Development of picosecond AFM and its application to the surface of semiconductors

本研究计画的目的是发展时间分辨扫描探针显微镜（SPM），并将其应用于半导体表面。为了实现皮秒级的时间分辨测量，需要将泵浦-探测技术与SPM相结合。本计画利用波长选择性瞬态反射光栅方法，研究矽中载子的快速热化过程。硅中光激发载流子的动力学过程包括载流子-载流子散射、载流子-晶格散射、复合、扩散和缺陷能级陷阱。通过分析瞬态反射光栅响应的波长依赖性，发现几皮秒的弛豫过程是由Si表面介电常数的变化引起的，而200皮秒的弛豫过程是由光激发载流子引起的热过程引起的。为了将泵浦-探测方法与扫描探针显微镜（SPM）相结合，选择了带光纤探针的扫描近场光学显微镜（SNOM）。在SNOM组态上实现了光热效应的测量。抽运激光的引入是利用光纤探头发出的倏逝光。然后，光的吸收引起折射率的变化，或热透镜，这是通过探测激光偏转检测。本文采用光纤探针发射的532 nm泵浦激光激发染料分子，并利用光纤探针发射的633 nm探测激光探测染料分子诱导的热透镜。这是第一次在SNOM结构中利用倏逝波激发探测热透镜。我们计划发展皮秒时间分辨的超近场光学显微镜，通过将脉冲激光引入光纤探针来探测光热现象，并将其应用于半导体表面。

The aim of this research project is to develop the time-resolved scanning probe microscope (SPM) and to apply it to the surface of semiconductors. To achieve the picosecond time-resolved measurement, the combination of the pump-probe technique and SPM is required. In this project, fast thermalization process of carrier in Si has been investigated using a wavelength-selective transient reflecting grating method. Dynamics of photo-excited carriers in Si involves carrier-carrier scattering, carrier-lattice scattering, recombination, diffusion, and trap into defect energy level. By analyzing wavelengh dependency in the transient reflecting grating response, it has been found that the relaxation process in several picosecond is attributed to change in dielectric constant at the Si surface and that one in 200 picosecond to thermally process induced by photo-excited carriers. This information is very useful to investigate fast phenomena of photo-excited carriers at the Si surface.To combine the pump-probe methods with SPM, scanning near-field optical microscope (SNOM) with the fiber probe was chosen. We have realized the measurement of photothermal effect in SNOM configuration. The introduction of pump laser to the sample was used the evanescent light from the fiber probe. And then the absorption of light induced the change of refractive index, or the thermal lens, which was detected by deflection of probe laser. In this work, the dye molecules were excited by 532nm pump laser emitted from the fiber probe, and the thermal lens induced were detected by the 633nm probe laser transmitted from the fiber one. This is the first time to detect the thermal lens by evanescent excitation in SNOM configuration. We have a plan to develop the picosecond time-resolved SNOM to detect the photothremal phenomena by introducing the pulse laser to the fiber probe and to apply it to the surface in semiconductors.