Atomistic simulations of the optical absorption of type-II CdSe/ZnTe superlattices
© Boyer-Richard et al.; licensee Springer. 2012
Received: 17 July 2012
Accepted: 19 September 2012
Published: 2 October 2012
We perform accurate tight binding simulations to design type-II short-period CdSe/ZnTe superlattices suited for photovoltaic applications. Absorption calculations demonstrate a very good agreement with optical results with threshold strongly depending on the chemical species near interfaces.
KeywordsType-II transition Superlattice ZnTe/CdSe Absorption Tight-binding 73.21.Cd 78.67.Pt 78.66.Hf
Tight-binding simulation of bulk materials
We consider the extended-basis sp3d5s* tight-binding (TB) model which has proved to provide a band structure description with a sub-milli-electron volt precision throughout the Brillouin zone of binary III-V semiconductors including quantum heterostructures and surfaces. We model CdSe, CdTe, ZnSe, and ZnTe in a cubic phase by fitting both the experimental band parameters and the first-principle electronic structures in the GW approximation. Strain effects are taken into account in the same way of smaller TB models using a recent generalization of Harrison’s d2 law for hopping integrals known to be reliable for strained III-V quantum well structures. The valence band offset (VBO) between the material constituents are taken from our own experimental measurements for the CdSe/ZnTe interface and from ab initio modeling for the interfacial bonds. Finally, the optical dipole matrix elements are derived from the TB Hamiltonian.
Superlattice absorption calculation
We have performed TB calculations to design the most suited type-II CdTe/ZnSe  configuration which fully maximizes the absorption in the solar spectrum. Associated SL band structures are found very sensitive to the VBO between CdTe and ZnSe. As knowledge of this VBO is scanty, we have performed photoluminescence measurements on a simple ZnTe/CdSe interface as a function of incident power. The extracted value is of 0.74 ± 0.02 eV, which is slightly different from the experimental result of 0.64 eV, but in agreement with the ab initio calculations. We have used a mesh of 1,200 points to sample the reduced Brillouin zone near the Γ-point. The discrete transitions are dressed with a Gaussian broadening of 0.005 eV to get smooth spectral functions. As CdTe and ZnSe do not share any common atom, three configurations have been simulated: CdTe-like or ZnSe-like terminations (symmetric D2d SL) and the CdSe/ZnTe interfaces (non-symmetric C2v SL).
Results and discussion
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).
Valence and conduction energy levels at Γ-point and dipole matrix element in transverse electromagnetic polarization
In conclusion, we have studied the optical properties of ultra thin II-VI quantum well structures suited for solar application and shown that a strong and stable optical process can occur at wavelengths of 885 nm. Further engineering of the electronic structure could be achieved by considering the different well thicknesses and alloyed materials in the superlattices. Our results show the usefulness of II-VI semiconductors to implement type-II band alignment in photovoltaic-based systems.
conduction (Electron) band state
Heavy-Hole valence band state
Light-Hole valence band state
Molecular Beam Epitaxy
Valence Band Offset.
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