Phonon emission by warm electrons in GaAs quantum wells: the effect of well width on the acoustic-optic crossover

We have made heat pulse measurements of the phonon emission by warm two-dimensional electrons in gallium arsenide quantum wells that range in width from 3 to 15 nm. As the power dissipated in the sample is increased. a characteristic change in the phonon time-of-flight traces is observed. This is identified with the onset of the emission of optic phonons at an electron temperature of about 50 K. We find that the onset of optic phonon emission occurs at a higher power in the narrower wells. We account for these observations in terms of the increase of the energy loss rate due to the emission of other modes owing to the relaxation of the perpendicular momentum conservation requirement in narrower wells. (C) 1999 Elsevier Science B.V. All rights reserved.