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

本研究项目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。为了实现皮秒时间分辨测量，需要将泵浦-探测技术与扫描探针显微镜相结合。在本项目中，利用波长选择的瞬时反射光栅法研究了硅中载流子的快速热化过程。光激载流子在硅中的动力学包括载流子-载流子散射、载流子-晶格散射、复合、扩散和陷阱进入缺陷能级。通过分析瞬时反射光栅响应的波长依赖性，发现几皮秒内的驰豫过程归因于硅表面介电常数的变化，而200皮秒内的驰豫过程归因于光激发载流子引起的热过程。这些信息对研究硅表面光激载流子的快速现象非常有用。为了将泵浦-探测方法与扫描隧道显微镜相结合，选择了带有光纤探针的扫描近场光学显微镜(SNOM)。实现了SNOM结构中光热效应的测量。将泵浦激光引入到样品中，利用光纤探头发出的逝去光。然后光的吸收引起折射率或热透镜的变化，这是通过探测激光的偏转来检测的。在本工作中，染料分子被光纤探头发射的532 nm泵浦激光激发，633 nm探头激光从光纤探头发射的633 nm探头激光探测到诱导的热透镜。这是首次在SNOM结构中通过瞬变激发探测到热透镜。我们计划将脉冲激光引入光纤探头，研制皮秒时间分辨近场光学显微镜来探测光致发光现象，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.