Nanocomposites and nanomaterials
Effect of annealing temperature on phase separation 
in GeHfOx thin films 
Bukatova D. 1,2, Lehninger D.3, M. Boisserie4, Yukhymchuk V.1, 
Portier X.4, Gourbilleau F.4, Heitmann J.3, Petrik P.5, Khomenkova L.1,2

1 National university “Kyiv-Mohyla Academy”, Skovorody str. 2, Kyiv 04070, Ukraine 
2 V. Lashkaryov Institute of Semiconductor Physics, Prospect Nauki, 45, Kyiv 03028, Ukraine; E-mail: khomen@ukr.net
3Institute of Applied Physics, TU Bergakademie, Freiberg, Germany 
4CIMAP, Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, Caen, France 
5Institute for Technical Physics and Materials Science (MFA), Center for Energy Research (EK), Hungarian Academy of Sciences (MTA), Budapest, Hungary 

The impact of isovalent dopants on optical and structural properties of HfO2 matrix attracts significant attention due to unique optical and electrical properties of such materials. In the present work the effect of annealing temperature on the properties of GeHfOx thin films is investigated by spectroscopic ellipsometry, Raman scattering, XRD and TEM methods.
The GeHfOx films were grown by radio-frequency top-down magnetron co-sputtering of two, Ge and HfO2, space-apart targets. The Ge content in GeHfOx films was about 20 at%. Besides, pure Ge and pure HfO2 films were prepared for comparison. The samples were annealed in rapid-thermal tool at T=300–1000 °C during 30 s in nitrogen atmosphere. 
As-deposited Ge-HfO2 films are amorphous and chemically homogeneous in the depth. Ellipsometry data of as-deposited GeHfOx films were perfectly modeled using the effective medium theory. X-ray diffraction data shows that as-deposited GeHfOx films are amorphous contrary to pure HfO2 films that are polycrystalline. This fact confirms that Ge incorporation stabilizes amorphous HfO2 phase. It was found to be stable up to 600 °C. 
The formation of Ge nanoclusters was observed at 600–800 °C that is accompanied by the formation of tetragonal HfO2 phase. However, annealing at T>800°C results in the considerable Ge out-diffusion from the films that causes appearance of monoclinic HfO2 phase. The mechanism of phase transformation is discussed.