A systematic study of the surface morphology of sputtered YBa2Cu3O7 films, as a function of temperature, was performed by means of atomic force microscopy. The investigated films display spiral-shaped islands with a central terrace that is typically much larger than the subsequent terraces. A careful analysis of the observed spirals shows that the change in terrace width occurs within a narrow range of angles. Possible causes for this cross-over, e.g. the role of dislocation stress or the existence of an Ehrlich-Schwoebel barrier, are discussed. Furthermore, the temperature dependences of the central terraces, as well as of the outer terraces, are found to follow an Arrhenius law. These findings are discussed within the framework of a recently developed two-component growth model, where a rate-limiting metallic species (Y, Ba or Cu) is deposited in a reactive gas (in our case O2). The model indicates that the supersaturation of the components involved is much less than assumed previously.@en

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.@en

We discuss the hetero-epitaxy of complex oxides taking YBa2Cu3O7-δ as an example. Special emphasis is on films grown by pulsed laser deposition (PLD), but other growth techniques are also discussed. Lattice mismatch in three directions is responsible for the fact that a real single crystalline YBa2Cu3O7-δ thin film has not yet been obtained. Nevertheless, growth features typically observed on single crystals (e.g. growth spirals as well as repeated nucleation and growth) are often found on YBa2Cu3O7-δ thin films. We discuss how surface diffusion and supersaturation influence the surface morphology (as observed by atomic force and scanning tunnelling microcopies (AFM/STM)) of complex oxide thin films. In particular, we conclude that the kinetic energy of the impinging adatoms affects the surface diffusivity and therefore the growth mode.@en

YBa₂Cu₃O_7-δ films on SrTiO₃ are characterized by a network of anti-phase boundaries (APB's), protruding from the interface to the film surface. We propose that the island morphology of these films is to a large extent determined by this network. At APB-outcrops deep trenches form which separate the growth islands. At the cross-section of three APB's deep holes are formed. It is found that the growth island size can be increased by increasing the substrate temperature or the oxygen pressure during growth.

@en

The surface morphology of pulsed-laser deposited YBa2Cu3O7-δ films is investigated by STM AFM. Instead of spiral growth, a 2D nucleation and growth behaviour is observed. As we find these 2D nuclei also on high-oxygen pressure DC sputtered films grown at a much lower growth rate, we conclude that the supersaturation is not a decisive parameter for the predominance of either growth mode. Instead, we attribute the absence of growth spirals to the non-steady state growth conditions inherent to the pulsed nature of the laser-ablation process. Growth spirals only develop, if a non-vanishing diffusional flow of adatoms towards the step edge is maintained. The number of growth spirals observed on a film is therefore not necessarily a measure for the number of screw dislocations. After wet-etching the films in Br-ethanol, we observe that etch pits are formed consisting of concentric steps. We conclude that these pits are due to repetitive nucleation around linear defects. The etchpit density identified in this way is of the order of 1 per μm2.@en

Epitaxial YBa2Cu3O7-δ films nucleate in c-axis oriented single-crystalline islands. The surface of the single-crystalline SrTiO3 substrates exhibit steps of one third of the YBa2Cu3O7-δ c-axis. These steps generate a mismatch in the island boundaries between the CuO2 superconducting blocks. We show that these defect regions are strong candidates for being the pinning centers responsible for the large critical currents observed in Laser Ablated and Sputtered thin films.@en

In an attempt to clarify the origin of the large critical current densities Jc observed in Laser Ablated and Sputtered YBa2Cu3O7-δ (YBCO) thin films, we make a systematic study of the low temperature Jc in samples carefully analysed using STM and AFM. Jc (B) is determined from torque-magnetometry performed in ring-patterned thin films. Epitaxial YBCO films nucleate in e-axis oriented single-crystalline islands with sizes ranging between 200 and 700 nm. We show that Jc can be mainly explained by vortex pinning loealised in the island boundaries.@en