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采用第一性原理方法对如何改善电荷俘获存储器的过擦现象进行了研究. 过擦是由于氮空位中Si原子对电荷的局域能力弱导致, 因此, 在Si3N4超胞中分别建立了以C, N, O替换氮空位中的Si原子的缺陷结构作为本文的研究模型. 分别计算了擦写之后体系的巴德电荷分布、相互作用能、态密度, 借以分析替位原子对过擦的影响. 巴德电荷分布的计算结果表明, Si3N4在O替位128号Si后的过擦现象被明显改善; C替位128号Si也可以改善过擦, 但由于C替位对电荷的局域作用变弱, 不利于电荷的存储实现; N替位128号Si则不能改善过擦; 而在162和196号Si位置, 三种原子的替换均无法改善过擦现象. 相互作用能的研究表明, 在128号Si位置, 三种原子都能够和氮空位形成团簇, 在体系中稳定存在. 特别地, O替位Si后, 体系中两缺陷的相互吸引作用最弱, 从而写入的电荷能够短暂的打破O团簇的稳定性, 实现电荷重构, 将电荷局域在O团簇周围. 此外, 态密度的分析结果表明O在128号Si位置能够在Si3N4禁带中引入深能级缺陷, 深能级局域电荷的能力强. 以上分析证明, O替位可以很好的改善Si3N4中的过擦现象. 本文的研究结果为电荷俘获存储器改善过擦提供了一种方法, 对提高器件的电荷保持特性和优化存储窗口具有指导意义.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.
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Keywords:
- first-principles/
- over-erase/
- N vacancy/
- Si3N4
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