文档介绍：4 Excitons
The concept of excitons
Free excitons
Free excitons in external fields
Free excit
Excitonic ansorption of GaAs among 21 K and 294 K. The dashed line is an attempt to fit the absorption edge with a value of Eg equal to eV which is appropriate for GaAs at 294 K
(a )
(b )
(a) The dissociation of exciton is mainly caused
by collision with longitudinal optic (LO) phonons;
The Coulomb interaction between the electron and hole still enhances the absorption rate considerably.
Excitonic absorption of ultra pure GaAs at K. The data clearly show the hydrogen-like energy spectrum of the exciton in the vicinity of the band gap. The energies of the n=1, n=2, and n=3 excitons are , and eV respectively. Eg= eV and RX = meV can be fitted from these data.
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Free excitons in external fields
Electric fields
A DC field can push the oppositely charged electrons and holes away from each other.
The electric field between electron and hole in the ground state exciton:
Field ionization as E > Ee-h, then the exciton will break apart.
Vbi: the build-in voltage 1V;
Li: the intrinsic region thickness V;
E = 106 V/m.
The excitons will be ionized before the bias applied
6105 V/m
Field ionization of the free excitons in GaAs at 5 K. The data was taken on a GaAs p-i-n diode with an i-region thickness of 1 m. The solid corresponds to ‘ flat band’(forward bias of +, where E 5 105 V/m. No exciton lines are resolved at zero bias.
The physics effect of bulk semiconductors in field is dominated more by the effect of the field on the band states—the Franz-Keldysh effect, rather than by the exciton effect.
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Magnetic fields
The cyclotron energy of excitation in magnetic fields:
Two field regimes:
Weak field limit: RX >>hC (< 2T)
Strong field limit: RX << hC (> 2T).
Weak field:
For n=1, the exciton has no net magnetic moment due to spherical symm