J. W. Robinson

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).

Time-resolved dynamics in single InGaN quantum dots

We present measurements of photoluminescence decay dynamics for single InGaN quantum dots. The recombination is shown to be characterized by a single exponential decay, in contrast to the nonexponential recombination dynamics seen in the two-dimensional wetting layer. The lifetimes of single dots in the temperature range 4 to 60 K decrease with increasing temperature.

Temporal variation in photoluminescence from single InGaN quantum dots

We report measurements of optical transitions in single III/V (InGaN) quantum dots as a function of time. Temporal fluctuations in microphotoluminescence peak position and linewidth are demonstrated and attributed to spectral diffusion processes. The origin of this temporal variation is ascribed to randomly generated local electric fields inducing a Stark shift in the optical emission peaks of the InGaN quantum dots.

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.

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.

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.

Photoluminescence studies of exciton recombination and dephasing in single InGaN quantum dots

This paper reports on time-integrated and time-resolved microphotoluminescence (/spl mu/-PL) measurements of single InGaN quantum dots (QDs). The linewidths of the /spl mu/-PL peaks originating from single metal-organic vapor phase epitaxy-grown III/V InGaN QDs are measured, implying dephasing times of at least 5 ps. Temporal fluctuations of the QD emission energy are observed, and these are explained in terms of randomly generated local electric fields inducing a Stark shift in the optical emission of the InGaN QDs. Time-resolved measurements demonstrate that decay dynamics from single InGaN QDs are exponential in nature. Measurements of the effect of temperature upon the recombination times in individual InGaN QDs have been performed from 4 to 60 K.

Modeling the nonlinear photoluminescence intensity dependence observed in asymmetric GaN quantum discs with AlGaN barriers

By means of a 3D self-consistent numerical simulation we have calculated the wavefunctions and energies of the states in an asymmetric GaN quantum disc (Q-disc) system. Overall, good agreement between the modeling and experimental results were observed. Furthermore, modeling has provided an insight into the carrier dynamics of the Q-disc system. In particular, results from the modeling supports the view that the nonlinear relationship between PL intensity and excitation power was due to electron tunneling between the asymmetric GaN Q-discs.

Simulation of the quantum-confined Stark effect in a single InGaN quantum dot

By means of a 3D self-consistent numerical simulation we have calculated the effect of an externally-applied lateral electric field upon a single InGaN quantum dot. Overall, good agreement between the modeling and experimental results was observed. Modeling results support the observation that the quantum-confined Stark effect has both permanent dipole moment and polarizability components.