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

Abstract

Kelvin probe force microscopy in ultrahigh vacuum is a powerful technique for the quantitative characterization of structural and electronic properties of semiconductor surfaces and interfaces on a nanometer scale. In chalcopyrite heterojunction solar cells the interfaces play a crucial role for the performance of the device. We studied chalcopyrite heterostructures based on epitaxial CuGaSe 2 thin films prepared by MOVPE. Lateral variations of the contact potential difference and the surface photovoltage (SPV) were investigated after different process steps, including the deposition of n-CdS or n-ZnSe buffer layers and the n +-ZnO window layer. Measurements on the CuGaSe 2 absorber material show terraces with preferential orientation in the [1 1 0] direction in the topographic image. A negative SPV of −300 mV on the as-grown CuGaSe 2 absorber could be attributed to a highly doped p +-Cu 2− x Se surface layer of a few nm thickness, which was removed by a KCN etch, resulting in a flat band condition. The deposition of the buffer layer alone does not lead to a significant band bending at the CuGaSe 2/buffer interface and the deposition of the ZnO window layer seems to be crucial for the development of the band bending within the absorber.