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 mu similar to mu T-o(alpha), and the exponent alpha varies also systematically with Si orientation and carrier type. The main finding is the presence of two temperature ranges with specific exponent values alpha(1) and alpha(2) 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. (c) 2006 American Institute of Physics.