Nearly all semiconductor devices are made of hetero structures, stacked layers of different materials deposited with different techniques. Thicknesses of these layers are in the range of micrometers down to nearly single atomic thickness. For the correct operation of an electronic device, the electronic configuration at layer interfaces is essential. Even though the electronic properties of such interfaces can be simulated, many physical effects caused by the deposition process and layer interactions influence the interface in a certain and often non predictable way.
Here, we present a powerful way of performing an in situ analysis of hetero structure interfaces using an organic solar cell structure as an example. The method and the work presented here were developed and performed by Saive et al. . Interfaces have been electrically characterized by Scanning Kelvin Probe Force Microscopy (SKPM). Like in a conventional kelvin probing system, the SKPM method provides information about the work function (chemical potential) difference between a sensing tip and the sample surface. For SKPM, an atomic force microscope (AFM) tip is used as a kelvin probe sensor, which allows to measure work function variations with extremely high lateral resolution, in some cases down to atomic level. The work function is determined by the intrinsic potential of a material, the electronic configuration like carrier density and type, and an eventually applied electrical potential. Performing an SKPM measurement on a cross section of a hetero structure, results in absolute measurements of work function differences between the different layers. This also enables the direct measurement of the voltage drop between the different layers if an external voltage is applied across the hetero structure.
The surface of an organic semiconductor is very sensitive to air exposure, so these semiconductors have to be encapsulated in air tight containers. To perform high resolution electrical characterization, exposed surfaces have to be protected from air contamination. The combination of the Carl Zeiss AURIGA crossbeam work station and the Carl Zeiss AFM option for MERLIN allows preparation and investigation of the solar cell without the need to expose the prepared structure to air.
The precise navigation of the AFM tip directly to the region of interest is ensured by high capability zoom of the SEM. In this way, it is possible to access information about topography, mechanical, electrical and magnetic properties of the surface with nanometer resolution.
1. Rebecca Saive, Michael Scherer, Christian Mueller, Dominik Daume, Janusz Schinke, Michael Kroeger, Wolfgang Kowalsky, Imaging the electric potential within organic solar cells, Advanced Functional Materials Volume 23, Issue 47, December 2013, Pages 5854-5860