Clustering-induced phonon linewidth anomalies inAlxGa1−xAs

We assume that ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As layers may be classified into two general groups of different configurations: one consisting of mesoscopic-size clusters differing in their cation content, but otherwise fairly homogeneous (I), while in the other the cation concentration fluctuates statistically on a microscopic scale about a well-defined average (II). In this way we explain the distinct behavior of these two classes of samples. 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 asymmetric, no luminescence and no second-order Raman could be detected. Structural damage induced by ion implantation shifts down the phonon frequency and broadens the line in II as expected. As opposed to that, and most striking, for excitation above the direct energy band gap, the phonon 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 clusters and thereby create a lower-frequency phonon with a narrower line shape.