DEVELOPMENT OF SCANNING TUNNELING MICROSCOPE CAPABLE OF CHEMICAL ANALYSIS

为了开发一种扫描隧道显微镜（STM）系统，能够对成像区域的轻元素进行化学分析，解决了以下问题。(1)对新设计的光致发光x射线激发电子能谱（PXEEES）的定量考虑表明，如果从非常尖锐的STM尖端获得足够的场发射电流，则基于电子产生x射线的阈值能量的元素分析是可能的。(2)为了制造PXEEES测量所需的可重复尖锐尖端，开发了一种新型尖端制造方法——超快电流切断法。事实证明，即使在高达500 V的高偏置电压下，制造的尖端也是稳定的。(3)石墨样品的初步实验表明，能量低至100eV的电子辐照可以引起离子碳的蒸发，这使我们得出结论，对于一些易受这种低能电子辐射损伤的材料，PXEEES测量是不可能的。(4)作为一种替代方法，利用STM可以在纳米尺度上进行空间分辨光吸收光谱测量。对砷化镓样品进行了成功的测试。(5)电场调制光谱与STM耦合可以在纳米尺度上测量半导体（以低温生长的砷化镓作为测试样品）的能带结构。(6)我们提出了一种新的红外吸收光谱方案，该方案利用了STM隧道结中非线性电流电压关系和尖端增强电磁场产生的差频产生的红外光。

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.