Optische und optoelektronische Eigenschaften von Nanostrukturen auf Basis breitlueckiger Halbleiter

Abstract

Nowadays, solid state light sources (SSLS) are omnipresent in applications such as optical storage technology as well as communication via optical fibre networks. In the green to UV spectral range, research on SSLS has focused to the group-III nitrides, as this material system has evolved as the most promising among the wide-bandgap semiconductors in the last decade. Recently, more and more research is done on nitride nanostructures in order to control the electrical and optical properties in a more sophisticated manner for optimized light-matter-interaction and extraction efficiency.In this context, the work presented in this thesis has been carried out, concentrating on two main topics. One aspect is the characterization of InGaN quantum dots (QDs). QDs possess a unique atom-like density of states for electrons, allowing for generation and manipulation of discrete electronic states.This thesis contains the analysis of QDs embedded in optoelectronic devices such as LEDs. Measurements of the electroluminescence (EL) of QD ensembles as well as single QDs are presented. Especially QD EL obtained at higher temperatures up to 150 K is a main achievement of this work. Furthermore, the photoluminescence (PL) of QD multilayer structures has been examined and discussed in detail. These multilayer structures are of particular interest for the incorporation as active material into laser structures. Therefore, experiments on the optical amplification in these multilayers have been carried out for the first time, yielding a maximum optical gain of g = 50/cm.Another main aspect of solid state lighting is the efficient light extraction from light sources. For this purpose, pillar microcavities based on nitrides have been investigated. This type of optical resonator represents the optical analogon of a QD. It possesses a discrete optical mode structure due to the three-dimensional optical confinement in these structures. For optimal light-matter coupling conditions, this leads to an enhanced extraction efficiency.In this context, studies on QD pillar microcavities (MCs) processed by focused ion beam milling from planar MC structures are presented. After a detailed analysis of the photonic properties of these pillar MCs, a temperature-variation method to tune the cavity in resonance with QD emission is demonstrated, yielding a five-fold enhancement of the extraction efficiency. These experiments were carried out on selenide-based structures which possess a very high structural quality. For nitride pillar MCs, growth is still very demanding. Their latest state of development is described, and the present achievements and challenges are presented and discussed.An alternative approach to achieve semiconductor waveguides is the growth of self assembled nanowires, as this kind of nanostructure possesses promising crystalline properties, and no processing is necessary to obtain pillar-like geometries. However, the integration of heterostructures, e.g. bragg mirrors or quantum wells, is still very demanding. An example of this bottom-up approach is presented and discussed. This part of work contains an analysis of the optical properties of nanowires. Furthermore, the influence of doping on these structures is investigated by electron microscopy and PL measurements. Also the integration of quantum well heterostructures is presented and examined.These results demonstrate the potential of semiconductor nanostructures for future applications in optoelectronic devices. If the remaining challenges can be solved, a wide range of new applications become possible by exploitation of the benefits from nitride nanostructures, such as electrically driven single photon emitters in the ultraviolet to green spectral region operating at room temperature.