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September 2005

Electron-beam-enhanced Recrystallization of Amorphous SrTiO3

Figure 1 Cross-sectional TEM images: (a) the amorphous layer of ~ 10 nm in thickness as indicated by the arrow; the dashed circle marks the electron irradiation area. (b) The crystallization of amorphous layer following the e-beam irradiation, as indicated by the reduced thickness of the amorphous layer marked by the dashed circle and the arrows. Full Image.

Single crystal strontium titanate (SrTiO3) is used in the microelectronics industry as insulating layers in dynamic random access memories, ferroelectric thin film structures, and high-Tc superconductor devices. Strontium titanate and other titanate ceramics have also been proposed for immobilization of nuclear waste. In many of these applications, knowledge of dynamic recovery and nanostructure evolution is critical. The current work studied the response of ion-beam induced amorphous layers in SrTiO3 to electron beam (e-beam) irradiation.

Amorphous surface layers were formed on single crystals of SrTiO3 by 1.0 MeV irradiation with gold ions at 400 K. The microstructural features, electron diffraction patterns, high-resolution transmission electron microscopy (TEM) images and recrystallization rates of the samples were determined at 300, 353 and 393 K using TEM. Following exposure of the amorphous layers to TEM e-beams, the amorphous thickness decreases with exposure time, indicating in-situ epitaxial recrystallization at the amorphous/crystalline interface. In the central area of the cross-sectional TEM image in Figure 1, where the e-beam is the most intense, more amorphous material recrystallizes, and the amorphous/crystalline interface moves toward the surface. On the other hand, at both edges of the cross-sectional image where the e-beam density is lower, recrystallization is not as significant. This indicates a strong dependence of the recrystallization process on e-beam flux.

Thermal and irradiation-enhanced recrystallization are separate processes. A few thermal epitaxial regrowth regimes have been previously observed in SrTiO3, summarized in Figure 2 as dashed lines. The two data points on each line indicate the observed upper and lower temperature limits. To compare the thermal regrowth rates with the current data, the fitted lines are extrapolated to lower temperatures. The extrapolated lines indicate that all these thermal regrowth mechanisms will be negligible below 400 K, especially for the case of thermal annealing in vacuum, which is most similar to the conditions of the present study. The recrystallization rates under e-beam irradiation were measured at three different temperatures, also shown in Figure 2. The significantly higher recrystallization rates at these temperatures under different e-beam fluxes indicate that the recrystallization processes are primarily attributed to e-beam enhanced recrystallization rather than thermal regrowth.

Extrapolation of the lines for e-beam enhanced recrystallization to the temperature regime of thermal annealing in vacuum indicates that both recrystallization processes are important in the temperature regime between 600 and 700 K. As temperature increases above 700 K, thermal epitaxial regrowth processes dominant.

Figure 2 Arrhenius plots for the solid phase regrowth. The recrystallization rate due to the e-beam induced regrowth is indicated by the solid lines. Previous thermal recrystallization results are also included as dashed lines for comparison. Full Image.

Previous photoluminescence measurements have indicated that electron irradiation of SrTiO3 at room temperature causes negligible permanent defects. The mechanism of significant e-beam enhanced recrystallization is probably not caused by defect production and annihilation. During e-beam irradiation, the incident electrons primarily transfer energy by ionization processes that produce localized electronic excitations. The localized electronic excitations affect local atomic bonds and may effectively lower the energy barriers to recrystallization processes, which may involve local atomic hopping or rotation of atomic polyhedra.

This work was conducted by Y. Zhang, C. M. Wang, M. H. Engelhard, and W. J. Weber, PNNL, and J. Lian, R.C. Ewing, University of Michigan

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