Carrier mobility in inversion layers of Si–thin Ta 2O 5 structures

Abstract

The behaviour of carrier mobility in the inversion channel of gateless p-MOSFETs with thin (7–50 nm) Ta 2O 5 layers, having a dielectric constant of (23–27) and prepared by rf sputtering of Ta in an Ar–O 2 mixture, has been investigated. It is shown that independently of the high dielectric constant of the layers, the transport properties in the channel are strongly affected by defects in Ta 2O 5/Si system in the form of oxide charge and interface states. These defects act as scattering centers and are responsible for the observed minority carrier mobility degradation. Both, the oxide and the interface state charges are virtually independent on the oxygen content (in the range 10–30%) during the sputtering process. A reduction of the oxide charge and the density of interface states with increasing Ta 2O 5 film thickness was found, which results in the observed increase of the inversion channel mobility with thickness. It is assumed that the bond defects (broken or strained Ta-bonds as well as weak Si–O bonds in the transition region between Ta 2O 5 and Si) are much more probable sources of defect centers rather than Ta and O vacancies or impurities.