AVS 52nd International Symposium
    Thin Films Monday Sessions
       Session TF+EM-MoM

Paper TF+EM-MoM6
Surface Energies and Surface and Grain Boundary Nanochemistry of Cu(In,Ga)Se@sub 2@

Monday, October 31, 2005, 10:00 am, Room 306

Session: Thin Films for Photovoltaic and Energy Applications
Presenter: A. Rockett, University of Illinois
Authors: C. Lei, University of Illinois
D. Liao, University of Illinois
A. Hall, University of Illinois
I.M. Robertson, University of Illinois
A. Rockett, University of Illinois
Correspondent: Click to Email
A combination of angle-resolved photoelectron spectroscopy, atomic force microscopy, and analytical high-resolution transmission electron microscopy including nanoprobe energy dispersive spectroscopy (EDS) and angular darkfield imaging have been used to characterize the surfaces and grain boundaries in a wide variety of Cu(In,Ga)Se@sub 2@ epitaxial single crystal and polycrystalline thin films. We have observed the formation of a wide variety of nanoscale and microscale voids in polycrystalline grain boundaries and in heteroepitaxial interfaces. It is argued that these are Kirkendall voids. Likewise, trapped internal voids within grains are observed in dislocation cores and at twin termination boundaries. These have been analyzed and a Wulff construction developed to characterize the surface energies in the material. The results show that the polar metal-terminated (112) planes are the lowest energy surfaces, followed by the Se-terminated (112) planes. These results are consistent with the surface morphology of growing epitaxial layers of various orientations, indicating that the surface faceting is a largely equilibrium rather than kinetically-determined result. Other planes are stable when covered by a surfactant layer of Cu@sub 2@Se, which occurs naturally when the film is deposited in an average Cu-rich condition. Grain boundaries are also found to exhibit faceted growth. Surprisingly, in spite of these surfaces being polar, no chemistry change is observed with nanoprobe EDS in the grain boundaries relative to the bulk grains for films grown at high temperatures. For low temperature depositions (below 450°C), non-equilibrium grain boundary compositions are observed. Clean metal-terminated (112) surfaces are shown to have Fermi energies higher in the energy gap when treated in various ways than the corresponding Se-terminated surfaces. The results are related to solar-cell device performances, the primary application of these materials.