Investigations On The Application Of TiO₂-Al₂O₃ Composite In The Charge Trapping Memory Devices

Compared with the conventional Floating-gate flash memory,SONOS type charge trapping memory(CTM)device has been widely used in digital camera,mobile phone and other portable digital products due to its outstanding performance of low power consumption,fast prograrm/erase speed,good endurance and long retention time.In SONOS type CTM devices,the charges are stored in the nitride layer,which are not compatible with further dimensional scaling,leading to poor performance in retention.With the increased integration of semiconductor industry,the SONOS type CTM device is unable to meet the demand of better storage performance.To confront this challenge,some high-k materials,for example HfO2,TiO2,and ZrO2,have been proposed as the charge trapping layer of the CTM devices in order to acquire better storage performance and retention characteristics.In these high-k dielectrics,oxygen vacancy is verified as main defect.Recently,the CTM devices with multilayered high-k dielectrics as the charge trapping layer also have been fabricated,which show better charge storage and retention characteristic while compared with those employing single layer charge trapping structure.The charge trapping behaviors in these multilayered memory devices were mainly attribute to the generation of inter-diffusion at the interface of different high-k oxides.The transverse structure of the CTM device contained multilayered high-k dielectrics is still too thick to satisfy the further scaling down,thus we investigated the memory properties of CTM devices consisting of high-k composite charge trapping layer.In this dissertation,the microstructure,the charge trapping properties and the reliability of the CTM devices based on TiO2-Al2O3 composite have been studied systematically,and the main achievements are as the following:1.A memory structure of P-Si/SiO2/TAO/Al2O3/Pt was fabricated by using the atomic layer deposition and RF-magnetron sputtering techniques.A memory window of 8.83 V under the working voltage of ±9 V was observed.At the same time,the CTM device also shows good endurance and retention characteristics.The result of X-ray photoelectron spectroscopic(XPS)suggestes that a portion of Ti4+ ions transform to Ti3+,and the charge trapping mechanism should be attributed to the defects generated due to the inter-diffusion between TiO2 and Al2O3.The CTM device shows an obvious response to an applied voltage pulse with an amplitude of 10V and a width of 10-5s in its flat band shift.2.The effects of different proportion of TiO2:Al2O3 composite on the charge trapping ability,P/E speed,endurance and retention for CTM devices were also investigated.Three samples with the same thickness but the different mole ratio of TiO2:Al2O3(8:1,14:3 and 2:1,respectively)have been fabricated.The data collected from tests under the same sweeping voltage shows that the storage density increases with the increase in the proportion of TiO2,this phenomenon should be ascribed to the effective inter-diffusion between TiO2 and Al2O3.It was also observed that the programing/erasing speed increases with the increase of composition of TiO2 in the high-k composite.It was believed that the increase of TiO2 in the high-k composite lovers the bottom of the conduction band of the high-k composite,thus resulting in the tunneling probability of charged carriers.3.The CTM devices based on different TiO2/Al2O3 nano-laminated charge trapping layers were fabricated and studied.The intercalation layer of Al2O3 leads to the increase of the storage density due to the increase in the defect density.In addition,the additional Al2O3 intercalation is able to reduce the loss of the stored electrons,so as to improve the device retention characteristic.