GaN-based Resonant Cavity LEDs Fabricated by Photo-Electrochemical Etching and Micro-Transfer Printing

Abstract

Resonant cavity LEDs (RCLEDs) exhibit excellent temporal and spatial coherence with narrow spectral linewidth and small divergence angle, which is of great importance for micro-displays. In this paper, we demonstrate a novel method to create GaN-based RCLEDs by using photo-electrochemical etching and micro-transfer printing (MTP) technology. Through systematic optimizing of the etching conditions, highly selective etching of an InGaN multiple quantum well sacrificial layer is achieved with parasitic etching of adjacent layers being completely suppressed. The roughness of the underside of the exfoliated GaN film is only 3.3 nm. Raman spectroscopy shows that the residual stress in the released material is reduced from 0.74 GPa for the as-grown sample to -0.15 GPa. Using the MTP method, GaN coupons with a deposited upper dielectric mirror were transferred onto target substrates covered with either an Al mirror or dielectric distributed Bragg reflector to form two types of blue RCLEDs. The electroluminescence spectra of the two RCLEDs show a much narrower linewidth, reduced from 32 nm in the conventional LED to ~5 nm, together with stable peak wavelength with increasing current density, with the shift reduced from 9.3 nm to less than 1 nm. The far-field pattern is influenced by the bottom mirror, and the far-field divergence angle can be decreased to only 52{\deg} by matching the cavity and the quantum well exciton modes. This scalable approach is highly promising for the realization of compact resonant cavity devices and their use in displays and in communications.