Optoelectronic measurements are usually performed by the measurement of I/V curves with an external light focused on a sample under investigation. The light with the energy exceeding the electronic band gap of the sample, excites electrons from valence to conductive band resulting in an increased conductivity. When there is an externally applied electric field (bias voltage) along the sample surface, the photo-generated electron-hole pairs are transferred to opposite sides and finally injected into external electrical contacts. In this way, the photo-generated charge carriers establish a photocurrent. Usually, photocurrents are measured macroscopically, between two electrical contacts patterned on the sample surface. Still, in order to fully elucidate optoelectronic properties of nanostructures, it is highly desirable to correlate their local morphology and photocurrents. For that purpose, atomic force microscopy (AFM) is a natural choice since the well established conductive AFM (C-AFM) technique enables simultaneous measurements of the sample morphology in contact mode and electrical currents. In order to extend capabilities of standard C-AFM towards measurements of photocurrents, it is necessary to couple external light sources with AFM and to focus the external light onto the tip-sample contact area. Here we describe the upgrade of the standard C-AFM setup of our NTEGRA system to photoconductive AFM (PC-AFM) by coupling external lasers [1, 2] to the optical components of our NTEGRA system.

Optical components of the initial AFM setup are depicted in Fig. 1(b). The optical path is defined by three mirrors (mirror1-3). In order to provide photo-assisted AFM measurements, on the input part displayed in Fig. 1(c), we put the laser with a manual beam shutter. At the output part depicted in Fig. 1(a), in order to efficiently focus the input laser, we added an objective with a large working distance. Since the objective has to be removable, the platform with the objective can be both translated and rotated. This property is very important since it enables easy handling of the AFM system. When it is necessary to remove AFM head, place a new sample, and/or exchange AFM probes, the stage with objective should be just rotated in the backward direction.