Two-component model to describe the growth of physical-vapour-deposited YBa2Cu3O7 films

Abstract

Although the relationship between superconducting and morphological properties of YBa2Cu3O7 films has recently been highlighted [Nature (London) 399 (1999) 439], only few models exist that describe the actual growth mechanisms of these complex materials on an atomic scale. While the existing models focus on a mono-atomic approach to describe the growth of high-temperature superconducting films, we present here a two-component model to study the growth of physical-vapour-deposited (PVD) YBa2Cu3O7 films. Within this model, we assume that the growth of YBa2Cu3O7 can be described by the deposition of a rate-limiting metallic species (Y, Ba or Cu) in a reactive gas (in our case O2). From the equilibrium conditions for such a vapour and the corresponding solid, we calculate the equilibrium concentration of adatoms neq on the surface of the film. Away from equilibrium, we use kinematic approach to derive nmax, the maximum concentration of adatoms on the surface of the film. We highlight the fact that for films grown in the desorption-free limit, such as PVD YBa2Cu3O7 films, the back-stress effect plays an important role. As a consequence, the morphology of existing spiral-shaped islands depends on the reduced supersaturation on the surface of the film rather than on the actual supersaturation of the vapour. Having derived neq and nmax, we then show that, within our model for PVD YBa2Cu3O7 films, the supersaturation of the vapour increases with decreasing temperature T, increasing oxygen pressure pO2 and/or increasing flux F of metallic particles. This has a direct impact on the surface morphology of PVD thin films, whether grown in the regime of two-dimensional nucleation or spiral growth. Finally, we propose a method to experimentally test our model and predict how the terrace width L of spiral-shaped islands grown on PVD YBa2Cu3O7 films is expected to vary as a function of deposition parameters such as temperature T and oxygen pressure pO2.