Ballistic Transport Through Nanostructures

Ballistic transport through nanostructures

Christian Bobisch und Rolf Möller

The Nanoprobe is a scanning tunneling microscope equipped with three autonomous scanning tunneling units. All three tunneling units can be operated simultaneously. Thus opens up new possibilities for the analysis of dissipation mechanisms on surfaces. For analysing transport phenomena two different methods are applied within the Nanoprobe lab:

  • The ballistic electron emission microscopy (BEEM)
  • The scanning tunneling potentiometry

BEEM

In ballistic electron emission microscopy the ballistic transport through metal films with a thickness of a few nanometers is analysed. Electrons which do not lose energy as a result of a scattering process are called ballistic. Thereby BEEM provides information about scattering centres and energy loss at the injection, within the film or at the interface.

Method

Using BEEM a thin metal film on a semiconducting substrate is needed. The films typically have got a thickness of about 3-5nm. Then at the metal semiconductor interface a Schottky barrier arises. One tip contacts the metal film setting it to ground potential and another one injects electrons into the film.

Principle of BEEM: One tip contacts the metal film (right side of picture), another tip injects the electrons.

Considering the grounded film and the injecting STM-tip, this circuit acts as an ordinary STM circuit. In addition BEEM allows simultaneous measurements of the tunneling current and the resulting ballistic current reaching the semiconductor. The ballistic current, known as BEEM-current, is composed of electrons, which passed the metal film without any energy loss.

By analyzing the BEEM-current one will gain information about scattering processes and energy dissipation in the metal film. Furthermore the height of the Schottky barrier can be determined with high lateral resolution. Hence, BEEM provides a lateral resolution in the sub-nanometer range.

3-5nm Wismuth auf Si(111), topography.
3-5nm Wismuth auf Si(111), BEEM-current.

Conclusions

Ballistic transport through the system bismuth on silicon was studied. On a Si(111)-7x7 substrate an inflated BEEM-current occurs at step edges of the Bi-film. This is a result of the overlap of the band structure of the metal layer with the truncated bulkstates of the silicon substrate at the interface.