Improvement of Solid Through Improved Solutions and Gels (2): The Other Methods

Abstract

In Chap. 12, we demonstrated that solid films with good properties can be formed at lower temperatures through modification of the solution composition by adding the third element of amines. Here, in Sect. 13.1 we introduce another option: solution amelioration. That is, solvothermal treatment of the solution was found to be very effective to ameliorate the solution, imparting it with improved processability for solidification. We refer to this treatment as “solvothermal synthesis” of a precursor solution. In this section, we introduce the effects of solvothermal synthesis on the insulating properties of LaZrO films. Detailed structural analyses of the precursor solutions, dried gels, and annealed solids were extensively carried out. The analytical results show a substantial improvement of properties achieved by solvothermal treatment of solutions. We confirmed that the structural modification of metal–organic precursors in solution enhanced the processability of the solution in solidification, resulting in a final solid oxide with good properties and a good crystal structure.

We observed that hybrid clusters with inorganic cores coordinated by organic ligands were the typical metal–organic precursor structures. Structural unification of the cluster core was achieved by the solvothermal treatment. Greater uniformity of clusters facilitates the formation of a higher quality solid. The thus-made solid maintains features similar to those of the core structure of the cluster, even after annealing at high temperatures. These results demonstrate the importance of designing and ameliorating the cluster structure in solution.

As a novel method for producing device-quality oxide semiconducting thin film at temperature as low as 200 °C, solution combustion synthesis (SCS) was developed by Kim MG, Kanatzidis MG, Facchetti A, Marks TJ, Nat Mater 10:382, 2011. In Sect. 13.2, the SCS method is introduced. The self-generated heat of combustion synthesis provides a localized energy supply, eliminating the need for high, externally applied processing temperatures. In addition, the atomically local oxidizer supply can efficiently remove organic impurities without coke formation. Here, a redox-based combustion synthetic approach is applied to indium tin oxide (ITO) thin film using acetylacetone as a fuel and metal nitrate as oxidizer (Tue PT, Inoue S, Takamura Y, Shimoda T, Appl Phys A Mater Sci Process 122(6):1–8, 2016). The structural and electrical properties of SCS-ITO precursor solution and thin films were systematically investigated with changes in tin concentration, indium metal precursors, and annealing conditions such as temperature, time, and ambient. After that, the optimized SCS-ITO thin film was applied for source/drain (S/D) electrodes in a total solution-processed amorphous oxide TFT. The performance and stability of the SCS-ITO TFT were evaluated and compared to those of the TFT with sputtered-ITO S/D electrodes.