Page 384 - Electrical Properties of Materials

366 Optoelectronics

20
Absorption (10 5 /m -1 ) 15

10

Fig. 13.26
Measured absorption spectrum of a
GaAs quantum well of 10 nm 0 1.46 1.54 1.62 1.7
thickness. Photon energy (eV)

the aid of the electric field must be high. However, when the electric field is
applied perpendicular to the layers, then the good intentions of the electric
field in trying to separate the particles are frustrated by the presence of the
walls. Provided the well is narrow enough, the exciton is not field ionized.
Thus, the exciton resonance is still there albeit with reduced amplitude due to
the increased separation of the electron–hole pair. There is also a shift in the
position of the resonance due to the electrostatic energy (this is nothing else
but the energy of a dipole in an electric field, something we have discussed
before).
The expectations based on the above qualitative argument are borne out
by the experimental observations, as shown in Fig. 13.27, where the absorp-
tion spectrum measured for a GaAs–AlGaAs quantum well structure is plotted
against photon energy. This phenomenon is known as the Quantum Confined
Stark Effect. The excitons are obviously quantum confined, and the Stark

4 (i)
(ii)

(iii)
log transmission 2 (iv)

Fig. 13.27 (v)
Measured absorption spectrum of a
quantum well for electric field applied
perpendicularly to the layers,
–1
4
–1
5
(i) 1.6 × 10 Vm , (ii) 10 Vm ,
5
–1
(iii) 1.4 × 10 Vm , 0
–1
5
(iv) 1.8 × 10 Vm , 1.42 1.46 A 1.5
–1
5
(v) 2.2 × 10 Vm . Photon energy (eV)