(39e) Atomic Layer Deposition of Nanoscale Materials for Energy Conversion

For
a number of technologies, including microelectronics and energy conversion
devices, feature sizes are becoming increasingly small, and the need for
precise control over costly materials continues to grow.  In addition to decreasing
length scales, it is important that limited natural resources are carefully used. 
In fact, many photovoltaic and fuel cell components are comprised of a variety
of materials that may be expensive, toxic and/or rare.  Placing such materials
only where needed to derive the most desirable properties will be essential in
reducing the costs of energy conversion devices.  Atomic layer deposition (ALD)
is uniquely poised to accomplish these goals.

 Atomic
layer deposition is a growth technique consisting of self-limiting surface
reactions, which allow for sub-nanometer control of growth.  The ability to
finely tune growth allows for careful control over material properties.  Furthermore,
ALD provides excellent capabilities for depositing a variety of materials on
high surface area substrates including those that are highly structured.

This
work focuses on the ALD and materials characterization of transparent
conducting oxides (TCOs) and surface passivation techniques that direct the
growth of functional materials.  Transparent conducting oxides (TCOs) are highly
conductive and transparent materials used in thin film solar cells, serving as
electrical contacts, chemical barriers and antireflection coatings.   One of the
most common and robust TCO materials, indium-doped tin oxide (ITO), has many
attractive properties; however, alternatives must be found  to address the
diminishing supplies, increasing demands and subsequently escalating costs of
indium.  Replacing indium with zinc to create zinc tin oxide (ZTO) provides a
cost-effective and earth-abundant alternative.  Various material properties of
ZTOs are not well understood and there is little control over stoichiometry
across growth systems.  At present, it is known that ZTO takes on an unstable
trigonal ilmenite structure ( ZnSO₃) and stabilizes into the cubic spinel
(Zn₂SnO₄) and tin dioxide (SnO₂) under high
temperature conditions. 

By
developing and carefully characterizing a ZTO ALD system, these structures will
be better understood and eventually optimized for use as TCOs.  Atomic layer
deposition reaction conditions for tin oxide (SnO_x) and zinc oxide
(ZnO), which are alternative TCOs, and ZTO will be described, including growth
rates and temperature windows in our custom built reactors.  Material properties
of the ZTO films will be characterized with x-ray photoelectron spectroscopy
(XPS), scanning Auger electron spectroscopy (AES),  spectroscopic ellipsometry,
scanning electron microscopy (SEM), x-ray diffractometry (XRD) and four-point
probe measurements providing insight into surface, structural and electronic
properties.