Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Fowler-Nordheim Tunneling in TiO2 for room temperature operation of the Vertical Metal Insulator Semiconductor Tunneling Transistor (VMISTT) Lit Ho Chong,Kanad Mallik, and C H de Groot School of Electronics and Computer Science University of Southampton, UK Funding: EPSRC, UK Overview Motivation: operation of metal oxide tunnel transistor (MOTT) Design of the VMISTT Fabrication of the tunnel barrier Characterization of the tunnel barrier Conclusions Motivation Downscaling of MOSFET Faster chips More transistors per unit area gate Gate oxide As feature sizes decrease, the MOSFET has problems: Short Channel Effects Fluctuation of threshold voltage due to random dopants in channel Gate tunneling high-k dielectrics source drain Metal Oxide Tunnel Transistor (MOTT) Operating Principle Fowler-Nordheim Tunneling through the oxide barrier. The tunnel barrier width is modulated by gate bias. Gate Gate Oxide Drain Bias only (without gate) fB Source Metal Oxide e e Drain source Fujimaru et al. Drain Bias & Positive Gate Bias tunnel barrier drain Appl. Phys. 85 (1999) 6912, Snow et al. Appl. Phys. Lett. 72 (1998) 3071. MOTT: Advantages & Disadvantages Advantages: Scalability to nanoscale High speed No short channel effects no single crystal Si Disadvantages: uncoventional fabrication Schottky Emission fB e Poole Frenkel e FN Tunneling operation at 100K only no complimentary device Source Tunnel Barrier Drain Vertical metal insulator semiconductor tunnel transistor VMISTT Silicon drain complimentary device possible requires correct tunnel barrier height and metal workfunction Vertical structure: Tunnel barrier better controlled surrounding gate gate oxide Source Metal oxide Drain gate Tunnel Barrier Criterion for tunnel barrier: Fabrication process: low barrier height for Fowler-Nordheim tunneling high barrier height to suppress Schottky leakage required: 0.4-0.6 eV for both bands TiO2 has optical bandgap of 3eV with 1eV per band Thermal oxidation of vacuum evaporated Ti. Device for characterization: Metal-insulator-Semiconductor (MIS) capacitors/diodes Fabrication of the Tunnel Oxide aluminium 10nm Ti titanium dioxide p-type Si Evaporation of 7nm or 10nm Ti on Si substrate Oxidation at 450oC, 500oC, and 550oC for 30min. Metallization Cross-section of a Typical TiO2 Layer TiO2 Si ~20 nm Current Voltage Measurement higher oxidation temperature for stoichiometry of TiO2 symmetry of the positive and negative bias indicates small interfacial layer. but also larger SiO2 interfacial layer estimated interfacial SiO2 layer(~1nm). alloy/anneal causes Al diffusion and larger leakage (not plotted) Capacitance-Voltage Measurement Capacitance-Voltage Measurement no saturation in accumulation decreasing capacitance with increasing oxidation quantum effect: accumulation layer width higher dielectric constant thicker interfacial layer dielectric constant: k~30 similar to TiO2 grown by chemical vapor deposition Campbell et al. IEEE Trans. E D 44 (1997) 104. Fowler-Nordheim Tunneling Fowler Nordheim Tunneling Dominant current transport mechanism for > 2 V Al/TiO2 and Si/TiO2 barrier height ~ 0.4eV for 10nm film. Ti[nm] T [oC] k φ[eV] 7 450 36 0.32 7 500 33 0.30 7 550 27 - electron tunneling assumed,but to be confirmed by changing the workfunction of the metal 10 450 28 0.36 10 500 24 0.46 Observed at RT, and confirmed at low temperature. Temperature dependence due to semiconductor carrier injection 10 550 22 0.40 Hopping Conduction sole source of leakage at low temperature Schottky-barrier Emission room temperature leakage mechanism Temperature Dependence of Current Temperature dependence independent of mechanism Si carrier injection limited Boron incomplete ionization recombination time Atlas device simulations show large temperature dependence of Fowler-Nordheim tunneling Conclusions Vertical Metal Insulator Tunnel Transistor (VMISTT) complementary device with easy fabrication based on gate modulation of Fowler-Nordheim tunneling TiO2 layers grown by thermal oxidation of evaporated Ti The Al/TiO2 and Si/TiO2 tunnel barrier height ~ 0.4 eV. Schottky-barrier emission leakage at room temperature. hole and electron tunneling to be investigated by using n-type and p-type Si and Al and Pt metal Fabricate transistor!