Beyond SiO2 technology: Simulation of the impact of high-κ dielectrics on mobility

A critical challenge for the microelectronics industry is the need for higher permittivity dielectrics to replace silicon dioxide. A number of different high-κ materials have been proposed and analyzed as SiO2 replacements 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 all lead to mobility degradation due to, among other factors, the coupling of carriers to surface soft-optical (SO) phonons. A severe mobility degradation in the presence of high-κ becomes evident when comparing the vertical field dependence of mobility for a wide range of high-κ materials against SiO2. As oxides containing Hf presently appears to be the leading high-κ contenders, we have performed a detailed analysis of Hf-based gate stacks, exploiting alternative structures and compositions. The introduction of a SiO2 interfacial layer between the channel and the HfO2 reduces the detrimental mobility degradation resulting from the mobility SO phonon scattering, but increases the equivalent oxide thickness (EOT) of the gate dielectric. A possible material of choice for the first commercial introduction of high-κ gate stacks is hafnium silicate (SixHf1−xO2), being thermally stable and offering a good compromise between small EOT and large electron mobility.