Improvement of the Electrical Properties of YMnO 3 Thin Films in a Metal/Ferroelectric/Si Structure
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films deposited directly on a semiconductor, it is
necessary to reduce the interface trap density (D,,) at the ferroelectric/semiconductor interface as well as the effective interface charge (Q,) and mobile ionic charge (Q,) in the ferroelectrics for normal FET operation. YMnO 3 is one of promising candidates for application to MFSFET because of its low dielectric constant and no volatile constituent elements. It has been reported that highly (0001)-oriented YMnO 3 thin films can be grown directly on n-type Si(100) substrates by a chemical solution deposition (CSD) at a high annealing temperature of 850 'C [1]. The rapid thermal annealing (RTA) process with final conventional furnace annealing (CFA) at the high temperature can deteriorate the ferroelectric properties of the film by causing thermodynamic reactions between the film and the substrate, thus increasing D,, Q,, and Qm as shown in the previous report [1]. A reduction of annealing temperature is desirable for direct integration of ferroelectrics with semiconductors. We report in this paper improved structural and electrical properties of YMnO 3 thin films grown at a temperature as low as 650 'C by CSD with a modified precursor solution and RTA. Detailed capacitance-voltage (C- V) characteristics due to the ferroelectric polarization were discussed in relation to Q, and Qm. D, and leakage current density were measured through a conductance method and current-voltage (I-V) measurement, respectively. Also, the stability of ferroelectric polarization is discussed in the MFS structure by investigating the polarization-voltage (P-V) characteristics as well as the C-V ones. It is shown that this MFS structure with YMnO 3 thin film is applicable to MFSFET nonvolatile memory devices.
305 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society
EXPERIMENT Sample Preparation To prepare the YMnO 3 precursor solution, manganese (III) acetyl-acetonate was dissolved in acetylaceton and refluxed for 24 h. Then, the solution was refluxed again in 2-methoxyethanol for 24 h. Tri-i-propoxyyttrium was mixed in the solution and refluxed for 72 h. The final precursor solution was transparent and stable. The Si substrates were dipped in 7:1 buffered HF solution for 3 min to remove the native Si0 2 on Si. The solution was deposited by spin coating followed by the RTA process in air at 650 VC for 40 s for each layer. The processes were repeated to obtain films of desired thickness. The samples prepared through different processes are listed in Table I, where Ml, M2, M3, and M4 are the sample names. Electrical Measurements To measure the electrical properties, the samples were prepared in a MFS structure with a circular gold electrode (7 X 10'4 cm 2) on the surface of the YMnO 3 thin films as gate. On the back surface of the substrate (Si doner doping concentration: 8.7x1014 cm 3 ), gold was evaporated and annealed at 350 'C in N 2 atmosphere for 10 min for better ohmic contact. The electrical measurements were carried out at room temperature (RT) in a light-proof m
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