Growth and characterization of epitaxial silver indium diselenide
Photovoltaics (solar cells) are a key player in the renewable energy frontier, and will become increasingly important as their cost per watt continues to drop, especially if fossil fuel costs increase. One particularly promising photovoltaic technology is based on chalcopyrite-structure semiconductors. Within the chalcopyrite compounds the highest efficiency thin film solar cell absorber material to date is Cu(In,Ga)Se2 (CIGS). While current efficiency records are over 21% for single-junction cells, there is still room for improvement. Replacing some of the Cu with Ag has been shown to be beneficial in CIGS devices. However, the Ag- containing chalcopyrites are still relatively unknown in terms of their growth mechanism, energetics, and surface atomic and electronic properties. These are best inferred through study of epitaxial films, yet they have little mention in literature and have not been the subject of a detailed study. This work describes the growth of epitaxial AgInSe2 (AIS) on GaAs substrates, studying the morphology, structure, and surface properties to understand how growth takes place. It also seeks to experimentally determine the surface electronic and atomic structure at the atomic scale to gain insight into the part of the material that forms the heterojunction that collects photon energy in the device. Finally, this work seeks to compare and contrast these findings with what is known about CIGS to determine where similarities and, more importantly, the differences may lie. This study has found that single phase tetragonal AIS can be epitaxially grown on GaAs, as illustrated by x-ray diffraction (XRD), transmission electron microscope (TEM), and surface morphology data. Like CIGS, the close packed polar (112) planes have the lowest energy. The morphology points to a difference in step dynamics, leading to less faceted, straight edged island shapes compared to CIGS. Epitaxial temperature as a function of growth direction shows a different trend in AIS than in CIGS. Interdiffusion of the group III elements across the substrate interface was found to result in an epitaxial intermixed layer between the film and substrate in some cases, which may help mediate the lattice mismatch. At the atomic scale, scanning tunneling microscopy (STM) was used to observe details of the surface morphology, which indicated growth of the (112)A orientation of AIS by a screw dislocation mechanism (other surfaces were not examined by STM but are expected to show similar results). The surface atomic structure was directly imaged for the first time, revealing an arrangement similar to that expected from a bulk terminated surface. The electronic structure shows a gap in surface electronic states with a width comparable to bulk AIS, n-type conduction, and a tail of states near the valence band edge that decay well into the gap. The conduction and valence bands show fluctuations as a function of position on the surface, with greater magnitude in the valence band. The fluctuations in both bands are less than those observed on the surface of CIS by STM. It seems to indicate a reduction in band tails, both in magnitude and spacial extent, in AIS compared to CIS, likely tied to a reduction in point defect concentration at the surface.
chalcopyrite, epitaxy, photovoltaics, scanning tunneling microscopy
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign