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Materials ChvzisrrJ, and PkJ,sics, 20 (I 988) 38 1 -396 381
'fin 0;:ide thin films, specially antkony doped tiri oxide iA'l'0)
and i'luorine doped tin oxide (FTO) have attracted considerable
attention over the past r"e~r years became of their possible
ph0toVoitaic applications. The se i"il!r!s a rz transparent as ueil
as conducting and in addition, have the abiiity to i'orhl very
hi& basriers on silc':z seniconductors as silicon. In the oresent
stitdy a nodified spray nyroiysis CiC?thOd is ?isci?,ssed 0-y y!:icl~
the 3nO 2 2ihs can bs prepared conveniently anL reproduciuiy.
tin tile uasis oi the pyroij-sis reaction 03.' LnGl. ii 511*0, s.:1oot1:,
transparent and closely ad!iemr,-c r'ii::o 3re 0btaineG m glacs.
I;‘rO!Ei tila -4 LL-- *a? uif?raction 5Lude:; the fiims arc found to be
polycTystallinc in nature. i?‘he electricai and o$ical propi‘rties
oi these f'ims are stuciec: to optkise tr.e uegosition pl;r&haters
-'Or acl,ieving tile :naiAmim _'igure of writ i"or solar cel.1 ap3lica-
tion. .T.
i~:aXir:iLl?Il f. igwe 01 ri.L _ vf?rit obtained for A',"0 Pilrn is G.qAO -3 -1 R .
The corrcsnonding Kim properties are ;;i Sk!
= &O (sheet resistance),
‘i’( 600) -I 337; for the lilns Loped with 3 ~101 $ of SbCl 3
and 2000 3
thick. 'The maximm figme ol^ merit obtained 1'01‘ I?TG I'ilni is
?.L@xlo-2L$ (H I Sh
= 2O",'cs, T( 600) = Y3 corresponding to a
I"ilm. ti:i.ckr.ess ol I$%0 a ;iith 1.2 sit ;"b i;h 1 F.
+?resent address Orissa (India)
: Regionai Research iaooratory, Ljhubaneswar,
0 Elswier S~quoi~/Print~d in The Netherlands
'Tin oxide is a wide band gsp scrziconductor (9; = 2.3 eii) and
has round widespread use because of its hi@ piectricai conAuctivity
in combination with transparency in the visible anti near inirared
v:avelen@h rafit;es. Its optical and electrical properties can be
tailored to suit the application o? interest by aglropriate doping.
ksearch on transparent COndilCtifig tin oxide filrx kive 'oeen
intensified reccctly o::in:; to their prociicing applications in the
fsbricstion ol" hnterojunction solar cells. Particularly the Sn02/Si
uhotovoltaic system [I ,2,3_5 ::ith weil-known attractiva ie,-itures
iksve dram consitief~ble attention. 'ihe hetarojunction sol::r ceil
offers the possiuility of fabrication ol‘ low cost solar cells rrith
perfor.;!ance characteristics saltable ior l:irge scale terrestrial
applications. One possibility for cost redilction lies in the
clethod of junction fabrication.
There are many techniques, inciudin;; sputterin;; CL+], Tlash
evaporation [L+J, reactive evaporation [5] chemical vapour deposi-
tion [67_ and spray pyrolysis 171 Zor depositing these nlaterials.
ii'lle inZluence of spray conditions on the quality of tile conduct-
ing ?i.l:r, has also been studied [Sl . The spray deposition method -_
is particularly attractive because of its sinlplicity. It is Yast,
inexpensive and does not require a vacuim. The spray p;rrolysis
1)rocess is suitable <or LEIS~ production. Eowever, problems with
uniiorkty and reproducibility are occasiona;ly encountered. On
the other hand, the usual chemical vapour deposition in a standard
tube furnace can 1236 to :aore uniform and reproducible r^ilms. In
the present study, combined snra.7 , and CVD methoGs based on SnCl 4
511,O hydrolysis reactions are used to deposit the r^ilms conveniently
and reprohcibly.
3:peri;iental set-up and r"ilm deposition
The spray pyrolysis of the Sn02 Tilms were carried out in a
specially fabricated spray/C'JD apparatus. Figure l(a) depicts the
schexxtic disgrm or" ti?s r"xrnaW asse!ilbiy. Figure l(b) shows the
spray ci used ior spraTyin&. Spraying was cond.ucted in a quartz
383
TO VACUUM
f/-- 5.5. TUBE
+-THERMOCOUPLE /If MULLITE TUBE
INSULATING MATERIAL
SPRAY NOZZLE
100 ml CONICA FLASK
Y=
(al FL SPRAY NOZZLE (bl
1.
Pli. 1. \a) hchexatic ui.agra.;l of the spray asse:,ibly.
(b) Spray gm used.
tube kept centrally in the furnace. The smple llolder,rrhicl~ is
a stainless steel tube, was also kept centrally in the qilartz tube.
The sample holder was connected to a vacua line to hold the sa!:iple.
An iron-constantan thermocouple was attached verjr close to the
xu'ostrate to monitor temperature during deposition. The relative
positions of the sample holder, the quartz tube and spray gun
can be varied to optimise the distance or" the s.ubstrate fror;i the
spray nozzle for efficient deposition. The nozzle diaxeter ol" the
spray gun was 1 .mn. An Slice air compressor which delivers dry oil
free air at a pressure or" 1 kg/cm* was used to spray solution on to
the substrate.
To obtain a homogeneo?Ls _'ilm, th Lroplcts ol the spray :;ere
evaporated b,?I'ore reaching th? hexted substrate. By spraying
upward in the tube furnace at a slow rate, all the droplets were
evaporated beYore reaching the substrate.
384
Prior to deposition, the glass slides (25 .mm x 25 mm) were
treated in a chromic acid bath for 24 hrs to make them grease free.
They xere then ::ashed with distiilcd water and dried. Finally
they were ultrasonically cleaned in acetone.
Different solution mixtures were investigated for the deposition
of Sn02 films. For the deposition of FTO films the spray solution
containing SnLl i;, 5H20 and kdh4F in ethyl alcohol and water was used.
'The concentrations of the SnCl li, 5H,O and XHq? were varied to have
different doping density. Composition of a typical spray solution
containing 1.5 vrt $ NH 4 F was
SnCl 4' 5y - 14.250 &!TlS
NH&F - 0.750 grill
IG0 - 1g.000 gns
c211501i - 15.800 Ems
IICl - 0.200 &!Tl
For AT0 film deposition the
and SbCl 3
in ethyl acetate.
PO deposit films of uniform
spray nozzle and the substrate
spray solution was 0.7X SnCl 4,' 5"*0
thickness the distance between the
was optimized and found to be 21 cm.
Glass substrates were kept in the hot zone of the furnace maintained
at 51;O'C for about 10 minutes before the spraying period (about
I set), followed by a 30 sec. wait to avoid excessive cooling of
the hot substrate during spraying.
Characterisation
Thickness of the films was determined by weighing the substrate
before and after deposition in a ic!ettler balance having a least
count of 0.00001 gym and taking the density of SnO2 as 6.99 gms/c.c.
The A-ray diffraction measurements were performed using a Philips
automatic power diffractometer with Cu and MO target, graphite mono-
chromator autodivergence slit, scatter slit and receiving slit
assembly. The target was operated at 40 KV, 20 mA for Cu-target
385
‘I?!e Tarticle size ;ras &temi.ned using the debye Sciierrer
equation
ir = iXl.l width at half !naxirn-ml ( Fyl;;Ii4) P h = 0.709 A for i.:oi< cI radiation
= 1.541 a ior LkJ_K~ rakLiation
l;or zindin g out the F;?iki, the peaks ::ere scanned wi'ch a rate o_"
6.25’ 28/iG_n.
Tile eLectrica characteristics of the filr:is sue!1 as conductivity
type and sheet resistance were determined by using hot probe and
four-point probe methods respectively.
The optical properties ol' SnO, fibs Vjere studied by recording ."
the trans!:iission o_' t!!r. iil.r~is dEposited onto t;ILass substrates for
various thichess.
'i'he transmission ratio ior two I'ilrzs 0T different thickne ss
is
where AC! is the difference in the thickness of the two I"ilms.
A kary 17, ;&:ici? is a double beani spectrophotonater rrias used in
this investigation. .4 properly cleaned blank &lass sitbstrate ol
tile sa!;le size was used as reference. 'l'he optical densities of the
different filns with refermce to the blank substrate were recortied
at roozi teztperature in the wavelen@A range 280 m to 750 n:;i. -l':ile
transmissions ol" the di fferent filn thicknesses were carried oilt at
600 nm to find out the figure 0Z !ilerit using a PYE UNiL:Ai*I Uii and
visible spectrophotorneter.
Yoth the conductivity and tranmission ol the fiims si~o~ulc! be as
hi& as po3sibI.e for solar cell applications. However, they are
inversely proportional to each other. Hence the optimm values or"
these two paraEleters shouid be established asing a figure of merit.
386
The most comonly iXf?d definition of figure 0Z merit $4Tc of a
trai-Lsparent coz&cting fil!:l given by :;aacke [jc] is
$Tc + (31 S!l
where ‘7 is the tranmission at a particular wavelength. 'The trans- . . .
mlsslon 1s a function of T;ravelength,an, ri therefore use of T (average)
lor calculation ol" gTc is inappropriate because the solar flux is
concentrated in a small wavelength range near green. The spectral
response of Sn02/Si has been reported pi] to be !naxi%in near
600 nm. Therefore, the transmission T(bO0) at a wavelength of
600 nm was used to calculate pTc in this study.
RSSULTS Alvll DISCUSSION
Smooth, transparent and closely adherent films of SnO2:F(FTO)
and Sn02:SbtATO) were obtained. Below l+50°G substrate temperature
the fiti appeared to be patchy. Figure 2 shows that the thickness
increases as a linear Cnction 05 the deposition the. it is also
seen from Fig.2 that the growth rate of the Sn02 film at the consi-
dered temperature is independent oZ film thickness. 'T!?ese results
6000- o AT0 Films
A FTO Films
so00 -
4000 -
3000 -
2000 -
lOOO-
I I I I I 4 8 12 16 20 24
NO. OF SPRAY
Fig. 2. Increase of film thickness as a linear function of the deposition time.
387
i:.licate t,5,Jt th.7 I lil!.i ,;ro?!tl? __ 'i'OCZSS 011 the >nC; 2 surface progess-
ed 'u;Ci>'A;,- ;‘: ;;itl-j ti<,qe ‘_i _ u . T!I 3 3 ~?_Jl’o:~i.mt~2 Lgl‘C:7th rste obtained i^ro!n
li.2. 2 Por AT0 and FTC arc 250 t/spray respectively.
Both AT0 an< FTO fiI!:ls mre f oiind to be polycrystalline in
nature as eviticnt from the dL-r3.~y <iffraction pattems in Pig. 3.
b 50 I
- 40 28 30 i p10
30 2o 28
1
Fig. 3. L-ray diffraction patterns (a) CuK, diffraction pattern of FTO film, (b) MoK, d iffraction pattern of FTO fil!n, and (c) MOK (L diffraction pattern of AT0 film.
It has been observed that the pyrolytic spray nethod induces an
ano:.alous textiu-e as seen in the suppression of (110) and (101)
iines which are norma_Lly strongest. In Sn02 powder, the lines
(IlO), (101) and (211) are the strongest with relative intensities
100, 81 and 63 (131 respectively). These results are only
sli&htly different from those obtained by Karlsson et al. p2] -I_
and by Kohatgi et al. 13 13.
tising Cuba. radiation for undoped Sn02
388
Zii!:is obtained by a s‘ pray p-,rolj;;is mthod. ~ELohatgi. et al. jj 3-j
Zound the (119) iinz to b- txice as stron- " iis the (211) ,Vrhereas
Karlsson et al.[l21 Zounc! --_ the (211) to be the strongest line
?oll~~wed by (IOI), (110) and (200). in oilr result the (211) line
is observed to be strongest follo?:ed by (200)) (110) and (101) for
FTO I"ilm. For AT0 films the (110) and (211) lines are seen eqilally
strong followed by (101) and (201) lines. This difference in resuit
may be attributec? to higher substrate tenperature Ts=jL+O'i; as com-
pared to Ts=/+OO'i I"or other irorkers.
Particle size corresponding to reflection Irom (110) plane was
calculated. using eqn.(l). As shovm in Fig. L+ particle size increas-
es with thickness. The increase in the residence time of the
3 50
04
23Oc
0 AT0 Films
A FTO Films
THICKNESS 18 1
Fig. I.+.. Variation of particle size with the film thickness both for AT0 and FTO films.
389
3 )c ;i:!en in ch2 furnace ior t!:n tllicker fiims niight be a re3son
_^.ji' t!!? inc:*o,is2 0P griiri siz3. FTO _'ii::is have larger grain size
compared to the AT0 films. The observed grain size is comparable
with the reported vaiue available in the literature I19
Lath FTO and AT0 f'ilms l::ere found to be n-type as expected l:hen
tested by the hot probe technique.
It has been observed that the sheet resistance (ils,l) decreases I
considerably 3s thz substrate temperature increases jFig.5). This
0 AT0 Films A FTO Films
350 400 450 500 550 600
Ts t°C 1
Fig. 5. Variation of film sheet resistance with the substrate temperature.
decrease in Hsh is owing to the improvement in the crystallinity
oi' the films at higher Ts. Also at low substrate temperature some
SnO and/or Sn30k (which are both highly resistive) may be present
in the film due to incomplete reaction resulting in a relatively
!ligh R sh PI-
Shr:Tt r?sist,jncl? of both FTC and AT0 films decroxs~ s with
increasing thickness of the iiLxs jFig.6). 'This is due to the
increase in grain size 3,.:ith thickness p 5j whicn is verified from the particle size measurements carried out by I-ray diffraction
methods.
90-
- 80- 0
2 70-
5 60- a _
z so-
2 =; 40- si
ii 30-
L w z zo-
lo-
o AT0 Films
A FTO Films
0 1000 2000 3000 4000 5000 6000 7000
THICKNESS d 1
Cig. 6. iariation of sheet resistance with t!ie thickness of the film.
Sheet resistance vs doping concentration is plotted in Fig. 7(a) -
and Fig. 7(b) for both AT0 and FTO films. As shown in Fig. 7(a)
R sh
initially -decreases, goes to a minimum and then increases
again as the antimony content increases. Lowest sheet resistance
is obtained for 3 mol$ of SbCl 3’
The increase in the carrier
concentration as a result of doping is responsible for the initial
decrease of R,h. However, since the ionic radius of antimony is
391
70-
(b)
30 -
101 I I I I I _I
0.0 0.5 1.0 1.5 2.0 2.5 3.0
wt (%I
Yi;. 7. Sheet resistance versus dopinE concentration plot (a) for -.-
.+'I0 :ims and (b) I"or P'i'o :ii.:ls, prepared at the substrate temp. 5!!+.ooc; for 8 spray sec;i?ences.
about 1.2 times that of tin, the crystal defects which create the txp
leve.Ls in the forbidden gap also increase with doping [12]. Thus
above a criticai antimony content the trap concentration doxlinstes
the concentration or" donated electrons, and on 5urther dopinz the
nntirnon-y introduces traps rather than donors. Hence 1% Sh
increases
with antimony content above 3 nol,; ol" SbCl . 'this phenomenon ~13s
also observed by other workers [lo] and thz7 sug.C;ested that the
pronounced increase in resistivity at larger dopant concentration
is probably caused by the incrc:ise or" impurity scattering. In case
ol" FTO films the rfiininum sheet resistance is for 1.5 wt.,: M4F and
392
tic 2 s,l increases very SlOVJl; at higher doping concentration. I
Ti:is decrease in R sh
and its subsequent s:iturstion can be attribut-
cd to ti?s loWsin& of the E;rai.n boundary potential due to incorpora-
tion of F atoms into the grain boundaries PSI. L ,-
The films are alrlost perfectly transparent in the visible region
and the fundamental absorption begins at about 0.3~)rn extending to
a shorter wavelength. For about 1000 A" thickness both FTO and AT0
film show inore than 95;; transniission at 600 nm. As the thickness
of the filn increases, the transparency of AT0 films decreases
drastically (317: for a thickness of 5100 i). For PTO filns the
variation in the transparency (70,; for 4700 dgj is less compared to
ATC ?il:ns.
In both the cases transpasency decreases as the dopins concen-
tration increases. Figares 8(a) and 8(b) show the energy depen-
tiance of CL 2
i'or AT0 and ST0 films respectively. 2
'The piots of iL
a;;ainst h$ have a linear region, and extrapolation of the Stl*aiC;ilt
line to a = 0 gives ti:e direct band gap. EC is 3.875 eti for AT0
and 4.065 eV for FTO filrls. These values of direct band gap are
cozparable with the values reported by Arai 116-J and Bhmradwaj
et al.[Ill . - -_ Figures 9(a) and 9(b) depict the figure of merit of AT0 films
for different film thickness and doping concentrations respectively.
Figure of merits for FTO film are plotted in Fig. IO(a) and Fig.
IO(b).
In case of AT0 films $JTc is almost constant for the film thick-
ness in the range 1000-2000 I: and decreases sharply as the fila
thickness increases. pTc increases rapidly up to the film thick- 0
ness of 1900 A for FTO films and falls rapidly as the film thick-
ness further increases. P, Tc increases initially because the Rsll L
decreases rapidly while T decreases slowly. At higher thickness
beyond 2000 8 Rsh decreases slowly but T decreases rapidly and thus
pTc falls rapidly.
The figure of merit increases slowly for AT0 film up to 3 mol$
of SbCl 3
and rapidly for FTO filns up to 1.5 wt$ of NH F and 4
finally both fall rapidlyy if the doping concentrations are increas-
ed further. R sh initiaily decreases ver:/ rapidly and transmission
20
16
"E : 12 AT0 Films (D 0 x 8
"ti 4
0 6.20 4.13 3.10 2.40 2.07 1.77 1
PHOTON ENERGY (ev)
J I.55
60
N
E 60 ”
h 0 40
” 20
3 0 6.20 413 3.10 2.48 2.07 1.77 1.55
PHOTON ENERGY IeV I
1000 2000 3000 4000
THICKNESS d I
Fig. '3. variation or" r^igdre or" r3eri.t _'or AT0 iiirns (aj with r^ilm thickness at the &oping cont. concentration : film thickness
3 3ol$ of S'cC$ and (b) with doping = 2000 8.
394
lo-!
2 - -I4 210
8
lo-5
\ (a) (b)
I I I 01 ’ I I I I
1000 2000 3000 4000 5000 1 2 3 G 5
THICKNESS (i 1 Mel L % I
6-
Fig. IO. Variation of figure of r,lerit for PTO filzs (aj with thickness at the doping concn. oi' 1.5 wt,; oi' XH doping concentrations at the fi.Ln thickness = 1
decreases slowly. Therefore $ITc increases as the doping concentra-
tion increases. However at higher doping concentration RsI1 increa-
ses and the transmission decreases causing vTc to fall rapidly.
14aximuq $JTc obtained for ATO Elm is 6.96x10-3P-". The corres-
ponding film properties are RsII=&O-x/a , T(600) = 88$ ?or 3 mo$
or” jb 1 3
and 2000 1 film thickness. The maximu! fig.llre of merit
obtained for the FTO film is 24.19~10 -3,C--l L-R Sh
= 2o+,, , T(600)
= 93% -7 corresponding to a Film thickness of 1900 2 with 1.2 wt$
NHLF. The results are comparable with those reported in the
literature vlO,12J .
The modi?ied spray pyrolysis technique was adopted to deposit
fluorine-doped and antimony-doped Sn02 conducting films. Growth
3 A.ijl:aradwaj, IL.S.iialonia, A.Zaza, A.i(.Sllarma, a.X.Gunta and
O.P. O&hot&. Solar Gel13 5 (19S2) 305. *y-3- 4 J.L.wossen, R.C.A. Rev., 22 11971) 289.
5 J. 1 lanifacier, tii.i)eu_wcia, J .?.Filland and S. Yicario, 'Thin
Solid Films, 41 (19771 127. -
6 S. bluranaka,Y.Bands and T.Tskada , Thin solid FilE, 86 (1381) -
Il.
7 3. L!cls!zeirner and 3. Ziegler, Thin Solid Films 109 (19831 72. e..-_~I -
8 T. Karlsson, A. Roos and i;.G. Ribbing, Solar Enerf;Y I;laterials,
11 (lSS5) 469.
9 K. Ishi~uro, T. Sasaki, T, Arai and T. Amai, J.Phys.Soc. Japan,
13 0958) 296.
10 G. Haacke , Appl. Phys. Ls., 3 61976) 622.
I? A. Zlaradwaj, U.K.Gupta, A. Raza, A.K. Sharma and O.P. Ognihotri,
Solar Cells 5 t 1982) 39. _-..-Pf _
12 T. Karlsson, A. Roos and C.G.Ribbing, Physica Scripta, a (19532)
772
13 A. Kohatt;i, T.R. Viverito and L.H. Slack, Rev. Hi&-Temperature
eaterials,m (1976) 139.
IL+ i;i.Srinivas Wr2;hy and S.R.Jawalakar, Thin Solid Films, 102
t 1983) 283.
396
15 K.L.dhopra and S.Xajor ) Proc.Irlt..r ','orkshop on Thin Solid Film II.- --~_I
Technolo,-,y r;nd Applications, Hex Delhi 19-30 110~. 1964, Tata
XcGraw-Kill Publishing Co., India, p. 224-236.
16 T. Arai, J.Phys.Soc. Japan, 2 (1960) 916. .---