Near-field photocurrent studies of temperature and polarization dependence in relaxed silicon germanium films on silicon substrates

Near-field scanning optical microscopy (NSOM) is used to examine the local photoexcitation of carriers within fully relaxed SiGe films on Si substrates. Two NSOM techniques were applied to this problem and their results are presented. The still developing field of NSOM allows for the subdiffraction-limited resolution imaging of sample features through the use of a subwavelength aperture. Optical resolution in our experiments is ∼100nm. We describe a novel variable (cryogenic) temperature near-field scanning optical microscope (VT-NSOM) that was designed specifically for submicron imaging of materials and devices over a temperature range of 12K to 300K. Our design features a large scan area (35μm x 35μm) at low temperatures and rapid temperature changes. The VT-NSOM is capable of imaging a single submicron feature over the entire temperature range. The near-field photocurrent (NPC) of individual threading dislocations (low room-temperature contrast and a negative temperature coefficient) in relaxed SiGe films indicates that the electrical activity of these defects comes from shallow centers. In another experiment we use NSOM tips as a source of linearly polarized light. We acquire topographic and NPC images with orthogonal linear polarizations. Correlation of these images show that both threading dislocations and surface crosshatch have significant polarization dependent image contrast. Particularly striking is the change in NPC observed in the crosshatch for orthogonal <110> polarizations. Through comparison with images generated from a numerical calculation based on a realistic model, we show that the contrast is due to a directionally dependent change in the indirect bandgap due to local strain.