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Transparent conducting oxides (TCOs) play a crucial role in thin film solar cells.  Central to the operation of such cells is the ability to have solar radiation penetrate the cell and be absorbed, with the light efficiently converted to an electrical current that can be extracted.  To accomplish this, the outer body of the cell needs to be optically transparent yet electrically conducting.  TCOs are the enabling technology in this regard.  There is an inherent competition between optical transparency and electrical conductivity, and great efforts are being expended to achieve low resistivity TCOs that can be doped reproducibly both n-type or p-type.  Since the interest in TCOs lies in their electronic properties, a comprehensive study of their electronic structure is of vital importance in improving their synthesis, and discovering new TCOs.  Our goal is to measure the surface and bulk electronic structure of novel TCOs.  We use the combination of high resolution soft x-ray emission, soft x-ray absorption, and photoemission spectroscopy to measure the detailed electronic structure of a selection of new thin film TCOs and TCO parent materials.  We measure the element and site specific partial density of states, band dispersions, chemical state, and orbital bonding in the films.  This combination of soft x-ray emission and photoemission spectroscopy is very powerful, and our program provides unique information on the electronic structure of TCO thin films.

We have studied the electronic structure of In2O3,[1] ZnO,[2] CdO,[3] CuCrO2,[4] and Cu2O.[5]  In each system, the element specific partial density of states has been measured.  Resonant inelastic x-ray scattering (RIXS) was used to measure band dispersion in CdO and ZnO,[2] as well as localized band gap excitations in Cu2O.[5]  A particular highlight was a study of the surface electronic structure of thin film single crystal CdO, where angle resolved photoemission spectroscopy (ARPES) revealed the existence of an intrinsic charge accumulation layer near the surface.  Furthermore, the electrons in the accumulation layer were found to exist in intrinsic nested quantum well states.  Such states have been observed earlier for InN,[6] but have never previously been seen in a metal oxide.

Six papers have been published so far.[1-5]  Seven invited talks have been delivered by the PI that included work from this program:

1.    “Observation of Intrinsic Quantum Effects in Electron Accumulation Layers
Department of Chemistry, University of Nevada, Las Vegas; (May 29, 2009)
2.    “New Insight on Hybridization, Bonding and Structural Distortions in Metal Oxides from Soft X-Ray Emission and Photoemission Spectroscopies”
    Department of Materials Physics, KTH - Kungliga Tekniska Högskolan [Royal Institute of Technology], Stockholm, Sweden; (March 13, 2009)
3.    “Observation of Intrinsic Quantum Effects in Electron Accumulation Layers”
    Centre for Research on Adaptive Nanostructures and Nanodevices, Trinity College Dublin, Ireland; (March 10, 2009)
4.    “Soft X-Ray Emission and Resonant Inelastic X-Ray Scattering: Novel Probes of Electronic Structure in Complex Materials”
Advanced Materials and Nanotechnology Conference (AMN4), University of Otago, Dunedin, New Zealand; (February 9, 2009)

5.    “New Insight on Hybridization, Bonding and Structural Distortions in Metal Oxides from Soft X-Ray Emission and Photoemission Spectroscopies”
        Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand; (February 5, 2009)
6.    “An Introduction to Resonant Inelastic Soft X-Ray Scattering and Soft X-Ray Emission Spectroscopy as Probes of Electronic Structure in Solids, at Interfaces, and at Surfaces”
        RIXS-08 Conference, Uppsala University, Uppsala, Sweden; (June13/14, 2008)
7.    “Soft X-Ray Emission and Resonant Inelastic X-Ray Scattering: Novel Probes of Electronic Structure in Complex Materials”
Departments of Physics and Chemistry, University of Warwick, U.K. (January 9th, 2008).

References:

[1]    L.F.J. Piper, et al., Appl. Phys. Lett. 94 (2009)  022105; A. Walsh, et al., Phys. Rev. Lett. 100 (2008)  167402.
[2]    A.R.H. Preston, et al., Phys. Rev. B 78 (2008)  155114.
[3]    L.F.J. Piper, et al., Phys. Rev. B 78 (2008)  165127.
[4]    T. Arnold, et al., Phys. Rev. B 79 (2009)  075102.
[5]    J.P. Hu, et al., Phys. Rev. B 77 (2008)  155115.
[6]    L. Colakerol, et al., Phys. Rev. Lett. 97 (2006)  237601.