The results of a preliminary study of two dendritic web samples are presented. The structure and electrical activity of the defects in the silicon webs were studied. Optical microscopy of chemically etched specimens was used to determine dislocation densities. Samples were mechanically polished, then Secco etched for approximately 5 minutes. High voltage transmission electron microscopy was used to characterize the crystallographic nature of the defects

Ast, D.

Cunningham, B.

Strunk, H.

English

NASA Technical Reports Server

  
 
 
N O T I C E 
 
THIS DOCUMENT HAS BEEN REPRODUCED FROM 
MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT 
CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED 
IN THE INTEREST OF MAKING AVAILABLE AS MUCH 
INFORMATION AS POSSIBLE 
https://ntrs.nasa.gov/search.jsp?R=19810002943 2020-03-21T16:10:58+00:00Z
`19 =-4^I
October '940
J?^ Cortrac. No. 95+852
(SAS A-CR-163709) DEFECT STRUCTURE OF WEB
	 N81-11451
SILICON GIBBON (Cornell Univ., Ithaca, N.Y.)	 23 p HC A02 /M1c
 A01	 CSCL 10A
Unci as
G3/44 29107
t	 by
B. Cunningham, H. Strunk and D. Ast
Materials Science and Engineering
Bar- Hall, Cornell University
,thaca, Nev York 144'53
The J?L lcw-cost solar array prole:., is sponscred b;. the L.S. Derar--e:. cf
Energy and ferias tar-, of the	 PhotovoltaicCcrversion Pro.ra:-. to
- n-'-.'ate
 
a naior effort toward the develovnent of low-cost solar arrays.
This vc,rk was rerformed for the Jet Prop •,ilsior. Laboratory, Californ:a
Ins.:tote of Technology by agreement between. NASA and DOE.
R 	 DEC 1980
L" RECEIVE6
^;`+ ,^,, WO S FACT
,g^
. IWM DEFT
n s
I"This report was prepared as an account of work
sponsored by the United States Govenment. Neither the
United States nor the United States Department of
Eaerby, nor a-.y of their employees, nor any of their
contractors, subcontractors, or their employees, makes
any warranty, express or implied, or assumes any lean]_
liability or responsibility for the accuracy, complete-
ness or usefulness of any information, apparatus,
product or process disclosed, or represents that its
use would not infringe privately owned rights."
r
Web silicon ribbon has recently emerged as a materiel for the production
of nigh efficiency solar cells. Since defects introduced d•--ing growth may
influence locally =inority carrier 7ecombination rates, there is now a need to
examine the defect structure in detail and to correlate it with "electrical
activity". This work describes initial observations made on web material b ­
ERIC and HZ,MMA.
Although EBIC investigations have shown that dislocations emerging at she
web surface enhance minority carrier recombination rates, their density is low
enough (typically 105cm-2 ) to have only a small effect on the efficiency of
the material as a solar cell. Since a condition for dendritic web growth is
that the dendrites contain at least two twin boundaries it is usual to find
that some of these boundaries extend into the web. These boundaries are formed
parallel to the (111) growth surface and are found to be sites of strong elec-
trical activity.
HVEM has been used to study the defect structure at the twin boundary.
Two types of dislocation networks lying on different {111} planes have been
observed, presumably corresponding to two adjacent twin boundaries. One network
consists of an hexagonal array of twin boundary partial dislocations interacting
with Lomer-Cottrell locks. The other network is made uv of both verfect and
partial dislocations running in the [2111 growth direction.
:tr
L^
_^e grog „n L_ :___con r _Docns c; V.:o de::o_ V^ i.. web -zr :ze:.e was __rs„ ....c-
V us s ea	 Der^..a ., an,. r auE- , _) ..nc ' n Q.r = an . Bennet-_ ' Z ' \  . _ .• :.e ear-,	 ' s .
The ..went interest in s ilicon web is due a 	 ; ertial .:se as a 5;.. r .r_:e
material for the economic production of high efficiency scla_r cells. Seidenst' er,3:
has shown that the characteristics of solar cells fabricate; from dendri..ic web are
virtually identical to tnose made from the more co=only grown. ^.zochralski materia. .
Since crystalline defects in semiconductor materials frequently affect the
electrical properties, e.g. minority carrier =ifetime, the structure and ele:etrical
activity of the defects in silicon web were the subjects of the uresent ^..vestigation.
This report summariz^ the results of a Treliminary Study of two I*7es,!nzhoase
dendritic web samples, identification numbers v'160- 3.4C and 0-135-3.6.
2. Exverimental Techniques
Optical microscopy of chemically etched specimens was used to determine dis-
location densities. Samples were mechanically polished, then Secco(L) etched for
approximately 5 mins.
High voltage transmission electron microsco-m , (HI'TMI ) was used to character-
ize the crystallographic-nature of the defects. The twin planes in web material
are parallel to the surface (and at or near the center of the ribbon). The web
thickness is only ti150um and, for this reason, it is difficult to rrepare a
specimen for Tom; where the twin boundaries can be viewed end-on. Snecimen5 for
the present investigation were therefore thinned as symmetrically as possible from
both surfaces with IMO. :M':HAc (5:1:1) until a hole with tapered edges was formed.
The centrally located twin boundaries were, however, generally not contained in
the very thin (d < 0.5um) annulus around the hole that can be inspected by
0
con ien.._..:a:	 :.se .,_ a
vc _.age ..icroscope (: :e: wi ^» _.s large :ten= .rat:	 _ : ver of se:-era:
roved .: be essential .o -_* =age  the twin - :ane or _lanes at 	 apprcx_m_.e ce:.er
cf the ribbon.
The electronic properties of 	 material were studied using a scanting
electron microscope ( SLr:) onerating in the B C :code ( electron bea.:. i.-' uce cur-
rent,). in this mode the electron beam is used to in.' ect minor ity carriers which
are collected at the specimen surface to form the ?BiC image. At regions of en-
hanced minor -f ty carrier recombination the collected current is less than r.. defer.
free regi.)ns. Thl a images at these points therefore appear dark ir- the IC micro-
graphs.
Finally, a nulti-wire, real time Laue back reflection x-ray camera was used
to determine twinning relationships in a variety of unthinned web material.
3. Results
a. Web-Dendrite Geometry
The crystallographic orientation of the web and dendrites is shown
schematically in Figure 1. The dendrites must contain at least two planes before
dendritec growth of a diamond lattice structure can occur(3) and, although it is
not necessary, it is common to find some of these twin planes extending into the
web. Both the 27180-3.4C and J135-3.6 webs were Laue x-rayed to determine the
orientation relationship between opposite growth faces. Figure 2 shows an x-ray
pattern taken with the beam striking the W180-3.4C web end-on. The (111) twin
relationship between opposite faces is evident showing that the web contains an
odd number of twin planes. X-ray patterns from the J135-3.6 web determined that
this sample contained an even number of twin planes.
)RIGINAL PAGE IS
)F POOR QUALITY
0
e+e: i ^- --. C was e_ ed F_nd 	 en c.
war:cus p=siti,=s aCress thA e surface. - nese sr e s :: vw in _ z- ,re Z t.-zet..er -tn
a s ke _-h -f the web Surface ind-catinE 't -e Erea€ C,rreS;:nd- ^.L _. the ..., _ .gra^ :5.
_	
s :ows the relationsh _ between _c_iticn and et....-. -. densi , . -n e
de ns^ty is t5 105	 r r	 dr^	 E	 ^^^	 `he edges...	 ,,.^4 ^.. the center o. the we	 _,.ppir. -„c ^...^.. a„ „
is izzenera,11y acceptedi5,c` that most of the dislocations in Silicon webs crigi:ate
from high:y stra ,-ned regions in the dendrites, caused by li quid entrapment during
growth. Such a region is shop,n in Fig,_re ^, with the stir plane traces
c
visi.-e. ?'he maxim= c  about
	
sl.cat_.n r r er cr. 0Cser:•e d it '	 .re
is cue tc thermal stresses generated during gro g, -th causing :^cst of tee iwslccetic:.s
to sli p into the center or the wet. :f the stresses generated at a dis:cca;,icn
source are insufficient to promote slip, then these dislocations wi:l re-mai n at
regions close to the dendrites. This explains the secondary maxima observed in
Figt;re _Q) .
Fisu_re 5 shows an EBIC micrograph of the web surface. ^he dark s pots on
the micrograph are caused by enhanced minority-carrier recombination at disloca-
tions inclined to the surface. ':he density of electrically active defects observed
in ERIC is the rare as the etch-pit density observed in optical microscopy, sug-
gesting that most, if not all, of the dislocations intersecting the web surface
are electrically active. Solar
 cells with relatively high efficie ncies have been
fabricated from silicon web containing dislocation densities of the order of
magnitude observed in the present study(;), indicating that either the electrical
activity associated with these defects is not strong enough to significantly detract
from cell uerformances or that the defect densities are at a tolerable level.
i^
"'he electrical ac r_: i`, cf -z he twin r.....car:es w&s st::__e»
and cc_-shing a svecimen	 at a s :°..l_ow angle, 7-- c , _. : he	 e;:r-
face.	 C, see Figure :, shows ,oat a	 dens;', cf dislrcEt_.ns is . znze ntra-
ted -t the twin boundaries and that in t is region stronz minority carrier reccn-
.:nation rrocesses occur . It is thong t the, the thermal stresses i:enerated
inz growth also cause dislocation€ to stir tw	 o, i.-eerier of the we:, ::ere the
twin planes act as obstacles tc further slir processes. Figure 1 is an crtical
microgrPph c:' the same area as the E3IC image after the specimen had been etched
to remove the : chottky diode and delineate the defect. As ir. the case of ..isloca-
ticns at the wib surface, there is a close agreement between the densit y cf dark
ERIC features in Figure C and the optical etch tit density in Figure t. is direct
correspondence between. Figures 6 and 1 is not possible since the etching crocess
removes several pm of material from the surface. Both ERIC and optical micrographs
show that two twin boundaries exist in the web, i.e., a microtwin, and that the
dislocation density on one side of the microtwin is higher than cn the ether.
The fine line marked A in Figure r is thought to be a small angle grain
boundary; however, this has not yet been confirmed. The width and contrast features
are not typical of surface scratches normally visible in ERIC micrographs and the
contrast is probably due to the surface tcpolog ,.%• rather than to an associated
electrical activity. yaue x-ray diffraction patterns from either side of the
line failed to reveal any difference in orientation, indicating that, if the ob-
served feature is indeed a low-angle boundary, the mismatch is <1 0 . Similar
features have been observed by the authors in other web specimens where they are
visible on the surface races.
n M
. igWC & ::^ ^: r^." ^iCrci ap tafen frc._ si—iC... we: =a_-- _^_ 	 _s
where the disloca te ^.s are a:=cst s,.Lrace.s:'6.._^e_. The	 are :en-.ered
arcund the active bc ,=dar:r shown in the =-_,i.1- a cr the	 _s ^r.:bab_.
a tw_ boundary_, ;; ic .as been forced out cf the •c:eb during Er-cwt  The -_ _....
ticns associated with it are also strcngl;^ electrical'_,- active.
d. Defect Structure at the Twin Planes
Pre.-ious investigators (5) identified the defect structure of twin
boundaries in sil icor. web as comzlicared arrays of dislocations displa,:i^g <12.0>
Burgers vectors, with the dislocation lines primarily directed along ;2' ar,^
<110> directions. The present study has confirmed the existence of these dis-
locations and in addition has identified two sets of misfit d-1.s1 cations. Ficares
9(a) - 9(d) are IPM, icrographs of web J135-3.6 shoving an hexagonal array of
twin boundary dislocations imaged with different diffraction conditions. From
contrast analysis the Burgers vectors of these dislocations were determined as be-
ing of the <2l1> type. Each set can be extinguished by using the appropriate
{220} reflection. The observed Burgers vectors are in agreement with. the prediction
of the Bollman theory(9), that intrinsic dislocations in coherent twin boundaries
are Shockley partials, b = 1/6 <21l>. These dislocations take up a small twist com-
ponent in the (111) twinning plane. The twin relationship could be easily verified
i
from selected area diffraction. The twist component estimated from a f: Ib is in
the order of 0.01 0 and is therefore too small to be detected by electron diffraction.
The second set of misfit dislocations, Figure 10, consists of slightly curved
dislocations running in the [211] direction. They are also contained in a (111)
twin plane and take up a small tilt component. Most of these dislocations are 600
partials having Burgers vectors = 1/6[121], all of the same sign of b as revealed
by +g, -g, while others are perfect dislocations with Burgers vectors of '4[011], 90°
it  I (; AL 
PAGE,)r Pl x
►R {^LT^II ^,
R 0
a..	 _ °.	 _re ..:osrpwwi^.. •ec	 ^^ 'rtw.re	 'was w..ww en :._.. ..:s.
n ono	
Or ^	
LL	 J	 C	 Lwas ar wialE were out of ccntra+sw.	 +r` an! _gzhV ' tt!'e, .,i.^ •._n a :.=rge `r ev
ni p i r t ch.n n	 •z^	 2nC• _. r on	 e^	 ^..	 ..srf •s^axnir.b os.. _:a,.o	 e...... _.:e ,^w_. _....w__i .. sae age .._ _ 	 _ec 
wions but did not resolve the c:° partials. They &Isc rep _ tei _is1. cs:_w..s ::: w..
_­•gers vectors ;101; and t1-o3 ype ,•ing in the	 d rect_..n, a:t :az
also noted that such dislocations sho_-"d not slip read'-_;:, as cJ_csed t: she ^nes
reucrted above.
Figure 10 also show*s straight dislocations runnir. n 	 rl ions	 g i the <1^0> di ections,
and part of an hexagonal network similar tc that shown in °'icire P. Sterec mic=c^-
graphs show that these dislocation are nct contained in the sa=e (_ ^' _lane as
the curved dislocations and must therefore belong to another twin _;pane, _.e.,
the other side of a microtwn. A strong interaction occurs between tae 4-islcca-
tions running in the <110> directions and the Network of parti szs. The stra,-g.::t
dislocations have <110> type Burgers vectors and are edge sessile. 'They form
when dislocations slipping on either (111), (111) or (fll) planes intersect
dislocations in the twin planes and form Lomer sessile dislocations. C"Nara and
Schwuttke proposed that the reaction for dislocations slip ping on the (.11) plane
would be
'z a ;0113 + ': a "1013 = -a a [110]
(111) plane (111) plane	 Loner dislocation
The ' a "110] Loner dislocations, shown in Figure 10, lying along the {1103
direction, can then dissociate into a Cottrell- yomer lock by the following reaction.
4 a [llo3 = 1/6 a [1123 + 1/6 a [ 1103 + 1/6 a [1123 .
It is likely that the dissociated partials are responsible for the interactions
with the hexagonal network.
a
^_...__ar rearv_Cns	 _:" fcr ^i$.v28t_, :^ £_ s ^_	 cn -.he
anes x_... .he	 :.=er	 .ia. _ng 	 =®rs vectcrs	 E y ++.
and	 a	 and ne d;rectic ns y;,l_, and	 respectively.	 ereas
ana c_c:aTnT».t.''.e _dent-f_e^. A a' CCp—t.io,S directed £-'one 	 an,;  	 he ^: r6-Ee ::J
investigation has identifi ed lomer dislocations1;. _..s in all ,,.._ ee of the
directions contained in the ( :} twin plane.
in both the etch and B'B_C studies it was noted that the dislo^,et_on densit;:
was higher or. one side of the r.icrotwin than on the other. The F=— ! studi_-_ : aie
also showy. that most of the dislocation reactions occur on one of the twin planes.
.t is likely that an irhowogeneous stress distribution is present in the reterial
d:zring growth, e.g. by buckling of the web.
4. Conclusions
The present study has shown that the defect structure in silicon web enhances
minority-carrier recombination rates. The dislocations in the bulk of the .aterial
are electrically active.Ilnpirically it is known tha 	this activity does not
significantly reduce the characteristics of solar cells. Since com plete devices
have not yet been investigated it is not known if the de fect density is ciTMply
too low or if annealing reduc^zs the electrical activity of dislocations(10). The
surface varallel twin, boundaries typical of this material are associated with a high
density of dislocations. EBIC shows that these dislocations are sites of high
=.*nority carrier recombination and a reduction in solar cell efficiency is there-
fore expected when twin boundaries lie in the active region of a solar cell.
The defect structure of the twin planes has been investigated using ::VBM:, and
several dislocation-types not previously reported in this material have been identified.
i
ii^
7
3
C.-der
	r'.;.._, _.:er£ tan! the de`ect s	 tune : f r	 ..n tee: an .::
e°_eat cn recomlbi nation processes, studies are nderva;- 	 e-"&-.e -.-e nst :re :f
t^e dislocations cbserred at the twin ^lanes and their aaeccia. ed elec..__car
ac,i^•,ty.
 The occurrence cr se:-erai `.ypes of dislocations and the relat_:e_,
large spacing between then. makes it an _dear ca ndidate for !sect
	 and
correlations. In addition, an attemrt will be made to cuanti°; the 	 sig ::._s,
thereby allouring a better comparison between the electrical ac,._. ity of different
yti1pes c: defects.
It
TC
N. :er»atis and
	: aeEt, n ^^C .. ..nOeLE
^Ew CC': A ..^ _Or .'^: : I' ,
	 C0=ur._2at_„n£ E_ev _rc n:..-
G. 2' ara and A.	 Bennett, J.
3.	 3. Seidersticker, J. Crust. Jros,	 39,	 ;197
F. Secco d';.ragona, J. Electrochem. Soc.	 _, 9,8 (197%).
5. S. O'Iieza, J. App-. Phys. 35, L09
6. u. L. Barrett, E. H. Myers, D. R. Hemilton and A. 1. Bennett, j . M ectrccaem.
Soc. 113, 952 :1971)•
7. C. S. Duancan et al. , "Silicon ;deb Process Development", „pl Iec n!cw Fepert,
Contract :vo. .;AS 95=.^ c5L,'^,'1, Westin ghouse R&D Cen`er , 1978.
8. S. O'Hare and G. B. Schwuttke, J. App1. PhNs. 36, 24',5 (196;`
9. W. Bollman, Crystal De f ects and Crystalline Interfaces, Springer 'rer'_ag, : es:
York: (1970).
10. H. Alexander, J. Physique Colloo. 40, C6-1 (1979)
I tt
c^ a^at_. s o»in5 the . _;•stallograp .... :r-* tat__.. rf »ec-den ,._ _	 s.___....
._	 'Back reflection :,aue x-ra,.: utter n	 -t-w:..	 _in .re:
?,. Zmt_cal wicrographs of etch ^;ts.
4. :.ptic	 micrograph of a dis? ocatior. source.
^. t;KIC microgragr. of a web surface.
c. ZBIC W_crograpr of a bevelled surface showirg electrical act4:-it;y -ear
-r^^ andaries.
Optical micrograph of same region, as shown in Figure 6.
8. ZBIC micrograph of surface-parallel dislocations.
micrographs of a dislocation network. g: diffr action. vector.
2 0. I?V'^EM micrographs 'see text for details) .
i
T---	 ni- GS
Dendrite Seed
Button
Lam' Bounding Dendrites
Web
Figure 1 Schematic Section of Web Growth
(l:xer Demean et al	 .:eo 'rrocess Develop.lw t:,,
Report No.; 4v5u-785;1
 (157-).
ri 0
/
Col , ^
/1^
i
N ^^[. YAh F; 1:
Ii
I^
I
I
r	 1
1	 ?	 ^	 5
=F'_
r	 F
d	 , .	 • . e	 L . +.•^ ,^^ ^^^
AV
4
s
Dislocation etch pits in avet:-clendri;i; silicon
Fig.3
^r►,^r.	 r,	 ASt
^v h
b c
I	 55i0
5 10 4
1
EV
1
r
.C%Jurr
^^^•	 ^; .^^ ^. Teti.`, .^.:; :^'.k:-r•
^^ •,
	
., •' ^1^ •' i
t
 • ^ t • ^• / - i r^^-'^
5
1
^er;.r,:e	 web
Djslc: :ion etch pits in web-Cei;jritic silicon
	
Fig 3	 9	 ' :)3 45t
r.
•	
006	 0
 
OF
}	 •
Fi g. 4
	 100Nm
n ••
iFi g.5
	 20Nm
1	 r	 ! • • JI
IV
•4L
•
OA
^ t
	
^
0 . . 0
50Nm
^ m
Fig• 6
r.
1Fig.7	 00ym
L X14
f -.
. .41 a-
66
JA
JX
It
^ ^	 its
lie
O
co
rn
LL.
:Iwo
cc
Q
IN
O
Al
^yam	 •-. ^ __ ••• - 	
- -
' 	O
_ _	 r
do
1-^ t _ •;t. ti t	 s i.l TT


Defect structure of web silicon ribbon

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server