Browsing by Author "Janotti, Anderson"
Now showing 1 - 18 of 18
Results Per Page
Sort Options
Item Band alignment and p-type doping of ZnSnN2(American Physical Society, 2017-05-31) Wang, Tianshi; Ni, Chaoying; Janotti, Anderson; Tianshi Wang, Chaoying Ni, and Anderson Janotti; Wang, Tianshi; Ni, Chaoying; Janotti, AndersonComposed of earth-abundant elements, ZnSnN2 is a promising semiconductor for photovoltaic and photoelectrochemical applications. However, basic properties such as the precise value of the band gap and the band alignment to other semiconductors are still unresolved. For instance, reported values for the band gap vary from 1.4 to 2.0 eV. In addition, doping in ZnSnN2 remains largely unexplored. Using density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional, we investigate the electronic structure of ZnSnN2, its band alignment to GaN and ZnO, and the possibility of p-type doping. We find that the position of the valence-band maximum of ZnSnN2 is 0.39 eV higher than that in GaN, yet the conduction-band minimum is close to that in ZnO, which suggests that achieving p-type conductivity is likely as in GaN, yet it may be difficult to control unintentional n-type conductivity as in ZnO. Among possible p-type dopants, we explore Li, Na, and K substituting on the Zn site. We show that while LiZn is a shallow acceptor, NaZn and KZn are deep acceptors, which we trace back to large local relaxations around the Na and K impurities due to the atomic size mismatches.Item Correct Implementation of Polarization Constants in Wurtzite Materials and Impact on III-Nitrides(American Physical Society, 2016-06-20) Dreyer, Cyrus E.; Janotti, Anderson; Van de Walle, Chris G.; Vanderbilt, David; Cyrus E. Dreyer, Anderson Janotti, Chris G. Van de Walle, and David Vanderbilt; Janotti, AndersonAccurate values for polarization discontinuities between pyroelectric materials are critical for understanding and designing the electronic properties of heterostructures. For wurtzite materials, the zincblende structure has been used in the literature as a reference to determine the effective spontaneous polarization constants. We show that, because the zincblende structure has a nonzero formal polarization, this method results in a spurious contribution to the spontaneous polarization differences between materials. In addition, we address the correct choice of “improper” versus “proper” piezoelectric constants. For the technologically important III-nitride materials GaN, AlN, and InN, we determine polarization discontinuities using a consistent reference based on the layered hexagonal structure and the correct choice of piezoelectric constants, and discuss the results in light of available experimental data.Item Crystal structure and electrical and optical properties of two-dimensional group-IV monochalcogenides(Physical Review B, 2023-08-07) Querne, Mateus B. P.; Bracht, Jean M.; Da Silva, Juarez L. F.; Janotti, Anderson; Lima, Matheus P.Two-dimensional (2D) semiconductor materials offer a platform for unconventional applications such as valleytronics, flexible nanoelectronics, and hosts of quantum emitters. Many of these materials and their electronic properties remain to be explored. Using ab initio simulations based on the density functional theory, we investigate group-IV monochalcogenides MQ (M=Si, Ge,Sn and Q=S, Se, Te), an emerging class of 2D materials, with two competing crystal structures: (i) phosphorenelike (Pmn21), which has already been synthesized, and (ii) SiTe-type (P¯3m1), which has been much less explored. Except for Sn, we find that the SiTe type is the lowest-energy structure and has higher structural stability, motivating efforts to synthesize this less explored P¯3m1 phase. Regarding the optoelectronic properties of these two phases, in the P¯3m1 phase, MQ compounds have band gaps around the sunlight spectrum peak and show narrower variations in band gap with the composition and higher absorption coefficients for lighter chalcogens. In contrast, in the Pmn21 phase, MQ compounds have wider band gaps and show a band gap variation of up to 72% with composition, higher absorption coefficients with Te atoms, and potential for valleytronics. In particular, SiS shows interesting high optical anisotropy among all the investigated materials. Furthermore, the optical spectra present peaks that are particular to each phase or composition, making the refractive index a distinguishing parameter for identifying the different MQ compounds. Finally, a phase transition from monolayer to bulk due to an interaction between the layers is observed. Thus, the present results straighten out the role of the crystalline phase in the optoelectronic properties of these monochalcogenides.Item Defects as qubits in 3C- and 4H-SiC(American Physical Society, 2015-07-20) Gordon, L.; Janotti, Anderson; Van de Walle, Chris G.; L. Gordon, A. Janotti, and C. G. Van de Walle; Janotti, AndersonWe employ hybrid density functional calculations to search for defects in different polytypes of SiC that may serve as qubits for quantum computing. We explore the divacancy in 4H- and 3C-SiC, consisting of a carbon vacancy adjacent to a silicon vacancy, and the nitrogen-vacancy (NV) center in 3C-SiC, in which the substitutional NC sits next to a Si vacancy (NC-VSi). The calculated excitation and emission energies of the divacancy in 4H-SiC are in excellent agreement with experimental data, and aid in identifying the four unique configurations of the divacancy with the four distinct zero-phonon lines observed experimentally. For 3C-SiC, we identify the paramagnetic defect that was recently shown to maintain a coherent quantum state up to room temperature as the spin-1 neutral divacancy. Finally, we show that the (NC-VSi)− center in 3C-SiC is highly promising for quantum information science, and we provide guidance for identifying this defect.Item Determination of the Mott-Hubbard gap in GdTiO3(American Physical Society, 2015-08-06) Bjaalie, L.; Verma, A.; Himmetoglu, B.; Janotti, Anderson; Raghavan, S.; Protasenko, V.; Steenbergen, E. H.; Jena, D.; Stemmer, S.; Van de Walle, Chris G.; L. Bjaalie, A. Verma, B. Himmetoglu, A. Janotti, S. Raghavan, V. Protasenko, E. H. Steenbergen, D. Jena, S. Stemmer, and C. G. Van de Walle; Janotti, AndersonThe band gaps of rare-earth titanates are commonly reported to be 0.2–0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report experimental and theoretical results on GdTiO3 (GTO) indicating that the gap is significantly larger. Photoluminescence (PL) measurements show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. First-principles calculations, based either on density-functional theory (DFT) with a hybrid functional or on DFT+U, consistently show that the gap is close to 2 eV. We also propose an interpretation of the previously reported optical absorption spectra. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials.Item First-principles study of surface charging in LaAlO3/SrTiO3 heterostructures(American Physical Society, 2015-08-19) Krishnaswamy, K.; Dreyer, C. E.; Janotti, Anderson; Van de Walle, Chris G.; K. Krishnaswamy, C. E. Dreyer, A. Janotti, and C. G. Van de Walle; Janotti, AndersonThe two-dimensional electron gas (2DEG) observed at the interface between LaAlO3 (LAO) and SrTiO3 (STO) is known to be very sensitive to the proximity of the LaAlO3 surface and the conditions to which the surface is exposed. We use first-principles calculations to study surface reconstructions on LAO films, taking into account that the LAO surface can be charged. The results for the charged surfaces and for the coupling between the surface and the 2DEG enable us to account not only for the behavior of the 2DEG as a function of thickness of the LAO layer, but simultaneously determine the stable terminations and reconstructions on the LAO surface under a variety of conditions. Our studies of charged surfaces are based on an extension of the methodology of A. Y. Lozovoi et al. [J. Chem. Phys. 115, 1661 (2001)]. From the calculated electronic structure of the unreconstructed (but relaxed) AlO2 and LaO surface terminations of LAO, we find surface states having excess holes (AlO2 termination) or excess electrons (LaO termination). This result is central to understanding the mechanism of 2DEG formation, and is consistent with a 2DEG of density 3.3 × 1014 cm−2 being intrinsic to the LaO-TiO2 interface in the LAO/STO system. We explore the effects of the Al-adatom, O-vacancy, and H-adatom surface reconstructions on the 2DEG density, and find that the stability of different reconstructions is tied to the thickness of the LAO layer as well as the surface exposure conditions. We find that including the effects of charging of the surface significantly stabilizes the AlO2 termination versus the LaO termination. Overall, our methodology has the advantage of decoupling first-principles calculations for the interface from those for the charged surface, and constitutes a general approach that can be applied to the commonly occurring problem of charge exchange between the surface and the interface of a thin film with a substrate, or between the surface and defects/impurities in the bulk of a material. DOI: 10.1103/PhysRevB.Item (InxGa1−x)2O3 alloys for transparent electronics(American Physical Society, 2015-08-31) Peelaers, Hartwin; Steiauf, Daniel; Varley, Joel B.; Janotti, Anderson; Van de Walle, Chris G.; Hartwin Peelaers, Daniel Steiauf, Joel B. Varley, Anderson Janotti, and Chris G. Van de Walle; Janotti, Anderson(InxGa1−x )2O3 alloys show promise as transparent conducting oxides. Using hybrid density functional calculations, band gaps, formation enthalpies, and structural parameters are determined for monoclinic and bixbyite crystal structures. In the monoclinic phase the band gap exhibits a linear dependence on alloy concentration, whereas in the bixbyite phase a large band-gap bowing occurs. The calculated formation enthalpies showthat the monoclinic structure is favorable for In compositions up to 50% and bixbyite for larger compositions. This is caused by In strongly preferring sixfold oxygen coordination. The formation enthalpy of the 50:50 monoclinic alloy is much lower than the formation enthalpy of the 50:50 bixbyite alloy and also lower than most monoclinic alloys with lower In concentration; these trends are explained in terms of local strain. Consequences for experiment and applications are discussed.Item Large Rashba spin splittings in bulk and monolayer of BiAs(Physical Review Materials, 2024-05-20) Zubair, Muhammad; Evangelista, Igor; Khalid, Shoaib; Medasani, Bharat; Janotti, AndersonThere is great interest in developing new materials with Rashba split bands near the Fermi level for spintronics. Using first-principles calculations, we predict BiAs as a semiconductor with large Rashba splitting in bulk and monolayer forms. Bulk BiAs has a layered crystal structure with two atoms in a rhombohedral primitive cell, derived from the structure of the parent Bi and As elemental phases. It is a narrow band gap semiconductor, and it shows a combination of Rashba and Dresselhaus spin splitting with a characteristic spin texture around the L point in the Brillouin zone of the hexagonal conventional unit cell. It has sizable Rashba energies and Rashba coupling constants in the valence and conduction bands at the band edges. The 2D monolayer of BiAs has a much larger band gap at Γ, with a circular spin texture characteristic of a pure Rashba effect. The Rashba energy and Rashba coupling constant of monolayer BiAs are large compared to other known 2D materials and rapidly increase under biaxial tensile strain.Item Light and microwave driven spin pumping across FeGaB–BiSb interface(Physical Review Materials, 2021-12-16) Sharma, Vinay; Wu, Weipeng; Bajracharya, Prabesh; To, Duy Quang; Johnson, Anthony; Janotti, Anderson; Bryant, Garnett W.; Gundlach, Lars; Jungfleisch, M. Benjamin; Budhani, Ramesh C.Three-dimensional (3D) topological insulators (TIs) with large spin Hall conductivity have emerged as potential candidates for spintronic applications. Here, we report spin to charge conversion in bilayers of amorphous ferromagnet (FM) Fe78Ga13B9 (FeGaB) and 3D TI Bi85Sb15 (BiSb) activated by two complementary techniques: spin pumping and ultrafast spin-current injection. DC magnetization measurements establish the soft magnetic character of FeGaB films, which remains unaltered in the heterostructures of FeGaB-BiSb. Broadband ferromagnetic resonance (FMR) studies reveal enhanced damping of precessing magnetization and large value of spin mixing conductance (5.03×1019m–2) as the spin angular momentum leaks into the TI layer. Magnetic field controlled bipolar DC voltage generated across the TI layer by inverse spin Hall effect is analyzed to extract the values of spin Hall angle and spin diffusion length of BiSb. The spin pumping parameters derived from the measurements of the femtosecond light-pulse-induced terahertz emission are consistent with the result of FMR. The Kubo-Bastin formula and tight-binding model calculations shed light on the thickness-dependent spin-Hall conductivity of the TI films, with predictions that are in remarkable agreement with the experimental data. Our results suggest that room temperature deposited amorphous and polycrystalline heterostructures provide a promising platform for creating novel spin orbit torque devices.Item Observation of a topologically non-trivial surface state in half-Heusler PtLuSb (001) thin films(Nature Publishing Group, 7/27/16) Logan,J. A.; Patel,S. J.; Harrington,S. D.; Polley,C. M.; Schultz,B. D.; Balasubramanian,T.; Janotti,A.; Mikkelsen,A.; Palmstrom,C. J.; J.A. Logan, S.J. Patel, S.D. Harrington, C.M. Polley, B.D. Schultz, T. Balasubramanian, A. Janotti, A. Mikkelsen & C.J. Palmstr�m; Janotti, AndersonThe discovery of topological insulators, materials with bulk band gaps and protected cross-gap surface states in compounds such as Bi2Se3, has generated much interest in identifying topological surface states (TSSs) in other classes of materials. In particular, recent theoretical calculations suggest that TSSs may be found in half-Heusler ternary compounds. If experimentally realizable, this would provide a materials platform for entirely new heterostructure spintronic Developmentices that make use of the structurally identical but electronically varied nature of Heusler compounds. Here we show the presence of a TSS in epitaxially grown thin films of the half-Heusler compound PtLuSb. Spin- and angle-resolved photoemission spectroscopy, complemented by theoretical calculations, reveals a surface state with linear dispersion and a helical tangential spin texture consistent with previous predictions. This experimental verification of topological behaviour is a significant step forward in establishing half-Heusler compounds as a viable material system for future spintronic Developmentices.Item Predicting band gaps and band-edge positions of oxide perovskites using density functional theory and machine learning(Physical Review B, 2022-10-28) Li, Wei; Wang, Zigeng; Xiao, Xia; Zhang, Zhiqiang; Janotti, Anderson; Rajasekaran, Sanguthevar; Medasani, BharatDensity functional theory (DFT) within the local or semilocal density approximations, i.e., the local density approximation (LDA) or generalized gradient approximation (GGA), has become a workhorse in the electronic structure theory of solids, being extremely fast and reliable for energetics and structural properties, yet remaining highly inaccurate for predicting band gaps of semiconductors and insulators. The accurate prediction of band gaps using first-principles methods is time consuming, requiring hybrid functionals, quasiparticle GW, or quantum Monte Carlo methods. Efficiently correcting DFT-LDA/GGA band gaps and unveiling the main chemical and structural factors involved in this correction is desirable for discovering novel materials in high-throughput calculations. In this direction, we use DFT and machine learning techniques to correct band gaps and band-edge positions of a representative subset of ABO3 perovskite oxides. Relying on the results of HSE06 hybrid functional calculations as target values of band gaps, we find a systematic band-gap correction of ∼1.5 eV for this class of materials, where ∼1eV comes from downward shifting the valence band and ∼0.5eV from uplifting the conduction band. The main chemical and structural factors determining the band-gap correction are determined through a feature selection procedure.Item Small polarons and point defects in barium cerate(American Physical Society, 2015-12-28) Swift, Michael; Janotti, Anderson; Van de Walle, Chris G.; Michael Swift, Anderson Janotti, and Chris G. Van de Walle; Janotti, AndersonBarium cerate (BaCeO3) is a well-known ionic conductor of both hydrogen and oxygen. In applications, it is frequently doped (for instance with Y) to increase stability and promote diffusion. However, the effects of doping and native defects are not fully understood. Computational studies have been stymied by the nature of the conduction band, which is made up of cerium 4f states. These states present a challenge to ab initio techniques based on density functional theory within the standard approximations for exchange and correlation. Using a hybrid functional, we investigate the effects of hydrogen impurities and native defects on the electrical and optical properties of BaCeO3. We discuss the tendency of excess electrons or holes to localize in the form of small polarons. We also explore the interactions of polarons with hydrogen impurities and oxygen vacancies, and their impact on luminescence properties.Item Structural and electronic properties of SrZrO3 and Sr(Ti,Zr)O3 alloys(American Physical Society, 2015-08-11) Weston, L.; Janotti, Anderson; Cui, X. Y.; Himmetoglu, B.; Stampfl, C.; Van de Walle, Chris G.; L. Weston, A. Janotti, X. Y. Cui, B. Himmetoglu, C. Stampfl, and C. G. Van de Walle; Janotti, AndersonUsing hybrid density functional calculations, we study the electronic and structural properties of SrZrO3 and ordered Sr(Ti,Zr)O3 alloys. Calculations were performed for the ground-state orthorhombic (Pnma) and high-temperature cubic (Pm3m) phases of SrZrO3. The variation of the lattice parameters and band gaps with Ti addition was studied using ordered SrTix Zr1−x O3 structures with x = 0, 0.25, 0.5, 0.75, and 1. As Ti is added to SrZrO3, the lattice parameter is reduced and closely follows Vegard’s law. On the other hand, the band gap shows a large bowing and is highly sensitive to the Ti distribution. For x = 0.5, we find that arranging the Ti and Zr atoms into a 1 × 1 SrZrO3/SrTiO3 superlattice along the [001] direction leads to interesting properties, including a highly dispersive single band at the conduction-band minimum (CBM), which is absent in both parent compounds, and a band gap close to that of pure SrTiO3. These features are explained by the splitting of the lowest three conduction-band states due to the reduced symmetry of the superlattice, lowering the band originating from the in-plane Ti 3dxy orbitals. The lifting of the t2g orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. Our results demonstrate how short-period SrZrO3/SrTiO3 superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk SrTiO3.Item Structural Phase Transitions between Layered Indium Selenide for Integrated Photonic Memory(Advanced Materials, 2022-04-18) Li, Tiantian; Wang, Yong; Li, Wei; Mao, Dun; Benmore, Chris J.; Evangelista, Igor; Xing, Huadan; Li, Qiu; Wang, Feifan; Sivaraman, Ganesh; Janotti, Anderson; Law, Stephanie; Gu, TingyiThe primary mechanism of optical memoristive devices relies on phase transitions between amorphous and crystalline states. The slow or energy-hungry amorphous–crystalline transitions in optical phase-change materials are detrimental to the scalability and performance of devices. Leveraging an integrated photonic platform, nonvolatile and reversible switching between two layered structures of indium selenide (In2Se3) triggered by a single nanosecond pulse is demonstrated. The high-resolution pair distribution function reveals the detailed atomistic transition pathways between the layered structures. With interlayer “shear glide” and isosymmetric phase transition, switching between the α- and β-structural states contains low re-configurational entropy, allowing reversible switching between layered structures. Broadband refractive index contrast, optical transparency, and volumetric effect in the crystalline–crystalline phase transition are experimentally characterized in molecular-beam-epitaxy-grown thin films and compared to ab initio calculations. The nonlinear resonator transmission spectra measure of incremental linear loss rate of 3.3 GHz, introduced by a 1.5 µm-long In2Se3-covered layer, resulted from the combinations of material absorption and scattering.Item Thermal transport across metal silicide-silicon interfaces: An experimental comparison between epitaxial and nonepitaxial interfaces(American Physical Society, 2017-02-22) Ye, Ning; Feser, Joseph P.; Sadasivam, Sridhar; Fisher, Timothy S.; Wang, Tianshi; Ni, Chaoying; Janotti, Anderson; Ning Ye, Joseph P. Feser, Sridhar Sadasivam, Timothy S. Fisher, Tianshi Wang, Chaoying Ni, and Anderson Janotti; Ye, Ning; Feser, Joseph P.; Wang, Tianshi; Ni, Chaoying; Janotti, AndersonSilicides are used extensively in nano- and microdevices due to their low electrical resistivity, low contact resistance to silicon, and their process compatibility. In this work, the thermal interface conductance of TiSi2, CoSi2, NiSi, and PtSi are studied using time-domain thermoreflectance. Exploiting the fact that most silicides formed on Si(111) substrates grow epitaxially, while most silicides on Si(100) do not, we study the effect of epitaxy, and show that for a wide variety of interfaces there is no dependence of interface conductance on the detailed structure of the interface. In particular, there is no difference in the thermal interface conductance between epitaxial and nonepitaxial silicide/silicon interfaces, nor between epitaxial interfaces with different interface orientations.While these silicide-based interfaces yield the highest reported interface conductances of any known interface with silicon, none of the interfaces studied are found to operate close to the phonon radiation limit, indicating that phonon transmission coefficients are nonunity in all cases and yet remain insensitive to interfacial structure. In the case of CoSi2, a comparison ismade with detailed computational models using (1) full-dispersion diffuse mismatch modeling (DMM) including the effect of near-interfacial strain, and (2) an atomistic Green’ function (AGF) approach that integrates near-interface changes in the interatomic force constants obtained through density functional perturbation theory. Above 100 K, the AGF approach significantly underpredicts interface conductance suggesting that energy transport does not occur purely by coherent transmission of phonons, even for epitaxial interfaces. The full-dispersion DMM closely predicts the experimentally observed interface conductances for CoSi2, NiSi, and TiSi2 interfaces, while it remains an open question whether inelastic scattering, cross-interfacial electron-phonon coupling, or other mechanisms could also account for the high-temperature behavior. The effect of degenerate semiconductor dopant concentration onmetal-semiconductor thermal interface conductance was also investigated with the result that we have found no dependencies of the thermal interface conductances up to (n or p type) ≈1 × 1019 cm−3, indicating that there is no significant direct electronic transport and no transport effects that depend on long-range metal-semiconductor band alignment.Item Tuning bad metal and non-Fermi liquid behavior in a Mott material: Rare-earth nickelate thin films(American Association for the Advancement of Science (AAAS), 2015-11-06) Mikheev, Evgeny; Hauser, Adam J.; Himmetoglu, Burak; Moreno, Nelson E.; Janotti, Anderson; Van de Walle, Chris G.; Stemmer, Susanne; Evgeny Mikheev, Adam J. Hauser, Burak Himmetoglu, Nelson E. Moreno, Anderson Janotti, Chris G. Van de Walle, Susanne Stemmer; Janotti, AndersonResistances that exceed the Mott-Ioffe-Regel limit (known as bad metal behavior) and non-Fermi liquid behavior are ubiquitous features of the normal state of many strongly correlated materials. We establish the conditions that lead to bad metal and non-Fermi liquid phases in NdNiO3, which exhibits a prototype bandwidth-controlled metalinsulator transition. We show that resistance saturation is determined by the magnitude of Ni eg orbital splitting, which can be tuned by strain in epitaxial films, causing the appearance of bad metal behavior under certain conditions. The results shed light on the nature of a crossover to a non-Fermi liquid metal phase and provide a predictive criterion for Anderson localization. They elucidate a seemingly complex phase behavior as a function of film strain and confinement and provide guidelines for orbital engineering and novel devices.Item Tuning Excitonic Properties of Monochalcogenides via Design of Janus Structures(Journal of Physical Chemistry C, 2024-07-25) Querne, Mateus B. P.; Dias, Alexandre C.; Janotti, Anderson; Da Silva, Juarez L. F.; Lima, Matheus P.Two-dimensional (2D) Janus structures offer a unique range of properties as a result of their symmetry breaking, resulting from the distinct chemical composition on each side of the monolayers. Here, we report a theoretical investigation of 2D Janus Q′A′AQ P3m1 monochalcogenides from group IV (A and A′ = Ge and Sn; Q, Q′ = S and Se) and 2D non-Janus QAAQ P3̅m1 counterparts. Our theoretical framework is based on density functional theory calculations combined with maximally localized Wannier functions and tight-binding parametrization to evaluate the excitonic properties. The phonon band structures exhibit exclusively real (nonimaginary) branches for all materials. Particularly, SeGeSnS has greater energetic stability than its non-Janus counterparts, representing an outstanding energetic stability among the investigated materials. However, SGeSnS and SGeSnSe have higher formation energies than the already synthesized MoSSe, making them more challenging to grow than the other investigated structures. The electronic structure analysis demonstrates that materials with Janus structures exhibit band gaps wider than those of their non-Janus counterparts, with the absolute value of the band gap predominantly determined by the core rather than the surface composition. Moreover, exciton binding energies range from 0.20 to 0.37 eV, reducing band gap values in the range of 21% to 32%. Thus, excitonic effects influence the optoelectronic properties more than the point-inversion symmetry breaking inherent in the Janus structures; however, both features are necessary to enhance the interaction between the materials and sunlight. We also found anisotropic behavior of the absorption coefficient, which was attributed to the inherent structural asymmetry of the Janus materials.Item Untangling the effects of octahedral rotation and ionic displacements on the electronic structure of BiFeO3(Physical Review B, 2021-07-29) Laraib, Iflah; Carneiro, Marciano A.; Janotti, AndersonThe electronic structure and related properties of perovskites ABO3 are strongly affected by even small modifications in their crystalline structure. In the case of BiFeO3, variations in the octahedral rotations and ionic displacements lead to significant changes in the band gap. This effect can possibly explain the wide range of values (2.5–3.1 eV) reported in the literature, obtained from samples of varied structural qualities, including polycrystalline films, epitaxial films grown by pulsed-laser deposition and molecular beam epitaxy, nanowires, nanotubes, and bulk single crystals. Using hybrid density-functional calculations, we investigate the dependence of the electronic structure on the crystal lattice distortions of the ferroelectric-antiferromagnetic BiFeO3, disentangling the effects of the ferroelectric ionic displacements and the antiferrodistortive octahedral rotations on the band gap and the band-edge positions. The band gap is shown to vary from 3.39 eV for the rhombohedral ground-state (R3c) structure down to 1.58 eV for the perfect cubic (Pm¯3m) structure, with changes in the conduction band being much more prominent than in the valence band. The gap varies linearly with the ferroelectric ionic displacements, but nonlinearly with the octahedral rotations around the pseudocubic [111]c axis, and this is explained in terms of the different interactions between Bi 6s,6p, Fe 3d, and O 2p bands. We argue that such large variation of the band gap with structural changes may well explain the large scattering of the reported values, especially if significant deviations from the equilibrium crystal structure are found near domain boundaries, extended defects, or grain boundaries in polycrystalline films.