Simultaneous Optical and Electrical Characterization at Nanoscale with Scanning Probe Microscope
Hiroo Omi, Ilya Sychugov, and Yoshihiro Kobayashi
Materials Science Laboratory
Optical and electrical properties of nanostructures can be addressed using electromagnetic radiation or electrical current as a probe. In general, a near-field type of electromagnetic interaction is necessary for an optical probe to enter nanoscale regime in spatial resolution by overcoming the optical diffraction limit (〜1 µm). A typical aperture scanning near-field optical microscope (aperture-SNOM) provides such an opportunity both for the excitation and collection of light. However, this instrument employs a dielectric fiber tip as an aperture, which makes it unsuitable for electrical measurements. An apertureless SNOM, on the contrary, has a metallic, needle-like opaque tip. The local field enhancement at the tip apex due to the plasmon effect enhances signal in the light scattering experiments. But, unlike the case of aperture-SNOM, the far-field optics alignment is necessary and the scattered light from the tip shaft can distort the measurements. In addition, the aperture-SNOM can operate in the combination with a scanning tunnelling microscope (STM) and it was demonstrated that such a combination can yield superior resolution for SNOM imaging compared to the conventional shear-force approaching method. On the other hand, the STM, which is capable of atomic-resolution measurements using electrical current, can also cause an optical response in materials. The collection of light in ordinary STM machines with opaque metal tips is realized by far-field optics. When the carriers are tunnelled to the sample it is the excitation (tunnelling) area that limits the spatial resolution; the achieved resolution of such an instrument was reported to be as small as 〜10 nm. The collection efficiency of the emitted light can be improved by employing a transparent STM tip, where the light is coupled directly to the tip rather than to a far-field detector. However, only the electrical probing of nanostructures was proven feasible in such a scanning tunnelling microscope luminescence (STML) experiment. We expand the STML approach to show an ultimate STM-based configuration with the aperture metal tip (Fig. 1), where both optical and electrical excitations are available (Fig. 2). This method enabled us to characterize the same area in near-field of the sample through the aperture of transparent tip covered with metals .
 I. Sychugov, H. Omi, T. Murashita, and Y. Kobayashi, Nanotechnology 20 (2009) 145706.
Fig. 1. Aperture metal tip.
Fig. 2. Electro- and photoluminescence from the
GaAs wafer on a nanoscale.
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