Depletion induced depolarization field in Hf_1−xZr_xO₂ metal-ferroelectric-semiconductor capacitors on germanium

Germanium Metal-Ferroelectric-Semiconductor (MFS) capacitors based on ferroelectric Hf_1−xZr_xO₂ (HZO) with clean, oxide free Ge/HZO interfaces emerge as an interesting layer structure for the fabrication of ferroelectric field effect transistor (FeFET) non-volatile memory devices. It is shown that, at low temperature (<160 K), a semiconductor depletion forms in Ge near the interface, resulting in an increase in coercive voltage by about 2 V, accompanied by a distortion of the ferroelectric hysteresis with subloop asymmetric behavior, which becomes more severe at higher frequencies of measurement. At higher temperatures, the Ge surface near the ferroelectric is easily inverted due to the low energy gap of Ge, providing sufficient screening of the polarization charge by minority free carriers, in which case, nearly ideal, symmetric hysteresis curves are recovered. The depolarization field is experimentally extracted from the coercive voltage and the capacitance measurements, is found to be ∼ 2.2 MV/cm in the low temperature range, comparable to the coercive field, then rapidly decreases at higher temperatures, and effectively diminishes at room temperature. This makes Ge MFSs good candidates for FeFETs for low voltage non-volatile memory with improved reliability.