ON THE ELECTRICAL ACTIVITY OF THE ((-1)11)-(1(-1)2) STEPS IN THE SILICON Σ = 3 TWIN BOUNDARY

HAL Id: jpa-00230360

https://hal.archives-ouvertes.fr/jpa-00230360

Submitted on 1 Jan 1990

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ON THE ELECTRICAL ACTIVITY OF THE ((-1)11)-(1(-1)2) STEPS IN THE SILICON Σ = 3

TWIN BOUNDARY

Jean-Luc Maurice

To cite this version:

Jean-Luc Maurice. ON THE ELECTRICAL ACTIVITY OF THE ((-1)11)-(1(-1)2) STEPS IN THE

SILICON Σ = 3 TWIN BOUNDARY. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-581-C1-

586. �10.1051/jphyscol:1990191�. �jpa-00230360�

COLLOQUE DE PHYSIQUE

Colloque Cl, suppl6ment au nol, Tome 5 1 , janvier 1990

ON THE ELECTRICAL ACTIVITY OF THE (ill)-(iiz) STEPS IN THE SILICON

z

= 3 TWIN BOUNDARY

J.-L. MAURICE

Laboratoire de Physique des Materiaux, CNRS, 1 Place A. Briand, F-92195 Meudon, France

Resume - Des mesures de courant i n d u i t p a r faisceau d161ectrons (EBIC) dans un microscope e l e c t r o n i q u e i balayage, o n t 6tB r e a l i s e e s s u r des marches (ill)-(112) de l a macle Z = 3 du s i l i c i u m . Les deux plans d ' i n t e r f a c e a i n s i que l e s deux types de d i s l o c a t i o n s qui l e s separent ne presentent frequemment aucune a c t i v i t e e l e c t r i q u e d e t e c t a b l e . Une t e l l e a c t i v i t 6 a p p a r a i t cependant quand l a d e n s i t 6 de marches e s t elevee. La microscopie e l e c t r o n i q u e en transmission (TEM) montre a l o r s que des nano-prCcipites decorent une grande p r o p o r t i o n des d i s l o c a t i o n s formant l e s aretes des marches.

A b s t r a c t - E l e c t r o n beam induced c u r r e n t (EBIC) measurements i n t h e scanning e l e c t r o n microscope have been performed on a 2 = 3 t w i n boundary w i t h (ill) - (li2) steps i n s i l i c o n . The two i n t e r f a c e planes, t o g e t h e r w i t h t h e two d i f f e r e n t d i s l o c a t i o n s a t t h e i r i n t e r s e c t i o n s , f r e q u e n t l y show no d e t e c t a b l e e l e c t r i c a l a c t i v i t y . Such an a c t i v i t y however appears when t h e step d e n s i t y i s high. I n t h i s case, transmission e l e c t r o n microscopy (TEM) shows t h a t a l a r g e p r o p o r t i o n o f t h e step d i s l o c a t i o n s i s decorated by n a n o - p r e c i p i t a t e s .

1

-

INTRODUCTION

Grain boundaries (GBs) i n s i l i c o n u s u a l l y e x h i b i t an e l e c t r i c a l a c t i v i t y , t h e o r i g i n o f which may be i n t r i n s i c (due t o t h e s t r u c t u r a l mismatch) o r e x t r i n s i c (due t o t h e i m p u r i t i e s ) /l/. Among t h e p o s s i b l e e l e c t r i c a l l y a c t i v e s t r u c t u r e s , t h e s i l i c o n d a n g l i n g bond has been shown t o p l a y a n o t i c e a b l e r o l e /2/. However, many o t h e r bond c o n f i g u r a t i o n s ( w i t h angle and l e n g t h d i s t o r s i o n s ) a r e a l s o present i n t h e GBs and t h e question a r i s e s wether they a l s o induce an e l e c t r i c a l a c t i v i t y ( i . e . deep l e v e l s ) . A l a r g e amount o f work has been performed i n t h i s r e s p e c t on t h e stepped 2 = 3 t w i n boundary, because t h i s GB has t h e p e c u l i a r i t y o f c a r r y i n g h i g h l y d i s t o r t e d bonds w i t h i n t h e frame o f t h e most simple coincidence r e l a t i o n s h i p /3-9/. The (ill) - (li2) step i s c h a r a c t e r i z e d by t h e presence o f t h e "incoherent"

kiz)

t w i n plane and by t h e f a c t t h a t t h e r e i s a r i g i d body t r a n s l a t i o n (RBT) associated t o t i s f a c e t w h i l e t h e r e i s none f o r t h e Ill

,

which induces t h e presence o f d i s l o c a t i o n s a t t h e step edges. The atomic s t r u c t u r e o

C

t h e s t e p has been described by B o u r r e t and Bacmann /5/ and can be summarized as f o l l o w s : (i) dangling bonds are absent, ( i i ) t h e (li2) f a c e t and t h e d i s l o c a t i o n s i n c l u d e r e c o n s t r u c t i o n s along t h e <110> t i l t axis, and ( i i i ) t h e d i s l o c a t i o n s have two d i f f e r e n t core c o n f i g u r a t i o n s . The e l e c t r o n i c l e v e l ^S associated t o t h e (li2) boundary /6-8/ and t o t h e whole step ( i n c l u d i n g the d i s l o c a t i o n s ) /g/, have been t h e o b j e c t s o f computer s i m u l a t i o n s which i n d i c a t e d t h a t t h e y should n o t f a l l i n t o t h e band gap. The aim o f t h i s paper i s t o present t h e e l e c t r i c a l a c t i v i t y observed on t h i s boundary i n a r e a l m a t e r i a l , by means o f t h e e l e c t r o n beam induced c u r r e n t (EBIC) mode o f t h e scanning e l e c t r o n microscope (SEM) /10/. We have mentioned, i n a previous paper /11/, EBIC r e s u l t s obtained on a stepped Z = 3, where indeed some o f t h e steps showed no e l e c t r i c a l a c t i v i t y . The example presented i n /11/ i s f u r t h e r developped here, and i t s imp1 i c a t i o n s a r e d e t a i l e d . Moreover, t h e effects o f an e l e c t r i c a l l y a c t i v a t i n g anneal on t h i s GB are a l s o discussed.

2

-

EXPERIMENTAL DETAILS 2.1. Experimental procedure

We observed stepped 2 = 3 boundaries i n two d i r e c t i o n n a l l y s o l i d i f i e d l a r g e grained p o l y c r y s t a l s , w i t h e i t h e r boron o r aluminium doping. However, we present h e r e a f t e r only one r e p r e s e n t a t i v e case, where EBIC measurements and transmission e l e c t r o n microscopy (TEM)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990191

COLLOQUE DE PHYSIQUE

could be carried out on the same regions. This twin boundary comes from the Al-doped material (N,,

= 2 X

1017cm-3) /12,13/. The activating anneal is made at 900°C for 2 h in vacuum in a sealed ampoule, it is followed by a fast cooling

(-

5°C S-l); this procedure has been shown to have the effect of increasing the GB electrical activity by creating metallic precipitates in the GB-plane, the metals (Cu and Ni) coming mainly from contamination during sample preparation /11,14/. This treatment is then applied to the stepped

Z =

3 to check the behaviour of the steps regarding the precipitation phenomenon. In this respect, two neighbouring samples with exactly the same GB patterns - thanks to the columnar structure of the ingot

-

have been studied together, one in the as-grown state and the other in the annealed state. A Schottky contact has been deposited by aluminium evaporation on both of them in the same run and the EBIC experiments have been performed with exactly the same beam and imaging conditions. The ohmic contacts are obtained with deposition on the back surface of an A1 -Ga alloy after scrapping; all preparations are carried out at room temperature.

2.2

Electron beam induced current

Electron-hole pairs are created by the incident electron beam of the SEM and they are separated in the electric field of the Schottky junction to form the electron beam induced current (EBIC), which is used, in turn, to modulate the image intensity of the CRT display of the SEM. Recombining defects then appear on the image with varying grey levels, on a white background, depending on their recombi nati on strength. The EBIC contrast can be interpreted provided it is obtained in low injection conditions (excess carrier density small compared to the

equilibrium

carrier density) /10/. The minimal detectable density of intrinsic states can be evaluated supposing that the minority carrier capture cross section of these states is similar to the one currently assigned to the dangling bonds (10-l6 cm2 l ) . In the case of a grain boundary, using the computed numerical relationships between the density of states and the recombination velocity given by Dugas /15/, it can be estimated to be about 10"

;

while in the case of a dislocation, Kittler and Seiffert have found it to be - ²

^X

108cm-I /16/. The EBIC technique is then particularly efficient to detect electrical activity in the GB case, since the average atomic density in a silicon plane (1015cm-2) and the density of states associated to dangling bonds in

/2/

(1012cm'2) are higher than the detection limit; however it is less interesting in the dislocation case, because the mean atomic density along a line in silicon

(-

3

X

107cm'l) is lower than the detection limit. In this last situation, only deep levels generated by segregated or precipitated impurities can be detected, since they exhibit higher density and capture cross sections /16/. This fact will be of prime importance in the discussion.

2.3 Transmission electron microscopy

The samples are mechanically thinned by the back surface to

11

30

W,

and then ion milled on both sides, until the EBIC-studied zones are transparent to the electrons. The microscope used (JEOL 2000 FX) has a point to point resolution of 0.28 nm, which allows to obtain lattice images along the <110> GB tilt axis,

3

-

^RESULTS

We use the same notations as Bourret and Bacmann /5/ to refer to the two kinds of step dislocations. Following this, the dislocation situated at point

P

in fig.8 of ref.5, is called

"P"

here and is situated at the left edges of the (li2) boundary planes in all the figures presented in this text (arrow

"l"

in fig.l), and the one situated at point Q in ref.5 is called "Q" here and lies on the right hand side of the (li2) facets in the figures of this paper (arrow

" t "

in fig.1).

3.1 Electron beam induced current

Figure 1 shows different images of the stepped P

=

3, recorded at a primary beam voltage of

20 kV. The strong contrast on the secondary electron image (Fig.1.a) is due to the chemical

etching prior to the Schottky contact deposition, it can be verified that it induces no EBIC

dark contrast. The two EBIC images (as-grown, fig.lb; annealed, fig.1.c) show that the

anneal systematically darkens the contrast in zones that where already active, while it very

seldom gives contrast to zones previously inactive; when a dark spot is present in only one

of the two images (1.b or l.c), it indicates more likely a difference between the two sample

microstructures than one between the two states of treatment.

Zone (A, B) i n f i g . 1 shows EBIC f e a t u r e s t y p i c a l o f most o f t h e steps: no EBIC c o n t r a s t on t h e ( i l l ) i n t e r f a c e plane, a v a r y i n g one on ( 1 i 2 )

,

and none a l s o on t h e "P" s i d e o f t h e ( l i 2 ) i n t e r f a & ( L ) and a s t r o n g one on t h e "Q" side o f t h i s f a c e t . Zone C shows an o r i g i n a l step w i t h no EBIC c o n t r a s t a t a l l before annealing ( f i g . 1 . b ) ; a c t u a l l y , t h e two kinds o f planes and t h e two k i n d s o f d i s l o c a t i o n s appear t o be e l e c t r i c a l l y i n a c t i v e i n t h i s case.

F i g . l

-

SEM images o f t h e Z = 3 stepped boundary, ( a ) secondary e l e c t r o n s , ( b ) EBIC on t h e same a r e a i n t h e as-grown sample, and ( c ) EBIC i n t h e annealed sample. Arrows i n d i c a t e exemples o f s t e p d i s l o c a t i o n s - " P " ( & ) and "Q"(?)

-

which show no e l e c t r i c a l a c t i v i t y . See TEM o f zone (A,B) i n f i g s . 2 and 3 . (The p l a n e i n d i c e s r e f e r t o t h e l e f t hand s i d e c r y s t a l ) .

3 . 2 Transmission e l e c t r o n microscopy

Zone (A,B) ( f i g . 1 ) from t h e as-grown sample has been observed by TEM. F i g u r e 2 shows i t s w e l l recognizable shape (compare t o f i g . 1 . a ) . A l i g h t EBIC spot i n t h e ( 1 i 2 ) i n t e r f a c e ( f i g . l .b) thus appears t o be associated t o p o i n t - l i ke TEM s t r a i n c o n t r a s t s (fig.2.A).

Microsteps are a c t u a l l y e x i s t i n g on t h e l e f t hand s i d e o f t h i s ( 1 i 2 ) boundary, as witnessed by a s l i g h t d e v i a t i o n t o t h e exact plane o r i e n t a t i o n (fig.2.A), which do n o t induce a recombination d e t e c t a b l e by EBIC ( f i g . l .b). The recombining p a r t o f t h e boundary (fig.2.B) i s a curved i n t e r f a c e n e a t l y f a r t h e r from t h e ( 1 i 2 ) exact o r i e n t a t i o n . A l a t t i c e image o f t h i s zone ( f i g . 3) shows t h a t i t i s made o f no o t h e r s t r u c t u r e s than ( i l l ) - ( l i 2 ) steps, and f u r t h e r , t h a t t h e s t r a i n c o n t r a s t s v i s i b l e i n f i g . 2 are associatedwithprecipitates. The p a r t i c l e shown i s l i k e l y amorphous; i t seems t o have nucleated i n a "Q" d i s l o c a t i o n core and t o have grown a t t h e expenses o f t h e nearby (112) i n t e r f a c e .

Cl-584 COLLOQUE DE PHYSIQUE

F i g . 2

-

Low magnification TEN images o f zones A and 5 o f f i g . l ( t h e plane indices r e f e r t o the lower g r a i n ) .

F i g . 3

-

High magnification image of zone 5 o f f i g s . I and 2.

4

- DISCUSSION

AND

CONCLUSION

4.1 The precipitate nucleation at the

2 = 3

steps

In the as-grown sample, the high magnification observations show that precipitates nucleate exclusively on the step dislocations, and more frequently on the "Q" type of these defects.

The EBIC contrast after annealing indicates that precipitation (metallic /14/) occurs also in these regions, and not at the

(li2)

"incoherent" boundary planes.

4.2 The origin of the observed e7ectrical activity at the

2 = .3

steps

The correlation between the EBIC and TEM images shows that precipitates are always present in the

2 =

3 steps when an EBIC contrast is observed. Moreover the presence of these precipitates appears to be associated to that of the dislocations at the step edges. On the other hand, all the different components of the step may be observed with no EBIC contrast.

One is then tempted to conclude that the

2 =

3 step is electrically active only when decorated by precipitates, i.e. that it exhibits no intrinsic recombination activity.

4 . 3

On a possible passivation by the dopant (Al) and the metallic impurities

Aluminium is known to passivate the GB electrical activity after diffusion below the eutectic-formation temperature (577'C) /17/. However no such mechanism is reported when aluminium is added to the melt where, on the contrary, a strong increase of the GB electrical activity is even observed after annealing /18/; moreover, we have also observed electrically inactive

2 =

3 steps in the B-doped material. Therefore Al-passivation is not likely in the studied samples.

Cu is also known to passivate GBs /19/ and dislocations /20/, after low-temperature diffusion, and is found to be one of the contamination metals in the preparation procedure /14/. However, it is known to precipitate at the defects during the heat treatment and to induce there a strong recombination activity /14/. As a number of non-recombining areas subsist after annealing, this hypothesis seems unlikely, i.e., the non-activity observed seems to be indeed of intrinsic origin.

4 . 4

About the possibility of recombination below the EBIC sensibility

As an isolated dislocation would not show intrinsic recombination activity in EBIC, even if there were one dangling bond per atom in its core /16/, the step dislocations "P" and "Q"

would also show none when isolated between two inactive facets. In order to check their intrinsic activity, one then needs a noticeable concentration of these defects with no precipitates, a situation which is rare in current polycrystalline silicon. A definite conclusion thus cannot be drawn on this particular point.

In the case of the "incoherent" (li2) interface however, the EBIC sensibility is high and the experiments show unambiguously that this interface has no recombining activity, i.e., that its deep level density, supposing a capture cross section of 10-16cm2, must be lower than approximately 1011cm-2 (lO-& Si monolayers).

The author would like to acknowledge a fruitful discussion with A. Bourret during the conference.

REFERENCES

/ l/ Maurice, J.-L., Rev. Phys Appl., 22 (1987) 613.

/

2/

Lenahan, P.M. and Schubert, W.K., Phys. Rev. B 30 (1984) 1544.

/ 3/ Rocher, A. and Labidi, M., Rev. Phys. Appl. 21 (1986) 201.

/ 4/ Bourret, A. and Bacmann, J.-J., Surface Sci. 162 (1985) 495.

/ 5/ Bourret, A. and Bacmann, J.-J., Proc. of "JIMIS-4", suppl. to Trans. Jpn. Inst. Met.

(1980) 125.

/

^6/

Mauger, A., Bourgoin, J.C., All an, G., Lannoo, M., Bourret, A. and Bill ard, L., Phys. Rev. B 35 (1987) 1267 (Ge).

/ 7/ Paxton, A.T. and Sutton, A.P., J. Phys. C 21 (1988) L 481.

/

8/

Kohyama, M., Yamamoto, R., Watanabe, Y., Ebata, Y. and Kinoshita,

M . ,

J. Phys. C. 21 (1988) 3205.

/ 9/ Kohyama, M., Ebata, Y., Kose, S., Kinoshita, M. and Yamamoto, R., this conf.

/10/ See, e.g., review by Leamy, H.J., J. Appl. Phys. 53 (1982) R51.

COLLOQUE DE PHYSIQUE

/11/ Maurice, J.-L. and C o l l i e x , C., Springer Proc. Phys.

3

(1989) 83.

/12/ Maurice, J.-L., Pastol, J.-L., Bourree, J.-E., Lava1,J.-Y., Revel, G. and Rodot, M-, i n P o 7 y - m i c r o - c r y s t a l l i n e and amorphous semiconductors, e d i t e d by P. Pinard and S.

K a l b i t z e r (Les E d i t i o n s de Physique, Paris, 1984) p.323.

/13/ Revel, G., Hania, D. and Pastol, J.-L., same as /12/, p.147.

/14/ Maurice, J.-L. and C o l l i e x , C., Appl. Phys. L e t t .

55

(1989) 241.

/15/ Dugas, J., Doctoral Thesi S, U n i v e r s i t y o f M a r s e i l l e s , M a r s e i l l e s , 1985.

/16/ K i t t l e r , M. and S e i f f e r t , W., Phys. S t a t . Sol.(a)

66

(1981) 573.

/17/ Sundaresan, R., Burk, D.E. and Fossum, J.G., J . Appl. Phys.

55

(1984) 1162.

/18/ Kazmerski

,

^L. L. and Russel

,

P. E., J. Phys. Suppl

.

^{(Pari S) Q} (1982) Cl-171.

/19/ Zehaf, M., Mathian, G., P a s q u i n e l l i , M. and M a r t i n u z z i , S., same as /12/, p.137.

/20/ Lee, Jae-Gwang and Morrison, S. Roy, J. Appl. Phys., &(1988) 6679. l