Atomistic simulations of the optical absorption of type-II CdSe/ZnTe superlattices

  • Soline Boyer-Richard1Email author,

    Affiliated with

    • Cédric Robert1,

      Affiliated with

      • Lionel Gérard2,

        Affiliated with

        • Jan-Peter Richters3,

          Affiliated with

          • Régis André2,

            Affiliated with

            • Joël Bleuse3,

              Affiliated with

              • Henri Mariette2,

                Affiliated with

                • Jacky Even1 and

                  Affiliated with

                  • Jean-Marc Jancu1

                    Affiliated with

                    Nanoscale Research Letters20127:543

                    DOI: 10.1186/1556-276X-7-543

                    Received: 17 July 2012

                    Accepted: 19 September 2012

                    Published: 2 October 2012

                    Abstract

                    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.

                    Keywords

                    Type-II transition Superlattice ZnTe/CdSe Absorption Tight-binding 73.21.Cd 78.67.Pt 78.66.Hf

                    Background

                    A photovoltaic cell is typically built onto three parts: a light absorber surrounded by an n-type and a p-type layer to separate and collect the photo-generated charge carriers (Figure1). In an ideal case, the n-type layer conduction band shall be aligned with the conduction band of the absorber while forming a barrier for holes in the valence band. Respectively, the p-type layer valence band shall be aligned with the valence band of the absorber while forming a barrier for electrons in the conduction band. Such a three-material system does not exist for semiconductors, and we propose here to mimic it by using a type-II short-period superlattice (SL) made of two materials with a type-II band alignment. The material with the lowest conduction band will then be used as the n-doped contact and the other one as the p-doped contact. Type-II material systems built with III-V semiconductors are already available and mainly known for mid-infrared detectors (for a review see[1]). CdSe and ZnTe bulks have been chosen to this scope because they are almost lattice-matched and exhibit a type-II interface. Furthermore, this SL first optical transition value can be designed to emphasize the solar spectrum absorption. In this letter, we propose atomistic modeling of the optical absorption of type-II CdSe/ZnTe superlattices. The ZnTe and CdSe layer thicknesses are optimized to maximize absorption in the solar spectrum and threshold is studied as function of the interface-related properties.
                    http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-7-543/MediaObjects/11671_2012_Article_1056_Fig1_HTML.jpg
                    Figure 1

                    Schematic band alignment of a p-i-n structure for photovoltaic application.

                    Methods

                    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[2] including quantum heterostructures[3] and surfaces[4]. 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[5]. The valence band offset (VBO) between the material constituents are taken from our own experimental measurements for the CdSe/ZnTe interface[6] and from ab initio modeling for the interfacial bonds[7]. Finally, the optical dipole matrix elements are derived from the TB Hamiltonian[8].

                    Superlattice absorption calculation

                    We have performed TB calculations to design the most suited type-II CdTe/ZnSe [001] 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[9], but in agreement with the ab initio calculations[7]. 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).

                    Table1 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: [110] (x), [−110] (y), and [001] (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- and y- planes, respectively. For the associated superlattice, we found for the fundamental transition a polarization degree E Py E Px E Py + E Px http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-7-543/MediaObjects/11671_2012_Article_1056_IEq1_HTML.gif of 17% (canceled for symmetric SL in agreement with point group D2d), and this is consistent with photoluminescence measurements[10]. As seen in Table1, 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 Figure2. 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.
                    Table 1

                    Valence and conduction energy levels at Γ-point and dipole matrix element in transverse electromagnetic polarization

                     

                    LH1(eV)

                    HH1(eV)

                    E1(eV)

                    ΔE(eV)

                    EPx(eV)

                    Polarization

                    Non-symmetric SL

                    −0.212

                    −0.062

                    1.340

                    1.402

                    3.08

                    17.5%

                    ZnSe termination

                    −0.255

                    −0.101

                    1.330

                    1.430

                    2.56

                    -

                    CdTe termination

                    −0.195

                    −0.048

                    1.292

                    1.339

                    3.24

                    -

                    The zero level is taken at the bulk CdSe valence band.

                    http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-7-543/MediaObjects/11671_2012_Article_1056_Fig2_HTML.jpg
                    Figure 2

                    Schematic diagram of the band alignment for the (CdSe) 7 /(ZnTe) 7 SL 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 miniband in three interfaces cases: (b) non-symmetric case, (c) ZnSe interfaces, and (d) CdTe interfaces. Envelope functions are plotted along the [001] axis (molecular average between the charge densities on cation and anion sites) for clarity reasons and to better evidence the location of electronic states in the structure.

                    Figure3 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 Figure4. 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 (Figure5).
                    http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-7-543/MediaObjects/11671_2012_Article_1056_Fig3_HTML.jpg
                    Figure 3

                    Absorption coefficient of the (CdSe) 7 /(ZnTe) 7 SL for three types of interface, as a function of energy.

                    http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-7-543/MediaObjects/11671_2012_Article_1056_Fig4_HTML.jpg
                    Figure 4

                    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 [001] and [110] directions.

                    http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-7-543/MediaObjects/11671_2012_Article_1056_Fig5_HTML.jpg
                    Figure 5

                    Photoluminescence spectra of (CdSe) 7 /(ZnTe) 7 SL grown by MBE.

                    Conclusions

                    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.

                    Abbreviations

                    E: 

                    conduction (Electron) band state

                    HH: 

                    Heavy-Hole valence band state

                    LH: 

                    Light-Hole valence band state

                    MBE: 

                    Molecular Beam Epitaxy

                    SL: 

                    Superlattice

                    TB: 

                    Tight-Binding

                    VBO: 

                    Valence Band Offset.

                    Declarations

                    Authors’ Affiliations

                    (1)
                    Université Européenne de Bretagne, INSA, FOTON, UMR 6082
                    (2)
                    Nanophysics and Semiconductors Group, Institut Néel-CNRS
                    (3)
                    Nanophysics and Semiconductor Group, CEA/Université Joseph Fourier, CEA/INAC/SP2M

                    References

                    1. Tournié E, Baranov AN: Mid-infrared semiconductor lasers: a review. In Advances in Semiconductor Lasers. Volume 86. Edited by: Coleman JJ, Bryce AC, Jagadish C. San Diego: Academic Press; 2012:183–226.View Article
                    2. Scholz R, Jancu JM, Beltram F, Bassani F: Calculation of electronic states in semiconductor heterostructures with an empirical spds* tight-binding model. Phys Status Solidi B 2000, 217: 449–460. 10.1002/(SICI)1521-3951(200001)217:1<449::AID-PSSB449>3.0.CO;2-BView Article
                    3. Sacconi F, Carlo AD: Full band approach to tunneling in MOS structures. IEEE T-ED 2004, 51: 741–748. 10.1109/TED.2004.826862View Article
                    4. Jancu J-M, Girard J-C, Nestoklon M, Lemaître A, Glas F, Wang Z, Voisin P: STM images of subsurface Mn atoms in GaAs: evidence of hybridization of surface and impurity states. Phys Rev Lett 2008, 101: 196801.View Article
                    5. Jancu J-M, Voisin P: Tetragonal and trigonal deformations in zinc-blende semiconductors: a tight-binding point of view. Phys Rev B 2007, 76: 115202.View Article
                    6. Mourad D, Richters J-P, Gérard L, André R, Bleuse J, Mariette H: Determination of the valence band offset at cubic CdSe/ZnTe type II heterojunctions: a combined experimental and theoretical approach. [cond-mat.mtrl-sci] 2012, arXiv:1208.2188.
                    7. Van de Walle CG, Neugebauer J: Universal alignment of hydrogen levels in semiconductors, insulators and solutions. Nature 2003, 423: 626–628. 10.1038/nature01665View Article
                    8. Boykin T, Vogl P: Dielectric response of molecules in empirical tight-binding theory. Phys Rev B 2001, 65: 35202.View Article
                    9. Yu ET, Phillips MC, McCaldin JO, McGill TC: Measurement of the CdSe/ZnTe valence band offset by X-ray photoelectron spectroscopy. J Vac Sci Technol B 1991, 9: 2233. 10.1116/1.585726View Article
                    10. Su W, Ya M, Chiu Y, Chen Y: Polarized optical properties in type-II ZnTe/CdSe multiple quantum wells induced by interface chemical bonds. Phys Rev B 2002, 66: 113305.View Article
                    11. André R, Bleuse J, Mariette H: Hétérostructure semi-conductrice de type-II et cellule photovoltaïque comprenant une telle hétérostructure. Patent FR1153146 2011.

                    Copyright

                    © Boyer-Richard et al.; licensee Springer. 2012

                    This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://​creativecommons.​org/​licenses/​by/​2.​0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.