Flow Modulation Epitaxial Lateral Overgrowth of Gallium Nitride on Masked 6H-Silicon Carbide and Sapphire Surfaces
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ABSTRACT Selective Area Flow Modulation Epitaxial growth of GaN is carried out in a low pressure Organometallic Vapor Phase Epitaxy reactor. This process is known to enhance reactant surface migration lengths on patterned group III-arsenide and phosphide growth surfaces. With this process, high quality laterally overgrown GaN epitaxial materials result. Under the ammonia rich growth conditions used, enhanced migration (by flux modulation) across masked regions of the substrate has not been observed. The mask materials were silicon dioxide and silicon nitride, both deposited on GaN/AlGaN buffer structures on sapphire and SiC substrates. Window stripes were patterned parallel and perpendicular to the (1100) crystal directions to observe the orientation dependence of the lateral growth rate. Structures exhibited heights above the mask surface as large as 30 microns and atomically smooth surfaces. With a periodic array of stripe window openings in the mask, planarized laterally overgrown surfaces are achieved after roughly 4 microns of overgrowth. Chemical assisted ion beam etching with chlorine gas was used to delineate defects in the selectively grown layers. Additional evidence on the defect reduction is given by Atomic Force and Scanning Transmission Electron Microscopies. INTRODUCTION Epitaxial lateral overgrowth of GaN on masked surfaces has received considerable attention as a method of reducing defects caused by the severe mismatch of III-V nitride epitaxial thin films and available substrates. It has been established that defect free GaN, within the limitations of TEM, can be obtained in regions were lateral overgrowth occurs over a Si0 2 mask on SiC substrates [1]. In addition to the mask material blocking dislocations from the buffer, there is also evidence that defects within the window openings bend towards the lateral growth direction, preventing propagation of these defects to the surface [1,2]. More recently, " 'proved reliability of UV laser diodes fabricated on top of GaN epitaxial lateral overgrowth regions on sapphire has been demonstrated [3]. All these trends seem to indicate, that in the near future, epitaxial lateral overgrowth will emerge as the preferred growth technique of nitride based material systems. In this paper, we present selectively grown GaN structures on both sapphire and 6HSiC patterned substrates by flow modulation epitaxy. Coalesced planar layers were grown on several stripe geometries aligned parallel to the crystal direction. Growth was also suspended prior to coalescence to observe surface structure evolution. Structures grown beyond the point of coalescence are also examined. Material is evaluated by scanning electron (SEM), scanning transmission electron (STEM), and atomic force (AFM) microscopies as well as chemical assisted ion beam etching (CIBE). Finally, energy bandgaps were determined using 300K photoluminescence to indicate the relative amount of residual stress in each film. 59
Mat. Res. Soc. Symp. Proc. Vol. 512 © 1998 Materials Research Society
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