Temperature (5.6-300K) Dependence Comparison of Carrier Transport Mechanisms in HfO2/SiO2 and SiO2 MOS Gate Stacks

Temperature dependent measurements have been used to examine transport mechanisms and energy band structure in MOS devices. In this study, a comparison between high-k HfO2 dielectrics and conventional SiO2 dielectrics is made to investigate dielectric specific thermally activated mechanisms. Temperature dependent measurements on large area n/pMOSFETs composed of SiO2 and HfO2/SiO2 gate dielectrics were performed from 5.6K to 300K. A large increase in the gate leakage current is observed at the formation of the minority carrier channel. The data indicate that gate leakage current prior to the formation of the minority channel is carrier rate limited while gate leakage current is tunneling rate limited above the threshold voltage. Gate leakage current measurements show two distinct Arrhenius transport regimes for both SiO2 and HfO2 gate dielectrics. The Arrhenius behavior of the gate leakage current is characterized by a strong temperature dependent regime and a weak temperature dependent regime. The activation energy of the strong temperature regime is found to vary with the applied gate voltage. Frenkel-Poole or other electric field models are able to explain the gate voltage dependence of the gate leakage current for the lowtemperature/voltage regime investigated. The data suggest that the variation of the activation energy for the Arrhenius behavior is weakly electric-field driven and strongly voltage, or Fermi energy level, driven. The weak electric field and strong voltage dependence of the thermal characteristics of the gate leakage current may point to trap densities within the HfO2 that vary in energy (hence applied voltage) as responsible for the observed activation energies. Trap assisted tunneling (or hopping) could be implicated as the transport mechanism.