Anas F. Jarjour

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.

Cavity-enhanced blue single-photon emission from a single InGaN∕GaN quantum dot

The authors report on the generation of single photons in the blue spectral region from a single InGaN∕GaN quantum dot. The collection efficiency was enhanced by embedding the quantum dot layer in the middle of a low-Q microcavity. The microphotoluminescence is observed to be approximately ten times stronger than typical InGaN quantum dot emission without a cavity. The measurements were performed using nonlinear excitation spectroscopy in order to suppress the background emission from the underlying wetting layer.

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.

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.

Electrically driven single InGaN/GaN quantum dot emission

Electroluminescence from single nitride-based quantum dots is reported. Clear single quantum dot emission is observed, which persists up to ∼85 K. This is achieved through the study of a quantum dot layer in the intrinsic region of a forward-biased vertical p-i-n diode. The current-voltage characteristic of the devices is examined at 4.3 K and observed to exhibit electrical bistability phenomena, which is explained in terms of charge accumulation in the InGaN layer. The dependence of the emission properties on current injection conditions are presented and related to the electrical properties of the device.

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.

Two-photon autocorrelation measurements on a single InGaN/GaN quantum dot

We report on the use of interferometric autocorrelation measurements to investigate the non-linear absorption processes evident in single InGaN/GaN quantum dots. The near quadratic excitation intensity dependence of the photoluminescence signal in conjunction with the asymmetric collinear autocorrelation trace unambiguously confirms the process as being one involving two photons via an intermediate virtual state. These results highlight the inherently non-linear optical properties of these structures.

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

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.

Progress in the optical studies of single InGaN/GaN quantum dots

The great success that GaN-based structures have enjoyed in implementing efficient optoelectronic devices has fostered a rapidly expanding interest amongst researchers aimed at understanding their underlying physics. There has been an active debate on the mechanisms that give rise to efficient luminescence in these materials. In this paper we approach these questions through optical studies of single InGaN/GaN quantum dots in the context of the available experimental and theoretical understanding of InGaN structures in general, and of three-dimensional localization in this material in particular. We will also show how it is possible to exploit the various unique properties that nitride-based materials offer, such as the strong inbuilt electric field, in a controlled manner. Such control may in the future prove essential for the implementation of single quantum dot devices in applications such as quantum information processing. We also show how nonlinear spectroscopy provides an invaluable tool in suppressing background luminescence effects inherent in this material.

Materials challenges for devices based on single, self-assembled InGaN quantum dots

Builiding on earlier studies of single InGaN quantum dots (QDs), we are considering their potential for use in blue- and green-emitting single photon sources. Envisaging a device based on a resonant cavity light emitting diode, we have studied the effect of growing QDs on an underlying AlN/GaN distributed Bragg reflector, and have shown that enhanced single QD emission may be obtained. Additionally, we have studied the effect of the growth and activation of a p-type cap on an underlying QD layer and have shown that the QDs survive the anneal process.