Kelvin probe force microscopy for the characterization of semiconductor surfaces in chalcopyrite solar cells

Kelvin probe force microscopy in ultrahigh vacuum is a powerful technique f or the quantitative characterization of structural and electronic propertie s of semiconductor surfaces and interfaces on a nanometer scale. In chalcop yrite heterojunction solar cells the interfaces play a crucial role for the performance of the device. We studied chalcopyrite heterostructures based on epitaxial CuGaSe2 thin films prepared by MOVPE. Lateral variations of th e contact potential difference and the surface photovoltage (SPV) were inve stigated after different process steps, including the deposition of n-CdS o r n-ZnSe buffer layers and the n(+)-ZnO window layer. Measurements on the C uGaSe2 absorber material show terraces with preferential orientation in the [110] direction in the topographic image. A negative SPV of -300 mV on the as-grown CuGaSe2 absorber could be attributed to a highly doped p(+)-Cu2-x Se surface layer of a few nm thickness, which was removed by a KCN etch, re sulting in a flat band condition. The deposition of the buffer layer alone does not lead to a significant band bending at the CuGaSe2/buffer interface and the deposition of the ZnO window layer seems to be crucial for the dev elopment of the band bending within the absorber. (C) 2001 Elsevier Science B.V. All rights reserved.