Carrier dynamics of InxGa1−xN quantum disks embedded in GaN nanocolumns

Abstract

Time-integrated and time-resolved microphotoluminescence studies have been performed on Inx Ga1−xN quantum disks at the tips of GaN nanocolumns. The results are analyzed in the context of current theories regarding an inhomogeneous strain distribution in the disk which is theorized to generate lateral charge separation in the disks by strain induced band bending, an inhomogeneous polarization field distribution, and Fermi surface pinning. It is concluded that no lateral separation of carriers occurs in the quantum disks under investigation. Internal field screening by an increased carrier density in the QDisks at higher excitation densities is observed via a blue-shift of the emission and a dynamically changing decay time. Other possible explanations for these effects are discussed and discounted. Cathodoluminescence studies have also been carried out on the nanocolumns to provide insight into the physical origin of the luminescence.

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

“…It has been shown that a radial strain gradient exists across the InGaN layer from its center to edge with its outer shell largely relaxed. 14,24 As such and noting that increasing the accelerating voltage of the electron beam increases its penetration depth into the nanorods, the changes in the observed CL with electron beam position and accelerating voltage should reflect the strain variation across the SQD.…”

“…11 The formation of a nanorod structure, either by bottom-up epitaxial growth [2][3][4][5] or by dry etching a pre-existing epitaxial III-nitride layer, [6][7][8][9][10] has been proposed as a mean of alleviating strain due to the free surface presented by the sidewalls enabling the lattice to relax. [12][13][14] In the case of homogeneous nanorods formed by dry etching from c-plane homo-epitaxial GaN templates, the degree of strain relaxation in the overall structure will depend on the diameter and height of the nanorods, with complete strain relaxation occurring when their height exceeds their diameter irrespective of the type of substrate used. 15 When such etched nanorods contain an InGaN QW or a MQW oriented in the c-plane changes in the optical transition energy have been observed and are frequently ascribed to the impact of the strain relaxation in the nanorod on the piezoelectric contribution to the quantum-confined Stark effect (QCSE).…”

Carrier dynamics in site- and structure-controlled InGaN/GaN quantum dots

“…(2) and τ using Eq. (6). The understanding of the correlation is straightforward: In a QD with low E, the potential barrier height φ B is large.…”

“…Note that we do not consider the contribution of the surface Fermi-level pinning [6,28], because it bends the conduction and valance bands in the same way and, hence, has negligible effects on the exciton potential profile.…”

How much better are InGaN/GaN nanodisks than quantum wells—Oscillator strength enhancement and changes in optical properties

“…Unfortunately, a large, strain-induced electric field in III-N heterostructures often severely suppresses the oscillator strength of the exciton and, hence, the radiative decay rate and the internal quantum efficiency (IQE). Since strain is relaxed near free surfaces, nanodisks (NDs) in nanowires, which has a large surfaceto-volume ratio, have been widely considered as a promising solution for improving the IQE of InGaN/GaN photonic devices [4][5][6][7][8][9][10][11] . The accompanying improvement in the radiative decay rate is also important for realizing ultrafast singlephoton sources using InGaN/GaN QDs 3,12 .…”