Toward reliable photoconductive atomic force microscopy measurements

Abstract

Photoconductive atomic force microscopy (PC-AFM) allows one to study the influence of illumination on the conductive properties of different samples at the nanoscale. However, for such measurements to be considered reliable, one has to be sure that illumination does not impact the tip–sample contact force. The change of the contact force may be caused by the influence of radiation on the atomic force microscope's position sensitive detector (PSD) as well as the probe and sample. There are certain approaches to reduce this illumination influence; however, they cannot always be implemented. Therefore, it is important to perform a detailed study of the influence of illumination on the PSD, the probe, and the sample during measurements in the PC-AFM mode. In this paper, this was done by studying the mechanism by which elevation is formed in topographic images of the cleaved GaAs substrate surface under lateral continuous wave (CW) laser illumination, with λ = 1064 and 404 nm. It was demonstrated that the illumination results, mainly, in the thermal expansion of the tip and the sample, as well as the generation of a thermal stress in the cantilever. The authors consider the cases in which these effects can increase the tip–sample contact force during PC-AFM measurements and suggest simple approaches by which the increase in force can be minimized. Also, the authors developed the model of thermal expansion of the GaAs sample fixed in the holder under CW laser illumination. Based on this model, the approach was suggested for estimation of the thermal contact conductance value for different solid–solid interfaces.