Aqueous solution-based synthesis and deposition of crystalline In-Ga-Zn-oxide films with an enhanced mobility

In-Ga-Zn-oxide (IGZO), in its amorphous state, is known to have a high electron mobility and low off-state current inside transistor devices, which may even be further improved by crystallization. Thin films of the IGZO superlattice structure require an optimal layer homogeneity in addition to precise control of the stoichiometry, which can be achieved by using a solution-based process. In this work, an aqueous precursor system is developed, starting from the respective metal (hydr)oxides. A stable multimetal precursor is obtained, which is ideally suited for solution-deposition via spin-coating. Through an optimized multi-step thermal treatment, crystalline thin films of IGZO are obtained that show a preferential c-axis orientation after rapid-thermal annealing at 1000 °C in inert conditions. The resulting film shows a good optical transparency (>70%) and an improved carrier mobility (27.2 cm²/Vs) compared to typical solution-processed amorphous IGZO films, and is therefore promising for further application. Top: Overview of the precursor synthesis, which consists of the metal (hydr)oxides as a starting product, citric acid (ligand) and ammonia (pH adjustment and bridging ion). Middle: Schematic route of the film processing. The precursor is applied on a substrate via spincoating. Intermediate hotplate steps (200–400–600 °C) decompose the precursor into an oxide film. Finally, a thermal treatment (1000 °C) inside an RTP (double-substrate setup) crystallizes the oxide films. Bottom: Thermogravimetric plot of the thermal decomposition of the precursor solution // Plane view SEM micrograph of the film morphology after annealing at 1000 °C for 500 s // XRD diffractogram of the film after annealing at 1000 °C for 500 s. Highlights Solution-based deposition starting from an aqueous multimetal citrate precursor system Crystalline IGZO films that show a preferential c-axis orientation after annealing at 1000 °C An improved carrier mobility compared to typical solution-processed amorphous IGZO films A good optical transparency has been observed in the visible-light range