M. Grant Norton

Observation of the early stages of growth of superconducting thin films by transmission electron microscopy

A method for the direct observation of the early stages of growth of superconducting films by transmission electron microscopy (TEM) is reported. The technique uses well‐characterized, single‐crystal TEM foils as substrates for the deposition process. Ultrathin films of YBa2Cu3O6+x (YBCO) were prepared by pulsed laser deposition from stoichiometric bulk samples directly onto (001) oriented MgO thin‐foil substrates. Observation of the film by TEM is possible without any post‐deposition specimen preparation. The epitactic nature of the film growth is shown by analysis of the moire fringe pattern and by selected area diffraction. In addition to an interconnected film, copper oxide particles and stoichiometric ablated clusters were observed.

Surface preparation for the heteroepitactic growth of ceramic thin films

The morphology, composition, and crystallographic orientation of the substrate influence the nucleation and growth of deposited thin films. A method for the preparation of controlled, characteristic surfaces is reported. The surfaces are suitable for the heteroepitactic growth of thin films. When used in the formation of electron‐transparent thin foils, the substrates can be used to investigate the very early stages of film growth using transmission electron microscopy. The substrate preparation involves the cleaning and subsequent annealing to generate a surface consisting of a series of steps. The step terraces are formed on the energetically stable surface, and controlled nucleation and growth of films at step edges is found. The substrate materials prepared using this technique include (001) MgO, (001) SrTiO3, and (001) LaAlO3.

Oriented aluminum nitride thin films deposited by pulsed‐laser ablation

Single‐phase aluminum nitride thin films with preferred crystallographic orientations have been grown on single‐crystal sapphire by pulsed‐laser ablation. The orientation of the films was found to be determined by the atmosphere and the nitrogen pressure during deposition and the substrate temperature. The films were examined by x‐ray diffraction, and scanning electron microscopy.

The nucleation and heteroepitactic growth of YBa2Cu3O7 − δ thin films on MgO

Abstract The nucleation and growth of YBa 2 Cu 3 O 7 − δ films on single-crystal (001)-oriented MgO have been studied using transmission electron microscopy (TEM). The films were formed by pulsed-laser ablation from a stoichiometric bulk sample. Observations of the early stages of film growth were made by utilizing a new specimen-preparation technique in which the films were deposited directly onto specially prepared electron-transparent substrates. The films were found to grow by an island mechanism with the nucleated islands growing to form an interconnecting film. A number of these islands are rotationally misaligned resulting in the formation of a polycrystalline, mosaic structure. The grains were predominantly oriented with their c -axis perpendicular to the film-substrate interface, although a smaller fraction of the film contained grains oriented with their c -axis in the film-substrate plane. These grains were found to grow in orthogonal directions along the [001] and [010] crystal axes of the MgO. The highly anisotropic growth of YBa 2 Cu 3 O 7 − δ , fastest along the a − b plane, is an important factor in determining the microstructure of the film.

Microstructure of hardened and softened zirconia after xenon implantation

Ion channeling and transmission electron microscopy (TEM) techniques were used to examine the microstructure of single-crystal Y2O3 stabilized cubic zirconia (YSZ) after implantation with 240 keV Xe+ ions. The observed microstructure was related to Knoop indentation hardness measurements. These measurements showed an increase in hardness for low ion-doses, reaching some maximum value, then a decrease in hardness at higher doses. In the hardening regime, below 7.5 x 10{sup 15} Xe+/cm{sup 2}, point defects and dislocation networks were observed by TEM. Ion-channeling showed a corresponding increase in damage as a function of ion-dose. For doses between 7.5 x 10 to the 15th power and 3 x 10 to the 16th power Xe+/cm2 the hardness falls, and the amount of damage, measured with ion-channeling, reaches a limiting value at less than complete damage. In this dose range the Xe concentration continues to increase beyond the dose where the amount of damage saturates. For high doses, greater than 3 x 10 to the 16th power Xe+/cm2, where softening of the zirconia occurs, additional reflections appear in the electron diffraction pattern that are consistent with the lattice parameter of solid Xe. A diffuse ring is also visible; this is believed to be due tomorexa0» the presence of fluid Xe. Both ion-channeling and TEM show that a significant amount of monocrystalline zirconia remains even up to doses of 1 x 10 to the 17th power.«xa0less

Growth mechanism of YBa2Cu3O7−δ thin films on vicinal MgO

Abstract YBa2Cu3O7-δ thin films have been grown on vicinal MgO substrates by pulsed-laser ablation. The substrate topography has been shown to affect the microstructure of the film. The step edge act as preferential sites for island nucleation, controlling the in-plane alignment of the islands. The resulting microstructure consists of c-axis oriented grains which have a large aspect ratio. The formation of a well-defined substrate surface enables the growth of highly oriented YBa2Cu3O7-δ films, on a substrate where there is a large lattice mismatch. The films have been characterized by both X-ray diffraction techniques and transmission electron microscopy.

Effect of substrate temperature and oxygen pressure on the microstructure of thin YBa2Cu3O7-δ film deposited on MgO by pulsed-laser ablation

Abstract The microstructure of thin YBa 2 Cu 2 O 7-δ films, prepared by pulsed-laser ablation, at a range of oxygen pressures and substrate temperatures has been investigated. The thin films were deposited directly onto specially prepared, electron-transparent thin-foil substrates of (001)-oriented MgO. Independent variation of the two parameters was found to affect the microstructure of the thin film. For example, at low substrate temperatures (600°C), the film was found to consist of small ( 100 nm diam.) isolated islands. These larger islands were faceted parallel to low index planes (e.g. {100} and {110}). The differences observed in the microstructure of films grown at the two temperatures was related to the degree of mobility of the ablated species on the substrate surface.

Direct observation of grain orientation in YBa2 Cu3 O7-δ thin films

Abstract Using a recently developed specimen-preparation technique to examine heteroepitactic growth in ceramics, the early development of grain orientation in thin YBa2 Cu3 O7-δ films prepared by pulsed-laser ablation can be determined. The technique involves deposition directly on to a specially prepared electron-transparent substrate. The films are examined during the early stages of growth by transmission electron microscopy. Three different orientations corresponding to different alignments of the c axis of the YBa2 Cu3 O7-delta; unit cell with the MgO substrate could be determined from the particle morphology in the image, but were not detected by selected-area diffraction due to their small size.

Transmission-electron microscopy study of ion-beam implanted single-crystal ceramics

Abstract The microstructural changes in single-crystal MgO following implantation with 200 keV Xe+ ions, at a range of ion fluences, have been studied using transmission-electron microscopy (TEM) and selected-area diffraction (SAD). By the application of a novel specimen preparation technique, the microstructure of the implanted specimen can be observed directly, without the possibility of introducing artifacts which may arise during conventional methods of post-implantation specimen preparation for TEM analysis. At low ion fluences the microstructure remained crystalline and the damage was confined to small point-defect clusters and dislocation networks. At higher ion fluences, > 5 × 102ions/cm2, solid noble-gas inclusions were identified by examination of SAD patterns and evidence was obtained for recrystallization from an amorphous phase.

Growth and Microstructure of Aluminum Nitride Thin Films

The synthesis of aluminum nitride thin films by pulsed-laser ablation is demonstrated. The films were formed on single-crystal sapphire and graphite substrates. A number of techniques were used to characterize the films: transmission and scanning electron microscopy, Rutherford backscattering spectrometry and x-ray diffraction.