TEM study of basal-plane inversion boundaries in Sn‐Doped ZnO

Abstract

Various dopants are known to produce inversion boundaries (IBs) in ZnO, causing polarity inversion across the interface. These are found either in basal or pyramidal planes and can be of head-to-head or tail-to-tail configuration with respect to the orientation of the polar c-axis. The dopants, known to produce IBs in ZnO are: In2O3, Fe2O3, Mn2O3, Ga2O3, SiO2, SnO2, TiO2 and Sb2O3. While some of IBs have been studied in detail, many IB structures remain unresolved. In our study we investigated structure and chemistry of basal plane inversion boundaries in SnO2-doped ZnO. The formation of IBs in this system was first reported by Daneu et al. (2000)1 and using high-resolution transmission electron microscopy the same group attempted to solve structure and chemistry of Sn-rich IBs.2 Implementing an innovative analytical approach based on acquiring multiple EDS spectra with concentric electron probes they showed that Sn4+ ions do not occupy the full IB layer, but rather one half of the layer. This suggested an average oxidation state of III+ for the cations comprising the IB plane. Based on electron micro-diffraction and HRTEM study Daneu et al.1,2 reported that Sn-rich IBs are head-to-head oriented with interfacial cations located in trigonal prismatic sites, however, the exact atomic arrangement of Sn along IB has not been determined. To identify the translation state and atomic arrangement on Sn-rich IBs in ZnO we performed quantitative HRTEM and HAADF-STEM analysis of SnO2-doped ZnO/Bi2O3 ceramics. IBs in ZnO grains were observed for two low-index orientations, [100] and [120], to obtain a 3D information on the translation state of the IB plane. The coordination site of interfacial cations was shown to be octahedral. 472 Translation obtained from images is compared with three different, so far known, types of the head-to-head IB translations with octahedral coordination of cations at IB-plane: (i) IB with stacking of the cation sublattice, as observed with Sb doping, (ii) IB with, as observed with In and Fe doping and (iii) IB with as observed with Mn doping. For Sn-doped ZnO translations turned out to be (ii) , the same as the one occurring in In- and Fe-doped ZnO. Based on experimental observations of local atomic arrangement, HRTEM and HAADF-STEM image simulations were performed for two different in-plane distributions of Sn and Zn atoms, to better understand experimental image contrast on IBs. The generated models will be further used for ab-initio calculations aimed at determining the electronic structure of IBs.