Characterization of nanostructured surfaces

The optics section develops optical instrumentation and methods suitable for solar-cell applications.

In particular, experiments are underway to study light scattering from various wafer surface preparations using Mueller matrix scattering ellipsometry, while various near infra-red polarimetric imaging systems are currently adapted to study strain in multicrystalline silicon wafers. We are also developing a sensitive optical method (photo-reflectance spectroscopy) to be applied in order to study intermediate band gap solar cells and alike, produced within the solar cell group of applied physics.

Spectroscopic Ellipsometry (SE) is a particularly surface sensitive technique that can be used both in-situ and ex-situ in order to study many of the "thin film" stages in the process of producing high quality solar cells. SE may also successfully be applied to study the formation of nanopillars, nanorods or nanocones, which can be used as light harvesting surfaces (both antireflective and absorbing depending on the material). SE can be quantitatively used when appropriate optical models are applied to analyze the measurements. We have particularly focused on studying the self-organized formation of densely packed GaSb cones forming upon low ion energy sputtering, see Figure 1. By studying this material with real time in-situ ellipsometry and appropriate in-house developed optical models, we have been able to understand the formation mechanism, see Figure 2. The optical results have been thoroughly verified by using alternative techniques such as Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), see Figure 1, and Transmission Electron Microscopy (TEM). Our results have been used alongside with computer simulation models for the formation process, from collaborating institutes, and have resulted in a new theory and understanding of the formation of GaSb nanocones and alike.

Fig. 2. Height evolution, as determined from in-situ spectroscopic ellipsometry, of GaSb nanocones forming during sputtering at normal incidence. Ref. Nerbø et al. APL 94, 213105 (2009).Illustrasjonsbilde/FOTO

Contact: Morten Kildemo

Fig. 1. Example of cross-section SEM image of GaSb nanocones formed upon sputtering at normal incidence (top figure), and at 45 degrees incidence (bottom figure).