Optical Properties of GaN Nanorods Containing a Single or Multiple InGaN Quantum Wells

Abstract

Measurements of light emission from GaN nanorods of diameter between 80 and 350 nm, containing either a three-well multiple InGaN quantum well or a single quantum well, have been performed by photoluminescence (PL) and cathodoluminescence (CL) hyperspectral imaging. The PL underwent a Stark shift to the blue as the nanorod diameter was reduced, indicating substantial relaxation of the compressive strain in the quantum wells. The intensity of the nanorod emission per unit area can exceed that of the planar starting material. The CL measurements revealed that the wavelength of the quantum well emission varied with radial position in the nanorod. Simulations by a modal expansion method revealed that the light extraction efficiency varies with radial position and the variation is dependent on nanorod diameter. Finite difference time domain simulations showed that Bloch mode formation in the buffer layer below the nanorods impacts on the light extraction.

A Quasi-Analytic Modal Expansion Technique for Modeling Light Emission From Nanorod LEDs

“…The relative extent to which a dipole source of known strength and location excites the various modes is given by [15]. However, the locations and relative strengths of the dipoles in nanorod LEDs are not known, nor are they insignificant, as shown in [8]. One way to learn more about the location and relative strength of emission within nanorods is to compare the predictions of the modal expansion model with experimentally observed far-field patterns.…”

“…The space where z − < z < z + is occupied by the optical fibre, which consists of a cylindrical core of radius R and refractive index n core surrounded by an infinite cladding of refractive index n clad . This simplification neglects the refractive index steps that occur at any heterojunctions between the basic nanorod substructure and either an embedded disc-shaped multiple quantum well (MQW) emission region perpendicular to the z axis [7], [8] or a shelllike active layer grown over the cylindrical core [16], [17]. This is justified because these refractive index changes are typically quite small [18]- [20] compared with the step changes at z = z + and z = z − [21], [22] and therefore will marginally perturb the waveguiding properties.…”

“…Advantages of such structures include increased internal quantum efficiency due to strain relaxation at the nanorod walls [2], [8], low defect densities [2], [5] and increased light extraction efficiency [8]. The top-down controlled etching approach used by [5]- [7] has the additional advantages of increasing extraction efficiency further via photonic crystal diffraction effects [7], [9], [10].…”

“…In addition, the amount of power coupled into these two classes of modes may be influenced by the location of the dipole within the active region of the nano-LED and the orientation of the dipole with respect to the nanorod geometry [8]. The cylindrical shape of nanorods can be used as the basis for such a model by treating a nanorod as an optical fibre of finite length, in which light is reflected and diffracted at the nanorod ends.…”

Influence of stress on optical transitions in GaN nanorods containing a single InGaN/GaN quantum disk

“…13,16 One theoretical study has predicted not only the widely observed line shifts associated with the QCSE, but also multiple transitions associated with electrons and holes being localized in different positions across the radius of the nanorod. 13 Recently, evidence has emerged to support these predictions, 17 but a comprehensive measurement-based study is still required to identify the combined influences of the effects of axial and radial strain on light emission from c-plane oriented QWs, notably contributions to transition energies from the bulk deformation of the InGaN layer(s) and the QCSE.…”