Crystallinity Improvement of Ferroelectric BiFeO3 Thin Film by Oxygen Radical Treatment

BiFeO₃ (BFO) thin film has been developed for realizing high-performance ferroelectric memory and/or sensors.^[1-3] Furthermore, BFO film has received attention for realizing high open-circuit voltage solar cells.^[4] Unlike other conventional ferroelectric materials such as PZT, BFO film does not contain Pb, which is a distinctive advantage to use BFO film. BFO thin films were mainly formed by the sol-gel method, pulsed laser deposition and sputtering deposition. After the BFO thin film deposition, there are oxygen vacancy defects in the BFO thin film. On the other hand, we developed high-performance ferroelectric Sr₂(Ta,Nb)₂O₇ (STN) thin films obtained using the combination of magnetron sputtering deposition of STN film and subsequent oxygen radical treatment using microwave-excited high-density Kr/O₂ plasma, where the oxygen radical treatment could dramatically reduce oxygen vacancy defects and improve crystallinity.^[5]In this study, we applied this method to the BFO film formation, and discuss the effect of oxygen radical treatment on the BFO film property.

BFO thin films (200 nm) were deposited on Pt(111)/Ta/SiO₂/Si substrate at room temperature using the magnetron sputtering. BiFeO₃ ceramic target was used. Before the sputtering deposition of BFO films, Ta (20 nm) and Pt (100 nm) were deposited on Si substrate with thermally grown SiO₂ film (500 nm). In the BFO film depositions, a working pressure was 4 Pa (Ar), and plasma was excited by applying a 13.56-MHz rf power to target with a power density of 1.0 W/cm². After the BFO thin film deposition, the radical oxidation treatment was carried out using the 2.45-GHz microwave-exited high-density plasma at 400 °C for 10 min at a working pressure of 133 Pa. An applied microwave power density was 1.0 W/cm². After the oxygen radical treatment, BFO thin film was annealed at 600 °C for 10 min in the oxygen ambient. The crystalline structures of deposited films were analyzed by X-ray diffraction (XRD).

Fig. 1 shows XRD patterns of BFO films with and without the oxygen radical treatment. The peaks of BFO (110) and (200) were clearly observed in the case with the oxygen radical treatment, while those peaks did not appear in the case without oxygen radical treatment. The results indicated that the perovskite phase could be formed in the case with oxygen radical treatment, and also, oxygen vacancy defects in the as-deposited film were oxidized by a large amount of oxygen radicals in the Kr/O₂ plasma, which promoted crystallization of the BFO film. Further reduction of crystallization temperature can be expected by optimizing the condition of oxygen radical treatment. We consider that oxygen radical treatment is effective to develop ferroelectric memory and/or sensors.

This work was supported by JSPS KAKENHI Grant Number 25630117.

Reference

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