Corridor of EEIC, The University of Tokyo

Nanometer-scale world investigated by nano-probes – Looking at invisible things -: Takuji Takahashi

Multi-directional Characterization of Solar Cell Materials

For solar cells, both high performance and low cost are required. Although polycrystalline or microcrystalline materials are superior in terms of substrate fabrication cost, photoexcited carriers are considered to recombine at the grain boundaries (interfaces between different crystal grains) that those crystals inevitably contain. Therefore, it is important to clarify the behavior of grain boundaries through microscopic observation. For such purposes, the nano-characterization technology using nano-probes that we are developing is very helpful.
In the Kelvin probe force microscopy (KFM) that can measure local surface potential of a sample, local photovoltaic measurement is realized when we operate KFM under light illumination. By applying this method to solar cells based on a multicrystalline silicon or a compound semiconductor like Cu(In,Ga)Se₂ [CIGS], we are investigating various physical properties of them, such as photovoltaic property as well as lifetime, diffusion length, and mobility of minority carrier, from multi-directional viewpoints. In particular, it has been confirmed that the CIGS-based materials exhibit high potential as solar cells although they consist of microcrystals with a diameter of about micrometer, and therefore their material properties such as the behavior of grain boundaries should be carefully investigated. Based on deep understanding of the local properties around the grain boundaries through local characterization using nanoprobes, we aim at improving of solar cell performance.

If, on the other hand, the photoexcited carriers (electrons and holes) in the solar cell recombine non-radiatively, electrical energy cannot be extracted and consequently the solar cell performance should be degraded. Since such non-radiative recombination releases heat, we have developed a photothermal mode AFM (PT-AFM), in which minute amount of thermal expansion at a sample surface induced by light illumination is measured by an atomic force microscopy (AFM). Using PT-AFM, we are conducting experimental research to clarify the relationship between grain boundaries and non-radiative recombination property of photoexcited carriers in the solar cell materials like CIGS.

Development of Novel SPM Methods

Since AFM with a conductive tip enables us to measure the electrostatic force between a tip and sample induced by an external electric bias voltage, the depletion phenomenon near the surface and the surface potential distribution can be examined through the local measurements of the electrostatic force. In particular, we are developing a new method that enables detailed examination of the depletion phenomenon by applying multiple AC voltages having different frequencies and by extracting a high-order frequency component in the electrostatic force.

In addition, the nano-probes have achieved extremely high spatial resolution, but they usually have the disadvantages that the operating speed is slow and the observation throughput is low because the height of the tip must be constantly controlled using feedback system. In order to realize high-speed image acquisition by overcoming these drawbacks, we proposed a new data acquisition mode, in which the displacement amount of the probe is sequentially captured by a sample-and-hold circuit. Using this mode, the scanning speed about 30 times faster than in the normal mode was achieved. Thus we are continuing our efforts to make the SPMs more convenient.

Characterization of Carbon Nanotube FETs

Field effect transistors (FETs) using carbon nanotubes (CNTs) as channels are expected to exhibit high performance because a CNT should act as a pure one-dimensional conductor ideally. Although high speed operation is expected in multi-channel CNT-FETs, one of the current issues is to figure out how the characteristics for each CNT channel are uniform. We have developed a method for quantitatively evaluating current through observation of a current-induced magnetic field with magnetic force microscopy (MFM), which acts as a magnetic field sensor with high spatial resolution. By applying this method to observe the　CNT-FET, we succeeded in analyzing the FET operation on a single CNT channel among multiple CNT channels.

Physics in Quantum Nanostructures

By using the light illumination STM method that we have originally developed, optical properties such as light absorption characteristics in a single InAs wire structure have been investigated.
In addition, regarding the electrostatic force detection by AFM as well as the surface potential measurement by KFM using the electrostatic force, we have proposed the sampling method that improves the sensitivity to the electrostatic force and the intermittent bias application method that improves the spatial resolution. Consequently, KFM with very high measurement accuracy has been achieved. Using those methods, furthermore, we examined the charge accumulation effect in a single InAs quantum dot.