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In earlier experiments1 it was demonstrated that superconductor tunnel diodes are devices of potential use in the investigation of phonon transport and interaction properties of materials possibly up to 1012 Hz. Several experiments2S using tin phonon generators, however, strongly supported the assumption that most of the relaxation phonons with energies n> 26.sn , the energy gap of tin (corresponding to 280 GHz), are reabsorbed by pair breaking, thus not leaving the generator films. Now using aluminum, a superconductor with less strong electron-phonon interaction, we have been able to observe the escape of 870-GHz relaxation phonons, corresponding to n = 246.A1 • The aluminum phonon generator (bottom film, 1300 A thick; 26.A1 =0.31 meVat 1 K) and a tin detector (26. Sn = 1.16 me V) were evaporated on opposite faces of a silicon substrate (length 2.3 mm, [111] direction) doped with 1018_cm3 0 16 impurities, as roughly estimated from infrared data. 6 The system Si:016, exhibiting a far-infrared absorption line7 at 29.3 cm-1 (878 GHz), was used as a frequency reference, at the same time demonstrating the possibility of tunable phonon spectroscopy in the terahertz frequency range. Because of the spectral sensitivity threshold of the tin detector which responds only to phonons of n ;;. 26. Sn""'86.Ab we observed a steep signal increase at a generator bias voltage V = (26. sn+ 26.A1)/ e. As we have already pointed out in a former communication,8 this confirms that the most distinct feature of the emitted relaxation phonon spectrum is a sharp high-frequency cutoff2 at n =eV 26.AlJ resulting from relaxation of quasiparticles injected at energies E ~eV 6.A1 to the gap edge E = 6.A1• Small spectral contributions occur, however, at higher energies resulting from processes discussed in more detail in Ref. 8. This leads to a weak Signal onset already at e V

Evidence for 870-GHz Phonon Emission from Superconducting Al Tunnel Diodes through Resonant Scattering by Oxygen in Silicon