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In this study, the process of nanosphere lithography was developed and applied to LED surface texturing and nanorod device fabrication. We observed a texture-size-dependent improvement of total light output. While the increase of output optical power from the textured LEDs can be attributed to surface roughening in the GaN-air surface and to the increase of internal quantum efficiency as the strain is relaxed with the surface texturing, the size-dependent device performance is related to the interaction of generated photons with the textured surface. We further etched through the p-GaN and quantum well region to form p-i-n nanorods on the sample. By inserting a spacer to prevent p-type contact from shorting the n-GaN, we successfully demonstrated nanorod LED arrays. For such a device, a narrower radiation profile was demonstrated from the nanorod LED array as compared with that from the planar LED. The result is associated with the vertical guiding effect along the nanorod cylinder and the Bragg scattering of photons extracted from the sidewall by the rest of the rods. Furthermore, the electroluminescence spectra showed a nearly constant peak wavelength of the nanorod LED arrays, which is due to the suppression of the effect of quantum confined Stark effect.

We investigated the carrier transient response of the nanopatterned silicon heterojunction photodiodes using ZnO as the n-type semiconductor. The results show that under the constant light illumination intensity, the planar structure has faster carrier response than the nanopatterned amorphous silicon (intrinsic) (a-Si(i)) diodes. It is attributed to a higher number of generated carriers in the nanostructure (due to the lower surface reflectivity) that increases the probability of collisions. On the other hand, the shortest response time of the device with nanopatterned p+-Si suggests that carriers can be effectively transported vertically and horizontally through the p-i(intrinsic)-n structure. Furthermore, the wavelength-dependent rise time is correlated to the different transport distance between electrons and holes at different excited wavelengths.

The polarization behavior of the light-emitting diodes (LEDs) with nanorods surrounding the p-mesa is investigated. The nanorods were fabricated using a natural nanosphere lithography and are intended to diffract laterally propagated light. In the horizontal direction, s-polarized light is dominated since the injected carriers choose to fill up the lowest energy state in a direction parallel to the quantum-well layers. The p/s-polarized ratio starts to increase with the increase of radiated angles and eventually saturates. Since the Bragg diffraction of laterally propagated p-polarized mode by nanorods is more efficient than the s-polarized light, the p/s-ratio of the device with nanorods is higher than that without rods. The p/s-ratio of the LED with nanorods is 1.96 at 90deg, and is 1.52 when the integrating intensity between 0deg and 90deg is considered.

We designed and fabricated GaN-based light emitting diodes encompassed with self-aligned nanorod arrays of three different distribution densities. The radiation profiles show that the device with less dense nanorod distribution has the highest optical power enhancement factor. By regarding the nanorod arrays as sidewall reflectors, the enhancement is due to the light diffraction of the laterally guided mode to radiative modes in the surface normal direction. As for the densest nanorods in our experiment, the radiation profile shows a better emission directionality as the guided modes are phase matched to the radiation modes. However, they show less optical output enhancement since some of the laterally propagated light is diffracted back to p-mesa by the first several columns of nanorods.

A practical approach to fabricate textured GaN-based light-emitting diodes (LEDs) by nanosphere lithography is presented. By spin coating a monolayer of SiO2 nanoparticles as the mask, textured LEDs can be fabricated. Both textured p-GaN and textured indium tin oxide LEDs show significant improvement over conventional LEDs without damaging the electrical characteristics. The results show that the method is promising for manufacturing low-cost high-efficient GaN-based LEDs.