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Via the density functional theory, the phase diagrams of HZO thin film in MFM (M = TiN, F = HZO) and MFI (I = α -SiO2) structures are constructed with dependences on grain size and temperature. In both MFM and MFI, the region for orthorhombic phase (o-phase) in phase diagram grows when HZO thickness gets thinner. Comparing to MFM, HZO/ α -SiO2 interface in MFI suppresses the growth of tetragonal phase (t-phase) to the region of very small grain size ∼1.2 nm. The simulation results agree with the experimental observation by Cheema et al, that the enhanced FE property was obtained in a MFIS (S = silicon) device with ultrathin HZO film (<2 nm). Simulation model indicates more chemical bonds forming between HZO and α -SiO2 interface could stabilize o-phase to greatly enhance the FE property in a MFIS device. [ABSTRACT FROM AUTHOR]

In this work, the pathways for polarization switching of hafnium oxide (HfO2) orthorhombic phase were simulated via density functional theory. The energy barriers vary from 30.7 to 69.3 meV/atom depending on the transition states taken in the process of polarization switching. With further analysis, it is found that the pathway via transition state of the Pbcm phase is suitable for the domain-wall motion mechanism for polarization switching but bears the highest energy barrier. On the other hand, the pathways via the tetragonal-like transition state with the lowest energy barrier suggest the uniform switching mechanism. For HfO2 films composed of different sizes of grains, a uniform switching and domain-wall motion mechanism could be activated in small and large grains. [ABSTRACT FROM AUTHOR]

The strain effect on the phase stability of bulk HfO2 is theoretically investigated with the density functional calculations. The origin of the ferroelectric (FE) characteristic in HfO2 is considered to be the formation of a non-centrosymmetric polar orthorhombic (PO) phase. The centrosymmetric nonpolar orthorhombic (O) phase is an antiferroelectric (AFE) structure, which has a smaller energy than PO-phase at unstrained condition. Applied biaxial strain can manipulate the relative stability among phases and also the ferroelectricity. Compressive strain is favored to transform HfO2 from monoclinic to PO-phase. The PO-phase HfO2 possesses the ferroelectricity with the remnant polarization of 66 µC/cm2 and the energy barrier of 70 meV/atom for the polarization switching from up to down. Tensile strain is found out to raise the relative energy of AFE state with respect to FE state. Furthermore, the Al doping in HfO2 could lower the energy barrier for polarization switching and also enhance the population of PO-phase. [ABSTRACT FROM AUTHOR]