We use a near-field scanning optical microscope (NSOM) to perform local optical beam induced current measurements on strain-relaxed GeSi films on Si substrates. Topographic and near-field optical beam induced current (NOBIC) images are acquired simulataneously, enabling us to correlate topographic features and electrical activities of near surface defects. We demonstrate spatial resolution of approximately 100 nm in the NOBIC images. The resolution in NOBIC imaging of defects is examined using a two-dimensional carrier diffusion model with finite-size generation. We find that carrier diffusion lenght does not limit the resolution as previously thought. The ultrahigh resolution achieved in NSOM is due to reduction of the excitation volume and of the carrier lifetime near defects. Recent experiments using linearly polarized NSOM light reveal field variations associated with threading dislocations as well as with the cross-hatch patterns.
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