First-principle calculations of electronic and positronic properties of AlGaAs2
Introduction
Isovalent ternary semiconductors AlxGa1−xAs, formed by mixing the zinc-blende (ZB) constituents AlAs and GaAs, constitute a group of technologically important materials for several photonic devices (laser, light-emitting diodes, and photo-voltaic converters) [1]. The GaAs is the most technologically important and the most studied material [2]. AlAs is an important electronic and optoelectronic material [3], [4], [5]. Extensive experimental studies on these materials are available [6]. Ternary AlGaAs2 semiconductors represent part of Pnictides family type III–III–V2 compounds, which crystallize in the tetragonal space group . Even though some studies concerning several kinds of ordered structures in the III–V ternary semiconductors, such as CuAu [7], [8], [9] chalcopyrite [10], [11], [12] and CuPt [13], [14] types have been reported.
The positrons annihilation in solids is a useful probe to obtain information about the momentum distribution of electrons in a material [15]. It is an important tool for investigating the electronic structure of metals and for understanding the behavior of electrons in crystals. However, to understand the results of positron annihilation, it is necessary to have an insight about how positrons will annihilate in semiconductors by means of electron and positron distributions for two different kinds of semiconductors. The investigations of the electronic properties of solids using electron and positron charge densities are of increasing importance for semiconductors and their alloys. So far, this investigation has been concerned with the electronic charge densities which were useful for understanding chemical bonds and the modification of band structures by interstitial impurities [16], [17]. Positron charge densities provide complementary information; they have been used quite extensively in a variety of materials [18], [19]. The great success of recent developments in this field motivated us to look for a better understanding of the charge densities. The foundation for the modern electronic structure calculations of solids is the density functional theory (DFT) based on the work by Hohenberg and Kohn [21] and by Kohn and Sham [22]. During the last two decades, ab initio methods have been developed rapidly. Nowadays, most of the important basic properties of solids, such as the structure and cohesion, can be calculated by ab initio methods and without adjustment for experimental results [29].
In this work, we have studied the structural, electronic, and positronic properties of AlGaAs2 semiconductor using the full potential linearized augmented plane wave (FPLAPW) method [20] based on DFT [21], [22], [23] within a local density approximation (LDA) [24], as implemented in WIEN2k code [25]. The organization of this paper is as follows: Section 2 deals with the CuAu structure, and Section 3 describes the theoretical procedure adopted to obtain the band structures and total energies. In Section 4, we will present the structural properties, where we have used a full total energy minimization by obtaining, firstly, the equilibrium c/a ratio; and secondly, the equilibrium volume, bulk modulus, and the bulk modulus derivative for the calculated c/a ratio. In Section 5, we will illustrate the electronic and positron band structures, the density of states, and the charge densities. Section 6 shows our conclusions.
We have already mentioned that, the ABC2 semiconductor crystallizes in the CuAu-I (CA) structure, which is related to the ZB structure. It has nearly the same arrangement of anions but differs in the ordered distribution of the cations, which makes the unit cell tetragonal with the c-axis about twice the a-axis of the ZB type unit cell. Each anion is coordinated by one A and B cation, while each cation is tetrahedrally coordinated by two anions [26]. The atomic positions are A (0, 0, 0), B , C (0, , η u) and (, 0, η (1—u)), where η is the c/a ratio and a and c are the lattice parameters. Note that the ideal η value for ZB structure is 1, and u is the internal distortion parameter for C atoms. There are only two different nearest-neighbors bond lengths in the alloy:andFor a set of a, η, and u values, the total energy is calculated (see Section 3).
Section snippets
Calculational methodology
Self-consistent calculations of the total energies and electronic structures based on the non-scalar relativistic full-potential (FP) ‘‘linearized augmented plane wave+local orbitals’’ (LAPW+lo) method were carried out using the WIEN2k packages [25]. This is a very accurate and efficient scheme to solve the Kohn-Sham equations of density functional theory (DFT) in which the exchange and correlation effects are treated, within LDA [24]. The electron density is obtained by summing over all
Results and discussion
Since the electronic properties of AlGaAs2 are strongly dependent on the internal parameter u, its determination from the total energy is necessary to understand all the other physical properties. In this way, we have used the total energy approach to determine the internal parameter u and the c/a ratio. We have fixed these two equilibrium parameters for calculating the equilibrium volume using Murnaghan's equation of state [28]. Various attempts have been made to systematize the internal anion
Conclusion
We have performed first-principle calculations of the electronic and positron band structures of AlGaAs2 alloy. The structural properties of the ternary AlGaAs2 alloy are in good agreement with other theoretical works and with available experimental data. The electronic band structures obtained with the FPLAPW method agree with other theoretical calculations and, for both cases, the energy gap is underestimated compared to the experimental data. From our calculations, AlGaAs2 has a
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