In Situ Time-Resolved Studies of Sub-Millisecond Metastable Phase Formation in Thin-Film Oxide Materials via Optical Imaging and Synchrotron X-Ray Diffraction

The ability to trap metastable phases in thin-film oxide materials via millisecond-scale Laser Spike Annealing (LSA) has been of significant interest to the combinatorial community in recent years. Analysis of films post-thermal spike annealing provides only details on the final quenched structures. The details leading to the final phase selection, and critical details of the structure, depend on the sequence and nature of transient phases that occur during the heating and quench. On these short timescales, detailed <i>in situ </i>characterization of such transient structures presents a significant experimental challenge.<br/><br/>Through the unique experimental configuration of LSA, the temporal evolution of the metastable phases in these thin-film oxide materials is mapped to the spatial domain (mm scale) in a frame of reference where the sample is translating relative to the incident laser irradiation. Optical microscopy of this steady-state thermal field during the anneal process has been used to identify the sequences of metastable phase formation<i> in situ</i> for a variety of oxide films as a function of both time and temperature, providing key insights into the thermal evolution. We present also initial data from <i>in situ</i> wide-angle x-ray diffraction data tracking these metastable phases using a micron-focused X-ray beam at the Cornell High Energy Synchrotron Source. Coupling X-ray diffraction with optical changes allows key phase and texturing information to be unambiguously resolved. Thin-film Bi₂O₃ films show initial nucleation in all cases of the δ-phase prior to subsequent conversion (within the millisecond time-frame) to β- or α-phases for longer anneals. Similar transformations as a function of temperature from an initially nucleating γ-phase Ga₂O₃ to the stable β-phase are also reported.