The influence of La/Al atomic ratio on the dielectric constant and band-gap of stack-gate La–Al–O/SiO₂ structure

The influence of La/Al atomic ratio on high-dielectric-constant of La–Al–O thin films deposited by atomic layer deposition is investigated. The uniform surface morphology of the bilayer films with different La/Al cycle ratio (La/Al ratio) is observed after rapid thermal annealing. With the help of high-frequency capacitance–voltage measurement, the dielectric constant drops greatly with decreasing La/Al ratio. It is caused by the formation of La(OH)₃ in La–Al–O thin film, which has been verified by X-ray photoelectron spectroscopy (XPS) measurement. For the reduced interface dipoles at the high-k/SiO₂ interface, the flat band voltage (V_fb) shift is not obvious in La–Al–O film. By ellipsometry measurements, it can be found that the band-gap of the thin film increases with increasing Al component, especially when Al component in La–Al–O gate dielectric is small. The research results here show that lower Al doping in La–Al–O can keep high dielectric constant and relatively large band-gap in the thin film, which is very useful for the high-k process development.