Electrodeposition of indium onto Mo/Cu for the deposition of Cu(In,Ga)Se2 thin films

Article abstract of DOI:10.1016/j.electacta.2007.11.069

A study of the electrodeposition and the oxidation process of indium on Mo/Cu substrates from a bath containing 0.008 M InCl₃, 0.7 M LiCl at pH 3 is described in this work. The voltamperometric study showed a reduction process which corresponds to the conversion of In³⁺ to In⁰ and an oxidation process which takes place in different steps. Utilizing the chronoamperometric technique the total efficiency of process, the number of monolayers, the film thickness and the diffusion coefficient were evaluated. The analysis of current transients, using theoretical growth model, showed that the electrodeposition of indium adjusts to a three-dimensional growth under instantaneous nucleation limited by diffusion. The kinetic growth parameters were evaluated through a non-linear fit. The films were characterized by X-ray diffraction and scanning electron microscopy techniques. These studies showed that the films were of crystalline in nature with compact and uniform surface, even for the film with a deposition time of 1 min.

Full text of DOI:10.1016/j.electacta.2007.11.069

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 3714–3721
Electrodeposition of indium onto Mo/Cu for the deposition
of Cu(In,Ga)Se2 thin films
a
R.C. Valderrama , M. Miranda-Hernandeza,1, P.J. Sebastiana,∗, A.L. Ocampob
´
a Centro de Investigacio´n en Energ´ıa, Universidad Nacional Auto´mona de Me´xico, Temixco 62580, Morelos, Mexico
b Facultad de Qu´ımica, Universidad Nacional Auto´noma de Me´xico, Ciudad Universitaria 04510, Mexico, D.F., Mexico
Received 30 July 2007; received in revised form 17 November 2007; accepted 19 November 2007
Available online 4 December 2007
Abstract
A study of the electrodeposition and the oxidation process of indium on Mo/Cu substrates from a bath containing 0.008 M InCl3, 0.7 M LiCl
at pH 3 is described in this work. The voltamperometric study showed a reduction process which corresponds to the conversion of In3+ to In0 and
an oxidation process which takes place in different steps. Utilizing the chronoamperometric technique the total efficiency of process, the number
of monolayers, the film thickness and the diffusion coefficient were evaluated. The analysis of current transients, using theoretical growth model,
showed that the electrodeposition of indium adjusts to a three-dimensional growth under instantaneous nucleation limited by diffusion. The kinetic
growth parameters were evaluated through a non-linear fit. The films were characterized by X-ray diffraction and scanning electron microscopy
techniques. These studies showed that the films were of crystalline in nature with compact and uniform surface, even for the film with a deposition
time of 1 min.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Indium; Electrodeposition; Nucleation and growth; CuIn alloy; Cu(In,Ga)Se2
1. Introduction
not well understood, as it was very difficult to incorporate the
Ga into the standard range of potential in which the CIS elec-
trodeposition was commonly carried out. Bhattacharya et al. [4]
were the first to demonstrate the Ga incorporation by ED, but at
very low levels. To get the device quality films it was necessary
to supplement the films by physical vapor deposition of Cu, In,
and Ga up to 50% of the total film thickness. Calixto et al. [5]
reported that bath concentration affects the composition of elec-
trodeposited CIGS films. A bath ratio of [Se4+]/[Cu2+] equals to
1.75 was required for maintaining the suitable deposit levels of
Se and Cu, while [In3+] could be adjusted to control the deposits
of In and Ga.
With the purpose of knowing more about the film formation
mechanism of Cu(In,Ga)Se2 obtained through the ED tech-
nique, a systematic study on the growth and incorporation of
the elements using the electrocrystallization theories reported
in the literature [6–10] is suggested. It is proposed to study the
nucleation and growth process of: In, Ga, Cu–Se, In–Ga–Se
and Cu–In–Ga–Se. In this paper, the electrodeposition of In is
presented. The study was carried out not only because of our
interest in In as a component in CIGS films, but also because
of its relevant role as In oxide [11–15] due to its application
Despite a number of publications that refer to the electrode-
position (ED) of CuInSe2 (CIS) thin films, only a few reports
have been published on a systematic study of the growth and
incorporation of the elements during the ED of the films. Mishra
and Rajeshwar [1] were the first to publish the CIS film forma-
tion mechanism, concluding that CIS films grow on an initial
deposit of Cu2−xSe. Another important contribution was made
by Thouin et al. [2,3], who suggested that the stoichiometry of
the CIS film is controlled by the ratio between selenium and cop-
per fluxes ([Se4+]/[Cu2+]) on the electrode which in turn controls
the insertion of In by the Kroger’s mechanism between excess
Se in the copper selenide (CuxSey) film and In ions. The CIS
film is precursor to the quaternary phase Cu(In,Ga)Se2 (CIGS)
and for a long time the formation of CIGS by ED in one step was
∗
Corresponding author at: Centro de Investigacio´n en Energ´ıa, Universi-
dad Nacional Auto´mona de Me´xico, Xochicalco s/n, Temixco 62580, Morelos,
Mexico. Tel.: +52 55 56229841; fax: +52 55 56229742.
E-mail address: sjp@cie.unam.mx (P.J. Sebastian).
ISE member.
1
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.11.069

R.C. Valderrama et al. / Electrochimica Acta 53 (2008) 3714–3721
3715
in optoelectronic devices. Another motivation for this study is
that it has rarely been studied as a pure metal [16–18]. This
work provides a systematic study of the electrodeposition of In
utilizing electrochemical method, its characterization by X-ray
diffractionandscanningelectronmicroscopy(SEM)techniques.
2. Experimental details
The experimental system for the deposition of the indium
films constituted an electrochemical cell of three electrodes con-
nected to a PGZ301 potentiostat, for application of the potential
difference. The data acquisition was carried out by means of the
Voltamaster 4 software. For CIGS growth on Mo it is important
to make a good ohmic contact between the semiconductor and
the substrate. However, [In3+] does not present good adhesion on
Mo, and for that reason it was necessary to deposit a layer of Cu
on Mo substrate (Mo/Cu) in order to deposit In. The electrodepo-
sition of Cu layer on Mo was carried out using a bath containing
1 M CuCl2, 0.7 M LiCl, pH 1.7 and an applied potential of
−0.30 V/SCEfor1 min. Inordertoensurereproducibleinterface
conditions, a fresh sample of Mo/Cu was used for each study.
The counter electrode used was a platinum mesh of area larger
than the working electrode, and the reference electrode was a sat-
urated calomel electrode (SCE, E0 = −0.24 V). The electrolyte
consisted of 0.008 M InCl3 and 0.7 M LiCl at pH 3, prepared
with de-ionized water (Ultrapure Milli-Q). The electrolyte was
de-oxygenated for 30 min with pure N2, before initiating the
measurements and was maintained during the experiment.
The cyclic voltammetry technique was used to describe the
potential intervals where the reduction and oxidation processes
of In3+ appear. For these tests a scan of different potential val-
ues starting from the negative limiting potential (E−λ) was done.
That is, from the rest potential (E0 = 0.0 V vs. SCE) increases
by each 20 mV in the negative direction until reaching the
Fig. 1. Typical voltammograms obtained during In deposition on Mo/Cu elec-
trodes in 0.008 M InCl3, 0.7 M LiCl (pH 3) bath, at a scan rate of 8 mV/s, for
different negative limiting potentials, E−λ. (a) (—) −0.70 V and (– · –) −0.76 V,
E
−λ = −1.0 V versus SCE at a scan rate of 8 mV/s, whereas the
positive limiting potential stayed constant at 0.05 V versus SCE.
The chronoamperometric technique was used to evaluate the
different stages of electrodeposition of In on Mo/Cu substrate,
by applying a double pulse potential for 20 s, which varies
the electrocrystallization potential. The potentials were selected
considering the results of the voltammetric study.
The structure of the films was studied by means of X-ray
diffractometer (Rigaku, DMAX 2200) using Cu K␣ radiation
(1.54056 A). The morphology of the films was analyzed by SEM
inset: (
) −0.55 V and (×) −0.65 V; (b) (ꢀ) −0.80 V, (—) −0.84 V, (×)
−0.88 V, ( ) −0.92 V and (ꢁ) −0.96 V.
Fig. 1 shows the typical voltammogram of Mo/Cu electrode
in 0.008 M InCl3, 0.7 M LiCl and pH 3 during the interval of
−0.1 to −0.76 V (Fig. 1a) and for 0.0 to −0.96 V (Fig. 1b) at a
scan rate of 8 mV/s. In the inset of Fig. 1a, for E−λ ≥ −0.65 V an
important response of negative current density (−j) associated
with a capacitive behavior was observed. During the reverse
potential scan, the current density was of low magnitude, but
different from zero, so that at these potentials the In deposition
on Mo/Cu still does not take place. The In metallic deposition
begins at −0.76 V < E−λ ≤ −0.70 V versus SCE (Fig. 1a) with
the manifestation of an important cathodic current density and a
crossing potential (EC1) at −0.65 V [19,20]. Also, an oxidation
peak (A1) at −0.61 V was observed which is attributed to the
dissolution of recently formed In.
˚
(Leo 1450VP) at 20 kV. The XRD and SEM characterization of
the films were done within 24 h of their deposition.
3. Results and discussion
3.1. Voltamperometric study
In order to select the interval of potential in which the
reduction of In3+ would be carried out, the cyclic voltammetry
technique was used, by varying the negative limiting potential
(E−λ). The purpose of varying this potential in small increments
was to systematically describe the evolution of the electrodepo-
sition process.
Fig. 1b shows a reduction peak (C1) with one current density
value practically independent of the applied potential, E−λ (see
Table 1), the return scan presents a second crossing potential

3716
R.C. Valderrama et al. / Electrochimica Acta 53 (2008) 3714–3721
Table 1
The potential and current density values corresponding to the reduction peak, C1, and the oxidation peaks: A1, A2, and the parameter m1, for the different imposed
negative potentials, E−λ, during In deposition on Mo/Cu
E
−λ
(V vs. SCE)
Cathodic responses
C1
Anodic responses
A1
m1
A2
E (V vs. SCE)
−j (mA cm−2
)
E (V vs. SCE)
j (mA cm−2
)
E (V vs. SCE)
j (mA cm−2
)
E (V vs. SCE)
j (mA cm−2
)
−0.70
−0.72
−0.74
−0.76
−0.78
−0.80
−0.82
−0.84
−0.86
−0.88
−0.90
−0.92
−0.94
−0.96
−0.98
−1.00
−0.67
−0.69
−0.69
−0.74
−0.73
−0.76
−0.74
−0.74
−0.76
−0.75
−0.74
−0.75
−0.75
−0.74
−0.75
−0.76
−1.0
−1.3
−1.4
−1.6
−2.3
−2.2
−2.1
−2.1
−2.0
−2.2
−2.0
−2.0
−2.2
−2.1
−2.1
−2.2
−0.61
−0.61
−0.61
−0.61
−0.60
−0.61
−0.61
−0.59
−0.59
−0.59
−0.60
−0.60
−0.61
−0.62
−0.62
−0.62
1.8
0.6
0.9
1.0
1.4
1.0
1.5
2.9
2.3
2.6
1.3
1.3
0.8
0.5
0.2
0.2
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1.2
1.2
1.4
1.0
0.9
–
–
–
–
–
–
–
–
−0.52
−0.51
−0.51
−0.52
−0.52
−0.52
−0.51
−0.51
−0.51
−0.50
−0.50
−0.50
−0.49
−0.52
0.2
0.5
0.8
0.2
0.5
0.5
0.8
0.8
0.7
1.2
1.2
1.5
2.0
0.7
−0.56
−0.56
−0.56
−0.56
−0.56
–
–
–
–
(EC2) at ∼−0.70 V, hence it may be inferred that the formation
ofInmetallicnucleusislimitedbyadiffusionprocessoftheelec-
troactive species [19,20]. While, in the anodic region between
A1 and A2 oxidation peaks, which were previously described, a
current plateau (m1) was defined.
it can be observed that the potentials are very similar and their
values agree with the magnitude of the conditional potential
of the system where it is taken into account the experimental
conditions, as expected for the redox pair of a system [9,10].
Whereas, A2 oxidation peak appears when In0 has been
formed at −0.74 V and considering the spontaneous formation
of CuIn alloy at room temperature [24,25], therefore A2 is asso-
ciated to In dissolution from the CuIn alloy which was formed
during the interval of potential shown in Table 1.
The results shown in Fig. 1 indicate that the In electrodepo-
sition/dissolution mechanism, in the present work was carried
out in two steps depending on the potential E−λ, so that, when
the energy conditions are higher than that for the instantaneous
deposition of In0 part of this was diffused to the bulk Cu to
form the CuIn alloy, fact by which the occurrence of two current
maxima may be justified. Therefore, increase of positive current
density for A2 peak at more negative potentials than −0.74 V
was recorded [26].
With the purpose of to be able to describe the different
steps which presented during the reduction/oxidation processes,
Table 1 presents the values of potential and the current den-
sity that correspond to the cathodic and anodic responses
for the imposed values of E−λ. The electrodeposition of In
on Mo/Cu begins from −0.70 and goes up to −1.0 V, so
that, at −0.70 V it was under mixed control [21,22] and at
−0.80 V < E−λ ≤ −0.70 V it was controlled by diffusion pro-
cess, at −1.0 V < E−λ ≤ −0.80 V it was limited by diffusion,
confirmingtheresultsdescribedinthepreviousvoltammograms.
Ontheotherhand, theoxidationprocesswascarriedoutindiffer-
ent steps, which depends on the potential at which the metallic
In was deposited. The oxidation process A1 was presented at
every potential interval studied; the value of the current density
The potential interval of −0.70 to −0.85 V versus SCE from
the previous voltamperometric study was selected to analyze
the nucleation processes of In3+ in greater detail using the
chronoamperometric technique.
for A1 showed an important behavior: it was increased for E
−λ
in the potential interval −0.70 to −0.84 V and was decreased
for E from −0.86 to −1.0 V, hence it may be inferred that
−λ
A1 corresponds to the dissolution of the In0 according to the
reaction:
3.2. Chronoamperometric study
In0 − 3e− ↔ In3+
The potential for reaction (1) in agreement with the Nernst
equation is
(1)
In order to find out the different stages of the electrocrystal-
lization process of In3+ on Mo/Cu substrates, double potential
pulses were applied, so that, during the direct pulse the In depo-
sition was carried out, which was dissolved during the reverse
pulse.
Fig. 2showsafamilyofpotentiostaticcurrenttransientsofthe
electrodeposition and oxidation for the In on Mo/Cu substrates
in 0.05 M InCl3, 0.7 M LiCl (pH 3) bath. The imposed direct
pulses varied from −0.7 to −0.85 V, while the reverse pulse was
0.06
E = E0 +
log |In3+
|
(2)
3
where |In3+| = 0.008 M and E0 = −0.61 V versus SCE at pH 3
[23], therefore E = −0.65 V versus SCE. If one compares the
E0 value with the crossing potential EC1 = −0.65 V versus SCE

R.C. Valderrama et al. / Electrochimica Acta 53 (2008) 3714–3721
3717
Fig. 2. The potentiostatic current transients for the deposition and oxidation
of In on Mo/Cu substrates in 0.05 M InCl3 and 0.7 M LiCl (pH 3) bath. The
imposed direct pulses were (ꢁ) −0.70 V, (ꢀ) −0.75 V, (×) −0.80 V and (—)
−0.85 V vs. SCE, the inverse pulse stayed constant at +0.10 V vs. SCE.
Fig. 3. The falling part of the transients from Fig. 2 plotted, −j vs. t−1/2, accord-
ing to the Cotrell’s equation predicting the linearity for diffusionally controlled.
The symbols are experimental values and the lines represent the linear adjust-
ments.
The film thickness was calculated according a Faraday’s law
normalized to a unit surface [14]:
maintained constant at +0.10 V, the pulse time was 20 s. During
the In deposition (first 20 s) the shape of the transients is a typical
response of an electrochemical nucleation and growth process.
A sudden increase of current until reaching a maximum value,
then it decreases asymtotically until reaching a constant current
value.
Considering the current transients (Fig. 2), the efficiency of
the process, the number of monolayers, the film thickness and
the diffusion coefficient were evaluated. In order to evaluate the
efficiency of the process, the charges associated with the reduc-
tion and oxidation processes, QC and QA, respectively, were
obtained by integrating the cathodic and the anodic branches
of the current transients. Table 2 shows the values for QA, QC
MQ
F =
(3)
ρzh
where the number of electrons exchanged z = 3 (Eq. (1)),
charge Q = QC, h is the thickness, indium molar mass
M = 114.82 g mol−1 and density ρ = 7.31 g cm−3. Substituting
these values in Eq. (3) one gets h as a function of QC, which is
MQ
h =
= (5.426 × 10−5)QC
(4)
ρzF
Table 2 indicates that both the number of monolayers and the
value of h slowly increase with the imposed potential, with the
exception of the values that correspond to E = −0.70 V.
The diffusion coefficient (D) was evaluated by analyzing the
experimental data that corresponds to the fall of the direct pulse
transients. This was plotted as −j versus t−1/2 (Fig. 3) and was
observedthat atlongertimes, the −j values tendto cometogether
in one point, and all responses can be fitted through a straight
line with almost the same inclination, except for the case in
which E = −7.0 V. This concurs with the linearity predicted by
the Cotrell’s equation. Thus, the linear fit was done according
to the Cotrell’s equation:
and the total efficiency of the process (QA/QC ratio) for the dif-
∼
ferent imposed potentials. These values, QA/QC 1, indicate
=
a high degree of reversibility for the In electrodeposition and
oxidation.
An approximate way to evaluate the monolayers that were
formed during the In deposition was formulated as follows:
considering the number of In atoms required to cover the geo-
metric area of the working electrode (1 cm2) and if the area of
an In atom is ≈8.66 × 10−16 cm2, therefore 1.15 × 1015 atoms
of In will form a monolayer, thus the quantity of charge to
form the monolayer would be Qmonolayer = 0.185 mC. Finally,
QC/Qmonolayer ratio is equal to the number of monolayers
(Table 2).
zFD1/2
π1/2
c
j =
= mt−1/2 + b
(5)
Table 2
The dependence of the reduction QC and oxidation QA charge, efficiency of process, QA/QC, the number of monolayers, and the film thickness, h, on the potential
obtained from the current transient analysis presented in Fig. 2
E (V vs. SCE)
QC (mC cm−2
)
QA (mC cm−2
)
QA/QC
No. of monolayers
h (␮m)
−0.70
−0.75
−0.80
−0.85
82.9
116.8
124.2
127.6
85.3
121.7
129.4
122.1
1.03
1.04
1.04
0.96
448
631
671
690
4.50
6.33
6.74
6.92

3718
R.C. Valderrama et al. / Electrochimica Acta 53 (2008) 3714–3721
Table 3
The dependence of current, Jm, and time, tm, maxima, the diffusion coefficient, D, and the number density of nucleation active sites, N0, on the potential obtained
from the current transients analysis presented in Fig. 2
E (V vs. SCE)
Jm (mA cm−2
)
tm (s)
D (×10−6 cm2 s−1
)
N0 (Eq. (7)) (×105 cm−2
)
Cotrell’s equation
Eq. (7)
−0.70
−0.75
−0.80
−0.85
4.80
8.89
12.35
15.04
3.04
1.55
1.08
0.85
0.21
3.82
4.01
3.93
1.90
4.67
4.39
4.19
5.86
2.99
6.69
11.22
where z and F were defined in Eq. (3), c is the bulk concentration
of species (c = 5 × 10−5 mol cm−3). Therefore D is give by,
and
Jm = 0.6382zFDc(KN0)1/2
For the progressive case:
(11)
ꢀ
ꢁ
2
m(πt)−1/2 + bπ−1/2
14.473 C cm−3
D =
(6)
ꢃ
ꢄ
1/2
4.6733
AN0πKD
tm
=
(12)
(13)
In Table 3, the D values are shown as a function of potential.
The average experimental value was D = 3.9 × 10−6 cm−2 s−1
.
and
In order to be able to study and describe the electrocrystal-
lization process of In, the potenciostatic transients of Fig. 2 was
analyzed. It has been reported in the literature similar responses
to these transients, which have been evaluated through mathe-
matical models that correspond to a three-dimensional growth
[6,7]. In accordance with voltamperometric study and the evalu-
ation of D; the In deposit was controlled by a diffusion process.
Thus, the transients in Fig. 2 can be described through a three
dimensional growth (3D) for an instantaneous nucleation:
Jm = 0.4615zFD3/4c(KAN0)1/4
The non-dimension expression of Eqs. (7) and (8) for the instan-
taneous case is
ꢃ
ꢄ
ꢆ
ꢂ
ꢃ
ꢄꢅ
2
2
J
1.9542
t/tm
t
=
1 − exp −1.2564
(14)
Jm
tm
And for the progressive case is
ꢇ
ꢀ
ꢁꢈ
2
ꢃ
ꢄ
ꢃ
ꢄ
2
2
zFD1/2
J
1.2254
t/tm
t
=
1 − exp −2.3367
(15)
J =
[1 − exp(−N0πKDt)]
(7)
π1/2 1/2
t
Jm
tm
Or for the progressive type nucleation:
Fig. 4 shows the normalized cathodic transients from Fig. 2
(symbols) together with the theoretical curves (lines) for instan-
taneous and progressive 3D nucleation and growth, obtained
ꢂ
ꢃ
ꢄꢅ
zFD1/2
c
2AN0πKDt2
J =
1 − exp
−
(8)
π1/2 1/2
t
3
N0 is the number density of nucleation active sites (cm−2). A is
the steady-state nucleation rate per active site (s−1). The other
terms are defined in Cotrell’s equation, except K which is the
non-dimensional growth rate constant of a nucleus, defined as
ꢃ
ꢄ
1/2
8πMc
K =
(9)
ρ
To determine whether the nucleation is instantaneous or pro-
gressive, an initial observational diagnostic was done, that
involves comparing the experimental and theoretical transients
in non-dimensional coordinates. For that, the experimental and
theoretical transients are normalized by their corresponding
maximum values of current (Jm) and time (tm). In Table 3, the
experimental values of Jm and tm are given. Theoretical maxi-
mum values can be evaluated by equating the first derivative of
Eqs. (7) and (8) to zero, then finding the values of tm and Jm. So
for the instantaneous case:
Fig. 4. A comparison of the theoretical non-dimensional plots, (j/jm)2 vs. (t/tm)
forinstantaneousequation(14)(solidline)andprogressiveequation(15)(dashed
line) nucleation with experimental data of the potentiostatic transients from
Fig. 2. The imposed potentials were: (ꢁ) −0.70 V, (ꢀ) −0.75 V, (ꢁ) −0.80 V
and ( ) −0.85 V.
1.2564
N0πKD
tm
=
(10)

R.C. Valderrama et al. / Electrochimica Acta 53 (2008) 3714–3721
3719
using Eqs. (14) and (15), respectively. By comparing the exper-
imentally obtained data with the theoretical curves, the figure
shows that at a very short time, before reaching the maxi-
mum points, all of the experimental data follow the tendency
of instantaneous nucleation. After for longer periods, during the
current fall, the experimental curves were separated and when
the imposed potential is more negative, the transient fall between
the limits of instantaneous and progressive nucleation. Thus, it
appears that the In deposition was carried out under a 3D growth
with instantaneous nucleation.
In order to prove that the experimental transients correspond
to a 3D growth with instantaneous nucleation, a non-linear fit
of Eq. (7) was performed. Table 3 gives values for: D and
N0, simulated from the fit of Eq. (7). In this table, values for
the experimental D obtained through the Cotrell’s equation,
are also reported. By comparing the experimental D with the
simulated one, similar values were found with the exception
of E = −0.70 V. On the other hand, the kinetic parameter of
nucleation N0 increases while the applied potential pulse was
more negative. This behavior was expected, since the density of
active sites was favored when increasing the energetic conditions
(potential), without considering the value of E = −0.70 V.
Finally, to show that the parameters reported in Table 3 cor-
rectly describe the experimental results, the simulated values for
D and N0 were substituted in Eq. (7), together with the known
constants, including K = 0.14 (Eq. (9)). In this way the theo-
retical current transient was generated for each of the applied
potentials (Fig. 5).
Fig. 5 shows a comparison between the simulated transients
with the experimental ones obtained for the bath 0.05 M InCl3
and 0.7 M LiCl (pH 3). When E = −0.70 V, the experimental
values were adjusted to the model for t < 4 s, but for t > 4 s it
is more adequate to use the D value obtained by the Cotrell’s
equations. The remaining experimental transients were adjusted
very well with the simulated transients. Thus, the In deposition
on Mo/Cu substrates in 0.05 M InCl3, 0.5 M LiCl and pH 3 was
Fig. 5. A comparison of the simulated transients for the 3D instantaneous nucle-
ation equation (7) (solid lines) with the experimental transients of the deposition
of In on Mo/Cu substrates in 0.05 M InCl3, 0.7 M LiCl at pH 3. The imposed
potentials were: (ꢁ) −0.70 V, (×) −0.75 V, (ꢀ) −0.80 V and (♦) −0.85 V vs.
SCE.
carried out under a 3D growth with instantaneous nucleation
limited by diffusion, which concurs with the reports by other
authors [16,17] for the In system.
In all of the analyses carried out in the chronoamperometric
study, it was found that the experimental transient that corre-
sponds to −0.70 V differs from the values and the behavior
reported for the other current transients. According to what was
found in the voltamperometric analysis, the response at −0.70 V
is presented under mixed control, therefore, the D value is very
small compared to the rest and the adjustment was not reached.
To complement this study, In films, deposited at the same
potentials and in the same electrochemical bath in which the
chronoamperometric study was carried out, were analyzed using
Fig. 6. The SEM images of In films deposited at different potentials on Mo/Cu substrates in 0.05 M InCl3, 0.7 M LiCl and pH 3. (a) At −0.70 V vs. SCE for 1 min.
The images of (b)–(e) were deposited from −0.70 to −0.85 V vs. SCE for 30 min.

3720
R.C. Valderrama et al. / Electrochimica Acta 53 (2008) 3714–3721
physical characterization techniques, in order to know the mor-
phology and structure of the In films in terms of applied
potential.
3.3. Physical characterization
To study the structure and morphology of the materials
deposited, X-ray diffraction and scanning electronic microscopy
were employed.
Fig. 6 shows five SEM images of In films obtained apply-
ing a pulse for different cathodic potentials, between −0.70
and −0.85 V versus SCE, at two different time intervals (1 and
30 min) on Mo/Cu substrates immersed in 0.05 M InCl3, 0.7 M
LiCl and at pH 3. Fig. 6a corresponds to the film deposited
at −0.70 V for 1 min. In Fig. 6a, a uniform deposit consist-
ing of crystals of approximately 0.5 ␮m was observed. The
samples show traces of the polishing lines, since the sub-
strates were polished during the cleaning. Fig. 6b corresponds
to the deposit at −0.70 V for 30 min. In this figure, a mas-
sive deposit was observed, which does not present a defined
crystalline geometry. However, for the deposit at −0.75 V for
30 min (Fig. 6c), well-defined crystalline structures, separated
with grain sizes that vary between 2 and 10 ␮m were observed.
With the increase of the deposition potential (Fig. 6d and e),
the crystals come together forming agglomerates that make up
a compact surface. Also, when comparing the SEM image for
the film deposited for 1 min (Fig. 6a) with those of 30 min, it
is evident that the deposition time is a function of In grain size
growth.
The results of the SEM analysis concur with those obtained
from the study of current transients, since: 3D growths with
instantaneous nucleation produce massive deposits, and also
concurs with the calculation obtained for the thickness of these
films (h = 6 ␮m).
Fig. 7 presents the XRD spectra for the In films deposited
at different pulses of cathodic potentials between −0.70 and
−0.85 V versus SCE for two deposition time intervals (1 and
30 min) on Mo/Cu substrates immersed in 0.05 M InCl3, 0.7 M
LiCl and pH 3. The XRD analysis was done at an incident angle
of 1.5◦. In Fig. 7a, the films deposited at different potentials
for 1 min are indicated within the figure. The XRD spectrum
in Fig. 7a exhibits as first phase the CuIn alloy at 34.459◦ and
43.097◦, according to the JCPDS card 35-1150. The peak that
corresponds to the In phase at 32.985◦ is barely observable.
Also, two peaks at 43.372◦ and 50.494◦ that correspond to the
Cu substrate were observed.
Fig. 7b shows the XRD spectra for the films deposited at
different potentials. This figure indicates that the films are poly-
crystalline in nature for all the deposition potentials. The first
phase was In, without considering the known phase of Cu, in
the (1 0 1) plane. This diffraction spectrum corresponds to the
tetragonal system according to the JCPDS card 05-0642. The
CuIn phase was presented as the second phase of importance.
Considering the results of XRD and SEM, the films deposited
for 1 min presented a compact crystalline layer with In nucleuses
which reacts with Cu forming CuIn alloy, on this the In film was
deposited.
Fig. 7. The XRD patterns of In films deposited on Mo/Cu substrates in 0.05 M
InCl3, 0.7 M LiCl and pH 3 at different potentials from −0.70 to −0.85 V vs.
SCE for two different times: (a) 1 min and (b) 30 min. Angle of incidence is
1.5◦.
4. Conclusions
The voltamperometric study showed a reduction process
(peak C1) which corresponds to the reaction of In3+ to In0. The
oxidation peak A2 corresponds to In dissolution from the CuIn
alloy. Also, it can be noted that the experimental cross poten-
tial of In3+ to In0 (EC1 = −0.65 V vs. SCE) was very close to
the value reported in the literature (E0 = −0.61 V vs. SCE). The
nucleation of In was limited by the diffusion process.
The current transients were utilized to evaluate the charge
associated with the oxidation and reduction process. From this
it was observed that the efficiency of the process In3+ to In0
was 100%. Also the physical parameters like number of mono-
layers (670 monolayers), the thickness of the film, and the
diffusion coefficient (D = 3.8 × 10−6 cm2 s−1) were evaluated.
The analysis of the reduction part of the current transients, using
theoretical growth model, showed that the electrodeposition of
indium adheres to a 3D growth under instantaneous nucleation
limited by diffusion. The kinetic growth parameters N0 was eval-

R.C. Valderrama et al. / Electrochimica Acta 53 (2008) 3714–3721
3721
uated through a non-linear fit. In addition to that the diffusion
coefficient was also evaluated.
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From the structural and morphological characterization it
may be concluded that the films are of polycrystalline in nature
with compact and uniform surface. In the first minute of depo-
sition a compact layer of CuIn alloy was formed on that the In
film was grown. After a 30 min deposition, large crystallites of
In was formed and as the potential was increased agglomerates
were formed, resulting in compact and massive surfaces.
The current transients showed 3D growth with instanta-
neous nucleation producing massive deposits. This is in good
agreement with the results obtained from the physical charac-
terization. A detailed study of the electrodeposition of In was
carried out and the results obtained from this will help in ana-
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of CIS and CIGS.
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The authors gratefully acknowledge Ing. Francisco Ginez-
Carbajal for helping with the electrodeposition of In films. This
workwasfundedbyDGAPA-UNAMunderthegrantES113107.
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