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

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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