Broadening of interband resonances in thin AlAs barriers embedded in GaAs

AlAs barriers embedded in GaAs were studied by spectroscopic ellipsometry and resonant Raman scattering. Heterostructures with AlAs barrier widths ranging from 2 to 30 nm were grown by molecular-beam epitaxy at growth temperatures between 410 and 660 Degree C. For layer widths below 10 nm the E(ind 1) and E(ind 1)+ Delta(ind 1) critical point resonance in the dielectric function of the AlAs was found to broaden and to be smeared out completely for a width of 2 nm. Resonant Raman scattering by the AlAs LO phonon reveals for layer widths equal or smaller than 10 nm a considerable broadening of also the E(ind 0) interband transition in the AlAs. The magnitude of the critical point broadening and redistribution of oscillator strength, however, was found to be independent of the growth temperature and thus of the cation intermixing observed by Raman spectroscopy for growth temperatures equal or greater than 600 Degree øC. Therefore, the observed critical point broadening is not caused by th e formation of graded composition (AlGa)As barriers. Instead, the broadening of interband resonances is attributed to a spread of the carrier wave functions into the surrounding GaAs, which are not confined within the AlAs barrier for neither the E(ind 0) nor the E(ind 1) and E (ind 1) + Delta(ind 1) interband transitions.