Strain relaxation and formation of screw dislocations in YBCO films on MgO substrates

We have studied the growth of the first monolayers of YBa₂Cu₃O₇ (YBCO) films on MgO substrates by reflection high-energy electron diffraction (RHEED), atomic force and scanning tunneling microscopy (AFM, STM) and scanning electron microscopy (SEM). Up to a nominal thickness of 1.5 nm we find island growth which is pseudomorphic to MgO. Between 1.5 and 4 nm there is a transition regime with relaxation of the lattice constant and coalescence of the islands. AFM and SEM show a coexistence of islands and contiguous patches while RHEED shows a broad distribution of lattice constants. On coalescence, we observe the formation of pairs of screw dislocations. We propose that the island growth results from the lattice misfit of 9% between YBCO and MgO, and that disorder arising during the relaxation of the strained islands is responsible for the early appearance of screw dislocations on MgO.