Monte carlo study of mobility in Si devices with HfO2HfO2-based oxides

HfO2HfO2-based high-κκ dielectrics are among the most likely candidates to replace SiO2SiO2 and the currently favoured oxinitride in the next generation of MOSFETs. High-κκ 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 HfO2HfO2 and SiO2SiO2, the severe degradation in mobility in the presence of high-κκ becomes evident. The introduction of a SiO2SiO2 interfacial layer between the channel and the HfO2HfO2 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-κκ gate stacks is Hafnium Silicate (SixHf1-xO2)(SixHf1-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.