Elsevier

Physica B: Condensed Matter

Volume 410, 1 February 2013, Pages 93-98
Physica B: Condensed Matter

The structural, electronic and optical response of IIA–VIA compounds through the modified Becke–Johnson potential

https://doi.org/10.1016/j.physb.2012.09.050Get rights and content

Abstract

The structural, electronic and optical properties of IIA–VIA compounds are performed, by using the full-potential linearized augmented plan wave (FP-LAPW) method within DFT, by using the (PBEsol-GGA 2008) version. We have compared the modified Becke–Johnson (mBJ) potential to LDA, GGA and EV-GGA approximations. The IIA–VIA compounds have rock salt structure (B1) and zinc-blend structure (B3). The results obtained for band structure using mBJ show a significant improvement over previous theoretical work and give closer values to the experimental results. The bandgaps less than 3.1 eV are used in the visible light devices applications, while those with bandgaps bigger than 3.1 eV, used in UV devices applications. Optical parameters, like the dielectric constant, refractive indices, reflectivity, optical conductivity and absorption coefficient are calculated and analyzed. Refractive index lesser than unity (vg=c/n) shows that the group velocity of the incident radiation is greater than the speed of light.

Introduction

The IIA–VIA compounds have been studied intensively due to their valuable use in optoelectronic commerce. These compounds are used in established marketable electronic and optoelectronic devices working in the entire spectral regions. They are used in visual displays, high-density optical memories, transparent conductors, solid-state laser devices, photo detectors, solar cells etc. These compounds have rock salt (B1) and zinc-blende (B3) structures [1], [2]. The optical properties of IIA–VIA materials are important for the investigation of optoelectronic devices. Due to these applications, the optical properties of IIA–VIA compounds have extensively been studied experimentally as well as theoretically.

The IIA–VIA chalcogenides are technically very important having applications ranging from catalysis to microelectronics. These compounds are used in the region of luminescent devices [3]. The IIA–VIA chalcogenides have potential applications in light emitting diodes and laser diodes. After the investigation of the structural phase transition, metallization, cohesive and elastic properties under high pressure, IIA–VIA chalcogenides achieved great importance during last twenty years [4]. The study of these compounds under high pressure persuaded the interest with the targeted physical properties. Due to unusual properties and wide band gap under high pressure these compounds have greatly attracted the research community. The IIA–VIA chalcogenides have no “d” electron in their valance band, which is an important feature in the research field. The compressed behavior and the metallization phenomena study have become possible due to the static pressure techniques [5].

These chalcogenides have mostly p-type doping concentration and long lifetime, which make them better than the other compounds of the II–VI. There are certain compounds which allow us to explore fundamental devices fabrication and spintronics physics e.g. CrZnTe [6] and CrBeTe [7]. Groh et al. investigated that beryllium oxide (BeO) has a wide bandgap and its applications as an UV transparent conduction oxide in flat-panel displays and solar cells is very important [8]. Hassan et al. investigated that, BeS, BeSe and BeTe compounds show covalent bonding with large bandgaps, due to which they are very important for laser emitting and blue green laser diodes [9]. It was also confirmed by Hassan et al. that BeS, BeSe and BeTe have zinc-blende (B1), BeO and MgTe have wurtzite (B3) and the rest show rock salt structure (B1) [9]. According to their findings, BeX compounds are more covalent, the Phillips ionicities range from 0.169 in BeTe to 0.286 in BeS, So they transform to the sixfold-coordinated hexagonal NiAs (B8). The structural studies at equilibrium and under high pressure were investigated experimentally to verify that the stable crystalline phase is zinc-blende [8] and to confirm that structural phase transition exist from the zinc-blende (B3) to NiAs (B8) type structure for BeS, BeSe and BeTe at pressures of 51±8 GPa, 56±5 GPa and 39±5 GPa respectively [10].

Magnesium chalcogenides are the important elements of earth's lower layer. The MgO, MgS and MgSe have rock salt structure and can be changed into the CsCl structure when pressure is increased [11], while MgTe possess wurtzite structure [12]. The MgTe compound has wurtzite structure and can change to NiAs structure at the energy range of 1 GPa–3.5 GPa, the unloading and annealing process can maintain this structure [13]. Bhardwaj et al. investigate that MgO, MgS, MgSe and MgTe compounds have wide bandgaps and are technologically very important. Baltache studied magnesium oxide “MgO” (almost 80% in the earth's lower layer) experimentally as well as theoretically and stated that the B1 structure remains constant up to 227 GPa pressures. It is predicted that the pressure to convert the material from B1 phase to B2 at T=0 is as low as 205 GPa [14] and as high as 1050 GPa [15]. Thus the excellent approximation is 510 GPa, due to which 40% of earth's pressure increased in its center. Zha et al. investigated that due to the strange structural stability, MgO is used as a pressure standard in high pressure experiments. First create MgO-based pressure scale, which has been undertaken using experiment [16] and molecular dynamics simulations [17] based on an accurate representation of inter atomic interactions.

Calcium chalcogenides has the conversion from B1 to B2 phase, at 61 GPa and 135 GPa [18] respectively. Bhardwaj et al. investigated the high-pressure experiments on CaO and CaTe and showed that the heavy alkaline earth chalcogenides form the second largest group of the partially ionic crystals undergoing B1–B2 transition after alkali halides [19]. Calcium chalcogenides are the most important, because they have high cation and anion radius ratio in high phase transition pressures. The studies of these chalcogenides through far above the ground pressure have been performed experimentally, using x-ray diffraction method to show the phase transition from B1 to B2 phase [20] at 40, 38 and 33 GPa pressure for Ca sulfide, selenides and telluride respectively. This technique has also used to observe the phase transition and Pressure–Volume relationships in CaTe (Pt=35 GPa) and SrTe (Pt=12 GPa) [21] which have the smallest cation and anion radius ratio among the AEC [22]. The phase transition pressure has also been observed [21], [22], [23] to be in between 33–35 GPa in CaTe. The pressure in CaSe (8%) and CaTe (4%) has a volume fall down which is comparatively smaller. Due to such a smaller volume fall down these compounds have been approved as huge difference in their ionic radii and this is the reason that the repulsive force among the anions oppose volume fall down at the phase shift in CaSe and CaTe [20].

The strontium chalcogenides show technological importance in the luminescence devices, imaging, radiation dosimetry and infrared devices [24]. Pandey et al. investigated that the band gap of SrSe is direct [25]. Labidi et al. investigated that if the hetero structures of II–VI are known, then their quaternary ternary alloys with direct band gap can be find out. Majority of the alloys having high absorption coefficients used for the fabrication of thin film hetero junction photovoltaic devices [26].

At high-pressure the compounds (BaX, X=S, Se, Te) go through a phase shift from rock salt structure to wurtzite structure. Arya et al. investigated the first order phase transition and showed that the interesting phenomenon of these compounds is metallization which occur by further increasing the pressure. Under high-pressure the “d” state in the CB and “p” state in the valance band (VB) overlap with each other, causing metallization [4]. Barium chalcogenides are important for optoelectronic devices in the blue light wavelength region. Hassan et al. investigated that the metallization process occurs in the CsCl structure, if we increase pressure, the bandgaps of these materials will be decreased and the “d” CB drops to the “p” VB [9]. Hassan et al. also calculated the elastic parameters for these compounds in B1 and B2 structures [9].

In present work we have used (FP-LAPW) method within LDA, GGA, EV-GGA and mBJ-GGA for the calculation of structural, electronic and optical properties of IIA–VIA compounds.

Section snippets

Method of calculation

In present work, we have used the FP-LAPW method [27] within the framework of the density functional theory (DFT) [28] as implemented in the WIEN2K code [29] that has been shown to give reliable results for the structural, electronic and optical properties of various solids. The exchange-correlation potential for structural, electronic and optical properties was calculated by local density approximation (LDA) [30], PBEsol generalized gradient approximation (GGA) [31], Engel and Vosko scheme

Results and discussion

The calculated bandgaps of IIA–VIA compounds with LDA, GGA, EV-GGA and mBJ-GGA are given in Table 1. To review the accuracy of the designed bandgaps, its comparison with experimental results and other calculated values are also given in Table 1.

The beryllium chalcogenides have rock salt structure, in which “BeO” gives direct band gap in EV-GGA and mBJ-GGA approximation. It is clear from Table 1 that in BeSe compound both the conduction and valance band pinned at the Fermi level and there are no

Optical properties

The dielectric function of the electron gas, with its strong dependence on frequency has a significant effect on the physical properties of solids. It describes the collective excitations of the Fermi Sea, such as the volume and surface plasmons. The dielectric function depends on the electronic band structure of a crystal, and its investigation by optical spectroscopy is a powerful tool for the determination of the overall band behavior of a solid. It has two parts, real and imaginary [42]:ε(ω)

Conclusions

Theoretical study of structural, electronic and optical properties of IIA–VIA compounds are performed by “FP-LAPW” method, applying LDA, GGA, EV-GGA and mBJ approximations. These compounds have rock salt structure except MgS, MgSe and MgTe, which have wurtzite structure. The bandgaps nature of these materials is mostly indirect (Γ–X). It is concluded that mBJ is an efficient theoretical technique for the calculation of the band structures of IIA–VIA chalcogenides. The results expect that mBJ

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