2, National Institute of Standards and Technology, Boulder, Colorado, United States
3, National Taiwan University, Taipei, , Taiwan
The mechanism of stability of the phases of HfO2, ZrO2, and HZO (HfxZr1-xO2) were systematically investigated with density functional theory molecular dynamics (DFT-MD) to determine the mechanism for HZO having a much larger process window for formation the ferroelectric phase as compared to doped HfO2 or ZrO2. For the bulk states, the monoclinic phase (“m”) is about 80 mV per formula unit more stable than either the orthorhombic ferroelectric (“f”) phase or tetragonal (t-phase) for all three oxides. The surface free energies of the (001), (110), and (111) surfaces of all three oxides were calculated using an identical DFT technique. For all three oxides, the (111) face has the lowest surface free energies consistent with experimental data on columnar HZO grains showing  is the preferred growth direction. However, the surface free energy for all direction are nearly degenerate between HfO2, ZrO2, and HZO; therefore, even for nanocrystal formation the surface free energy does not favor f-phase formation. The effect of stress/strain was calculated by determining the free energy of formation as a function of the volume of the unit cell. When the oxides are grown in the low density amorphous phase but a post deposition anneal is perform for crystallization. The crystalline forms are more dense than the amorphous forms and the DFT calculation show that a higher surface area per unit cells will greatly favor f-phase formation. However, the effect is nearly identical for HfO2, ZrO2, and HZO; this is consistent with experiments showing the molar volumes of HfO2 and ZrO2 being within 2%. Instead, formation of nanocrystalites is hypothesized to be the source of the enhanced processing window for HZO. Experimental data is consistent with partial phase separation in HZO. Atom probe tomography imaging of the chemical composition of TiN/5 nm HZO/Si(001) ferroelectric films show an asymmetric distribution of the Hf and Zr within the HZO layer with the Zr being concentrated near the TiN/HZO interface; this is consistent with ZrO2 having a 100C lower crystallization temperature than HfO2 and therefore initiate the crystallization starting on the TiN(111) surface. It is hypothesized that the nanocrystals which template on TiN(111) can produce the interfacial stress/strain needed to stabilize f-phase formation; high resolution TEM shows regions of epitaxial alignment between HZO and TiN consistent with this mechanism.
Funding by LAM Research is gratefully acknowledged.