We first test our TB model by calculating the electronic properties of non-symmetric (CdSe)7/(ZnTe)7 superlattices and found a strong in-plane anisotropy of the optical spectrum. The energy subbands are calculated at the Γ-point and labeled according to their dominant bulk-state component: conduction (e), heavy-hole (hh), and light-hole (lh).
reports on the dipole matrix elements squared (EP
in electron volt) between the first Γ-like valence and conduction band states for transverse electric polarization in the CdSe/ZnTe superlattices. In a non-symmetric C2v
configuration, interfaces are characterized by forward and backward bonds lying in the (110) (or x-
) and (−110) (or y-
)planes respectively, giving the definition of optical axes here considered:  (x
), [−110] (y
), and  (z
). In addition the growth sequence in the simulation is as follows: Se-Cd=Se-Cd=Se….Cd=Te-Zn=Te…where ‘-’ and ‘=’ indicate chemical bonds in the x-
planes, respectively. For the associated superlattice, we found for the fundamental transition a polarization degree
of 17% (canceled for symmetric SL in agreement with point group D2d
), and this is consistent with photoluminescence measurements
]. As seen in Table
, the e1-hh1 transition strongly depends on the chemical species at SL terminations, which underlines that relevant active states are mainly located in the surrounding of interfaces. Very interestingly, the CdTe-like terminations allow for a lower absorption threshold due to the very small VBO between CdTe and ZnTe. This explanation can be illustrated from the calculation of the charge densities as shown in Figure
. Obviously, the ground-state wave function is maximized in CdTe layers compared to ZnSe. The CdTe termination mimics larger ZnTe layers increasing the energy level of hh1. This type of interface allows for a stronger overlap between the valence and conduction subbands, which enhances the optical matrix elements of the band edge.
Valence and conduction energy levels at Γ-point and dipole matrix element in transverse electromagnetic polarization
Figure 2 Schematic diagram of the band alignment for the (CdSe)7/(ZnTe)7SL and electronic wave functions. Schematic diagram of the band alignment for the (CdSe)7/(ZnTe)7 SL (a) and electronic wave functions of the upper valence miniband and the lower conduction (more ...)
shows the absorption coefficient calculated for each type of superlattice. In the simulation, we considered six conduction and 12 valence subbands. Consequently, the calculated spectral function is valid near the center of the reduced Brillouin zone up to 2 eV above the valence band maximum. In the same way of optical transitions, the absorption threshold is found strongly dependent on the chemistry at interfaces. According to these calculations, the CdTe interfaces should be favored to increase absorption in the solar spectrum. However, they are very difficult to control during the sample growth by molecular beam epitaxy (MBE). The major steps correspond to the different conduction minibands. The peaks around 1.52 and 1.83 eV for CdTe terminations, and around 1.9 eV for the non-symmetric SL, correspond to the curvature inversion observed in the valence miniband around −0.6 eV for the non-symmetric SL as shown in Figure
. Absorption measurements have not yet been performed on such samples but photoluminescence measurements at 4 K for the same SL grown by MBE show a maximum value around 1.42 eV, in good agreement with the simulated absorption thresholds (Figure
Absorption coefficient of the (CdSe)7/(ZnTe)7SL for three types of interface, as a function of energy.
Band diagram of the CdSe/ZnTe SL with non-symmetric interfaces. Band diagram (black lines) of the CdSe/ZnTe SL with non-symmetric interfaces in the reduced Brillouin zone along the  and  directions.
Photoluminescence spectra of (CdSe)7/(ZnTe)7SL grown by MBE.