Characterization of insulator surfaces after ultra-precision machining by STM/STS with high-frequency pulses

本研究的目的是开发一种高频脉冲扫描隧道显微镜(STM)/扫描隧道光谱（STS）系统，以观察绝缘体表面的原子分布或电子结构。此外，对超精密加工后的绝缘子表面进行了评价，并对加工条件进行了优化。为了用STM观察绝缘子表面，我们必须施加足够高的偏置电压，使电子隧穿到绝缘子表面的导带中，并且电压必须以交流短脉冲（< 1 msec）的形式施加，以避免通电和绝缘子表面的破坏。此外，我们必须开发反馈电路，其源不是由杂散电容产生的位移电流，而是两者之间的隧道电流。一个探针和一个样本。隧道电流谱对于阐明绝缘体的缺陷状态也是必要的。我们开发了一种RC电路，可以在不受矩形短电压脉冲产生的位移电流阻碍的情况下检测隧道电流。构建了高频脉冲驱动的STM系统原型，在该系统中，高频矩形电压脉冲（频率大于10khz，幅值可达±10V）可作用于隧道结。隧道电流检测电路的输出经二极管整流后，由反馈回路控制信号。我们已经成功地利用所构建的体系获得了高取向热解石墨（HOPG）表面的原子图像。然而，原子分辨的图像尚未获得，无论是本征硅表面或硅片覆盖了天然氧化物。为了观察这些表面的原子图像，电流放大器的信噪比必须在高频（> 100khz）区域得到改善。同时，我们要实现反馈系统更高的截止频率。我们设计了一种新的高频STM反馈控制系统。新系统配备了一个在高频区域工作的电流放大器和一个锁相放大器，以便在施加矩形脉冲的特定频率上放大隧道电流的成分。该系统不需要二极管整流电路，提高了信噪比。我们已经确认新反馈控制系统的截止频率为1.0 kHz。预计现在可以在原子尺度上解决绝缘体表面的问题。少

The purpose of this study is to develop a scanning tunneling microscopy (STM)/ scanning tunneling spectroscopy (STS) system operated by high-frequency pulses in order to observe either atomic distributions or electronic structures of insulator surfaces. Furthermore, we aim at evaluating insulator surfaces after ultra-precision machining processes, and optimizing the condition of the machining process. In order to observe insulator surfaces with STM, we have to apply bias voltages high enough for electrons to tunnel into the conduction band of the insulator surface, and the voltage has to be applied as alternating short pulses (< 1 msec) to avoid the electrification and the destruction of the insulator surface. In addition, we have to develop the feedback circuit of which the source is not a displacement current originated from a stray capacitance but tunneling current between. a probe and a sample. Tunneling current spectroscopy is also necessary to elucidate defect states of insulator … More surfaces.We have developed the RC circuit in which tunneling current can be detected without being hindered by displacement current induced by a rectangular short voltage pulse. The prototype of STM system operated by high-frequency pulses has been constructed in which high-frequency rectangular voltage pulses (frequency : more than 10 kHz, amplitude: up to ±10V) can be applied to the tunneling junction. After the output of the detecting circuit of tunneling current is rectified by a diode, the signal has been controlled by a feedback loop. We have succeeded in obtaining atomic images of highly oriented pyrolytic graphite (HOPG) surfaces by the constructed system. However, atomically resolved images have not been obtained yet of either an intrinsic Si surface or a Si wafer covered with native oxides. In order to observe atomic images of these surfaces, the ratio of signal to noise of a current amplifier must be improved at a high-frequency (>100 kHz) region. At the same time, we have to achieve the higher cut-off frequency of the feedback system. We have designed a new feedback control system for high-frequency STM. The new system equips a current amplifier operated at the high-frequency region, and a lock-in amplifier in order to amplify the component of tunneling current at the specific frequency of applied rectangular pulses. This system does not need a rectifying circuit by a diode, which enhances the ratio of signal to noise. We have confirmed that the cut-off frequency of the new feedback control system is 1.0 kHz. It is expected that insulator surfaces can now be resolved in an atomic scale. Less