Structural changes of the interface, enhanced interface incorporation of acceptors, and luminescence efficiency degradation in GaAs quantum wells grown by molecular beam epitaxy upon growth interruption
A comparison of the luminescence of GaAlAs/GaAs/GaAlAs quantum wells (QW) grown by molecular beam epitaxy with and without a 1–100 s interruption of the growth at the interfaces is presented. Well widths of 2 and 5 nm are studied as model systems. The luminescence from the noninterrupted samples consists of Gaussian shaped doublets. A line shape theory is outlined, allowing for the first time a determination of the distribution of the microscopic chemical and crystallographic disorder of the interfaces from the spectra. All samples were grown under nominally identical conditions and all show the same Gaussian distribution of the interface position. The full width at half maximum of the distribution function is 1.25 Å. The interfaces in these samples are believed to have an island-like character with a typical island size of much less than 17 nm. Interruption of the growth by a few seconds changes the luminescence line shape function qualitatively. The spectrum splits into doublets, with doublet fine structure. The energies of the two main components agrees with that expected from excitonic recombination in wells with widths Lz equal to m, and (m+1) times the thickness of a GaAs layer, where m is an integer. The interfaces are therefore believed to smooth with increasing growth interruption time and islands much larger than 17 nm form rapidly at the free GaAs surface. The luminescence associated with carbon acceptors on interface sites increases linearly up to a factor of 10 with increasing growth interruption time up to 100 s and the total luminescence efficiency deteriorates rapidly for times larger than 30 s. It is argued that the efficiency decrease does not result from incorporation of carbon, which we clearly observe, but from the incorporation of other nonradiative centers. Growth interruption was never observed to cause an increase of the quantum efficiency by more than 20%. Quantum wells with interface disorder but perfect bonds seem to have a quantum efficiency which is close to the maximum possible.