HEAT PULSE STUDIES OF THE ENERGY RELAXATION BY PHONON EMISSION OF A HEATED 2DEG IN A GAAS (ALGA)AS QUANTUM-WELL STRUCTURE/

Heat pulse techniques have been used to study the energy relaxation vi a phonon emission of hot electrons in a 200 Angstrom wide GaAs/(AlGa)A s quantum well structure in which two 2D electronic subbands were occu pied. The electrons were heated by applying 20 ns electrical pulses to the device and the emitted phonons were detected on the opposite surf ace of the substrate using superconducting aluminium bolometers. It is found that at excitation levels above about 3 pW per carrier the ener gy relaxation process is basically the same as for a device based on a single heterostructure: that is, the energy relaxation is dominated b y the emission of optic mode phonons which ultimately decay to acousti c phonons. However, as the excitation power was reduced below 3 pW per carrier there was a marked difference between this system and heteros tructures, in that a strong longitudinal acoustic mode phonon peak is observed in the emission signal. It is argued that the longitudinal ph onon emission is associated with intersubband electronic transitions.