3D structure and growth of inversion domain boundaries in nanowires of I n RO 3(ZnO)m (R = Al, Fe, Ga, In) ‐ an electron microscopy study

Abstract

Materials based on zinc oxide with additions of other transition and main group metal oxides offer a broad range of applications such as varistors, transparent conducting oxides (TCOs), gas sensors and dye‐sensitized solar cells. They have good semiconducting and optical properties at low costs and easy availability.[1] The prime example is IGZO (indium gallium zinc oxide, InGaZnO 4 ), which has garnered widespread attention for its use in flat‐panel TFT displays. While the structure of these materials has been studied extensively for close to 50 years,[2] a model comprehensively describing the formation and growth of its unique structural features has not been proposed thus far. It is crucial to gain an understanding of the atomic arrangement and the growth mechanisms of basal and pyramidal inversion domain boundaries (IDBs). ZnO nanowires (NWs) were grown on fused silica substrates via a thermal evaporation method and a metal‐seeded growth mechanism.[3] Conversion of said NWs to IAZO, IGZO or IFZO NWs was performed by spin‐coating with solutions of indium nitrate and aluminium nitrate, gallium nitrate or iron(III) acetylacetonate, respectively, in 2‐methoxyethanol. For thermal decomposition of the solution droplets and subsequent reaction of the various oxide particles with ZnO at the NW surface, specimens were then annealed in a furnace in ambient air at 1000 °C. High‐angle annular dark field (HAADF) and BF/ABF STEM imaging at high resolution as well as spectroscopic analyses were performed using an advanced analytical TEM/STEM system (JEOL JEM‐ARM 200CF equipped with a cold FEG, probe C s corrector, X‐ray (JEOL Centurio) and electron spectrometer (GATAN GIF Quantum ERS) attachments).[4] The aforementioned synthesis method yields faceted ZnO NWs of various growth directions including [10‐1 0] and [10‐1 1]. Due to their distinct morphology, they offer two preferential sites for the reaction with R 2 O 3 particles: large, planar {2‐1‐1 0} surfaces and kinks between {0001} and {10‐1 1} facets. This allows to image the initial formation of basal and pyramidal IDBs and their growth into the bulk of the NW with the viewing direction either perpendicular or parallel to the direction of growth (see figure 1). In perpendicular direction, the location where the basal IDB and two ZnO {0002} lattice planes meet (dotted line, fig. 1a) appears sharp. Adjacent ZnO layers are displaced in direction of the polar c axis by up to 0.9 Å at the boundary. In parallel direction, the IDB appears to be sandwiched tightly between two ZnO planes, its image contrast gradually fading towards the outer edges. The surrounding atomic columns of ZnO appear slightly distorted. In a three‐dimensional representation, it is visualized that both phenomena are the result of projections from different depth regions in the NW (see figure 2). Elemental distributions of cations were mapped by X‐ray spectroscopic imaging. From spatially resolved EDS analyses, we conclude that both trivalent cations (In and R) occupy exclusively those sites most energetically suitable for them. R‐decorated pyramidal IDBs originate as a flat defect parallel to and adjoining the In‐decorated basal IDB, while the surrounding wurtzite‐structured ZnO domains are unaffected. From there, the flat defect grows into a dome‐shaped and, finally, a pyramid‐shaped defect. This process requires a displacement of R cations via cation vacancies and entails a considerable distortion of all surrounding tetrahedral cation sites (see figure 3).