With the further scaling down of device dimensions, charge trapping memory with high k materials HfO2 serving as capture layer shows good endurance and high storage capacity. Its relatively simple process and complete compatibility with the conventional semiconductor process furthermore make it widely studied. The oxygen vacancies in HfO2 are studied using the first-principles calculation in order to learn their influence on the storage properties of charge trapping memory. Write and erase operations of memory devices are simulated via changing the number of electrons in the super cell with defects. The results show that basically the distance between oxygen vacancies has no effect on charge trapping, but the number of oxygen vacancies does affect it. The more the number of oxygen vacancies, the stronger the electron capture ability is. Moreover, four-fold coordinated oxygen vacancy (Vo4) has lager capability for trapping charge than three-fold coordinated oxygen vacancy (Vo3). The analysis of density of states shows that Vo4 induces a large number of quantum states with deep energy levels which is little affected by distance and has large possibility of trapping charges. The results show that oxygen vacancy defects in HfO2 tetravalent coordination are conducive to improving the storage characteristics of charge trapping memory.
The first-principles method has been used to explore how to minimize the over-erase phenomenon in charge trapping memory. Over-erase phenomenon originates from the nitrogen vacancy due to its weak localization of charge on Si atoms. Therefore, we develop a defect model for studying Si3N4 supercells. The defect model consists of an N vacancy and a substitutional atom on the Si site. The substitutional atoms can be C, N, and O atoms, respectively. The Si site belongs to the N vacancy. Then, the Bader charge distribution after program/erase operation, the interaction energy and density of states are calculated for the model so as to analyze the effects of the substitutional atoms on the over-erase phenomenon. The obtained results of the Bader charge distribution show that the substitution of O for the 128th Si can minimize the over-erase phenomenon in Si3N4, and the replacement of the 128th Si by C can also reduce the over-erase phenomenon. However, the model represents a weak localization of charge due to the replacement by C, which is not preferable for charge storage. And the results also reveal that the substitution of N for the 128th Si completely fails to reduce the over-erase phenomenon. With regard to the 162th and 196th Si sites, the substitutions of the three atoms for the two sites cannot minimize the over-erase phenomenon. Furthermore, the analysis of the interaction energies indicates that the combination of each of the three atoms with the N vacancy can form stable clusters on the 128th site in the model. In particular, the attractive interaction between O and N vacancy is the weakest of the three so that the injected charge can temporarily break the stability of the O cluster to rearrange the charge distribution, realizing the localization of charge around the O cluster. And then, the results of the density of states designate that subtitutional O atom at the 128th Si atom site produces a deep-level trap in the band gap, which has a powerful ability to localize the charge. The above results suggest that substitution of O for Si is an excellent solution for the minimization of over-erase phenomenon in Si3N4. This work can provide a method for the minimization of over-erase phenomenon in charge trapping memory and also can be helpful to the improvement of charge retention and optimization of memory window in the charge trapping memory.
HfO2, as a gate dielectric material for the charge trapping memory, has been studied extensively due to its merits such as high k value, good thermal stability, and conduction band offset relative to Si, etc.. In order to understand the reason why the charge trapping efficiency is improved by high k capture layer with respect to charge trapping type memory, the variation of HfO2 crystal texture induced by oxygen vacancy and the influences of it are investigated using the first principle calculation based on density functional theory. Results show that the distance of the nearest neighbor oxygen atom from oxygen vacancy is markedly reduced after optimization, whereas the decrease of distances between the next nearest neighbor oxygen atom from oxygen vacancy and hafnium is less. The change of local crystal lattice is caused by optimized oxygen vacancy for it significantly changes the local lattice, but rarely influences the far lattice. Deep energy level and density of electron states in conduction band are contributed by Hf atoms, while the density of electron states in valence band is contributed by O atoms. The local density of electron states in each element and the total density of electron states in the optimization system are all larger than those in the system without optimization, and the sum of the local densities of electron states is less than the total density of electron states. The trapped charges are moving mainly around the oxygen vacancy and the adjacent atoms of oxygen in the optimization system, but the charges are without optimization throughout the system. The local energy of charge is increased in optimized defect system, while the local energy of charge is conspicuously reduced in the system without optimization, i.e. lattice variation without saturation characteristic has a large effect on the local energy of charge. Results further prove that the change of crystal lattice induced by oxygen vacancy has strong ability to capture charge, which helps improve the features of memory.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.