The effect of donor/acceptor nature of interface traps on Ge MOSFET characteristics

In this paper, the acceptor and donor nature of interface traps are investigated using conductance and interface trap time constant measurements on Ge n- and p-type metaloxidesemiconductor field-effect transistors (N- and PMOSFETs). The presence of acceptor-type interface trap states in the valence-band side of Ge band gap is confirmed by these measurements. Electron trapping by the acceptor-type interface states and their effect on Ge N- and PMOS performance are discussed. The high density of the acceptor-type interface traps found to be degrading Ge NMOSFET performance, while it is not a concern for Ge PMOSFETs because of the position of charge neutrality level in Ge. Trapped charge calculations show that reducing the interface trap density by the ozone oxidation mitigates the electron trapping by the acceptor-type traps, which otherwise degrade Ge NMOSFET performance. By engineering the gate dielectric interface of Ge NMOSFETs, 40% improvement in inversion electron mobility is reported. Improvement of 2.5× over universal hole mobility is achieved for Ge PMOSFETs.