Pulsed optical excitation in combination with scanning tunnel microscopy resolves charge dynamics at the gallium arsenide surface.

We present a combination of pulsed optical excitation and scanning tunneling microscopy with a highly flexible pulse generation method. A high frequency arbitrary wave generator drives a gigahertz electro-optical modulator, which processes a continuous-wave laser beam of a low-noise laser diode into the desired wave shape. For pump-probe excitation we generate optical pulse series in an all-electronic way. Thereby we can easily adapt parameters like pulse amplitude, width, or repetition cycle to the demands of the experiment. This setup is used to study different dynamic processes at the GaAs(110) surface. Separating thermally induced effects from electrically induced effects allows us to quantify the thermal contribution of the optical excitation in STM experiments. Time-resolved decay spectra of the photo-generated electron-hole pairs demonstrate the nanoscale spatial resolution.

The miniaturization of future electronic devices is intimately connected to the ability to control electric fields on the atomic scale. In a nanoscopic system defined by a limited number of charges, the combined dynamics of bound and free charges become important. Here we present a model system based on the electrostatic interaction between a metallic tip of a scanning tunnelling microscope and a GaAs(110) semiconductor surface. The system is driven out of equilibrium by optical excitation, which provides ambipolar free charge carriers, and by an optically induced unipolar tunnel current. This combination enables the active control of the density and spatial distribution of free and bound charge in the space-charge region, that is, modifying the screening processes. Temporal fluctuations of single dopants are modified, meaning we are able to control the noise of the system. It is found that free charge carriers suppress the noise level in field-controlled, nanoscopic systems.

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In this thesis, the successful implementation of optical excitation for time-resolved Scanning Tunneling Microscopy (STM) is presented. The fruitful combination of these two experimental methods allows investigating photo-induced dynamic processes on the nanosecond time scale with atomic resolution. The optical setup provides a great versatility regarding the adjustment of excitation parameters such as optical pulse height, pulse width or pulse repetition rate to the experimental needs. Moreover, for the first time, it is possible to disentangle and quantify thermally induced effects, e.g. thermal expansion of the STM tip, from the originally inquired signals, resulting from photo-triggered charge dynamics at the sample surface.Using continuous wave optical illumination at the Gallium-Arsenide(110) (GaAs) surface, it is proven in this thesis, that the presence of excited holes, accumulating in the tip-induced Space Charge Region (SCR) beneath the STM tip not only creates a Surface Photovoltage (SPV) but also opens an additional tunneling channel. Current dependent studies show that this extra tunneling process exhibits a high impact on the charge concentration at the surface. Assuming a steady state between charge generation via optical excitation and charge annihilation via the tunneling process, parameters such as the diffusional or field driven transport rate towards the surface are determined.The build-up and relaxation of an SPV is connected to various microscopic processes, e.g., charge transport or carrier recombination. With pulsed optical excitation, these mechanisms can be probed and disentangled. Whereas for low tunnel rates the decay of the system after optical excitation is mainly determined by the charge annihilation via the tunneling process, for high rates the recharging of dopants becomes visible. Studies of this ionization process for donors, positioned at different depths beneath the surface, reveal a significant local inhomogeneity. By applying a field driven ionization mechanism, it turns out, that the dopants cannot be treated independently from each other. Instead, the ionization dynamics have to be treated as an interacting network of coupled, locally fixed and randomly distributed charge centers.