CHANGEOVER FROM ACOUSTIC TO OPTIC MODE PHONON EMISSION BY A HOT 2-DIMENSIONAL ELECTRON-GAS IN THE GALLIUM-ARSENIDE ALUMINUM GALLIUM-ARSENIDE HETEROJUNCTION

We have used heat pulse techniques to study the energy relaxation of a hot two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterojuncti on. The 2DEG was heated by applying short (almost-equal-to 100 ns) ele ctrical pulses to the drain-source contacts of the device. The electro ns lost energy by emitting phonons which were detected by a CdS bolome ter on the opposite side of the GaAs substrate. A change in the nature of the phonon signal occurring at an excitation level of about 5 pW p er electron indicated a change in the phonon emission process. The cor responding electron temperature, T(e), at which optic phonon emission is expected to become the dominant energy relaxation process was estim ated to be about 60 K. At powers well below the change-over, we found that the energy loss rate per electron, P(e), due to acoustic phonon e mission is proportional to T(e)3. At much higher powers, P(e) is-propo rtional-to exp(-HBAR-omega(LO)/kT(e)), where HBAR-omega(LO) is the lon gitudinal optic phonon energy. We obtained a value of 3.3 ps for the e lectron-optic phonon scattering time, which is consistent with the ran ge of values found in the literature.