Optical studies of focused ion beam fabricated GaN microstructures andnanostructures

Abstract

In this thesis, Gallium Nitride (GaN) micro- and nanostructures were fabricated based on focused ion beam (FIB) milling. The starting wafer is an epitaxial structure containing InGaN/GaN multi-quantum wells. High crystal quality structures such as the nano-cone, nanopillar array and single pillar were fabricated based on the FIB method. During the fabrication process, various approaches were designed to minimize FIB damage caused by Gallium ion bombardment.

The fabrication process for nano-cone is a combination of mask preparation by FIB with subsequent reactive ion etching (RIE). For fabricating nanopillar arrays, the nanopillars were patterned directly using FIB with an optimized beam current followed by wet etching process to remove the damage. On the other hand, the single pillar is achieved by gradually decreasing the ion beam current as the diameter of the pillar becomes smaller.

The first order Raman spectra for the nanopillar array reveal a strong additional peak when the diameter of the nanopillars is less than 220 nm. This peak can also be observed in GaN pillars without MQW and is clearly assigned to the surface optical (SO) mode originated from the A1 phonon in wurtzite GaN. The frequency of this SO mode is found to be sensitive with the diameter and surface roughness of the nanopillars. Temperature-variable photoluminescence (PL) measurements show that a broadband emission in the as-grown sample split into the two well-resolved bands for nanopillars and the emission band at the higher energy side quickly thermally quenched.

Room temperature PL measurements on the single pillars exhibit an increasing blue-shift of the peak emission with the decreasing of the pillar diameter. Additional simulation data and excitation power dependent PL studies confirm the observation of strain relaxation in the pillar’s MQW due to FIB fabrication. The temperature variable PL on the single pillar shows a monotonous blue shift as the temperature arises to 300 K.