Light extraction enhancement on GaN based LEDs using laser assisted debonding and electrodeless photochemical etching

Abstract

Detailed investigations of laser-debonded GaN-based light emitting diodes (LEDs) with InGaN/GaN multiple quantum wells (MQWs), grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates, have been performed. The debonded surface was roughened by photo-electrodeless chemical (PEC) etching in a mixture of KOH and K2S2O8 solution, with an aim to improve the light extraction efficiency. The power for the laser-assisted debonding process has been optimized using the numerical thermal analysis technique and systematic experimental investigations. The data showed that as long as the laser energy density does not exceed the optimal debonding value, there is no degradation in the I-V characteristics and the brightness of the device. The structural and electrical properties of the GaN films and LEDs were analyzed using atomic force microscopy (AFM), transmission electron microscopy (TEM), the x-ray diffraction technique (XRD) and low-frequency noise measurement, before and after the laser debonding process. Numerical thermal analysis using the heat transfer equation was employed to estimate the optimum laser debonding power and the temperature distribution within the GaN film. The calculated results showed that a threshold energy density of 400 mJcm-2 is required to cause thermal decomposition of GaN, and the thickness of the sacrificial layer was estimated to be 200 nm. A cross-sectional TEM study revealed that laser debonding at the threshold fluence caused structural changes of the GaN material that are highly localized within a region of 180 nm from the GaN/sapphire interface, which is consistent with the prediction based on the thermal analysis. Detailed low-frequency noise characterizations were conducted to investigate the defect properties over the entire active area of the device. Experimental data on the voltage noise power spectra measured from the same device before and after laser debonding indicated no significant change in the magnitudes of the flicker noise over a wide temperature range. For PEC etching, systematic variation of the KOH concentration was performed to roughen the debonded LED surface. Significant roughening was obtained under the etching conditions at 2M KOH for 30 minutes or more, forming hexagonal pyramid structures. The experimental results demonstrated strong dependencies of the luminous intensity of the device on the roughness of the debonded surface. A 60% improvement in the luminous intensity of the debonded and roughened LED was observed compared to the original on-sapphire device. This increase in the extraction efficiency is attributed to the reduction in the total internal reflection at the roughened GaN/air interface.