TiN/GaN Metal/Semiconductor Multilayer Nanocomposites Grown by Reactive Pulsed Laser Deposition
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J21.4.1
TiN/GaN Metal/Semiconductor Multilayer Nanocomposites Grown by Reactive Pulsed Laser Deposition
Vijay Rawat and Timothy D. Sands School of Materials Engineering and School of Electrical and Computer Engineering, Purdue University, IN 47906, U.S.A. ABSTRACT TiN/GaN multilayers with periods ranging from 5 nm to 50 nm were grown by reactive pulsed laser deposition (PLD) using elemental metal targets in an ammonia ambient at 20mtorr onto Si(100), MgO(100) and sapphire(0001) substrates. For growth on Si and MgO substrates, an epitaxial 40 nm thick TiN buffer layer was deposited prior to deposition of the multilayers. An epitaxial 150 nm GaN buffer layer was grown on sapphire substrates. For all substrates, layer thicknesses and periods investigated, x-ray diffraction and cross-sectional transmission electron microscopy revealed {0001} texture for GaN, and {111} texture for TiN in the multilayers. Both TiN layers and GaN layers thicker than ~ 2nm appear to be continuous, with no evidence of agglomeration. Both phases are crystalline, with lateral grain sizes comparable to the layer thickness. These results suggest that epitaxy will not be necessary to fabricate pinhole free metal/semiconductor multilayers in the nitride system. INTRODUCTION Metal/semiconductor multilayers with nanoscale periods and tailored barrier heights are expected to exhibit novel thermal and electronic cross-plane transport properties. Theoretical investigations have suggested that metal/semiconductor multilayers with controlled interface roughness have the potential to yield solid-state thermionic energy conversion devices with efficiencies that are substantially greater than those of conventional thermoelectric materials[1]. Thermoelectric/thermionic devices for direct thermal energy conversion are expected to work at high temperatures; hence nitrides with high melting points and excellent corrosion resistance are potentially well suited for such applications. Furthermore, the properties (e.g., barrier height and thermal conductivity) of metallic and semiconducting nitrides and their interfaces can be tuned by alloying. In this study, TiN/GaN metal/semiconductor multilayers have been chosen for initial investigations of nitride metal/semiconductor multilayer growth mechanisms,morphologies, orientation relationships, stability and defect structures. Previous research on nitride multilayers and superlattices was performed mainly with the aim of fabricating superhard and corrosion resistant coatings[2] and for making superconducting nitride multilayers[3]. In the present work, we have fabricated nitride multilayers in reactive ammonia ambient with the aim of utilizing the wide range of electronic and thermal properties exhibited by group III nitrides and transition metal nitrides, in energy conversion devices. A longer-term aim of present work is to stabilize the rocksalt phase of GaN in order to fabricate a rocksalt metal/semiconductor superlattice. TiN was chosen for this phase stabilization effort
J21.4.2
since its lattice parameter i
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