| Silicon-based semiconductor memory devices will remain for a long time to lead the memory industries with technological breakthroughs.The innovation of the structures and the applications of new materials greatly improve the performance of storage devices.However,the physical limit under small size make it difficult to pursuit higher storage density,lower power consumption,faster program/erase speed,better endurance and retention properties in the development of semiconductor memory devices.Charge-trapping memory devices such as SONOS CTM devices are compatible with traditional CMOS technology.The replacement of poly silicon by Si3N4 as charge trapping layer also implements the charge storage density and increases the reliability of the device while new challenges appear with the continual down-scaling of the cell size.The employment of high-k material made a compensation for Si3N4 as charge storage layer in CTM devices.Al2O3 was always chosen as the tunneling and blocking layers because of its higher dielectric constant,larger band offsets with Si,good chemical and thermal stability.A variety of high-k materials and their laminated structures,such as HfO2,TiO2 and ZrO2,Y2O3,La2O3,HfO2/Al2O3/HfO2,ZrO2/Al2O3/ZrO2,were used as charge storage layer replacing the traditional silicon nitride,in order to obtain better endurance and retention characteristics.In recent years,high-k composite oxide materials were used as charge-trapping layer in CTM devices due to its high density of defect states generated from the inter-diffusion among the oxides,which is crucial for the charge-trapping efficiency.In addition,researchers believed that the charge storge density and other properties could be enhanced by reducing the band offsets between Si substrate and high-k materials and increasing the dielectric constant of composition.Ta2O5 and TiO2 have been studied adequately when their dielectric constant and crystal structure were related to the deposition methods.According to the first-principles calculations by Robertson,Ta2Os has a gap of 4.4 eV while its band offsets with Si is about 0.3 eV.The gap turns out to be 3.1 eV(similar to BaTiO3),and the bottom of conduction band is near to Si for TiO2.So we believe the the composition of Ta2O5 and TiO2 would perform better when employed as charge-trapping layer in CTM devices.We focus on the study of the relationship between the composition and electrical characteristics when(Ta205)x(TiO2)1-x was used as charge-trapping layer in CTM devices and try to analysis the machnism.Besides,we also studied the relationship between the properties of memory devices and the thickness of tunneling layer.Then three-terminal memory devices were fabricated using IGZO as channel and(Ta2O5)0.5(TiO2)0.5 as charge-trapping layer materials for study.The main results are summarized as follows:1.A series of CTM devices were fabricated using(Ta2O5)x(TiO2)1-x as charge-trapping layer with x=0.9,0.75,0.5 and 0.25 while devices with Ta2O5,TiO2 and Al2O3 as CTL were also fabricated to make a comparision.The retention performance under different tempreture of the devices proved the machnism of charge loss.Memory window reached to 11.9 V under a ± 12 V sweeping voltage when x=0.5 corresponding to charge storge density of 3.84×1013 cm-2,and a 32%degradation in its initial stored charges after 10 years was obtained.The better electrical performances in TTO(1:1)were explained by the band aligment which means the charge-trapping properties could be improved by adjusting the dielectric constant and band gap of composition.2.CTM devices with different thickness(2 nm,3 nm,4 nm and 6 nm)of tunneling layer were fabricated for researching.The results showed that memory window reached to 8.37 V,8.34 V,8.28 V and 7.86 V at an applied voltage of ±12 V for T2,T3,T4 and T6 respectively.The differences of charge-trapping characteristics could be explained by the machnism of quantum tunneling and band bending,which also gave a reason for the contrast of program/erase speed and retention properties.3,Three-terminal devices with IGZO as channel and Al2O3/(Ta2O5)0.5(TiO2)0.5/Al2O3 as charge-trapping structure were fabricated for further study. |
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