The influence of Coulomb centers located in HfO2/SiO2 gate stacks on the effective electron mobility

Abstract

In this paper, we present an experimental and theoretical study on the reduction in electron mobility in metal-oxide-semiconductor field-effect transistors (MOSFETs) with a TiN/HfO2/SiO2 gate stack. Through low temperature mobility measurements down to 100 K, it is shown that the scattering mechanism responsible for the mobility degradation in MOSFETs with gate stack integrating HfO2 material is weakly dependent on temperature. Furthermore, the effect of vicinity of HfO2 is demonstrated through two SiO2 interfacial layers (ILs) of thicknesses 1 and 2 nm showing that the electron mobility is further reduced by decreasing the IL thickness. Both of these observations lead us to identify the remote-Coulomb scattering (RCS) as being the main factor limiting low-field mobility. In order to investigate more deeply the effect of Coulomb centers located in the gate stack, we have developed and used a RCS-limited mobility model. This model includes image charge, inversion layer quantization with upper subbands, a finite IL thickness, and dielectric screening. The induced Coulomb scattering potential is calculated for various high-κ permittivities and charge locations inside the gate stack. The impact of the amount and location of fixed charges, inside the gate dielectric as well as the increase in the IL dielectric constant for thin SiO2 layer on the RCS-limited mobility are investigated and discussed. Good agreement is obtained between theory and experiment over a large range of temperatures (100–300 K) and IL thicknesses (1–2 nm). As a result, by combining both experimental and theoretical data we confirm that the mobility degradation is mainly explained by a charge density located at the HfO2/SiO2 interface. At room temperature the charge density was evaluated to be 1.5×1013 cm−2.