Mechanisms of inhomogeneous broadening in InGaN dot-in-wire structures

Abstract

Inhomogeneous broadening of semiconductor nanostructures results from structural and chemical variations between different nanostructure entities. Inhomogeneous broadening can have profound impacts on the optical properties of a nanostructure array. In this work, various inhomogeneous broadening mechanisms in wurtzite InGaN/GaN dot-in-wire (DIW) structures were investigated, both experimentally and theoretically. Using lithographically defined nanostructures, the microscopic variations including random alloy fluctuations and atomic-scale thickness fluctuations can be isolated from macroscopic variations such as size, shape, and alloy nonuniformity. An epitaxial InGaN/GaN quantum well sample was patterned into an array of sparsely spaced dot-in-wire structures and measured by confocal microphotoluminescence (PL) at 10 K. Both static (photon energy) and dynamic (carrier lifetime) properties were measured. The PL measurement results were compared to a theoretical model based on the k-dot-p method under the effective mass approximation and including the excitonic effect and surface recombinations. Random alloy fluctuations, atomic-scale thickness fluctuations, and size variations of the quantum dots were separately analyzed. It was found that both the diameter variation and random alloys dominate the inhomogeneous broadening of photon energies, while the random alloys dominate the inhomogeneous broadening of decay rates. The piezoelectric field in InGaN materials plays a minor role in increasing the effect of random alloys but helps suppress the inhomogeneous broadening due to well-width fluctuations by keeping the electrons toward the center of the dots.