JApplPhys_107_034906.pdf

Scanning tunneling microscopic analysis of Cu?In,Ga…Se2 epitaxial layers Marie A. Mayer,1 Laura B. Ruppalt,2 Damon Hebert,1 Joseph Lyding,2 and Angus A. Rockett1,a
1Department of Materials Science and Engineering, University of Illinois, 1304 W. Green Street, Urbana, Illinois 61801, USA 2Department of Electrical and Computer Engineering and The Beckman Institute, University of Illinois, 3065 Beckman Institute, 405 N. Matthews St., Urbana, Illinois 61801, USA
Received 2 June 2009; accepted 7 January 2010; published online 8 February 2010 Scanning tunneling microscopy
STM measurements have been made on single-crystal epitaxial layers of CuInSe2 grown on GaAs substrates. Results were obtained for as-grown, air-exposed, and cleaned surfaces; in situ cleaved surfaces; surfaces sputtered and annealed in the STM system; and samples prepared by a light chemical etch. Conventional constant-current topographs, current-voltage curves, and current imaging tunneling spectroscopy
CITS scans were obtained. Topographic images show that the surfaces appear rough on the atomic scale and often exhibit regular features consistent with a previously proposed surface ad-dimer reconstruction. CITS scans show a spatially varying energy gap consistent with band-edge fluctuations on a scale of a few atomic spacings. Energy variations were observed in both band edges. Although quantitative description of the magnitude of these fluctuations is difficult, the fluctuations on the atomic scale appear much larger than observed by methods such as photoluminescence, which average over larger volumes. ? 2010 American Institute of Physics. doi:10.1063/1.3304919 I. INTRODUCTION Photovoltaic materials are receiving significant attention due to increasing concerns regarding fossil fuel resource life- times and the need for distributed power generation. Photo- voltaics based on Cu
In1?xGaxSe2
CIGS alloys with x
0.3 have the highest performance of any thin-film solar cells and show excellent lo