Monte carlo study of mobility in Si devices with -based oxides

HfO₂-based high-κ dielectrics are among the most likely candidates to replace SiO₂ and the currently favoured oxinitride in the next generation of MOSFETs. High-κ materials allow the use of a thicker gate dielectric, maintaining the gate capacitance with reduced gate leakage. However, they lead to a fundamental mobility degradation due to the coupling of carriers to surface soft (low-energy) optical phonons. Comparing the vertical field dependence of the mobility for HfO₂ and SiO₂, the severe degradation in mobility in the presence of high-κ becomes evident. The introduction of a SiO₂ interfacial layer between the channel and the HfO₂ mitigates this degradation, by increasing the effective distance between the carriers and the SO phonons, thus decreasing the interaction strength, this does though lead to an increase in the equivalent oxide thickness (EOT) of the gate dielectric. The material of choice for the first commercial introduction of high-κ gate stacks is Hafnium Silicate (Si_xHf_1-xO₂). This alloy stands up better to the processing challenges and as a result suffers less from dielectric fluctuations. We show that as the fraction of Hf increases within the alloy, the inversion layer mobility is shown to decrease due to the corresponding decrease in the energy of the surface optical phonons and increase in the dielectric constant of the oxide.