Air-spaced GaN nanopillar photonic band gap structures patterned by nanosphere lithography

Abstract

We report on the fabrication of ordered hexagonal arrays of air-spaced GaN nanopillars by nanosphere lithography. A self-assembled two-dimensional silica nanosphere mask was initially formed by spin-coating. Prior to pattern transfer to the GaN substrate, a silica-selective dry etch recipe was employed to reduce the dimensions of the nanospheres, without shifting their equilibrium positions. This process step was crucial to be formation of air-spaced hexagonal arrays of nanospheres, as opposed to closed-packed arrays normally achieved by nanosphere lithography. This pattern is then transferred to the wafer to form air-spaced nanopillars. By introducing air gaps between pillars, a photonic band gap (PBG) in the visible region can be opened up, which is usually nonexistent in closed-packed nanopillar arrays. The PBG structures were designed using the plane wave expansion algorithm for band structure computations. The existence and positions of band gaps have been verified through optical transmittance spectroscopy, which correlated well with predictions from simulations. From photoluminescence (PL) spectroscopy, a fourfold increase in PL intensity was observed and compared to an as-grown sample, demonstrating the effectiveness of well-designed self-assembled PBG structures for suppressing undesired optical guiding mode via PBG and for promoting light extraction. The effects of defects in the nanopillar array on the optical properties are also critically assessed.