A phonon emission study of quasi-1D electron gases

This thesis is a theoretical and experimental investigation into acoustic phonon emission from a quasi-1D electron gas. The theoretical investigation commences with a discussion of acoustic phonon emission from a truly ID electron gas in the single quantum limit. The effect of changing electron density and temperature has been considered, and the contributions from both phonon coupling mechanisms to transverse and longitudinal phonons calculated. As the number of occupied subbands is increased, the oscillation in the density of states at the Fermi energy produces a corresponding oscillation in the emitted phonon power. A detailed study into the angular distribution of phonon intensity found, at low temperatures, that the distribution of emission angle can be simply calculated from the conservation laws. Increasing the temperature or the number of occupied subbands changes the angular distribution to become perpendicular to the growth direction, similar to emission from a 2DEG. There is a distinct difference in the angle of emission between intra and inter subband transitions because of the changing wavevector dependence of the form factor. Heat pulse experiments have been used to detect directly the phonons emitted due to energy relaxation in 2DEG's and quasi-1D electron gases. In a 2DEG a change in the nature of phonon emission is detected at about 0.5pW per electron. This occurs at lower power than previous studies suggest (2-7pW per electron). We attribute this change in the phonon emission process to the onset of optic phonon emission and present evidence that the crossover measured previously is too large because of the analysis technique. In a wide wire array (> 150nm) there is very little change from the phonon emission process in a 2DEG. As the wire is narrowed the acoustic-optic crossover moves to lower electron power. The acoustic phonon emission process becomes less efficient because reducing the number of occupied subbands reduces the phase space available for phonon emission. However, in narrow wires there are few occupied subbands and the crossover rises to higher power. This is because the phase space that has been removed by the reduction in the number of subbands is partially returned by momentum non-conservation. The experimental results are found to agree well with the theory once consideration of the changing electron system has been taken into account. (author)