Scanning Electroluminescence Microscopy on Nanostructures

During the last decade, nanostructured materials became a topic of large interest due to their favorable mechanical, electrical and optical properties. As basic feature, optically active nanostructures are normally embedded into a host material, as it is for example the case for InAs self assembled quantum dots in GaAs and GaAs/AlGaAs heterostructures. The corresponding matrix material always has an essential influence on the electrical and optical behavior of the embedded nanostructures and thus, we propose in this project to study the photoemission properties of embedded nanostructures by scanning electroluminescence microscopy with focus on the possible usability of the nanostructured materials for photon emitters in the near infrared regime. To achieve our aim, a scanning probe microscope (SPM) shall be employed as a nanopositioning system to address the lateral position of a single nanostructure inside a sample. Once a single nanostructure has been addressed, the electrical and photoemission properties of the nanostructure can be investigated by locally injecting a current into the sample by means of a conductive SPM tip. As starting point, efficient ways for detecting single InAs self assembled quantum dots buried in epitaxial GaAs/AlAs layers shall be investigated. Later, the electronic and optoelectronic properties of single InAs quantum dots will be determined by electroluminescence spectroscopy and imaging in the near infrared regime. Local electroluminescence experiments shall be performed to explore the usability of embedded nanostructures for near infrared photon emitters. In addition, an already operative setup for local, AFM based photocurrent spectroscopy can be applied to get complementary information on the electronic level structure and the carrier life time. As the suggested method for local electroluminescence are not restricted to InAs quantum dot samples only, our local photoemission studies shall later on be extended to other nanostructured materials such as Si nanowires or wet chemically grown nanocrystals. Again, the focus will be the possible usability of the nanostructured materials for photon emitters in the near infrared regime and the influence of the surrounding host material on the emission properties.

The goal of this project was to develop methods for local electroluminescence measurements on nanostructures in the visible and near infrared. Using a conductive tip of an Atomic Force Microscope (AFM), an electrical current can be locally injected into a semiconductor sample with nm resolution. Any emitted electroluminescence radiation should be detected using a confocal microscope setup connected to two spectrometers for the respective wavelength regimes. As the current injection area is very small due to the small dimensions of the AFM tip anyway, the confocal microscope approach looked very promising and automatically compatible with the current injection by the AFM. The experimental setup which was based on a "Witec 300" confocal scanning Raman microscope offered two beam paths for a topside and backside detection of any light emitted from the sample. The sensitivity of the experimental setup was very high and allowed single photon detection in principle. Because an optical cryostat could not be integrated into the "Witec"-setup, all measurements were designed for room temperature operation only. For this project, the system was tested successfully by recording the tree-dimensional emission pattern of a commercial semiconductor laser in the Fresnel regime with impressive spatial resolution. For the electroluminescence experiments, three different sample systems were investigated: InAs quantum dots embedded in a GaAs matrix, PbS colloid nanocrystals, and towards the end of the project, organic light emitting diodes. All samples were carefully analyzed using local photocurrent spectroscopy by conductive AFM and also advanced photocurrent simulations were carried out using a two-dimensional Poisson solver in order to study tip geometry effects. Although the photocurrent spectroscopy data looked promising on all samples, we were facing severe problems with electroluminescence on all investigated samples. On InAs dots, the quantum efficiency was obviously too low at room temperature, and the colloid nanocrystals degraded too quickly. Only the organic light emitting diodes had sufficiently high emission intensities in electroluminescence. As these samples became only available at the end of the project, however, it was no more possible to demonstrate electroluminescence by AFM on these samples in time.