The effect of sputter deposition conditions on the growth mechanism of YBa2Cu3O7-delta thin films studied by scanning tunnelling microscopy
TL;DR: In this article, the surface topography of as-grown films of YBa2Cu3O7- delta (YBCO) deposited by off-axis RF magnetron sputtering on MgO and SrTiO3 single-crystal (100) substrates, has been investigated by scanning tunnelling microscopy (STM)/scanning tuning spectroscopy.
Abstract: Surface topography of as-grown films of YBa2Cu3O7- delta (YBCO) deposited by off-axis RF magnetron sputtering on MgO and SrTiO3 single-crystal (100) substrates, has been investigated by scanning tunnelling microscopy (STM)/scanning tunnelling spectroscopy (STS) operated in air at ambient temperature. Both c-axis- and a-axis-oriented YBCO films have been investigated. In the case of c axis thin films, we have directly observed spiral-shaped growth terraces, which emanate from screw dislocations of this layered superconducting oxide. The growth steps are generally seen to possess a step height close to or multiples of the unit cell height of the YBCO crystalline structure. In the case of a axis growth, the substrate as well as the deposition temperature determines the grain orientation. Furthermore, our STS data revealed that the surface layer of the film is semiconducting and the tunnelling spectrum varies its shape depending on the tip-to-sample distance. A superconducting gap appears clearly in the spectra when the STM tip is placed closer to the surface than the normal position of the scanning mode. This suggests that the semiconducting layer is confined in the topmost surface region of the as-prepared film, while the layer beneath it is superconducting in character.
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TL;DR: In this article, a magnetic probe microscopy study of levitation and atomic-scale friction is reported for Fe on YBCO in the temperature range 65-293 K. The results indicate that contact electrification and static electricity may play a significant role in the non-superconducting phase.
Abstract: A magnetic probe microscopy study of levitation and atomic-scale friction is reported for Fe on YBCO (Tc = 92.5 K) in the temperature range 65–293 K. Below Tc, the friction coefficient is constant and exhibits no correlation with the strength of superconducting levitation forces. Above Tc, the friction coefficient increases progressively, and nearly doubles between Tc and room temperature. The results are discussed within the context of the underlying atomic-scale electronic and phononic mechanisms that give rise to friction, and it is concluded that contact electrification and static electricity may play a significant role in the non-superconducting phase. Given that the properties of YBCO can be finely tuned, the results point the way to a variety of interesting studies of friction and superconductors.
23 citations
TL;DR: In this article, the structure of TiO 2 (110)-(1 x 1) surface, the anisotropic transport of formate anions on the surface, and the dynamic visualization of surface gas-phase reaction at 800 K.
14 citations
TL;DR: In this paper, the properties of high T c cuprates for gas sensing applications were examined for gas-sensing properties by incorporating them in a FET-sensor structure.
Abstract: We have discovered an interesting property of high T c cuprates, which are popularly known as high temperature superconductors, for gas sensing applications. Metallic films of YBCO and BSCCO compounds are examined for gas-sensing properties by incorporating them in a FET-sensor structure. These multi-component ceramic oxides exhibited room temperature gas detection capability in comparison to conventional metal-oxides, which are sensitive to gases only at elevated temperatures. The sensor responses to NH 3, H 2, CO, CO 2, NO, NO 2, and hydrocarbon gases are presented and discussed. The results evince that thin film of high T c cuprates are highly sensitive to NH 3 and NO 2 at and around room temperature and less sensitive to other gases. These films, therefore, can be used to realize a room temperature low power FET-gas sensor.
11 citations
TL;DR: In this article, surface characterization of superconducting Nd1Ba2Cu3Oy (Ndl23) thin films using an ultrahigh vacuum scanning tunneling microscope/spectroscopy (UHV-STM/STS) and an atomic force microscope (AFM) system operated in air is reported.
Abstract: Recently, superconducting Nd1Ba2Cu3Oy (Ndl23) thin films with high superconducting transition temperature (T
c) have been successfully fabricated at our institute employing the standard laser ablation method. In this paper, we report the results of surface characterization of the Nd123 thin films using an ultrahigh vacuum scanning tunneling microscope/spectroscopy (UHV-STM/STS) and an atomic force microscope (AFM) system operated in air. Clear spiral pattern is observed on the surfaces of Nd123 thin films by STM and AFM, suggesting that films are formed by two-dimensional island growth mode. Contour plots of the spirals show that the step heights of the spirals are not always the integer or half-integer number of thec-axis parameter of the structure. This implies that the surface natural termination layer of the films may not be unique. This result is supported byI-V STS measurements. The surface morphology of the Nd123 thin films is compared with that of thec-axis-oriented Y1Ba2Cu3Sy thin films. Surface atomic images of the as-prepared Nd123 thin films are obtained employing both STM and AFM. STS measurements show that most of the surfaces are semiconductive. The results of STS measurements together with the fact that we are able to see the surface atomic images using scanning probe microscopes suggest that exposure to air does not cause serious degradation to the as-prepared surfaces of Nd123 thin films.
3 citations
TL;DR: In this article, the surface morphology of YBa2Cu3O7−δ (YBCO) films is planarized by setting the relative position and angle of the heater to the gun.
Abstract: The surface morphology of YBa2Cu3O7−δ (YBCO) films is crucial for applications such as multilayering and optical detection. In this article, we showed that the resputtering effect due to the rf plasma can planarize the surface morphology of YBCO films under specific conditions. By setting the relative position and angle of the heater to the gun, the plasma density over the substrate can be altered from an asymmetric to a symmetric distribution. When the substrate was near the plasma, the negative oxygen ions resputtered part of the mobile atoms from the surface of the film back into the plasma, which caused compositional distortion, delaying the merge of grains and leaving uncovered holes. With a longer distance, the resputtering effect was suppressed, and precipitates appeared on the surface of films, resulting in a rougher surface. At an optimum heater-to-gun configuration, the function of the resputtering effect produced a polishing effect on the surface of films, which made a smooth and precipitate-fr...
1 citations
References
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TL;DR: In fact, the existence of a critical finite supersaturation for further growth has only been established for a few materials, and then for individual faces of individual crystals, being different from case to case as discussed by the authors.
Abstract: The kinetic theory of the nucleation of new phases, developed especially by Volmer,l by Farkas,2 by Kaischew and Stranski3 and by Becker and DOring4 indicates that under typical conditions * the self-nucleation from vapour of new crystals, new liquid drops and fresh two-dimensional monolayers of molecules on a “ saturated ” crystal face require respectively supersaturations of the vapour by factors of typically 10, 5 and 1.5 respectively in order to proceed at appreciable rates. Experimentally, the first two of these figures are apparently correct : but the third is much larger than the actual supersaturation required to cause further growth of a crystal already formed. In fact, the existence of a critical finite supersaturation for further growth has only been established for a few materials, and then for individual faces of individual crystals, being different from case to case ; at the most it is about I yo. Volmer and S c h ~ l t z e , ~ who found a critical supersaturation of 0.8 yo for the growth of an iodine crystal from the vapour, interpreted this as the critical supersaturation for two-dimensional nucleation : but the quantitative discrepancy is far too great (for details of the growth rate formulzJ see the contributions of Burton and Cabrera to this Discussion). However, this discrepancy is not in the least surprising. One ought not to expect that any visible crystal will exhibit a completed perfect face needing fresh two-dimensional nucleation in order to grow. Investigation of the mechanical properties of solids shows that no macroscopic specimen ever exhibits the theoretical strength of the perfect crystal; and this enormous discrepancy (a factor of 100, say, and more for “good”
888 citations
TL;DR: The surface microstructures of c-axis-oriented films of YBa2Cu3O7, deposited by off-axis magnetron sputtering on MgO and SrTiO3 single crystal (100) substrates, have been investigated and there is strong evidence that the films nucleate as islands and grow by adding material to the edge of a spirally rising step.
Abstract: The surface microstructures of c-axis-oriented films of YBa(2)Cu(3)O(7), deposited by off-axis magnetron sputtering on MgO and SrTiO(3) single crystal (100) substrates, have been investigated with scanning tunneling microscopy and atomic force microscopy. There is strong evidence that the films nucleate as islands and grow by adding material to the edge of a spirally rising step. This results in columnar grains, each of which contains a screw dislocation at its center. This microstructure may be of significance in determining superconducting properties such as critical current, and represents a significant difference between thin films (especially those grown in situ) and bulk materials.
387 citations
TL;DR: In this article, the free energy of the tetragonal-to-orthorhombic transition occurring upon cooling from the deposition temperature was analyzed for ultrathin films.
Abstract: The microstructure of ultrathin films of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ on MgO has been studied for film thicknesses from 1.2 to 12 nm. For thicknesses of 1.2, 2.4, and 7.2 nm, the films appear to be in the tetragonal modification and consist of interconnected, low-aspect-ratio (thickness to diameter ratio \ensuremath{\le}0.1) nuclei. The dominant growth mechanism is propagation in the a-b plane of one-unit-cell-high ledges by attachment onto the ledge face, resulting in a large difference between the c- and a-direction growth rates and forcing a two-dimensional appearance in the initial deposition. The 12-nm films are also tetragonal but provide complete coverage of the substrate. Their microstructure has passed the initial nucleation and growth stage and is similar to that of standard (thickness \ensuremath{\ge}100 nm) films. By analysis of the free energy of the tetragonal-to-orthorhombic transition occurring upon cooling from the deposition temperature, we note a film-thickness dependence of the energy available for the transition. We postulate this as an explanation for the lack of orthorhombicity in ultrathin films.
120 citations