ABSTRACT
In-plane structural anisotropy is characteristic of nonpolar (1120) a-plane GaN (a-GaN) films grown on r-plane sapphire substrates. The anisotropic peak broadenings of X-ray rocking curves (XRCs) are clearly observed with M- or W-shaped dependence on the azimuth angles. We investigated the optical properties of both M- and W-shaped a-GaN samples with room and low-temperature photoluminescence (PL) measurements. The W-shaped a-GaN film showed higher PL intensity and more compressive strain compared to the M-shaped a-GaN film, whereas the XRC peak widths of the M-shaped a-GaN film on the azimuth angles are lower than those of W-shaped specimens, indicating that better crystalline quality was obtained. We speculate that the PL intensity and strain state of a-GaN layers may be more influenced by the crystallinity of a specific crystal orientation or direction, especially along the m-axis as opposed to the c-axis. This occurrence is most likely due to anisotropic defect distributions, resulting from differences in dangling bond densities of (0001) and {1-100} facets.
ABSTRACT
The authors report on an improved light extraction method from large-area vertical light emitting diodes (VLEDs) with deep hole-patterns fabricated using nanosphere lithography. In order to produce the ordered deep-hole patterns on the n-type GaN surface, a 150 nm thick Ni dot mask formed via a lift-off process of the Ni coated onto a 500 nm diameter polystylene bead array was employed to enable deep etching. Three VLEDs-one as a reference with no patterns, and two with periodic 360 nm diameter hole patterns, one with 1.0 microm and the other with 1.5 microm depths on the n-type GaN surface, were prepared for comparison. The light output power measured for the VLEDs with the hole-patterns increased by 4.13 and 4.86 times, respectively, as compared to the reference VLED. These enhancements are attributed to the multiple scatterings of the light from the sidewall of the hole-patterns and to the increased surface area to which the light can approach. The higher light output power obtained for the VLEDs with the deep hole patterns might be due to a photon reabsorption reduction within the n-GaN layer.
