Papers by Author: Raluca Savu

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Authors: Cezarina C. Mardare, Andréi I. Mardare, Raluca Savu
Abstract: Three types of bottom electrodes were deposited by RF magnetron sputtering on SiO2/Si substrates: LaNiO3 (LNO), Pt/Ti and LNO/Pt/Ti. The effect of different deposition and processing conditions for the LNO films on the ferroelectric properties of sputtered Pb(Zr0.52Ti0.48)O3 (PZT) capacitors was investigated. The LNO films were either deposited at room temperature and heattreated in O2 flow in the furnace at 500 or 600°C or made in situ in the range of 200-500°C. Other deposition parameters under study were the pressure, the RF power and the Ar:O2 ratio. The resistivity of the LNO films was measured and on some of the films with the lower values, amorphous PZT was deposited and then crystallized in the furnace. X-ray diffraction results show that the PZT films deposited over Pt/Ti had a preferential (100) orientation, while those deposited over LNO made in situ are strongly (100) oriented and the ones deposited over amorphous LNO do not exhibit any preferential orientation. The remanent polarization of the capacitors was around 28μC/cm2 when amorphous LNO or Pt/Ti electrodes were used and around 20μC/cm2 with LNO made in situ. Leakage currents were improved when LNO electrodes made in situ was used; a good ferroelectric fatigue performance of the capacitors when subjected to 1010 switching cycles was also observed
Authors: Raluca Savu, Ednan Joanni
Abstract: Nanocrystalline indium tin oxide (ITO) thin films were deposited on Si/SiO2 substrates by laser ablation from a ceramic target with a composition of 0.9 In2O3 . 0.1 SnO2. Samples were prepared in the pressure range from 10-1 to 5mbar, either in-situ at 500°C or at room temperature and heat-treated in air at 500°C. X-ray diffraction results show that the films are not oriented, except the ones made at high temperature which exhibit strong (400) orientation. AFM pictures show that the grains are round shaped and the sizes are in the range between 50 and 200nm, except for films made in-situ at 10-1mbar which are elongated and faceted. For higher pressures the grains tend to be small and to form agglomerates. The porosity of the films increases with the deposition pressure and the thicknesses reach a maximum of 2.8µm at 1mbar for the films made at room temperature and of 1.2µm at 2mbar for the ones made in-situ; for higher pressures the growth rate drop drastically, as revealed from SEM observations of cross-sections. The electrical resistance increases with the deposition pressure due to the increase in porosity, changing from 3.3k to 38.9M for films deposited at room temperature and from 20 to 265k for the ones made in-situ.
Authors: Andréi I. Mardare, Cezarina C. Mardare, Raluca Savu
Abstract: The bottom electrode crystallization (BEC) method was applied to the crystallization of Pb(Zr,Ti)O3 (PZT) thin films deposited by RF magnetron sputtering on Pt/Ti/SiO2/Si substrates. Using a proportional-integral-differential controller, the current flowing in the Pt/Ti films provided accurately controlled Joule heating for the crystallization of the PZT films. The temperature uniformity of the heat treatments was investigated by measuring the ferroelectric properties of PZT. Platinum and tungsten wires were alternatively used as electrical contacts. Scanning electron microscopy (SEM) images were used to inspect the electrical contact regions between the platinum films and different contact wires. The PZT films showed higher remanent polarizations and lower leakage currents near the electrical contacts when Pt wires were used; the ferroelectric properties were more uniform on the PZT films heat-treated with W contact wires. The BEC method can successfully replace the more conventional means for thin film crystallization, having the advantage of being a very precise, low cost and low power consumption technique.
Authors: José R. A. Fernandes, Ednan Joanni, Raluca Savu
Abstract: Thin films of PbZr0,52Ti0,48O3 (PZT) for applications in piezoelectric actuators were deposited by the pulsed laser deposition technique (PLD) over Pt/Ti/SiO2/Si substrates. The effect of different electrode and PZT deposition and processing conditions on the ferroelectric and piezoelectric properties of the devices was investigated. X-Ray diffraction results showed that the deposition temperature for the electrodes had a strong influence on the PZT orientation; the increase in the electrode deposition temperature changes the PZT orientation from random or (111) to (001) depending also on PZT deposition pressure. From scanning electron microscope (SEM) pictures one could also observe that the deposition pressure affects the porosity of the PZT films, which increases with the pressure above 1×10-1 mbar for films deposited at room temperature. The measurement of the ferroelectric hysteresis curves confirmed that the structural changes induced by different processing parameters affected the ferroelectric properties of the material. The best ferroelectric properties including fatigue endurance were obtained for electrodes made at high temperature and for PZT deposited at 2×10-2 mbar and heat treated at 675°C for 30 minutes in an oxygen atmosphere. The piezoelectric coefficient d33, measured using a Michelson interferometer, had values in the range between 20 and 60 pm/V, and showed a strong dependence on the thickness of the PZT films.
Authors: Cezarina C. Mardare, Pedro B. Tavares, Andréi I. Mardare, Raluca Savu
Abstract: A dense ceramic target of BiFeO3 was synthesized by the urea combustion method. X-ray diffraction indicates that this target is composed of a mixture of phases, the main one is BiFeO3, but Bi46Fe2O72 and Bi2Fe4O9 are also present in small amounts. The BiFeO3 target was used for depositing thin films on Pt/Ti/SiO2/Si substrates by the laser ablation technique. The depositions were made in oxygen atmosphere at pressures in the range between 5x10-3 and 2x10-2mbar, using a KrF laser. The substrate temperatures were 450 or 500°C and the laser energy, the frequency and the distance between the target and the substrate were kept constant at 125mJ, 10Hz and 4cm, respectively. After a deposition time of 30minutes the thickness of the films was approximately 400nm. Some of the films were heat-treated in situ, in 100mbar O2 for 30minutes, at the same temperatures used for deposition. X-ray diffraction results show the BiFeO3 phase, as well as some Bi46Fe2O72 and Bi2Fe4O9. The films were crystallized without any preferential orientation, but the ones made at 2x10-2mbar and 450°C were partially amorphous. For measuring the ferroelectric hysteresis loops, either Al top electrodes were deposited by thermal evaporation or Pt, by sputtering. The distorted shapes of the hysteresis loops obtained indicated that the films exhibit weak ferroelectric properties and high leakage current values.
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