Strain Dependence of Dielectric Properties and Reliability of High-k Thin Films
The effect of strain on both electronic and structural characteristics of HfO2 used for sub-50-nm semiconductor devices was analyzed by a quantum chemical molecular dynamics analysis. The band structure of HfO2 is strongly affected by strain in the film. Based on the change rate of band gap obtained from our simulations, the strain sensitivity of the relative permittivity of HfO2 film is predicted to about 2.4%/1%-strain. The predicted strain sensitivity was validated quantitatively by measuring the change of the capacitance of MOS capacitors using a HfO2 gate dielectric film. It is very important, therefore, to minimize the mechanical strain in the dielectric film to assure the reliability of MOS transistor.
Temperature- and Frequency-Dependent Ferroelectric Characteristics of Metal-Ferroelectric-Metal Capacitors with Atomic-Layer-Deposited Undoped HfO2 Films
In this study, we evaluated the temperature- and frequency-dependent ferroelectric characteristics of TiN/undoped HfO2/TiN metal-ferroelectric-metal (MFM) capacitors in which an undoped HfO2 film was deposited through atomic layer deposition (ALD). Successful ferroelectric characteristics were achieved after postdeposition annealing at 650 °C, which exhibited a remanent polarization of 8 μC/cm2 and a coercive electric field of 1.6 MV/cm at 25 °C (room temperature). The ferroelectric property was maintained at 200 °C and decreased as the temperature increased. The ferroelectric property was completely lost above 320 °C and fully recovered after cooling. The frequency dependency was evaluated by bias-dependent capacitance–voltage and s-parameter measurements, which indicated that the ferroelectric property was maintained up to several hundred MHz. This study reveals the ultimate limitations of the application of an undoped HfO2 MFM capacitor.
6Li and 7Li Magic‐Angle Spinning Nuclear Magnetic Resonance and In Situ X‐Ray Diffraction Studies of the Charging and Discharging of Li x Mn2 O 4 at 4 V
{sup 6}Li and {sup 7}Li magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and in situ X-ray diffraction (XRD) have been used to study lithium manganate cathode materials (Li{sub x}Mn{sub 2}O{sub 4}, 0 50% charging, from lithium in the tetrahedral sites of the spinel structure, the resonance shifting by no more than 8 ppm in this range (from its original position at ca. 520 ppm). When the cell is charged above 50%, a new resonance is observed at {approximately}645 ppm, due to Li{sup +} in a spinel local environment with nearby manganese ions in oxidation states close to +4(e.g., in a composition such as Li{sub 0.1}Mn{sub 2}O{sub 4}). The two resonances at {approximately}520 and 645 ppm are both observed in the range 70 to <80 and 80 to 90% for the samples annealed at 850 and 650 C, respectively. These coexistences occur over the same ranges as the two-phase coexistence, as observed by XRD during the first charging cycle. Resonances were observed at 830 and 930 ppm, for the samples annealed at 650 and 850more » C, respectively, at {ge}90% charging. These same resonances were seen after multiple charging cycles and are assigned to defect spinels with high manganese oxidation states. Additional resonances are observed for electrodes with high carbon contents at {minus}2 to {minus}13 ppm, from nonspinel phases, which grow in intensity with the number of charging cycles. The presence of the defect spinels and the additional impurity phases contribute to the reduced capacity of these materials. Three-phase behavior is observed by in situ XRD during the first discharging cycle. These results are discussed in terms of previous in situ XRD charging/discharging results.« less