The road towards the ferroelectric-FET -- Carrier density modulation by ferroelectric switching in BaTiO$_{3}$/Ge

Germanium, with its higher hole and electron mobility is a potential candidate to replace silicon as a channel material in a field effect transistor in the future. The ferroelectric high-k dielectric barium titanate (BTO) can be integrated on germanium (001) due to the small lattice mismatch between BTO and Ge and could therefore be a potential candidate for a ferroelectric memory. We report the epitaxial growth of BTO on a germanium (001) substrate with a thin STO buffer layer, which imposes compressive strain on BTO and causes it to be out of plane polarized. The BTO film crystallizes as-deposited which is monitored by RHEED. XRD measurements of the BTO film indicate an out-of-plane ferroelectric polarization which can be confirmed by piezoresponse force microscopy. Using microwave impedance microscopy we could show that we can effectively modulate the charge in germanium; this charge modulation constitutes the field effect, which is an important step towards the development of a ferroelectric-FET.