Titanium oxide nanotube arrays for high light extraction efficiency of GaN-based vertical light-emitting diodes

Abstract

TiO₂ nanotube (NT) arrays were fabricated on the surface of n-GaN through a liquid-phase conversion process using ZnO nanorods (NRs) as a template for high-efficiency InGaN/GaN multiple quantum well (MQW) vertical light-emitting diodes (VLEDs). The optical output power of the VLEDs with TiO₂ NTs was remarkably enhanced by 23% and 189% at an injection current of 350 mA compared to those of VLEDs with ZnO NRs and planar VLEDs, respectively. The large enhancement in optical output is attributed to a synergistic effect of efficient light injection from the n-GaN layer of the VLED to TiO₂ NTs because of the well-matched refractive indices and superior light extraction into air at the end of the TiO₂ NTs. Light propagation along various configurations of TiO₂ NTs on the VLEDs was investigated using finite-difference time domain simulations and the results indicated that the wall thickness of the TiO₂ NTs should be maintained close to 20 nm for superior light extraction from the VLEDs.