Kelvin Probe Force Microscopy on Semiconductors

Featuring Dr. Sascha Sadewasser
Division of Solar Energy, Hahn-Meitner Institut
Berlin, Germany

Hosted by Regina Ragan
Assistant Professor, Chemical Engineering and Materials Science

Location:  1114 Natural Sciences I
Open and free to the public

Abstract: 
To improve understanding of semiconductor materials and device functionality, detailed studies are required to provide information on materials characteristics on the nanometer scale. Kelvin probe force microscopy (KPFM) is a technique based on non-contact atomic force microscopy (NC-AFM) and is well suited for this task, as it measures the surface potential with high spatial resolution. We apply a resonance enhanced mode of operation that allows compensating electrostatic forces using detection ac-voltages below 100 mV; thus, the influence of the KPFM tip on the semiconductor is minimized [1].

This talk will first introduce the KPFM method. It will then be demonstrated that for correct height imaging, the KPFM method is superior to regular NC-AFM. Depending on the sample bias, even a contrast inversion can be observed in regular NC-AFM topography imaging [2]. Using KPFM, a variation in the surface potential at surface steps of III-V semiconductors is observed. In conjunction with simulations, the density of charged defects was quantitatively determined, corresponding to about 2% of the dangling bonds being charged [3]. For the application to chalcopyrite semiconductors (i.e. Cu(In, Ga)Se2) in thin film solar cells, it is shown that differently oriented facets of single semiconductor grains show a distinct work function [4]. The investigation of grain boundaries in these polycrystalline thin films shows a local band bending due to the presence of charges [5]. However, for an epitaxially grown Σ3 grain boundary, a charge neutral barrier to majority transport could be identified for CuGaSe2 [6].

 

[1] Ch. Sommerhalter et al., Appl. Phys. Lett. 75, 286 (1999).
[2] S. Sadewasser et al., Phys. Rev. Lett. 91, 266101 (2003).
[3] Y. Rosenwaks et al., Phys. Rev. B 70, 085320 (2004).
[4] S. Sadewasser et al., Appl. Phys. Lett. 80, 2979 (2002).
[5] D. Fuertes Marrón et al., Phys. Rev. B 71, 033306 (2005).
[6] S. Siebentritt et al., Phys. Rev. Lett. 97, 146601 (2006).