Theory of electroabsorption in asymmetric-graded-gap quantum wells

Abstract

Electronic and optical properties of asymmetric-graded-gap triangular quantum wells are studied in the presence of a uniform electric field applied parallel to the [001] growth direction. A triangular quantum well consists of finite barriers of AlyGa1−yAs surrounding a well composed of AlxGa1−xAs whose aluminum concentration x increases linearly with position from the left-hand to right-hand side along the growth direction. We study exciton transition energies, binding energies, and oscillator strengths as well as the total absorption coefficient as a function of applied electric field strength using a multiband effective mass theory, which takes valence subband mixing effects into account. All results are compared with the usual case of a square quantum well, where x is constant along the growth direction. In triangular quantum wells, the electron and hole charge densities are confined near the left-most interface and thus exciton binding energies and oscillator strengths are relatively insensitive to the applied electric field. In addition, the magnitude of the downward shift in the fundamental absorption edge is approximately linear in the applied field strength and the total absorption spectrum tends to retain its overall shape. This contrasts with the situation in square quantum wells, where electron and hole charge densities are more easily separated by the applied field (quantum confined Stark effect). In square-quantum wells, exciton binding energies and oscillator strengths are quite sensitive to the electric field, the shift in the fundamental absorption edge is nonlinear, and the total absorption spectrum varies in a complicated manner due to the varying strengths of the various excitonic transitions. For these reasons, asymmetric graded gap quantum wells would seem to offer possibilities for the design of optoelectronic devices which exploit such electroabsorptive effects in quantum wells.