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Novel Nanocrystalline Materials by Pulsed Laser Deposition J. Narayan, A.K. Sharma, A. Kvit, D. Kumar, and J.F.Muth1 NSF Center for Advanced Materials and Smart Structures, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC

27695-7916. 'Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695.

ABSTRACT We have developed a novel method based upon pulsed laser deposition to produce nanocrystalline metal, semiconductor and magnetic material thin films and composites. The size of nanocrystals was controlled by interfacial energy, number of monolayers and substrate temperature. By incorporating a few monolayers of W during PLD, the grain size of copper nanocrystals was reduced from 160nm (Cu on Si (100)) to 4nm for a multilayer (Cu/W/Cu/W/Si (100)) thin film. The hardness increased with decreasing grain size up to a certain value (7nm in the case of copper) and then decreased below this value. While the former is consistent with Hall-Petch model, the latter involves a new model based upon grain boundary sliding. We have used the same PLD approach to form nanocrystalline metal (Ni, Co, Fe embedded in ox-A120 3 and MgO) and semiconductor (Si, Ge, ZnO, GaN embedded in AIN and (x-A120 3 ) thin films. These nanocrystalline composites exhibit novel magnetic properties and novel optoelectronic properties with quantum confinement of electrons, holes and excitons in semiconductors. We review advanced PLD processing, detailed characterization, structureproperty correlations and potential applications of these materials.

INTRODUCTION Nanocrystalline (nc) materials with grain size 1-100nm range are found to exhibit improved hardness, increased ductility and toughness, superior magnetic and optoelectronic properties [ 1-5]. However, the properties of nc materials in general and mechanical properties in particular have been found to be controversial due to difficulties in compaction and unavailability of "artifact-free" nc samples [6]. In the present investigation, we have used a modified pulsed laser deposition method to control three-dimensional nucleation or island growth. The size of nanocrystals is determined by interfacial energy, number of monolayers and substrate temperature. We review processing, characterization and structure-property correlations of novel metallic, magnetic and semiconductor nanocrystalline materials.

1) Nanocrystalline Metallic Materials

J2.4.1

The basic idea behind processing of nanocrystalline materials is demonstrated in Fig. 1 which shows formation of three-dimensional Ge islands on Si(100) substrate at 400TC. This growth mode is induced when the average of film and substrate interaction energies (WAA + WOB)/2 exceeds the interaction energy between the film and the substrate (WAB). In the case of epitaxial Ge growth on Si(100) substrate, there is a transition from twodimensional layer-by-layer to three-dimensional island growth at a substrate temperature of 375 C. The high-resolution electron micrograph clear