Coherent phonon oscillations in GaAs/Al/sub x/Ga/sub 1-x/As multiple quantum wells: the x-dependence

Summary form only given.Femtosecond coherent phonon experiments are performed on a series of GaAs/Al/sub x/Ga/sub 1-x/As multiple quantum wells (MQWs) with varying x, at fixed well and barrier width of 100 /spl Aring/. For x=0.2, we have found clear beating in time domain. Fourier transformation reveals two distinct peaks at the LO and TO phonon positions, which is indicative of bulk-like plasmon-phonon oscillations in the growth direction. This is surprising, for the supposed carrier confinement due to the relatively large barrier width has been thought to forbid such oscillations. At x=0.5, little beating is seen and only the LO phonon frequency is found in Fourier transformation), in agreement with earlier results on a series of samples with fixed x=0.35. Our results are in agreement with the recent realization of the existence of anomalously large well-to-well coupling of carriers through thick AlGaAs barriers. In addition, for bulk-like, growth-direction plasmon-phonon oscillation to be possible, we believe that optical phonons also should be able to penetrate the barrier and couple with the phonons in the next well. Such a propagating phonon model has been proposed. Finally, our barrier and x-dependent studies show that the growth-direction plasmon-phonon coupling is a more sensitive function of x than of the barrier width.