Midinfrared absorption ofPbSe/Pb1−xEuxTequantum dot superlattices in IV-VI microcavities

Abstract

The midinfrared absorption of highly ordered self-assembled $\mathrm{PbSe}/{\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Te}$ quantum dot superlattices is investigated. A different approach is used for quantitative determination of the absorption spectrum of the dot superlattice by inserting it into a high-finesse microcavity structure with a small mode spacing and ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Te}/\mathrm{EuTe}$ Bragg mirrors. For such microcavities, we show a linear relation between the width of each cavity resonance and the extinction coefficient at the resonance energy. Thus, the absorption spectrum is experimentally determined from the resonance widths. It reveals a narrow peak arising from the quantum dots and a step from the two-dimensional wetting layers. The peak absorption coefficient of the dot ensemble amounts to $2.5\ifmmode\times\else\texttimes\fi{}{10}^{4} {\mathrm{cm}}^{\ensuremath{-}1}$ and is similar to that of the wetting layers. We also present calculations of the dispersion of the absorption coefficient based on model dielectric functions of quantum dots and wetting layers. From a fit to the experimental absorption spectrum, we deduce interband transition energies as well as corresponding oscillator strengths and level broadenings. The broadening of the dot transition due to dot size fluctuations of the large quantum dot ensemble is only 8.7 meV, confirming the exceptionally high dot size uniformity in the PbSe quantum dot superlattice.