Lowering the effective work function via oxygen vacancy formation on the GeO₂/Ge interface

The use of a GeO₂ interfacial layer (IL) between a high-k dielectric and a Ge substrate helps to reduce the interface state density in Ge MOS devices. We report that the presence of the GeO₂ IL changes the effective work function (eWF) of the gate stack when annealed after high-k dielectric deposition. The eWF is reduced from 4.31 eV to 3.98 eV for TaN and from 5.00 eV to 4.44 eV for Ni. Consequently, the threshold voltage (V_th) decreases from 0.69 V to 0.21 V for Ni after post deposition annealing. Our investigation confirms that the generation of oxygen vacancies in the GeO₂ IL near the Ge substrate is the main cause of the eWF modulation. In addition, the reliability of the GeO₂ IL is investigated via the conductance method and a constant-current stress test.