse-nic in CVD grown polysilicon has shown that there is substantial equilibrium segregation after annealing at temperatures between 700 and 1000 C. The varia-tion of extent of segregavaria-tion with temperature allows calculavaria-tion of the avera-ge binding energy of arsenic atoms to silicon grain boundaries (0.65eV/atom) and the average saturation concentration in the boundaries (12at%). 2 possible segregation sites are discussed with emphasis on the electrical effects of se-gregation to them.
1. INTRODUCTION.
The segregation of impurity and dopant species to grain boundaries in semiconductors is a phenomenon of some importance since it has been established that the electrical character of the grain boundaries can be influenced by such segregation /l/. Polycrystalline semiconductors have important applications as both fine grained interconnection paths and as large grained solar cell materials , and an understanding of the mechanism and extent by which segregation may change the electrical character of a bulk polycrystalline material would be valuable. Chemical modification of boundary properties is already practiced on polysilicon solar cells by the well known 'passivation treatment' where grain boundaries are exposed to atomic hydrogen /2,12/. However the effect of segregation of dopant species to grain boundaries has been relatively less fully investigated. Indirect measurements of the extent of segregation of arsenic to silicon grain boundaries /3,4/ requires the assumption that the segregation itself causes no change in the boundary electrical character. In the light of the results of hydrogenation /2/ and A1,0 and Ti segregation /l/ on the electrical properties of semiconductor grain boundaries this assumption is however hard to justify. Direct Scanning Transmission Electron Microscopy (STEM) analysis of the segregation of dopant species to grain boundaries in silicon has been sucessful in the calculation of the extent and thermodynamics of the segregation process /5,6/. This paper will briefly present the results of analysis of arsenic segregation to polysilicon grain boundaries /6/, and will then discuss the possible segregation sites in boundaries in covalently bonded materials , and the influence of segregation to these sites on the electrical properties of the boundaries.
C4-412 JOURNAL DE PHYSIQUE
2. EWERIMENTAL RESULTS.
These results are presented fully elsewhere 161 and so will be only briefly explained here. STEM analysis of arsenic profiles across grain boundaries in CVD grown polysilicon has shown substantial arsenic enhancement peaks at the boundaries after heat treatments designed to allow equilibrium segregation to occur. A typical arsenic profile across a grain boundary is shown in Figure 1. The sample from which this boundary was chosen was annealed at 8 0 0 ~ ~ for 60 hours. Samples were annealed at 700,800,900 and 1 0 0 0 ~ ~
,
and 10 randomly chosen boundaries analysed from each sample..1
BOUNDARY2 0 0 -300 -200 -100 0 100 200 300 400
DISTANCE ! ANG
Fig.1. A typical arsenic profile across a single grain boundary in heavily doped polysilicon.
The number of arsenic counts due to segregation at each of these boundaries is easily obtained from the STEM data and are shown in Figure 2. It can be seen that at each annealing temperature there is a wide range in the extent of segregation
,
and that there is also a trend of increasing segregation as the annealing temperature is reduced. Reannealing at 9 0 0 ~ ~ results in analysis of reduced segregation again as expected for an equilibrium segregation phenomenon. (Also plotted in Figure 2 are the bulk resistivity values for the polysilicon samples after the various annealing treatments.) By plotting the natural logarithm of grain boundary arsenic enhancements against 1/T four data points are obtained that lie on a curve as shown in Figure 3. The McLean Isotherm for segregation to an interface with a saturation segregation level XS can be fitted through these data points by fixing the two independent variables XS and QS the energy of segregation of an arsenic atom to a silicon grain boundary. The best fit is obtained with XS = 12 atomic percent,
and QS = 0.65eVlatom. We expect that both the density and the character of the segregation sites in grain boundaries will vary with the boundary structure,
and similarly that there are a number of different sites in any boundary to which segregation couldoccur
,
and to which the binding energy would be different. These calculated parameters are therefore averages characteristic of those of the 40 randomly chosen boundaries in this experiment,
and as such cannot be used to give any information on segregation of arsenic to particular sites in a boundary of particular structure.ANNEALING TEMPERATURE
Fig.2. The segregated As counts from 43 grain boundaries analysed after segregation anneals at temperatures between 700 and 1 0 0 0 ~ ~ . The bulk resistivities of the same silicon samples are also included.
3. SEGREGATION SITES IN COVALENTLY BONDED GRAIN BOUNDARIES.
High resolution electron microscopy has been used extensively to investigate the atomic structure of grain boundaries in silicon and germanium. Some of the results of these experiments can be summarized as follows ;
(1). Some highly symmetric boundaries (X=3,5,9,11 and 25) have boundary structures where the atoms are tetrahedrally coordinated
,
there are no dangling bonds /7-11/.(2). Intrinsic dislocation cores in boundaries close to C=3 may contain dangling bonds if of Frank or stair-rod type /7/.
(3). Oxygen segregation is often observed to these high symmetry boundaries even though these are no dangling bonds /7/.
Electron spin resonance (ESR) experiments on CVD polysilicon have shown strong evidence for the presence of 'trivalent silicon1 or dangling bonds /12/. The density of these defects in the grain boundaries has been estimated to be 1 0 ~ ~ c m - ~ . The fact that the HKEM experiments concentrate of necessity on boundaries of very high symmetry while the ESR measurements sample all grain boundaries in a bulk polysilicon sample sufficiently explains the discrepancy between the two sets of results. From these observations it seems possible to conclude that some high symmetry grain boundaries in covalently bonded materials have no dangling bond sites
,
but that more general interfaces,
or those containing sets of intrinsic or extrinsic dislocations,
must have some dangling bond sites to explain theJOURNAL DE PHYSIQUE
ESR r e s u l t s . H o r n s t r a 1131 has proposed models f o r g r a i n boundary s t r u c t u r e s i n c o v a l e n t m a t e r i a l s some of which a r e s i m i l a r t o t h o s e found i n HREM experiments (C=9 i n p a r t i c u l a r )
,
and i n some boundaries of lower symmetry h a s p r e d i c t e d s t r u c t u r e s t h a t c o n t a i n d a n g l i n g bonds a t t h e c o r e s of g e o m e t r i c a l l y n e c e s s a r y g r a i n boundary d i s l o c a t i o n s . The d e n s i t y of t h e s e d a n g l i n g bonds is approximately 1 0 l ~ c m - ~,
o r one every 5-10 boundary atoms assuming t h a t t h e boundary i s 0.5nm wide. It must a l s o be remembered t h a t oxygen s e g r e g a t e s t o boundaries t h a t have been shown t o have no d a n g l i n g bonds,
s o t h e l a c k of t r i v a l e n t l y bonded s i t e s is n o t a s u f f i c i e n t c r i t e r i o n f o r t h e r e t o be no s e g r e g a t i o n . The STEM experiments however c l e a r l y demonstrated t h a t coherent twin boundaries were never o b s e r v e d t o c o n t a i n any measurable l e v e l of s e g r e g a t e d a r s e n i c 161.SATURATION
LEVEL
Fig.3. The p l o t of t h e McLean I s o t h e r m through t h e 4 d a t a p o i n t s o b t a i n e d from t h e a v e r a g i n g of t h e A s enhancements a t 10 b o u n d a r i e s a t each of t h e 4 a n n e a l i n g t e m p e r a t u r e s . XS = 12 atomic p e r c e n t
,
QS = 0.65eVlatom.At t h i s p o i n t i t i s p o s s i b l e t o propose t h a t t h e r e a r e two kinds of s i t e s t o which a r s e n i c atoms may s e g r e g a t e i n s i l i c o n g r a i n boundaries. P o s i t i o n s i n t h e g r a i n boundary a d j a c e n t t o d a n g l i n g bonds w i l l a l l o w p e n t a v a l e n t c o o r d i n a t i o n f o r t h e a r s e n i c atoms and remove t h e d a n g l i n g bond. T h i s a r s e n i c atom w i l l t h e n become d i f f i c u l t t o i o n i z e
,
and s o w i l l l o s e t h e dopant c h a r a c t e r i t d i s p l a y s i n t h e g r a i n i n t e r i o r s . F i g u r e 4 i s a diagram ( a f t e r H o r n s t r a ) of a symmetric t i l t g r a i n boundary c o n t a i n i n g d a n g l i n g bonds. The c o r r e s p o n d i n g sites f o r p e n t a v a l e n t a r s e n i c c o o r d i n a t i o n a r e arrowed. The s t r u c t u r e s of some h i g h symmetry boundaries a n a l y s e d by HREM( C = l l i n p a r t i c u l a r I l l / ) do n o t a g r e e w i t h t h o s e proposed by H o r n s t r a , however F i g u r e 4 is conveniently i l l u s t r a t e s t h a t a d j a c e n t t o a dangling bond t h e r e w i l l be a p o s s i b l y f a v o u r a b l e a r s e n i c s e g r e g a t i o n s i t e .
A r s e n i c atoms w i l l a l s o be expected t o s e g r e g a t e t o some t e t r a h e d r a l l y c o o r d i n a t e d s i t e s a t s i l i c o n g r a i n boundaries. The 5 and 7 membered r i n g s observed i n C=9 / 7 , 8 / and C = l l b o u n d a r i e s 1111 a r e t h e most s t r i k i n g s t r u c t u r a l f e a t u r e s of t h e s e boundaries. At t h e s e s t r u c t u r a l u n i t s t h e r e i s a c o n s i d e r a b l e amount of bond d i s t o r t i o n 171 which may i n c r e a s e t h e i r
a t t r a c t i v e n e s s a s s e g r e g a t i o n s i t e s . The observed s e g r e g a t i o n of oxygen t o boundaries c o n t a i n i n g t h e s e s t r u c t u r a l u n i t s s u p p o r t s t h i s view. A r s e n i c atoms s e g r e g a t e d t o such s i t e s would r e t a i n t h e i r a b i l i t y t o be i o n i z e d .
Fig. 4. A diagram ( a f t e r H o r n s t r a ) of t h e proposed s t r u c t u r e of a symmetric t i l t boundary i n a c o v a l e n t l y bonded m a t e r i a l . 4 p o s s i b l e a r s e n i c s e g r e g a t i o n s i t e s a d j a c e n t t o d a n g l i n g bonds a r e arrowed.
4. ELECTRICAL IBFLUENCE OF SEGREGATION.
S e g r e g a t i o n of a r s e n i c atoms t o t h e two k i n d s of g r a i n boundary s i t e s w i l l have very d i f f e r e n t e f f e c t s on t h e e l e c t r i c a l c h a r a c t e r of t h e boundaries. I n r a t h e r l i g h t l y doped n-type s i l i c o n g r a i n boundaries c r e a t e p o t e n t i a l b a r r i e r s t o c a r r i e r t r a n s p o r t due t o i n t e r f a c i a l t r a p s t a t e s t h a t p i n t h e Fermi l e v e l i n t h e middle of t h e band gap 1141. These i n t e r f a c i a l s t a t e s a r e a s s o c i a t e d w i t h p e r t u r b a t i o n s of t h e s i l i c o n l a t t i c e a t t h e boundary
,
and can t h e r e f o r e be caused by d a n g l i n g bonds,
t h e 5 and 7 membered s t r u c t u r a l u n i t s,
o r any o t h e r boundary d e f e c t . A r s e n i c s e g r e g a t i o n a d j a c e n t t o d a n g l i n g bond s i t e s w i l l remove any i n t e r f a c i a l s t a t e a s s o c i a t e d w i t h t h e s e d e f e c t s ( a l t h o u g h i t may c r e a t e a new d e f e c t s t a t e t h a t lies e l s e w h e r e i n t h e band gap). It might be e x p e c t e d t h e r e f o r e t h a t s e g r e g a t i o n t o d a n g l i n g bonds would lower t h e p o t e n t i a l b a r r i e r a t t h e boundary. S e g r e g a t i o n of a r s e n i c atoms t o t e t r a h e d r a l l y c o o r d i n a t e d s i t e s a t g r a i n boundaries means t h a t a very high d e n s i t y of i o n i z a b l e a r s e n i c atoms w i l l be o b t a i n e d . (The s o l i d s o l u b i l i t y of a r s e n i c i n s i n g l e c r y s t a l s i l i c o n i s l e s s than 1 / 2 a t % ) . The measured a r s e n i c c o n c e n t r a t i o n i n t h e boundaries i n t h i s work was g r e a t e r t h a n 5 a t % . Thus a v e r y high p o s i t i v e charge d e n s i t y can be c r e a t e d a t t h e i n t e r f a c e,
and an e x t e n s i v e space c h a r g e r e g i o n around t h e boundary. T h i s s p a c e charge r e g i o n w i l l l i m i t c a r r i e r t r a n s p o r t a c r o s s t h e b o u n d a r y a n d s o i n c r e a s e t h e b u l k r e s i s t i v i t y . I n h e a v i l y doped p o l y s i l i c o n t h i s e f f e c t w i l l be t h e most s i g n i f i c a n t one on t h e e l e c t r i c a l c h a r a c t e r of t h e g r a i n boundaries s i n c e t h e p o t e n t i a l b a r r i e r h e i g h t of t h e boundaries w i l l be n e g l i g i b l y s m a l l /14/. The bulk r e s i s t i v i t y changes i n t h e p o l y s i l i c o n i n t h i s work a r e p l o t t e d a g a i n s t t h e average c o n c e n t r a t i o n of s e g r e g a t e d a r s e n i c i n F i g u r e 5C4-4 16 JOURNAL DE PHYSIQUE
for each annealing temperature. This shows clearly that the bulk resistivity increases with segregation as predicted by the charge sheet model of segregation of ionizable arsenic atoms to the grain boundaries. This does not mean that there is no segregation to dangling bond sites
,
only that any electrical effect of such segregation will be swamped by the strong effect of charge collection in these heavily doped specimens. In more lightly doped material segregation to dangling bonds (or similar carrier pinning sites) may lower the bulk resistivity by reducing the potential barriers associated with the boundaries.ATOMIC % SEGREGATED As
Fig.5. A plot of bulk resistivity against average arsenic enhancement levels showing the increase in resistivity with segregation.
CONCLUSIONS.
STEM analysis of arsenic segregation to silicon grain boundaries has measured the extent of segregation and calculated an average binding energy of the arsenic to the boundaries. Two kinds of sites have been proposed to be available for arsenic atoms in the boundaries, adjacent to dangling bonds
,
and to tetrahedrally coordinated sites in grain boundary structural units. The effect of segregation on the electrical properties of the boundaries has been predicted for the two different kinds of sites from simple models of the interaction of carriers with the boundaries.REFERENCES.
/I/. L.L.Kazmerski and P.E.Russel1. J.de Physique.c,Cl-171,1982. /2/. C.H.Seager,D.S.Ginley and J.D.Zook, Appl.Phys.Lett.~,831,1980. /3/. M.M.Mandurah,K.L.Saraswat,C.R.Helms,T.I.Kamins,J.Ap.Phys,51,5755,1981. /4/. A.Carebelas,D.Nobili and S.Solmi, J.de Phys. 43,Cl-187,1982.
/5/. J.M.Rose and R.Gronsky, Appl.Phys.Lett. 2,99r1983.
/6/. C.R.M.Grovenor,P.E.Batson,D.A.Smith and C.Y.Wong, to be publ.Phil.Mag.
/7/. A.Bourret,C.d'Anterroches and J.M.Penisson, J de Phys.=,C6-83,1982. /8/. O.K.Krivanek,S.Isoda and K.Kobayashi, Phil.Mag. 2,331,1977. /9/. J-J.Bacmann. J.de Physique. =,C6-93,1982'.
/lo/. A-Bourret and C.d'Anterroches. J.de Physique. 43,Cl-1,1982.
/11/. A.M.Papon,M.Petit,G.Silvestre and J-J.Bacmann. in Grain Boundaries in Semiconductors.(eds Leamy,Pike and Seager)North Holland 1982 p.27. /12/. N.M.Johnson,B.K.Biegelsen and M.D.Moyer, Appl.Phys.Lett. 60,882,1982. /13/. J. Hornstra, Physica 5,409,1959.
