Two-Dimensional Polaronic Behavior in the Binary Oxides<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>m</mml:mi><mml:mtext mathvariant="normal">−</mml:mtext><mml:msub><mml:mi>HfO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>m</mml:mi><mml:mtext mathvariant="normal">−</mml:mtext><mml:msub><mml:mi>ZrO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>

McKenna, Keith P.; Wolf, Matthew J.; Shluger, Alexander L.; Lany, Stephan; Zunger, Alex

doi:10.1103/physrevlett.108.116403

Cited by 63 publications

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“…The trapping of hole polarons on single O atoms in m-HfO 2 has been previously predicted theoretically [25]. Recent calculations using the cancellation of nonlinearity approach [54] predicted that holes can trap only at 3-coordinated O sites in the bulk of m-HfO 2 [26] with trapping energy of 0.18 eV and with much larger trapping energies at surfaces, featuring two-coordinated O sites.…”

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“…Shallow electron and hole polaron states have been predicted by theoretical calculations in crystalline monoclinic (m)-HfO 2 and m-ZrO 2 [25] and corundum [3]. Subsequent work [26] confirmed that holes can self-trap at three-coordinated O sites in m-HfO 2 and ZrO 2 with trapping energies of about 0.2 eV and in much deeper states at some low-coordinated O sites at the surface of ZrO 2 [27]. Recent resonant photoelectron spectroscopy measurements strongly suggest hole polaron trapping in HfO 2 thin films [28].…”

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Deep electron and hole polarons and bipolarons in amorphous oxide

et al. 2016

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Amorphous (a)-HfO2 is a prototype high dielectric constant insulator with wide technological applications. Using ab initio calculations we show that excess electrons and holes can trap in aHfO2 in energetically much deeper polaron states than in the crystalline monoclinic phase. The electrons and holes localize at precursor sites, such as elongated Hf-O bonds or under-coordinated Hf and O atoms and the polaronic relaxation is amplified by the local disorder of amorphous network. Single electron polarons produce states in the gap at ∼2 eV below the bottom of the conduction band with average trapping energies of 1.0 eV. Two electrons can form even deeper bipolaron states on the same site. Holes are typically localized on under-coordinated O ions with average trapping energies of 1.4 eV. These results advance our general understanding of charge trapping in amorphous oxides by demonstrating that deep polaron states are inherent and do not require any bond rupture to form precursor sites.Electron and hole states with energy levels lying deep in the bandgap impair the dielectric quality of insulating layers. In particular, electron transitions facilitated by these states account for multitude of degradation phenomena including enhanced leakage current and charge trapping eventually leading to the dielectric barrier failure and breakdown. Routinely, however, these deep electron states are seen as not inherent to the perfect material but rather associated with the presence of defects and/or impurity centers in the atomic network of an insulator. This belief offers some hope that imperfections can be eliminated by using more clean and optimized synthesis and proper processing of the insulators. On the other hand, self-trapping of excess charges in the form of small electron and hole polarons is well known to occur even in perfect crystalline oxide insulators. However, it is usually shallow, with trapping energies of the order of 0.2 eV (see e.g. [1-3]). As a result, the electron and hole polarons are mobile at room temperature in crystalline reduced TiO 2 and NiO [4], CeO 2 [5, 6], doped ZrO 2 [7,8], and in plethora of other oxides (see e.g. [1,[9][10][11][12]). The intrinsic localization of excess electrons and holes in noncrystalline materials and liquids has also been a subject of extensive experimental and theoretical studies pioneered in [13]. Structural disorder typically induces shallow electron states near the bottom of the conduction band, below the so-called mobility edge (see, e.g. [14] . In these systems, the polaronic relaxation is amplified by the local disorder of amorphous network.Here we turn to amorphous (a)-HfO 2 , which represents a wide class of high dielectric constant oxides recently emerged as the major contenders to replace SiO 2 in a broad spectrum of nano-electronic devices ranging from deep-scaled transistors to DRAM and non-volatile memory cells (see, e.g. [21,22]). Amorphous oxides make the backbone of most electronic devices and charge trapping appears to be the key factor determining devic...

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Deep electron and hole polarons and bipolarons in amorphous oxide

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“…Valentin and Selloni described this model for anatase [82] (see also refs [83,84] ) and we have used it extensively in understanding different modifications of TiO2 with nanoclusters [81,[85][86][87] . Figure 7 shows the spin density from the relaxed triplet state for a selection of free and supported TiO2 nanoclusters; ref [81] shows a more extensive set of nanoclusters and see also ref [88] for high level DFT results on electron-hole localisation in TiO2 nanoclusters, showing consistency between the different DFT approaches and models.…”

Section: Tio2 Nanocluster Modified Rutile Tio2 As a Model System For mentioning

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Design of Novel Visible Light Active Photocatalyst Materials: Surface Modified TiO₂

et al. 2016

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Type of publicationArticle (peer-reviewed) AbstractThis progress review describes our work on the design of new TiO2 based photocatalysts. The key concept is the formation of composite structures through the modification of anatase and rutile TiO2 with molecular-sized nanoclusters of metals oxides. Our density functional theory (DFT) level simulations have been compared with experimental work synthesizing and characterizing surface modified TiO2. We use DFT to show that nanoclusters of metal oxides such as TiO2, SnO/SnO2, PbO/PbO2, ZnO and CuO are stable when adsorbed at rutile and anatase surfaces and can lead to a significant red shift in the absorption edge which will induce visible light absorption; this is the first requirement for a useful photocatalyst. We determine the origin of the red shift and the fate of excited electrons and holes. For p-block metal oxides the oxidation state of Sn and Pb can be used to modify the magnitude of the red shift and its mechanism. We describe comparisons of recent experimental studies of surface modified TiO2 that validate our DFT simulations. These nanocluster-modified TiO2 structures 2 form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with a correct choice of nanocluster modified can be applied to other reactions.

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“…It may be supported by polaronic lattice deformations that act as precursors for self-trapped electron (STE) and self-trapped hole (STH) type polaronic charges. 35 Next we explain the main parameters of the electronic structure of Ga 2 O 3 in Figs. 5 and 6.…”

Section: A Atomic Model Of Intrinsic Defect Statesmentioning

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Intrinsic electronic defects and multiple-atom processes in the oxidic semiconductor Ga2O3

Schmeißer

Henkel

2018

Journal of Applied Physics

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We report on the electronic structure of gallium oxide (Ga2O3) single crystals as studied by resonant photoelectron spectroscopy (resPES). We identify intrinsic electronic defects that are formed by mixed-atomic valence states. We differentiate three coexisting defect states that differ in their electronic correlation energy and their spatial localization lengths. Their relative abundance is described by a fractional ionicity with covalent and ionic bonding contributions. For Ga2O3, our analyses of the resPES data enable us to derive two main aspects: first, experimental access is given to determine the ionicity based on the original concepts of Pauling and Phillips. Second, we report on multi-atomic energy loss processes in the Ga2p core level and X-ray absorption data. The two experimental findings can be explained consistently in the same context of mixed-atomic valence states and intrinsic electronic defects.

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Two-Dimensional Polaronic Behavior in the Binary Oxidesm−HfO2andm−ZrO2

Cited by 63 publications

References 44 publications

Deep electron and hole polarons and bipolarons in amorphous oxide

Deep electron and hole polarons and bipolarons in amorphous oxide

Design of Novel Visible Light Active Photocatalyst Materials: Surface Modified TiO₂

Intrinsic electronic defects and multiple-atom processes in the oxidic semiconductor Ga2O3

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Two-Dimensional Polaronic Behavior in the Binary Oxidesm−HfO2andm−ZrO2

Cited by 63 publications

References 44 publications

Deep electron and hole polarons and bipolarons in amorphous oxide

Deep electron and hole polarons and bipolarons in amorphous oxide

Design of Novel Visible Light Active Photocatalyst Materials: Surface Modified TiO2

Intrinsic electronic defects and multiple-atom processes in the oxidic semiconductor Ga2O3

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Design of Novel Visible Light Active Photocatalyst Materials: Surface Modified TiO₂