N. T. Pelekanos

Growth kinetics and optical properties of self-organized GaN quantum dots

Self-organized GaN islands of nanometric scale were fabricated by controlling the Stranski–Krastanov growth mode of GaN deposited by molecular beam epitaxy on AlN. Evidence for ripening of dots under vacuum has been observed, resulting in changes in dot size distribution. We also show that in superlattice samples, consisting of multiple layers of GaN islands separated by AlN, the GaN islands are vertically correlated provided that the AlN layer thickness remains small enough. The luminescence peak of GaN dots is blueshifted with respect to bulk emission and its intensity does not vary with temperature, both effects demonstrating the strongly zero-dimensional character of these nanostructures.

Giant electric fields in unstrained GaN single quantum wells

We demonstrate that, even in unstrained GaN quantum wells with AlGaN barriers, there exist giant electric fields as high as 1.5 MV/cm. These fields, resulting from the interplay of the piezoelectric and spontaneous polarizations in the well and barrier layers due to Fermi level alignment, induce large redshifts of the photoluminescence energy position and dramatically increase the carrier lifetime as the quantum well thickness increases.

Self-assembled InGaN quantum dots grown by molecular-beam epitaxy

Self-assembled InGaN islands were grown by molecular-beam epitaxy on GaN, following a Stranski–Krastanow growth mode. Atomic force microscopy revealed that their dimensions were small enough to expect zero-dimensional quantum effects: the islands were typically 27 nm wide and 2.9 nm high. Strong blue-violet photoluminescence of the dots is observed, persisting up to room temperature. The temperature dependence of the photoluminescence is analyzed and compared to that of InGaN quantum well and bulk samples.

Improved quality GaN grown by molecular beam epitaxy using In as a surfactant

The surfactant effect of In during the growth of GaN by molecular beam epitaxy has been investigated. It has been found that the presence of In modifies the diffusion kinetics in the growing GaN surface, leading to the observation of persistent reflection high energy electron diffraction intensity oscillations, characteristic of layer-by-layer growth. Electron microscopy studies revealed drastic modifications of the GaN structural properties associated with the presence of In during the growth. When grown in the presence of In, GaN exhibits an intense band edge luminescence, free of the component at 3.41 eV which is characteristic of defects associated with growth in N-rich conditions.

A GaAs polariton light-emitting diode operating near room temperature

The increasing ability to control light–matter interactions at the nanometre scale has improved the performance of semiconductor lasers in the past decade. The ultimate optimization is realized in semiconductor microcavities, in which strong coupling between quantum-well excitons and cavity photons gives rise to hybrid half-light/half-matter polariton quasiparticles. The unique properties of polaritons—such as stimulated scattering, parametric amplification, lasing, condensation and superfluidity—are believed to provide the basis for a new generation of polariton emitters and semiconductor lasers. Until now, polariton lasing and nonlinearities have only been demonstrated in optical experiments, which have shown the potential to reduce lasing thresholds by two orders of magnitude compared to conventional semiconductor lasers. Here we report an experimental realization of an electrically pumped semiconductor polariton light-emitting device, which emits directly from polariton states at a temperature of 235 K. Polariton electroluminescence data reveal characteristic anticrossing between exciton and cavity modes, a clear signature of the strong coupling regime. These findings represent a substantial step towards the realization of ultra-efficient polaritonic devices with unprecedented characteristics.

Zinc-blende MnTe: epilayers and quantum well structures

Epilayers of the previously hypothetical zinc‐blende MnTe have been grown by molecular beam epitaxy. Epitaxial layers (0.5 μm thick) of MnTe were characterized using x‐ray diffraction and transmission electron microscopy; optical reflectance measurements indicate a band gap of ∼3.2 eV. A series of strained single quantum well structures was fabricated with zinc‐blende MnTe forming the barrier to CdTe quantum well regions; photoluminescence spectra indicate optical transitions corresponding to strong electron and hole confinement.

ROOM-TEMPERATURE EXCITON ABSORPTION IN (ZN,CD)SE/ZNSE QUANTUM WELLS AT BLUE-GREEN WAVELENGTHS

Excitonic absorption has been investigated in multiple quantum wells of (Zn,Cd)Se/ZnSe in an effort to increase the electron‐hole Coulomb interaction. Well‐defined absorption peaks for the n=1 heavy hole exciton are observed at room temperature showing evidence for a reduced exciton‐optical phonon scattering rate.

Self-assembled zinc blende GaN quantum dots grown by molecular-beam epitaxy

Zinc blende (ZB) GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy on AlN buffer layers using 3C-SiC(001) substrates. The two- to three-dimensional growth mode transition is studied by following the evolution of the reflection high-energy electron diffraction pattern. ZB GaN island layers are further examined by atomic force microscopy and transmission electron microscopy, extracting a mean island height of 1.6 nm and a mean diameter of 13 nm at a density of 1.3×1011 cm−2. Embedded ZB GaN quantum dots show strong ultraviolet photoluminescence without any thermal quenching up to room temperature.

High-reflectivity GaN/GaAlN Bragg mirrors at blue/green wavelengths grown by molecular beam epitaxy

Highly-reflective GaN/GaAlN quarter-wave Bragg mirrors, designed to be centered at blue/green wavelengths, have been grown by molecular beam epitaxy. The reflectivity for a mirror centered at 473 nm was as high as 93% and the bandwidth reached 22 nm. Detailed x-ray diffraction measurements allowed us to characterize the structural parameters of the Bragg mirrors. We show that, in spite of substantial strain relaxation occurring in our samples, high reflectivity is still possible. In addition, we show that growth interruption at the heterointerfaces is crucial for achieving high reflectivities.

Preferential nucleation of GaN quantum dots at the edge of AlN threading dislocations

We report on transmission electron microscopy and optical characterization of GaN dots embedded in an AlN matrix. GaN dots were grown by molecular beam epitaxy on top of an AlN layer deposited on (0001) sapphire. We found that the GaN dots (average diameter of 16 nm, average height of 4 nm) are coherently grown on AlN, but nucleate next to threading edge dislocations that were propagating in AlN. The presence of these adjacent dislocations does not inhibit the optical emission of the GaN dots: contrary to the photoluminescence of thick GaN layers or GaN/AlGaN quantum wells, the photoluminescence of these GaN dots is found to be temperature independent.