Spectroscopy of gap states in oxide insulators
Defects with energy levels in the bandgap of high-k dielectrics critically affect electron device performance and reliability in a variety of ways ranging from the leakage current to the trapping-induced charge instabilities. The scope of the research development program is to apply the unique exhaustive photodepopulation spectroscopy (EPDS) technique to quantify the density and energy distribution of deep charge traps in dielectrics and correlate these with electrical properties of MOS devices. In combination with shallow trap analysis using thermally-induced charge de-trapping, this approach allows one to quantify gap states in the energy range covering the entire insulator bandgap width. Research will address dielectrics of interest with scaled thickness (for which the minimum value has to be experimentally determined) on Si, SiGe and Ge semiconductor channels. To advance a comprehensive understanding, EPDS will be complemented by electrical characterization aiming at identification of specific defect contribution to the overall charging behavior of the insulating stack of interest, e.g., by separating interface, near interface, and bulk oxide traps. Furthermore, the impact of a given trap distribution on the device electrical behavior strongly depends on the energy position of particular traps relative electron bands in the MOS device channel and electrodes. This factor is of critical importance in dielectric stacks used in devices for memory applications where the band offsets have a critical impact in determining the memory window and retention. Also, in logic devices the energy offset between the bandgap edges in the semiconducting channel and insulating stack determines the trapping behavior for a given range of the device operation conditions. In order to reliably determine the energy position of trapping sites distribution relative MOS electrode band edges, a complete study of the gap states energy distribution and of the trapping behavior of an insulating stack will be complemented by Internal PhotoEmission (IPE) experiments which enable direct determination of interface barriers and band offsets at the interfaces of semiconductors and metals with dielectric materials.