Herein, we present simulations of conductive filament formation in resistive random-access memory using a finite element solver. We consider the switching material, which is typically an oxide, as a two-phase material comprising low- and high-resistance phases. The low-resistance phase corresponds to a defective and conducting region with a high anion vacancy concentration, whereas the high-resistance phase corresponds to a non-defective and insulating region with a low anion-vacancy concentration. We adopt a phase variable corresponding to 0 and 1 in the insulating and conducting phases, respectively, and we change the phase variable suitably when new defects are introduced during voltage ramp-up for forming. Initially, some defects are embedded in the switching material. When the applied voltage is ramped up, the phase variable changes from 0 to 1 at locations wherein the electric field exceeds a critical value, which corresponds to the introduction of new defects via vacancy generation. The applied voltage at which the defects percolate to form a filament is considered as the forming voltage. Here, we study the forming-voltage uniformity using simulations, and we find that for typical planar-electrode devices, the forming voltage varies significantly owing to the stochastic location of the initial defects at which the electric field is “crowded.” On the other hand, a protruding electrode can improve the switching uniformity drastically via facilitating the deterministic location of electric-field crowding, which also supported by the reported experimental results.
An evaluation of Ti-based gate metals ͑Ti, TiN, and TiB 2 ͒ on Hf-silicate gate dielectric prepared by atomic layer deposition has been reported. The effective metal work functions, calculated by taking an interface layer and interface charge into consideration, were 4.27, 4.56, and 5.08 eV for Ti, TiN, and TiB 2 , respectively. Regardless of gate electrodes, the conduction mechanism of the samples was fitted with the Poole-Frenkel model, which is related to oxygen vacancies in the film. A Ti gate electrode was found to be more favorable for n-channel metal oxide semiconductor ͑MOS͒ devices, and TiB 2 gate electrode can be used for p-channel MOS devices with Hf-silicate dielectrics.Many high-k materials are currently being considered as a potential replacement for SiO 2 -based dielectrics in future complementary metal oxide semiconductor ͑CMOS͒ technology. 1-7 Among the many candidate materials, nitrogen-incorporated hafnium silicate has received intense attention as a new gate dielectric material due to its reasonable permittivity ͑15-25͒, relatively large bandgap ͑5.68 eV͒, and thermodynamic stability with Si. 1-11 Nitrided Hf-silicate is also more resistant to boron diffusion from poly-Si gate through the dielectric. 12 However, the incompatibility between the poly-Si gate electrode and the Hf-based dielectric, such as poly Si depletion effect, Fermi energy pinning, and sheet resistance constraint, limit its usefulness in advanced CMOS devices, especially beyond the 45 nm technology node. 13-15 Therefore, advanced high-performance devices require high-dielectric-constant ͑high-k͒ gate dielectrics and dual work function metal gate electrodes. 13 Dual metal gate CMOS integration requires two different metals with work functions near the Si bandedges, around 4.0 eV for n-channel metal oxide semiconductors ͑NMOS͒ and 5.0 eV for p-channel metal oxide semiconductors ͑PMOS͒. 13 Consequently, the evaluation of various single metal and/or metal alloys on nitrogen-incorporated Hf-silicate dielectric could be advantageous for future advanced CMOS technology development.Because of good thermal stability and minimal interaction with high-k films, titanium-based materials could be an attractive candidate for metal gate applications. 16,17 Ti-based refractory materials such as TiN and TiB 2 have been previously considered as diffusion barriers in metal contact formation due to their high electrical conductivity and excellent chemical inertness at high temperature. 18 In this article, the electrical characteristics of metal oxide semiconductor ͑MOS͒ devices with Ti-based metal gates ͑Ti, TiN, and TiB 2 ͒ on nitrogen-incorporated Hf-silicate dielectric are investigated. The effective metal work functions are extracted in order to examine their compatibility with NMOS or PMOS devices. In addition, the conduction mechanism of Ti-based metal/Hf-silicate dielectric/p-type Si MOS devices is investigated.HfSiO x films were deposited directly on p-type ͑100͒ precleaned ͑SC1, 1% HF solution and deionized water rinse͒ 8 in. Si substrat...
An evaluation of TiB2 gate metal on Hf-silicate dielectric prepared by atomic layer deposition method has been reported. The extracted effective metal work function for TiB2 gate was about 5.08eV. The work function showed almost identical values and the sharp interface between metal and dielectric was confirmed after postdeposition annealing at 1000°C. The work function lowering (4.91eV) at 1100°C was caused by metal-dielectric intermixing and oxygen vacancy formation. TiB2 gate electrode was found to be suitable for use in p-channel metal oxide semiconductor device.
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