M. Seyfried

Composition mapping in InGaN by scanning transmission electron microscopy

We suggest a method for chemical mapping that is based on scanning transmission electron microscopy (STEM) imaging with a high-angle annular dark field (HAADF) detector. The analysis method uses a comparison of intensity normalized with respect to the incident electron beam with intensity calculated employing the frozen lattice approximation. This procedure is validated with an In(0.07)Ga(0.93)N layer with homogeneous In concentration, where the STEM results were compared with energy filtered imaging, strain state analysis and energy dispersive X-ray analysis. Good agreement was obtained, if the frozen lattice simulations took into account static atomic displacements, caused by the different covalent radii of In and Ga atoms. Using a sample with higher In concentration and series of 32 images taken within 42 min scan time, we did not find any indication for formation of In rich regions due to electron beam irradiation, which is reported in literature to occur for the parallel illumination mode. Image simulation of an In(0.15)Ga(0.85)N layer that was elastically relaxed with empirical Stillinger-Weber potentials did not reveal significant impact of lattice plane bending on STEM images as well as on the evaluated In concentration profiles for specimen thicknesses of 5, 15 and 50 nm. Image simulation of an abrupt interface between GaN and In(0.15)Ga(0.85)N for specimen thicknesses up to 200 nm showed that artificial blurring of interfaces is significantly smaller than expected from a simple geometrical model that is based on the beam convergence only. As an application of the method, we give evidence for the existence of In rich regions in an InGaN layer which shows signatures of quantum dot emission in microphotoluminescence spectroscopy experiments.

Strong coupling in monolithic microcavities with ZnSe quantum wells

Strong light-matter coupling is demonstrated in monolithic microcavities with only three ZnSe quantum wells embedded. Basis for this observation is the excellent structural quality of the sample as confirmed by scanning transmission electron microscopy measurements. A comparative large energy splitting between the upper and lower polariton (LP) of about 19 meV is observed by reflectivity measurements in real and k-space. Efficient polariton relaxation is shown by photoluminescence measurements at low temperatures in k-space. These beneficial properties of the sample result in a nonlinear increase of the lower polariton population in excitation density dependent measurements before the photonic cavity emission becomes dominant.

Optical properties of photonic molecules and elliptical pillars made of ZnSe-based microcavities

The influence of the geometric shape of optically confining structures on the emission properties of ZnSe-based microcavities is studied. Elliptical as well as coupled circular structures were fabricated with quantum wells or quantum dots as optical active material. For the elliptical pillars a lifting of the polarization degeneracy of the resonator modes is observed as it is favorable to control the polarization state of the emitted photons. The influence of the ellipticity on the polarization splitting of the fundamental mode as well as on the quality factor of the sample is discussed. For the coupled pillar microcavities the effect of their distance on the energy splitting of the fundamental resonator mode is analyzed. Furthermore, detailed measurements of the spatial mode distribution in elliptically shaped pillars and photonic molecules are performed. By comparing these results to the calculated mode distribution their analogy to a diatomic molecule is illustrated. It turns out that the observed mode splitting into localized bonding and delocalized antibonding states in ZnSe-based microcavities is more pronounced for elliptical geometries. The realization of delocalized mode profiles is favorable for the coupling of spatially separated quantum dots.

Optical properties of InGaN quantum dots in monolithic pillar microcavities

The integration of InGaN quantum dots into GaN-based monolithic microcavities grown by metal-organic vapor-phase epitaxy is demonstrated. Microphotoluminescence spectra reveal distinct spectrally sharp emission lines around 2.73 eV, which can be attributed to the emission of single InGaN quantum dots. The samples are structured into airpost pillar microcavities. The longitudinal and transversal mode spectra of these cavities are in good agreement with theoretical calculations based on a vectorial transfer-matrix method. Quality factors up to Q=280 have been achieved.

Blue monolithic II-VI-based vertical-cavity surface-emitting laser

We report on laser operation of optically pumped, fully epitaxial blue vertical-cavity surface-emitting lasers. The ZnSe-based structures posses a bottom and top distributed Bragg reflector consisting of ZnMgSSe as high-index material and a short-period superlattice of MgS/ZnCdSe as low-index material. The cavity has an optical thickness of λ and contains three ZnSe quantum wells surrounded by ZnMgSSe barriers. To illustrate the specific issues related to this demanding material combination, we compare two epitaxial structures of different crystalline quality. A minimum threshold of 5 pJ is observed for laser emission at 443 nm comparable to values reported for nitride based vertical-cavity surface-emitting lasers.

A (S)TEM and atom probe tomography study of InGaN

In this work we show how the indium concentration in high indium content InxGa1-xN quantum wells, as they are commonly used in blue and green light emitting diodes, can be deduced from high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. This method bases on introducing normalized intensities which can be compared with multislice simulations to determine the specimen thickness or the indium concentration. The evaluated concentrations are compared with atom probe tomography measurements. It is also demonstrated how the quality of focused ion beam prepared TEM- lamellas can be improved by an additional etching with low energy ions.

Polarized light emission from CdSe/ZnSSe quantum-dot monolithic pillar microcavities

Small-size II-VI micropillars with asymmetrical cross sections are presented as a way to achieve more than 95% of the emitted light from single quantum dots having one particular linear polarization state. We show that the detected PL intensity of the QD is increased by optimizing the spectral overlap between QD emission and resonator mode. The polarization of the emitted light is defined by the polarization of the resonator mode. Moreover, the internal mode structure in photonic molecules is investigated by studying their far field pattern. The observed field distribution opens the possibility of coupling individual quantum dots to each other via the mediation of the electromagnetic field.

Optical Properties of InGaN Quantum Dots in Monolithic Pillar Microcavities

InGaN quantum dots were successfully implemented into fully epitaxially grown nitride‐based monolithic microcavities. Measured discrete modes of airpost pillar microcavities prepared out of the planar sample are compared to theoretical simulations based on a vectorial‐transfer matrix method. Quality factors of up to 280 have been achieved and the emission of a single quantum dot was traced up to a temperature of 125 K.

Manipulating the optical properties of CdSe/ZnSSe quantum dot based monolithic pillar microcavities

A customization of the optical properties of pillar microcavities on the desired applications is essential for their future use as quantum-optical devices. Therefore, all-epitaxial cavities with CdSe quantum dot embedded in pillar structures with different geometries have been realized by focused-ion-beam etching. The quality factors of circularly shaped pillar microcavities have been measured and their dependence on the excitation power is discussed. As a possibility to achieve polarized light emission, asymmetrically shaped microcavities are presented. Examples of an elliptically shaped pillar as well as of photonic molecules are investigated with respect to their photoluminescence characteristics and polarization.