Book Description

"Atomic force microscopy (AFM) was developed in the mid 1980's to measure the topography of a sample with atomic resolution. Since the first reported atomic resolution images, AFM has constantly been developed further to gain more insights into structure and property at the nanometer scale. Its great advantage is the capability to spatially resolve the tip-sample interaction at a sub-nanometer scale. Extensive research and development was conducted over the past two decades to not only measure the structure of a sample but also to extract information about the local properties. Kelvin Probe Force Microscopy is an example of such a technique, enabling the measurement of the local contact potential between the AFM tip and the sample. In this thesis, AFM is used to spatially resolve the surface potential generated upon illumination of a sample with light.A new technique to accurately measure the change of the contact potential difference under pulsed illumination was developed and implemented. This new measurement technique was needed since we reached the limit of currently available methods. These did not allow the measurement of the surface photovoltage as a function of illumination wavelength or time. This new method allows a much more accurate determination of surface potential differences. Resolving the surface photovoltage on a nanometer length scale with AFM can be of great interest in particular if one can additionally gain information about the temporal response of the sample. To address this, we developed a method to study the decay of the surface photovoltage by non-contact AFM, which is only limited by the underlying physics process. The approach used to achieve fast time resolution measurements is discussed in a general context. We demonstrate that the well known fundamental sensitivity limits of force detection also govern the achievable time resolution. The time resolved methods developed in this thesis can be adapted to measure time resolved ion diffusion, thermal response and electronic pulse propagation. As a proof for the novel measurement method, the ultra-fast decay time of the photocarriers in low temperature grown GaAs of about 1 ps was measured by AFM. These experiments were implemented by combining a traditional optical pump-probe modulated excitation with localized readout by AFM, The spatial resolution is therefore given by the AFM setup and not the optical excitation." --