YMnO 3 and YbMnO 3 Thin Films for fet type FeRam Application
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Ferroelectric RAMs with a metal-ferroelectric-semiconductor field effect
transistor (MFSFET) structure have the advantages of saving electrical power and decreasing memory cell size, because the electrostatic charge induced by remnant polarization of ferroelectric materials controls the conductivity of Si substrates [3]. For the MFSFET operation, it is necessary for the film to have not only a ferroelectric property but also a small interface state density between Si and ferroelectric films. The interdiffusion between ferroelectric films and Si [4] and the formation of a SiO2 layer with a low dielectric constant at the film/Si interface, however, make it very difficult. The use of fluoride ferroelectric materials such as BaMgF4 has been proposed to resolve this problem [5]. Although C-V hysteresis with a 237 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society
polarization type has been obtained, fatigue and retention problems still exist. On the other hand, a metal-ferroelectric-insulator-semiconductor field effect transistor (MFISFET) structure has also been demonstrated using various buffer layers such as CeO2 and MgO [6-10]. The applied voltage cannot be effectively used for polarization, however, due to the existence of a buffer layer with a dielectric constant lower than that of typical ferroelectric films. We have proposed the use of RMnO, (R: rare earth elements) thin films for non-volatile memories [11-18].
Because RMnO3 has an hexagonal structure with a unipolarization axis
along [0001], it has low permittivity, and has no volatile elements, it is expected to have several advantages over PZT and SBT, especially for the application to MFF(I)SFET. This paper describes the progress of YMnO 3 and YbMnO 3 on various substrates and the possibility to the FET type FeRAM application is discussed.
EXPERIMENTAL
YMnO3 and YbMnO3 thin films were deposited
on (ill)MgO,
Al
doped
(0001)ZnO/(0001)Sapphire, ( lI )Pt/(0001)Sappire, SiO/(100)Si and Si (I 11) substrates using a pulsed-laser deposition. A KrF excimer laser (Lambda Phisik) with a 248 nm wavelength was used at a pulse rate of 1-5 Hz and laser energy density of 1-5 J/cm 2 . Sintered ceramic pellets of stoichiometric or Mn-rich YMnO3 and YbMnO3 were used for targets. The Si substrates were boiled in ethanol for 5 min and rinsed with pure water. They were then soaked in 30 wt% HNO3 solution for 5 min, rinsed with pure water, and dipped in 2.5 wt% HF solution for 30 s. sequence was repeated twice.
This
The substrate temperature was varied from 700 'C to 850 'C.
The structure of the film was evaluated by X-ray diffraction (XRD, Shimazu XD-3A), reflection high-energy electron diffraction (RHEED, JEOL2000FXII), and transmission electron diffraction (TED, JEOL2000FXII).
Surface morphology and composition of the films
were evaluated using a field emission type scanning electron microscope with energy dispersion X-ray spectroscopy (FE-SEM, HITACHI S-4500). To measure the dielectric properties, circular Pt electrodes (0.01-0.1
mm2)
were
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