Download Vertical Metal Oxide Tunneling Transistor (VMOTT

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!