|AVS 58th Annual International Symposium and Exhibition|
|Energy Frontiers Focus Topic||Wednesday Sessions|
|Session:||Thin Film Chalcogenide Solar Cells (CIGS, CZTS, CdTe and Related Materials)|
|Presenter:||P. Peña Martin, University of Illinois at Urbana-Champaign|
|Authors:||P. Peña Martin, University of Illinois at Urbana-Champaign
J.W. Lyding, University of Illinois at Urbana-Champaign
A. Rockett, University of Illinois at Urbana-Champaign
|Correspondent:||Click to Email|
Chalcopyrite semiconductors such as (Cu,Ag)(In,Ga)(S,Se)2 show great promise in thin film solar cells as they exhibit high optical absorption and excellent performance even as polycrystalline layers. The alloy AgInSe2 (AIS) is a promising candidate for solar applications, as it has a nearly ideal energy gap (1.2 eV), high absorption coefficient, and shows sharper photoluminescence emissions than do the Cu-containing alloys. The surface of the material forms the heterojunction and determines many aspects of device performance. These semiconductors also contain a large number of intrinsic point defects, which are probably responsible for minority carrier recombination in the depletion region. Therefore understanding the surface and near surface nanostructure and nanochemistry are critical to device optimization. To characterize the structure and buried point defects near the surface, we used ultra high vacuum scanning tunneling microscopy (UHV-STM) to obtain atomic-scale topographic and electrical information.
We report the first atomically-resolved STM images and current-voltage profile measurements of AIS. Epitaxial layers were grown on epi-ready substrates of p-GaAs(111)A by hybrid technique, in which Ag and In are sputtered concurrently with effusion cell evaporation of Se. The resulting film was transported in a N2 ambient to the STM laboratory, mounted under normal lab air as quickly as possible, and introduced to vacuum, with a total atmosphere exposure of about 20 minutes. The sample was degassed at ~100°C to drive off water and other contaminants. Electrochemically-etched W tips were used for scanning. The STM measurements were carried out in a home-built system with a base pressure of 1.2x10-8 Pa (9x10-11 Torr). Topographic images reveal atomically-resolved regions on the surface with the periodicity expected for the polar metal-terminated  plane, 0.34 and 0.36 nm along rows oriented 60° from each other. This indicates that the surface does not reconstruct, unless by swapping one type of metal atom for another. Current-voltage spectra confirm that the material exhibits n-type behavior with an energy gap close to the bulk value of 1.2 eV. Some regions exhibit more fluctuations in the bandgap for a series of current spectra taken along a different line, indicating that there are variations in the electronic properties due to defects. We attempt to correlate these with topographic features in order to identify the defect responsible. Understanding and controlling these defects should lead to improved device performance, and some of the results may even carry over to CIGS devices.