Ge Nanostructures Embedded in ZrO2 Dielectric Films for Nonvolatile Memory Applications

Zirconia and hafnia dielectrics are promising high-k materials to replace SiO₂ gate in CMOS devices.  However, in ultrathin film approach, the structure and the properties of these materials depend strongly on the deposition conditions and the post deposition treatment. It was predicted from the first principles that dielectric constant of these materials can be improved via silicon and/or germanium doping¹. Recently it was demonstrated that silicon/germanium doping allows the tetragonal phase to be stabilized^2,3. Besides, the formation of Si- and/or Ge-nanoclusters was observed.

Nowadays, germanium has attracted a wide interest because of its improved hole mobility in comparison to silicon and the 0.6-eV-bandgap making it attractive for electronic applications. However, Ge oxides show an unstable behavior whereas for success application a good passivation of the Ge surface is mandatory. In this regard, high-k dielectrics are a promising candidate for Ge surface passivation. In this work, we will present the results obtained for Ge-doped ZrO₂ dielectrics fabricated by RF magnetron sputtering in terms of the effect of deposition conditions on GeZrO_x solid solution formation as well as the effect of annealing treatment on its phase separation and modification of Ge and ZrO₂phases’ properties.

The Ge-doped ZrO₂ composite films as well as Ge-ZrO₂/ZrO₂ multilayer structures were investigated. The samples were fabricated by confocal RF magnetron sputtering of pure Ge and ZrO₂targets in Ar plasma. The structural, optical and electrical properties of the samples were studied by means of spectroscopic ellipsometry, C-V (I-V), Raman scattering, FTIR and TEM methods versus deposition conditions and annealing treatment.

It was observed that Ge-doped ZrO₂ materials demonstrate phase separation upon thermal treatment. At the same time the “start point” of this phase separation depends significantly on Ge content in Ge-doped-ZrO₂ materials and/or Ge-doped sublayer thickness in multilayer structures. Although the phase separation begins at about 600-700°C, the higher Ge content and/or thicker Ge-doped sublayer result in the lowering of the annealing temperature stimulated phase separation and an appearance and crystallization of Ge nanoclusters. Along with this the formation of tetragonal ZrO₂ occurs that offers an achievement of higher dielectric constant. In addition, this leads to an orientation relation between the crystallized ZrO₂and Ge nanosclustes. We will discussed in details the mechanism of phase separation that is the first step towards an in-situ passivation of the Ge surface. The obtained results demonstrate that the RF magnetron sputtering is a promising technique for the production of different structures and devices fully based on high-k dielectrics.

 This work is supported by DAAD grant (A/14/02380).

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