Growth of Oriented Gallium Nitride Films on Amorphous Substrates by Self Assembly
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Growth of Oriented Gallium Nitride Films on Amorphous Substrates by Self Assembly Hongwei Li, Mahendra K. Sunkara*, Department of Chemical Engineering, University of Louisville, Louisville, KY 40292, U.S.A. *E-mail: [email protected] ABSTRACT C-plane oriented thin films of gallium nitride (GaN) were grown on both amorphous quartz substrates and single crystalline c-sapphire substrates at sub-atmospheric pressures by exposing molten gallium thin films to electron cyclotron resonance (ECR) microwave generated nitrogen plasma. Gallium nitride crystals nucleated from molten gallium and self-aligned with respect to each other due to the mobility of nitrogenated gallium and formed textured film directly on amorphous substrates. Scanning electron microscopy (SEM) images and X-ray Diffraction (XRD) spectra confirmed the orientation among crystals. Micro-Raman spectra exhibited a FWHM of 3 cm-1. Self-assembled, nanocrystalline GaN thin films were obtained when spincoated gallium thin films (< 1 µm) on quartz substrates were nitrided. INTRODUCTION Gallium nitride (GaN) is a direct, wide band gap semiconductor that finds applications in light emitting diodes (LED)[1-2], laser diodes (LD)[3-4], high temperature/power field effect transistors (FET)[5] and UV optical devices. Since both GaN substrates and lattice matched substrates are unavailable, hetero-epitaxy on substrates such as sapphire or SiC is currently employed using metal organic chemical vapor deposition (MOCVD)[6-7], Molecular beam epitaxy (MBE)[8-9] or Hydride vapor phase epitaxy (HVPE)[10-11] techniques. The lattice mismatch and different thermal expansion coefficients between GaN and substrates cause high density of dislocations and stresses in the film [12-13]. Lateral Epitaxial Overgrowth (LEO) [14] and Pendeo-epitaxy [15] have achieved significant reduction in the dislocation density, however homoepitaxy on high quality GaN substrates would be preferred [16]. Bulk gallium nitride growth under high nitrogen pressure (~15kbars) from gallium melt has low dislocation densities (106cm-2), but the area of these crystals remains at about 3cm2 [17]. Bulk nucleation and growth of GaN from gallium melt at sub atmospheric pressure had been demonstrated using atomic nitrogen [18]. However this process produced epitaxial growth on (0001) single crystal sapphire substrate and has dislocation density in excess of 1010cm-2 [19]. Here, we report a different approach that uses low pressure bulk synthesis to get large area, self-oriented, high quality thin film GaN which avoids epitaxy using either thin molten gallium film as a buffer layer or directly growing on an amorphous substrate. Thin films of liquid gallium are spread onto a single crystalline, polycrystalline or amorphous substrate, and then nitrided in an ECR–MW nitrogen plasma environment. Since nitrogen containing gallium melts wet solid surfaces very well [20-21], the flow of nitrided gallium melts will allow growing GaN platelets crystals to self-orient with respect to each other. The selfalignment o
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