Lineshape Analysis of Intersubband Transitions in Multiple Quantum Wells

Conduction intersubband transitions between the ground and first excited states in Al0.3Ga0.7As/GaAs multiple quantum wells (MQWs) are studied as a function of the twodimensional electron gas density (0.75 × 1012 ≤ σ ≤ 3.75 × 1012 cm−2) and temperature (5 ≤ T ≤ 300 K). There is no electron tunneling between the wells and well regions are uniformly doped with silicon donors. Theoretically, we have solved the Schrödinger equation containing the self-consistent Hartree potential, in which the z-dependencies of both electron effective mass and dielectric constant, as well as the non-parabolicity in the conduction energy subband dispersion have been taken into consideration. By applying many-body theory which includes the depolarization-shift from a collective dipole moment and the excitonicshift from the negative exchange interaction, we calculate the absorption spectrum as a function of the incident photon energy hw for different values of T and σ. From this, we can quantitatively analyze both T- and σ-dependencies of the peak position and the full width at half-maximum (FWHM) of peak values. The blue-shift or red-shift in the absorption peak position are quantitatively reproduced as either T or σ is reduced. The exchange interaction which depends on σ, will modify the energy subband dispersion. Therefore, the absorption peak will be broadened by the exchange interaction. The T-dependence of broadening from the optical-phonon scattering is also taken into account by a phenomenological model. From the calculated absorption spectrum as a function of o, we have successfully reproduced and explained the σ-dependence of FWHM measured in recent experiments.