Formation of HgSe thin films using electrochemical atomic layer epitaxy

Article abstract of DOI:10.1149/1.2047547

The growth of HgSe using electrochemical atomic-layer epitaxy (EC-ALE) is reported. EC-ALE is the electrochemical analog of ALE, where electrochemical surface-limited reactions referred to as underpotential deposits, generally result in the formation of an atomic layer of an element, under controlled potential. HgSe thin films were formed on gold substrates using two reactant solutions: a solution of Hg²⁺ complexed with ethylenediaminetetraacetic acid and a HSeO₃^- ion solution. X-ray diffraction analysis showed a zinc blende structure for the deposits, with a strong (111) preferred texture, and an average grain size of 425Δ. Electron probe microscope analysis showed near-stoichiometric deposits. Fourier transform infrared spectroscopy reflection absorption measurements suggest two bandgaps: 0.42 and 0.88 eV.

Full text of DOI:10.1149/1.2047547

Journal of The Electrochemical Society, 152 ͑11͒ C751-C755 ͑2005͒
C751
0013-4651/2005/152͑11͒/C751/5/$7.00 © The Electrochemical Society, Inc.
Formation of HgSe Thin Films Using Electrochemical Atomic
Layer Epitaxy
Mkhulu K. Mathe,a Steve M. Cox,b Venkatram Venkatasamy,a Uwe Happek,b
,z
and John L. Stickneya,
*
aDepartment of Chemistry, and bDepartment of Astronomy and Physics, University of Georgia, Athens,
Georgia 30602, USA
The growth of HgSe using electrochemical atomic-layer epitaxy ͑EC-ALE͒ is reported. EC-ALE is the electrochemical analog of
ALE, where electrochemical surface-limited reactions referred to as underpotential deposits, generally result in the formation of an
atomic layer of an element, under controlled potential. HgSe thin films were formed on gold substrates using two reactant
solutions: a solution of Hg2+ complexed with ethylenediaminetetraacetic acid and a HSeO−3 ion solution. X-ray diffraction analysis
showed a zinc blende structure for the deposits, with a strong ͑111͒ preferred texture, and an average grain size of 425⌬. Electron
probe microscope analysis showed near-stoichiometric deposits. Fourier transform infrared spectroscopy reflection absorption
measurements suggest two bandgaps: 0.42 and 0.88 eV.
© 2005 The Electrochemical Society. ͓DOI: 10.1149/1.2047547͔ All rights reserved.
Manuscript submitted September 10, 2004; revised manuscript received June 8, 2005.
Available electronically September 30, 2005.
HgSe is a II-VI compound with possible applications in opto-
electronics: IR detectors, IR emitters, tunable lasers, and thermo-
electric coolers.1 HgSe has been formed by molecular beam epitaxy
͑MBE͒,2 chemical bath deposition,1,3,4 the cold traveling heater
method ͑CTHM͒,3,5 as well as MBE using GaAs substrates.2
The nature of HgSe as a metal or semiconductor continues to be
debated, exemplified by the work of Gawlik et al.,6 in which they
determined a direct bandgap of 0.42 eV for HgSe, suggesting a
semiconductor. The band structure of HgSe has been studied using
Fourier transform spectroscopy by von Truchsess et al.,7 where they
concluded that HgSe is a semimetal with an inverted structure and a
bulk gap of −0.274 eV. Other studies of HgSe have reported band-
gaps of 1.42 eV,3 2.50 eV,8 and recently a direct bandgap of
0.81 eV and an indirect bandgap of 0.45 eV were also reported for
the same deposit.1
The electrodeposition of II-VI compounds, in general, has been
reviewed.9,10 The majority of previous compound electrodeposition
work has been directed toward the formation of cadmium chalco-
genides such as CdS, CdSe, and CdTe,9,11-20 by a variety of elec-
trodeposition methods. The electrodeposition of cadmium chalco-
genides has been studied extensively,21-26 mainly due to their
possible applications in the formation of optoelectronic devices such
because of the free energy of compound formation, the formation of
a surface compound or alloy. When the surface is covered, the driv-
ing force for formation of the compound is no longer present. In the
case of codeposition, the low concentration chalcogenide will limit
the deposition rate, while the metal quickly deposits at an underpo-
tential on top of any chalcogenide atoms that deposit, as the metal is
present at a significant excess. This means that each chalcogenide
atom reacts with one metal ion, resulting in control of the deposit
stoichiometry, 1:1. In the case of codeposition of the Hg chalco-
genides, however, Hg is more noble than the chalcogenide, not the
other way around, and the chalcogenide underpotential deposit
͑UPD͒ on Hg has slow kinetics, making it difficult to achieve 1:1
stoichiometry.
The work in this group focuses on the development of electro-
chemical atomic-layer epitaxy ͑EC-ALE͒ for the formation of com-
pound semiconductor thin films.35-38 It is the electrochemical analog
39-42
of atomic layer epitaxy ͑ALE͒
or atomic layer deposition
͑ALD͒,43-48 both of which are gas- or vacuum-based methods for the
formation of compounds an atomic layer at a time, using surface-
limited reactions. As noted, UPD is an electrochemical surface-
limited reaction and is used in EC-ALE to promote monolayer-by-
monolayer growth, and promote epitaxy. Most EC-ALE studies have
involved growth of II-VI compounds,16,38,49-52 although some
III-V53 and IV-VI54-56 compounds have also been formed. The III-V
compounds InAs53 and InSb have been formed, and some initial
studies into GaAs and GaSb deposition have been performed.57 Re-
as photovoltaics. Studies of zinc chalcogenide electrodeposition
22,24,26
͑ZnTe and ZnSe͒
have been reported as well.
Studies of the electrodeposition of mercury chalcogenides are
few,1,3,27-31 primarily because of the differences in potentials needed
to deposit Hg and the chalcogenides, electrochemically. Hg is a
noble metal and begins to deposit from solutions of Hg2+ at a fairly
positive potential ͑Eo = 0.851 V vs standard hydrogen electrode͒.
Chalcogenide reduction is much less noble, and generally displays
slow deposition kinetics,30,32-34 typically depositing below 0.0 V in
neutral solutions.
The fastest and simplest method for compound electrodeposition
is codeposition, where both elements are deposited simultaneously
from the same solution. This has been used with good results in the
commercial formation of CdTe deposits for photovoltaics, for in-
stance. In general, the method involves use of a metal ion that de-
posits at potentials below those needed to deposit the chalcogenide.
A low concentration of the chalcogenide can be used, which is de-
posited at an overpotential. However, this same potential corre-
sponds to an underpotential for the metal ion. At underpotentials, an
atomic layer of one element deposits on a second, at a potential prior
to ͑under͒ that needed to deposit the element on itself. This happens
cently, IV-VI compounds such as PbSe and PbTe and their superlat-
54-56
tices ͑PbSe/PbTe͒
have also been formed using EC-ALE. We
report here the first studies of the formation of HgSe using EC-ALE.
Experimental
The deposition system used here for the EC-ALE formation of
thin films has been described previously.38,58,59 Basically, it consists
of pumps, valves, cell, and a potentiostat, all computer controlled
using a program written in Lab View. The pump heads, valves, and
tubing were confined inside an N2-purged Plexiglass box to limit
oxygen, which can affect deposit quality.
The electrochemical flow cell was of a thin-layer design, to pro-
mote laminar flow over the deposit. A gasket placed between a Au
on glass substrate and an ITO auxiliary electrode defined the depo-
sition area of 3.25 cm2. The gaskets were generally silicon rubber,
which provided a good seal. A transparent ITO auxiliary electrode
was used and allowed the deposition process to be visually fol-
lowed. The reference electrode, Ag/AgCl ͑3M NaCl͒ ͑Bioanalytical
Systems, Inc., West Lafayette, IN͒, was positioned at the cavity
outlet to limit contamination.59
*
Electrochemical Society Active Member.
z E-mail: Stickney@chem.uga.edu
Solutions used included pH 2, 0.2 mM HgCl2, complexed with
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Journal of The Electrochemical Society, 152 ͑11͒ C751-C755 ͑2005͒
Figure 1. Cyclic voltammogram of 0.2 mM Hg2+, pH 2.0 on Au. Scan
Figure 2. Cyclic voltammogram of 0.25 mM HSeO−3, pH 3.0 on Au. Scan
rate = 5 mV/s, Au electrode surface area = 3.25 cm2.
rate = 5 mV/s, Au electrode surface area = 3.25 cm2.
10 mM ethylenediaminetetraacetic acid ͑EDTA͒, as well as
0.25 mM HSeO−3, pH 3, and a pH 4.0 blank solution. Solution pH
was adjusted with H2SO4 and a supporting electrolyte of 0.5 M
Na2SO4 was used in all solutions. The water used was supplied from
a Nanopure water filtration system ͑Barnstead, Dubuque, IA͒
hooked to the house distilled water system. The chemicals were
reagent grade or better.
Substrates were glass microscope slides with a 3 nm Ti adhesion
layer and 600 nm of Au. Substrates were formed by first etching the
glass in 15% HF for 60 s, then rinsed in ultrapure water, and in-
serted into the vapor deposition chamber. They were then annealed
at 400°C for 12 h under a 10−6 torr vacuum before deposition, as
well as during deposition.54 Each resulting substrate was etched in
concentrated nitric acid for 5 min and then annealed in a hydrogen
flame prior to use.
The EC-ALE cycle used for film growth on the Au substrates
was as follows: the HSeO−3 solution was first pumped into the cell
for 2 s ͑50 mL/min͒. The solution was then held for 15 s, without
pumping, for deposition, and then flushed from the cell by pumping
the blank solution for 3 s. This was followed by a 2 s fill with the
Hg2+ solution, and holding for 15 s for deposition. The cycle was
then completed by flushing the Hg2+ solution from the cell with 3 s
of the blank, and refilling with the HSeO−3 solution. This cycle,
ideally, results in the deposition of one monolayer of the compound.
The thickness of a deposit was then determined by the number of
times the cycle was performed.
The deposits were initially inspected with a Jenavert metallo-
graphic microscope. The thickness was measured using a single
wavelength Ellipsometer ͑Sentech SE 400͒. Glancing angle X-ray
diffraction patterns were acquired on a PAD V diffractometer with
Cu K␣ radiation ͑␭ = 1.5418⌬͒, equipped with a thin film attach-
ment. Electron probe microanalysis ͑EPMA͒ was run on a Joel 8600
wavelength dispersive scanning electron microprobe. AFM studies
were performed using a Dimension 3100 ͑Digital Instruments, Santa
Barbara, CA͒. Absorption measurements were performed in reflec-
tion mode, using a Fourier transform infrared ͑FTIR͒ spectropho-
tometer ͑Bruker FTS-66v, Bruker Optics, Inc.͒.
major oxidation features, AIII, AII, and AI. AIII is bulk Hg oxidation,
suggesting that the formal potential is between AIII and CIII, or about
0.25 V for this solution. This formal potential was the result of
complexation of the Hg2+ ions with EDTA, which shifts the bulk
deposition potential from about 0.7 V. AII is probably UPD strip-
ping, but could also be the dissolution of Hg from an amalgam ͑an
alloy of Au and Hg͒. The difference between UPD and alloy forma-
tion is simply that in an alloy the depositing metal can go subsur-
face. The driving forces are the same for both: a decrease in free
energy. The large oxidative current above 0.8 V, AI, was oxidation
of EDTA. Upon reversing to a negative going scan again, a small
reduction peak, CI, is evident, and may be a UPD feature. It has
been noted in these and other studies of Hg deposition that the
voltammetry can be quite variable, probably the result of the time
dependence of amalgam formation and stripping. This has led to
increased uncertainty in the assignments just described.
The negative going HSeO−3 scan, shown in Fig. 2, was started at
0.4 V and showed an initial reduction feature at 0.25 V with a peak
at 0.05 V ͑CI͒. In addition, there are reduction features at −0.4 ͑CII͒
and −0.7 V. In the subsequent positive going scan, oxidation begins
about 0.4 V, and results in two peaks at 0.6 V ͑AII͒ and
0.75 V ͑AI͒. The feature at −0.7 V is hydrogen evolution, but may
also involve some selenium reduction to selenide. CI has the appear-
ance of a drawn out UPD peak, or a surface-limited process, as does
CII. CII, however, is larger than expected for a normal UPD process,
where the formation of a single atomic layer of an element would be
expected. Previous studies60,61 have shown this peak to correspond
to the formation of a couple of monolayers of Se, probably due to
the nature of the Se film. Bulk Se is normally composed of chains of
chalcogenide atoms, held together by van der Waals forces.62 Elec-
tron transfer may limit the rate of deposition after the first couple of
monolayers, accounting for CII,60,63 because the initial atomic layer
deposition was on the Au substrate, while subsequent deposition
occurs on bulk Se. In addition, given that the formal potential for
HSeO−/Se is about 0.4 V, both CI and CII are overpotential peaks,
not un3derpotential peaks. This is again the result of slow deposition
kinetics for Se in general. Peak AII corresponds to bulk Se oxida-
tion, but is smaller than expected, a result of the reduction of most
bulk Se during the hydrogen evolution reaction at −0.7 V, forming a
soluble selenide species which diffuses away. Peak AI corresponds
to oxidation of the first Se atomic layer, or UPD stripping.
Results and Discussion
The starting potentials for steps in the development of an EC-
ALE program are usually determined from cyclic voltammograms.
Typical voltammograms for Hg2+ and HSeO3− on Au on glass sub-
strates are shown in Fig. 1 and 2, respectively. The Hg scan ͑Fig. 1͒
was started negatively from 0.7 V and shows two reduction features,
CII and CIII, in the initial negative going scan. The first peak appears
to be a UPD feature while the second is presumed to be bulk Hg
deposition. On the subsequent positive going scan, there were three
From experience, it has been shown that by keeping the potential
above −0.4 V, most bulk Se deposition can be avoided. However,
given that the formal potential for Se deposition is about 0.4 V,
thermodynamically, bulk Se is stable at any potential less than
0.4 V, and peaks CI and CII are not classic UPD features, but instead
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Journal of The Electrochemical Society, 152 ͑11͒ C751-C755 ͑2005͒
C753
Table I. Thickness and stoichiometry data of HgSe deposits.
No. of
cycles
E ͑V͒ vs
Ag/AgCl
Se/Hg
ratio
Sample
Th ͑nm͒
HgSe6/11/032
HgSe6/16/03
HgSe6/4/03
HgSe6/5/03
HgSe6/5/032
HgSe6/8/03
HgSe6/9/03
HgSe6/9/031
HgSe6/9/032
50
50
100
100
50
Se −0.18 and Hg 0.15
Se −0.30 and Hg −0.20
Se −0.60 and Hg −0.60
Se −0.25 and Hg −0.20
Se −0.28 and Hg −0.20
Se −0.30 and Hg −0.25
Se −0.30 and Hg −0.25
Se −0.30 and Hg −0.25
Se −0.25 and Hg −0.20
1.11
1.13
1.02
0.99
0.90
0.89
0.99
0.99
0.97
15.64
24.95
33.03
45.74
11.65
26.50
34.51
66.72
59.40
50
100
200
200
are formed at overpotentials.15,60,64-67 Use of such overpotentials for
EC-ALE cycles, where the deposition is predominantly surface-
limited, has been successful. Drawbacks are that some small amount
of bulk chalcogenide is being deposited, so the deposition time used
for Se makes a difference in the amount of bulk Se deposited, as it
is not a purely surface-limited process. This factor also increases the
influence of mass transfer on the deposit, where for purely surface-
limited processes the importance of mass transfer should be mini-
mal.
From the voltammetry shown in Fig. 1 and 2, starting potentials
for the initial EC-ALE cycle program were selected. For the Se step,
−0.15 V ͑Fig. 2͒ was used, so that the first atomic layer “UPD”
peak, CI, would be included, while avoiding CII, where multiple
layers of Se might be deposited. The initial Hg deposition potential
selected was 0.275 V, just prior to initiation of bulk Hg formation.
The initial HgSe cycle started with a 2 s Hg solution fill step at
0.275 V, followed by 15 s of deposition with no solution flow. The
cell was then rinsed through with blank for 3 s, filled for 2 s with
the HSeO−3 solution, and held at −0.15 V for 15 s for deposition.
Finally, the HSeO−3 solution was flushed from the cell with 3 s of the
blank. The intent was for each cycle to result in the deposition of a
HgSe compound ML, with the deposit thickness increasing linearly
with the number of cycles.
By following the current time traces for the deposition, it was
evident that the depositions dropped off over the first 10 or so
cycles. The LabView program used to control the depositions al-
lowed potentials to be shifted from cycle to cycle. Experience has
shown that potentials used to form atomic layers on the Au substrate
are frequently not the potentials needed to deposit atomic layers
after the deposit has started to grow. This appears to be related to
differences in the thermodynamics of UPD on the substrate metal vs
on the growing compound. By shifting the potentials after each
cycle, for the first 5–10 cycles, steady-state potentials can generally
Figure 4. Thickness plot of different number of cycles of HgSe deposits.
be reached, potentials which do not need to be shifted, yet maintain
deposition at 1 ML/cycle. Linear or logarithmic changes in potential
can be used for the first few cycles, to better maintain consistent
deposition from cycle to cycle. The largest potential steps are usu-
ally needed after the first cycle, as electronic effects from the Au
substrate drop off rapidly as the compound layers are formed. The
steady state potentials used for the first deposits were 0.15 V for Hg
and −0.18 V for Se.
The principle of EC-ALE is that underpotentials are used to pro-
mote 2D or layer-by-layer growth. STM studies of Hg deposition on
Au suggest that UPD does occur.3,8,29,32,34 Current time traces ob-
served in the formation of HgSe suggested that Hg deposition oc-
curred in a UPD-like surface-limited reaction, while from current
time traces, Se deposition appears to have two components, a fast
surface-limited deposition, along with a small background current
attributable to the slow deposition of bulk Se. This is expected,
given the discussion of Se deposition above. To try and better opti-
mize film quality, a series of deposits were formed using different
steady-state potentials. Deposit quality was monitored using EPMA
for stoichiometry ͑Table I͒, XRD for structure ͑Fig. 3͒, and coulom-
etry for elemental coverages.
Coverages from coulometry at the steady-state potentials aver-
aged 0.37 ML for Hg and 0.52 ML for Se, where one atom per Au
͑111͒ surface atom corresponds to 1.0 ML. Deposits were Se-rich as
Figure 3. XRD diffraction pattern for HgSe 50803, showing the present
HgSe peaks. The elemental peaks for Hg and Se were not present.
Figure 5. HgSe absorption spectra with a dip around 1500 cm−1
.
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C754
Journal of The Electrochemical Society, 152 ͑11͒ C751-C755 ͑2005͒
Acknowledgments
Financial support from the NSF divisions of Chemistry and Ma-
terials is gratefully acknowledged.
The University of Georgia assisted in meeting the publication costs of
this article.
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1/2
␣͑h␯͒ = A͑h␯ − Eg͒
͓2͔
where ␣͑h␯͒ is the absorption coefficient, h␯ is the photon energy, A
is a proportionality constant, and Eg is the direct energy gap. Figure
6 shows a plot of the square of the absorption data vs energy for the
100-cycle deposit. The plot of absorption measurements showed a
direct bandgap of 0.42 eV and an indirect bandgap of 0.88 eV, in
good agreement with literature values of 0.45 and 0.81 eV.1
Conclusions
The formation of HgSe thin films by EC-ALE has been shown
for the first time. The growth conditions, from current time traces,
are in agreement with EPMA results, showing that deposits were
selenium-rich when positive Se potentials were used. The deposition
mechanism for HgSe will be the subject of future STM and EQCM
studies. XRD patterns showed that the HgSe deposits were polycrys-
talline, with a strongly preferential ͑111͒ orientation. Bandgap mea-
surements showed the existence of direct and indirect energy gaps of
0.42 and 0.88 eV, respectively.
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