N. Chandrasekhar

Nucleation and growth of DyBa2Cu3O7−x thin films on SrTiO3 substrates studied by transmission electron microscopy and atomic force microscopy

The microstructure of superconducting films influence their properties, requiring optimization of processing conditions. The early stages of nucleation and growth determine the final microstructure of the film. We report our studies of the initial stages of epitaxial growth of DyBa2Cu3O7−x films on SrTiO3 substrates by transmission electron microscopy (TEM), atomic force microscopy (AFM), and reflection high energy electron diffraction (RHEED). Prethinned, electron transparent, reconstructed (100) and (110) SrTiO3 substrates were used to deposit 40‐, 100‐, and 1000‐A‐thick DBCO films by ozone‐assisted molecular beam epitaxy (MBE). The reconstructed substrates have a stepped surface and the film nucleates preferentially at the steps giving rise to an interconnected network of islands. The growth mechanism appears to be nucleation at the step edges and propagation of these steps. In situ reflection high energy electron diffraction yielded identical conclusions. Islands saturate at a certain size and further...

Fabrication of high-Tc superconductors using ozone-assisted molecular beam epitaxy

Abstract Ozone-assisted molecular beam epitaxy (MBE) has been developed for growing high-Tc superconducting films. It involves the co-evaporation of their elemental constituents together with the use of ozone as a source of reactive oxygen. The advantage of using ozone over other forms of reactive oxygen is that it is stable enough to be prepared and delivered to the substrate in a very pure form using very simple apparatus, providing a well-characterized flow of oxidizing gas. Using ozone, a post-deposition anneal is not needed to produce films with high transition temperatures, and growth can be carried out at relatively low system pressures. As a consequence, the surface analysis using reflection high energy electron diffraction is possible during film growth. Recent refinements of ozone-assisted MBE directed at the YBa2Cu3O7-σ family of films are described here. The results indicate that this technique is very competitive for growing the highest quality superconducting films and should be ideal for fabricating structures.

Growth of DyBa2Cu3O7−x studied by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to investigate the microstructure of DyBa2Cu3O7−x films grown on (100) SrTiO3 by ozone‐assisted molecular beam epitaxy. The surface roughness is found to be higher than that reported for sputtered films of YBa2Cu3O7−x. Intensity oscillations were observed in the specular spot of in situ reflection high energy electron diffraction patterns, a result which suggests that surface smoothness is not a requisite for the observation of such intensity oscillations. The observation of spiral morphology in STM pictures indicates the presence of screw dislocations, in excess of 109 cm−2. A zeroth order approximation has been used to evaluate the shapes of the spirals and reasonable agreement is obtained with experiment.

The growth of DyBa2Cu3O7−x on stepped surfaces

Abstract Classical growth models, such as that of Burton, Cabrera and Frank, assume step motion driven by the diffusion of atomic species with the steps acting as sinks. The presence of steps on both reconstructed and unreconstructed surfaces of substrates used for the growth of superconducting films, such as SrTiO 3 , and observations of nucleation along such steps, provide an opportunity to investigate the applicability of growth models, with suitable modifications for higher volume supersaturations encountered in processes such as molecular beam epitaxy. Diffraction information suggests the formation of an unstable phase as a precursor of the 123 phase. Although the kinetics of formation of the 123 phase remains unclear, the presence of the unstable phase suggests that kinetics may be governed by additivity of cation fluxes. Analysis yields a rate of advance of steps of the order of 1nm/s. Island size and misorientation distributions are presented for films of two different thickness. Reasonably good agreement of island sizes is obtained with experiment for ultrathin films of DyBa 2 Cu 3 O 7− x with thickness of 40 and 100 A.

Growth of DyBa2Cu3O7−x thin films on vicinal (100) SrTiO3 substrates

We report studies of the initial stages of epitaxial growth of DyBa2Cu3O7−x films on vicinal (100) SrTiO3 substrates using in situ reflection high energy electron diffraction (RHEED), transmission electron microscopy, and atomic force microscopy. Two sets of substrates, with misorientations of 2.2° and 4.4° off (100) towards (110) were used. These substrates were prethinned and annealed prior to deposition. DyBa2Cu3O7−x films were grown by ozone‐assisted molecular beam epitaxy. The annealed substrates were reconstructed and had stepped surfaces. Films nucleating preferentially at the steps giving rise to an interconnected network of islands, as observed in plan view transmission electron microscopy. In situ RHEED showed intensity oscillations of the specular spot consistent with such an interpretation, and exhibited behavior dominated by surface diffusion. The higher density of nucleation sites on vicinal surfaces appears to reduce the fraction of a‐axis outgrowths. The films grown on vicinal substrates a...

Surface step density oscillation during the growth of YBa2Cu3O7−x and DyBa2Cu3O7−x superconducting thin films

Intensity oscillations of the specular reflection high‐energy electron diffraction (RHEED) beam observed during the growth of YBa2Cu3O7−x and DyBa2Cu3O7−x thin films on (100) SrTiO3 substrates have been analyzed within the framework of an analytical model. Monte Carlo studies have also been carried out. Results of both of these studies are in agreement with experimental data and support the proposition that the RHEED intensity oscillations in these systems are a consequence of changing surface step density, caused by cycles of nucleation and coalescence. Differing time periods for RHEED oscillations of the Dy and Y compounds are shown to be the consequence of differing vapor pressures at the substrate temperature.