Basic Study of Advanced Materials

Atomic level characterization and elucidation of material’s structures and electronic properties

The ultimate miniaturization of electronic devices requires that we characterize electronic materials at the sale of atoms and analyze various quantum effects of electrons, in order to understand and optimize functions of nanoelectronics devices. Our research focuses on observing atomic structures of material surfaces using scanning tunneling microscopy (STM), measuring surface characteristics on an atomic level, and elucidating local electronic properties of materials in order to gain further insight on their functions in nanoelectronics.

We are also interested in atom-sized contacts and wires of metals, which have many unique electronic and mechanical properties, not seen in macroscopic contacts. A simple breaking-junction technique allows us to study single-atom contacts of various metals and alloys, foreseeing their potential application as interconnect in nanoelectronics.

Atomic scale characterization of materials using scanning probe microscopy (SPM)

Spatial fluctuations in electrostatic potential in semiconductors, caused by ionic adsorbates and dopant atoms, are considered to make significant influence on the performance of nano-scale devices, and their characterization is thus a matter of primary importance. We are carrying out atomic-level observation of the local electrostatic potential utilizing the “STM barrier-height (STM-BH) imaging”, which combines the ultimate spatial resolution of STM and its capability of probing local tunneling barrier height. Since the barrier-height is related to the local surface potential, we can obtain, with STM-BH, high-resolution mapping of electrostatic potential, and perform pinpoint potential measurements around individual charged adsorbates and dopants. We also develop Scanning Tunneling Potentiometry (STP) and observe local electronic conductivity in thin films and (metal-semiconductor and semiconductor-semiconductor) heterojunctions, in order to visualize electronic current paths in nm resolution.

Atom-sized contacts of metals and alloys

A single-atom contact of metals, consisting of one atom bridging between electrodes, is the smallest of all contacts but exhibits a variety of interesting properties. Single-atom contacts of noble metals, for example, have a tensile strength comparable to that of ideal crystals and show a ballistic electron transport, leading to a universal conductance which nicely agrees with the conductance quantum 2e2/h (e and h are elementary charge and Planck constant, respectively). We are studying single-atom contacts of noble metals and alloys under high-bias/high-current conditions, employing various breaking junction techniques for forming them. Our aim is to elucidate how much current they can sustain, how they can be destabilized by bias/current-induced effects, and how they differ depending on their chemical species and alloy compositions. Answering these questions is not only important for potential applications of single-atom contacts as interconnects in atomic and molecular electronics but also quite interesting in it own right.