Effect of TiN electrodes and Gd-doping on HfO2 structural properties

The ferroelectric properties evidenced on undoped hafnium oxide, which are similar to those of doped HfO2, are rather unexpected since it is believed that doping is a prerequisite for stabilizing HfO2 ferroelectric behavior. Two different origins were invoked to account for this behavior. The first is related to the thermodynamic conditions of orthorhombic/ferroelectric HfO2 phase formation, favored upon the monoclinic/non-ferroelectric one by its lower surface energy in very small crystallites, i.e., in very thin layers. The second one is related to the existence of mechanical stress within the HfO2 layer, which allows stabilization of the non-centrosymmetric orthorhombic/ferroelectric phase. The mechanical stress is either due to doping or the top and bottom metal layers in the metal/HfO2/metal stack. The origin of undoped HfO2 ferroelectric behavior is still under debate. In this work, additional evidence of the role of mechanical stress in stabilizing the HfO2 orthorhombic/tetragonal (o/t) phase is presented. The effect of top and bottom TiN electrodes on the crystallization of Gd-doped and undoped HfO2 layers was studied as a function of HfO2 layer thickness. The results show that the HfO2 o/t phase of undoped HfO2 disappears when the bottom TiN electrode is absent. In contrast, Gd-doped HfO2 layers show the presence of a high o/t phase amount, even without the TiN bottom layer. The mechanical stress role is further evidenced by atomic plane interplanar distance measurements, which show large deformation between in-plane and out-of-plane x-ray diffraction geometries, only in the case of Gd-doped HfO2.