Structural, Electrical, and Optical Properties of the Tetragonal, Fluorite

Jun 30, 2015 - We report the discovery of Zn0.456In1.084Ge0.460O3, a material closely related to bixbyite. In contrast, however, the oxygen atoms in t...
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Structural, Electrical, and Optical Properties of the Tetragonal, Fluorite-Related Zn0.456In1.084Ge0.460O3 Karl Rickert,†,§,⊥ Nazmi Sedefoglu,†,‡,⊥ Sylvie Malo,§ Vincent Caignaert,§ Hamide Kavak,∥ and Kenneth R. Poeppelmeier*,† †

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Department of Physics, Osmaniye Korkut Ata University, Osmaniye 80000, Turkey § Laboratoire CRISMAT, CNRS ENSICAEN, 6 bd Maréchal Juin, Caen 14050, France ∥ Physics Department, Cukurova University, Adana 01330, Turkey ‡

S Supporting Information *

ABSTRACT: We report the discovery of Zn0.456In1.084Ge0.460O3, a material closely related to bixbyite. In contrast, however, the oxygen atoms in this new phase occupy 4 Wyckoff positions, which result in 4 four-coordinate, 24 sixcoordinate (2 different Wyckoff positions), and 4 eight-coordinate sites as compared to the 32 six-coordinate (also 2 different Wyckoff positions) sites of bixbyite. This highly ordered material is related to fluorite, Ag6GeSO8, and γUO3 and is n-type with a bulk carrier concentration of 4.772 × 1014 cm−3. The reduced form displays an average room temperature conductivity of 99(11) S· cm−1 and an average optical band gap of 2.88(1) eV. These properties are comparable to those of In2O3, which is the host material for the current leading transparent conducting oxides. The structure of Zn0.456In1.084Ge0.460O3 is solved from a combined refinement of synchrotron X-ray powder diffraction and time-of-flight neutron powder diffraction and confirmed with electron diffraction. The solution is a new, layered, tetragonal structure in the I41/amd space group with a = 7.033986(19) Å and c = 19.74961(8) Å. The complex cationic topological network adopted by Zn0.456In1.084Ge0.460O3 offers the potential for future studies to further understand carrier generation in ∼3 eV oxide semiconductors.

1. INTRODUCTION The major application of transparent conducting oxides (TCOs) is the transparent electrode, a necessary component for flat panel displays and photovoltaic devices.1,2 As a result of the ubiquitous nature of these devices, TCO optimization has been of great interest in the last few decades. This has led to significant improvements in device performance and the characterization of several TCOs, including Sn:In2O3 (ITO), amorphous indium zinc oxide (a-IZO), F:SnO2, and Al:ZnO. Of these, ITO and a-IZO are the most widely used when high conductivity is required.3 Indium is not a common metal, however, and the high cost of its extraction makes new TCOs with a reduced indium content (compared to indium’s ∼90% cationic content in ITO and a-IZO) highly sought after commodities.4 One method of reducing the indium content is to cosubstitute indium with two different metals which maintain complementary oxidation states.5 This rationale has successfully produced TCOs in MxIn2−2xSnxO3, where M = Mg, Ca, Ni, Cu, Zn, or Cd and In2‑xX2x/3Sbx/3O3, where X = Zn or Cu.5−12 Understandably, these investigations have focused on the successful In2O3 system and use substitutes that are common in known TCOs, but cosubstitution has also been applied to other TCO materials such as ZnO.13−16 As Freeman et al. suggest, © 2015 American Chemical Society

however, there are many TCO materials and solid solutions that are undiscovered and outside the thoroughly researched domains.17 Germanium, for example, has received little attention as a tetravalent cation when compared to tin, even though recent thin film studies demonstrate that it performs similarly as a dopant.18−20 Aside from these studies, few bulk investigations of Ge:In2O3 have been performed, and although these materials show higher conductivity than In2O3, the solubility limit of Ge is unusually low (53% in GITO). In contrast, other multication, In-containing materials with a much higher composition of In (i.e., ITO with ∼90% In) do not display a complex cationic network, instead adopting the bixbyite structure of In2O3. ZIGO’s relationship with fluorite is observable from its fully occupied, cubic close packed cation lattice and an anion lattice that only occupies the tetrahedral holes of the cation lattice. As shown in Figure 2, the cation

Figure 1. Rietveld refinements of synchrotron XRD (top) and TOF ND (bottom) used to solve the structure of ZIGO. The peak markers are for the ZIGO phase. For the sake of clarity, the peak markers for In2O3 and Zn2GeO4 are not shown but are available in the Supporting Information.

corroborated by bond valence calculations (provided in the Supporting Information). The product is not a single phase, however, as two secondary phases were also identified: In2O3 and Zn2GeO4.36,37 The Zn2GeO4 peaks displayed an unusual peak shape and were fitted with two phases with identical crystallographic parameters and different shape parameters. The weight percent of each secondary phase is low (≈ 0.67% and ≈1.03%, respectively) but demonstrates that there is a slight Zn deficiency in ZIGO. Notably, a Zn deficiency is also reported in the compositionally similar Zn/Sn cosubstituted In2O3 system (ZnxIn2−2xSnxO3).38,39 Although the accuracy of mass percent determinations from diffraction data is poor for the trace amounts of secondary phases that are present in this sample, the mass percents were combined with the demonstrated Zn deficiency to determine the final composition of ZIGO (Zn0.456In1.084Ge0.460O3).40 This stoichiometry agrees 5074

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fluorite, it is readily apparent that the 16f site adopts an octahedron that is formed by removing two anions on a body diagonal, whereas the 8e site has two anion vacancies on a face diagonal. The remaining anions then distort toward these vacancies, producing the different octahedra present in ZIGO. An identical situation is present in bixbyite In2O3, where the same environments are present and are denoted as the b- and d-sites, respectively (this arises from their Wyckoff positions). In ZIGO, the f-site (b-site analogue) is occupied by Zn, In, and Ge, but the e-site (d-site analogue) is exclusively occupied by In. Similarly, the b-site in In2O3 is the energetically favored site for substitution and is the site occupied by Sn in ITO.43,44 In Zn/Sn cosubstituted In2O3, however, the Zn and Sn randomly occupy both the b- and d-sites.45 Thus, in both of the former cases, the cation site with the body-diagonal anion vacancies is more amenable to containing multiple cationic species. Interestingly, the ZIGO structure not only maintains these various coordination environments and complex occupancies but also clearly separates them into unique layers. As shown in Figure 4, the regular octahedra of the Zn/In/Ge f-sites form

Figure 2. Unit cell of ZIGO as viewed in the [114] direction. Zn/In/ Ge is gray, Ge is orange, In is blue, and O has been omitted for clarity. The purple planes are the cubic close-packed cation layers.

layers are not perfectly orthogonal to a face diagonal as they are in fluorite because ZIGO maintains a tetragonal unit cell instead of the cubic unit cell of fluorite. Furthermore, ZIGO has a 25% anion deficiency when compared to that of fluorite, as it has an anion-to-cation ratio of 3:2 (compared to fluorite’s 2:1). This is the same as that of In2O3, but whereas In2O3 only contains 6-coordinate cation sites, ZIGO has 4-, 6-, and 8coordinate cation sites, and the 8-coordinate sites are not the perfect cubes of fluorite. Instead, they have 4 short bonds and 4 long bonds, making them dodecahedra, or distorted cubes, and they could also be identified as tetrahedra if only the short bonds are considered. The 4- and 8-coordinate sites exist in the same abundance in ZIGO, however, which make an overall average coordination number of 6. ZIGO’s 6-coordinate sites are particularly interesting, as only one of ZIGO’s two 6-coordinate sites (16f and 8e) maintains a multiple occupancy of three different elements, whereas the other is occupied solely by In. Although the coordination number is the same in both cases, the actual coordination environment is different, as depicted in Figure 3. When compared to the ideal cubic coordination environments of

Figure 4. ZIGO unit cell, as viewed along the a axis, depicting the 4coordinate (orange), 6-coordinate In (blue), 6-coordinate Zn/In/Ge (gray), and 8-coordinate (green) cation environments.

corner-sharing, zigzag chains of rows of edge-sharing octahedra. Furthermore, these chains are perpendicular to those of the preceding and following Zn/In/Ge layers. As conductivity has been linked to corner- and edge-sharing octahedra, these layers are likely beneficial to ZIGO’s TCO properties.46,47 The other layers, however, are composed of the octahedra of the e-site intermixed with tetrahedra and dodecahedra. Moreover, the esite octahedra alternate with the tetrahedra and dodecahedra, so that these octahedra are never corner- or edge-sharing. The

Figure 3. Coordination environments of the two different 6-fold sites in ZIGO (left) and the ideal cubic analogies (right). Zn/In/Ge is gray, In is blue, and O is red. The site with a body-diagonal (top) is approximately a perfect octahedron, whereas the site with a facediagonal (bottom) is a distorted octahedron. 5075

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to the same percent In composition (x > 0.4 yields secondary phases in ZnxIn2−2xSn2O3, but x = 0.458 corresponds with the % In in ZIGO).53 The low conductivity is partially a result of the low doping inherent in this material, as the offstoichiometry of Zn and Ge result in a Ge4+ dopant of 0.2%. Dopants in In2O3 are typically on the order of 10% to optimize the conductivity.55 Sample diffuse reflectance spectra of ZIGO1 before and after reduction are provided in Figure 6 (see

tetrahedra and dodecahedra are ordered, such that any given row within a layer has e-sites and only tetrahedra or dodecahedra. The isolation of the e-site is the reverse of what is found in bixbyite, as bixbyite’s b-site is isolated by adjacent dsites, and the d-sites are corner-and edge-sharing. The complex cationic network of ZIGO, however, is not without precedent. Both Ag6GeSO8 and γ-UO3 maintain structures that bear similarities to ZIGO, but the difference in the average cation oxidation state of ZIGO (≈ 3+) compared to that of Ag6GeSO8 (2+) and γ-UO3 (6+) result in ZIGO sharing properties of both structures, as shown in Figure 5.48,49

Figure 5. Structures, as viewed along the a axis, of γ-UO3 (left), Ag6GeSO8 (middle), and ZIGO (right), with U (black), Ag (white), S (yellow), Ge (orange), In (blue), Zn/In/Ge (gray), and O (red). The anion lattices of γ-UO3 and ZIGO and the cation lattices of Ag6GeSO8 and ZIGO are similar.

Whereas Ag6GeSO8 has a similar cation lattice to that of ZIGO, γ-UO3 has a similar anion lattice. The most appropriate description is that it has the anion lattice of γ-UO3 and the cation lattice of Ag6GeSO8. That γ-UO3 should be similar to the fluorite-related structure of ZIGO is understandable, as rareearth oxides have a vast library of fluorite-related structures, often separated only by small changes in compositions.50−52 Property Characterization. As a substituted In 2O 3 derivative with corner- and edge-sharing octahedra, ZIGO’s TCO properties are of interest. Hall measurements show that nonreduced ZIGO1 has a bulk carrier concentration of 4.772 × 1014 cm−3, which is in the same order as In2O3 thin films and lower than reduced In2O3.53,54 Hall measurements of the ZIGO1 samples indicate that they have n-type conductivity, which is consistent with the bixbyite Zn−In−Ge−O phases of Cheng et al. and In-containing TCOs in general.24 The average room temperature conductivity of ZIGO1 is 0.08(1) S·cm−1 before hydrogen reduction and 147(4) S·cm−1 after reduction. The average room temperature conductivity of ZIGO2 is 0.08(1) S·cm−1 before hydrogen reduction and 99(11) S·cm−1 after reduction. These values are either comparable or superior to the conductivity of In2O3, as reported values display a wide degree of variation.10,54 This is also of the same order of magnitude as the conductivity values for ZnxIn2−2xSn2O3, if the conductivity trend of the ZnxIn2−2xSn2O3 system is extrapolated

Figure 6. Sample transformed diffuse reflectance spectra of ZIGO1 (a) before reduction and (b) after reduction showing the linear extrapolations used to approximate the band gap.

Supporting Information for sample spectra of ZIGO2). An average approximate optical band gap of 2.82(6) eV is obtained before reduction and 2.91(3) eV after reduction for ZIGO1 and average approximate optical band gaps of 2.82(2) eV and 2.88(1) eV, respectively, for ZIGO2. These band gaps are at the point that is desired for TCOs (∼3.1 eV to avoid absorbing visible light), and the reduced samples have superior band gaps as well as superior conductivity. As these are n-type materials, the increase in optical band gap upon reduction can be attributed to a Burstein−Moss shift.56,57 Although the reduction step caused the color of the samples to change from pale yellow to a pale gray-green, powder XRD patterns (provided in the Supporting Information) of as-synthesized and reduced samples are not distinguishable, demonstrating that structural changes and decomposition do not occur as a result of the reduction process. 5076

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4. CONCLUSIONS In summary, we have reported the fluorite-, Ag6GeSO8-, and γUO3-related tetragonal structure adopted by the new material Zn0.456In1.084Ge0.460O3 (ZIGO). The structure maintains layers of multiple occupancy, corner- and edge-sharing octahedra separated by layers of single occupancy tetrahedra, octahedra, and dodecahedra. As in In2O3, a 6-coordinate site with a bodydiagonal of anion vacancies is the preferred site for cations other than In, as compared to the 6-coordinate site with a facediagonal of anion vacancies. The TCO properties of ZIGO are comparable to those of In2O3, which is remarkable given its significantly lower (54.2% vs 100% of In2O3) indium content. ZIGO maintains a similar carrier concentration, and the reduced form of ZIGO maintains a similar average room temperature conductivity of 99(11) S·cm−1 and a band gap of 2.88(1) eV. With the low doping concentration of 0.2% inherent in ZIGO as a result of its natural Zn deficiency, further doping of this material could be of interest to improving its TCO properties.

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by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. This work made use of the OMM Facility and the J. B. Cohen X-ray Diffraction Facility, both of which are supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University (NU). A portion of this work was supported by the NU Keck Biophysics Facility and a Cancer Center Support Grant (NCI CA060553). We acknowledge Ashfia Huq (ORNL) and Saul Lapidus (ANL) for their assistance in obtaining neutron and synchrotron X-ray diffraction data, respectively. N.S. acknowledges Serdar Akbayrak for his help with property measurements.

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ABBREVIATIONS TCO, transparent conducting oxide; ITO, tin-doped indium oxide; a-IZO, amorphous indium zinc oxide; ZIGO, Zn0.456In1.084Ge0.460O3; ND, neutron diffraction; TOF, timeof-flight; XRD, X-ray diffraction; GSAS, general structure analysis system; TEM, transmission electron microscopy; ED, electron diffraction; EDS, energy dispersive spectroscopy; GITO, Ga3‑xIn5+xSn2O16

ASSOCIATED CONTENT

S Supporting Information *

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Description of van der Pauw geometry; detailed conductivity and band gap calculations; atomic positions and thermal parameters of ZIGO; typical ED pattern; dual refinement showing all peak markers; bond valence calculations; stoichiometric ZIGO XRD pattern; before and after reduction XRD patterns; and ZIGO2 diffuse reflectance spectra. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01724.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].. Author Contributions ⊥

K.R. and N.S. contributed equally to this work.

Funding

K.R. recognizes that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. D GE-1324585. K.R. also recognizes that this material is based upon research supported by the Chateaubriand Fellowship of the Office for Science & Technology of the Embassy of France in the United States. N.S gratefully acknowledges that this study was partially supported by the Council of Higher Education (CoHE) of Turkey. K.R., N.S., and K.R.P. gratefully acknowledge additional support from the Department of Energy Basic Energy Sciences Award No. DE-FG02-08ER46536. Notes

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A portion of this research was performed at Oak Ridge National Laboratory’s (ORNL’s) Spallation Neutron Source at POWGEN, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, and the U.S. Department of Energy. Use of 11BM at the Advanced Photon Source at Argonne National Laboratory (ANL) was supported 5077

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