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Journal of Electronic Materials, Vol. 11, No. 5, 1982
E FF EC TS O F M ON OC HL OR OS IL AN E O N T HE P RO PE RT IE S
O F P L AS M A D E PO S IT E D H Y DR O GE N AT E D A M OR P HO U S S I LI C ON *
A . E. D e la h oy , 1 R . W . G r if f lt h , 2 F.J. K a mp a s a n d P.E. V a ni e r
D iv is io n of M et al lu rg y a nd M at er ia ls S ci en ce
B r oo k ha v en N a ti o na l L a bo r at o ry
U pt on , N ew Y o r k 1 19 73
( R e ce i v e d O c t ob e r 23, 1981)
C on ce nt ra ti on s o f m on oc hl or os il an e ( Si H3 CI ) of the o rd er
o f 2 50 0 p pm h av e b ee n d et ec te d i n some c om me rc ia l t an ks o f
s il an e b y ma ss s pe ct ro me tr y a nd o pt ic al e mi ss io n s pe ct ro s-
copy (OES). This impurity is shown to depress the posi-
tion of the Fermi level in a-Si:H, resulting in lower
p h ot o co n du c tl v it y a n d s o la r c e ll e f fi c ie n cy .
K ey w or ds : A mo rp ho us s il ic on , m o no ch lo ro si la ne , o pt ic ale mi ss io n s pe ct ro sc op y, m as s s pe ct ro me tr y, p ho to co nd uc tl -
v it y, s ol ar c e l ls .
i p re s en t a d dr e ss : C h ro n ar C o rp o ra t io n
3 30 B ak er s B a s i n Rd.
T re nt on , N J 0 8 6 382 p re s en t a d dr e ss : U.S. A R O
E l e c tr o n i cs D i v is i o n
P.O. B ox 1 22 11
R es ea rc h T ri an gl e P ar k, N C 2 7 70 9
*This research was performed under the auspices of the
U.S. Department of Energy under Contract No. DE-ACO2-
76CH00016.
8 6 9
0361-5235/82/1105-86953.00 1982 AIME
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870 Delahoy, Griffith, Kampas and Vanier
Introduction
T he s tu dy o f h yd ro ge na te d a mo rp ho us s il ic on h as b ee n
h am pe re d f or a l on g time b y the v ar ia bi li ty o f the p ro pe r-
t ie s o f s am pl es p ro du ce d in d if fe re nt l ab or at or ie s. E ve n
w he n c om pa ri so ns are m ad e o nl y b et we en s am pl es p ro du ce d b y
a p ar ti cu la r d ep os it io n m e th od ( fo r i ns ta nc e, r f g lo w d is -
charge in silane) the properties of films described by
different groups cover a wide range. Some of the dif-
f er en ce s b et we en s am pl es c an b e a tt ri bu te d to c on tr ol la bl e
d ep os it io n p ar am et er s s uc h a s s ub st ra te t em pe ra tu re , t ot alrf power, gas pressure and flow rate. Other variations
m ay ar ise in m ore su bt le w ay s fr om the de tai le d ge ome tr y
o f t he d ep os it io n c h am be r, w hi ch d et er mi ne s the c ha ra ct er -
i st ic g as f lo w p at te rns a nd the d is tr ibu ti on o f rf po we r
density. Even when all these parameters are kept con-
s ta nt, w e h av e ob se rve d l ar ge d if fe ren ces i n el ec tro ni c
p ro pe rt ie s b et we en f il ms p re pa re d f ro m d if fe re nt t an ks o f
silane. In this paper we will show that such variations
i n f il m p ro pe rt ie s c an b e c au se d b y d if fe re nc es i n c o nc en -
tr ati on o f mo no chl or osi lan e (S iH 3CI ) w h i c h s ome ti mes is
p re se nt as a n i mp ur it y i n s i la ne .
The presence of this impurity in one particular
silane tank was first indicated when a peak at 281 nm in
t he g lo w- dl sc ha rg e e mi ss io n s pe ct ru m w as t en ta ti ve ly a t-
tributed to the species SiCl (i). The assignment was
r ea so na bl e, s in ce s il an e i s o f te n s yn th es iz ed b y r ed uc ti on
of chlorlne-contalnlng compounds. It was also noticed
immediately t h a t the f il ms p re pa re d f ro m t hi s t an k s ho we dp h ot o co n du c ti v e b e ha v io r that w a s s i gn i fi c an t ly d i ff e re n t
f ro m p re vi ou s d ep os it io ns . W lt h i mp ro ve me nt s i n the o pt i-
c al s ys te m, t hi s e mi ss io n p ea k w as r es ol ve d into two com-
ponents expected for SIC1 and found to occur, but with
lo wer i nte ns ity , w hen s ev er al ot he r ta nks o f s il ane w er e
e mp lo ye d (2). F ur th er mo re , the a mo un t o f c hl or in e in t he
deposited film, as measured by secondary ion mass spec-
t ro me tr y ( SI MS ), w as f ou nd to c or re la te w it h t he i nt en si ty
o f th e S iC l em iss io n. W e h av e n ow c om pl ete ly es ta bli sh edt ha t the e mi tt in g s pe ci es is S IC 1, i de nt if ie d the p ar en t
co mpo un d a s m ono ch lor os ila ne (S IH 3CI ), a nd s tu di ed the
effects of this impurity upon the properties of a-Si:H
f il ms a nd s ol ar c el ls .
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Effects o f Monochlorosilane on Amorphous Silicon 871
Experimental
T he o pt ic al e mi ss io n s pe ct ro me te r c on si st s o f a S pe x
0 .7 5- m m on oc hr om at or , a c oo le d H am am at su R 9 5 5 P p h ot om ul ti -
plier tube, and a Spex PC-I p h o t o n c ounter. The m o n o -
chromator h as a d is pe rsi on o f i.i n m/m m and w as o pe ra ted
w it h 0 .5 -m m s li ts .
T he p ar en t c om po un d of the S iC I e mi ss io n w as i de nt i-
fied as SiH3CI using a m o d e l IO0-C UTI quadrupole mass
sp ect ro met er o per at ed a t an i on iz ati on e ne rgy o f 70 eV.W ith n o rf p ow er a pp li ed, th e d ep os iti on sy ste m wa s us ed
as a co ntr ol lab le s our ce o f si la ne an d w as set a t a p re s-
su re of 1. 0 T or t. A V ee co P V-1 0 p ie zo- el ect ri c v al ve w as
u se d to m od ul at e t he g as f lo w b et we en the d ep os it io n s ys -
t em a nd t he m as s s pe ct ro me te r, w hi ch w as d if fu si on p um pe d.
The output of the mass spectrometer was detected by an
I th ac o 3 9 7 E 0 l oc k- in a mp li fi er .
D a rk c o nd u ct i vi t y and p h ot o co n du c ti v it y m e as u re m en t s
were performed on coplanar ~amples in which the elec5
trodes, consisting of a 300 X layer of Cr and a I000 A
l ay er o f Mo, w er e f ir st e va po ra te d o nt o C or ni ng 7 05 9 g l as s
substrates leaving a 1.6 ~m gap. The a-Si:H was then
deposited in an rf capacitatlve system as a 0.4 ~m thick
layer bridging the gap and forming ohmic contacts with
t he M o. T he rf po wer w as set at the l ow est le vel at wh ic h
a d is ch ar ge c ou ld b e m ai nt ai ne d, w hi ch in this s ys te m w as
15 W. The silane flow rate was I00 sccm at a pressure of
I00 m T o r r and the substrate temperature was 225C. Dep-
o si ti on s w er e c ar ri ed out b ot h w it h a nd w it ho ut a p ro pr i-
e ta ry c hl oro si lan e fi lte r pr ov ide d b y M at he son G as C om -
pany. A Keithley 616 electrometer was used for dc mea-
s ur em en ts , and t he I th ac o l oc k- in a mp li fi er w as u se d f or
m od ul at ed p ho to co nd uc ti vl ty e xp er im en ts . M on oc hr om at ic
l ig ht w it h a w av el en gt h o f 6 00 n m and an i nt en si ty o f 1 01 4
photons cm-2s - I was chopped at 7 Hz in the latter case.
T he a pp li ed v ol ta ge f or b ot h d ar k c on du ct iv it y a nd p ho to -
c on duc ti vit y me asu re men ts w as i0 0 V. Fu rth er de tai ls ont he de pos iti on s yst em a nd me asu re men t t ech ni que s ca n be
f ou nd in a p r ev io us p ub li ca ti on (3). T he C l c on ce nt ra ti on
in th e a -S i:H f il ms w as d ete rm ine d b y s ec on dar y- lon m as s
s pe ct ro me tr y ( SI MS ) o f 35CI u si ng a C s+ p ri ma ry b ea m (4).
In order to study the effect of SiH3CI on devices,
diagnostic solar cells were fabricated in the p-i-n
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872 Delahoy, Griffith, Kampas and Vanier
c onf igu ra tio n fr om the s am e tank of s il ane b oth w it h a nd
w it ho ut t he c hl or os il an e f il te r. T he a -S i: H w as d ep os it edto a n omi na l t hic kn ess of 60 00 A o n st ain le ss s te el sub-
strates held at 180C. J ~ e p- and n-type layers were
r es pec ti vel y I 00 ~ a nd 4 4 v = in t hic kn ess , a nd w er e p re-
p ar ed u si ng g as p ha se c on ce nt ra ti on s o f i 00 0 p pm B 2H 6 a nd
150 ppm PH3, respectively. Cells 2 ~ 2 in area were
defined u%ing as a top contact either a metallic film
( e.g . 65 A Pd) o r a q~ art er -wa ve tr an spa re nt c ond uc tin g
oxide layer (e.g. 660 A In203). The latter layer was
e -b ea m e va po ra te d f ro m a s ln te re d s ou rc e o f I n2 03 u si ng al ow g un v ol tag e (4 kV) to a vo id da ma gin g the a- Si: H. Th e
d ep osi ti on w as p er fo rme d wi th a pa rti al p re ssu re of d ry
o xy ge n o f 5 x 1 0- 4 T o rr , a s ub st ra te t em pe ra tu re o f 1 4 0 C,
and at a rate of 0.3 ~ s-I.
The current-voltage (J-V) characteristics of the
c e ll s w e re d e te r mi n ed u n de r a p pr o xi m at e ly A M I i l lu m in a ti o n
provided by a Xenon lamp solar simulator. The J-V data
w er e a cq ui re d a ut om at ic al ly u si ng 12 b it A /D a nd D /A c on -
v er te rs c on tr ol le d b y a T ek tr on ix 4 05 1 g ra ph ic s c om pu te r.
S in ce t he I n2 03 l ay er s w er e o ft en f ou nd to h av e n on -a br up t
edges, the J-V data were obtained using a mask placed in
c on ta ct w it h t he c el l to a cc ur at el y d ef in e the i ll um in at ed
a re a a nd to p re ve nt p er ip he ra l c ol le ct io n. F or this p ur -
pose t he I n2 03 d ep os it io n m as k i ts el f w as u se d.
Results
E m is s io n S p ec t ro s co p y a n d M a s s S p e ct r o me t r y
A n e mi ss io n s pe ct ru m of a d ep os it io n u si ng the s il an e
tank with the highest chlorosilane content is shown in
Fig. i. T he s ila ne p re ssu re w as i0 0 m T o r r and a n r f p owe r
of 15 W was employed. Emission from two different elec-
tr oni c t ran si tio ns ( B' 2A- ~ X2 ~ a nd B 2 E + ' ~ X 2 H ) of SI C1 is
identified (5). The state X2R is the electronic ground
state of SIC1. The pairs of numbers in parenthesis in
Fig. i g iv e the v ib ra ti on al q ua nt um n um be rs o f the e xc it eda nd g ro und s ta tes , r es pec tiv el y. Th e s ym bo ls QI' PI' a nd
P 2 g iv e t he b ra nc h o f t he s pe ct ru m, w hi ch is d et er mi ne d b y
t he c ha ng e i n r o t at io na l q ua nt um n um be r.
Al so s ho wn is em iss io n fr om at om ic s il ico n, tr ans i-
tions I and UV 43 (6). The emission line UV 43 is the
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Effects of Monochlorosilane on Amorphous Silicon 873
I i l l T-~ i i : rSi uv43
500 t~m SLITS2000 5 nm/ rnin
SiCI I seclC
e' 2A*x2n
"6 SiC1QI I P2 A2]~.~X2 H
i IOOC Si Iz
-z (LO) I~,o) (i, ~o,o1 oyJ~(~ . _ ~ j ~ u ~ [ ,i,,,,,2)i /I (o,2)
V \ A I/UI# ~,2)/(o,4)~ V , J v p , / ~
[
W/e,VELENG13~ Into)
Fig. i. Em iss io n sp ect ru m fr om an r f d isc ha rge in s il an e
s ho wi ng S IC 1 a nd Si l in es . S ee text f or c on di ti on s.
s tr on ge st i n t he s ll an e g lo w d is ch ar ge s pe ct ru m a nd h ad a n
intensity of approximately 1.5 x 105 counts s-I for the
s p ec t ru m s h ow n .
The mass spectrum of the silane from the tank with
the largest SlCl emission had peaks at m/e values of 63,
64, 65, and 66 (e.g. 28Si35CIIHx or 28Si37CIIHx, x=O,l)
w ith a mp lit ude s in the a pp rox ima te ra tio s of 2: I0: 2: 3.
The mass spectrum of a tank with no detectable SiCI emis-
si on d id n ot h ave th es e pe ak s. T he s ign al -to -n ols e r at io
for the peak at m/e of 66 was approximately five. Peaksat m/e values of 67 and 68 (e.g. 28S137CIIHx, x=2,3) were
either not present (implying H 2 is split off in the ioni-
ze r), or we re too we ak to be d et ec ted . Th e s tro ng est p ea k
(at m /e of 64) ha d a n am pli tu de of ap pr oxi ma tel y 1 /4 00 o f
the principal peak from silane (at m/e of 30). Since the
d ete cto r ef fi cle nc les o f t he se s pe ci es ar e n ot kn own , w e
can only estimate the concentration of SiH3CI to be i in
400, i .e . 2 50 0 ppm, w it hi n a n or de r o f m ag ni tu de . N o p e ak s
were seen for m /e values of 98-103, as would be expectedfo r dl-, tr i-, a nd te tra ch lor os ila ne (7). A s a c he ck , t he
mass spectrum of a mixture of 1% dichlorosilane in argon
w as m ea su red an d th e pe ak s at m/ e o f 9 8- 10 3 w er e d et ec ted .
F ur th er mo re , n o S IC I e mi ss io n w as o bs er ve d w he n I000 p pm o f
d lc hl or os il an e w as a dd ed to a h ig h p ur it y s il an e d is ch ar ge .
T he se r es ul ts e st ab li sh c on cl us iv el y that the p ar en t c om -
p o un d o f t h e SiCI e m is s io n is m o no c hl o ro s il a ne .
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874 Delahoy, Griffith, Kampas and Vanier
D a rk C o nd u ct i vi t y a n d P h ot o co n du c tl v lt y
A rr he ni us p lo ts o f the d ar k c on du ct lv lt ie s o d o f two
films are s hown in Fig. 2. The dashed llne represents a
film prepared from a silane tank w h i c h w as found to con-
t ai n S IH 3C I. T he a ct iv at io n e ne rg y f or the straight sec-
tion of the plot above r o o m temperature is EG = 0.96
eV. A no th er e st im at e o f the r oo m t em pe ra tu re F er mi e ne rg y
E F relative to the mobility edge is given by AE = kT in
(Go/Gd(T)) = 0.78 eV (at T = 300 K), where the minimum
metallic conductivity is assumed to be GO = 200 ~ - Ic m -I. T he so li d l ln e r epr es ent s a f il m p rep are d f ro m the
same t an k of s il an e u n d er i de nt ic al d ep os it io n c on di ti on s
except that the s il an e w a s f ir st p as se d t hr ou gh the f il te r
f or r em ov in g c hl or os il an es . (OES m ea su re me nt s s ho we d that
th e fi lt er w as ef fec ti ve in re duc ing the S iC l p ea ks b e l o w
the l ev el o f d et ec ta bi li ty .) In this c as e, t he two e st i-
m a t e s of the Fermi level position were E~ = 0.75 eV a nd
AE = 0.55 eV. By either method of estimating EF, the
r em ov al o f S IH 3C I f ro m the g as s tr ea m r es ul ts i n a n u pw ar ds hi ft o f E F b y a bo ut 0 .2 2 e V i n the f il ms d ep os it ed .
In Fig. 3 the photoconductlvlties GD of the same
two films are plotted against reciprocal temperature.
T he re a re t wo s tr ik in g d if fe re nc es b et we en t he se c ur ve s.
First, the filtered silane produces a film with a room
t em pe ra tu re p ho to co nd uc ti vl ty t hr ee o rd er s o f m ag ni tu de
h ig he r t ha n i s o bt ai ne d u si ng u nf il te re d s il an e. S ec on d,
t he f il m p r od uc ed w it h the f il te r e xh ib it s a s in gl e v al le y
in the photoconductlvity p lo t w he re as the f il m d ep os it ed
f ro m u nf il te re d s il an e s ho ws a m or e c om pl ic at ed t em pe ra -
t u re d e pe n de n ce , w i th two v a ll e ys .
By adding various amounts of B2H6 to silane from a
ta nk w h i c h co nt ain s no S iH 3C I , we f oun d th at 1 p pm B2 H 6 in
t he s il an e p ro du ce s a -S i: H f il ms w ho se d ar k c on du ct iv it y
and photoconductivity is v er y s im ila r to th e f il ms de pos -
i te d f ro m the s il an e c on ta in in g 2 50 0 p pm S iH 3C I.
E ff ec ts o f S IH BC I o n D e vi ce s
We turn now to the effects of SIH3Cl on device per-
formance. In Fig. 4 we compare the illuminated J-V c h a r -
a ct er is ti cs o f t wo I n2 03 /P -l -n s ol ar c el ls , d ep os it ed w it h
and without the chlorosilane filter. The illumination
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Effects o f M o n o c h l o r o s i l a n e o n A m o r p h o u s S i l i c o n 8 7 5
- 3 I I I I
- 4
--~" - 5
T - 6
b
~ - 8
- 9 \ ~,---Wl THO UT FI LT ER
- I O 1 I \ I I I
3 4
I O 0 0 / T (K - I)
Fi g. 2. A rr he n iu s pl ot s o f o d f or tw o f il m s g ro wn f ro m
t he s am e t an k of s i la ne u nd er id en t ic al re a ct or co nd i -
t io ns . S ol id l in e - s il an e w as f il te re d to r em ov e c hl or o-s il an es . B ro ke n l in e - wi th ou t f il te r.
- 3I ' I I I I I I I I
- 4
- " - 5'E
T - 6
b
- 9 "~
I I I I I I I I I
- 1 0 2 3 4 5 6 7' 8 9 I 0 I I 12I O 0 0 / T (K -I) "
Fig. 3. Ar r he n iu s pl o ts o f a p f or th e s am e tw o fi lm s
s h o w n i n Fig. 2. P h o t on f l u x ffi 10 1 4 c m - 2 s-l; X ffi 6 0 0 nm.
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876 Delahoy, Griffith, Kampas and Vanier
In203/p-i-n/ss15W, IOOmTorr, 180=C
-I.0
J (mAcrn 2)12.0
/
l.OVWITHOUT . / /
FILTER / / / /
~ ' ~ WITH FILTER~
- 1 2 0
Fig. 4. I llu mi nat ed J -V c har act er ist ic s of I n20 3/ P-i -n
solar cells a and d in Table I, showing the effect of
monochlorosllane.
intensities were 99.2 mW cm-2 and 97.6 mW cm-2, respec-
tively. The photovoltalc parameters of these cells are
given in T a b l e I (cells a and d). Short-clrcult current
de nsi ti es ( Jsc 'S ) a re no rma li zed to ex ac tly I 00 m W c m- 2
i nc ide nt in ten si ty. We see that th e c el l d ep os it ed f rom
u nf il te re d s ll an e e xh ib it ed a p oo r f il l f ac to r ( 0. 34 0) and
a s ome wh at lo w cu rre nt d ens it y ( 6.9 6 m A c m- 2). Up on fll-
tering out the SiH3CI , however, the flll factor FF in-
c re ase d dr am ati ca lly to 0 .5 39 a nd Js c i nc re ase d t o 7 .4 6
m A c m -2, r e su l ti n g in a n i n cr e as e o f c o nv e rs i on e f fi c ie n cy
f rom 1. 90 % t o 3. 34% . (The In 20 3 w as de pos it ed o nt o c ell s
a , c, a nd d si mu lta ne ous ly, so d if fe ren ce s be twe en th ese
c el ls c an no t b e a tt ri bu te d to v ar ia ti on s In the t ra ns mi s-
s io n o r r es is ti vi ty o f t hl s l ay er ; t he I n2 03 o n cell b w as
d e po s it e d s e pa r at e ly , a l th o ug h u n de r t h e s a me c o nd i ti o ns . )
Similar results regarding Jsc and FF were found for
Pd/p-i-n cells (8). The respective Jsc'S of In203 andP d c el ls d epo si ted fr om fi lte re d sl lan e w ere t yp ic al of
cells deposited from tanks of sllane that produced no
d ete ct abl e S iCI e mi ss io n. In t ho se c el ls, SI MS a na ly sis
o f the a -S i: H i nd ic at ed a CI c on ce nt ra ti on of the o rd er o f
2 pp m. In c el l d, de po sit ed fr om the un fil te red sl la ne,
4 00 -6 00 p pm CI w as f ou nd .
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Effects of Monoch iorosilan e on Amorphous Silicon 877
TABL E I. Parameters of In203/P-i-n a-Si:H Sola r Cells as a
Function of SIH3CI Concentration in the Pla sma
a-Si :H
Dep. T hi ck - Av. Dep.
Cell Silane Time hess Rate Voc Jsc FF ~]
(rain) (X) ( ~ s-11 (V) (mAcro-21 (%)
a Filtered 210 6000 0.48 0.830 7.46 0.539 3.34
b Partially 170 7400 0.73 0.826 8,81 0.526 3.83
Filtered
c Partially 210 9300 0.74 0.816 9.77 0.423 3.37
Filtered
d Unfiltered 210 9600 0.76 0.804 6.96 0.340 1.90
At s om e p oin t d uri ng the de pos it ion of the a -S I: H for
c e l l s b and c, SiCI emission appeared. By the end of the
r un th e i nt ens it y h ad sl owl y in cre as ed to ab ou t that f ou nd
fo r un fi lte re d si lan e, th us si gna ll ing s at ur ati on o f the
filter. (A f reshly charged filter w as used for cell a.)
I t is e st im at ed t ha t the C I c on ce nt ra ti on i n t h es e c el ls is
gr ad ed in s om e fa shi on f rom
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878 Delahoy, Griffith, Kampas and Vanier
as i ppm B2H 6. Monochlorosilane also increases the film
de pos it ion r ate , l ike di bo ran e (9). T hi s re sul t i mpl iesth at the p os iti on o f the gr ow ing fi lm F e r m i l ev el a ffe cts
the deposition rate, since it is unlikely that B2H 6 and
S iH sC I h av e a ny c at al yt ic p ro pe rt ie s i n c o mm on .
T he l ow er p ho to co nd uc tl vi ty o f a -S i: H f il ms d ep os it ed
f ro m u nf il te re d s il an e is a c on se qu en ce of t he l ow er F er mi
le vel po si tio n (3). T he va ll ey i n the t em pe rat ure d ep en-
dence of Gp (with filter) is largely a result of competi-
t io n b et we en two d is ti nc t t yp es o f r ec om bi na ti on c en te rs .A schematic of the density of states in the gap needed to
e xp la in this type o f d at a as w el l a s the r el at ed p he no me no n
o f i nf ra re d q ue nc hi ng (I0) i s s ho wn i n Fi g. 5. In a dd it io n
t o e xp on en ti al b an d t ai ls t he re a re two d if fe re nt t yp es of
states in the gap labeled I and 2. States 2 have a larger
electron capture cross-section than states i. A lower
Fe rmi le vel r es ult s in a hi ghe r co nc ent ra tio n o f h ol es i n
states 2. The resulting higher recombination rate then
lowers the photoconductivity. The origin of the secondv al le y i n G p ( wi th ou t f il te r) i s n ot c le ar w it ho ut f ur th er
e x pe r im e nt s ( i i) .
A simple explanation of the device results shown in
F ig . 4 i s t ha t t he l ow er ed p ho to co nd uc ti vi ty o f f il ms m ad e
f ro m u nf il te re d s il an e r e su lt s i n a l ar ge s er ie s r es is ta nc e
(4100 ~ cm2) in the device. However, spectral response
s tu die s h ave sh ow n t ha t th e si tua ti on i s mo re c omp li cat ed
(8). As far as the variations in J sc for the cells of
T a b l e I are concerned we conclude the following. The in-
crease in Jsc in passing from cell a to b to c is due to
en han ce d l igh t ab so rpt io n (for ~ > 600 nm) re sul ti ng f ro m
the increased cell thickness. In competition with this
trend is the progressive reduction in zero-bias quantum
e ff ic ie nc y f or % < 6 00 n m w it h i nc re as in g S iH sC I c on ce nt ra -
t io n i n the p la sma . E ven tua ll y t hi s e ff ect d omi na tes an d
accounts for the lower Jsc of cell d. The progressive
r ed uc ti on i n F F f or c e ll s a -d is d u e to the p ro gr es si ve r e-
du cti on i n q uan tu m e ff ic ien cy f or ~ < 60 0 n m wh en a fo rwa rd
b ia s i s a pp li ed . T he se r ed uc ti on s i n q ua nt um e ff ic ie nc y in
p -i -n c el ls a re c au se d b y i nc re as ed e le ct ro n r ec om bi na ti on
(8) a s i nd ic ate d by t he de cr eas ed p ho toc ond uc tlv it y. Fi -
n al ly we r em ark t ha t in t he ca se of n- i-p c el ls ( li gh t in -
c id ent on the n l aye r) it i s po ss ibl e that a l ow ere d F erm i
l ev el w ou ld i mp ro ve h ol e t ra ns po rt to t he b ac k of the c el l.
I n t ha t c as e the p re se nc e o f S iH 3C I m a y b e b en ef ic ia l.
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Effects o f Monochlorosilane on Amorphous Silicon 879
\ \- \
\ \ \ \
\ I
t
1!6 I 112 I O I J 0 . 4 0 . 0
AE[eV)
Fig. 5. S ch em at ic o f p ro po se d d en si ty of s ta te s i n hy dr o-
g e na t ed a m or p ho u s s i li c on .
All of our results can be explained by the weak p-
type doping caused by monochlorosilane. However, the
m ec ha ni sm b y wh ic h this o cc ur s is u nc le ar . W e c on si de re d
t he p os si bi li ty that a C I c on ta in in g s pe ci es in t he p la sm a
et che d t he AI el ec tro de s t her eb y i nt rod uc ing AI in to th efi lms . H ow eve r, SI MS a nal ys es of a- Si: H f il ms de po sit ed
fr om v ar iou s t an ks of si la ne r ev ea led n o c or rel ati on be -
tw een S iH 3CI co nce nt rat io n i n t he s il an e a nd A I co nce n-
tration in the film (which ranged from roughly 2 ppb to
2 p pm ).
A st ud y of the p ro pe rti es o f a -Si :(H ,C I) de po sit ed
from SiC14 + H 2 (12) is not useful in understanding the
effects we report here. Those films contained 1.5 to 8%
ch lor in e a nd h ad ac tiv at ion en er gie s ra ng ing f ro m 0 .3 to
0.8 eV. (It was apparently assumed that the films were
n-type.) In the same study a-Si=H films were prepared
from silane in a different deposition system. The dif-
f er en ce s i n d ep os it io n s y st em s, s ta rt in g g a se s, a nd d ep o-
s it io n p ar am et er s m ak e it i mp os si bl e to d ed uc e the e ff ec ts
o f C l a s a n i mp ur it y i n a -S i: H f ro m t ho se r es ul ts .
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880 Delahoy, Griffith, Kampas and Vanier
A n ef fec t o f ch lo ros ll ane i mp ur iti es i n d is il ane on
a-Si:H film photoconductivlty was reported by Scott et
al. (13). They also found that films containing small
amounts of chlorine have lower photoconductivity than
f il ms w it h n o d et ec ta bl e c hl or in e.
T he l ow d op ing e ff ici en cy o f Si H3C I i ndi cat es that
o nl y a sm all f ra cti on o f the ch lor in e a to ms c an b e ac tin g
as a cc ep to rs . It m ay a ls o b e that the d op in g m ec ha ni sm is
i nd ir ec t a nd t ha t t he p re se nc e o f S iH 3C I in the d is ch ar ge
alters the concentrations of non-chlorlne containingdefects, thus moving the Fermi level. The fact that
S iH 3CI is no t a vai lab le co mme rc ial ly r ai ses a l ar ge o b-
s t ac l e to f u rt h er i n ve s ti g at i on .
Conclusion
T he e ff ec ts of m on oc hl or os il an e i mp ur it y o n g l ow d is -
c ha rg e d ep os it ed a -S i: H f i lm s c an l ar ge ly be u nd er st oo d in
t erm s o f i ts p -t yp e d opa nt pr ope rt ies . Th e d ete ct ion oft his i mpu ri ty in se ve ral c om me rci al ta nk s of si la ne a nd
the deleterious effect it has on solar cell efficiency
w ou ld in di cat e a need f or c ar e o n the part of the s il ane
u se r as we ll as t he s il ane m an ufa ct ure r. Two a nal yt ica l
t ec hn iq ue s, m as s s pe ct ro me tr y and g lo w d is ch ar ge o pt ic al
e m is s io n s p ec t ro s co p y, a r e a d eq u at e i n s e ns i ti v it y .
Acknowledgments
We sh oul d l ik e to t ha nk Dr. Zvl Ov ady ah u fo r a dv ice
c on ce rn in g the d ep os it io n o f I n2 03 a nd M r. R ob er t G re mm e
for t e c hn i c a l a s s i s t an c e .
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Effects of Monochlorosilane on Amorphous Silicon 881
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