The Shallow Implantation of Bismuth During the Growth of Bismuth Nanocrystals in Al 2 O 3 by Pulsed Laser Deposition
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The shallow implantation of bismuth during the growth of bismuth nanocrystals in Al2O3 by pulsed laser deposition A. Suárez-García, J-P. Barnes1, R. Serna, A. K. Petford-Long1, C. N. Afonso and D. Hole2 Instituto de Optica, C.S.I.C., Serrano 121, 28006, Madrid, Spain 1 Department of Materials, Oxford University, Parks Road, Oxford, OX1 3PH, UK 2 University of Sussex, Pevensey Building, Brighton, BN1 9QH, UK ABSTRACT The effect of the laser energy density used to deposit Bi onto amorphous aluminum oxide (a-Al2O3) on the growth of Bi nanocrystals has been investigated using transmission electron microscopy of cross section samples. The laser energy density on the Bi target was varied by one order of magnitude (0.4 to 5 J cm-2). Across the range of energy densities, in addition to the Bi nanocrystals nucleated on the a-Al2O3 surface, a dark and apparently continuous layer appears below the nanocrystals. Energy dispersive X-ray analysis on the layer have shown it is Bi rich. The separation from the Bi layer to the bottom of the nanocrystals on top is consistent with the implantation range of Bi species in a-Al2O3. As the laser energy density increases, the implantation range has been measured to increase. The early stages of the Bi growth have been analyzed in order to determine how the Bi implanted layer develops.
INTRODUCTION The technique of pulsed laser deposition (PLD) is especially suited for the growth of complex oxide thin films with good adhesion and high density.1, 2 The high kinetic energy species (up to several hundred eV) present in PLD processes have been typically considered a key factor in explaining the formation of films with such good properties. PLD can also be used to grow nanocomposite films of metal (Cu, Bi, Ag) nanocrystals (NCs) embedded in an oxide matrix by alternate ablation of metal and oxide targets.3, 4, 5 The NCs size and in depth distribution can be controlled as a function of the number of pulses on the targets. Nevertheless, the influence of the energy density for ablation on the structure of such nanocomposite films has not been analyzed in detail. Recently, we have reported the influence of laser energy density on the growth of Bi NCs embedded in an amorphous aluminum oxide (a- Al2O3) matrix. 6 In addition to the NCs, evidence of Bi implantation was found in TEM images of a cross section sample. The possibility of implantation during PLD processes has been discussed before, mainly in respect to interfacial mixing and metastable phases formation observed in multilayer systems. However, since the typical layer thickness in a multilayer film is greater than that of the layers of nanocrystals in a nanocomposite film, shallow implantation is harder to discriminate from interfacial mixing in such systems. 7, 8 The discontinuous nature of the layers of nanocrystals is another useful advantage in observing any implantation. In this work, our aim is to further investigate the Bi implantation effects related to the energy density used to ablate the Bi target
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