The Fabrication And Memory Characteristics Of Ge Nano-crystals In High-k Dielectrics

Since S.Wiwari demonstrated a nano-crystal memory device by using Sinano-particles in Applied Physics Letters, more and more attentions have been paid on this new kind of non-volatile memory. The main practical goal is to replace the traditional poly-Si floating gate in flash memory.At the sub 90nm technology node, the gate oxide thickness is expected to be 12-15 Angstrom. When the gate dielectric film becomes this thin and thinner, gate direct tunneling generates large leakage current across the gate dielectric. This severely hampers the ability to turn the gate on and off, and affects the speed and performance of the device. To reduce this significant leakage current, high-k dielectric materials are the ideal choice for the advanced gate dielectric films.Focusing on the fabrication of nano-crystal floating gate memory using high-k dielectrics, and according to the demands of National Natural Science Foundation of China and the Shanghai Nanotechnology Promotion Center Foundation, we have made a series of investigations. Main efforts are made on the following subjects: Fabrication and memory characteristics of Ge nano-crystals embedded in high-k dielectrics, such as Al2O3, HfO2 and ZrO2 by high vacuum electron-beam evaporation. Main results are shown as below:(1) Ge nanocrystals embedded in Al2O3 dielectrics by high vacuum electron-beam evaporation at room temperature were fabricated. Good memory effects and low leakage currents were obtained: The flat-band voltage shift (△Vfb) was 4.1V and leakage current density was 5.3 X 10~7A/cm~-2 when the applied voltage was 20V.(2) Uniform and self-aligned Ge nanocrystals embedded in high-k dielectric material HfO2 were successfully obtained by high vacuum electron-beam evaporation. The influences of annealing conditions on structure and electrical performances of the nano-floatinggate have been investigated systematically. Excellent memory characteristics and very low leakage current density were obtained. The optimal annealing condition was 800 C, 30min, when the leakage current density was 1.7X 10'7A/cm"2 under 20V gate voltage and stored charge density reached 2.34* 1012 cm"2. Distinct negative photoconductivity was observed through the analysis of Current-Voltage and Capacitance-Voltage curvp? in the dark and under illumination respectively.(3) For the first time, Ge nanocrystals were fabricated under relatively low temperature when substrate was heated at 600°C. Large A Vfb (6V) and very low leakage current density(5.57X 10~9A/cm~2) under 20V gate voltage was obtained.(4) For the first time, HKVSiCh stack was used as tunnel oxide, which not only prevented the Ge penetration to the substrate, but helped to obtain excellent interface. The optimal annealing condition was 800 C. 45min, when the leakage current density was 1.29X 10'8A/cm'2 under 5V gate voltage and stored charge density reached 3.3><1012 cm"2. Compared with the structure with single H1D2 tunnel oxide, the floating gate structure with HfCVSiCh stack as tunnel oxide possessed better charge storage performance and less leakage current density.(5) Double Ge nanocrystal layers in HfO2 dielectrics were studied. The charges in the double nano-floating gate had not been lost after 2,000s, which showed better charge retention performance than structure with single Ge nanocrystals. The influences of normal annealing and rapid annealing conditions on the Ge nanocrystals were studied and optimal annealing condition was obtained. The electric uniformity of MIS structure after rapid annealing was better than that after normal annealing.(6) Ge nano-crystals embedded in Z1O2 were fabricated through high vacuum electron-beam evaporation when the substrate's temperature was 600' C. Good stored charge density (2.41 *1012 cm"2) and low leakage current density were obtained, which was 2.45X10"7A/cm2 under illumination and 9.03 X 10"8 A/cm2 in the dark respectively when gate voltage reached 20V.