Scientists Identify Optical Phonon Mode Involved in Electron Transport Through Thin Insulating Films

Scientists from EPFL and Max Planck have discovered for the first time the phonon mode generated when electrons are transported by hopping through thin insulating films.

Phonons are quasiparticles that arise from vibrations within the atomic lattice of materials. Two types of phonons that gather much interest are the acoustic and the optical phonons. Acoustic phonons arise from atoms moving from side-to-side in unison, while optical phonons are formed in the crystal lattice when a positively charged ion is moving to one direction and its neighbor with negative charge to the opposite direction. Now, scientists from EPFL and Max-Planck have clearly identified the longitudinal optical phonon mode involved in the electron transport through a thin insulating film. The work is published in Physical Review Letters.

Wolf-Dieter Schneider, with colleagues from the Fritz-Haber-Institute of the Max-Planck Society, used a scanning tunneling microscope to study electron transport in thin calcium oxide film (CaO), 25 atomic monolayers in thickness. For ultrathin films, scanning tunneling microscopes work through the principle of quantum tunneling: A metal tip is brought close to the surface of the tested sample. An applied bias voltage between the surface and the tip allows electrons to flow through the vacuum, registering changes in current as the tip passes over the surface, and generating an image on a computer.

In this study, the researchers used thin insulating films made of CaO. The film thickness has been increased to more than 20 monolayers so that the electrons from the microscope’s tip cannot tunnel through the film anymore but only move by “hopping” through the conduction band of the CaO film. The scientists observed that this hopping was accompanied with the generation of longitudinal optical phonons in the thin film’s lattice. In addition, the authors were able to spatially resolve a “softening” of this phonon mode around lattice irregularities in the oxide film.

The work offers the first experimental proof that electron transport in dielectric films is mediated through optical phonons arising from the film’s lattice vibrations. It also demonstrates the power of scanning tunneling microscopy to "view" phonon softening locally at lattice dislocations and for small islands in the nanometer regime.