Microstructure and Electrical Properties of Multicomponent Doped In₂O₃-Based Ceramic Targets

Silicon heterojunction (SHJ) solar cells have a high conversion efficiency, which drives the development of the photovoltaic industry. The transparent conductive oxide (TCO) films with a high electron mobility are needed to improve the conversion efficiency of SHJ solar cells. The mobility of the conventional 10% (in mass) SnO₂ doped In₂O₃ films is 30 cm²/(V·s), which cannot meet the demand of high mobility. Some reports indicate that the Ce, Ti, Zr, W, Hf, Mo, and Ta dopants with a low doping content can improve the mobility. A high density ceramic target is a key material for depositing TCO films. However, a low doping content is unconducive to sintering densification of ceramic target. Therefore, multiple oxide dopants with harmless sintering additive are used to enhance sintering densification and decrease resistivity. In this paper, an In₂O₃-based ceramic target doped with ZrO₂, CeO₂, Ta₂O₅, and TiO₂ was designed, and CeO₂ and TiO₂ were acted as sintering additives. In addition, the phase structure, microstructure, density, and resistivity of targets sintered at different sintering temperatures and times were also investigated.

High purity (99.99%) In₂O₃, CeO₂, Ta₂O₅, ZrO₂, and TiO₂ nanopowders were used as raw materials. The doping contents of CeO₂, Ta₂O₅, ZrO₂, and TiO₂ powders were 0.3%, 0.3%, 0.3% and 0.1%, respectively. Anhydrous ethanol was used as a dispersant, and the weight ratio of zirconia balls to powders was 3:1. The slurry was ground in a planetary ball mill at 360 r/min for 6 h, and then dried at 80 ℃ for 24 h. The dried powders were added with polyvinyl alcohol (PVA) at 5% aqueous solution, and then granulated through an 80-mesh sieve. The granulated powders were pressed into green bodies with a relative density of approximately 58%, a diameter of 13 mm, and a thickness of 1.5 mm. The green bodies were dewaxed at 600 ℃ for 3 h and then sintered in an oxygen atmosphere at a flow rate of 5 L/min. The targets were sintered at 1400, 1450, 1500, 1500 ℃ and 1600 ℃ for 4 h and 6 h, respectively.

The phase structure was analyzed by X-ray diffraction (XRD). The microstructure and elemental distribution were characterized by field-emission scanning electron microscopy (FE-SEM). The grain size and distribution were analyzed and statistically calculated from FE-SEM images using a software named Nano measure. The density was determined based on the Archimedes principle. The resistivity was evaluated using a four-point probe. The sintering densification behavior during the heating process was investigated by a dilatometric analyzer.

The densification and shrinkage of target during heating process can be divided into three stages. The target exhibits a rapid shrinkage after 1324 ℃, and its shrinkage rate at 1500 ℃ is 10.7%. This confirms the sintering additive function of dopants. The XRD patterns show the formation of a cubic bixbyite phase, indicating that Ce⁴⁺, Zr⁴⁺, Ta⁵⁺ and Ti⁴⁺ are dissolved in the lattice of In₂O₃. The grains are densely packed together, and the grain size gradually increases with increasing sintering temperature and holding time. All the targets show a dense internal microstructure and transgranular fracture behavior. The average grain size of targets sintered for 4 h increases from 1.44 μm at 1400 ℃ to 5.31 μm at 1600 ℃. A more substantial increase also occurs from 1.95 μm at 1400 ℃ to 10.41 μm at 1600 ℃ in the targets sintered for 6 h. Elements Ce, Ta, Zr and Ti exhibit a uniform distribution in In₂O₃ matrix, which helps to suppress an abnormal discharge during magnetron sputtering.

The density of targets sintered for 6 h is higher than that of targets sintered for 4 h. The density firstly increases and then decreases as the sintering temperature increases from 1400 ℃ to 1600 ℃. The optimal density of targets sintered for 4 h and 6 h is 99.05% and 99.39%, respectively. The resistivity of targets sharply decreases when the sintering temperature increases from 1400 ℃ to 1500 ℃, and then slightly increases with further increasing sintering temperature to 1600 ℃. The optimal resistivity of targets sintered at 1500 ℃ for 4 h and 6 h is 1.214×10^–3 Ω·cm and 1.181×10^–3 Ω·cm, respectively. The resistivity increase of target sintered at 1600 ℃ is due to the volatilization of In₂O₃ and density decrease of target at a high sintering temperature.

This work investigated the effects of sintering temperature and time on the microstructure and properties of In₂O₃-based targets doped with CeO₂, Ta₂O₅, ZrO₂, and TiO₂. The results showed that all the targets have single cubic bixbyite phase, dense microstructure. The transgranular fracture characteristics and uniform grains with sizes of <10 μm could be obtained. The doped multiple oxides exhibited a sintering additive function, promoting sintering densification. The sintering densification process could be accelerated after 1324 ℃. Raising sintering temperature and prolonging holding time could effectively improve the density and reduce the resistivity. The target sintered at 1550 ℃ for 6 h had a relative density of over 99% and a resistivity of less than 1.181×10^–3 Ω·cm. Therefore, multiple oxides doped In₂O₃-based targets with a low doping content could achieve high density and conductivity.