Coupled quantum wells

So far, we have considered single quantum wells or multiple quantum well structures with thick barriers for which there is practically no coupling between subsequent quantum wells.

(a) (b)

(c) (d) E

V E (e)

(f) (g)

E

E1 Ea

Eb

Eb Ea Ea

Eb Ea Ea

Figure 3.5: Coupling of quantum wells. (a) Symmetric double quantum wells with large barrier width (negligible coupling). (b) The small barrier width leads to a coupling of the quantum wells. The corresponding states are the binding |b and the anti-binding |astate with an energy splitting ∆E =E_a−E_b. (c) Asymmetric double quantum well. (d) Resonant situation with applied bias. (e) Fan chart for an asymmetric double quantum well structure with applied biasV. (f) Wavepacket |l= ^|b+|√₂^a. (g) Wavepacket afterT_osc/2,|r= ^|b−|√₂^a.

In this case, the quantum wells can be treated independently. When the barrier thickness becomes smaller, coupling between the wells has to be taken into account. This is schemat-ically depicted in Figs. 3.5 (a) and (b), where the well widths are equal. In Fig. 3.5 (a) the barrier is very thick so that coupling can be neglected and the eigenstates are localized in the respective wells. In Fig. 3.5 (b) the barrier is thin so that there is coupling between the two wells. As a consequence, an energy splitting with energies E_a and E_b occurs and the respective electron states, the so-called binding (|b) and anti-binding (|a) states, are delocalized over both wells. The energy splitting or tunnel coupling ∆E ≡ E_a−E_b is de-termined by the barrier thickness and the barrier height. Such a resonant situation can also be obtained for asymmetric, coupled double quantum wells with applied bias. In Fig. 3.5 (c) the lowest two states of an asymmetric, coupled double quantum well are shown. The lowest state is mainly localized in the wide well and the other state is mainly localized in the narrow well. Due to the coupling of the two wells the two states have nonzero probability density in both wells. Under a suitable bias these two states become resonant [Fig. 3.5 (d)]

and we observe the same behavior as described above. In Fig. 3.5 (e) the energies of the two states as a function of the applied bias are schematically shown (fan chart). The minimal energy splitting ∆E is observed at resonance.

After the coherent excitation of an electronic wavepacket, e.g., by optical excitation with a pulse which is spectrally broader than ∆ω = ∆E/¯h, an oscillatory motion is expected.

Such a wavepacket consisting of a coherent superposition of the binding and the anti-binding state reads

|ab, φ(t) = 1

√2

|b+|aeⁱ(^∆_¯h^Et+φ) (3.1)

In Fig. 3.5 (f) a wavepacket localized in the left well

|l=|ab,0(0) = |b√+|a 2

is shown. As time elapses, this wavepacket tunnels through the barrier and after a time ^Tosc₂ (tunneling time) it is localized in the right well [Fig. 3.5 (g)]

|r=|ab,0(T_osc

2 ) = |b − |√ a 2

After one oscillation period the wavepacket is localized again in the left well

|l=|ab,0(T_osc)

From Eq. (3.1) it can be easily seen that the oscillation period is directly connected with the energy splitting by the relation

T_osc= h

∆E

Assuming the energy eigenstates|aand|bas a single, homogeneously broadened 2-level system dephasing leads to an exponential damping of the oscillation with a damping rate Γ = 1/T₂. The normalized spatial displacement of the wavepacket in z-direction yields

z(t) =e^−t/T2 cos(ωosct+φ), ωosc= 2π T_osc

Figure 3.6: Exponentially damped oscillation z(t) = e^−t/T2 cos(2π t/Tosc) as a function of time. The oscillation period isT_osc= 410 fs corresponding to an energy splitting of ∆E = 10 meV. (a) Oscillation for a damping time T₂ = _π⁴ T_osc, i.e., 2 ¯hΓ = 0.25·¯h ω_osc = 2.5 meV.

(b) Oscillation for a damping time T₂ = _π¹Tosc, i.e., 2 ¯hΓ = ¯h ωosc= 10 meV. (c) Oscillation for a damping time T₂ = ₄¹_πT_osc, i.e., 2 ¯hΓ = 4·¯h ω_osc = 40 meV. Inset: Fourier transforms of the respective transients.

as shown in Fig. 3.6 (a). The Fourier transform of the exponentially damped oscillation yields a Lorentz line (Insets of Fig. 3.6)

˜

z(ω) = 1/T₂

(1/T₂)²+ (ω₀−ω)²

with the homogeneous linewidth 2 Γ = 2/T₂. The coherence of the oscillation strongly depends on Γ. For Γ ≥ ω_osc the oscillation is heavily damped [Fig. 3.6 (c)] so that the original wavepacket is completely dephased after one oscillation. We define electron transport as coherent if Γ< ω_osc [Fig. 3.6 (a)] and incoherent if Γ> ω_osc [Fig. 3.6 (c)].

So far, we have only considered the case of resonant tunneling. Also non-resonant tunnel-ing plays an important role for transport in semiconductor nanostructures. For simplicity, we will not discuss non-resonant tunneling now but we will come back to this point in the discussion of the transport measurements on quantum cascade structures.

In the following we want to discuss some important experimental studies related to vertical electron transport in semiconductor nanostructures.

Tunneling in biased quantum well structures was first observed by Oberli et al. [93]. The authors used a biased, asymmetric, coupled double quantum well structure based on the GaAs/AlGaAs material system. Carriers were generated in one of the quantum wells via sub-picosecond interband photoexcitation and the subsequent luminescence was measured time-resolved. The tunneling time was identified with the photoluminescence decay time.

As expected, a strong reduction of the tunneling time was observed if the two states in the respective quantum wells are resonantly coupled. Wavepacket oscillations between the wells of a biased, asymmetric GaAs/AlGaAs double quantum well were observed by Leo et al. [94]. They created an electronic wave packet in the wide well via ultrafast interband photoexcitation and traced the oscillatory motion of this wave packet by degenerate four-wave mixing and by pump-probe spectroscopy. In a further experiment by Roskos et al.

Figure 3.7: Measured coherent electromagnetic transients emitted from an asymmetric dou-ble quantum well for different bias fields [95].

[95] such a wave packet oscillation was directly observed by time-resolved detection of the THz emission from this spatial charge oscillation as shown in Fig. 3.7. In resonance, several oscillations were observed at carrier densities on the order of 10⁹ cm⁻². From the decay of the coherent electromagnetic signal at resonance a dephasing time ofT₂ ≈7 ps was deduced compared to an oscillation period ofT_osc≈1 ps.

Tunneling was also studied in superlattice structures with many coupled quantum wells.

Here, wavepacket oscillations—the so-called Bloch oscillations—were observed. [96, 1]. The first Bloch oscillations were observed with four-wave-mixing experiments in a semiconductor superlattice by Feldmann et al. [14]. The oscillations have been induced by femtosecond interband excitation and monitored in real-time. Martini et al. [97] studied the coherent THz emission from interband excited Bloch oscillations as a function of excitation density.

Pronounced oscillatory motions with an oscillation period T_osc ≈ 600 fs were found for electron concentrations of up to several 10⁹ cm⁻², whereas a rapid damping of the oscillations occurs at higher carrier densities as shown in Fig. 3.8 (a). For the homogeneous linewidth of the oscillation 2 Γ = 2/T₂ [Fig. 3.8 (b)] as a function of the excitation density n_ex they derived the linear expression

Γ(n_ex) = Γ(0) +ν ·n_ex, Γ(0) = 1

¯

h1.2 meV , ν = 1

¯

h9 ·10⁻¹¹ meV·cm²·nwell (3.2) For n_ex < 10¹⁰ cm⁻² the dephasing times are higher than the oscillation period and the transport is coherent. For higher carrier densities the transport is incoherent.

So far, we have seen that transport in double quantum wells and superlattices can be described in terms of tunneling. It is coherent for electron sheet densities <10¹⁰ cm⁻². In electrically driven device structures like quantum cascade lasers, however, the carrier sheet

(a) (b)

Figure 3.8: (a) Measured coherent electromagnetic transients emitted from a GaAs/AlGaAs superlattice structure for various optical excitation powers. (b) Homogeneous linewidth Γ and decay time constant (inset) of the THz pulses as a function of excitation density [97].

densities are typically ≈4×10¹¹ cm⁻². Extrapolating the data measured by Martini et al.

using equation (3.2) one would expect dephasing times of ≈30 fs at such elevated electron densities. For a typical energy splitting on the order of 10 meV (h/10 meV = 410 fs) this suggests that the transport in such structures is expected to be fully incoherent. However, such dephasing times have never been measured experimentally.

It is important to note that in the experiments described above, electronic wavepackets were generated via interband excitation, where the dephasing times are strongly influenced by electron-hole interaction. To study electron transport in unipolar devices, a measurement avoiding the generation of both electrons and holes at the same time is necessary. This has not been done so far.