Naoka Nagamura

Scanning photoelectron microscope for nanoscale three-dimensional spatial-resolved electron spectroscopy for chemical analysis.

In order to achieve nondestructive observation of the three-dimensional spatially resolved electronic structure of solids, we have developed a scanning photoelectron microscope system with the capability of depth profiling in electron spectroscopy for chemical analysis (ESCA). We call this system 3D nano-ESCA. For focusing the x-ray, a Fresnel zone plate with a diameter of 200 μm and an outermost zone width of 35 nm is used. In order to obtain the angular dependence of the photoelectron spectra for the depth-profile analysis without rotating the sample, we adopted a modified VG Scienta R3000 analyzer with an acceptance angle of 60° as a high-resolution angle-resolved electron spectrometer. The system has been installed at the University-of-Tokyo Materials Science Outstation beamline, BL07LSU, at SPring-8. From the results of the line-scan profiles of the poly-Si/high-k gate patterns, we achieved a total spatial resolution better than 70 nm. The capability of our system for pinpoint depth-profile analysis and high-resolution chemical state analysis is demonstrated.

Variable-temperature independently driven four-tip scanning tunneling microscope

The authors have developed an ultrahigh vacuum (UHV) variable-temperature four-tip scanning tunneling microscope (STM), operating from room temperature down to 7 K, combined with a scanning electron microscope (SEM). Four STM tips are mechanically and electrically independent and capable of positioning in arbitrary configurations in nanometer precision. An integrated controller system for both of the multitip STM and SEM with a single computer has also been developed, which enables the four tips to operate either for STM imaging independently and for four-point probe (4PP) conductivity measurements cooperatively. Atomic-resolution STM images of graphite were obtained simultaneously by the four tips. Conductivity measurements by 4PP method were also performed at various temperatures with the four tips in square arrangement with direct contact to the sample surface.

Direct observation of charge transfer region at interfaces in graphene devices

Nanoscale spectromicroscopic characterizing technique is indispensable for realization of future high-speed graphene transistors. Highly spatially resolved soft X-ray photoelectron microscopy measurements have been performed using our “3D nano-ESCA” (three-dimensional nanoscale electron spectroscopy for chemical analysis) equipment in order to investigate the local electronic states at interfaces in a graphene device structure. We have succeeded in detecting a charge transfer region at the graphene/metal-electrode interface, which extends over ∼500 nm with the energy difference of 60 meV. Moreover, a nondestructive depth profiling reveals the chemical properties of the graphene/SiO2-substrate interface.

Observation of rebirth of metallic paths during resistance switching of metal nanowire

To clarify the mechanism of resistance-switching phenomena, we have investigated the change in the electronic structure of a Ni nanowire device during resistance-switching operations using scanning photoelectron microscopy techniques. We directly observed the disappearance of density of state (DOS) at the Fermi level (EF) in a high-resistance state and recovery of a finite DOS at EF in a low-resistance state. These results are direct evidence that the Ni nanowire is fully oxidized after switching to the high-resistance state and that Ni-metal conductive paths in the oxidized nanowire are recovered in the low-resistance state.

Microscopically-Tuned Band Structure of Epitaxial Graphene through Interface and Stacking Variations Using Si Substrate Microfabrication

Graphene exhibits unusual electronic properties, caused by a linear band structure near the Dirac point. This band structure is determined by the stacking sequence in graphene multilayers. Here we present a novel method of microscopically controlling the band structure. This is achieved by epitaxy of graphene on 3C-SiC(111) and 3C-SiC(100) thin films grown on a 3D microfabricated Si(100) substrate (3D-GOS (graphene on silicon)) by anisotropic etching, which produces Si(111) microfacets as well as major Si(100) microterraces. We show that tuning of the interface between the graphene and the 3C-SiC microfacets enables microscopic control of stacking and ultimately of the band structure of 3D-GOS, which is typified by the selective emergence of semiconducting and metallic behaviours on the (111) and (100) portions, respectively. The use of 3D-GOS is thus effective in microscopically unlocking various potentials of graphene depending on the application target, such as electronic or photonic devices.

Chemical potential shift in organic field-effect transistors identified by soft X-ray operando nano-spectroscopy

A chemical potential shift in an organic field effect transistor (OFET) during operation has been revealed by soft X-ray operando nano-spectroscopy analysis performed using a three-dimensional nanoscale electron-spectroscopy chemical analysis system. OFETs were fabricated using ultrathin (3 ML or 12 nm) single-crystalline C10-DNBDT-NW films on SiO2 (200 nm)/Si substrates with a backgate electrode and top source/drain Au electrodes, and C 1s line profiles under biasing at the backgate and drain electrodes were measured. When applying −30 V to the backgate, there is C 1s core level shift of 0.1 eV; this shift can be attributed to a chemical potential shift corresponding to band bending by the field effect, resulting in p-type doping.

Pinpoint operando analysis of the electronic states of a graphene transistor using photoelectron nanospectroscopy

Graphene is a promising material for next-generation devices owing to its excellent electronic properties. Graphene devices do not, however, exhibit the high performance that is expected considering graphenes intrinsic electronic properties. Operando, i.e., gate-controlled, photoelectron nanospectroscopy is needed to observe electronic states in device operation conditions. We have achieved, for the first time, pinpoint operando core-level photoelectron nanospectroscopy of a channel of a graphene transistor. The direct relationship between the graphenes binding energy and the Fermi level is reproduced by a simulation assuming linear band dispersion. This operando nanospectroscopy will bridge the gap between electronic properties and device performance.

Observation of nanoscopic charge-transfer region at metal/MoS2 interface

2D materials are promising for next-generation device applications, such as flexible transistors. However, the devices using 2D materials as active layers cannot exhibit good performance. One of the causes is that electronic properties are influential to the interface with, for instance, metal contact due to an ultra-thinness of the 2D materials. We have, therefore, performed core-level photoelectron microscopy measurements to investigate the local electronic states at interfaces in a MoS2 field-effect transistor (FET). We detect a charge-transfer region (CTR) at the MoS2/metal-electrode interface, which expands over ~500 nm with the electrostatic potential (energy shift) variation of ~70 meV, which causes band bending in the MoS2 electronic structure with a Fermi level shift. The observed potential variation of the CTR is well reproduced by a simple calculation using Poissons approximation. Our results point to a potential way of understanding the interfacial effect of the MOS2/metal electrode on the device characteristics and performance.

Electronic structure of Li2Fe1−xMnxP2O7 for lithium-ion battery studied by resonant photoemission spectroscopy

In order to clarify changes in the electronic structures, especially Fe partial density of states (DOS), of Li2Fe1−xMnxP2O7 with Mn substitution, we have performed x-ray absorption spectroscopy and resonant photoemission spectroscopy (RPES) experiments for Li2Fe1−xMnxP2O7. Using RPES teqniques, we have succeeded in extracting the Fe2+ partial DOS. We have found the systematic shift to higher binding energy and broadening of Fe 3d t2g down-spin states accompanying with the Mn substitution. The peak shift of the Fe 3d t2g down-spin states is matched very well to the change of Fe3+/Fe2+ redox potential, suggesting that the origin of high Fe3+/Fe2+ redox potential in Li2Fe1−xMnxP2O7 is the shift of the Fe 3d t2g down-spin states to the higher binding energy with Mn substitution.