Normal-state tunnel junction as a tunable quasimonochromatic phonon source

The frequency distribution of the acoustic phonons emitted from a biased metal-insulator-metal normal-state tunnel junction has been studied. Electrons tunneling across the insulating layer form an excited population with a range of energies up to ∼${\mathit{eV}}_{\mathit{B}}$ above the Fermi level and then relax, producing a phonon spectrum with a frequency cutoff at the same characteristic value. Normal-state tunnel junctions made from aluminum have been produced and phonon spectra determined by using the stress-tuned splitting of boron acceptors in silicon as a spectrometer. It has been shown that, in the range ∼1–4 meV (∼250–1000 GHz), the second differential of the signal with respect to junction bias (${\mathrm{\partial }}^2$P/∂${\mathit{V}}_{\mathit{B}}^2$) is quasimonochromatic, having a spectrum of an approximately Gaussian form with the peak occurring at an energy of ${\mathit{eV}}_{\mathit{B}}$ and a half width of ∼6${\mathit{kT}}_{\mathit{e}}$.