Multi-wavelength emission from a single InGaN/GaN nanorod analyzed by cathodoluminescence hyperspectral imaging

Gunnar Kusch,Paul R Edwards,Martin J Cryan,Peter J Parbrook,Haoning Li,Robert W Martin,Chao Zhao,Boon S Ooi,Jon Pugh,Michele Conroy

doi:10.1038/s41598-018-20142-5

Abstract

Multiple luminescence peaks emitted by a single InGaN/GaN quantum-well(QW) nanorod, extending from the blue to the red, were analysed by a combination of electron microscope based imaging techniques. Utilizing the capability of cathodoluminescence hyperspectral imaging it was possible to investigate spatial variations in the luminescence properties on a nanoscale. The high optical quality of a single GaN nanorod was demonstrated, evidenced by a narrow band-edge peak and the absence of any luminescence associated with the yellow defect band. Additionally two spatially confined broad luminescence bands were observed, consisting of multiple peaks ranging from 395 nm to 480 nm and 490 nm to 650 nm. The lower energy band originates from broad c-plane QWs located at the apex of the nanorod and the higher energy band from the semipolar QWs on the pyramidal nanorod tip. Comparing the experimentally observed peak positions with peak positions obtained from plane wave modelling and 3D finite difference time domain(FDTD) modelling shows modulation of the nanorod luminescence by cavity modes. By studying the influence of these modes we demonstrate that this can be exploited as an additional parameter in engineering the emission profile of LEDs.

Highlights

High quality InGaN/GaN LEDs are key to future lighting technologies, promising considerable reduction in worldwide power consumption
Their findings show that strong coupling exists between light emitted from m-plane nonpolar QWs and the optical modes and that the resonant wavelengths can be tuned by the nanorod diameter
This paper presents our investigation of light emitted from spatially separated active regions, with different polarities (c-plane and semipolar), of a single InxGa1−xN/GaN nanorod from an ultra-dense nanorod array, using cathodoluminescence (CL) hyperspectral imaging

Summary

Results and Discussion

CL and scanning transmission electron microscopy (STEM) investigation of the ultra dense array revealed that the growth conditions resulted in two different, spatially separated active zones showing emission from 395 to 480 nm from the semipolar sidewalls and between 490 and 650 nm from c-plane QWs in the apex of the nanorods[10]. From the calculated mode positions it can be seen that the agreement between the mode position and the experimentally observed peaks gets worse the longer the wavelength and the lower the mode number is This is most likely due to limitations of the plane wave model or a deviation in the actual refractive index n from the indexes given in ref.[23]. Due to this limitation of the plane wave model we have employed 3D FDTD modelling[25] to investigate the mode structure (using the same values for the refractive index n) For this a dipole source emitting from 400 nm to 700 nm was placed into the pyramidal top of the nanorod (see Additional Information). The presence of optical modes, which can be engineered by changing the diameter and height of the nanorod (see Additional Information Figs. 1 and 2), as well as two QW regions emitting from the blue to the red spectral region, potentially allows the design of true white LEDs without the need of light conversion using phosphors

Methods

Author Contributions

Additional Information