delahoy 1982 effects of siclh3 on pecvd a-sih j electr mat

8/2/2019 Delahoy 1982 Effects of SiClH3 on PECVD a-SiH J Electr Mat

1/14

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


2/14

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 .


3/14

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


4/14


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


5/14

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 .


6/14


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


7/14

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.


8/14


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 .


9/14

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


10/14


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.


11/14

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 .


12/14


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 .


13/14

Effects of Monochlorosilane on Amorphous Silicon 881

References

(i) R. W. Griffith, F. J. Kampas, P. E. Vanier, and M.

D. H ir sc h, J. N on -C ry st . S ol id s 3 5/ 36 , 391 ( 19 80 ).

(2) F. J. Kampas and R. W. Griffith, Solar Cells 2, 385

(1980).

(3) P. E. V an ie r, A. E. D el aho y, an d R. W. Gr lff it h, J.

A pp l. P hy s. 5 2 , 5235 ( 19 81 ).

(4) SIMS analyses were performed by Charles Evans and

A s so c ia t es , S a n Ma t eo , CA.

(5) R . W . B. Pe ars e a nd A . G. G ay don , Th e I de nt ifi ca tio n

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e d n. , 1 9 76 .

(6) C. E. M oo re , S el ec te d T a bl es o f A t om ic S pe ct ra S i I,

NSRDS-NBS 3, Section 2, Reference Data Series,N at io na l B ur ea u o f S ta nd ar ds , W as hi ng to n, DC, 1 96 7.

(7) V. S. Ban, Mat. Res. B ul l. IO , 8 1 ( 19 75 ).

(8) A. E. D el ah oy a nd R. W. G ri ff ith , in P roc . 1 5t h I E EE

P ho to Vo lt ai c S pe ci al is ts C on fe re nc e, K is si mm ee , F L

( IE EE , N ew Y or k, 1 98 1) , pp. 7 04 -7 12 .

(9) J. C. Knights, J. Non-Cryst. Solids 35/36, 159

(1980).

(lO) P. E. Va ni er, A. E. D el aho y, an d R. W. G ri ffi th , inT et ra he dr al ly B on de d A mo rp ho us S em ic on du ct or s, A IP

C on fe re nc e P ro ce ed in gs , N um be r 73, e di te d b y R. A.

Street, D. K. Biegelsen, J. C. Knights (ALP, New

Y o rk , 1 9 81 ) , pp. 2 2 7- 2 32 .

(Ii) R ec en t p ho to th er mo el ec tr ic p ow er m ea su re me nt s i nd i-

c at e t ha t the a pp ea ra nc e o f t he t hi rd p ea k i n p ho to -

co ndu ct ivi ty n ear r oo m te mpe ra tur e i s a sso ci ate d

w it h h ol e c on du ct io n (see Fig. 3, a nd A. E. D el ah oy

and P. E. Vanier, Bull. Am. Phys. Soc. 27(3), 268

(1982)).


14/14


(12) W. W. Kruehler, R. D. Plaettner, M. Moeller, B.

Rauscher, and W. Stetter, J. Non-Cryst. Solids35/36, 333 (1980).

(13) B. A. Scott, M. H. Brodsky, D. C. Green, P. B.

Kirby, R. M. Plecenik, and E. E. Simonyi, Appl.

P hy s. L et t. 3 7, 7 25 ( 19 80 ).

delahoy 1982 effects of siclh3 on pecvd a-sih j electr mat

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