Society,Inc.
Inc.
J. Electrochem. Soc.,
144, No.
No. 7,
7, July
July 1997
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© The
Electrochemical
ElectrochemicalSociety,
J.
Soc., Vol.
Vol. 144,
2342
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Chemistry and
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138,
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138, 3592 (1991).
Cadmium Sulfide
Bath Deposited
Deposited Cadmium
ofChemical
Chemical Bath
Optimization of
Thin Films
Films
Isaiah 0.
0. Oladeji
Oladeji and
and Lee
Lee Chow*
Isaiah
USA
Orlando, Florida
Florida32816,
32816, USA
Central Florida,
Florida,Orlando,
of Physics,
Physics, University
University of Central
Departmentof
Department
ABSTRACT
ABSTRACT
We
report the optimization of
grown by
by chemical
chemicalbath
bath deposition
deposition where
wherehomogenous
homogenousreactions
reactionsare
areminminof CdS thin film grown
We report
imized.
The optimum
optimum parameters
parameters have
have enabled
enabled us
us to
to maximize
maximizethe
the thickness
thicknessofofthe
thedeposited
depositedfilm
filmininaasingle
singledip
dipand
andtoto
imized. The
grow thicker films
replenishing the
the concentration
of reactants
reactants while
the substrate remains
remains continuously
continuously
concentration of
while the
films by periodically
periodically replenishing
improved optical
CdS films
films exhibit
dipped in
reveal the
the deposited
deposited CdS
exhibit improved
optical and
and elecelecCharacterization results
results reveal
in the
the reaction
reaction bath.
bath. Characterization
trical properties.
properties.
trical
Introduction
Introduction
known' to be an excellent
excellent hetsulfide (CdS)
(CdS) is known'
Cadmium sulfide
ero junction partner
partner of
of p-type
p-type cadmium telluride
telluride (CdTe)
or
(CdTe) or
erojunction
(CuInSe2 ) due
copper indium
indium diselenide
diselenide (CuInSe2)
p-type copper
due essentially
essentially
window
widely used
used as
as aa window
to its high
high electron
electron affinity.
affinity. It is widely
material
thin-film solar
based on
on
solar cells
cells based
material in high-efficiency
high-efficiency thin-film
CdTe
or CuInSe
CuInSe2
owingtotoits
itstransparency
transparency and
and photoconphotoconCdTe or
2 owing
ductivity
of CdS
CdS
properties. The
The thickness
thickness of
ductivity among
among other
other properties.
film
required for
for the
the fabrication
fabrication of
of these
these cells
cells isis in
in the
the
film required
4
range' of
However,ininother
otherapplications3
applications2'
of 0.05
0.05 to 0.1
0.1 p.m.
,um. However,
meters,
cells, light meters,
CdS, which include
include photochemical
photochemical cells,
of CdS,
well
working thickness
thickness is
image intensifiers,
image
intensifiers, etc.,
etc., the
the working
is well
above
above 11 p.m.
pm.
The
used methods
methods for
for depositing
depositing CdS
CdS thin
The commonly
commonly used
'5
sputtering, spray
spray pyrolyfilms
are vacuum
vacuum evaporation,
evaporation, sputtering,
films" are
pyrolysis,
The latter
latter
(CBD). The
chemical bath
bath deposition
deposition (CBD).
sis, and
and chemical
' 6
method"6
low-cost and
and a scaleable
technique for
for the
scaleable technique
method , isis aa low-cost
CdS films.
films.
deposition of high quality CdS
Although
CBDisis ideally
ideally suited
suited for
for the
the fabrication
fabrication of
of
Although CBD
window material in thin-film solar cells,
cells, it cannot
cannot be
be readreadrequiring thicker
thicker highused for
for other
other applications
applications requiring
ily used
highquality
CdS films
films
is because, (i)
(i) the thickness of CdS
quality films.
films. This
This is
7
formed
by CBD
and
0.05 and
to saturate"63
saturate"' between
between 0.05
formed by
CBD tends
tends to
0.2
p.m,and
and (ii)
(ii)atatlonger
longerreaction
reaction time
timethe
the resulting
resulting films,
films,
0.2 Vtm,
though
have a duplex
This structure
structure
duplex structure.7
structure.7 This
though thicker,
thicker, have
consists
of an inner
is very
very adherent
adherent and
and an
an
layer which
which is
consists of
inner layer
outer
its visual
visual
less adherent
adherent despite
despite its
outer layer
layer which
which is less
appearance. These
is a limlimbecause CBD
CBD is
These limitations exist
exist because
heterogeneous
process. In addition, the heterogeneous
ited source growth process.
reaction (or atom-by-atom7
of the film
film on
on the
the subatom-by-atom7 growth of
reaction, which
which
the homogenous
homogenous reaction,
strate) competes
competes with the
strate)
further depletes
form CdS
CdS colloids
colloids in the
depletes the
the reactants
reactants to
to form
bulk of
reachomogenous reacof the
the solution.
solution. A
A predominantly
predominantly homogenous
tion often
reaction, and
and the
the
heterogeneous reaction,
often terminates
terminates the heterogeneous
by adsorption67
adsorption6 7' of
then grows
grows by
film then
of colloids,
colloids, leading
leading to
to a
porous overlayer.
overlayer.
porous
Here, we
we report
optimization of
of chemical
chemical bathHere,
report the
the optimization
deposited
reaction is
homogenous reaction
CdS thin films
films where
where homogenous
deposited CdS
minimized
and the thickness
of film
film deposited
deposited in
in aa single
single
minimized and
thickness of
dip maximized.
For aa concentration
concentration of
of cadmium
cadmiumsalt
saltthat
that
maximized. For
is
good quality 0.5
0.5 p.m
jim thick
thick film was
0.005 M,
M, a good
is as low
low as 0.005
obtained.
It isis also
these optimization
optimization
also shown,
shown, using
using these
obtained. It
deposited by
results, that
that thicker
thicker CdS
CdS film
film can be deposited
by periodperiodically replenishing
replenishing the
ically
the reactants
reactants while
while keeping
keeping the
the subsubstrate continuously
continuously dipped
dipped in
in the
the reaction
reaction bath,
bath, or
or by
by the
the
multiple dips
dips method.
method.
traditional multiple
heated
Review
of the
the CdS
mechanism.-In aa heated
CdS growth
growth mechanism—In
Review of
' 68
aqueous alkaline
alkaline solution, CdS
CdS is grown
grown"6"
' by
bythe
thereaction
reaction
between
cadmium salt
salt (e.g.,
acetate or
or
(e.g., cadmium
cadmium acetate
between cadmium
Cd(CH3COO),),
thiourea(SC(NH,)2),
(SC(NH,),),and
andammonia
ammoniawater
water
Cd(CH 3 COO) 2 ), thiourea
(NH4OH),
(NH4 OH), with
withammonium
ammoniumsalt
salt (e.g.,
(e.g.,ammonium
ammoniumacetate
acetate or
or
NH4CH,COO)
buffer.From
Fromthe
the cadmium
cadmium side,
side, the
the
NH 4 CHCOO) asasa abuffer.
main
equilibria involved
involved inin the
the formation
formation of
main chemical
chemical equilibria
either CdS
are as
as follows
follows
CdS films or colloids
colloids are
NH ++ OHOHNH
'- NH3
NH3 + H,O
H2 0O
-
[1]
[1]
Cd2
Cd 2+ +
+ 4NH,
4NH3, -÷ Cd(NH,)42
Cd(NH3)42 +
[21
[2]
however, promotes
promotes the
The presence
presence of ammonium
ammonium salt, however,
The
forward
reaction in
in Eq.
Eq. 1.
This then
then reduces
reducesthe
the pH,
pH, or
or
1. This
forward reaction
OW
in the reaction
according to the
concentration, in
reaction bath according
OH concentration,
6
relation
relation6
pH = pK0
pKa + log ([NH3]0/[NHfl0)
([NH 3 ],/[NH4]o)
Here, [NH3]0
[NH 3]0 and
areare
thetheinitial
Here,
and[NHI+]o
[NHflO
initialammonia
ammoniawater
waterand
and
** Electrochemical
Electrochemical
Society
Society Active
Active Member.
Member.
ammonium
salt concentrations,
respectively,and
andpKa
P'a is
ammonium salt
concentrations, respectively,
J. Electrocherri.
Electrochem. Soc., Vol.
Vol. 144,
144, No.7,
No. 7, July
July1997
1997The
© The
ElectrochemicalSociety,
Society, Inc.
Inc.
J.
Electrochemical
2+
reported6 to
25°C. As
reported6
to be
be 9.2
9.2 at
at 25°C.
Asaaresult,
result, Cd
Cd''
is
is predomipredominantly
in Eq.
Eq. 22 to
toform
form cadmium
cadmium tetraammine
tetraammine
nantly used up in
complex ions
especially if
if NH3
NH3 concentration
complex
ions (Cd(NH,
(Cd(NH,)),
concentration
3 )+), especially
2+]
sufficiently high.
is sufficiently
high. These
Theseensure
ensurethat
thatthe
theproduct
productof
of[Cd
[Cd']
9
[OH ] is
is less
less than
thanthe
thesolubility
solubilityproduct9
product of
and [OW]
ofCd(OH),
Cd(OH)2
(1.2 x 10
4) and prevent
(1.2
prevent the equilibrium
equilibrium reaction
reaction
i0')
Cd'
Cd 2+++2 2OW
OH-°'+ Cd(OH)3
Cd(OH)2 (s)
[3]
(s)
[3]
place in the bulk of
from taking place
of the solution.
solution.
For CdS
CdS colloid
colloid formation
formation we
we have,
have, in
in addition
addition to
to Eq.
Eq. 1
and 2
(NH,)CS + OHOH(NH,)CS
-
SW ++ OW
SHOH
SH
SH- + H,O + H,CN,
H2CN2
2
°- S
S' + H,O
H2O
[4]
[4]
[5]
[5]
and
Cd'
Cd 2+ +
+ 52
S2 -+ CdS
CdS (s)
(s)
[6]
[61
Equation 6
6 takes
takes place
place if the
the cadmium
cadmium and sulfide
sulfide ions
ions
Equation
ionic product is
is greater than
than the
the solubility
solubility product9
product 9 of CdS
ionic
(7.1 x 10-28).
(7.1
11128).
Mechanisms of
Mechanisms
of CdS
CdS film
film growth
growth on
on substrates
substrates have
have been
been
by many
many authors.57'1011
authors. 67' 01
, The
explicit,
howproposed by
Themost
most
explicit,
however, is
of Ortega-Borges
Ortega-Borges and
Lincot, 6 which
ever,
is that
that of
and Lincot,6
which in
in addiaddiEq. 11 and 2 is
tion to Eq.
is as follows
follows
(A) Reversible
(A)
Reversibleadsorption
adsorption of
of cadmium
cadmium hydroxide
hydroxide species
species
+
Cd[NH
Cd[NH,ff
+ 22 OHOH ++ site"
4NH, [7]
site [Cd(OH)3]ath
[Cd(OH)2]ad, ++4NH,
[7]
3 ]2 +
<-
(B) Formation
(B)
Formation of
of surface
surface complex
complex with
with thiourea
thiourea
[Cd(OH),]4th
SC(NH,), -*
[Cd(OH)2]ads ++ SC(NH2)2
- [Cd(SC(NH2)2)(OH)2]a,,
[Cd(SC(NH2)2)(OH)2]a, , [8]
(C) Formation
(C)
Formation of
of CdS
CdSwith
with site
site regeneration
regeneration
[Cd(SC(NH,),)(OH),]4dS
[Cd(SC(NH2))(OH)2]d,—*
- CdS
CdS + CN,H,
CN2H 2 + 2H,O
2H20 + site [9]
[9]
is inferred from these
equations that CdS
It is
these equations
at
CdS film grows
grows at
To ensure this, NH3
NH3 must
a lower rate. To
must be
be present
present in suffisufficient
amount to
to bind
bind Cd
Cd'2 inincadmium
cient amount
cadmium tetraammine
tetraammine comcomions, but OHOH and
and NH3
NH3 are
both introduced
introduced in the
plex ions,
are both
solution
the form
form of
solution in the
Therefore,toto allow
allow the
the
of NH4OH.
NH 4 OH. Therefore,
surface of
slow process
process at
at the surface
of the
the substrates
substrates descrihed
by
described by
Eq. 77 through
through 99 to
to take
take place
place predominantly
predominantly over
over the
Eq.
(reactions 4 and 5)
direct hydrolysis of
of thiourea (reactions
in the
the bulk
bulk
5) in
of the reaction
reaction bath, ammonium
ammonium salt
of
salt must
must be present
to
present to
control OHcontrol
OW concentration
and stabilize
stabilize the
the tetraammine
tetraammine
concentration and
complex ions.
complex
ions. Consequently,
Consequently, CdS
CdS colloid
colloid formation
formation isis minminimized, which
imized,
which is
is the subject of our study.
study.
Experimental
Experimental
The choice
choice of
of deposition
depositionparameters
parameters has
has been
been guided
guided by
by
kinetic studies
studies carried
carried out
outby
byother
otherauthors.6'7'10"
authors., 71 0" Each
kinetic
Each
experimental solution
solution contained
contained 300
experimental
300 ml
ml of
of deionized
deionized water.
This was
was prepared
adding into
This
prepared by
by adding
into the beaker
beaker containing
containing
the stirred
85°C,
stirred deionized
deionized water
waterat at
85°C,Cd(CH,COO)2,
Cd(CH 3 COO)2,
NH4 CH3 COO, NH
NH4CH,COO,
NH4OH,
andSC(NH
SC(NH,),
from 0.5,
0.5, 2,2, 14.8,
14.8,and
and
4 OH, and
2), from
0.5 M
M stock
stock solutions,
solutions, respectively,
respectively, in
order. The
0.5
in that
that order.
The
cleaned glass or SnO2/glass
SnO2 /glass substrates,
cleaned
substrates, held
heldby
by aa substrate
substrate
holder, were
holder,
were immersed
immersedinin the
the solution
solution prior
prior to
to adding
adding
The substrate
substratesize
thiourea. The
sizewas
3.9cm.
cm.The
Thesubstrate
substrate
was 3.9
3.9 ><
X 3.9
cleaning steps
steps were:
were: (i)
(i) washing
Liquinox soap,
soap, (ii)
(ii)
cleaning
washing with Liquinox
ultrasonic cleaning
cleaning in Liquinox soap
soap solution, (iii) washing
ultrasonic
washing
in distilled water;
water, (iv)
(iv) soaking in
in chromic
chromic acid,
acid, (v)
(v) washing
in
in acetone, (vi)
(vi) washing
in
washing in
in methanol,
methanol, (vii)
(vii)ultrasonic
ultrasonic cleancleaning in distilled
distilled water,
water, (viii)
(viii) ultrasonic
ing
ultrasonic cleaning
cleaning in
in isoisopropanol, (ix)
(ix) washing
washing in deionized
deionized water,
water, (x)
(x) ultrasonic
ultrasonic
propanol,
cleaning in deionized
deionized water,
cleaning
water, (xi)
(xi) boiling
boiling in
in deionized
deionizedwater
water
until used. The last six steps
steps were
were also
also used
used for
for cleaning
cleaning the
the
substrate holder;
holder, the
and the thermometer
substrate
the stirrer,
stirrer; and
thermometer after
after
10% HC1
rinsing with 10%
HC1 solution.
solution. The
The experiments
experimentswere
werecarcarin a hood.
hood. The deposition
deposition time
ried out in
time ranged
ranged between 30
30
240 min.
During the
the deposition
deposition the approximate
and 240
mi During
approximate startstarthomogeneous and
ing times
times of the homogeneous
and heterogeneous
heterogeneous reactions
reactions
observed. Homogeneous
were observed.
Homogeneousreaction
reactionwas
was marked
marked by
by a
and opaque reaction bath. A
turbid and
A predominant
predominant heterogeheterogeneous reaction
neous
reaction was
was characterized
characterized by
byaa clear
clear reaction
reaction bath,
bath,
2343
2343
while yellowish
yellowish CdS
while
CdS film
film formed
formed on
on the
the substrate.
substrate. After
After
each
deposition, the
coated substrate
substrate was
each deposition,
the coated
was ultrasonically
ultrasonically
cleaned
boiling deionized
deionized water and
and dried.
cleaned in boiling
Before the
Before
the multiple
multiple dip
dip or
or continuous
continuous dip
dip deposition,
deposition, the
the
growth of CdS
CdS films
filmsusing
using optimal
optimal parameters
parameters was
was studied
studied
to find the saturation point of
mulof the film thickness.
thickness. For multiple dip deposition,
deposition, the
was taken off
tiple
the substrate was
off the
the reacreaction bath
about the
the saturation
tion
bath at about
saturation point
point and
and the
the process
process
was repeated using
using the same
same substrate.
substrate. In the case
of conconcase of
tinuous dip
dip deposition,
deposition, reactants
tinuous
reactants were
were replenished
replenishedperiodperiodically at
saturation point
ically
at about
about the saturation
point of
the initial deposiof the
deposicycle. Reactant
tion cycle.
Reactant concentrations
in the
concentrations in
the replenishing
replenishing
solution were the same
as those
those in the initial cycle,
same as
cycle, except
except
that of
of ammonium
ammonium acetate which
that
changed.
which was changed.
Thickness,
Thickness, optical
optical transmission,
transmission, composition,
composition,and
and dark
dark
conductivity of the deposited films
films were determined.
determined. The
thickness was
film thickness
was measured
measuredusing
using an
ana-step
a-step profilometer.
profilometet
X-ray fluorescence
fluorescence spectroscopy
spectroscopy was
X-ray
was used
used to determine
determine
compositions of
the compositions
of CdS
CdS films.
films. Optical
Optical transmission
transmission of
of the
the
films was measured using a Perkin-Elmer TJV-visible
films
specUV-visible spectrophotometer. For dark conductivity measurement, ohmic
trophotometer.
contacts were
were made
made by evaporating
evaporating two coplanar
coplanar Tn/Ag
In/Ag
contacts
electrodes onto
electrodes
onto the
the surface
surface of
of aa CdS
CdS film
film deposited
deposited on
on
glass. The
13 mm,
mm, the bias
glass.
The contact
contact separation was 13
bias voltage
voltage
9.9 V,
was 9.9
V, and
andthe
the current
current was
was measured
measured using
using aa Keithley
Keithley
614
electrometer.
614 electrometer.
Results and Discussion
Discussion
Optimization.-Figure 11 shows
shows the
the dependence
dependence of
of
Optimization.—Figure
grown CdS film
grown
film thickness on
on [NH,]
[NH 3] introduced as
as NH4OH
NH 4OH
the reaction
reaction bath
in the
bathwith
with[Cd(CH,COO),]
[Cd(CH3 COO)2] == 0.002
0.002 M,
M,
[NH4CH,COO]
[NH 4 CH 3COO] = 0.04 M,
M, and
[SC(NH) 2] =
0.012 M.
and [SC(NHj,]
= 0.012
M. We
We
note that
that[Cd(CH,COO),]:[SC(NH,),]
[Cd(CH 3 COO)2]:[SC(NH2)2] = 1:6. The
The thickness
thickness
peaks at about
about 0.6
0.6 M
M of
peaks
of ammonia.
ammonia.ItIt isis relatively
relativelythin
thin atat
ammonia concentration
low ammonia
concentration and
and decreases
decreasesrapidly
rapidly at
at highhigher ammonia
ammonia concentrations.
concentrations.
the same
same growth
growth conditions,
conditions, Fig.
For the
Fig. 22 shows
showsthe
the obobserved
approximate homogeneous
homogeneous and heterogeneous
heterogeneous
served approximate
reaction starting
starting times.
times. At
At low
low [NH,],
reaction
[NH 3], the
the homogeneous
homogeneous
reaction starts
reaction
starts shortly
shortly after
after thiourea
thiourea isis introduced
introduced to
to the
the
reaction bath. This is because
reaction
there is insufficient
insufficient NH
NH,3 to
because there
2+
Cd 2+ into
into Cd(NH,)t
bind Cd'
Cd'
together
Cd(NH3)42+ .And
And excess
excess Cd
together with
with
available S
satisfy the
available
S'2- satisfy
the conditions
by Eq.
Eq. 6.
conditions described
described by
6.
Therefore,
homogenous nucleation
CdS colloids
Therefore, the
the homogenous
nucleation of
of CdS
colloids in
in
the solution
the
solution dominates
and the film
dominates and
film growth
growth takes
takes place
place
mainly by adsorption of
of colloids.
colloids.
increase in [NH,],
[NH 3 ], however;
however, increases
The increase
increases the
the approxiapproxihomogeneous reaction
mate homogeneous
reactionstarting
starting time
time and
and allows
allowsthe
the
heterogeneous reaction
heterogeneous
reactiontotobe
beobserved
observedfirst.
first.At
At[NH
[NH,]
3]
0.6 M,
0.6
M,though,
though,there
thereisisaatime
timedelay
delayininthe
thestarting
startingof
ofboth
both
reactions, the
time difference
difference isis about
reactions,
the time
about 66 min
mm and
and appears
appears
to be
to the
be constant.
constant. This
This time
time difference
difference corresponds
corresponds to
the
during which
which heterogeneous
heterogeneous reaction
period during
reaction isis predomipredomi-
0.25
0.25
I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0
i
'0
4)
4)
C
0~~~~
0.20
0.20
C)
'.0
F_
015
0.15
L
0.00
0
0.0
I
_I
I
)
0.20
0.60
0.80
0.40
0.60
0.80
Ammonia
Ammonia concentration (M)
(M)
1.00
1.00
Fig. 1.
1. Influence
Influence of ammonia concentration
concentration on
onCdS
CdS film
filmthickness.
thickness.
Fig.
Growth conditions:
Growth
0.002 M,
conditions: T
T ==85°C,
85°C,ECd(CH3COO)2]
[Cd(CH 3COO) 2] == 0.002
M,
[Cd(CH 3COO)2]:[SC(NHJ)2] == 1:6,
[Cd(CH3COO)2]:ISC(NHj2]
1:6, [NH4CH
[NH4CII3COO]
M, time
time ofof
3COO] == 0.04 hI,
deposition 30
30 mm.
min.
J. Electrochem.
Electrochem. Soc.,
Soc., Vol.
Vol. 144,
J.
Electrochemical
144, No.
No.7,
7, July
July1997
1997The
© The
ElectrochemicalSociety,
Society,Inc.
Inc.
2344
15
15
adsorption of
of colloids.
colloids. In region
region II,
however, the
partly by adsorption
II, however,
the
is low,
low, but very
very minimum
minimum homogeneous
homogeneous reacgrowth rate is
reaction takes place.
may imply
imply that
that CdS
CdS films
grow prepreplace. This
This may
films grow
dominantly in this region
region by
by heterogeneous
heterogeneous reaction.
dominantly
reaction. In
In
addition,
growth rate
rate isis due
due to
to reduced
reducedthiourea
thiourea
addition, this lower
lower growth
concentration. These
concentration.
Theseresults
resultscorroborate
corroboratethe
thefact
fact that
that
atom-by-atom
is favored
favored by
by slow
slowgrowth
growthrate
rate and
and
atom-by-atom growth
growth is
colloid
by fast
fast growth
growth rate.
rate.
colloid formation
formation by
The
regionisisofof interest
interestinin this
this work.
work. For
For our
our
The second
second region
study,
the selected
ratio of cadmium
acetate to
to thiourea
study, the
selected ratio
cadmium acetate
thiourea
that minimizes
minimizes the homogeneous
homogeneous reaction
reaction throughout
throughout the
the
deposition period is 0.5.
0.5.
deposition
C
E4)
'U
(a)
10
.2
C
1
C
C
C
U
U
55
I4)
0.
C.
0.
C.
0
n
____________________________________________
_
0.0 0
0.00
0.20
0.40
0.60
0.80
0.20
0.40
0.60
0.80
Ammonia concentration (M)
1.00
1.00
Fig. 2.
2. Effect
Effect of
of ammonia
ammonia concentration
concentration on the approximate reacreacFig.
tion starting
start ingtime:
time:(a)
(a)homogenous
homogenous reaction
reaction and
and(b)
(b)heterogeneous
heterogeneous
tion
reaction.
reaction. Growth conditions
conditions same
same as in
inFig.
Fig. 1.
1.
place. This means
0.6 M
M is
t; aking place.
means that
that [NH
[NH3]
is sufsufnantly taking
3 ] > 0.6
2t
ficient
ficient to
tto bind
bind Cd2
Cd into
intocomplex
complex ions
ions needed
needed for slow
slow
growth described
described by Eq.
Eq. 77toto9.9.However,
However, excess
excess NH3
NH 3
growth
complex ions
reduces the
the growth
growth
overstaydilizes the
overstabilizes
the complex
ions and
and reduces
Thiis explains why
why the film thickness
rate. This
thickness decreases
decreases rapidrapid[NH3 ] =
= 0.6
0.6 M.
Therefore, under
growth
ly beyond
beyorid [NH3]
M. Therefore,
under this growth
ly
0.6 M
M ammonia
condition,
ammonia has
has been
been chosen
chosen as
as the
the concenconcenconditio n, 0.6
that stops
stops ororminimizes
minimizes the
theearly
earlyhomogeneous
homogeneous
tration that
reaction
reaction.
the homogenous
homogenous reaction
reaction eventually
On
On the
thee other hand, the
eventually
is sufficient
sufficient NH3,
NH3 , and
thus more
more
observedI when
observed
when there
there is
and thus
may be
due to
to excess
excess sulfide
sulfide ions.
ions. The
The
hydroxide
ions, may
be due
hydroxi le ions,
are produced
produced by
by rapid
rapid hydrolysis
hydrolysis of
excess S
excess
S22 are
of thiourea,
thiourea, as
as
1, 4,
5. This
shown in
i nEq. 1,
shown
4, and 5.
This rapid
rapid hydrolysis
hydrolysis may
may also
also be
be
aided by
b' y high concentration
reaction
aided
concentration of
of thiourea
thiourea in
in the2 reaction
the ratio
ratio of
of concentrations
concentrations of
Cd ` to
to S2
2
bath, since
si nce the
bath,
of Cd2
1:6. It
therefore important
important to
to find
find
precursors
precurscors isis 1:6.
It isis therefore
homogethat minimizes
minimizes the homoge[Cd(CH
[C.d(CH3COO)2]/[SC(NH2)3]
3 COO)j2/[SC(NH 2 ) 3] that
re action.
neous reaction.
shows the
theinfluence
influenceof [Cd(CH3COO)3]/
of [Cd(CH3 COO)2 ]/
Figure
Figure 33 shows
[SC(NH, )2] onon(a)
[SC(NH2)2]
(a)homogeneous,
homogeneous,(b)
(b) heterogeneous
heterogeneous reacreactimes, and
and (c)
(c) film
film thickness.
thickness. Here,
Here, concenconcenstaarting times,
tions starting
trations of
of cadmium
cadmium acetate,
acetate, ammonium
ammonium acetate,
acetate, and
and
trations
ammonia (NH4OH)
(NH 4OH) are
ammonia
are fixed
fixed at
at 0.002,
0.002, 0.04,
0.04, and
and 0.6
0.6 M,
M,
respectively. The
respectively.
The ratio
ratio of
of cadmium
cadmium acetate
acetate to
to thiourea
thiourea is
by varying
varying the thiourea
thiourea concentration.
concentration. This
This figure
varied by
figure
can be
be divided into regions
regions I and II.
II. The
The first corresponds
corresponds
to
where the film is thick and where homogeneous
to a region where
homogeneous
reaction is
is present.
present. The
The thick film
film obtained
obtained here
reaction
here might
might
have been
been partly
partly deposited
deposited by
by atom-by-atom
have
atom-by-atom growth
growth and
and
45.0
II
V
- (b)
-(b)
I
A (C)
0.60
0.3
0.3
.,E
0.3
Q 15.0 I4)
n^
[I I1
Ca
v
c
.9
.n
0.1
0.1
'I
,'
"I • I
.
o
E 0.40
'a
4
C
a
0.30
0.30
C)
0.20
:
_
_
0.50
0.80
0.50
0.80
[Cd(CH3COO)2]/[SC(NH 2)2]
[Cd(CH3COO)2]/[SC(NH2)2]
n^
— 0.0
g$
uI
1.10
1.10
Fig.
Influence ofof[Cd(CH3COO)21/[SC(NH2)2]
[Cd(CH 3COO) 2]/[SC(NH2) 2] on
Fig. 3. Influence
onthe
the approximate
approximate
reaction starting
starting time:
time:(a)
(a)homogenous
homogenous reaction,
reaction,(b)
(b)heterogeneous
heterogeneous
reaction
reaction,
reaction, and (c)
(c)the
the effect
effect of
ofthe
the same
same reactants
reactants ratio
ratio on
onCdS
CdS film
film
thickness.
thickness. Growth
Growth conditions:
conditions: T
T=85°C,
85°C,[Cd(CH3COO)2]
[Cd(CH 3COO)2] == 0.002 M,
M,
3
[NH4CH3COO] = 0.04
0.04M,
M,[NH3]
[NH ]==0.6
0.6M,M,time
timeofofdeposition
deposition45
45mm.
min.
[NH4CH3COOJ
,
/
I,
., /
'
I
I '
0
0
,'
0j /0
0/
0
O/
./
/
e(a)
B/
I,
. . /.."o
0.00
.',
0.00
00
,,'
/o
/
!
(b)
(b) ''
,
0.10
0.10
Al- -1
0.20
0.20
Growth of thicker films.-Maximum
films—Maximum concentrations
concentrations of
of
primary cadmium
cadmium and
sulfur precursors
precursors reported
by
primary
and sulfur
reported
by
Ortega-Borges and
Lincot 6 to
CdS
Ortega-Borges
and Lincot6
to produce
produce high-quality
high-quality CdS
films are
times those
films
are about
about six to ten times
those used
used in
in this study
study to
pm film.
deposit 0.5 p.m
film.This
Thismeans
meansthat
that there
there is
is room
room to
to grow
grow
micron-range
micron-rangegood
goodfilm
filminin aa single
single dip
dip using
using our
our optiopti-
0.50
0.50
0.2
0.2
i:
growth
curves are
are similar
similar to
to those
thosereported
reported by
byother
otherworkworkgrowth curves
ers.6
Theyare
are characterized
characterizedby
by an
an incubation
incubation or
or nuclenucleers. 6 They
ation
region, and
and aa saturation
ation region,
region, aa linear
linear growth
growth region,
saturation
region.
The shorter
shorter incubation
period and
and the slightly
region. The
incubation period
slightly
steeper linear growth
region indicate
indicate that
that (b)
(b) has
has aa higher
higher
growth region
growth
observed that
that (a)
(a) and
and (b)
(b)satsatgrowth rate
rate than
than (a).
(a). It
It is
is also
also observed
urate
respectively. Since
Since the
the cadcadurate at
at about
about0.5
0.5 and
and0.4
0.4 p.m,
Ilm, respectively.
mium
and sulfide
concentrations are
are the
the same,
same, one
one
mium and
sulfide source
source concentrations
would
expect the
the saturation
saturation thickness
thickness of
of the
the latter
latter to
to also
would expect
also
be
This did
reacsome reacbe about
about 0.5
0.5 p.m.
Vxm. This
did not
not occur,
occur, because
because some
tants
tants were
were lost
lost to
to colloid
colloid formation
formation as
as expected.
expected. Other
Other
authors612
reportedconcentrations
concentrationsofof cadmium
cadmium salt
salt and
authors 1'2 reported
and
thiourea
higher than
than those
those used
used here
here to
to obtain
obtain film
film
thiourea much
much higher
thickness
that saturates
saturates at
at about
or less.
thickness that
about 0.2
0.2 p.m
ptm or
less. This
This
shows
reaction
shows that
that minimization
minimization of
of the
the homogeneous
homogeneous reaction
with
encourwith the
the deposition
deposition conditions
conditions stated
stated previously
previously encourages
atom-by-atom growth
thickness.
ages atom-by-atom
growth and
and maximizes
maximizes film
film thickness.
'(a)
* (a)
I
A
reactants
reactants are
are obtained
obtained from
from the
the quoted
quoted ratios.
ratios. These
These
0.4
0.4
IIII
E
_ 30.0
We mentioned
rowth mechanism
We
mentioned in
in the
the CdS
CdS growth
mechanism review
reviewsecsecconfirmed in
investigations that
tion and confirmed
in our
our investigations
that growth
growth rate
rate
decreases
with increase
increase in
in ammonium
acetate concentraconcentradecreases with
ammonium acetate
tion. Generally,
tion.
Generally, for
for our
ouroptimal
optimalratios
ratiosofof[Cd(CH3COO)3]/
[Cd(CH3 COO)2 ]/
[SC(NH2)2]
with the
[SC(NH
0.5, [Cd(CH3COO)2]/[NH3]
[Cd(CH3 COO) 2]/[NH 3 ] = 0.0033, with
2)2] = 0.5,
the growth
growth rate
rate is
is reduced
reduced and
and
bath temperature
temperature at
at 35°C,
85°C, the
the homogeneous
reaction remains
remains minimum
minimum ifif the
the cadmicadmihomogeneous reaction
the interval
interval 00 <<x
< 0.12.
um-to-ammonium salts ratio lies in the
x<
0.12.
To buttress
point, we consider
To
buttress this
this point,
consider the
the growth
growth curve
curve in
in
4. The
The growth
growth conditions
conditions are:
Fig. 4.
are:(a)
(a)[Cd(CH3COO)3]
[Cd(CH 3COO)2] ==
0.005
0.07 which
0.005 M,
M, [Cd(CH3COO)21/[NH4CH3COO]
[Cd(CH3 COO)2]/[NH 4 CH 3 COO] = 0.07
which lies
lies
in
[Cd(CH3COO)2}
= 0.005
in the
the stated
statedinterval;
interval;(b)(b)
[Cd(CH 3COO)2 =
0.005 M,
[Cd(CH3COO)Z]/[NH4CH3COO]
0.13 which
not lie
lie in
in
[Cd(CH3 COO)2]/[NH 4 CH3 COO] == 0.13
which does
does not
the interval.
deposition
the
interval. The
The concentrations
concentrations of
of remaining
remaining deposition
. .
50
100
150
200
200
250
Time (mm)
(min)
Time
Fig. 4.
Growth curve
curve of
of CdS
CdS films
films grown
grown by
by CBD:
CBD: (a)
(a)
Fig.
4. Growth
[Cd(CH 3COO)2] == 0.005
0.005M,
M,[Cd(CH3COO)21/(NH4CH3COOI
[Cd(CH 3COO)2]/[NH 4CH3COO] = 0.07;
0.07;
[Cd(CH3COO)2]
= 0.005
(b)
[Cd(CH 3COO) 2]
=
M,
[Cd(CH 3COO) 2]/
[Cd(CH3COO)2]
[Cd(CH3COO)2]/
[NH4CH3COO] = =0.13.
85°C,
[NH4CH3COO]
0.13.Other
Other growth
growth conditions:
conditions:T7 = 85°C,
[Cd(CH
[Cd(CH3COO) 2]/[NH 3 ] = 0.0033.
0.0033.
[Cd(CH3COO)2]/[SC(NH)2]
0.5, [Cd(CH3COO)2]/[NH3)
3 COO)2]/[SC(NHJ)]== 0.5,
2345
2345
Electrochemical
J. Electrochem. Soc.,
Soc., Vol.
Vol. 144,
144, No.
No. 7,
7, July
July1997
1997The
© The
ElectrochemicalSociety,
Society, Inc.
Inc.
Table I.
I. Effect
in the replenishing
Table
Effect of [Cd(CH3COO),]/[NH4CH3COO]
[Cd(CH3COO)2]/[NH4CH3COO] in
reactants on
on the
the percentage
percentage thickness
thickness yield
yield per
per cycle.
cycle.
reactants
[Cd(CHCOO)]/ [NHCHCOO]
[Cd(CH,COo),]/[NH4CH,COc']
Yield/Cycle
Yield/Cycle
0.08
0.08
42%
42%
47%
47%
0.11
0.11
10.0
10.0
8.0
8.0
6.0
6.0
C
40
mization parameters.
parameters. In this study,
mization
study, the growth
growth of thicker
film
multiple dip and by continuous
film is carried out
out by multiple
continuous dip.
dip.
reaction bath, the thickness
thickness yield
yield per subsesubseFor a given
given reaction
quent dip
dip in
in multiple dip deposition
deposition is
is found to range from
100 to
to 120%
120% of
of the
the initial
initial thickness.
thickness. This
This agrees
agrees with
with the
100
results obtained
results
obtained by other
other authors.8
authors.8 The
The difference
difference here,
here,
nevertheless,
plm thick
nevertheless, isis that 11 p.m
thick film
film can
can be achieved
achieved in
in
dips.
two dips.
Though ammonium
Though
ammonium salt
salt acts
acts as
as a buffer in the chemical
chemical
bath deposition
deposition process,
process, it holds the key to the successful
successful
Table II shows
shows
deposition of thicker film by continuous dip. Table
the effect
ratio in
in the
effect of
of [Cd(CH,COO),]
[Cd(CHCOO) 2 ] to [NH4CH,COO]
[NHCHCOO] ratio
replenishing reactants
the percentage
percentage thickness
replenishing
reactants on the
thickness yield
yield
per cycle.
The reference
reference thickness
thickness here
here isisthat
that of
of the
the initial
initial
cycle. The
cycle.
cycle. The yields shown
shown are those
those where
where [Cd(CH,COO),]/
[Cd(CHCOO),]/
[NH4CH,COO]
liesininthe
theinterval
interval00<<x
seen in
in
[NHCHCOO] lies
x c< 0.12. As seen
the table,
table, when the ratio is
high growth rate
rate
is high there is a high
and hence high yield per cycle.
However,when
whenthe
theratio
ratio isis
cycle. However,
outside
reaction takes
takes place,
outside this interval,
interval, homogeneous
homogeneous reaction
place,
leading to the
the growth
growth of
of aa porous
porous overlayer.
overlayer. Despite
leading
Despite the
the
fact that
that this
thisnew
new method
method isis limited
limited by
by low
low yield owing to
the constraint
constraint placed
placed on
salt concentraon the
the ammonium
ammonium salt
concentration, it isis the
thicker
the least
least laborious
laborious method
method of growing
growing thicker
film.
film.
Characterization.-Figure 5 5shows
optical
Characterization.—Figure
showsaa typical
typical optical
transmission
transmission spectrum of our films.
films. Generally,
Generally, the optical
transmission
transmission of
of CdS
CdS film
film depends
depends on
on the
the thickness'3
thickness"3
because
of the increase
from the
because of
increase in diffuse
diffuse scattering
scattering from
adsorbed colloids,
especially when
when the
the film
filmisisdeposited
depositedinin
colloids, especially
the presence
reaction. Film
Film thickthickpresence of
of heavy
heavy homogeneous
homogeneous reaction.
ness investigated
investigated ranged
0.05 to
pIm. The
ness
ranged from 0.05
to 11 p.m.
The optical
optical
transmission
transmission of
of these
these films
films below
below the bandedge
bandedge of CdS
CdS
film ranged
20 to
ranged from
from 60
60 to 100%.
100%. Nair et al.'4
al."4 reported
reported 20
50%
optical transmission for
chemically depositdeposit50% optical
for aa 0.5
0.5 p.m
ptm chemically
ed CdS
whereasfor
for the
the same
same or
or higher
higher film
film
CdS film
film at 85°C,
85'C, whereas
thickness
thickness deposited
deposited at
at the
the same
we have
same temperature,
temperature, we
have
transmission that is
is well
well above
recorded a transmission
60%. This
Thissugsugabove 60%.
gests
gests that
that minimization
minimization of
of the
thehomogenous
homogenous reaction
reaction
improves the
transmission below
below the
improves
the optical transmission
the bandedge.
bandedge.
The
vs.
The plot of
of the square
square of
of the absorption
absorption coefficient
coefficient vs.
photon energy
energy is
is shown
shown in
in Fig.
Fig. 6.
Curves a,a, b,b, and
and cc are
are
6. Curves
those of
of films
those
films deposited
deposited by
by continuous
continuous dip,
dip, single
singledip,
dip,and
and
100
100
a
o 60
60
.
a 40
3-
20
20
0n
300
300
400 500
500
400
600
700
800
Wavelength (nm)
Wavelength
900
900 1000
1000
Fig. 5.
5. Typical
Typical optical
optical transmission
transmission spectrum
spectrum ofofaaCBD-grown
CBD-grownCdS
CdS
thin film.
0.007 hI,
film. Growth
Growthconditions:
conditions:TT= =85°C,
85°C,ECd(CH3COO)21
[Cd(CH 3COO)
]2 == 0.007
M,
[NHCHCOO] = = 0.1
M, [Cd(CH,COO),]/[SC(NH2),]
[Cd(CHCOOj2]/[SC(NH2)2] == 0.5,
0.5,
[NH4CH,COO1
0.1 M,
[Cd(CH 2COO)2/[NH3] == 0.0033.
[Cd(CFI,COO)2)/(NH,1
0.0033. Film
Film thickness
thickness 0.6
0.6p.m.
p.m.
Fi9.
('4
0.0
0.0
2.30
2.30
2.40
2.40
2.50
2.50
2.60
hv (eV)
(cv)
Fig. 6. Plot
Plot of absorption
absorption coefficient
coefficient squared
squared vs.
vs. photon
photon energy:
energy:
Fig.
(a) continuous
(c)multiple dips. Growth
Growth condicondi(a)
continuous dip,
dip, (b) single
single dip, (c)
tions:
tions: (a)
(a) and
and (c) T = 85°C,
[Cd(CH 3COO)] = 0.005
0.005 M,
M,
85°C, [Cd(CH3COO)2]
ICd(CH3COO),]/ENH4CH,COO]
=
[Cd(CH 3COO)2]/[NH4CH,COO]
[Cd(CHCOO),1/
0.07,
[Cd(CH,COO),1/
ISC(NH2),]
[SC(NH2 ] == 0.5, [CdCHCOO)2]/[NH
fCd(CH,COO),]J[NH3]
0.0033; film
filmthicknesses
thicknesses
3] == 0.0033;
0.7
0.7 and
and 1 1Ism,
m,respectively;
respectively; (b)
(b)TT==85°C,
85°C,ICd(CH,COO)2]
[Cd(CH 3COO)2] ==
0.002
0.002 M,
M,[NH4CH,COO1
[NH4CH3COO] == 0.04
0.04M,M,ICd(CH,COO)2]/[SC(NH2)2]
[Cd(CHCOO)2]/[SC(NH2) 2] =
0.5, [Cd(CH,COO),/INH3]
[Cd(CH3COO),/[NH3] ==0.0033;
0.0033;Film
filmthickness
thickness0.2
0.2I.Lm.
I.m.
0.5,
About the
the characteristic
characteristic enerenermultiple dips, respectively.
respectively. About
multiple
CdS, they
similar optical
optical properties.
properties. As
As
gy of CdS,
they all
all exhibit similar
can be
films have
have 2.39
2.39 eV
eVasas bandgap,
bandgap, aa
be seen,
seen, deposited
deposited films
value
value that agrees
agrees with
with the
the 2.42
2.42 eV
eV value"'6
value "5 6 generally
generally
accepted
accepted for bulk CdS.
CdS.
From x-ray fluorescence
fluorescence spectrum,
spectrum, an
approximate
From
an approximate
of cadmium
cadmium to sulfur of 1:1
ratio of
1:1 was
was obtained.
obtained. This
Thisresult
result
2
is in line with
with that reported
reported in the literature.'2
literature.'
The dark
The
dark resistivities
resistivities of
of our
films are
our as-deposited
as-deposited films
are
between
and io
reported in
in the
the literaliterabetween 10'
10' and
104fl-cm.
fl-cm. Values
Values reported
'8 3
ture"3
to
ture" 'for
forCBD-grown
CBD-grown CdS
CdS film
film are in the range of 10'
10 to
10 2 fl-cm
10"
f-cm before
before any
any thermal
thermal annealing.
annealing. After
After annealing,
annealing,
the dark resistivities
typicallydrop
droptotoabout
about10-'
10' to 10'
(1resistivities typically
103 Qf2
cm.
high resistivity
resistivity""
films has
has
cm. This
This high
"'"ofofthe
theas—deposited
as-deposited films
been attributed
attributed to the
the chemisorbed
0, at
at the
the grain
grain boundboundchemisorbed 02
aries.
aries. Our
Our results
results show
show that
that by
by minimizing
minimizing the homogehomogeneous reaction,
reaction, as-deposited CdS
low resistivity
CdS film with low
obtainable. This could
is obtainable.
could mean
mean that under
under very
very minimum
minimum
homogeneous reaction,
homogeneous
reaction, little
little 0,2
0, chemisorption
chemisorption takes
takes place.
place.
Conclusion
In chemical
chemical bath
bath deposition of CdS
CdS films,
films, ifif ratios
ratios of
of varvarious reactants
reaction bath at
ious
reactants in the reaction
at 85°C
85'C satisfy the conditions
ditions [Cd(CH,COO),]/[SC(NH,),]
[Cd(CHCOO),]/[SC(NH2)2] = 0.5,
0.5, [Cd(CH,C00),]/
[Cd(CHCOO)2]/
[NH,]
= 0.0033,
lies in
in
[NH3] =
0.0033, and [Cd(CH,CO0),]/[NIEI4CH,C0O]
[Cd(CH3COO)2]/[NH4,CHCOO] lies
< x c< 0.12,
the interval 00 <x
0.12, the
the homogeneous
homogeneous reaction
reaction isis minminimized, the
deposited film
improved, and
imized,
the quality of deposited
film is improved,
and its
thickness maximized.
thickness
maximized. Using
Using these
these growth
growth conditions
we
conditions we
demonstrated for the
have demonstrated
the first
first time
time that
that thicker CdS
CdS films
films
could be
could
be deposited
deposited by
by continuous
continuous dip.
dip.
Acknowledgment
Acknowledgment
80
F-'
4.0
'Z 4.0
a
2.0
2.0
The
The authors thank
thank James
James Ross
Ross and Dr.
Dr. George
George Stegeman
Stegeman
of Center
Center for
for Research
Research and
and Education
in Optics
and
Education in
Optics and
Lasers, University
Lasers,
University of
of Central
Central Florida,
Florida, for
for the
the film
film thickness measurement;
measurement; and
and Dr.
Dr. Tom
Tom Li
Li of Lockheed-Martin
Lockheed-Martin
Corporation for
x-ray fluorescence
Corporation
for the x-ray
fluorescence spectra.
spectra. We
We also
also
acknowledge Dr.
Glen Cunningham
acknowledge
Dt Glen
Cunningham of
of the
the Department
Department of
of
Chemistry, University
Chemistry,
Universityof
ofCentral
CentralFlorida,
Florida, for
for the
the use
useofofthe
the
UV-vis spectrophotometer.
UV-vis
spectrophotometer.
Aug. 14,
14, 1996;
1996; revised
Manuscript submitted Aug.
revisedmanuscript
manuscript
received
received March
March 18,
18, 1997.
1997.
The University of
The
of Central
Central Florida
Florida assisted
assisted in meeting the
publication
of this
this article.
publication costs
costs of
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John Wiley
Wiley &
&Sons,
Sons, New
The Effect
Effect of
of pH
pH Changes
Changes on
on the
the Giant Magnetoresistance
Magnetoresistance of
of
The
Electrodeposited Superlaitices
Superlattices
Electrodeposited
and W.
W. Schwarzacher*
Schwarzacher*
Alpera and
M. Alper°
BS8
1TL,1 TL,
England
of of
Bristol,
Bristol,
H. H.
H. Wills
Wills Physics
PhysicsLaboratory,
Laboratory,University
University
Bristol,
Bristol,
BS8
England
S.
S.J.
J.Lane
Lane
Departmentof Geology,
Geology, Univeristy
Univeristy of
of Bristol,
Bristol, Bristol,
Bristol, BS8
BS8 1RJ,
1RJ, England
England
Department
ABSTRACT
ABSTRACT
We have
have studied
studied the
the effect
effect of
ofelectrolyte
electrolytepH
pHon
onthe
the"giant
"giantmagnetoresistance"
magnetoresistance"ininCo-Ni-Cu/Cu
Co-Ni-Cu/Cusuperlattices
superlatticesprepared
prepared
We
15% while,
while,
(GMR) of over
over 15%
giant magnetoresistance
magnetoresistance (GMR)
low pH
pH (1.8
(1.8
0.1) exhibited giant
Films grown
by electrodeposition.
electrodeposition. Films
0.1)
by
grown at low
anisotropic magnepredominantly anisotropic
(3.3
0.1) exhibited
exhibited either predominantly
films grown
grown at high pH (3.3
thicknesses, films
depending
magnedepending on layer thicknesses,
toresistance (AMR)
or much
much smaller
smaller GMR
GMRthan
than possible
possible at
at low
low pH.
pH. Also,
Also,the
thefilms
filmsgrown
grownatatlow
lowpH
pHwere
werefound
foundto
tohave
haveaa
(AMR) or
magnetization than those grown at high pH.
higher magnetization
pH. The
The different
different magnetic
magneticand
andmagnetotransport
magnetotransportproperties
propertiesobserved
observedfor
for
different values
of the
the electrolyte
electrolyte pH
pH are
are accompanied
accompaniedby
bychanges
changesininthe
theshapes
shapesofofthe
thecurrent
currenttransients
transientsrecorded
recordedduring
during
different
values of
film growth
growth and by changes
changes in
in the composition
of the
the superlattices.
superlattices. AA possible
possibleexplanation
explanationofofthese
theseresults
resultsisisthat
thatreducreduccomposition of
film
nonmagnetic layers
between the ferromagnetic
ferromagnetic and
more abrupt interface between
ing the electrolyte
electrolyte pH leads
leads to a more
and nonmagnetic
layers by
by suppresssuppressdissolution.
ing Co
Co dissolution.
Introduction
Introduction
the giant
giantmagnetoresistance
magnetoresistance
discovery of
of the
Since the
the discovery
of magnetic
(GMR) in
(GMR)
inFe/Cr'
Fe/Cr' superlattices
superlattices the
the properties
properties of
magnetic
superlattices
extensively studied,
studied, both
both because
because
been extensively
superlattices have been
of their importance
applications such
such as
as
importance for technological
technological applications
magnetoresistive
sensors and because
fundamenbecause of their fundamenmagnetoresistive sensors
tal scientific
such as
as
Magnetic superlattices
superlattices such
scientific interest.
interest. Magnetic
4
3
Fe/Cr,2 Co/Cu,
Fe/Cr,2
Co/Cu,3Co-Fe/Cu,
Co-Fe/Cu,4and
andNi-Co/Cus
Ni-Co/Cu5exhibit
exhibitparticuparticuroom temperature.
GMR at
larly large GMR
at room
temperature.
proare generally
generally propresent, GMR
GMR superlattice
superlattice films
At present,
At
films are
duced by vacuum
vacuum deposition
deposition methods;
methods; sputtering
sputtering and
and molmolAlthough electrodeposition
electrodeposition
beam epitaxy
epitaxy (MBE).
(MBE). Although
ecular beam
some obvious
has some
obvious advantages
advantages(in
(inparticular
particular the
the apparatus
apparatus
required is extremely cheap)
cheap) relatively
relatively little
little work
work has
has been
been
done using this technique.'1'
we have
have shown
shownthat
that
Recently, we
technique.6 ' " Recently,
superlattices with
with
it isis possible
to grow
grow well-ordered
well-ordered superlattices
possible to
repeat
and we
we have
A"2 and
have also
also
distances as short
short as
as —15
-15 A'2
repeat distances
demonstrated
both on
on
superlattices electrodeposited
electrodeposited both
demonstrated that superlattices
polycrystalline
Cu substrates
substrates and on
n-type GaAs
GaAs (100)
(100)
on n-type
polycrystalline Cu
exhibit significant
Of course,
course, as
as isis the
the case
case with
significant GMR.6-'
GMR. - 9 Of
properties of
techniques, the
superlattices grown by other techniques,
superlattices
the properties
of
electrodeposited
electrodeposited superlattices
superlattices generally
generally depend
depend on
on paraparameters
of the
the magnetic
magnetic and
and nonnonas the thicknesses
thicknesses of
meters such
such as
magnetic
layers and
and the
the choice
choice of
of substrate,
substrate, but
but in
in addiaddimagnetic layers
tion
potential, the
the
such as
as the
the cathode
cathode potential,
tion to these,
these, factors
factors such
concentration and
electrolyte used to grow the
and the
the pH
pH of
of the
the electrolyte
superlattices
et al."
Recently, Lenczowski
Lenczowski et
superlattices are important.
important. Recently,
superlattices elecelecproperties of
of Co-Cu/Cu
Co-Cu/Cu superlattices
studied the
studied
the properties
** Electrochemical
Electrochemical
Society
Member.
Society Active
Active Member.
FizikBolumU,
B1liimti, GdrUkle,
Girikle,
address: Uludag
UludagUniversitesi,
Universitesi,Fizik
Present address:
16059, Turkey.
Turkey.
Bursa, 16059,
trodeposited at pH 4 to 4.5
4.5 in
in detail
detail by
byvarying
varyingparameters
parameters
such
the Cu
Cu
thicknesses, the
substrate texture,
texture, layer
layer thicknesses,
such as the
the substrate
concentration
of additives.
additives.
electrolyte and the use of
concentration in the electrolyte
Cu
They
observed that
that superlattices
with relatively
relatively thin
thin Cu
superlattices with
They observed
layers (e.g.,
(e.g., 16
layers
16 A)
A)exhibited
exhibited anisotropic
anisotropic magnetoresistance
magnetoresistance
A)
(AMR)
whilethose
thosewith
with thicker
thicker Cu
Cu layers
layers (e.g.,
(e.g., 40
40 A)
(AMR) while
GMR.
exhibited GMR.
of the
elecIn this paper,
we concentrate
concentrate on the effect
effect of
the elecpaper, we
in electrodeposited
electrodeposited Co-Ni-Cu/Cu
Co-Ni-Cu/Cu
pH on
on the
the GMR
GMR in
trolyte pH
superlattices. We
superlattices.
We compare
comparethe
the structural,
structural, magnetic,
magnetic, and
and
at
of superlattices
magnetotransport properties
magnetotransport
properties of
superlattices prepared
prepared at
pH (3.3
(3.3 ± 0.1)
0.1) and
and discuss
discuss the
(1.8 ± 0.1)
0.1) and high pH
low pH (1.8
low
like
significant
differences observed.
observed.At
At the
the higher
higher pH,
pH, like
significant differences
Lenczowski
for some
AMR for
some films,
films, but
we observe
observe AMR
Lenczowski et al.," we
show
are grown
grown at
at low
low pH,
pH, superlattices
superlatticeswith
with
show that if they are
thin Cu layers exhibit larger GMR
than those
those with
with thicker
thicker
GMR than
Cu
Cu layers.
layers.
Experimental
Experimental
strong (100)
(100) texPolycrystalline copper
Polycrystalline
copper sheets
sheets having
having a strong
texture and aa thickness
ture
thickness of
of 0.5
0.5 mm
mm were
were used
used as substrates.
substrates.
These were
were polished
polished first
first mechanically
and then electroelectromechanically and
These
superlattices were
were
50% H3P04.
HPO4 . Our
chemically using
chemically
using 50%
Our superlattices
deposited
concell from an electrolyte
electrolyte condeposited in a three-electrode
three-electrode cell
taining nickel
Ni(SONH,), == 2.3 mol/liter;
mol/liter; cobalt
taining
nickelsulfamate
sulfamateNi(SO3NH2)2
cobalt
sulfate CuSO4
CuSO4 =
sulfate
0.4 mol/liter;
mol/liter; copper
sulfate CoSO4
CoSO4 = 0.4
copper sulfate
=
0.05
mol/liter using
HBO3 == 0.5
0.5 mol/liter
0.05 mol/liter;
mol/liter; and boric acid H3B03
using a
6 '3' 4
The
controlled by
bya acomputer.6'13"4
computer.
potentiostat controlled
Thecomputer
computer
also
integrates the net
current
deposition current
net current (i.e.,
(i.e., the deposition
also integrates
charge
minus any
dissolution current)
minus
any dissolution
current) toto give
give the
the net
net charge
passed
between the
the anode
anode and
and the
the cathode.
When this
this
cathode. When
passed between
charge
reaches the
the value
to the
the desired
desired
value corresponding
corresponding to
charge reaches