Off-resonant absorption in bound-to-continuump-typeGaAs/AlxGa1−xAsquantum wells: Overcoming absorption saturation with doping

Abstract

Optimum bound-to-continuum normal-incidence absorption in low-doped (less than $1\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{cm}}^{\ensuremath{-}2})$ p-type GaAs/AlGaAs quantum wells obtains for well widths for which the second light-hole (LH2) level is resonant with the top of the valence band quantum well near the center of the Brillouin zone. Experimentally we found that such absorption saturates at higher doping levels. For higher doping around $4\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{cm}}^{\ensuremath{-}2},$ our envelope-function approximation (EFA) model predicts that pushing LH2 deeper into the continuum avoids absorption saturation and at least doubles the photoresponse. The results are explained on the basis of an EFA calculation, which shows that saturation is due to the fact that the line of resonances in the continuum as a function of the in-plane wave vector eventually becomes a bound LH2 band in the well at some critical wave vector. By matching this critical wave vector (via well width and/or well depth adjustment) with the Fermi wave vector (determined by doping in the well) for the desired QWIP (i.e., cutoff wavelength), saturation can be avoided. This prediction is verified on a set of well-characterized samples. A re-entrant band behavior, in which a band is bound over a limited portion of the Brillouin zone, is also demonstrated.