Preparation and characterisation of ceramic and thin film Zn(_2)SnO(_4)

Abstract

Ceramic zinc stannate, Zn(_2)SnO(_4), was prepared from 1SnO(_2):2ZnO mixture using powders of the highest commercially available purity. The solid state reaction between the ZnO and the SnO(_2), thought to be an evaporation-recondensation mechanism, was found to start at ~ 900 ˚C (12 hours heating, rate 5 ˚C min(^-1)). However, the reaction did not go to completion in the timescale of the experiment unless the temperature was raised to~1300 C. In this case mono-phase, polycrystalline Zn(_2)SnO(_4) was produced, as confirmed by X-ray diffraction (XRD), scanning electron microscopy and energy dispersive X-ray analysis (EDAX). Further evidence for these reaction temperatures was obtained from thermal analysis experiments. As-sintered, Zn(_2)SnO(_4) was insulating (σ — 10(^19) Ω(^-1) cm(^-1)) although it could be made conductive, by a reduction heat-treatment. This entailed refiring the sintered pellets of Zn(_2)SnO(_4) in an atmosphere of mixture gas (25% H(_2) + 75% N(_2)) at ~ 450 ˚C for 14 hours (heating rate of 10 C min(^-1)). This reduced the conductivity to values of σ~1 x 10(^-2) Ω(^-1) cm(^-1) . XRD failed to reveal any changes in the phase of the material after the reduction treatment. Several dopants were investigated, the most successful of which was in, using a vapour phase method. Doping with In this way gave a significant change in the colour from white to dark grey together with a reduction in electrical resistivity, without recourse to further heating treatments. No change in the usual phase of the Zn(_2)SnO(_4) was detected. Doping with group V oxides, such as Nb(_2)O(_5), V(_2)O(_5) etc, produced changes in the colour from white to dark grey, but no reduction in the resistivity, unless further heating treatments were carried out in reducing ambients. When high concentrations of Nb were introduced an additional phase, possibly Nb(_2)Sn(_2)O(_7) was observed by XRD. Thin film Zn(_2)SnO(_4) was prepared by Electron Beam Evaporation using Zn2Sn04 sintered powder as the evaporant material. The thin films were deposited onto glass substrates, at a range of substrate temperatures between room temperature and 250 ˚C. XRD was used to confirm the formation of Zn(_2)SnO(_4), and provide estimates for the grain size, which varied from 20 to 25 nm. RHEED studies indicated that the grain size increased as the substrate temperature was increased. SEM revealed that the thin films were flat and uniform, with no cracks. The optical transmission of the thin films was about 88% for films deposited at 200 ˚C, but decreased significantly as the substrate temperature was decreased. The spectral dependence of complex refractive index (n&k) suggested that true thin film formation did not take place until the substrate temperature exceeded ~ 150 ˚C, and that the material was apparently a direct gap semiconductor with a band gap energy of ~1.95 eV. It was found that the main carrier transportation mechanism for doped, un- doped, and thin films of Zn(_2)SnO(_4) was variable range hopping, with a temperature dependence of the form exp(To/T)'^This result was consistent with Hall effect measurements, where high, temperature independent carrier concentrations of about 10(^17) cm(^-3) were obtained, along with low values of carrier mobility ( ~ 1 cm(^2) v(^-1) sec(^-1)) that obeyed the same temperature dependence as the conductivity, [exp(To/T)(^1/4)].