HYDROGEN CATALYZED CRYSTALLIZATION OF STRONTIUM-TITANATE

The crystallization rate of amorphous strontium titanate is enhanced b y more than an order of magnitude during thermal annealing in water va por as compared to a dry ambient. Time resolved optical reflectivity ( TRR) has been combined with Rutherford backscattering spectrometry (RB S) and ion channelling to investigate this effect. Thin amorphous film s (0.6 mum) were produced on single-crystal substrates of (100) stront ium titanate by bombardment with 1.9 or 2.0 MeV Pb ions. Specimens wer e annealed under controlled ambient conditions (H2O, D2O, vacuum, 265- 430-degrees-C) and the solid phase epitaxial crystallization monitored in situ by TRR (633 nm). The TRR data were calibrated ex situ by tran smission electron microscopy and RBS measurements. Isotope substitutio n, nuclear reaction analysis, and secondary-ion-mass spectrometry were utilized to reveal the uptake of hydrogen and oxygen into the implant ed layer. Hydrogen is identified as the only species which penetrates to the crystal/amorphous interface. It is shown that the crystallizati on 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 cry stallization from 2.1 to 1.0 eV. It is concluded that hydrogen, provid ed by the dissociation of water molecules at the surface, is a catalys t in the crystallization of amorphous strontium titanate.