Article pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Carrier Generation in p‑Type Wide-Gap Oxide: SnNb2O6 Foordite Akane Samizo,†,‡ Naoto Kikuchi,*,† Yoshihiro Aiura,† Keishi Nishio,‡ and Ko Mibu§ †
Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan ‡ Department of Materials Science and Technology, Tokyo University of Science, Katsushika 125-8585, Japan § Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
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S Supporting Information *
ABSTRACT: Wide-gap oxides with their valence band maximum (VBM) composed of s orbitals are essential for realizing practical p-type transparent oxide semiconductors. We prepared a new p-type wide-gap oxide, SnNb2O6 foordite, with its VBM composed of Sn 5s orbitals. To discuss carrier generation, we prepared both p-type and n-type SnNb2O6 by controlling the annealing conditions. The carrier mobility and density were 3.8 × 10−1 cm2 V−1 s−1 and 3.7 × 1018 cm−3, respectively, for the p-type sample and 9.9 cm2 V−1 s−1 and 7.5 × 1015 cm−3, respectively, for the n-type sample. The crystal structure of SnNb2O6 foordite consists of two types of alternating layers, Sn and Nb2O6 octahedra, where three nonequivalent oxygen sites exist. Six oxygens in the chemical formula of SnNb2O6 are distributed at the three sites in pairs, where the oxygens in three nonequivalent sites were named O1−O3. Hole and electron carriers were considered to be generated by Sn4+-on-Nb5+ substitutional defects (Sn′Nb) and oxygen vacancies of O1 and O2 that are not bonded to Sn (V•• ′ and V•• O1/O2), respectively. Therefore, we concluded that it is essential to control SnNb O1/O2 to control the semiconducting properties such as the carrier type and carrier density.
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INTRODUCTION The development of practical p-type wide-gap oxides is driven by the desire to fabricate innovative transparent electronic devices based on p−n junctions. For conventional oxides, because of the localized nature of their valence band maximum (VBM) composed of oxygen 2p orbitals, the realization of ptype wide-gap semiconductors with high mobility is challenging.1,2 To reduce the nature of localization of the hole carriers and to improve mobility, the delocalization of the VBM through hybridization with orbitals with a large spatial spread is considered to be indispensable.3 From a computational search including many binary and ternary oxides, those having s orbital-based VBM (hereafter termed s orbital-based oxides) were proposed as prospective candidates for p-type wide-gap oxides.4 Here, the s orbital-based oxides are defined as oxides containing metal ions with a formally closed-shell electronic configuration of (n − 1)d10ns2 (n is the principal quantum number) such as Sn2+ and Bi3+. As the s orbital of a heavy metal exhibits a large spatial spreading in addition to its isotropic nature, the low effective mass of the hole carriers and the low sensitivity for structural disturbance are expected from s orbital-based oxides. This approach has been demonstrated by SnO with a hole mobility of 2.4 cm2 V−1 s−1 and its pchannel thin-film transistor.5,6 In addition, the Ba1.3K0.7BiTaO6 double perovskite having a hole mobility as high as 30 cm2 V−1 s−1 was found recently.7 s Orbital-based oxides are promising © XXXX American Chemical Society
candidates for practical p-type wide-gap oxides. Moreover, s orbital-based oxides have the potential to be applied as practical amorphous thin films. However, until now, only a few reports of p-type s orbital-based oxides have been published. This is likely due to the difficult carrier generation in these oxides. Because s orbital-based oxides need to be annealed in a reducing atmosphere to prevent oxidative decomposition (e.g., SnO changes to SnO2), charge compensation of holes by electrons generated by the formation of oxygen vacancies takes place. Hence, it is hard to obtain a p-type semiconductor. Therefore, the control of defect formation is important for realizing p-type conductivities in s orbital-based oxides. We focused on an s orbital-based oxide containing Sn2+ and recently succeeded in preparing p-type Sn2(Nb2−xTax)O7 by controlling the annealing conditions.8 From the mutual relation between the defect content and electrical properties, we found that hole carriers were generated by Sn4+-on-Nb5+/ Ta5+ substitutional defects (SnNb/Ta ′ according to the Kröger− Vink notation) in Nb2O6/Ta2O6 octahedra of Sn2(Nb2−xTax)O7. Sn2M2O7 (M = Nb or Ta), which is one of the s orbitalbased oxides, exhibits a pyrochlore crystal structure. In addition, dispersion-less bands, called flat bands, appear at its Received: August 9, 2018 Revised: October 23, 2018
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DOI: 10.1021/acs.chemmater.8b03408 Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
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VBM.9 These flat bands at the VBM are a disadvantage for increasing hole mobility. SnNb2O6 is known as a foordite,10,11 and its crystal structure consists of two alternating layers of Sn and Nb2O6 octahedra along the a axis as shown in Figure 1. Previous band
Article
EXPERIMENTAL SECTION
SnNb2O6 foordite samples were prepared by a solid-state reaction. The starting materials used were SnO (Kojundo Chemical Laboratory, 99.5% pure) and Nb2O5 (Kojundo Chemical Laboratory, 99.9% pure). They were mixed in an agate mortar with ethanol. The mixed sample was dried in air for 24 h to evaporate the ethanol, followed by uniaxial pressing at 350 MPa to form discs with a diameter of 12 mm and a thickness of 3−4 mm. Then, the disks were calcined at 1173 K in an alumina tube furnace under N2 at a flow rate of 150 mL min−1. The calcined discs were ground again in an agate mortar, followed by mixing with a 2 wt % poly(vinyl alcohol) aqueous solution and ethanol. After drying in air for 24 h, the dried samples were sieved to obtain narrow sieve fractions of
