Broadening of interband resonances in thin AlAs barriers embedded in GaAs

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Abstract
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 °C. For layer widths below 10 nm the E1 and E1+Δ1 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 ≤10 nm a considerable broadening of also the E0 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 ≥600 °C. Therefore, the observed critical point broadening is not caused by the 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 E0 nor the E1 and E1+Δ1 interband transitions.

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