Intersubband absorption in ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As multiple quantum wells

The intersubband absorption in ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As multiple quantum wells having various well widths, barrier widths, and doping densities is investigated. As the well width increases from 35 to 200 Å, the energies of the absorption peaks decrease from 300 to 100 meV. For a quantum well of 130 Å, two sharp absorption peaks are observed, corresponding to the transitions from the second to the third subbands ${\mathit{E}}_{32}$ and from the first to the second ${\mathit{E}}_{21}$. This well-width dependence of the peak energy is in good agreement with theoretical calculations of the subband structure and the Fermi level. Line broadening with the well width is discussed in terms of one-monolayer fluctuation and the energy spread of minibands. The oscillator strength and the dipole matrix element are also evaluated. The barrier width and doping dependencies of the ${\mathit{E}}_{21}$ and ${\mathit{E}}_{32}$ transitions are studied for the 130-Å-well samples. A decrease in the barrier width contributes to line broadening via the energy spread of minibands, while this has no influence on the oscillator strength. Up to the electron density ${\mathit{N}}_{\mathit{s}}$=8×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$, only the ${\mathit{E}}_{21}$ peak is observed. For ${\mathit{N}}_{\mathit{s}}$ above 1.8×${10}^{12}$ ${\mathrm{cm}}^{\mathrm{-}2}$, the ${\mathit{E}}_{32}$ transition is found to be observed. This behavior is in good agreement with the calculation of the Fermi-energy position. In addition, depolarization and many-body effects are also briefly discussed.