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Evidence of defect band mechanism responsible for band gap evolution in (ZnO)(1-x)(GaN)(x) alloys
Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
Univ Oslo, Ctr Mat Sci & Nanotechnol, Dept Phys, POB 1048, N-0316 Oslo, Norway..
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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. Vol. 100, no 16, article id 165201
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-262950DOI: 10.1103/PhysRevB.100.165201ISI: 000489037500006Scopus ID: 2-s2.0-85073711420OAI: oai:DiVA.org:kth-262950DiVA, id: diva2:1374969
Note

QC 20191203

Available from: 2019-12-03 Created: 2019-12-03 Last updated: 2019-12-12Bibliographically approved
In thesis
1. Optical and Electronic Properties of WO3 and Zn Chalcogenides Alloys: A Theoretical study
Open this publication in new window or tab >>Optical and Electronic Properties of WO3 and Zn Chalcogenides Alloys: A Theoretical study
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:nbn:se:kth:diva-265539 (URN)978-91-7873-394-1 (ISBN)
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

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