Abstract :
The very initial growth stage of Y Ba2Cu3O7−δ (YBCO) thin films by pulse laser deposition (PLD) on (001) SrTiO3 (STO) substrates was investigated in situ using a home-made RHEED system. At a relatively high temperature at 820 °C and low repetition rate of 1 Hz in 22 Pa oxygen, the specular spot intensity shows three oscillations of unsteady period, and then decreases gradually. Clear laser pulse modulations can be resolved on the curve. Fitting the intensity recovery data after each single pulse by the expression I = I 0 [ 1 − e x p ( − t / t 0 ) ] results in an increasing diffusion time constant t 0 . In contrast, t 0 for the homoepitaxial STO film is roughly constant. Meanwhile at comparable growth conditions, after the first dozens of laser pulses, the RHEED pattern shows 3D spots revealing a cubic-like structure with a lattice parameter ∼4.3 Å, 10% larger than the a -axis of YBCO. We ascribe it to a perovskite-like structure with Y and Ba atoms randomly occupying the A-sites, due to the limitation of the materials available at the very beginning. Because of this layer, the epi-strain in the on-top-grown YBCO 123-structure is rather large, consequently the migration of YBCO adatoms on the surface is hindered, leading to a failure of the layer-by-layer growth mode.
