SelfConsistent Hybrid Functional Calculations: Implications for Structural, Electronic, and Optical Properties of Oxide Semiconductors
 Daniel Fritsch^{1}Email authorView ORCID ID profile,
 Benjamin J. Morgan^{1} and
 Aron Walsh^{1}
DOI: 10.1186/s1167101617799
© The Author(s) 2017
Received: 19 July 2016
Accepted: 9 December 2016
Published: 6 January 2017
Abstract
The development of new exchangecorrelation functionals within density functional theory means that increasingly accurate information is accessible at moderate computational cost. Recently, a newly developed selfconsistent hybrid functional has been proposed (Skone et al., Phys. Rev. B 89:195112, 2014), which allows for a reliable and accurate calculation of material properties using a fully ab initio procedure. Here, we apply this new functional to wurtzite ZnO, rutile SnO_{2}, and rocksalt MgO. We present calculated structural, electronic, and optical properties, which we compare to results obtained with the PBE and PBE0 functionals. For all semiconductors considered here, the selfconsistent hybrid approach gives improved agreement with experimental structural data relative to the PBE0 hybrid functional for a moderate increase in computational cost, while avoiding the empiricism common to conventional hybrid functionals. The electronic properties are improved for ZnO and MgO, whereas for SnO_{2} the PBE0 hybrid functional gives the best agreement with experimental data.
Keywords
Density functional theory Hybrid functionals Semiconducting oxides Dielectric functionsPACS Codes
71.15.Mb 71.20.Nr 78.20.BhBackground
Metal oxides exhibit many unique structural, electronic, and magnetic properties, making them useful for a broad range of technological applications. Metal oxides are exclusively used as transparent conducting oxides (TCOs) [1], find applications as building blocks in artificial multiferroic heterostructures [2] and as spinfilter devices [3], and even include a huge class of superconducting materials. To develop new materials for specific applications, it is necessary to have a detailed understanding of the interplay between the chemical composition of different materials, their structure, and their electronic, optical, or magnetic properties.
For the development of new functional oxides, computational methods that allow theoretical predictions of structural and electronic properties have become an increasingly useful tool. When optical or electronic properties are under consideration, electronic structure methods are necessary, with the most popular approach for solids being density functional theory (DFT). DFT has proven hugely successful in the calculation of structural properties of condensed matter systems and the electronic properties of simple metals [4]. The earliest developed approximate exchangecorrelation functionals, however, face limitations, for example severely underestimating band gaps of semiconductors and insulators.
Over the last decade, several new, more accurate, exchangecorrelation functionals have been proposed. Increased predictive accuracy often comes with an increased computational cost, and the adoption of these more accurate functionals has only been made possible through the continued increase in available computational power. One such more accurate, and more costly, approach is to use socalled hybrid functionals. These are constructed by mixing a fraction of HartreeFock exactexchange with the exchange and correlation terms from some underlying DFT functional. Calculated material properties, such as lattice parameters and band gaps, however depend on the precise proportion of HartreeFock exactexchange, α. Typical hybrid functionals treat α as a fixed empirical parameter, chosen by intuition and experimental calibration. A recently proposed selfconsistent hybrid functional approach for condensed systems [5] avoids this empiricism and allows parameterfree hybrid functional calculations to be performed. In this approach, the amount of HartreeFock exactexchange is identified as the inverse of the dielectric constant, with this constraint achieved by performing an iterative sequence of calculations to selfconsistency.
Here we apply this new selfconsistent hybrid functional to wurtzite ZnO and rutile SnO_{2}, both materials with potential applications as TCOs, and MgO, a wide band gap insulator [6]. We examine the implications of the selfconsistent hybrid functional for the structural, electronic, and optical properties. In the next section, we present the theoretical background, describe the selfconsistent hybrid functional, and give the computational details. We then present results for the structural, electronic, and optical properties for ZnO, SnO_{2}, and MgO, and compare these to data calculated using alternative exchangecorrelation functionals and from experiments. The paper concludes with a summary and an outlook.
Methods
Density functional theory and hybrid functionals
The Hartree potential, v _{H}(r), and the external potential, v _{ext}(r), are in principle known. The exchangecorrelation potential, \(v_{\text {xc}}(\vec {r},\vec {r}^{\prime })\), however, is not and must be approximated. Most successful early approximations make use of the local density approximation and the semilocal generalised gradient approximation (GGA), for example, in the parametrisation of Perdew, Burke, and Ernzerhof (PBE) [7]. These approximations already allowed reliable descriptions of structural properties within the computational resources available at the time, but lacked accuracy when determining band energies, especially fundamental band gaps, and d valence band widths of semiconductors. These properties are particularly important for reliable calculations of electronic and optical behaviours of semiconductors.
In recent years, socalled hybrid functionals have gained in popularity. In a hybrid functional, some proportion of the local exchangecorrelation potential is replaced by HartreeFock exactexchange terms, giving a better description of electronic properties. The explicit inclusion of exactexchange HartreeFock terms make these calculations computationally much more demanding compared to the earlier GGA calculations, and hybrid functional calculations have become routine only in recent years. The fraction of HartreeFock exactexchange admixed in these hybrid functionals, α, is usually justified on experimental or theoretical grounds, and then fixed for a specific functional. This adds an empirical parameter and forfeits the ab initio nature of the calculations. One popular choice of α=0.25 is realised in the PBE0 functional [8].
which may then be computed in a selfconsistent cycle [5, 12]. This iteration to selfconsistency requires additional computational effort, but removes the empiricism of previous hybrid functionals and restores the ab initio character of the calculations. The utility of this approach, however, depends on the accuracy of the resulting predicted material properties. Here, we are interested in the implications for the structural, electronic, and optical properties of oxide semiconductors, and consider ZnO, SnO_{2}, and MgO as an illustrative set of materials.
Computational Details
The calculations presented in this work have been performed using the projectoraugmented wave (PAW) method [13], as implemented in the Vienna ab initio simulation package (VASP 5.4.1) [14–16]. For the calculation of structural and electronic properties, standard PAW potentials supplied with VASP were used, with 12 valence electrons for Zn atom (4s ^{2}3d ^{10}), 14 valence electrons for Sn (5s ^{2}4d ^{10}5p ^{2}), 8 valence electrons for Mg (2p ^{6}3s ^{2}), and 6 valence electrons for O (2s ^{2}2p ^{4}), respectively. When calculating dielectric functions, we have used the corresponding GW potentials, which give a better description of highenergy unoccupied states.
To evaluate the performance of the selfconsistent hybrid approach, we have calculated structural and electronic data using three functionals: GGA in the PBE parametrisation [7], the hybrid functional PBE0 [8], and the selfconsistent hybrid functional [5], which we denote scPBE0.
Structural relaxations were performed for the regular unit cells within a scalarrelativistic approximation, using dense k point meshes for Brillouin zone integration (8×8×6 for wurtzite ZnO, 6×6×8 for rutile SnO_{2}, and 10 × 10 × 10 for rocksalt MgO). For each material, we performed several fixedvolume calculations, in the cases of ZnO and SnO_{2} allowing internal structural parameters to relax until all forces on ions were smaller than 0.001 eV Å^{−1}. Zeropressure geometries were determined by then fitting a cubic spline to the total energies with respect to the unit cell volumes.
To evalutate the selfconsistent fraction of HartreeFock exactexchange, α, the dielectric function ε _{ ∞ } is calculated in an iterative series of full geometry optimisations. To calculate ε _{ ∞ }, for each of the ground state structures, the static dielectric tensor has been calculated (including local field effects) from the response to finite electric fields. For noncubic systems (ZnO, SnO_{2}), ε _{ ∞ } was obtained by averaging over the spur of the static dielectric tensor \(\frac {1}{3}\left (2\epsilon _{\infty }^{\perp }+\epsilon _{\infty }^{\parallel }\right)\). We have considered ε _{ ∞ } to be converged when the difference between two subsequent calculations falls below ±0.01 [5].
Results and Discussion
Structural properties
For our scPBE0 calculations, for each material, the static dielectric constant converged in three iterations (Fig. 1, lower panels). Here, computationally the most expensive part is the full geometry optimisation using the PBE0 functional. Each subsequent step in the selfconsistent loop to determine the amount of HartreeFock exactexchange starts from optimised crystal structures of the previous step and reduces the computational costs considerably.
Ground state structural parameters for wurtzite ZnO, rutile SnO_{2}, and rocksalt MgO obtained with different approximations for the exchangecorrelation potential in comparison to lowtemperature experimental data
ZnO  GGA  PBE0  scPBE0  Exp. 
a [Å]  3.289  3.258  3.255  3.248 [20] 
c [Å]  5.308  5.236  5.230  5.204 [20] 
u  0.381  0.381  0.381  0.382 [21] 
ε _{ ∞ }  5.01  3.64  3.58  3.72 [22] 
α  −  0.25  0.28  − 
E _{gap} [eV]  0.715  3.132  3.425  3.4449 [17] 
SnO_{2}  GGA  PBE0  scPBE0  Exp. 
a [Å]  4.834  4.757  4.752  4.737 [23] 
c [Å]  3.244  3.193  3.190  3.186 [23] 
u  0.307  0.306  0.306  0.307 [23] 
ε _{ ∞ }  4.71  3.76  3.72  3.92 [24] 
α  −  0.25  0.27  − 
E _{gap} [eV]  0.609  3.591  3.827  3.596 [18] 
MgO  GGA  PBE0  scPBE0  Exp. 
a [Å]  4.260  4.211  4.193  4.199 [25] 
ε _{ ∞ }  3.22  2.98  2.90  3.01 [26] 
α  −  0.25  0.34  − 
E _{gap} [eV]  4.408  7.220  8.322  7.833 [19] 
Electronic and Optical Properties
The scPBE0 calculations provide accurate structural properties and band gaps versus experimental data, and we can therefore be relatively confident when calculating properties less easily accessible directly by experiment. We have calculated the real (ε _{1}) and imaginary (ε _{2}) parts of the dielectric functions via Fermi’s Golden rule summing over transition matrix elements. For these calculations, we used the recommended VASP GW pseudopotentials and considerably increased the number of empty bands to ensure converged results.
Conclusions
We have presented a theoretical investigation on the application of a new selfconsistent hybrid functional to oxide semiconductors ZnO, SnO_{2}, and MgO. We have presented and compared calculated structural, electronic, and optical properties of these oxides to experimental data, and have discussed the implications of using the new selfconsistent hybrid functional. We find that the selfconsistent hybrid functional gives calculated properties with accuracies as good as or better than the PBE0 hybrid functional. The additional computational cost due to the selfconsistency cycle is justified by avoiding the empiricism of similar hybrid functionals, which restores the ab initio character of these calculations.
Abbreviations
 DFT:

Density functional theory
 GGA:

Generalised gradient approximation
 PAW:

Projectoraugmented wave
 PBE:

Perdew, Burke, and Ernzerhof
 TCOs:

Transparent conducting oxides
 VASP:

Vienna ab initio simulation package
Declarations
Acknowledgements
This research has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 641864 (INREP). This work made use of the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk) via the membership of the UK’s HPC Materials Chemistry Consortium, funded by EPSRC (EP/L000202) and the Balena HPC facility of the University of Bath. BJM acknowledges support from the Royal Society (UF130329).
Authors’ contributions
AW and BJM conceived the idea. DF performed the calculations, analysed the data, and wrote the manuscript. DF, BJM, and AW participated in the discussions of the calculated results. All the authors have read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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