Dong Chul Yoo

Improvement of electrical properties of ferroelectric gate oxide structure by using Al2O3 thin films as buffer insulator

Abstract SrBi2Nb2O9 (SBN) and Al2O3 thin films were prepared by r.f.-sputtering as a ferroelectric material and as a buffer insulator for metal/ferroelectric/insulator/semiconductor (MFIS) structure, respectively. The electrical properties of those films on Si substrate were studied. Coercive field that decisively affects the memory window was greatly increased by inserting an Al2O3 insulator between SBN and Si, and thus the memory window also increased with the increasing electric field to the SBN. The Al2O3 intermediate layer between the perovskite SBN film and Si substrate prevent SBN from the serious inter-diffusion into Si substrate. Memory windows of MFIS structure were in the range of 0.7–3.4 V when the gate voltage varied from 3 to 9 V. Memory windows of MFIS structure were found to be dependent on the thickness and stoichiometry of the buffer layer. We obtained the maximum memory window in the MFIS structure with an optimized insulator thickness of 11.4 nm.

Microstructure control of YMnO3 thin films on Si (100) substrates

Abstract The microstructure of the sputtered-YMnO 3 thin films on Si (100) substrates was controlled by only using thermal treatment processes and YMnO 3 thin films having a well-defined bi-layered microstructure were fabricated. These YMnO 3 thin films have two distinct layers, i.e. approximately 40 nm-thick top layer of {00 l }-oriented YMnO 3 and approximately 60 nm-thick bottom layer of polycrystalline YMnO 3 in the 100 nm-thick film. The abrupt change of the crystalline orientation from the {00 l }-preferred orientation to the random orientation is mainly due to a high stress induced by the {00 l }-oriented YMnO 3 layer, which was confirmed by a full width at half-maximum in Si (004) rocking curves. The controlled c -axis/polycrystalline YMnO 3 thin films showed a better memory window and low leakage current density than purely c -axis-oriented YMnO 3 thin film and the purely polycrystalline YMnO 3 thin film.

Atomic structure of random and c-axis oriented YMnO3 thin films deposited on Si and Y2O3/Si substrates

We have studied the atomic structure of YMnO3 deposited on Si and Y2O3 with high-resolution transmission electron microscopy and fast Fourier transforms-filtered lattice image analysis during furnace and rapid thermal annealing (RTA) processes. For the YMnO3/Si, it is found that the YMnO3 layer is c-axis oriented with an amorphous bottom region after furnace annealing at 850 °C for 1 h. In contrast, after RTA at 850 °C for 3 min the bottom region forms YMnO3 polycrystalline layer with the {1212} plane parallel to the surface. When an Y2O3 layer is interposed between YMnO3 and Si, a c-axis oriented YMnO3 layer grows on a [111]-oriented Y2O3 layer. Memory window and leakage current density of the c-axis YMnO3/[111] Y2O3 bilayers are strongly improved due to an aligned [0001] unipolar axis.

Hydrogen-induced atomic deformation in SrBi2Nb2O9 perovskite structure

The origin of hydrogen-induced structural deformation of ferroelectric SrBi2Nb2O9 (SBN) thin films is investigated by annealing in forming gas (3%H2–97%N2). High resolution transmission electron microscopy and fast Fourier transformation analysis reveal that the {115} planes are shifted upward and downward by 0.92A along {115} plane after forming gas annealing, resulting in (00l) planes inclined by 9.54°. This shifted distance of 0.92A means that the perovskite structure is distorted by 29.98% compared to the normal interatomic distance of 3.077A. This distorted perovskite structure results in degradation of ferroelectric properties. However, this lattice deformation and ferroelectric property of SBN films are recovered after annealing in oxygen ambient.

Crystallization Behavior and Ferroelectric Properties of YMnO 3 Thin Films on Si (100) Substrates

YMnO 3 thin films were sputtered on Si (100) substrates under different ambient conditions. After rapid thermal annealing process at 850 °C, the YMnO 3 film deposited in Ar ambient had random orientations and the YMnO 3 film deposited in Ar+O 2 ambient was crystallized with distinct two layers, i.e., c-axis oriented layer in top region and random oriented layer in bottom region. Relations between the microstructure and the electrical properties of Pt/YMnO 3 /Si capacitor were investigated. Memory window and leakage current depended on the orientation of the YMnO 3 thin films and the interfacial microstructure of the YMnO 3 /Si, respectively