LEDs on HVPE grown GaN substrates: Influence of macroscopic surface features

Sk S Rahman,F Scholz,J Wang,M Ishii,R A R Leute,M Klein,K Koyama,T Meisch,H Takeda

doi:10.1063/1.4890348

Abstract

We demonstrate the strong influence of GaN substrate surface morphology on optical properties and performance of light emitting devices grown on freestanding GaN. As-grown freestanding HVPE GaN substrates show excellent AFM RMS and XRD FWHM values over the whole area, but distinctive features were observed on the surface, such as macro-pits, hillocks and facets extending over several millimeters. Electroluminescence measurements reveal a strong correlation of the performance and peak emission wavelength of LEDs with each of these observed surface features. This results in multiple peaks and non-uniform optical output power for LEDs on as-grown freestanding GaN substrates. Removal of these surface features by chemical mechanical polishing results in highly uniform peak wavelength and improved output power over the whole wafer area.

Highlights

Gallium nitride (GaN) is an extremely promising material for optoelectronic devices such as light-emitting diodes (LEDs) and laser diodes (LDs).[1,2] GaN based white LEDs for solid state lighting are increasingly deployed due to their low energy consumption and very long lifetime.[3]
We demonstrate the strong influence of GaN substrate surface morphology on optical properties and performance of light emitting devices grown on freestanding GaN
We investigated the performance of test structures grown on freestanding as-grown substrates compared to polished substrates in order to identify the effects of macroscopic surface features and how the excellent crystalline quality of the GaN substrates can be used for high performance devices

Summary

INTRODUCTION

Gallium nitride (GaN) is an extremely promising material for optoelectronic devices such as light-emitting diodes (LEDs) and laser diodes (LDs).[1,2] GaN based white LEDs for solid state lighting are increasingly deployed due to their low energy consumption and very long lifetime.[3]. GaN layers on such foreign substrates, typically sapphire, suffer from a high dislocation density (108 − 1010 cm−2) which leads to many drawbacks such as the increase of the leakage currents in blue and green LEDs, reduced efficiency of nearUV LEDs and decreased lifetime of laser diodes.[4,5] homo-epitaxy is preferable. The fast HVPE growth of thick layers, even with homogeneously high crystal quality, on MOVPE templates introduces defects and pits which lead to a macroscopically rough surface. This interface between the pseudo bulk substrate and the subsequent MOVPE grown layers may be critical for the device performance. We investigated the performance of test structures grown on freestanding as-grown substrates compared to polished substrates in order to identify the effects of macroscopic surface features and how the excellent crystalline quality of the GaN substrates can be used for high performance devices

EXPERIMENTAL

RESULTS

CONCLUSION