Hydrogen catalyzed crystallization of strontium titanate

Abstract

The crystallization rate of amorphous strontium titanate is enhanced by more than an order of magnitude during thermal annealing in water vapor as compared to a dry ambient. Time resolved optical reflectivity (TRR) has been combined with Rutherford backscattering spectrometry (RBS) and ion channelling to investigate this effect. Thin amorphous films (0.6 μm) were produced on single-crystal substrates of (100) strontium titanate by bombardment with 1.9 or 2.0 MeV Pb ions. Specimens were annealed under controlled ambient conditions (H2O, D2O, vacuum, 265–430 °C) and the solid phase epitaxial crystallization monitored in situ by TRR (633 nm). The TRR data were calibrated ex situ by transmission electron microscopy and RBS measurements. Isotope substitution, nuclear reaction analysis, and secondary-ion-mass spectrometry were utilized to reveal the uptake of hydrogen and oxygen into the implanted layer. Hydrogen is identified as the only species which penetrates to the crystal/amorphous interface. It is shown that the crystallization rate is proportional to the concentration of diffusing hydrogen (H or D) at the interface. The data show that the effect of water vapor, or more precisely, hydrogen, is to reduce the activation energy of crystallization from 2.1 to 1.0 eV. It is concluded that hydrogen, provided by the dissociation of water molecules at the surface, is a catalyst in the crystallization of amorphous strontium titanate.