Absorption of quantum wells

For modelling the optical properties of QWs we use the method described by S. Chuang ([1]). The absorption coefficient at thermal equilibrium in a QW is given by:

\alpha_0(E) & = C_0(E) \sum_{n,m} |I_{hm}^{en}|^2 | \hat{e} \cdot \vec{p} |^2 \rho_{rmn}^{2D} \\
& \times \left[ H(E-E^{en} + E_{hm}) + F_{nm}(E) \right]

where |I_{hm}^{en}|^2 is the overlap integral between the holes in level m and the electrons in level n; H is a step function, H(x) = 1 for x>0, 0 and 0 for x<0, \rho_{rmn}^{2D} is the 2D joint density of states, C_0 a proportionality constant dependent on the energy, and F the excitonic contribution, which will be discussed later.

C_0 (E) & =    \frac{\pi q^2 \hbar }{n_r c \epsilon_0 m_0^2 E} \\
\rho_r^{2D} &= \frac{m_{rmn}^*}{\pi \hbar L}\end{aligned}

Here, n_r is the refractive index of the material, m_{rmn} = m_{en} m_{hm} / (m_{en} + m_{hm}) the reduced, in-plane, effective mass and L an effective period of the quantum wells. The in-plane effective mass of each type of carriers is calculated for each level, accounting for the spread of the wavefunction into the barriers as ([2]):

m_{\perp} =  \int_{0}^{L} m(z) | \psi(z) |^2\end{aligned}

This in-plane effective mass is also used to calculate the local density of states shown in Figure [fig:qw]b. In Eq. [eq:QW_abs2], | \hat{e} \cdot \vec{p} |^2 is the momentum matrix element, which depends on the polarization of the light and on the Kane’s energy E_p, specific to each material and determined experimentally. For band edge absorption, where k = 0, the matrix elements for the absorption of TE and TM polarized light for the transitions involving the conduction band and the heavy and light holes bands are given in Table [tab:matrix_elements]. As can be deduced from this table, transitions involving heavy holes cannot absorb TM polarised light.




3/2 M_b^2



1/2 M_b^2

2 M_b^2

Table: Momentum matrix elements for transitions in QWs. M_b^2=m_0 E_p /6 is the bulk matrix element.

In addition to the band-to-band transitions, QWs usually have strong excitonic absorption, included in Eq. [eq:qw_abs] in the term F_{nm}. This term is a Lorenzian (or Gaussian) defined by an energy E_{nmx, j} and oscillator strength f_{ex, j}. It is zero except for m=n \equiv j where it is given by Klipstein et al. ([3]):

F_{nm} &= f_{ex, j} \mathcal{L}(E - E_{nmx, j}, \sigma) \\
E_{nmx, j} &= E^{en} - E_{hm} - \frac{R}{(j-\nu)^2} \\
f_{ex, j} &= \frac{2R}{(j-\nu)^3} \\
R &= \frac{m_r q^4}{2 ( 4\pi \epsilon_r \epsilon_0)^2 \hbar^2 }\end{aligned}

Here, \nu is a constant with a value between 0 and 0.5 and \sigma is the width of the Lorentzian, both often adjusted to fit some experimental data. In Solcore, they have default values of \nu = 0.15 and \sigma = 6 meV. R is the exciton Rydberg energy ([1]).

Fig. [fig:QW_absorption] shows the absorption coefficient of a range of InGaAs/GaAsP QWs with a GaAs interlayer and different In content. Higher indium content increases the depth of the well, allowing the absorption of less energetic light and more transitions.