A metastable temperature-strain phase diagram of HfxZr1−xO2 thin films based on synchrotron-based in situ 2D GIXRD investigation

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The in-plane strain in the ferroelectric HfxZr1−xO2 (HZO) thin films has been considered to be the global factor behind many process parameters affecting the concentration of metastable polar-orthorhombic phase (O-phase Pca21) formed in the transformation pathway from tetragonal to monoclinic phase. However, the strain is generally effective in crystal phase nucleation and transition with the thermal budget and itself also changes with the thermal budget. The issue of how the O-phase is formed and changed in real time with effect of both thermal budget and in-plane strain has not been clarified, which is critical for engineering the O-phase concentration. Focusing on this issue, this work demonstrates the co-effect of strain and temperature on phase formation and transition in HZO by employing the synchrotron-based in situ two-dimensional (2D) grazing incidence x-ray diffraction (GIXRD) investigation. HZO thin films with different process parameters exhibit four types of phase transition processes during heating and cooling. Meanwhile, the in-plane strain magnitude and each phase concentration in the films during annealing are extracted. Based on both, the study established a universal temperature-strain phase diagram of HZO films and proposed a kinetic model for optimizing the ferroelectric O-phase formation. The study provides deep insights into O-phase engineering and ferroelectricity optimization in HZO thin films.