Scattering states of ionized dopants probed by low temperature scanning tunneling spectroscopy

N type InAs(110) is investigated by scanning tunneling spectroscopy at low temperature. dI/dV images at positive sample bias exhibit circular corrugat ions, which are caused by the scattering of electron waves at the attractiv e potential of ionized dopants. Normalizing the images by simultaneously re corded constant current images gives quasidirect access to the surface dist ribution of the corresponding energy selected scattering states. The normal ized images are in quantitative agreement with a WKB model. The energy depe ndence of the scattering cross section can be estimated from the model.