Effect of potential on bismuth telluride thin film growth by electrochemical atomic layer epitaxy

Article abstract of DOI:10.1016/j.electacta.2005.01.003

In the present study, bismuth telluride compound thin film was grown by means of electrochemical atomic layer epitaxy (ECALE) with an automated thin layer flow cell deposition system. The dependence of the Bi and Te deposition potentials on Pt electrode was studied. Because developing a contact potential between the substrate and the growing semiconductor, the deposition potential adjustment is necessary for the first 30 or more cycles of each component. The dependence of the deposit as a function of the deposition potential adjustment slope has been investigated. The results show that an excess elemental Bi existed at a slope of -2 mV/p (p indicates per cycle), indicating that this is a lack of deposition at the potential. Single-phase Bi₂Te ₃ compound could be obtained between -4 and -6 mV/p. Bi ₂Te₃ and Bi₄Te₃ coexistence is observed at a slope of -10 mV/p. The EDS data indicates that the stoichiometry of compound is consistent with XRD result. SEM studies show that the deposits are inhomogeneous and have an micron sized particles morphology.

Full text of DOI:10.1016/j.electacta.2005.01.003

Electrochimica Acta 50 (2005) 4041–4047
Effect of potential on bismuth telluride thin film growth by
electrochemical atomic layer epitaxy
W. Zhu, J.Y. Yang∗, X.H. Gao, S.Q. Bao, X.A. Fan, T.J. Zhang, K. Cui
State Key Laboratory of Die&Mould Technology, College of Materials Science and Engineering,
Huazhong University of Science and Technology, Wuhan 430074, PR China
Received 11 November 2004; received in revised form 7 January 2005; accepted 7 January 2005
Available online 2 February 2005
Abstract
In the present study, bismuth telluride compound thin film was grown by means of electrochemical atomic layer epitaxy (ECALE) with an
automated thin layer flow cell deposition system. The dependence of the Bi and Te deposition potentials on Pt electrode was studied. Because
developing a contact potential between the substrate and the growing semiconductor, the deposition potential adjustment is necessary for the
first 30 or more cycles of each component. The dependence of the deposit as a function of the deposition potential adjustment slope has been
investigated. The results show that an excess elemental Bi existed at a slope of −2 mV/p (p indicates per cycle), indicating that this is a lack
of deposition at the potential. Single-phase Bi2Te3 compound could be obtained between −4 and −6 mV/p. Bi2Te3 and Bi4Te3 coexistence is
observed at a slope of −10 mV/p. The EDS data indicates that the stoichiometry of compound is consistent with XRD result. SEM studies
show that the deposits are inhomogeneous and have an micron sized particles morphology.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: ECALE; UPD; Bismuth telluride; Thermoelectric material; Thin film
1. Introduction
In general, these methods are performed in vacuum and are
thermal methods, achieving compound formation by heating
the reactants and substrate.
Bismuth telluride based compounds have attracted consid-
erable interest as thermoelectric (TE) materials. These mate-
rials are widely used for Peltier coolers. The performance of
thermoelectric devices depends on the figure of merit (ZT)
of the material. Several possible approaches to enhancing ZT
have been investigated [1–3]. In comparison with bulk TE
materials, thin film TE materials offer tremendous scope for
ZT enhancement. Meanwhile, thin films of bismuth telluride
also can be applied to TE temperature controllers for sub-
miniature electronic devices. In the formation of high qual-
ity thermoelectric devices, a number of thin film formation
methodologies are used, including: molecular beam epitaxy
(MBE) [4], chemical vapor deposition (CVD) [5,6], flash
evaporation [7,8], co-evaporation [9,10], and sputtering [11].
Low temperature electrochemical deposition is desirable
for avoiding heat-induced interdiffusion of adjacent layers in
a structure, in addition, vacuum atmosphere is not longer a ne-
cessity. There are several methods presently used to form bis-
muth telluride compounds electrochemically, the most prac-
tical being codeposition [12,13]. The resulting deposits often
require post-deposition annealing procedures, which negate
the low temperature advantage of electrodeposition.
Atomic layer epitaxy (ALE), in general, is a thin film for-
mation methodology where growth is controlled using sur-
face limited reactions. Separate reactions are used for the
atomic layers of each of the component elements, and these
reactionsareruninacycle, witheachcycleproducingoneML
of the compound. The thickness of the deposit is determined
by the number of cycles performed. Electrochemical atomic
layer epitaxy (ECALE) [14–26] is the electrochemical analog
∗
Corresponding author. Fax: +86 27 87543776.
E-mail address: jyyang@public.wh.hb.cn (J.Y. Yang).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.01.003

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W. Zhu et al. / Electrochimica Acta 50 (2005) 4041–4047
valve bank to avoid problems with mixture of solution each
othervia siphonal phenomena. The secondapproachinvolved
the upturned flare of the channel at the inlet and outlet. The
structure utilized the buoyancy to dislodge trapped bubbles
automatically. The third approach was based on potential sta-
bilization between the reference electrode and the working
electrode. Here, the outlet stream of the cell was higher than
the reference electrode underside. The structure ensured the
reference electrode was contacted with solution adequately,
rather than the outlet stream was level with the reference elec-
trode underside, thereby potentially eliminated potential ex-
cursion. The cavity was defined by a Si(1 0 0) wafer substrate,
coated with 1 ␮m of Pt by magnetic sputtering and an auxil-
iary electrode, a plate of Pt. These electrodes were held apart
by a 5 mm-thick gasket, which defined a 0.7 cm × 3 cm rect-
angular opening. The plexiglass was transparent, allowing
the deposition process to be followed visually. The reference
electrode, saturated calomel electrode (SCE), was positioned
at the cavity outlet.
Solutions were prepared with high purity reagents and
twice-distilled water. All bismuth solutions consisted of
0.1 mM Bi(NO3)3·5H2O, and used 0.1 M HClO4 as a sup-
porting electrolyte, pH 1.5. Tellurium solutions were 0.1 mM
in TeO2, and also used 0.1 M HClO4 as a supporting elec-
trolyte. The pH 8.5 Te solutions were adjusted with ammonia.
Various blank rinse solutions were also utilized, with a pH
analogous to its respective deposition solution. All solutions
were deaerated by blowing purified N2 gas through and over
the solution for 30 min. All experiments were performed at
room temperature.
Fig. 1. The schematic drawing of an automated electrochemical thin-layer
flow deposition reactor.
of ALE. Electrochemical surface limited reactions are gener-
ally referred to as underpotential deposition (UPD) [27–30].
Electrochemical ALE (EC-ALE) is the result of combining
UPD with the principles of ALE to form a deposition cycle.
ECALE cycles involve switching deposition solutions and
potentials to form each component atomic layer. This is in
marked contrast to codeposition, where a single solution is
used to deposit all the elements, generally at a single poten-
tial. By using an ECALE cycle, the degrees of freedom in the
deposition process are expanded. Variables in the cycle in-
clude the deposition potentials, the rinsing procedures, flow
rates, deposition times, etc.
II–VI compounds such as CdTe [15–19], CdS [22–25,31],
and ZnSe [20] have been successfully formed using by
ECALE, as well as some III–V compounds: GaAs [32,33],
InAs [34]. Recently Oznuluer has reported the kinetics and
growth mechanism of VA–VIA compound Bi2S3 by ECALE
[35]. Despite the element sulfur and tellurium belong to VIA
family of periodic table, there are very significant differences
for their electrochemical aspects. However, no work has been
reported on the formation of bismuth telluride VA–VIA com-
pound thin film by ECALE. In the present investigation,
an automated computer controlled thin-layer electrochemi-
cal flow cell system is developed in this group (Fig. 1). The
dependence of the Bi and Te deposition potentials on Pt elec-
trode will be reported in this paper.
Energy dispersive spectroscope studies were performed
using a Oxford Inca EDS. The XRD patterns were obtained
with a Philips-PW 1710 X-ray diffractometer using Cu K␣
radiation. SEM images were taken with a commercial instru-
ment (Quanta 400)
3. Results and discussion
3.1. Electrochemical aspects of tellurium and bismuth
The curve shown in Fig. 2 is obtained from tellurium so-
lution on the Pt substrate. The scan proceeded from right
to left and resulted in two reduction peaks C1 and C2. In-
tegration of this peak C1 corresponds to 0.4 ML Te cover-
age approximately, suggesting an apparently surface limited
deposition. The UPD nature of this peak C1 is further sup-
ported by the fact that it disappears in the successive scans,
in which only bulk TeO2 reduction is observed (Fig. 3). Peak
C2 corresponds to Te bulk deposition on the Pt substrate. The
subsequent anodic stripping peaks A2 and A1 correspond to
stripping of the Te bulk and Te UPD, respectively.
2. Experimental procedure
The deposition instrument consisting of peristaltic pumps,
valves, programmable logistic computer (PLC), a flow cell
and potentiostat was used under the control of a computer
(Fig. 1). The electrochemical flow cell was similar to those
described by Stickney work group [15]. However, some im-
provement were progressed by this group and described be-
low. One involved the use of subminiature single-directional
Due to the slower kinetics of Te reductive UPD on Pt, one
method for forming the Te UPD layer involves first depositing
a full aliquot of TeO2 at −0.6 V, so that UPD and bulk Te
result, followed by reduction of the bulk Te, forming soluble

W. Zhu et al. / Electrochimica Acta 50 (2005) 4041–4047
4043
Fig. 2. Cyclic voltammogram of Te on Pt. The scanning rate is 10 mV/s.
Fig. 5. Anodic potentiodynamic curves, obtained on the bulk deposited Te
electrode in an ammonia buffer solution of pH 8.5. The electrode was po-
larized to the potential E (−1.1 V to −1.6 V) for 1.5 min, then the anodic
potential scanning was performed from E = 0.05 to 0.65 V. The potential
scanning rate was 10 mV/s.
towards more negative values, the re-reduction process of Te
(UPD) occurs giving a very narrow and sharp shape current
peak C2.
Since the Te UPD layer is more strongly bound to Pt sub-
strate and therefore it is expected to reduce at more negative
potentials. It is clearly shown from the cyclic voltammograms
recorded in the region of UPD stripping process after keep-
ing the electrode at increasingly negative potentials (Fig. 5).
Keeping the electrode at −1.1 to −1.3 V only occurs bulk de-
posited Te re-reduction. The underpotentially deposited Te,
however, remains on the electrode, thus having an anodic
striping peak of underpotential deposits in the region of UPD
stripping process. As a matter of fact, the underpotentially de-
posited Te starts to be re-reduced at the potential of −1.4 V.
Keeping the electrode at potentials as negative as −1.6 V
causes complete re-reduction of the previously underpoten-
tial deposited Te, thus having no anodic stripping peak of
underpotential deposits.
Fig. 3. Cyclic voltammograms of 0.1 mM TeO2 in an ammonia buffer solu-
tion of pH 8.5 on Pt. The numeral indicates consecutive scans, from E = −0.1
to −0.8 V. The scanning rate is 10 mV/s.
H2Te. This process is referred by Foresti and Stickney as
oxidative Te UPD [18–20]:
Te (UPD) + Te (bulk) + H+ + 2e · · · Te (UPD) + HTe−
(1)
Fig. 4 shows a cyclic voltammogram of a Pt electrode cov-
ered with overpotentially deposited Te (−0.6 V) in the pH 8.5
blank solution containing no Te redox system. The reduction
peak C1 (−0.9 to −1.05 V) only involves the re-reduction
process of bulk deposited Te. When the potential is scanned
Fig. 6 shows the current–potential curves for different Bi
concentration deposition and stripping on a Pt substrate. The
Fig. 6. Cyclic voltammograms of Bi on Pt obtained from 0.1 mM (dot line)
and 0.4 mM (solid line) Bi(NO3)3·5H2O in a prochloric acid solution. The
scanning rate is 10 mV/s.
Fig. 4. Cyclic voltammogram of the bulk deposited Te reduction to Te2− in
an ammonia buffer solution of pH 8.5. The scanning rate is 10 mV/s.

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W. Zhu et al. / Electrochimica Acta 50 (2005) 4041–4047
relatively broad cathodic peak between 0.5 and 0 V, labeled
C1, and the stripping peak, labeled A1, are practically coinci-
dent, in despite of the concentration change. This suggested
C1 and A1 to be conjugative deposition and stripping UPD
peaks. In fact, because the UPD process is surface-limited,
the charge involved in the UPD is independent of concen-
tration: increasing concentration from 0.1 to 0.4 mM causes
only the bulk redox process becomes more intensive. Fig. 6
shows a clear difference between cathodic and anodic charge,
which labeled C2 and A2 for the concentration variety, cor-
responding to the bulk reduction and oxidation, respectively.
ergy change involved in the formation of the different chalco-
genides [19]. That is, the underpotential depositions of Bi on
Te-covered substrate should reflect the heat of formation of
bismuth telluride. The more negative the heat of formation,
the more positive the potential at which UPD occurs. Bi2Te3
is the least stable Bi chalcogenide, therefore, Bi UPD on
Te-covered substrate should occur at less positive potentials.
Moreover, once the adsorbed Te atoms present, interactions
of Bi with the substrate become weaker than those involved
in Bi2Te3 formation could shift the underpotential deposi-
tion towards more negative potentials. As a consequence, the
potential 0.2 V chosen for deposition might become too pos-
itive to deposit a sufficient amount of Bi. Therefore, the Bi
underpotentials for the first 30 cycles needed to adjustment
gradually to lower, albeit in shorter and shorter steps, until
maintain a steady state. This was also observed in ECALE
of the CdTe system by Stickney and co-workers [15]. In fact,
it must be stressed that bulk electrodeposition processes are
scarcely influenced by the substrate. Thus, bulk Te reduction
on a Bi-coated Pt electrode occurs at the same potentials as
on the bare Pt electrode. After deposition of Te, it is a prob-
lem to shift the potential more positively for Bi deposition. If
the Bi underpotentials drop for the first 30 cycles very small,
the potential needed to shift back more positively for Bi de-
position and some of the previously deposited Te would be
stripped, though the Te UPD stripping potential could shift
to a little positive position for the lower pH of Bi deposi-
tion. On the other hand, a large potential drop used for the
first 30 cycles of Bi deposition will not result in loss of Te.
However, if the potential drop too large bulk Bi deposits will
occur. In the present study, potentials were not extensively
optimized, however, to investigate the problems, such as de-
scribed above, several potential drop which are adjustmented
negatively each step for the first 30 cycles, are tried.
3.2. ECALE film deposition
Initial Bi potential of 0.2 V on Pt is selected, along with
a Te potential equal to −0.6 V, for the first bismuth telluride
+
program. Due to the low concentration of HTeO2 and the
slow kinetics for Te deposition, very little Te (bulk) formed.
+
The HTeO2 solution is then exchanged for a blank elec-
trolyte solution, and a potential sufficiently negative (−1.1 V)
to re-reduce Te (bulk), but not negative enough to re-reduce
Te (UPD), is applied. The first cycle of the ECALE process
for the deposition is schematized in Fig. 7. However, dur-
ing the process of ECALE, using the potentials determined
by cyclic voltammograms such as those shown in Fig. 7, for
the deposition of elements on a Pt substrate, is a very sim-
plistic approximation. The deposition charges decreased over
the first few cycles if the potentials determined from Fig. 7
are used and kept constant during the ECALE process of the
Bi–Te system. This is because the underpotentials for depo-
sition of these elements on Pt are different from those for
deposition on each other or on the compound. In the ECALE
process of the Bi–Te binary system, the first Te deposition is
essentially UPD of the element on the electrode. Then, the
deposition of the Bi element occurs on a Te-covered Pt sub-
strate and forms a monolayer of the compound. Foresti has
suggested that this process is simply driven by the free en-
3.3. Structure control and morphology of the compound
growth
Fig. 8 shows XRD patterns of 200-cycles electrodeposited
compounds for different potential adjustment slopes of the
Fig. 8. X-raypatternof200-cycleselectrodepositonPtfordifferentpotential
adjustment slope of the first 30 cycles.
Fig. 7. Diagram of an ECALE cycle for Bi2Te3 formation.

W. Zhu et al. / Electrochimica Acta 50 (2005) 4041–4047
4045
Table 1
The change in composition of thin films as a function of potential adjustment
slope
Adjustment slope X (mV)
Bi (At.%)
Te (At.%)
−2
−4
94.74
42.51
39.70
49.60
5.26
57.49
60.30
50.40
−6
−10
first 30 cycles. Peaks of Si and Pt are observed at a slope of
−2 mV/p (p indicates per cycle), indicating that the coverage
of electrodeposited thin film is low. It suggested this is a
lack of deposition at the potential. Probably, as the above
mentioned, this value, 0.2 V, corresponds to the deposition
of Bi on Pt and had to be changed after the deposition of a
Te layer. Increasing the negative slope from −2 to −4 mV/p,
the XRD peak of Si disappears and the peaks of Pt weaken,
along with the peaks of Bi2Te3 become more evident. It can
be seen single-phase Bi2Te3 compound is obtained at a slope
of −6 mV/p except the prominent substrate [1 1 1] reflection.
Fig. 9. Plots of the current involved in UPD of bismuth and tellurium alter-
nately as a function of deposition time.
Bi2Te3 and Bi4Te3 coexist with a further decrease of the slope
to −10 mV/p.
Energy dispersive spectroscopy (EDS) is used as a prelim-
inary diagnostic tool to study the compositions of the films.
The variation in the composition of the films as a function
Fig. 10. SEM images of different examples: (a) naked substrate, (b) outlet of the flow cell, (c) center of the flow cell, (d) entrance of the flow cell.

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W. Zhu et al. / Electrochimica Acta 50 (2005) 4041–4047
of potential adjustment slope is summarized in Table 1. The
compositions shown above are roughly 2:3forBi:Te when the
slopes equal to −4 and −6 mV/p. Evident excess elemental
Bi is observed at −2 mV/p. The evident Bi excess probably
is due to a partial loss of Te in the Bi UPD process on the Te,
where Te atomic layer, however, is not stable at 0.14 V, and
will oxidatively strip from the surface under the positive po-
tential conditions. The approximate 1:1 stoichiometric ratio
for bismuth to tellurium at −10 mV/p suggested the Bi2Te3
and Bi4Te3 coexistence, which is consistent with XRD
result.
pound, is confirmed by EDS. SEM studies indicate that the
deposits are inhomogeneity, probably because the flow pat-
terns within the cell have been developed, as well as it have
correlated with defects in the substrates. It is evident that the
cycles still need some optimization, however, it is also clear
that EC-ALE can be used to grow Bi2Te3 with atomic layer
control.
Works on optimizing the ECALE growth conditions for
bismuth telluride are continuing. Different deposition condi-
tions are tuning in order to further retard the growth of the
films even further in order to improve the morphology and
crystallinity of the films, and to control the composition more
accurately.
In conclusion, the potential adjustment slope of −6 mV/p
seems successful and feasible for the Bi2Te3 compound
ECALE process. The current time traces for this procedure
were shown in Fig. 9. Using the current time traces we can ex-
amine the deposition process for per step in real time and the
current involved in each step is controlled. From integration
of the currents for deposition, the Bi and Te coverages per
cycle, from coulometry, averaged 0.6 ML of Te and 0.4 ML
of Bi. The film prepared at −6 mV/p is examined by scanning
electron microscopy (SEM). After 200 cycles, the deposited
filmshowsaninhomogeneousdistributionalongtheflowcell.
Film grown at outlet of the flow cell is very smooth as seen by
eye, like the naked substrate, and SEM image shows small,
relatively monodispersed feature sizes, as shown in Fig. 10b.
At center of the flow cell the film appear gray. Under the SEM
the film shows a number of micron sized crystallites on the
surface. However, the region appears smooth, and the crys-
tals are stoichiometric (Fig. 10c). The film at entrance of the
flow cell has rough morphology (Fig. 10d). As comparing,
the naked substrate image, which has not any deposits, also
shows in Fig. 10a. The inhomogeneous deposits are similar
to that of the binary CdTe [15]. Stickney work group has sug-
gested these results are related to the significant irreversibility
for Te, which results in a not completely surface limited pro-
cess. Given homogeneous conditions in the cell and the use
of optimal potentials, high quality deposits can be formed.
In future flow cells the 5 mm thick gasket has been limited
to 0.8 mm thick, helping to establish laminar flow earlier in
the flow channel. On the other hand, the presence of inhomo-
geneities also attributes to the poor quality of the substrate
by magnetic sputtering, which is generally polycrystalline,
and often accompany with some defects. Studies of bismuth
telluride growth using ECALE on single crystal substrates
are beginning, and should help to resolve the issue.
Acknowledgements
This work is co-financed by the National Basic Research
Program (2004CCA03200), the Chinese National Natural
Science Foundation (50401008), the National Foundation
for Quick Response to Fundamental Research of China, the
National Foundation of China for Returnee Student from
Abroad.
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