Distinct Nucleation and Growth Kinetics of Amorphous SrTiO₃ on (001) SrTiO₃ and SiO₂/Si: A Step toward New Architectures

Abstract

Integration of emerging complex-oxide compounds into sophisticated nanoscale single-crystal geometries faces significant challenges arising from the kinetics of vapor-phase thin-film epitaxial growth. A comparison of the crystallization of the model perovskite SrTiO (STO) on (001) STO and oxidized (001) Si substrates indicates that there is a viable alternative route that can yield three-dimensional epitaxial synthesis, an approach in which STO is crystallized from an amorphous thin film by postdeposition annealing. The crystallization of amorphous STO on single-crystal (001) STO substrates occurs via solid-phase epitaxy (SPE), without nucleation and with a temperature-dependent amorphous/crystalline interface velocity. In comparison, the crystallization of STO on SiO/(001) Si substrates requires nucleation, resulting in a polycrystalline film with crystal sizes on the order of 10 nm. A comparison of the temperature dependence of the nucleation and growth processes for these two substrates indicates that it will be possible to create crystalline STO materials using low-temperature crystallization from a crystalline seed, even in the presence of interfaces with other materials. These processes provide a potential route for the formation of single crystals with intricate three-dimensional nanoscale geometries.

Reconfiguration of Amorphous Complex Oxides: A Route to a Broad Range of Assembly Phenomena, Hybrid Materials, and Novel Functionalities

“…The eventual crystallization of the amorphous oxide will potentially provide for even higher stresses than are available through the reconfiguration of the amorphous structure as the thickness of amorphous SrTiO 3 thin films decreases by 13% during crystallization. [22] Reconfiguration-driven assembly is a versatile approach to impart large strains and strain gradients in a broad palette of complex oxides. This capability allows for manipulating and enhancing ferroelectricity, flexoelectricity, piezoelectricity, superconductivity, and ferromagnetism in complex oxides.…”

“…The density of SrTiO 3 was allowed to vary between 4 and 5 g cm −3 , the previously reported values for amorphous SrTiO 3 and the ideal density of crystalline SrTiO 3 . [22] The reflectivity simulation yielded the parameters shown in Table 1.…”

“…This result closely matches the volume contraction reported by Chen et al for SrTiO 3 /Si after 8 min at 600 °C. [22] Chen et al also estimated a characteristic nucleation time of 16 min for nanocrystallites of SrTiO 3 on nonepitaxial substrates at 600 °C. [22] The characteristic nucleation time (i.e., the time until the first measurable crystals appear) is significantly larger than the duration of the heating reported here, indicating that the SrTiO 3 has not undergone crystallization.…”

“…[22] Chen et al also estimated a characteristic nucleation time of 16 min for nanocrystallites of SrTiO 3 on nonepitaxial substrates at 600 °C. [22] The characteristic nucleation time (i.e., the time until the first measurable crystals appear) is significantly larger than the duration of the heating reported here, indicating that the SrTiO 3 has not undergone crystallization. Therefore, reconfiguration of the amorphous layer, rather than crystallization, is responsible for the densification of the SrTiO 3 and the stress that creates large-curvature assemblies.…”

“…Dramatic changes in the atomic configuration accompany interface formation, the relaxation of glassy states, and crystallization. [21,22] The bonding in amorphous form is incomplete yielding stress that varies during reconfiguration and relaxation. [23][24][25][26][27] Rolled-up NMs may serve as probes to measure film stress evolution during the structural transformation of complex oxides and provide insight into the fundamental mechanisms involved in such structural transformation.…”