Chemical stability of lanthanum germanate passivating layer on Ge upon high-k deposition: A photoemission study on the role of La in the interface chemistry

The chemical stability of La-containing passivating layers /high-k gate stacks on Ge substrates is investigated by x-ray photoelectron spectroscopy. It is found that upon exposure to oxygen, a La-covered Ge surface spontaneously converts to a stable LaGeOx passivating layer. However, in the presence of overlying metal layers such as Al and Hf or upon deposition of high-k metal oxide cap layers such as Al2O3 and HfO2, the LaGeOx layer strongly reacts with the overlayers forming LaAlOz and LaHfOz compounds. In the case of Al or Al2O3, the LaGeOx is completely dissociated, while in the case of Hf or HfO2, LaGeOx remains although with a change in composition. The results are interpreted in terms of oxygen densification according to which the strongly electropositive La drives the reaction toward the formation of oxides with the highest oxygen density. The results suggest that HfO2 has a better chemical compatibility with LaGeOx making it more suitable for the gate stack.