Study of HfAlO high-k gate dielectric thin films on compressively strained Si1-xGex

Abstract

To find a suitable high-k dielectric material in replacing SiO2 as gate dielectric is an urgent demand in the advanced complementary metal-oxide-semiconductor (CMOS) technology. However, the carrier mobility degradation due to the high-k gate dielectric induced phonon and Coulomb scattering effect limits the application of high-k gate dielectric materials. Compressively strained Si1-xGex (SiGe) surface channel may serve as a solution to the carrier mobility degradation problem. However, interfacial reaction, in particular the GeOx formation and Ge diffusion that result in large amount of interfacial traps and charge traps in the dielectric, is a main issue. On the other hand, HfAlO is a promising high-k gate dielectric as substitute for SiO2 due to its thermodynamic stability and low leakage current. In this project, synthesis and characterizations of HfAlO thin films on SiGe are studied. The reactions at the film-SiGe interface and the corresponding electrical properties of the MOS capacitors are investigated. The HfAlO high-k gate dielectric thin films were grown on compressively strained Si1-xGex (x=17%) by pulsed-laser deposition (PLD) technique. Structural and electrical properties of the films were investigated by x-ray photoemission spectroscopy (XPS), high-resolution transmission electron microscopy (TEM), and measurements of high-frequency (1MHz) capacitance-voltage (C-V), conductance-voltage (G-V) and leakage current-voltage (I-V). The ratio of Hf/Al in the HfAlO films, oxygen partial pressure and substrate temperature during film growth, and post-growth thermal annealing temperature are important factors to the film and interfacial properties. The results revealed that the ratio of Hf/A1 affects the flat-band voltage shift of the MOS capacitors, and thus the density of fixed charges and charge traps in the dielectric films. Low oxygen partial pressure results in extremely thin interfacial layer, while high oxygen pressure results in thicker interfacial layer. The post-growth thermal annealing is also found to be critical to the interfacial reaction. The optimized condition for the HfAlO film growth has been achieved. In order to study the merit of HfAlO compared to HfO2, interfacial reactions and electrical properties of HfO2 and HfAlO high-k gate dielectric films on strained SiGe fabricated by PLD are investigated. It is found that HfAlO films can reduce GeOx formation and reduce Ge segregation at the interfacial layer during the film annealing process. Such suppression effect is attractive since it can reduce defects and degradations, and thus improve the carrier mobility. In addition, the suppression of GeOx formation by utilizing the Si-cap on SiGe layer is investigated, and it revealed a more significant suppression effect when both HfAlO dielectric and Si-cap were associated.