DEVELOPMENT OF SCANNING TUNNELING MICROSCOPE CAPABLE OF CHEMICAL ANALYSIS

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In order to develop a scanning tunneling microscopic (STM) system that enables chemical analysis of light elements in the imaged regions, the following issues have been addressed.(1)Quantitative considerations on a newly devised photoluminescent X-ray excitation electron energy spectroscopy (PXEEES) revealed that element analysis based on the threshold energy of electrons for X-ray generation is possible if a sufficient amount of field emission current is obtained from very sharp STM tips.(2)To fabricate reproducibly sharp tips necessary for the PXEEES measurements, a novel method of tip fabrication, the ultra-fast current shut-off method, has been developed. The fabricated tips proved to be stable even at high bias voltages up to 500 V.(3)Preliminary experiments using graphite samples revealed that irradiations of electrons in energies as low as 100eV can cause evaporation of ionic carbons, which led us to conclude that the PXEEES measurements are not possible in some materials vulnerable to such low energy electron radiation damage.(4)As an alternative method, spatially-resolved photoabsorption spectroscopic measurements on a nanometer scale were found to be possible by using STM. Tests have been done successfully on GaAs samples.(5)Electric-field modulation spectroscopy coupled with STM revealed to be capable of measuring band structures of semiconductors (low-temperature grown GaAs used as test samples) on nanometer scales.(6)We have proposed a novel scheme of infrared absorption spectroscopy that utilizes infrared light generated by difference frequency generation due to the non-linear current-voltage relation and the tip-enhanced electromagnetic field in the STM tunneling junctions.