J. H. Na

Quantum dot emission from site-controlled InGaN/GaN micropyramid arrays

InxGa1−xN quantum dots have been fabricated by the selective growth of GaN micropyramid arrays topped with InGaN∕GaN quantum wells. The spatially, spectrally, and time-resolved emission properties of these structures were measured using cathodoluminescence hyperspectral imaging and low-temperature microphotoluminescence spectroscopy. The presence of InGaN quantum dots was confirmed directly by the observation of sharp peaks in the emission spectrum at the pyramid apices. These luminescence peaks exhibit decay lifetimes of approximately 0.5ns, with linewidths down to 650μeV (limited by the spectrometer resolution).

Quantum-confined Stark effect in a single InGaN quantum dot under a lateral electric field

The effect of an externally applied lateral electric field upon an exciton confined in a single InGaN quantum dot is studied using microphotoluminescence spectroscopy. The quantum-confined Stark effect causes a shift in the exciton energy of more than 5 meV, accompanied by a reduction in the exciton oscillator strength. The shift has both linear and quadratic terms as a function of the applied field.

Registration of single quantum dots using cryogenic laser photolithography

We have registered the position of single InGaAs quantum dots using a cryogenic laser photolithography technique. This is an important advance towards the reproducible fabrication of solid-state cavity quantum electrodynamic devices, a key requirement for commercial exploitation of quantum information processing. The quantum dot positions were registered with an estimated accuracy of 50 nm by fabricating metal alignment markers around them. Photoluminescence spectra from quantum dots before and after marker fabrication were identical except for a small redshift (~1 nm), probably introduced during the reactive ion etching.

Biexciton and exciton dynamics in single InGaN quantum dots

Time-resolved and time-integrated microphotoluminescence spectrometry of exciton and biexciton transitions in a single self-assembled InGaN quantum dot gives sharp peaks, with the biexciton 41 meV higher in energy. Theoretical modelling in the Hartree approximation (using a self-consistent finite difference method) predicts a splitting of up to 51 meV. Time-resolved microphotoluminescence measurements yield a radiative recombination lifetime of 1.0 ± 0.1 ns for the exciton and 1.4 ± 0.1 ns for the biexciton. The data can be fitted to a coupled DE rate equation model, confirming that the exciton state is refilled as biexcitons undergo radiative decay.

Time-resolved and time-integrated photoluminescence studies of coupled asymmetric GaN quantum discs embedded in AlGaN barriers

We have investigated exciton dynamics in asymmetric GaN quantum discs embedded in AlGaN barriers with an Al content of 50% using time-integrated and time-resolved micro-photoluminescence measurements. Emission from the quantum discs emerges at lower energy than that from the GaN nanocolumns, which suggests that GaN quantum discs are strongly affected by the built-in electric field. The lifetimes of localized excitons in quantum discs were obtained. Nonlinear emission from quantum discs under high excitation power was attributed to tunneling of carriers to larger discs from smaller discs.

Photoluminescence properties of a single GaN nanorod with GaN/AlGaN multilayer quantum disks

Photoluminescence (PL) properties of a single nanorod containing multiple GaN quantum disks separated by AlGaN potential barriers are investigated using micro-PL spectroscopy. Previous studies reported ensemble spectra from many nanorods. The PL spectra show different features depending on the region of the nanorod excited by the laser, including a sharp feature originating from the quantum disk region. The distinct differences between the PL from the different regions are discussed. The results imply that excitons are strongly confined in the quantum disks, and the authors suggest that small quantum disks can be regarded as quantum dots having a discrete density of states.Photoluminescence (PL) properties of a single nanorod containing multiple GaN quantum disks separated by AlGaN potential barriers are investigated using micro-PL spectroscopy. Previous studies reported ensemble spectra from many nanorods. The PL spectra show different features depending on the region of the nanorod excited by the laser, including a sharp feature originating from the quantum disk region. The distinct differences between the PL from the different regions are discussed. The results imply that excitons are strongly confined in the quantum disks, and the authors suggest that small quantum disks can be regarded as quantum dots having a discrete density of states.

Enhancement of free-carrier screening due to tunneling in coupled asymmetric GaN/AlGaN quantum discs

We present an investigation of free-carrier screening in coupled asymmetric GaN quantum discs with embedded AlGaN barriers using time-integrated and time-resolved microphotoluminescence measurements, supported by three-dimensional multiband k∙p computational modeling. We observe that with increasing optical excitation the carrier lifetime decreases and emission energy blueshifts. This originates from the screening of built-in piezo- and pyroelectric fields in the quantum discs by photogenerated free carriers. Due to nonresonant tunneling of carriers from the smaller disk to the larger disk, free-carrier screening is enhanced in the larger disk. Computational modeling was in good agreement with the experimental results.

Two-dimensional exciton behavior in GaN nanocolumns grown by molecular-beam epitaxy

We have investigated the behavior of excitons in GaN nanocolumns using time-integrated and time-resolved micro-photoluminescence measurements. In the weak confinement limit, the model of fractional-dimensional space gives an intermediate dimensionality of 2.14 for GaN nanocolumns, with an average diameter of 80 nm. Enhanced exciton and donor binding energies are deduced from a fractional-dimensional model and a phenomenological description. Time-integrated photoluminescence spectra as a function of temperature show a curved emission shift. Recombination dynamics are deduced from the temperature dependence of the PL efficiency and decay times.

Characterization of tunneling and free-carrier screening in coupled asymmetric GaN/AlGaN quantum discs

We present a systematic investigation of free‐carrier screening in coupled asymmetric GaN quantum discs with embedded AlGaN barriers using time‐integrated and time‐resolved micro‐photoluminescence measurements, supported by three‐dimensional multi‐band k⋅p computational modeling. Free‐carrier screening effects decreased with the barrier thickness, which indicates that the source of the free‐carriers were carriers tunneling through the barrier.

Two-photon excitation of asymmetric GaN/AlGaN quantum discs

By using two‐photon absorption spectroscopy we have performed time‐integrated and time‐resolved photoluminescence measurements on several coupled asymmetric GaN quantum discs, with embedded AlGaN barriers. We observe free‐carrier screening, with an enhancement due to tunneling between the coupled quantum discs.