Optically Pumped GaN-AlGaN Double-Heterostructure Lasers Grown by ECR-GSMBE and HVPE
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recording/reading, chemical sensing, and displays. While the development of efficient, long-lived diode lasers in this wavelength range is the eventual objective, optical pumping is a convenient technique for evaluating the optical properties of GaN laser structures without the added complications posed by electrical injection. Optically pumped lasing has been demonstrated [2-3], and there have been a number of reports of stimulated emission [4-8]; however, high threshold power densities observed in the optical pumping experiments indicate that the realization of a low threshold current density diode laser will require improvements in the material and laser structures. We have found that a limiting factor in the gain characteristics of GaN is the surface morphology of the epitaxial film, which is intimately dependent on the substrate used for epitaxial growth. Morphology affects the gain by contributing a scattering factor as the optical mode is guided through the lasing medium. The strength of this scattering depends on the characteristic length scale of the surface ripple. Unfortunately, the length scale of surface ripple of GaN heteroepitaxially grown on (0001) sapphire, at least by ECR-MBE, contributes to high scattering losses in the guided mode for GaN/AlGaN DHs in the UV wavelength range. This is true, at least in our experiments, despite the fact that the surfaces appear to be "mirror smooth" at visible wavelengths. These scattering losses should impact visible-wavelength lasers as well because of the wavelength reduction in the material due to the GaN refractive index. Here, we demonstrate that the morphology of the epitaxial GaN film can be improved by using a GaN buffer grown by HVPE on (0001) sapphire as the starting substrate. The advantage of the HVPE buffer is that the morphology is optically smoother in the UV than for GaN material grown by ECR-MBE on sapphire using a low-temperature buffer (two-step process). The HVPE The buffer enables homoepitaxial growth which replicates the starting surface morphology. morphology of the HVPE buffer at present exhibits a variety of growth features characteristic of 919 Mat. Res. Soc. Symp. Proc. Vol. 395 0 1996 Materials Research Society
the HVPE process, but by mechanically polishing these buffers, high-quality, planar surfaces can be obtained. It should be noted, however, that stimulated emission is observed from layers grown on both as-grown and polished surfaces. Using these buffers, we have observed, for the first time, optically pumped laser emission exhibiting cavity modes in GaN/A1GaN DHs grown by ECR-MBE. EXPERIMENTAL The details of the HVPE process have been published elsewhere [9]. The HVPE buffers, initially -15 jim in thickness, grown on (0001) sapphire, are mechanically polished to remove growth hillocks with a resulting thickness of -8 gim. The substrates are chemically cleaned using solvents and 1:1 H 2 SO 4 :H 2 0 2 to remove contaminants, and chemically etched with 1:1 H 2SO4: H3 PO 4 prior to loading into the MBE system. The MBE system is a V
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