Analysis of strain relaxation and emission spectrum of a free-standing GaN-based nanopillar

Abstract

We have made a GaN-based single nanopillar with a diameter of 300nm using the focused ion beam (FIB) technique. The micro-photoluminescence (µ-PL) from the embedded GaN/InGaN multi-quantum wells reveals a blue shift of 68.3 meV in energy. In order to explain the spectrum shift, we have developed a valence force field model to study the strain relaxation mechanism in a single GaN-based nanopillar structure. The strain distribution and strain induced polarization effect inside the multiple quantum wells is added to our self-consistent Poisson, drift-diffusion, and Schrodinger solver to study the spectrum shift of µ-PL.Keywords: GaN, InGaN, nanopillar, nanorod, strain relaxation, valence force field model INTRODUTIONThe III-nitrides have become very important materials for applications in optoelectronic 1-5 and high power electronics.6, 7 The III-nitrides are direct bandgap materials which can directly emit light easily. The bandgap of nitride alloys is from 6.2eV (AlN) to 0.7eV(InN) which covers from the UV light to the infrared. These properties make it promising for many applications. The III-nitrides have now been widely applied in the quantum well based light emitting diodes(LED) for UV and blue light sources. With the assistant of phosphors, it is possible to convert the UV or blue light into green and red light so that making a white light source for LED becomes feasible. However, it is difficult to directly make a green or a red quantum well LEDs with InGaN based materials. As we know, there is a large lattice mismatch ∼ 10% between InN and GaN. This large lattice mismatch leads to a strong piezoelectric polarization field inside the quantum wells for the normally used c-axis growth. As a result, the strong quantum confined Stark effect (QCSE) is observed in the quantum well structures. The crystal quality also worsens due to stronger strain as we push the emission to longer wavelengths by increasing the indium composition.The strain related issues are difficult to be avoided since it always appears in during the epitaxial growth of InGaN with an large area size. However, the development of nanotechnology has enabled the study of the optical properties of the nanostructures. GaN-based nanostructures, such as nanorods, nanocones, or nanopillars, have been fabricated via various methods.8-12 Several characterizations have been made on the properties of these structures. However, few studies have clarified the emission mechanism from one single nanopillar. In our recent work, 13 we have made a GaN-based single nanopillar with a diameter of 300nm using the focused ion beam (FIB) technique. The micro-photoluminescence (µ-PL) from the embedded GaN/InGaN multi-quantum wells (MQWs) reveals a blue shift of 68.3 meV in energy. The full width at half maximal (FWHM) is also broadened due to the distribution of strain relaxation in the nanopillar.In this paper, we have developed a valence force field model to to explain the µ-PL spectrum in detail and study the strain relaxation mechanism in a single GaN-based n...

The Magnetic Field Effects on Radially Symmetric Core-Shell-Shell Structure

“…A numerical Schrodinger-Poisson solver for radially symmetric nanowire core-shell structures was introduced by Lingquan Wang et al [9]. The strain distribution and strain induced polarization effect inside the multiple quantum wells was added to self-consistent Poisson, drift-diffusion, and Schrodinger solver to study the spectrum shift and spectrum broadening effect of micro-photoluminescence from the embedded GaN/InGaN multi-quantum wells [10].…”