电荷俘获存储器的过擦现象

基于密度泛函理论的第一性原理平面波超软赝势方法和VASP软件对电荷俘获存储器过擦现象进行了分析研究.通过形成能的计算,确定了含有氮空位缺陷的Si3N4和含有间隙氧缺陷的Hf O2作为研究的对象;俘获能的计算结果表明两种体系对电子的俘获能力比对空穴的大,因而对两体系擦写载流子确定为电子.分别计算了Hf O2和Si3N4擦写前后的能量、擦写前后电荷分布变化、吸附能和态密度,以说明过擦的微观机理.对能量和擦写电荷变化的研究,表明Si3N4相比于Hf O2,其可靠性较差,且Si3N4作为俘获层,在一个擦写周期后,晶胞中电子出现减少现象;界面吸附能的研究表明,Si3N4相比于Hf O2在缺陷处更容易与氧进行电子交换;最后,通过对态密度的分析表明Si3N4和Hf O2在对应的缺陷中均有缺陷能级俘获电子,前者为浅能级俘获,后者为深能级俘获.综上分析表明,Si3N4在氮空位的作用下,缺陷附近原子对电子的局域作用变弱,使得Si3N4作为俘获层时,材料本身的电子被擦出,使得擦操作时的平带偏移电压增大,导致存储器发生过擦.本文的研究结果揭示了过擦的本质,对提高电荷俘获存储器的可靠性以及存储特性有着重要的指导意义.

Research on charge trapping memory’s over erase

In this paper, charge trapping memory (CTM) is studied for analyzing the over-erase phenomenon, based on the first principles and VASP package. The nitrogen vacancy (V_N) in Si₃N₄ and the interstitial oxygen (I_O) in HfO₂ are selected as model, because of the formation energy. The result about trapping energy shows that the electrons are trapped more easily than holes in these models, so the electrons are selected as programming/erase object. The energy after programming/erase operation, Bader charge analysis, different charge densities, adsorption energy and density of states are all studied to explain the over-erase micro change. The energy and electron change show that HfO₂ as trapping layer makes CTM more reliable than Si₃N₄ as trapping layer; and after a programming/erase cycle, electrons in Si₃N₄ are erased more than programming ones; and the result of adsorption energy shows that the electrons can exchange more easily in Si₃N₄ than in HfO₂. Finally, the research on the density of states shows that Si₃N₄ has shallow trapping energy level, HfO₂ has deep trapping energy level. In conclusion, the essence of the over-erase in Si₃N₄ is that the atoms near the defect have weaker localized action on the electrons, resulting in the instinct electrons that are erased in erase operation. The over-erase essence is revealed, which is of benefit to improving the reliability and retention.
Keywords： over-erase 2')" href="#">HfO₂ 3N₄')" href="#">Si₃N₄ the first principles