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When the electric field is applied to the Quantum Well (QW) structure, the absorption is changed by Quantum Confined Stark Effect (QCSE). In the QW, electron and hole is confined in the well because of the energy difference between well and barrier, and the discrete energy level is formed. Without electric field, the wavefunction of electron and hole is symmetric about the center of the well, on the other hand, when the electic field is applied to the QW, electron wavefunction move to the left, and hole wavefunction move to the right. And at the same time, the energy level of electron is relatively decreased, and the energy level of hole is relatively increased. By the increase of applied electric field, the effective energy gap is decreased.
By comparing to the bulk structure, one of the important optical characteristic in the QW is the existence of room temperature exciton. Exciton is the bound state of eledctron and hole pair by the coulomb force. In the bulk GaAs, the binding energy of exciton is 4.2meV, the absorption peak of exciton is observed only ultra low temperature, and at the room temperature, the absorption peak is vanished because of the phonon scattering. On the other hand in the QW structure, binding energy of 2-dimensional exciton become 4 times larger than the 3-dimensional exciton. Hence the absorption peak of exciton exists even the room temperature.
When the electric field is applied to the bulk structure, the broadening of absorption edge occurs by the Franz-Keldysh effect. At the ultra low temperature, when the electic field is applied to the bulk structure, the binding energy of exciton is easily decreased because of the stark shift. On the other hand in the QW structure, when the electric field is applied to the vertical direction to the well, the barrier energy prevents the dissociation of exciton, and binding energy is kept, as a result, the absorption peak occurs stark shift without the degradation of absorption.