High dielectric constant gate oxides for metal oxide Si transistors

TLDR

Silicon dioxide gate dielectrics have become too thin, causing high leakage, and high‑κ oxides such as hafnium oxide and hafnium silicate—though promising—have been poorly studied and exhibit crystallization tendencies, defect densities, and dominant oxygen‑vacancy traps. The study aims to replace thin SiO₂ with thicker high‑κ oxides like hafnium oxide and hafnium silicate, requiring intensive research to develop them as high‑quality materials and integrate them with metal gate electrodes. The review examines oxide selection, deposition, structural and metallurgical behavior, atomic diffusion, interface reactions, electronic structure, defects, charge trapping, conduction mechanisms, mobility degradation, flat‑band voltage shifts, and work‑function control in metal gate electrodes.

Abstract

The scaling of complementary metal oxide semiconductor transistors has led to the silicon dioxide layer, used as a gate dielectric, being so thin (1.4 nm) that its leakage current is too large. It is necessary to replace the SiO2 with a physically thicker layer of oxides of higher dielectric constant (κ) or 'high K' gate oxides such as hafnium oxide and hafnium silicate. These oxides had not been extensively studied like SiO2, and they were found to have inferior properties compared with SiO2, such as a tendency to crystallize and a high density of electronic defects. Intensive research was needed to develop these oxides as high quality electronic materials. This review covers both scientific and technological issues—the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure and reactions, their electronic structure, bonding, band offsets, electronic defects, charge trapping and conduction mechanisms, mobility degradation and flat band voltage shifts. The oxygen vacancy is the dominant electron trap. It is turning out that the oxides must be implemented in conjunction with metal gate electrodes, the development of which is further behind. Issues about work function control in metal gate electrodes are discussed.