Whether Ge-Rich ZrO2 and Ge-Rich HfO2 Materials Have Similar Reaction on Annealing Treatment?

Abstract

Usually, HfO2 and ZrO2 are considered as twin-oxides that have to demonstrate similar reaction on their doping with group IV elements as well as stable tetragonal “high-k” phase. However, in most cases the films produced with different approaches were compared. In this work, both Ge doped HfO2 and Ge-doped ZrO2 films were prepared using the same equipment that allows the same Ge content in both materials to be achieved via monitoring of deposition conditions. Thus, the effect of Ge content and thermal treatment on optical and structural properties of these films was studied to obtain direct experimental confirmation whether the formation of corresponding solid solutions and their phase separation stimulated by thermal treatment is similar for both oxides.The films were grown on 6-inch Si wafers by confocal magnetron sputtering using the deposition unit with installed Ge, HfO2 and ZrO2 targets simultaneously. After the deposition, the wafers with the films were cut on smaller samples and rapid thermal annealing in nitrogen atmosphere was performed at 300-900ºC during 30 s. The samples were investigated by means of spectroscopic ellipsometry, Raman scattering, FTIR and TEM methods.As-deposited pure ZrO2 and Ge-ZrO2 films as well as those annealed at TA≤600°C were found to be amorphous and homogeneous. Annealing at higher TA stimulates a phase separation and a formation of Ge nanoclusters (Ge-ncs). They crystallize at TC=640-700°C showing the tendency to the TC decrease with Ge content rise. An appearance of tetragonal ZrO2 phase in Ge-rich films was observed at TC=680-700°C. Such behavior of ZrO2 host shows the possibility to form Ge-ncs in amorphous matrix at TA=640-680°C that offers success application of such films.Contrary to pure ZrO2 films, as-deposited pure HfO2 films were found to be crystalline. Their doping with [Ge]≥5at% reduces the degree of film crystallinity towards amorphous structure that was found to be stable for TA£600°C. At higher TA the formation of Ge-ncs and tetragonal HfO2 phase was detected. However, this latter was found to be formed at TC=600-670 °C, while the Ge-ncs crystallize at TC=700-800°C. Note that the formation of larger Ge-ncs in Ge-ZrO2 films than that in Ge-HfO2 ones at the same annealing conditions was observed.Obtained results support the theoretical consideration about stability of tetragonal “high-k” phases in annealed films. However, they show also the difference in the reaction of Ge-HfO2 and Ge-ZrO2 materials on thermal treatment followed by phase separation. Its mechanism will be discussed in details.