Pendeo-Epitaxial Growth and Characterization of GaN and Related Materials on 6H-SiC(0001) and Si(111) Substrates
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ABSTRACT Discrete and coalesced monocrystalline GaN and AlxGa1-xN layers grown via Pendeoepitaxy (PE) [1] originated from side walls of GaN seed structures containing SiNx top masks have been grown via organometallic vapor phase deposition on GaN/AlN/6HSiC(0001) and GaN(0001)/AlN(0001)/3C-SiC(111)/Si(111) substrates. Scanning and transmission electron microscopies were used to evaluate the external microstructures and the distribution of dislocations, respectively. The dislocation densities in the PE grown films was reduced by at least five orders of magnitude relative to the initial GaN seed layers. Tilting in the coalesced GaN epilayers was observed via X-ray diffraction. A tilt of 0.2° was confined to areas of mask overgrowth; however, no tilting was observed in the material suspended above the SiC substrate. The strong, low-temperature PL band-edge peak at 3.45 eV with a FWHM of 17 meV was comparable to that observed in PE GaN films grown on 6H-SiC(0001). The band-edge in the GaN grown on AlN(0001)/SiC(111)Si(111) substrates was shifted to a lower energy by 10 meV, indicative of a greater tensile stress.
INTRODUCTION It has been a necessity for investigators in the III-nitride community to grow films of GaN and related nitride materials using heteroepitaxial growth routes because of the dearth of bulk substrates of these materials. This results in films containing dislocation densities of 108-1010 cm-2 because of the mismatches in the lattice parameters and the coefficients of thermal expansion between the buffer layer and the film and/or the buffer layer and the substrate. These high concentrations of dislocations may also limit the performance of devices. Several groups [2-10], including the present authors, have conducted research regarding selective area growth (SAG) and lateral epitaxial overgrowth (LEO) techniques for GaN deposition, specifically to reduce significantly the dislocation density. Increased emphasis in this research topic was fueled in part by the announcement by Nakamura, et al. [11-13] of the dramatic increase in projected lifetime of their GaN- based blue lightemitting laser diodes fabricated on LEO material. Using these approaches, researchers have been able to grow GaN films containing dislocation densities of ≈ 105 cm-2 in the areas of overgrowth.
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However, to benefit from this reduction in defects, the placement of devices incorporating LEO technology is limited and confined to regions on the final GaN device layer that are located on the overgrown regions. Recently we have pioneered a new approach to selective epitaxy of GaN and AlxGa1-xN layers, namely, pendeo- (from the Latin: to hang or be suspended) epitaxy (PE) [1,14-19] as a promising new process route leading to a single, continuous, large area layer; multilayer heterostructures or discrete platforms of these materials. It incorporates mechanisms of growth exploited by the conventional LEO process by using an amorphous mask to prevent vertical propagation of thre
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