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Optical and Electronic Properties of WO3 and Zn Chalcogenides Alloys: A Theoretical study
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0001-9975-7131
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [sv]

I denna avhandling analyseras optiska och elektroniska egenskaper hos WO3 och Zn-relaterade legeringar med hjälp av täthetsfunktionalteori (DFT). Metoder som går utöver DFT, såsom GW-approximationen och hybridfunktionaler, används för att minimera det fel som genereras av det smala bandgapet som erhålls med konventionella DFT-funktionaler. WO3 har sex olika stabila termodynamiska faser i olika temperaturintervall. En triklinisk till monoklinisk fasövergång sker nära rumstemperatur, och därför innehåller experimentprover ofta båda faserna. Beräkningar av dessa två strukturer visar likheter i absorption och bandstruktur, med en liten skillnad på 0,1 eV vid påbörjan av absorption. Detta värde är relaterat till skillnaden i bandgap mellan de två faserna. Den monokliniska fasen vid låg temperatur uppvisar en annan banddispersion och ett bredare bandgap som påverkar huvudsakligen absorptionsstarten. Modellering av volframvakanser i superceller av WO3 uppvisar magnetiska moment i vissa kristallfaser, varvid effekten är starkare i strukturer med låg symmetri, nämligen de trikliniska och monokliniska. Det magnetiska momentet uppstår från de oparade elektronerna från syreatomer i anslutning till vakansen. Denna effekt är emellertid lokaliserad och genererar inte en hålmedierad ferromagnetisk fas i materialet. Studien av zinklegeringar utförs med superceller för att nå en önskad mix av grundämnen. För Zn(O,S)- och Zn(O,Se)-legeringar sker väsentliga minskningar av bandgapet med ∼1 eV vid koncentrationer nära 50%. För att beskriva beteendet hos bandgapet hos dessa legeringar föreslogs ett tillvägagångssätt som kombinerar två olika metodologier, där regionen nära binärerna beskrivs av bandets antikorsningsmodell, medan mellanregionen representerar legeringsbandets böjningsmodell. ZnO-GaN-legeringar visar också en bandgapsböjning och resultaten som erhållits genom beräkningarna är i god överensstämmelse med experimentella observationer. ZnTe uppvisar ett mellanband när det är dopat med en III-nitridförening, såsom GaN, AlN och InN. Denna effekt tros vara resultatet av resonansen mellan ZnTe-tillstånden och tillstånden härstammande från dopningselementen.

Abstract [en]

In this thesis, optical and electronic properties of WO3 and Zn related alloys are analyzed by means of density functional theory (DFT). Beyond-DFT methods such as the GW approximation and hybrid functional are employed in order to minimize the error generated by the low band gap obtained with conventional DFT functionals. WO3 has six different thermodynamic stable phases in different temperature regions. A triclinic to monoclinic transition occurs near room temperature, and therefore experimental samples often contain both phases. Calculations of these two structures show similarities in absorption and band structure, with a small difference of 0.1 eV between the absorption onset. This value is related to the band gap difference between the two phases. The low temperature monoclinic phase presents a different band dispersion and a wider band gap, affecting mainly the absorption onset. In the three cases, the joint density of states have onsets in a lower energy when compared to the absorption due to forbidden transitions at low photon energies. Modeling tungsten vacancies in supercells of WO3 reveals magnetic moments in some the crystalline phases, with the effect being stronger in the low symmetry structures, triclinic and monoclinic. The magnetic moment arises from the unpaired electrons of oxygen atoms adjacent to the vacancy. This effect, however, is localized and does not generate a hole-mediated ferromagnetic phase in the material. The study of zinc alloys is performed with supercells to reach the desired mixing of elements. For Zn(O,S) and Zn(O,Se) alloys, substantial reductions in the band gap by ∼1 eV are found for concentrations close to 50%. To describe the band gap behavior of these alloys, an approach combining two different methodologies was suggested, where the region close to the binaries is described by the band anti-crossing model, while the intermediate region is represented by the alloy band bowing model. ZnO-GaN alloys also display a band gap bowing and the results obtained by the calculations are in good agreement with experimental observations. ZnTe exhibits an intermediate band when doped with a III-nitrides compound, such as GaN, AlN and InN. This effect is believed to be the result of the resonance between the ZnTe states and the states originated from the dopants.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2020. , p. 188
Series
TRITA-ITM-AVL ; 2019:37
National Category
Condensed Matter Physics
Research subject
Physics, Material and Nano Physics
Identifiers
URN: urn:nbn:se:kth:diva-265539ISBN: 978-91-7873-394-1 (print)OAI: oai:DiVA.org:kth-265539DiVA, id: diva2:1377894
Public defence
2020-01-24, Kollegiesallen, Brinellvägen 8, Stockholm, 09:30 (English)
Opponent
Supervisors
Available from: 2019-12-20 Created: 2019-12-12 Last updated: 2019-12-20Bibliographically approved
List of papers
1. Evidence of defect band mechanism responsible for band gap evolution in (ZnO)(1-x)(GaN)(x) alloys
Open this publication in new window or tab >>Evidence of defect band mechanism responsible for band gap evolution in (ZnO)(1-x)(GaN)(x) alloys
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 16, article id 165201Article in journal (Refereed) Published
Abstract [en]

It is known that (ZnO)(1-x)(GaN)(x) alloys demonstrate remarkable energy band bowing, making the material absorb in the visible range, in spite of the binary components being classical wide band gap semiconductors. However, the origin of this bowing is not settled; two major mechanisms are under debate: Influence of the orbital repulsion and/or formation of a defect band. In the present work, we applied a combination of the absorption and emission measurements on the samples exhibiting an outstanding nanoscale level of (ZnO)(1-x)(GaN)(x) homogeneity as monitored by the high resolution electron microscopy equipped with the energy dispersive x-ray analysis and the electron energy loss spectroscopy; moreover the experimental data were set in the context of the computational analysis of the alloys employing density functional theory and quasiparticle GW approximation. A prominent discrepancy in the band gap values as deduced from the absorption and emission experiments was observed systematically for the alloys with different compositions and interpreted as evidence for the absorption gap shrinking due to the defect band formation. Computational data support the argument, revealing only minor variations in the bulk of the conduction and valence band structures of the alloys, except for a characteristic "tail" in the vicinity of the valence band maximum. As such, we conclude that the energy gap bowing in (ZnO)(1-x)(GaN)(x) alloys is due to the defect band formation, presumably at the top of the valence band maximum.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-262950 (URN)10.1103/PhysRevB.100.165201 (DOI)000489037500006 ()2-s2.0-85073711420 (Scopus ID)
Note

QC 20191203

Available from: 2019-12-03 Created: 2019-12-03 Last updated: 2019-12-12Bibliographically approved
2. Electronic and optical properties of nanocrystalline WO3 thin films studied by optical spectroscopy and density functional calculations
Open this publication in new window or tab >>Electronic and optical properties of nanocrystalline WO3 thin films studied by optical spectroscopy and density functional calculations
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2013 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 20, p. 205502-Article in journal (Refereed) Published
Abstract [en]

The optical and electronic properties of nanocrystalline WO3 thin films prepared by reactive dc magnetron sputtering at different total pressures (P-tot) were studied by optical spectroscopy and density functional theory (DFT) calculations. Monoclinic films prepared at low P-tot show absorption in the near infrared due to polarons, which is attributed to a strained film structure. Analysis of the optical data yields band-gap energies E-g approximate to 3.1 eV, which increase with increasing P-tot by 0.1 eV, and correlate with the structural modifications of the films. The electronic structures of triclinic delta-WO3, and monoclinic gamma- and epsilon-WO3 were calculated using the Green function with screened Coulomb interaction (GW approach), and the local density approximation. The delta-WO3 and gamma-WO3 phases are found to have very similar electronic properties, with weak dispersion of the valence and conduction bands, consistent with a direct band-gap. Analysis of the joint density of states shows that the optical absorption around the band edge is composed of contributions from forbidden transitions (>3 eV) and allowed transitions (>3.8 eV). The calculations show that E-g in epsilon-WO3 is higher than in the delta-WO3 and gamma-WO3 phases, which provides an explanation for the P-tot dependence of the optical data.

Keywords
Microscopic Surface-Roughness, Augmented-Wave Method, Tungsten-Oxide, Phase-Transitions, Crystal-Structure, Smart Windows, Trioxide, Electrochromics, Photoemission, Dependence
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-123618 (URN)10.1088/0953-8984/25/20/205502 (DOI)000318556100013 ()2-s2.0-84877635597 (Scopus ID)
Funder
Swedish Research CouncilSwedish Energy Agency
Note

QC 20130614

Available from: 2013-06-14 Created: 2013-06-13 Last updated: 2019-12-12Bibliographically approved
3. Optical band-gap determination of nanostructured WO3 film
Open this publication in new window or tab >>Optical band-gap determination of nanostructured WO3 film
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2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 6Article in journal (Refereed) Published
Abstract [en]

The optical band-gap energy of a nanostructured tungsten trioxide film is determined using the photoacoustic spectroscopy method under continuous light excitation. The mechanism of the photoacoustic signal generation is discussed. The band-gap energy is also computed by other methods. The absorption coefficient as well as the band-gap energy of three different crystal structures of tungsten trioxide is calculated by a first-principles Green's function approach using the projector augmented wave method. The theoretical study indicates that the cubic crystal structure shows good agreement with the experimental data.

Keywords
energy gap, light absorption, nanostructured materials, photoacoustic, spectroscopy, thin films, tungsten compounds, augmented-wave method, tungsten-oxide, energy
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-19208 (URN)10.1063/1.3313945 (DOI)000274516900023 ()2-s2.0-76749121539 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20110126Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2019-12-12Bibliographically approved
4. Nitride-doped zinc telluride for intermediate band optoelectronics
Open this publication in new window or tab >>Nitride-doped zinc telluride for intermediate band optoelectronics
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We explore ZnTe:Y co-doped with Y = BN, AlN, GaN, and InN as potential intermediate band (IB) semiconductor for optoelectronic applications, employing the hybrid exchange-correlation functional within the density functional theory. The four nitride dopants behave fairly similar in ZnTe, and the main trend is that the lighter group-III cations yield valence and conduction bands with somewhat less dispersion. The theoretical analyses reveal that Ga‒N doping implies proper balance between localized inter-dopant interaction in resonance with host conduction band, creating a well-defined IB with the width Δw ≈ 0.1 eV and the band gap energies Eg ≈ 1.3/1.6 eV and Eg2 ≈ 0.9/0.6 eV. The cation Ga donor induces the IB, while the anion N acceptor regulates the Fermi level, and together they form GaN pairs and Ga2N complexes but avoid further clustering. The absorption coefficient α(ω) exhibits proper optical efficiency for IB solar energy applications. ZnTe:GaN with concentration ratio 1 ≤ [Ga]/[N] ≤ 2 can provide distinct absorption onset and also exhibit good absorption in the energy region 0.5 to 2.8 eV due to the IB, and one can optimize both the electronic and optical properties. Attractively, for ZnTe:GaN we find a spin-separated IBs with magnetic character that can be utilized for spin-polarized IB optoelectronics.

Keywords
optoelectronics, intermediate band, spin polarized, band-gap engineering, electronic structure, absorption, density functional theory
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-265538 (URN)
Note

QC 20191218

Available from: 2019-12-12 Created: 2019-12-12 Last updated: 2019-12-18Bibliographically approved
5. Vacancy induced magnetism in WO3
Open this publication in new window or tab >>Vacancy induced magnetism in WO3
2013 (English)In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 86, no 6, p. 273-Article in journal (Refereed) Published
Abstract [en]

The possibility to obtain ferromagnetic (FM) phase from native defects in WO3 is investigated by theoretically analyzing six different crystalline structures. The local magnetic moment from vacancies is calculated using the projector augmented wave method in combination with the local spin density approximation including a Coulomb correction (LSDA + U) of the W d-states. We find that tungsten vacancies V-W can induce a magnetic phase of similar to 3.5 mu(B)/V-W with a local magnetic moment on the oxygen atoms of at most similar to 1 mu(B)/V-W, whereas corresponding oxygen vacancies V-O have no impact on the magnetic coupling. Intriguingly, although the six crystalline structures have very comparable bonds, the magnetic moment generated by the cation vacancies is different, showing higher local magnetic moments for WO3 structures with low crystalline symmetry. The results indicate that WO3:V-W cannot induce a hole-mediated FM phase, and instead V-W in WO3 induces a local magnetic moment on the unpaired states at surrounding O atoms.

Keywords
Augmented-Wave Method
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-125757 (URN)10.1140/epjb/e2013-40014-7 (DOI)000321446200033 ()2-s2.0-84898873846 (Scopus ID)
Funder
Swedish Energy AgencySwedish Research Council
Note

QC 20130814

Available from: 2013-08-14 Created: 2013-08-13 Last updated: 2019-12-12Bibliographically approved
6. Understanding the optical properties of ZnO1-xSx and ZnO1-xSex alloys
Open this publication in new window or tab >>Understanding the optical properties of ZnO1-xSx and ZnO1-xSex alloys
2016 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 119, no 4, article id 045704Article in journal (Refereed) Published
Abstract [en]

ZnO1-xYx with chalcogen element Y exhibits intriguing optoelectronic properties as the alloying strongly impacts the band-gap energy E-g(x). In this work, we analyze and compare the electronic structures and the dielectric responses of Zn(O,S) and Zn(O, Se) alloys by means of the density functional theory and the partially self-consistent GW approach. We model the crystalline stability from the total energies, and the results indicate that Zn(O, S) is more stable as alloy than Zn(O, Se). We demonstrate also that ion relaxation strongly affects total energies, and that the band-gap bowing depends primarily on local relaxation of the bonds. Moreover, we show that the composition dependent band-gap needs to be analyzed by the band anti-crossing model for small alloying concentration, while the alloying band-bowing model is accurate for strong alloying. We find that the Se-based alloys have a stronger change in the band-gap energy (for instance, Delta E-g(0.50) = E-g(ZnO) -(E)g(x = 0.50) approximate to 2.2 eV) compared with that of the S-based alloy (Delta E-g(0.50) = 1.2 eV), mainly due to a stronger relaxation of the Zn-anion bonds that affects the electronic structure near the band edges. The optical properties of the alloys are discussed in terms of the complex dielectric function epsilon(omega) = epsilon(1)(omega) + i epsilon(2)(omega) and the absorption coefficient alpha(omega). While the large band-gap bowing directly impacts the low-energy absorption spectra, the high-frequency dielectric constant epsilon(infinity) is correlated to the intensity of the dielectric response at energies above 4 eV. Therefore, the dielectric constant is only weakly affected by the non-linear band-gap variation. Despite strong structural relaxation, the high absorption coefficients of the alloys demonstrate that the alloys have well-behaved optoelectronic properties.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2016
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-183334 (URN)10.1063/1.4940700 (DOI)000369896300047 ()2-s2.0-84956618463 (Scopus ID)
Note

QC 20160308

Available from: 2016-03-08 Created: 2016-03-07 Last updated: 2019-12-12Bibliographically approved

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