Excitonic properties of type-I and type-II Si/Si_1-xGe_x quantum wells

The exciton properties of Si/Si_1-xGe_x quantum wells (QWs) are investigated within a variational approach, taking into account interface effects for two possibilities of the band alignment, type I and type II. For the latter, two-dimensional (2D) and three-dimensional (3D) variational wave functions are used in order to give a better description of the exciton confinement, where a large enhancement on the exciton binding energy (about 30% for a 70 Å well width) is found for the 3D case. The effects due to the presence of external electromagnetic fields on exciton energies are also analyzed. Our theoretical results explain the exciton energy blueshifts recently found in photoluminescence experiments for type-I Si/Si_0.82Ge_0.18 QWs when electric fields are applied in the growth direction. Exciton energy shifts due to magnetic fields are calculated, and a comparison between such shifts in the 2D and 3D approaches is performed for type-II wells. Numerical results show that interfacial layers of 15 Å thickness for a type-I (type-II) Si/Si_0.82Ge_0.18 (Si_0.70Ge_0.30) QW lead to a variation of about 20 meV (25 meV) in relation to the abrupt well for a 50 Å well width, yielding a blueshift on the exciton total energy.