We have carried out sensitive measurement of the vertical transport characteristics of several GaAs/AlAs “double-barrier” heterostructures pressurized just beyond the type-II transition, and in the presence of longitudinal magnetic fields of up to 15 T. Towards the upper field limit, clear periodic structure is observed in the second derivative current-voltage characteristic of the resonance attributed to the process (Formula presented) where (Formula presented) indicates the lowest quasiconfined subband associated with the transverse (Formula presented) minima in AlAs, and (Formula presented) is a zone center transverse optical phonon. The periodic structure is interpreted as a series of transitions to collector states of increasing Landau index, with the requirement for conservation of in-plane momentum being satisfied for any interlevel transition by the phonon emission. Quantitative analysis of the data yields a value for the Landau-level separation, and thus also a value for the two-dimensional geometric effective mass of the transverse (Formula presented) minima in AlAs. © 1998 The American Physical Society.
