Point Defects in InGaN/GaN Core–Shell Nanorods: Role of the Regrowth Interface

K Loeto,PA Shields,SM Fairclough,I Girgel,G Kusch,P-M Coulon,E Le Boulbar,RA Oliver

doi:10.1088/2632-959x/abe990

Abstract

Core-shell nanorod based light-emitting diodes (LEDs) with their exposed non-polar surfaces have the potential to overcome the limitations of planar LEDs by circumventing the quantum confined stark effect. In this experiment, InGaN/GaN core-shell nanorods were fabricated by a combination of top-down etching and bottom-up regrowth using metal-organic vapour phase epitaxy. When viewing the nanorods along their long axis, monochromatic cathodoluminescence maps taken at the GaN near-band-edge emission energy (3.39 eV) reveal a ring-like region of lower emission intensity. The diameter of this ring is found to be 530 (±20)nm corresponding to the ∼510 nm diameter nickel etch masks used to produce the initial GaN nanopillars. Thus, the dark ring corresponds to the regrowth interface. To understand the origin of the ring, scanning transmission electron microscopy (STEM) and cathodoluminescence (CL) hyperspectral mapping at 10K were performed. STEM imaging reveals the absence of extended defects in the nanorods and indeed near the regrowth interface. Monochromatic CL maps recorded at 10K show that the ring remains dark for monochromatic maps taken at the GaN near-band-edge emission energy (3.47 eV) but is bright when considering the donor-acceptor pair emission energy (3.27 eV). This peculiar anticorrelation indicates that the dark ring originates from an agglomeration of point defects associated with donor-acceptor pair emission. The point defects are incorporated and buried at the GaN regrowth interface from the chemical and/or physical damage induced by etching and lower the radiative recombination rate; limiting the radiative efficiency close to the regrowth interface.

Highlights

Vertically-aligned radial core-shell nanorods have been shown to provide means to overcome the limitations of conventional planar quantum well (QW) light-emitting diodes (LEDs) based on wurtzite GaN [1,2,3,4]
The nanorods investigated in this work can be seen, from the top, as a uniform hexagonal array in the secondary electron (SE) image shown in figure 2(b)
InGaN/GaN core-shell nanorods prepared by a top-down etching and bottom-up facet regrowth process were investigated using CL hyperspectral and scanning transmission electron microscopy (STEM)-high-angle annular dark-field (HAADF) imaging

Summary

Introduction

Vertically-aligned radial core-shell nanorods have been shown to provide means to overcome the limitations of conventional planar quantum well (QW) LEDs based on wurtzite GaN [1,2,3,4]. The hybrid approach, which will be the focus of this letter, provides advantages such as high uniformity of the nanorod array [5] This method can suffer from defect incorporation through etch related damage [2, 6]. For GaN-based planar LEDs, the mature growth direction is the polar [0001] c-direction which is hampered by inherent spontaneous and piezoelectric polarisation fields leading to a reduced electron-hole overlap in the active region, limiting device efficiency [7]. This is known as the quantum-confined Stark effect (QCSE). The nanorod geometry provides a reduced dislocation density due to dislocation bending and termination [9, 10], a strain relaxed structure that limits stress build-up hindering crack nucleation [11], and a high surface-area-to-volume ratio which reduces carrier density in the QWs limiting the effects of efficiency droop [12]

Methods

Results

Conclusion