Response analysis for identifying the origin of photo-modulated current contrasts in scanning tunneling microscopic imaging semiconductor surfaces

Fourier transform photoabsorption spectroscopy based on scanning tunneling microscopy

“…For spectroscopic measurements in the conventional LI scheme, 13 we used a wavelength-swept light from a grating monochromator ͑Horiba Jobin Yvon, Triax 320͒ equipped with an incandescent halogen lamp ͑150 W͒ as the white light source. The light chopped at a frequency was guided with an optical fiber and focused in a diameter of 1 mm onto the gap between the STM tip and sample at an angle of 55°w ith respect to the tip axis.…”

“…Most efforts in the succeeding works using the SPV as the signal arising from photoabsorption 9-12 were devoted to microscopic mapping of the magnitude of SPV in high spatial resolution, not to spectroscopic measurements of SPV signal as a function of illuminating photon energy. The present authors showed that STM-based spectroscopic measurements of photoabsorption can be conducted, 2,13,14 in various schemes including somewhat a sophisticated one, by detecting a change in the STMtip current due to the SPV induced when a spectroscopic light is illuminated on the tunnel junction.…”

“…In our previous studies, the photoinduced changes in the STM-tip current were detected with a lock-in ͑LI͒ amplifier synchronized with the light chopping. 2,6,13 This conventional technique for high S/N measurements, however, suffers from two difficult problems common in STM-based experiments. The first problem is with the spurious spectra that are often brought about by temporal instabilities and/or the positional drift of the STM tip that changes the tunneling current not with the wavelength but with time, although we need to sweep the wavelength of spectroscopic light with time in the LI detection.…”

“…The second problem arises when the intensity of spectroscopic light is modulated for LI detection: the intensity modulation causes a periodic photothermal expansion of the tip material, which leads to an undesirable excess component in the photomodulated tip current. 13 Several experimental ideas for avoiding the unwanted photothermal expansion have been proposed so far. Grafström et al showed that the photothermal expansion can be suppressed by superposing two lights chopped alternately in opposite phase, one a spectroscopic light and the other a light with a fixed wavelength close to the former.…”

The origin of spectral distortion in electric field modulation spectroscopy based on scanning tunneling microscopy

“…4. As reported previously [8], when a modulated voltage (regardless of the external bias modulation or a modulation of an internal origin such as modulated SPV induced by light chopping) is applied between a semiconducting sample and an STM-tip, the current flowing STM-tips can be consisted of two components, the true tunneling current and the displacement current through a parasitic capacitance between the sample and the tip. Since the tunneling current is absent out of the tunneling regime, no effect of tip-sample distance on the spectra (Fig.…”