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.