Study On Storage Characteristics Of Oxygen Defects In Charge Trap Layer Based On HfO₂

Along with the rapid development of manufacturing technology and integrated circuit technology, semiconductor device dimensions are continuously scaling down, the tunneling oxide layer thickness is also becoming thinner, hence the leakage current is increased. Besides, due to the decreasing storage unit and the reducing coupling between the floating gate and a control gate, the adjacent floating gate interference has enhanced. Non volatile memory of traditional floating grid structure is difficult to fulfill the requirements to maintain the retention characteristics of memory and thus difficult to meet the desired high-density storage required for the development of the information technology. High K trapping layer charge trapping memory based on transition metal oxides, inheriting the traditional floating gate structure, is fully compatible with the traditional CMOS technology because of its simple structure. Moreover, it takes advantage of the isolated charge trapping to achieve discrete charge storage, thus eliminating the adjacent floating gate interference and SILC effect, and effectively preventing the charge leakage of the traditional floating gate structure.The transition metal oxides (such as HfO2, ZrO2 etc.) has a relatively high dielectric constant, high thermal stability and wide band gap. When used in the charge trapping layer, it has better charge trapping efficiency, and this kind of memory has a large memory window, thus causing extensive studies of people. In CTM study, researchers mainly focus on improving memory performance or clarifying the microscopic characteristics associated with memory performance by first-principles calculations. However, less research has been conducted on the micro mechanism of CTM trapping layer charge trapping associated with oxygen deficient in HfO2, or characteristics of maintaining and writing speed.Oxygen defect characteristics of high k materials HfO2 as CTM trapping layer and the impact of introducing impurity Al into the manufacturing process on it have been studied in depth by using first-principles calculations in this paper.Firstly, the paper introduces the various types of improved and new non-volatile memory concepts, as well as their advantages and disadvantages, explains the background and significance of CTM, CTM memory principle and introduces the basic concepts of First Principles. Usually the size of the store window, the pace of erasing, as well as its tolerance and retention characteristics can be used to characterize the performance of the memory, and these characteristics are decided by the inherent mechanism of its materials. First-principles calculations can study the crystal structure, the nature.electronic distribution, energy bands, density of states, the optical properties (such as:the dielectric function, reflection, refractive index), and the influence of doping on the crystal, etc. The results may reflect the reading and writing speed of memory, retention characteristics and other properties. Overall understanding of the material properties can help a better stoichiometric control of the various elements in the manufacturing process.Secondly, the paper studies the oxygen vacancy defect of high-k materials HfO2 itself, the lattice structure changes caused by it and its impact on the performance of the trap layer through injecting charge to simulate write operation.The result shows the charge trapping ability of oxygen vacancy is substantially unaffected by the distance between the oxygen vacancies; but the number of oxygen vacancy can influence the capture of electrons.Tetravalent ligand oxygen vacancies has better electron trapping ability than the trivalent ligand oxygen vacancies. Moreover, state density analysis shows that deep level quantum state number introduced by the tetravalent ligand oxygen vacancies is bigger, less influenced by the distance between the oxygen vacancy and the electronic capture probability is larger. Oxygen vacancy defects of high coordination valence can produce more DOS and smaller energy level shift, indicating that the number of deep level quantum states introduced is larger and data retention characteristics is more stable; changes in the structure of the lattice caused by oxygen vacancy defects also significantly increase Charge localization energy, indicating that the charge-trapping capacity has been enhanced. Thus CTM storage performance can be improved through increasing oxygen vacancy defects of high coordination valence in manufacturing process.Thirdly, the effect of the interstitial oxygen defects of HfO2 on trapping layer characteristics has been researched. It studies the different forming process of the interstitial oxygen defects in different positions and compares their oxygen vacancy defects. The result shows It is easier to form interstitial oxygen defects under enriched oxygen; the defects in the place where interstitial oxygen defects are most likely to form can produce the donor level close to the conduction band and the valence band near the top of of the acceptor level; they are also able to capture both electrons and of and holes, but the probability of electron capture is smaller than the hole capture. The captured carriers mainly gather in the vicinity of interstitial oxygen atoms. When defect spacing increases, the attraction gradually turns into exclusion and then decreases, resulting in an obvious change of number and depth of the acceptor level. A significant increase in the number of quantum states enables the hole tunneling oxide current to increase.Finally, for the conduction band offset with respect to Si caused by Al in HfO2 and AI2O3 multilayer stack gate capturing layer, the increasing trap density relevant to Al and some other reasons, the band gap can be adjusted through adjusting the concentration ratio of Al and Hf so as to improve the HfO2 trapping layer characteristics in the manufacturing process. Therefore, this paper studies the characteristics of HfO2 supercell with Al substituted for Hf containing tetravalent oxygen vacancy defects. The study has found that Al makes the neutral oxygen vacancy defect formation easier, the trapped charge is mainly distributed in the vicinity of the minimum distance between the two oxygen atoms, the charge-trapping ability of oxygen vacancy defects is significantly enhanced, the defect levels deviate to the conduction band and a shallow level of a large number of quantum states is introduced near the valence band top, which indicates that the introduction of Al will help to improve the device P/E operating speed.