Effect of La and Y on Crystallization Temperatures of Hafnia and Zirconia
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Crystallization of amorphous Y- and La-doped HfO2 and ZrO2 nanophase powders was studied using thermal analysis and high-temperature x-ray diffraction. Substantial increase of crystallization temperature of amorphous hafnium and zirconium oxides could be achieved by alloying with La2O3. The crystallization temperature of Hf2La2O7 composition is higher than 900 °C, which makes it a candidate for advanced gate dielectrics. In contrast, Y-doping did not significantly raise the crystallization temperature.
Advanced electronics need replacement of amorphous SiO2 by a more effective dielectric.1–4 Hafnium oxide has been the subject of intensive research for this purpose.2–4 However, crystallization of amorphous HfO2 films at relatively low temperature (below 600 °C)4 poses a problem. Alloying with SiO2 and Al2O3 has been suggested to retard HfO2 crystallization,3,5 increasing thermal stability at the cost of lowering dielectric constant. Rare earth oxides have higher dielectric constants than Al2O3 or SiO2. We studied the crystallization of Laand Y-doped hafnia and zirconia to evaluate the effect of these dopants on crystallization temperature. Pure and doped amorphous hafnia and zirconia were prepared by precipitation from aqueous solutions. Appropriate volumes of approximately 0.3 mol/L solutions of hafnium or zirconium oxychlorides (HfOCl2 · 8H2O 98+% metal basis, 1.5 wt% Zr, and ZrOCl2 · 8H2O, 99.9%, Alpha Aesar, Ward Hill, MA) were mixed with approximately 0.3 mol/L solutions of lanthanum or yttrium chlorides in volumes of 18–27 mL. The solutions were added to approximately 10 mL ammonium hydroxide (NH4OH, 28%). The pH of the resulting solution was higher than 9. Precipitates were washed in deionized water followed by centrifugation. After several cycles of washing and centrifugation, the solid was dried at 140 °C. Removal of Cl− ion was confirmed by testing the wash water with AgNO3. Thermal analysis was performed on a Netzsch (Waldkraiburg, Germany) STA 449 at 20 °C/min in an oxygen flow of 40 mL/min in platinum crucibles. Thermogravimetry (TG) and differential scanning calorimetry
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(DSC) traces were recorded, and evolved gas was analyzed for some samples using a Bruker (Karlruhe, Germany) Equinox 55 infrared (IR) spectrometer attached to a thermal analyzer. On heating, samples lost 6–14 wt% from water and CO2, depending on composition and time of exposure to air after drying. Most of the weight loss took place below 500 °C and correlated with wide endothermic peaks on DSC. On further heating, weight loss did not exceed 1.1 wt%. Crystallization onset and enthalpy were determined from the exotherms. Sample weight at temperature after the exothermic peak on DSC was taken from TG traces for crystallization enthalpy calculation. Temperature calibration was done using the melting points of metal standards, and sensitivity calibration was performed using the heat capacity of sapphire standards before each series of experiments. Thirty to 100
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