Reserch activities

Analysis of quantum nano structure

~Photo-absorption current measurements on single quantum nanostructures by dual light illumination method in STM~

Local characterization of optical properties in nanostructures is very important but is difficult by conventional methods for optical measurements because of the resolution limit. Since SPM can overcome such a limit, we have performed photo-absorption measurements on InAs wires fabricated on GaAs (110) vicinal substrates, by STM under light illumination. For a purpose of accurate characterization of photo-absorption properties of the InAs wires at the surface by suppressing the influences of a depleted region and a consequent built-in field in the underlying GaAs layer, we propose a dual light illumination method in STM, in which both continuous and modulated lights are used to illuminate a sample surface. As a result, a change in STM current induced by the latter light, we called it a photo-induced current (PIC) signal, was successfully acquired on the InAs wires, from which the photo-absorption property in the single InAs wire will be discussed. In addition, we have found that the PIC signals acquired on the InAs wire with a width of 25 nm clearly depend on the polarization of the incident light, indicating a fact that this wire structure has an anisotropic photo-absorption property originating from the structural anisotropy. We have also found that such an anisotropy in photo-absorption is weakened in the wire with a width of 50 nm and that a step-like shape appears in the PIC signal spectrum which is given from the dependence of PIC signal on the photon energy of the incident light. Therefore we consider that this wider wire behaves as a quasi-quantum well rather than a quantum wire.

1-1) K. Takada, et al., Jpn. J. Appl. Phys., 41, 4990 (2002).

1-2) H. Masuda, et al., Ultramicroscopy, 105, 137 (2005).

1-3) S. Katsui, et al., Jpn. J. Appl. Phys., 48, 08JB03 (2009).

1-4) S. Katsui, et al., Jpn. J. Appl. Phys. (to be published).

Fig.1:(a):Surface topography and (b):photo-induced current (PIC) images of InAs on GaAs substrate obtaind by dual light illumination method in STM.

~ Improvements of Kelvin probe force microscopy (KFM) for precise surface potential measurements~

We are aiming to achieve accurate potential measurements by KFM. Through both numerical simulations and experiments we have pointed out that the electrostatic force variation during tapping operation of KFM cantilever should be carefully considered because the electrostatic force which is used to determine the potential is a long range force. In order to suppress the influence of the electrostatic force variation on the potential determination, we have proposed and demonstrated an intermittent bias application method, and the results indicate that this method is very effective to improve the quality of potential images obtained by KFM and that the estimated spatial resolution is better than 10 nm. In addition, sampling extraction of cantilever bending has been introduced to achieve very sensitive detection of the electrostatic force. Owing it, amplitude of a.c. modulation bias can be reduced around 10 mVp-p, which contributes to reduce a disturbance of potential measurements by the external bias application.

1-1) K. Takada, et al., Jpn. J. Appl. Phys., 41, 4990 (2002).
1-2) H. Masuda, et al., Ultramicroscopy, 105, 137 (2005).
1-3) S. Katsui, et al., Jpn. J. Appl. Phys., 48, 08JB03 (2009).
1-4) S. Katsui, et al., Jpn. J. Appl. Phys. (to be published).

Fig.2:(a)Surface topography, (b) potential and (c) error signal images observed by KFM with intermittent bias application method.

Current probe through magnetic field detection by magnetic force microscopy (MFM)

~Verification of current-induced magnetic field observation method in MFM~

Current-induced magnetic fields around artificial current networks have been observed by MFM to realize a nondestructive ammeter with high spatial resolution. We applied an a.c. bias to the current path to generate an ac current and detected the torsional displacement of the MFM cantilever synchronized with a frequency of the applied bias. Here we tuned the ac bias frequency to a torsional resonant frequency of the cantilever in order to enhance a response to the magnetic field, and adjusted a d.c. offset bias to compensate the intrinsic potential difference between the MFM cantilever and the current path in order to suppress an undesired influence of the electrostatic force acting between them. Owing to those improvements, the MFM observation of the magnetic field, induced by the current in the order of µA, has been achieved with good sensitivity and linearity. We have also confirmed that the spatial resolution of this method is better than 200 nm. Those results clearly indicate ability in current evaluation by our MFM method.

~Characterization of individual channels in a carbon nanotube field-effect transistor (CNT-FET) studied by MFM~

Individual channel properties of CNT-FETs were investigated through the current-induced magnetic field observation by MFM. We first modified the shape of a MFM cantilever to enhance its response to magnetic force and then observed the MFM signals around individual CNT channels. As a result, we found differences in the threshold gate bias and transconductance among different CNT channels and in the asymmetric conductance of a single CNT channel [Jounal_9]. Moreover, by comparison with the potential distribution along the CNT channel observed by Kelvin probe force microscopy (KFM), usefulness of the phase component in the MFM signal to distinguish the conductive channel from the whole channels is clarified. Actually, we succeeded in identifying a group of the CNT channels which act as a dominant current route in the CNT networks.

2-1) D. Saida, et al., Jpn. J. Appl. Phys., 44, 8625 (2005).
2-2) D. Saida, et al., IEEE Trans. Magn., 44, 1779 (2008).
2-3) M. Ato, et al., J. Appl. Phys., 106, 114315 (2009).

Fig.3:(a) FIB processed MFM cantilever for detecting current induced magnetic field, (b) Surface topography of CNT cahnel,(c)Current induced magnetic field signal image around CNT chanel,(d) Behavioral analysis of individual channels.

Characterization of solar cell materials by SPM

~Photovoltaic measurements by photoassisted Kelvin probe force microscopy (P-KFM) on multicrystalline silicon solar cells~

 It is very important to analyze photovoltage distribution around a grain boundary in a multicrystalline silicon solar cell material because the grain boundary is considered to degrade solar cell performances. Up to now, however, it has not been well investigated mainly due to a lack of spatial resolution in photovoltaic measurement methods. We have developed the method for surface photovoltaic measurements using KFM in the presence of light, which we refer to as ”photo-assisted KFM” (P-KFM) and observed the photovoltage distribution around the grain boundary. As a result, an abrupt change of photovoltage near the grain boundary and variation in photovoltage between different grains were observed, indicating that those photovoltage distribution degrades of the overall performance of solar cells. In addition, diffusion length and lifetime of minority carrier were evaluated from the dependence of photovoltage on incident light wavelength and from temporally averaged photovoltage as a function of modulation frequency, respectively, and the results indicate that the diffusion length and lifetime of minority carrier were apparently shortened around the grain boundary. Through those measurements, usefulness of P-KFM for solar cell characterization has been clarified.

3-1) T. Igarashi, et al., Jpn. J. Appl. Phys., 45, 2128 (2006).
3-2) M. Takihara, et al., Jpn. J. Appl. Phys., 46, 5548 (2007).
3-3) M. Takihara, et al., Appl. Phys. Lett., 93, 021902 (2008).
3-4) M. Takihara, et al., Appl. Phys. Lett., 95, 191908 (2009).

Fig.4:(a) Surface topography, (b) potential (nonirradiated) and (c) photo-induced voltage (890 nm laser).

Developing new methods of SPM

~Sampling method in AFM for fast scanning~

 We have proposed a novel imaging method to realize fast scanning in AFM by using a sample-and-hold (S/H) circuit. In this method, AFM is operated in the intermittent contact mode and the S/H circuit clips the deflection sensor signal at exact moments when the tip taps on the sample surface. When we choose relatively low feedback gain, fine topographic information is included in the clipped signal rather than in the piezo bias, and thus a quasi-topographic image can be formed from a direct trace of the clipped signals. In this case, precise feedback control of the tip height becomes unnecessary, and therefore very fast scanning is possible. Consequently, we have succeeded in observing AFM images of good quality in 10 x 10 µm2 area with 256 x 256 pixels at a line scanning rate up to 32 Hz. The result evidently indicates high performance of this sampling method.

4-1) T. Takahashi, et al., Jpn. J. Appl. Phys., 43, L582 (2004).
4-2) T. Takahashi, et al., Ultramicroscopy, 105, 42 (2005).

Figure:Images of quantum dots by high speed AFM.
Scan area:(a) 500 nm square,(b) 2 μm square.

Movie:Comparing normarl mode(left)and high speed mode(right)

~Photothermal spectroscopic measurements by AFM and its application to solar cell characterization~

A photothermal (PT) effect is an energy transfer phenomenon from photon to heat, and it allows us to investigate a photoabsorption property and a non-radiative recombination process across a bandgap and/or via some discrete levels, such as impurity levels. If AFM is applied to PT measurements, a very precise analysis will be realized. Therefore we have proposed a dual sampling method in AFM for the PT measurements, and confirmed the feasibility of this method through the excitation photon energy dependence of the PT signal measured on Si and GaAs. We also applied this method to the analyses on the multicrystalline Si solar cells and found that the PT signal was apparently enhanced near the grain boundary, which is attributable to fast non-radiative recombination at the boundary. On the other hand, we found that the non-radiative recombination does not occur so frequently in the CIGS solar cells. This result is well attributable to the spatial separation effect of electrons and holes owing to the band diagrams deduced from the P-KFM and STS measurements as described above.

3-5) K. Hara, et al., Jpn. J. Appl. Phys., 48, 08JB22 (2009).
3-6) K. Hara, et al., Proc. of IEEE PVSC35, 001387 (2010).