Adsorption-controlled growth of ferroelectric PbTiO_3 and Bi_4Ti_3O_12 films for nonvolatile memory applications by MBE

摘要

Epitaxial PbTiO_3 and Bi_4Ti-3O_12 thin films have been grown on (100) SrTiO_3 and (100) LaAlO_3 substrates by reactive molecular beam epitaxy (MBE). Titanium is supplied to the film in the form of shuttered bursts each containing a one monolayer dose of titanium atoms for the growth of PbTiO_3 and three monolayers or the depositing film in the form of shuttered bursts each containing a one monolayer dose of titanium atoms for the growth of PbTiO_3 and three monolayers for the growth of Bi_4Ti_3O_12. Lead, bismuth, and ozone are continuously supplied to the surface of the depositing film. Growth of phase pure, c-axis oriented epitaxial films with bulk lattice constants is achieved using an overpressure of these volatile species. With the proper choice of substrate temperature (600 -650 deg C) and zone backgound pressure (P_O_3= 2x10~-5 Torr), the excess of the volatile metals and ozone desorb from the surface of the depositing film leaving a phase-pure stoichiometric crystal. The smooth PbTiO_3 surface morphology revealed by atomic force microscopy (AFM) suggests that the PbTiO_3 films grow in a layer-by-layer fashion. In contrast the Bi_4Ti_3O_12 films contain islands which evolve either continuously or around screw dislocations via a spiral-type growth mechanism.