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

Although the relationship between superconducting and morphological propert ies of YBa2Cu3O7 films has recently been highlighted [Nature (London) 399 ( 1999) 439], only few models exist that describe the actual growth mechanism s 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 g rowth of physical-vapour-deposited (PVD) YBa2Cu3O7 films. Within this model , we assume that the growth of YBazCu307 can be described by the deposition of a rate-limiting metallic species CY, Ba or Cu) in a reactive gas tin ou r case O-2) From the equilibrium conditions for such a vapour and the corre sponding solid, we calculate the equilibrium concentration of adatoms n(eq) on the surface of the film. Away from equilibrium, we use kinematic approa ch 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-fre e limit, such as PVD YBa2Cu3O7 films, the back-stress effect plays an impor tant role. As a consequence, the morphology of existing spiral-shaped islan ds 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 YBa2Cu3O7 films, the s upersaturation of the vapour increases with decreasing temperature T, incre asing oxygen pressure p(O2) and/or increasing flux F of metallic particles. This has a direct impact on the surface morphology of PVD thin films, whet her grown in the regime of two-dimensional nucleation or spiral growth. Fin ally, 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 i s expected to vary as a function of deposition parameters such as temperatu re T and oxygen pressure p(O2). (C) 2001 Elsevier Science B.V. All rights r eserved.