Origin of Leakage Current of YMnO 3 Thin Films Prepared by the Sol-Gel Method
- PDF / 1,017,147 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 96 Downloads / 200 Views
ABSTRACT The preparation conditions of YMnO3 thin films by the sol-gel method using yttrium alkoxide were optimized to decrease the leakage current of the films. The leakage current of the films was decreased due to the dense microstructure of the films. Moreover, the heat treatment in hydrogen atmosphere and the zirconium doping resulted in a further decrease of the leakage current. The heat treatment in hydrogen atmosphere and the zirconium doping were effective in the decrease of carriers originating in the valence fluctuation of the Mn ions in YMnO3. INTRODUCTION YMnO3 thin films have been proposed for a field effect transistor type non-volatile ferroelectric memory device since they have low dielectric constant and sufficient remanent polarization, and do not contain volatile elements such as Bi or Pb [1-4]. However, a serious issue for YMnO 3 is its rather large leakage current due to the unstable valence state of Mn ions. Kamata et al., using thermogravimetric analysis, report that YMnO 3 sintered in air has the composition YMnO3 , [5], which is reported to be a p-type semiconductor [6,7], and Shimura et al. suggest that the excess oxygen in the bulk YMnO +, 3 enhanced hole generation [8]. Since the leakage current of YMnO3 must originate from the valence fluctuation of Mn ions, the atmosphere during the heat
treatment of YMnO 3 thin films is very important to control the leakage current. We reported the preparation of YMnO3 thin films by the sol-gel method from yttrium acetate [9-13] and yttrium alkoxide [14] as a starting material. When yttrium acetate was used, the ferroelectricity was observed in the films of metal-ferroelectric-metal and metal-ferroelectric-insulator-semiconductor structures [12]. However, the crystallization of YMnO 3 thin films prepared from yttrium acetate required process temperature of at least about 800'C. On the other hand, the preparation using yttrium alkoxide was found to be effective in lowering the crystallization temperature of YMnO 3 thin films; YMnO 3 thin films prepared from the alkoxide were crystallized at 5000C for 3 min. [14]. The films which were layer-by-layer crystallized at 700'C for 3 min. exhibited a high crystallinity with c-axis preferred orientation and ferroelectricity was observed in the films. However, the films prepared from the alkoxide had a rather large leakage current. This is due to columnar grains, and to the valence fluctuation of Mn ions in the films. In general, leakage currents of films are affected by the parameters including the microstructures [15], annealing conditions [16], compositions [17], and their correlations. Therefore, the improvement of the microstructure and the control of the valence state of Mn ions should be achieved through optimization of the preparation process. In this study, the optimum preparation conditions of the films fabricated from yttrium alkoxide have been investigated. The heat treatment process of the films was changed to increase the nucleation density and then obtain a dense microstructure. In addition, heat
Data Loading...
