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Chemistry Faculty:
John L. Stickney,
Ph.D.
 Professor and Head
Phone: 706-542-2726
E-mail: stickney@chem.uga.edu
Biographical Information
Ph.D., University of California,
Santa Barbara, 1984
Research Interests
Our group is interested in the study and growth of thin film materials. We
are developing the electrochemical form of atomic layer epitaxy (ALE) or
atomic layer deposition (ALD). ALE is a method where atomic layers of the
elements making up a compound are deposited in a cycle, using surface limited
reactions. In that way, growth is always layer by layer, promoting the
epitaxial growth high quality thin films. Electrochemical surface limited
reactions are generally referred to as underpotential deposition, or UPD. UPD
is the phenomenon where an element is deposited at a potential prior to
(under) that needed to deposit the element on itself. Deposition is
facilitated by the free energy of formation of a surface compound. That is, a
solution containing a precursor for a first element is reacted at a
controlled potential with a previously deposited atomic layer of a second
element, until the surface is covered, forming a compound monolayer. In
electrochemical ALE (EC-ALE), the solution is then exchanged for one
containing a precursor to the second element, and from which an atomic layer
of it is deposited at a controlled potential, completing the deposition of
one monolayer of the compound. Thin films are grown by repeating this cycle
as many times as desired.
Semiconductors investigated so far include II-VIs,
IV-VIs and III-Vs. Currently, the majority of our
work has involved growth of the II-VI compounds CdTe, CdSe, CdS, ZnTe, ZnSe,
ZnS, HgSe and HgTe. Some work investigating growth of the III-V compounds
GaAs, InSb, GaSb and InAs has been pursued, as has deposition of the turnery
compound CuInSe2. Recently, excellent cycles for the growth of
PbSe and PbTe have been developed.
There are hundreds of studies reported in the literature concerning the
electrochemical growth of compound semiconductors. There is even a commercial process for
the formation of CdTe for photovoltaics.
Most of these studies use a simpler, quicker method, referred to here
as co-deposition, where a single solution containing precursors for both of them,
is used to deposit both elements simultaneously at a controlled potential or
current density. In terms of
speed and simplicity, EC-ALE is not competitive with codeposition,
however, the degrees of freedom available in co-deposition are severely
limited compared to EC-ALE.
EC-ALE provides much increased control over deposit structure,
morphology, and composition, by having separately optimized solutions and
potentials for each element. In
addition, as EC-ALE is based on layer by layer growth, so that epitaxy can be
facilitated and atomic layer control over deposit thickness is a by product.
Most of the materials we are depositing are compound semiconductors, and
in general are optoelectronic materials.
These compounds are used to form emitters and detectors: light
emitting diodes, lasers, photovoltaics, and photon sensors. They are characterized by their band
gap, which determines the energies of photons emitted or detected by devices
made with these materials.
The advantages of atomic layer control are evident in the formation of nanostructured materials. Quantum confinement is when the
electronic states of a material are a function of the dimensions of the
material. That is, when an
optoelectronic material absorbs a photon, it creates an exciton,
an electron hole pair. The
nominal distance between the electron and hole is a function of the compound,
and has been referred to as the Bohr radius. If the dimensions of the material are
smaller then say the Bohr radius, the exciton is perturbed
by the walls of the material in which it is confined. Just like a particle in a box, the
separation between states increases the smaller the box. This frequently results in a blue
shift in the ban gap, a shift to higher energy of the photons emitted or
absorbed by the material. The
result is that if you can control the dimensions of the material in the
nanometer range, you can manipulate the band gap, i.e. band gap engineering.
One way to achieve this is to make a superlattice, a material where very
thin films of two materials are alternated. Frequently such films require
individual films which are a few monolayers thick, and that is where atomic
level control over the deposition process becomes useful. Using EC-ALE it is a simple process to
form four monolayers of one compound and then four of a second, and then
repeat this whole process in order to form a superlattice.
Superlattices are materials confined in only one dimension. By forming nanowires or nanoclusters,
where there are two or three dimensions of confinement, much stronger
confinement can be produced.
Electrochemical formation of wires or clusters,
becomes relatively easy with a template.
That is, one of the nice things about electrodeposition is that it
only takes place on an electrode, a conductive substrate, selective area
deposition. By using a nanostructured electrode, nanostructured
deposits are formed. Presently,
both superlattices and nanoclusters have been formed using EC-ALE. Spectroscopy of these materials is
carried out by Professor Uwe Happek in the Physics Department here at UGA.
Studies of EC-ALE are presently directed towards the growth of more and new
materials, as well as superlattices and nanocrystals. In addition, as EC-ALE is a process
based on surface limited electrochemical reactions, a significant effort is
directed towards the surface chemistry central to the formation of these
compounds.
Studies of film growth involve development of an
automated electrochemical flow cell reactors, in which electrochemical
ALE can be performed. Resulting deposits are characterized using X-ray
diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy
(AFM), electron probe micro analysis (EPMA), ellipsometry,
and inductively coupled plasma mass spectrometry (ICP-MS). In addition, as noted above, the
optical properties of these materials are being studied in Physics here at
UGA, using surface reflection FTIR, and photoconductivity. Photoelectrochemical studies are also underway
in the Stickney lab.
In atomic level studies, a number of surface sensitive techniques are used
to investigate the atomic layer formation. For many of these techniques,
ultra high vacuum (UHV) is required. UHV surface analysis chambers used for
these studies contain optics for low energy electron diffraction (LEED),
Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS),
thermal desorption spectroscopy (TDS), and scanning tunneling microscopy
(STM). In addition, electrochemical cells have been directly interfaced to
these chambers, so that substrates can be characterized before experiments,
and deposits can be characterized after, all without exposure to air. In
addition, STM is being performed in-situ, during deposition, to better
understand the origins of particular defect structures. Recently, an electrochemical flow cell
with a quartz crystal microbalance (QCM) has been used to follow both the
currents for deposition and the mass changes as each atomic layer is
deposited.
Publications
T.E. Lister, and J.L. Stickney, "CdSe
Deposition on Au(111) by Electrochemical ALE," Appl. Surface Sci.,
103 (1996) 153.
M.P. Soriaga, and
J.L. Stickney, "Vacuum Surface Techniques in Electroanalytical
Chemistry," Chem. Anal., N.Y.,139 (1996)
1.
T.E. Lister, and J.L. Stickney,
"Atomic Level Studies of Se Electrodeposition on Au(111)
and Au(110)," J. Phys. Chem., 100, (1996) 19568.
Thomas A. Sorenson, D. Wayne Suggs, Iris Nandhakumar, and John L. Stickney “Phase
Transitions in the Electrodeposition of Tellurium Atomic Layers on
Au(100)”, J. Electroanal.
Chem., 467 (1999) 270-281.
J. L. Stickney, “
Electrochemical Atomic Layer epitaxy (EC-ALE): Nanoscale control in
the electrodeposition of compound semiconductors”, in Advances in Electrochemical Science and
Engineering, Volume 7, editors: R. Alkire and
D. Kolb, Wiley-VCH, (2001), p. 1.
T. L. Wade, R. Vaidyanathan,
U. Happek, and J.L. Stickney, ”Electrochemical
Formation of a III-V Compound Semiconductor Superlattice: InAs/InSb”, J. Electroanal.
Chem., 500 (2001) 322.
T. A. Sorenson, K. Varazo,
D. W. Suggs, and J. L. Stickney, “Formation of and Phase Transitions in
Electrodeposited Tellurium Atomic Layers on Au(111)”, Surface Science, 470 (3) (2001) 197.
K. Varazo,
M.D. Lay, T. A. Sorenson, and J.L. Stickney* Formation of the
First Monolayers of CdTe on Au(111) by
Electrochemical Atomic Layer Epitaxy (EC-ALE): Studied by LEED, Auger, XPS, and In-situ
STM”, J. Electroanal.
Chem., 522 (2002) 104.
M.D. Lay, K. Varazo,
J. L. Stickney, “Structures of Cd atomic layers electrodeposited from
chloride electrolytes, studies by LEED, Auger, XPS and STM,” J.
Amer. Chem. Soc., 125 (2003) 1352.
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Research Interests
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