CLUSTERING-INDUCED PHONON LINEWIDTH ANOMALIES IN ALXGA1-XAS

We assume that AlxGa1-xAs layers may be classified into two general gr oups of different configurations: one consisting of mesoscopic-size cl usters differing in their cation content, but otherwise fairly homogen eous (I), while in the other the cation concentration fluctuates stati stically on a microscopic scale about a well-defined average (II). In this way we explain the distinct behavior of these two classes of samp les. In group I, phonon lines are broad but reasonably symmetric, room -temperature luminescence is present and a second-order resonant Raman spectrum is observed. In group II, phonon lines are narrow but asymme tric, no luminescence and no second-order Raman could be detected. Str uctural damage induced by ion implantation shifts down the phonon freq uency and broadens the line in II as expected. As opposed to that, and most striking, for excitation above the direct energy band gap, the p honon line in I narrows upon ion implantation. This is explained via a decrease in the electron-phonon interaction as a result of the phonon confinement, which allows the electrons to reach lower-potential clus ters and thereby create a lower-frequency phonon with a narrower line shape.