12.4 Recombination in Quantum Wells
379
Fig. 12.42 Photoluminescence spectrum (T = 2 K) (dots) of GaAs/ (Al,Ga)As quantum well grown by MBE with
120 s growth interruptions. Recombination is due to excitons in islands of 19, 18, and 17 monolayers (a 0 /2) height.
The solid line is a lineshape fit including lifetime broadening ( = 1.34 meV) and residual inhomogeneous broadening
(σ = 0.04 meV) due to (Al,Ga)As barrier alloy fluctuations. Note that the energy separation of the peaks is much larger
than kT . The peak doublet structure is discussed in [1199]. Adapted from [1199]
A potential fluctuation can localize an exciton laterally at low temperatures [1199] and behave like
a quantum dot (cf. Sect. 14.4). Localized and delocalized excitons are separated by a boundary called
the mobility edge [1209]. The transition between the two regimes is a Mott transition [1210].
Monolayer Growth Islands
Under certain growth conditions, quantum wells with piecewise very flat interfaces can be fabricated.
The thickness difference between such regions (with lateral extension in the µm range) is an integer
monolayer. Accordingly, the recombination spectrum yields several, typically two or three, discrete
lines (Fig. 12.42).
12.4.3 Quantum Confined Stark Effect
The quantum confined Stark effect (QCSE, Sect. 15.1.2) in quantum wells shifts energy levels when
electric fields are present along the width of the quantum well. A strong effect exists for pyroand piezoelectric material combinations such as c-axis oriented (In,Ga)N/(Al,Ga)N [1211, 1212] or
(Cd,Zn)O/(Mg,Zn)O [1213, 1214] quantum wells due to the built-in electric field (cf. Sect. 16.2). The
QCSE induced red-shift is larger in thicker quantum wells and goes beyond the bulk band gap of the
quantum well material (Fig. 12.43b). Also the wavefunction overlap is reduced with increasing well
width, leading to an increase of the radiative recombination lifetime as shown in Fig. 12.43. The pyroelectric field and the related modification of lifetime are absent in quantum wells grown on non-polar
directions such as [11.0] (Fig. 12.43a).
379
Fig. 12.42 Photoluminescence spectrum (T = 2 K) (dots) of GaAs/ (Al,Ga)As quantum well grown by MBE with
120 s growth interruptions. Recombination is due to excitons in islands of 19, 18, and 17 monolayers (a 0 /2) height.
The solid line is a lineshape fit including lifetime broadening ( = 1.34 meV) and residual inhomogeneous broadening
(σ = 0.04 meV) due to (Al,Ga)As barrier alloy fluctuations. Note that the energy separation of the peaks is much larger
than kT . The peak doublet structure is discussed in [1199]. Adapted from [1199]
A potential fluctuation can localize an exciton laterally at low temperatures [1199] and behave like
a quantum dot (cf. Sect. 14.4). Localized and delocalized excitons are separated by a boundary called
the mobility edge [1209]. The transition between the two regimes is a Mott transition [1210].
Monolayer Growth Islands
Under certain growth conditions, quantum wells with piecewise very flat interfaces can be fabricated.
The thickness difference between such regions (with lateral extension in the µm range) is an integer
monolayer. Accordingly, the recombination spectrum yields several, typically two or three, discrete
lines (Fig. 12.42).
12.4.3 Quantum Confined Stark Effect
The quantum confined Stark effect (QCSE, Sect. 15.1.2) in quantum wells shifts energy levels when
electric fields are present along the width of the quantum well. A strong effect exists for pyroand piezoelectric material combinations such as c-axis oriented (In,Ga)N/(Al,Ga)N [1211, 1212] or
(Cd,Zn)O/(Mg,Zn)O [1213, 1214] quantum wells due to the built-in electric field (cf. Sect. 16.2). The
QCSE induced red-shift is larger in thicker quantum wells and goes beyond the bulk band gap of the
quantum well material (Fig. 12.43b). Also the wavefunction overlap is reduced with increasing well
width, leading to an increase of the radiative recombination lifetime as shown in Fig. 12.43. The pyroelectric field and the related modification of lifetime are absent in quantum wells grown on non-polar
directions such as [11.0] (Fig. 12.43a).