Phonon-Assisted Tunneling in Bismuth Tunnel Junction

Abstract

We report the first observation of phonon-assisted tunneling in a semimetal-insulator tunnel junction, represented by remarkably fine structure in the conductance-voltage curve at low temperatures. This process gives rise not only to structure near zero bias, as in the case of the semiconductor tunnel diode, but also to several conductance peaks near the edge of a new band at voltages corresponding to the band edge plus the phonon energies. The existence of phonon-assisted tunneling, resulting from the conservation of the transverse momentum, thus provides a useful method for locating band-edge energies by tunneling spectroscopy. We establish, for the filled electron and hole bands of pure bismuth, four band edges: +90±5, +38±2, -35±3, and -60±5 meV; and less reliably, nine additional band edges: +650, +145, -80, -110, -140, -200, -600, -800, and -1600 meV, where + and - refer to below and above the Fermi level, respectively.