Activation energies for oxide- and interface-trap charge generation due to negative-bias-temperature stress of Si-capped SiGe-pMOSFETs

We investigate negative-bias temperature instabili-ties in SiGe pMOSFETs with SiO2/HfO2 gate dielectrics. The measured activation energies for interface-trap charge buildup during negative bias-temperature stress are lower for SiGe chan-nel pMOSFETs with SiO2/HfO2 gate dielectrics and Si capping layers than for conventional Si channel pMOSFETs with SiO2 gate dielectrics. Electron energy loss spectroscopy and scanning transmission electron microscopy images demonstrate that Ge atoms can diffuse from the SiGe layer into the Si capping layer, which is adjacent to the SiO2/HfO2 gate dielectric. Density func-tional calculations show that these Ge atoms reduce the strength of nearby Si-H bonds, and that Ge-H bond energies are lower still, thereby reducing the activation energy for interface-trap genera-tion for the SiGe devices. Activation energies for oxide-trap charge buildup during negative bias-temperature stress are similarly small for SiGe pMOSFETs with SiO2/HfO2 gate dielectrics and Si pMOSFETs with SiO2 gate dielectrics, suggesting that in both cases the oxide-trap charge buildup likely is rate-limited by hole tunneling into the near-interfacial SiO2.