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Phonon spectroscopy studies of semiconductor structures

Studies have been made on the phonon spectroscopy of low-dimensional semiconductor structures and amorphous films using a range of complementary techniques. The low-dimensional structures are single and double quantum wells (QW) grown by molecular beam epitaxy and amorphous samples. Studies have been made of phonon-assisted tunnelling through aluminium gallium arsenide barriers. This has been done by observing the change in tunnel current produced by an incident phonon pulse and measurements have been made on several samples, including samples with both 2-dimensional and 3-dimensional emitters. Studies have been made as a function of magnetic field strength and angle. The phonon-assisted tunnelling has been shown to be a one-phonon process so that the device can be used as a tunable phonon spectrometer. Photoluminescence studies have been on slightly asymmetric double quantum wells with thin barriers. Both direct and indirect exciton lines can be seen in such structures and information on acoustic phonon-assisted tunnelling can be obtained by studying their intensities as a function of bias voltage. The studies of the luminescence spectra of double QWs in the presence of nonequilibrium phonons showed the significant role of the elastic tunnelling processes in the near-resonance region. The work has concentrated on the phonon spectroscopy of amorphous silicon using optically generated heat pulses and a ruby phonon detector sensitive to 29 cm{-1} (870 Ghz) phonons. A result of particular interest is that in hydrogenated amorphous silicon, the scattering of the 29 cm{-1} phonons increases strongly with optical intensity and it is suggested that the scattering is by optically created long-lived very high frequency phonons. Preliminary measurements have also been made using intense far infrared radiation (FIR) from a free electron laser (FELIX). The results strongly suggest it may be possible to use such FIR excitation to generate monochromatic phonons in amorphous material.

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University of Nottingham
University Park
NG7 2RD Nottingham
United Kingdom
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