Carrier relaxation in doped quantum wells

Abstract

The relaxation time of an electron in a quantum well is derived within the random-phase approximation including full multi-subband and frequency dependent screening. The resulting expression encompasses both electron-electron and electron-phonon scattering taking into account the mutual interactions of the electrons and phonons. The intersubband relaxation time of an electron is numerically evaluated considering electron-electron and electron-phonon (bulk LO-phonon) scattering in a GaAs quantum well. It is shown that the intersubband relaxation time is significantly influenced by the electron density in the well. It is also shown that at room temperature it is necessary to use the finite temperature dielectric function to accurately determine the intersubband relaxation time. Scattering due to the coupled system of electrons and phonons is compared with the decoupled scattering where both electron-electron and unscreened electron-phonon scattering are considered separately. In addition, the above theory of carrier relaxation is applied to quantum well lasers. The gain saturation coefficient, c:, of InxGat-xAs/ Alo.2Gao.8As strained layer quantum well lasers (SL-QWLs) is calculated as a function of strain from carrier intrasubband relaxation times. The intrasubband relaxation times are calculated within the RPA including carrier-carrier as well as carrier-polar optical phonon interactions at a temperature of 300 K. The band structures are determined from the Luttinger-Kohn Hamiltonian and a multiband effective mass equation. It is demonstrated that the gain saturation coefficient increases with compressive strain in the active layer of the quantum well due to a corresponding increase of the intrasubband relaxation time. From this, a direct connection between strain and laser switching speed can be deduced.U of I Onlydissertatio