J.E. Medvedeva

Applied Physics Letters 121 (26), 261902 (2022)
https://doi.org/10.1063/5.0128941

Ab initio molecular dynamics liquid-quench simulations and hybrid density functional calculations are performed to model the effects of room-temperature atomic fluctuations and photo-illumination on the structural and electronic properties of amorphous sub-stoichiometric In₂O_2.96. A large configurational ensemble is employed to reliably predict the distribution of localized defects as well as their response to the thermal and light activation. The results reveal that the illumination effects on the carrier concentration are greater in amorphous configurations with shorter In–O bond length and reduced polyhedral sharing as compared to the structures with a more uniform morphology. The obtained correlation between the photo-induced carrier density and the reduction in the number of fully coordinated In-atoms implies that metal oxides with a significant fraction of crystalline/amorphous interfaces would show a more pronounced response to illumination. Photo-excitation also produces In–O₂–In defects that have not been previously found in sub-stoichiometric amorphous oxides; these defects are responsible for carrier instabilities due to overdoping.

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J.E. Medvedeva, K. Sharma, B. Bhattarai, M.I. Bertoni

ACS Applied Materials & Interfaces 14 (34), 39535-39547 (2022)
https://doi.org/10.1021/acsami.2c09604

The role of disorder and particularly of the interfacial region between crystalline and amorphous phases of indium oxide in the formation of hydrogen defects with covalent (In–OH) or ionic (In–H–In) bonding are investigated using ab initio molecular dynamics and hybrid density-functional approaches. The results reveal that disorder stabilizes In–H–In defects even in the stoichiometric amorphous oxide and also promotes the formation of deep electron traps adjacent to In–OH defects. Furthermore, below-room-temperature fluctuations help switch interfacial In–H–In into In–OH, creating a new deep state in the process. This H-defect transformation limits not only the number of free carriers but also the grain size, as observed experimentally in heavily H-doped sputtered In₂O_x. On the other hand, the presence of In–OH helps break O₂ defects, abundant in the disordered indium oxide, and thus contributes to faster crystallization rates. The divergent electronic properties of the ionic vs covalent H defects─passivation of undercoordinated In atoms vs creation of new deep electron traps, respectively─and the different behavior of the two types of H defects during crystallization suggest that the resulting macroscopic properties of H-doped indium oxide are governed by the relative concentrations of the In–H–In and In–OH defects. The microscopic understanding of the H defect formation and properties developed in this work serves as a foundation for future research efforts to find ways to control H species during deposition.

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A. Sil, M.J. Deck, E.A. Goldfine, C. Zhang, S.V. Patel, S. Flynn, H. Liu, P.H. Chien, Kenneth R Poeppelmeier, Vinayak P Dravid, Michael J Bedzyk, Julia E Medvedeva, Yan-Yan Hu, Antonio Facchetti, Tobin J Marks

Chemistry of Materials 34 (7), 3253-3266 (2022)
https://doi.org/10.1021/acs.chemmater.2c00053

In this contribution, the structural and electronic effects of fluoride doping in both crystalline and amorphous indium oxides are investigated by both experimental and theoretical techniques. Pristine crystalline and amorphous fluoride-doped indium oxide (F:In–O) phases were prepared by solution-based combustion synthesis and sol–gel techniques, respectively. The chemical composition, environment, and solid-state microstructure of these materials were extensively studied with a wide array of state-of-the-art techniques such as UV–vis, X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, ¹⁹F and ¹¹⁵In solid-state NMR, high-resolution transmission electron microscopy (HR-TEM), and extended X-ray absorption fine structure (EXAFS) as well as by density functional theory (DFT) computation combined with MD simulations. Interestingly, the UV–vis data reveal that while the band gap increases upon F^–-doping in the crystalline phase, it decreases in the amorphous phase. The ¹⁹F solid-state NMR data indicate that upon fluorination, the InO₃F₃ environment predominates in the crystalline oxide phase, whereas the InO₄F₂ environment is predominant in the amorphous oxide phase. The HR-TEM data indicate that fluoride doping inhibits crystallization in both crystalline and amorphous In–O phases, a result supported by the ¹¹⁵In solid-state NMR, EXAFS, and DFT-MD simulation data. Thus, this study establishes fluoride as a versatile anionic agent to induce disorder in both crystalline and amorphous indium oxide matrices, while modifying the electronic properties of both, but in dissimilar ways.

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A. Sil, E.A. Goldfine, W. Huang, M.J. Bedzyk, J.E. Medvedeva, A. Facchetti, Tobin Marks

ACS Applied Materials & Interfaces 14 (10), 12340-12349 (2022)
https://doi.org/10.1021/acsami.1c22853

Zirconium oxide (ZrO_x) is an attractive metal oxide dielectric material for low-voltage, optically transparent, and mechanically flexible electronic applications due to the high dielectric constant (κ ∼ 14–30), negligible visible light absorption, and, as a thin film, good mechanical flexibility. In this contribution, we explore the effect of fluoride doping on structure–property–function relationships in low-temperature solution-processed amorphous ZrO_x. Fluoride-doped zirconium oxide (F:ZrO_x) films with a fluoride content between 1.7 and 3.2 in atomic (at) % were synthesized by a combustion synthesis procedure. Irrespective of the fluoride content, grazing incidence X-ray diffraction, atomic-force microscopy, and UV–vis spectroscopy data indicate that all F:ZrO_x films are amorphous, atomically smooth, and transparent in visible light. Impedance spectroscopy measurements reveal that unlike solution-processed fluoride-doped aluminum oxide (F:AlO_x), fluoride doping minimally affects the frequency-dependent capacitance instability of solution-processed F:ZrO_x films. This result can be rationalized by the relatively weak Zr–F versus Zr–O bonds and the large ionic radius of Zr⁺⁴, as corroborated by EXAFS analysis and MD simulations. Nevertheless, the performance of pentacene thin-film transistors (TFTs) with F:ZrO_x gate dielectrics indicates that fluoride incorporation reduces I–V hysteresis in the transfer curves and enhances bias stress stability versus TFTs fabricated with analogous, but undoped ZrO_x films as gate dielectrics, due to reduced trap density.

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Julia E. Medvedeva, B. Bhattarai, I.A. Zhuravlev, F. Motti, P. Torelli, A. Guarino, Andreas Klein, Emiliano Di Gennaro, Fabio Miletto Granozio

Physical Review Materials 6 (7), 075605 (2022)
https://doi.org/10.1103/PhysRevMaterials.6.075605

Understanding the short-range structure of an amorphous material is the first step in predicting its macroscopic properties. Amorphous strontium titanate (a-STO) presents a unique challenge due to contradictory experimental findings regarding the local oxygen environment of titanium, concluded to be either tetrahedral or octahedral. To elucidate the discrepancy, 72 models of a-STO with density ranging from the crystalline value 5.12 to $3.07g/{\mathrm{cm}}^3$ were prepared using ab initio molecular dynamics liquid-quench simulations and characterized by extended x-ray absorption fine structure (EXAFS) for both Ti and Sr K edge. An excellent agreement between the calculated and two independent experimental EXAFS measurements demonstrates that the discrepancy in the Ti coordination stems from differences in the material's density. Next, density-dependent structural characteristics, including Ti-O and Sr-O coordination, distances, angles, polyhedral sharing, and vibrational density of states in a-STO are thoroughly analyzed and correlated with disorder-induced changes in the electronic properties that are calculated using a hybrid density functional. The obtained increase in the band gap and broadening of Ti-$d{e}_g$-orbital contributions in the conduction band are in excellent agreement with our x-ray absorption spectroscopy for Ti L-edge spectra and optical absorption measurements for crystalline and amorphous STO grown by pulsed laser deposition. The derived microscopic understanding of the structure-property relationship in amorphous “perovskite” serves as a foundation for further investigations of a-STO and related materials.

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J.E. Medvedeva, E. Caputa-Hatley, I. Zhuravlev

Physical Review Materials 6 (2), 025601 (2022)
https://doi.org/10.1103/PhysRevMaterials.6.025601

The unique response of amorphous ionic oxides to changes in oxygen stoichiometry is investigated using computationally intensive ab initio molecular dynamics simulations, comprehensive structural analysis, and hybrid density-functional calculations for the oxygen defect formation energy and electronic properties of amorphous ${\mathrm{In}}_2{O}_{3-x}$ with $x=0$–0.185. In marked contrast to nonstoichiometric crystalline nanocomposites with clusters of metallic inclusions inside an insulating matrix, the lack of oxygen in amorphous indium oxide is distributed between a large fraction of undercoordinated In atoms, leading to an extended shallow state for $x<0.037$, a variety of weakly and strongly localized states for $0.074<x<0.148$, and a percolation-like network of single-atom chains of metallic In-In bonds for $x>0.185$. The calculated carrier concentration increases from $3.3\times {10}^{20}{\mathrm{cm}}^{-3}$ at $x=0.037$ to $6.6\times {10}^{20}{\mathrm{cm}}^{-3}$ at $x=0.074$ and decreases only slightly at lower oxygen content. At the same time, the density of deep defects located between 1 and 2.5 eV below the Fermi level increases from $0.4\times {10}^{21}{\mathrm{cm}}^{-3}$ at $x=0.074$ to $2.2\times {10}^{21}{\mathrm{cm}}^{-3}$ at $x=0.185$. The wide range of localized gap states associated with various spatial distributions and individual structural characteristics of undercoordinated In is passivated by hydrogen that helps enhance electron velocity from $7.6\times {10}^4$ to $9.7\times {10}^4$ m/s and restore optical transparency within the visible range; H doping is also expected to improve the material's stability under thermal and bias stress.

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