The role of Si orientation and temperature on the carrier mobility in metal oxide semiconductor field-effect transistors with ultrathin $HfO2$ gate dielectrics

The effective carrier mobility in $HfO2$-based $n$- and $p$-metal oxide semiconductor field-effect transistors and in their control $SiO2$ devices has been investigated as a function of temperature for three different silicon crystal orientations (100), (111), and (110). For both $HfO2$ and $SiO2$⁠, the electron mobility is steadily reduced between these orientations, whereas the hole mobility exhibits the opposite trend. The mobility-temperature dependence follows a power law $μ∼μoTα$⁠, and the exponent $α$ varies also systematically with Si orientation and carrier type. The main finding is the presence of two temperature ranges with specific exponent values $α1$ and $α2$ occurring only for holes and for the (100) and (111) orientations. This crossover with rising temperature is explained by the progressive scattering of Si light holes that form the first excited states above the heavy-hole ground state. The same observation in $SiO2∕Si(100)$ points to scattering by acoustic phonons in bulk Si. In addition to the contribution of acoustic phonons, the systematic reduction of mobility in $HfO2$ devices as compared to $SiO2$ is attributed to remote soft optical phonon scattering. A detailed analysis allows us to determine the precise inversion charge density range (or effective electric field) where remote phonon scattering predominates.