Site-Selective Oxygen Vacancy Formation Derived from the Characteristic Crystal Structures of Sn–Nb Complex Oxides

Abstract

Divalent stannous oxide, oxide semiconductor, p-type, oxygen vacancy, EXAFS, Rietveld analysis ABSTRACT: Divalent tin oxides have attracted considerable attention as novel p-type oxide semiconductors, which are essential for realizing future oxide electronic devices. Recently, p-type Sn 2 Nb 2 O 7 and SnNb 2 O 6 were developed; however, enhanced hole mobility by reducing defect concentrations is required for practical use. In this work, we investigate the correlation between the formation of oxygen vacancy (V ! •• ), which may reduce the hole-generation efficiency and hole mobility, and the crystal structure in Sn-Nb complex oxides. Extended X-ray absorption fine structure spectroscopy and Rietveld analysis of x-ray diffraction revealed the preferential formation of V ! •• at the O site bonded to the Sn ions in both the tin niobates. Moreover, a large amount of V ! •• around the Sn ions were found in the p-type Sn 2 Nb 2 O 7 , thereby indicating the effect of V ! •• to the low holegeneration efficiency. The dependence of the formation of V ! •• on the crystal structure can be elucidated from the Sn-O bond strength that is evaluated based on the bond valence sum and Debye temperature. The differences in the bond strengths of the two Sn-Nb complex oxides are correlated through the steric hindrance of Sn 2+ with asymmetric electron density distribution. This suggests the importance of the material design with a focus on the local structure around the Sn ions to prevent the formation of V ! •• in p-type Sn 2+ oxides.

Bipolar Semiconducting Properties in α-SnWO4 Based on the Characteristic Defect Structure

“…This could be originated from the small effective masses ( m *) in the average via the theoretical calculation for α-SnWO 4 ; they are an order of magnitude lighter than those reported for SnO, Sn 2 (Nb 2– x Ta x )­O 7 , and SnNb 2 O 6 . Although the hole mobility for α-SnWO 4 is 10 times greater than that for Sn 2 (Nb 2– x Ta x )­O 7 at this hole carrier density (∼10 18 cm –3 ), there is a negligible difference between the α-SnWO 4 and SnNb 2 O 6 values. , This might be related to the reduced carrier-generation efficiency of α-SnWO 4 (∼0.2%) relative to SnNb 2 O 6 (∼1.4%) arising from the previously mentioned electrostatic-charge compensation produced by the V O •• . In other words, because the V O •• acts as scattering centers, the mobility is not improved as much as might be expected given the low values of m *.…”

Tuning of hole carrier density in p-type α-SnWO4 by exploiting oxygen defects

“…This is likely because oxygen vacancies form spontaneously to compensate for the positive charge 19,20,23 . For example, in α-SnWO 4 , the net hole carrier density is determined by a balance between the following defect formations, as shown by this Kröger-Vink notation: we assume the formation of an additional sublattice composed of and , as previously reported 24 .…”

“…is the Sn 4+ Sn(IV) ′′ W O × O Sn(IV) ′′ W substitutional defect at the W 6+ site, and corresponds not to V ′′ Sn(II) the tin vacancy due to evaporation from the material but to the resultant void due to the formation of 15,24 . Meanwhile, Sn(IV) ′′ W h • and denote the holes and electrons, respectively.…”

“…Meanwhile, Sn(IV) ′′ W h • and denote the holes and electrons, respectively. As a result, the e ′ hole carrier generation efficiencies of Sn 2+ -based oxide semiconductors from Sn 4+ substitutional defects are tremendously low, as low as 0.005-1.4 %; the efficiencies of layered-structure Sn 2+based oxide semiconductors are much greater than that of pyrochlore-type ones 13,14,24 . This is due to electrostatic interaction between electron densities at Sn 2+ ions and surrounded O 2ions 24 .…”

Trends in Bandgap of Epitaxial $\textit{A}$$_2$$\textit{B}$$_2$O$_7$ ($\textit{A}$ = Sn, Pb; $\textit{B}$ = Nb, Ta) Films Fabricated by Pulsed Laser Deposition

“…Therefore, we can fairly postulate that cation off-stoichiometry is negligible in our films. Besides the cation ratio, anti-site defects Sn 2+ /Sn 4+ (or Pb 2+ /Pb 4+ ), and oxygen vacancies are also important in terms of the electronic structures of pyrochlore oxides and are indeed discussed in several previous works with bulk samples especially for Sn 2 Nb 2 O 7 [17][18][19][20]. However, quantitative evaluation of those is quite challenging in thin-film samples, and thus would be left for future research.…”