Yutaka Mera

Time‐resolved D‐band luminescence in strain‐relieved SiGe/Si

Temporal decay characteristics of dislocation‐related luminescence bands (D1–D4) were explored in strain‐relieved epitaxial SiGe/Si(100). Close similarity of the decay profiles was observed not only between D1 and D2 bands but also between D3 and D4 bands. The decay transients of the D1 and D2 bands at low temperatures are characterized by long decay times, τ≳200 ns, whereas the D3 and D4 bands exhibit even sharper transients with τ<60 ns. Temperature dependence of ‘‘radiative’’ lifetimes implies a free‐to‐bound nature of the D1 and D2 bands, while a bound‐to‐bound character of the luminescence origins for the D3, D4 bands.

A reproducible method to fabricate atomically sharp tips for scanning tunneling microscopy

The electrochemical etching method by Ibe et al. [J. Vac. Sci. Technol. A 8, 3570 (1990)] to fabricate sharp tips for scanning tunneling microscopy was modified by shortening the cutoff time of the etching current after the material wire drops off at the air-electrolyte interface. The tip radius measured by field ion microscopy was successfully reduced to 8 nm when the cutoff time was shortened to 50 ns. The dependence of the field-emitting electron current from the sharpest tips was close to one expected from the Fowler–Nordheim formula with a reasonable value for the emitting area of the tip.

Dislocation glide motion in heteroepitaxial thin films of Si1−xGex/Si(100)

Abstract Measurements of dislocation glide velocity in heteroepitaxial Si1−xGex thin films grown on Si(100) substrates revealed that the velocity of threading dislocations penetrating the epitaxial layers depends almost linearly on the film thickness (dislocation length) in very thin films and shows saturation as the film thickness exceeds about 1 μm, in agreement with results of a similar experiment performed by Tuppen and Gibbings in 1990. The activation energy of dislocation motion is unaltered over this transition, which is incompatible with the view that such saturation is brought about by commencement of kink collision in long dislocations. This fact, together with other findings in the present study, supports an interpretation that the dislocation glide in bulk crystals of Si, even though the segment of straight dislocation is of a macroscopic dimension, proceeds without kink collision and is controlled solely by the formation rate of double kinks.

Probing ultrafast spin dynamics with optical pump-probe scanning tunnelling microscopy.

Studies of spin dynamics in low-dimensional systems are important from both fundamental and practical points of view. Spin-polarized scanning tunnelling microscopy allows localized spin dynamics to be characterized and plays important roles in nanoscale science and technology. However, nanoscale analysis of the ultrafast dynamics of itinerant magnetism, as well as its localized characteristics, should be pursued to advance further the investigation of quantum dynamics in functional structures of small systems. Here, we demonstrate the optical pump-probe scanning tunnelling microscopy technique, which enables the nanoscale probing of spin dynamics with the temporal resolution corresponding, in principle, to the optical pulse width. Spins are optically oriented using circularly polarized light, and their dynamics are probed by scanning tunnelling microscopy based on the optical pump-probe method. Spin relaxation in a single quantum well with a width of 6 nm was observed with a spatial resolution of ∼ 1 nm. In addition to spin relaxation dynamics, spin precession, which provides an estimation of the Landé g factor, was observed successfully.

In situ scanning tunneling microscopic study of polymerization of C60 clusters induced by electron injection from the probe tips

Polymerization of C60 clusters epitaxially grown on Si(111)-(7×7) substrates was found to be induced by electron injection from the probe tips of scanning tunneling microscopes (STM) as the sample bias was increased from +4.0 to +5.5 V, exhibiting an evolution behavior characterized by an incubation, a linear growth, and a saturation. The incubation time and the growth rate are dependent greatly on the sample site, which is explained by a model taking into account the pre-existing stress as the driving force of the polymerization and the internal stress built up as a consequence of polymerization producing a stress for backward reactions.

Nonthermal decomposition of C60 polymers induced by tunneling electron injection

Scanning tunneling microscopic (STM) studies of C60 films deposited on highly oriented pyrolytic graphite substrates revealed that the electron injection from the STM tip induces the decomposition (isomerization) of the C60 molecules that have been polymerized also by the electron injection into the films. Both reaction rates were characterized by a linear dependence on the injected tunneling current and a common threshold around 2 V in the sample bias dependence. We discuss two nonthermal mechanisms for the polymerization and decomposition reactions: electronic excitation mechanism by Auger decay and a hypothetical ionic reaction model.

Cluster reactions in C60 films induced by electron injection from a scanning tunneling microscope tip

Abstract Polymerization and decomposition of C 60 clusters induced by electron injection from a scanning tunneling microscope (STM) tip have been studied experimentally. The tunneling current dependence of polymerization rate and decomposition rate was found to be both linear, indicating that each reaction is caused by injection of a single electron from the STM tip. We propose a model that an electron injected from an STM tip into a C 60 crystal propagates in the electronic band of the C 60 crystal until it becomes localized at a specific sites where it causes an ionically enhanced [2+2] cyclo-addition reaction. The positional variation of polymerization efficiency and peculiar patterns of polymerized areas are interpreted in terms of the strain field built-in or built-up in the crystal.

Electric field modulation spectroscopy by scanning tunneling microscopy with a nanometer-scale resolution

Instrumentation has been devised by coupling electric field modulation spectroscopy (EFMS) with scanning tunneling microscopy (STM) that enables the investigation of electronic band structures in semiconductors with a nanometer-scale spatial resolution. Model experiments using low-temperature-grown GaAs (LT–GaAs) epifilms show that a difference as small as 0.01 eV in electronic energy gaps between the GaAs substrate and the LT–GaAs epilayers can be distinguished in the EFMS spectra, demonstrating the high energy, as well as spatial, resolution of the STM–EFMS.

Dislocation motion in semiconducting crystals under the influence of electronic perturbations

The radiation enhanced dislocation glide effect was confirmed in h-ZnO as well as in other semiconductors, which proved further the ubiquitous nature of this effect in semiconductors. Analysis of the radiation enhanced dislocation vibrations observed in the same h-ZnO led us to a tentative model that the effect is brought about by fluctuations of the charge state of point defects which exert electrostatic forces on the charged dislocations. The hydrogen-plasma enhanced dislocation glide was studied in Ge, SiGe and GaAs in addition to the previous reports on Si. The absence of the hydrogen effect in α-dislocations in n-GaAs and in thin films of SiGe is interpreted in terms of the soliton model, the essence of which is a strong binding between kinks and hydrogens.

Origin of low frequency noise and 1/f fluctuations of tunneling current in scanning tunneling microscopes

The generation mechanism of conspicuous low frequency noise (including 1/f fluctuations) observed in the tunneling current of scanning tunneling microscopes was studied experimentally by investigating combinations of Pt–Ir tips and Au films or crystalline graphite samples. For 1/f noise, the noise intensity at a fixed current increases with increasing bias voltage or increasing tip–sample distance z. The noise power varies from place to place in a sample surface of Au film. The spatial variation of the noise has a positive correlation with the magnitude of the tunneling barrier height φ measured by the z modulation method. The demodulated signal in the z modulation experiments also exhibits fluctuations with a 1/f spectrum of the relative magnitude nearly equal to that in the current noise. The above features are observed also in high vacuum (10−7 Pa), ruling out the possibility that the current noise is brought about by absorption/desorption of gas molecules in the ambient. Tips capable of imaging graphi...