Advantages and remaining issues of state-of-the-art m-plane freestanding GaN substrates grown by halide vapor phase epitaxy for m-plane InGaN epitaxial growth

Advantages and remaining issues of state-of-the-art m-plane freestanding GaN (FS-GaN) substrates grown by halide vapor phase epitaxy (HVPE) for m-plane In_xGa_{1 x}N epitaxial film growth by metalorganic vapor phase epitaxy are described. Because of the low threading dislocation and basal-plane stacking fault densities, improved quantum efficiency and short radiative lifetime are achieved for the near-band-edge emission of 200250 nm thick m-plane pseudomorphic In_xGa_{1 x}N (x 0.14) epilayers. Values of full-width at half-maximum for the x-ray -rocking curves remain unchanged as the substrate values being 80 and 60 arcsec for the (100) diffraction with 0001 and azimuths, respectively, and 80 arcsec for the (102) diffraction. As the surface flatness is greatly improved, the In-incorporation efficiency (^In _inc) is lower than the cases for conventional c-plane growth and m-plane growths on defective GaN bases. The former originates from nonidentical surface kinetics, and the latter is due to the reduction in the area of inclined and tilted planes. Sub-micrometer-wide zonary patterns parallel to the c-axis and 2 m long axis figure-of-eight patterns parallel to the a-axis are clearly visualized in the monochromatic cathodoluminescence (CL) intensity images. Because the spatio-time-resolved CL measurement reveals very little spatial variation of low-temperature radiative lifetime, the slight peak energy variation is interpreted to originate from nonidentical ^In _inc for the growing surfaces exhibiting various miscut angles. The figure-of-eight patterns are ascribed to originate from the anisotropic, severe m-plane tilt mosaic along the a-axis of the GaN substrate, and the zonary patterns may originate from the m-plane tilt mosaic along the c-axis. Further reduction in the tilt and twist mosaics is necessary for HVPE of FS-GaN substrates, in order to grow homogeneous InGaN epilayers.