The Studies On The Application Of High-k Composites In Charge Trapping Memory Devices

Silicon-oxide-nitride-oxide-silicon(SONOS)-type memory device as one of the charge-trapping memory devices(CTM)has received considerable interest.With continuous down-scaling the cell dimension to obtain high data-storage density,high programing/erasing speeds,low operating voltage and low power consumption,some intrinsic limitations make this kind of memory rapidly approach the scaling limit.One of the challenges is that the number of electrons stored in the charge-trapping dielectric of SONOS-type memory device decreases significantly with the continual down-scaling of the cell size.An improvement of the charge-trapping capability and retention performance can be obtained by using high-k materials or band-gap-engineered charge-trapping layer to replace traditional Si3N4.However,they still suffer the same few-storage-electron limitation.Recently,high-k composite dielectric has been employed as the charge-trapping layer.It was suggested that the excellent charge-trapping efficiency of high-k composite should be ascribed to the high density of defect states formed due to the inter-diffusion between two kinds of high-k oxides.In this work,we studied the density of the defect states in Hf0.5Zr0.5O2 high-k film and the storage performance of charge trapping memories employing Ta0.5Al0.5Ox(TaAlO-55)and Zr0.5Al0.5Ox(ZxAlO-55)high-k composites as the charge trapping layer.The storage performances of charge trapping memories employing Ta0.5Hf0.5Ox(TaHfO-55)and Ta0.3Hf0.7Ox(TaHfO-37)high-k composites as the charge trapping layer were studied systematically too.Finally,the density of the trapped charges in the charge trapping memories employing high-k composites with different valence states as the charge trapping layer were investigated.The main achievements are as followed:1.A memory structure Pt/Al2O3/Hf0.5Zr0.5O2/Al2O3/p-Si was fabricated by using atomic layer deposition and rf-magnetron sputtering techniques,and its microstructure has been investigated by using the high resolution transmission electron microscopy(HRTEM).By measuring the applied gate-voltage dependence of the capacitance for the memory structure,the planar density of the trapped charges in Hf0.5Zr0.5O2 high-k film was estimated as 6.63×1012 cm-2,indicating a body defect density of larger than 2.21×1019 cm-3.It was observed that the post-annealing process in N2 can reduces the defect density in Hf0.5Zr0.5O2 film,which was ascribed to the occupancy of oxygen vacancies by nitrogen atoms.2.The charge-trapping memory structures Pt/Al2O3/TaAlO-55/Al2O3/p-Si and Pt/Al2O3/ZrAlO-55/Al2O3/p-Si were fabricated by using rf-sputtering and atomic layer deposition techniques and their microstructures have been investigated by using the high resolution transmission electron microscopy(HRTEM).With a lower PBCB(potential at the bottom of the conduction band)difference between TaAlO-55 and p-Si than that between ZrAlO-55 and p-Si,TaAlO-55 CTM device shows a better charge-trapping performance.A density of trapped charges 2.88 ×1013 cm-2 at an applied voltage of±7V was obtained for TaAlO-55 CTM device,and it could keep about 60%of the initially trapped charges after 10 years.3.The charge-trapping memory structures Pt/Al2O3/TaHfO-55/Al2O3/p-Si and Pt/Al2O3/TaHfO-37/Al2O3/p-Si were fabricated by using rf-sputtering and atomic layer deposition techniques and their microstructures have been investigated by using the high resolution transmission electron microscopy(HRTEM).Under an applied voltage of±11 V,the memory window of TaHfO-55 CTM device is about 9.53 V and the density of the stored charges in TaHfO-55 CTM device is about 3.65 × 1013cm-2.The TaHfO-55 CTM device shows the maximum storage density under the same applied voltage due to the most effective inter-diffusion between Ta2O5 and HfO2.After 1×105 programming/erasing cycles,the memory windows of TaHfO-55 and TaHfO-37 CTM device decrease little.The programming/erasing speed test shows that TaHfO-55 and TaHfO-37 CTM devices respond to a voltage pulse with a pulse width of 10-4 s obviously.With the increase of the pulse width,the shift in the flat band voltage increases rapidly.After 10 years,the charge loss of TaHfO-55 devices was estimated as about 50%,and the charge loss of TaHfO-37 devices was estimated as about 60%.With a lower PBCB difference between TaHfO-55 and p-Si than that between TaHfO-37 and p-Si,TaHfO-55 CTM device shows a better charge-trapping performance.4.Hf0.5Zr0.5O2(ZrHfO-55)?Hf0.5Al0.5Ox(HfAl0-55)and Ta0.5Hf0.5Ox(TaHfO-55)CTM devices were fabricated to investigate the effect of different chemical valence of cations in individual high-k dielectric for high-k composite on the density of the stored charges of CTM devices with high-k composite as the charge storage layer.Under the same applied voltage,the density of the stored charges in ZrHfO-55 CTM device was the lowest,and the density of the stored charges in TaHfO-55 CTM device is the largest.A density of the trapped charges 3.59 × 1013 cm-2 at an applied voltage of±11 V was obtained for TaHfO-55 CTM device.Due to the similar structure between ZrO2 and HfO2,and the same chemical valence of Zr4+and Hf4+,the density of the defect state at the interface between both high-k dielectrics will not be large.Because of the different chemical valence between Al3+ and Hf4+,the space distributions of cations and oxygen ions will be quite different with those in amorphous Al2O3 and HfO2 dielectrics,and the charge distributions(or the distributions of electron cloud)also will be quite different with those in amorphous Al2O3 and HfO2 dielectrics.So when the high-k composite was formed,a new energy band structure quite different with those of amorphous Al2O3 and HfO2 dielectrics will be formed at the interface of both high-k dielectrics,resulting in the formation of the defect states with a high density in the band gap of high-k composite.For Ta2O5 and HfO2 composites,the situation is completely similar.Therefore,TaHfO-55 and HfAlO-55 CTM devices show higher densities of the trapped charges.The PBCB difference between TaHfO-55 and p-Si is lower about 0.5 eV than that between HfAlO-55 and p-Si,so TaHfO-55 CTM device shows a largest density of the trapped charges.