Photoconductive terahertz microprobes for high-resolution contact-free imaging of large-scale sheet conductivity distributions

Recently, the demand for non-destructive contact-free measurement tools which are able to measure conductivity distributions at spatial resolutions down to a few micrometres has strongly increased. This demand is caused by an increased application of micron-scale doping areas in solar cells (e.g. selective emitter structures fabricated by laser-doping [1]) and further semiconductor applications. Although, scanning-type (AFM-based) microscope solutions have been reported even with resolutions down to 100 nm [2, 3] a main drawback of this methods still is the limitation to small scan areas and (optically) flat samples. Several optical solutions (VIS or IR) like the recently presented confocal microphotoluminescence may offer micron-scale resolution and applicability to large-scale samples but they also suffer from strong scattering artefacts on rough samples and cross-sensitivity to carrier-lifetime. This is leading to data misinterpretations on samples with inhomogeneous carrier-lifetime distributions like for example present on laser-processed (locally melted and re-crystallized) surfaces. In this work a recently introduced approach based on the photoconductive (PC) near-field detection of terahertz (THz) light transmission [4] is demonstrated for the first time for sheet conductivity imaging on rough laser-doped multicrystalline silicon samples featuring doping microstructures.