Two-component model to describe the growth of physical-vapour-deposited YBa 2Cu 3O 7 films

Abstract

Although the relationship between superconducting and morphological properties of YBa 2Cu 3O 7 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) YBa 2Cu 3O 7 films. Within this model, we assume that the growth of YBa 2Cu 3O 7 can be described by the deposition of a rate-limiting metallic species (Y, Ba or Cu) in a reactive gas (in our case O 2). From the equilibrium conditions for such a vapour and the corresponding solid, we calculate the equilibrium concentration of adatoms n eq on the surface of the film. Away from equilibrium, we use kinematic approach to derive n max, 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 YBa 2Cu 3O 7 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 n eq and n max, we then show that, within our model for PVD YBa 2Cu 3O 7 films, the supersaturation of the vapour increases with decreasing temperature T, increasing oxygen pressure p O 2 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 YBa 2Cu 3O 7 films is expected to vary as a function of deposition parameters such as temperature T and oxygen pressure p O 2 .