Ferroelectric properties of c-oriented YMnO3 films deposited on Si substrates
Applied Physics Letters
We have proposed the use of RMnO 3 ͑R: rare earth elements͒ films for metal-ferroelectricsemiconductor field effect transistor ͑MFSFET͒-type ferroelectric random access memories ͑ferroelectric RAMs͒. This reports the production of YMnO 3 films on Si substrates for MFSFET with confirmation of the distinct ferroelectricity by P -E hysteresis and capacitance-voltage (C -V) measurement. ͑0001͒-oriented YMnO 3 films were obtained on a ͑111͒Si substrate using a pulsed-laser deposition method.
... the Pt/YMnO 3 /Si structure exhibits a very small remnant polarization of 1.2 nC/cm 2 , it has clear ferroelectric polarization switching type C -V characteristics with a memory window of 1.1 V. The dielectric constant and the dissipation factor were 27.8 and 0.035, respectively. The polarization switching characteristics are discussed. © 1998 American Institute of Physics. ͓S0003-6951͑98͒03429-9͔ Ferroelectric thin films, such as Pb(Zr x Ti 1Ϫx )O 3 ͑PZT͒ and SrBi 2 Ta 2 O 9 ͑SBT͒ have attracted much attention due to their application in nonvolatile random access memories ͑RAMs͒. 1,2 Ferroelectric RAMs with a metal-ferroelectricsemiconductor field effect transistor ͑MFSFET͒ 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 SiO 2 layer with a low dielectric constant at the film/Si interface, however, make it very difficult. The use of fluoride ferroelectric materials such as BaMgF 4 has been proposed to resolve this problem. 5,6 Although C -V hysteresis with a polarization type has been obtained, fatigue and retention problems still exist. On the other hand, a metal-ferroelectric-insulatorsemiconductor field effect transistor ͑MFISFET͒ structure has also been demonstrated using various buffer layers such as CeO 2 and MgO. 7-11 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. Previously, we have proposed the use of RMnO 3 ͑R: rare earth elements͒ thin films for nonvolatile memories. 12-14 Since RMnO 3 has a hexagonal structure with a unipolarization axis along ͓0001͔, has low permittivity, and is without volatile elements like Pb and Bi, it is expected to have several advantages over PZT and SBT, especially for the application to MF͑I͒SFET. Moreover, the ͑0001͒ epitaxial YMnO 3 films on ͑111͒MgO and ͑0001͒ZnO:Al/͑0001͒ sapphire substrates have already been successfully obtained. In the present study, we obtained ͑0001͒-oriented YMnO 3 films on a ͑111͒Si substrate using a pulsed-laser deposition method and demonstrated that the Pt/YMnO 3 /Si structure exhibits polarization switching behavior. YMnO 3 thin films were deposited on p-type Si͑111͒ substrates with 1 ⍀ cm resistivity using a pulsed-laser deposition method ͑ULVAC, ULP-1000͒. A KrF excimer laser ͑Lambda Physik͒ with a 248 nm wavelength was used at a pulse rate of 5 Hz and laser energy density of 2.5 J/cm 2 . Sintered ceramic pellets of stoichiometric YMnO 3 were used for targets. The substrates were boiled in ethanol for 5 min and rinsed with pure water. They were then soaked in 30 wt % HNO 3 solution for 5 min, rinsed with pure water, and dipped in 2.5 wt % HF solution for 30 s. This sequence was repeated twice. The substrate temperature was fixed at 830°C. The structure of the film was evaluated by x-ray diffraction ͑XRD, Shimazu XD-3A͒, reflection high-energy electron diffraction ͑RHEED, JEOL2000FXII͒, and transmission electron diffraction ͑TED, JEOL2000FXII͒. 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, a͒ Electronic mail: email@example.com FIG. 1. XRD pattern of the YMnO 3 film deposited on Si substrates with a 2-nm thick Y-Mn-O buffer layer.