Threshold voltage shifts in Si passivated (100)Ge p-channel field effect transistors:: Insights from first-principles modeling

An asymmetric shift of the capacitance-voltage characteristics of n-Ge/Si/SiOx/HfO2/TaN p-channel field effect transistor is reported, namely, a shift of the threshold voltage toward positive values in inversion, while the flatband voltage remains constant. First-principles calculations on silicon-passivated germanium surfaces reveal the formation of a dipole layer at the germanium/silicon interface, which leads to a decrease of the substrate work function/threshold voltage by 0.4-0.5 V. Silicon-induced surface states are also found in the germanium band gap. When the substrate Fermi level is located near these states, electrons are transferred to the silicon layer and compensate the work function shift, explaining the absence of flatband voltage shift. (C) 2007 American Institute of Physics.