Distribution of Fe and In dopants in ZnO: A combined EELS/EDS analysis

Abstract

Zinc oxide (ZnO) is a highly versatile material with unique combinations of optical, electronic and piezoelectric properties which can be controlled by the addition of Fe or In. Undoped ZnO crystallizes in the non-centrosymmetric wurtzite structure with alternating (0002) layers of tetrahedrally coordinated O²⁻ and Zn²⁺ stacked along the c axis. In the present investigation the distribution of dopants in ZnO is studied by a combined quantitative EELS/EDS analysis. In ZnO doped with Fe³⁺ or In³⁺ a characteristic inversion domain structure with planar inversion domain boundaries (IDBs) is observed on (0001) planes (basal IDBs) and $\{ 2 \bar 1 \bar 15\}$ planes (pyramidal IDBs), respectively (Fig.1). The number of IDBs is directly correlated to the local dopant concentration; quasi-periodic structures are observed at dopant concentrations ≥ 5 at.% of cations (Fig.2). The (0002) lattice planes are well resolved, albeit severely distorted in the vicinity of IDBs in Fe-ZnO. Elemental mapping in EFTEM indicates that dopants are essentially located within the IDBs (Fig.3), however, it does not allow for a quantitative assessment of dopant concentrations [1]. EEL spectra were acquired with high spatial resolution in diffraction/nanoprobe mode. Regions analyzed by EELS measurements are indicated in Fig.3a (open circles). Quantitative measurements, corrected for O-K EXELFS oscillations [2], yielded a solid-solubility < 0.4 at.% Fe in unaffected ZnO domains (ZSS), whereas Fe is depleted in inverted domains (Z₀). The Fe content in single basal IDBs was measured by EELS using the variable beam diameter method [1], yielding an effective boundary thickness δ ≈ 0.27 nm, corresponding to one closepacked monolayer of Fe³⁺.