Growth of Gallium Nitride Textured Films and Nanowires on Polycrystalline Substrates at sub-Atmospheric Pressures
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Growth of Gallium Nitride Textured Films and Nanowires on Polycrystalline Substrates at sub-Atmospheric Pressures Hari Chandrasekaran and Mahendra K. Sunkara* Department of Chemical Engineering University of Louisville Louisville, KY 40292 *Corresponding Address: [email protected]. Abstract Textured gallium nitride (GaN) films were grown on polished, polycrystalline and amorphous substrates in sub-atmospheric pressures, by direct nitradation of a thin molten gallium films using electron cyclotron resonance (ECR) microwave nitrogen plasma. C-plane texturing was achieved, independent of the substrate crystallinity. Single crystal quality GaN nanowires with diameters ranging from 40-50 nm were also synthesized using direct nitridation of thin gallium films with nitrogen plasma. Scanning electron microscopy (SEM), X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS) and Cross-sectional transmission electron microscopy (CS-TEM), high-resolution TEM (HRTEM) and Micro-Raman spectroscopy were used to characterize the synthesized gallium nitride films and GaN nanowires.
Introduction The need for short wavelength lasers for increased data storage capacity, bright blue light emitting diodes1 (LED) for flat screen display units, high temperature/power field effect transistors2 (FETs), UV photo detectors3, have proved to be an impetus for GaN research. Despite huge densities of dislocations, metal organic vapor phase epitaxy4 (MOVPE) and Molecular beam epitaxy5 (MBE) techniques are currently being used. Lateral epitaxial overgrowth6 (LEO) and pendo epitaxy7 produce crystals with greatly reduced densities of dislocations. The substrate’s pretreatment for these techniques are very rigorous. Homoepitaxy on high quality GaN substrate, would be an ideal solution to these problems. Crystals grown by high pressure8 (15kbars) bulk growth from gallium melt have low dislocation densities (106cm-2), but the areas of these crystals remain at about 1 cm2. Hydride vapor phase epitaxy9 (HVPE) a pseudo bulk growth technique achieves crystals of 2 inch diameter at a growth rate of 100µm/hr, but these crystals are prone to cracking. Bulk growth of GaN from gallium melt in activated nitrogen environment has been demonstrated by Angus et al.10 Oriented growth11 on [0001] single crystal sapphire substrate has been achieved. However, the resulting oriented films exhibited dislocation density of 1010/cm2 at substrate-film interface similar to that of MOCVD grown films.
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In this regard, we have been working on a method that uses micro-flow patterns, and interplatelet interactions to self-align high aspect ratio platelets, crystallized from the bulk at subatmospheric conditions using polished, polycrystalline and amorphous substrates. In this technique, a thin film of gallium melt is spread on a substrate and exposed to microwave nitrogen plasma. Wetting properties of gallium films on substrate vary and improve with nitrogen dissolution12. Hence, thin film wafers can be grown over large areas using polycrystalline or amorphous
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