Transcript pearton.mse.ufl.edu - Prof. Stephen J. Pearton's Research

Oxide-based Electric Devices :
Flexible Transparent Thin Film Transistors
Yu-Lin Wang
Department of Materials Science and Engineering , University of Florida
Background
Flexible displays are attractive for
portable devices such as cell phone,
PDA, laptop, e-book and wearable
due to their lightweight, low power
consumption, and being bendable.
Soldiers can use flexible display
computers on the battlefield for
communication and information
access.
Flexible display
Currently, these display either use a-Si TFTs or organic TFTs
(OTFTs) in the active matrix arrays.
To get a high resolution display, high mobility TFTs are
necessary. However, a-Si TFTs or OTFTs cannot fit this
application.
Material Comparisons
Which material is a better choice for the transparent
and flexible display?
IZO : Indium znic oxide
Advantages of the IZO Thin Film Transistors
1. High transparency : available for transparent TFTs
2. Room temperature process : available for plastics
substrate
3. Can be used as electrodes, or channel layers : by
adjusting O2 ratio
4. High field effect mobility : 10 ~ 50 cm2V-1S-1
5. Large area deposition : By sputtering machine
6. Rapid Process
7. Low cost
Conduction behavior of transparent
conductive oxides (TCOs)
Post-transition
metal oxide
(n≧4) Ex.
Indium oxide,
or Tin oxide
Si
Hosono et. al., Nature, 432, 488 (2004)
Requirements for TCOs in flexible
transparent TFTs applications
•
•
•
•
•
•
•
•
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Good conductivity
Bandgap energy Eg> 3 ev
High mobility
Room temperature deposited film
Amorphous film
Controllable carrier concentration
Non-toxic elements
Good reliability
Cheap processing
Various conductive oxides
TFTs on flexible substrates
Currently, most TFTs use α-Si or organic TFTs
(OTFTs). Two methods are used to fabricate these
TFTs on flexible substrates.
1. Directly deposit film on flexible substrates
(plastics) and fabricate TFTs.
2. Fabricate TFTs on hard substrate (glass)
and then transfer the TFTs to flexible
substrates (plastics).
InGaZnO TFTs on PET
PET : polyethylene terephthalate
Threshold voltage 1.3V
On/Off ratio >
105
Sub-threshold voltage swing ~0.24 V/decade
Field effect mobility ~10 cm2V-1S-1
gm ~0.03 mS/mm
Hosono et. al., Jpn, J. Appl. Phys., 45, 4303
(2006)
-3
1x10
21
60
50
40
1x10
19
1x10
17
1x10
15
1x10
13
30
20
2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
% O2 in O2 + Ar [%]
10
0
Resistivity (ohm-cm)
1x10
23
125W-ZnO 150W-In2O3
5 mTorr chamber pressure
Carrier concentration
Carrier mobility
Resistivity
Carrier Mobility (cm /V-sec.)
Carrier Concentration (cm )
IZO Film : Oxygen Ratio Dependence
By choosing different oxygen ratio, we can choose what
carrier concentration we need
Depletion mode TFTs
Plan to do
Fabricated
Channel : IZO or IGZO or SIZO
Channel : IZO
Gate dielectrics : SiO2, SiNx, Sc2O3
Gate dielectrics : SiO2, SiNx
Process sequences
Enhancement mode TFTs
Plan to do
Fabricated
Channel : IZO or IGZO or SIZO
Channel : IZO
Gate dielectrics : SiO2, SiNx, Sc2O3
Gate dielectrics : SiO2
Large Area Depletion Mode TFTs
Gate
Source
Au 80nm
Drain
Ti 20nm
Ti 20nm
Au 80nm
SiO2
Pt 20nm
SiO2 50nm
Au 80nm
SiO2
IZO 50 nm
Glass Substrate
S
IZO film : Carrier concentration ~1018 cm-3
Gate Width/Length =100um / 36um
G
D
Large Area Depletion Mode TFT Performance
Drain-Source Current (mA)
Gate W/L: 100 m / 36 m
0.25
Vg= 0V
0.20
0.15
Step= -1V
0.10
0.05
0.00
0
1
2
3
4
Drain-Source Voltage (V)
5
1/2
Drain-Source Current (A )
Gate W/L: 100 m / 36 m
0.8
0.014
0.7
0.012
0.6
0.010
0.5
I ds
0.008
0.4
gm
0.006
0.3
0.004
0.2
0.002
0.1
0.000
-9
0.0
-8 -7
-6
-5
-4
-3
-2
-1
0
Extr. Transconductance (mS/mm)
Large Area TFT Performance
Gate Voltage (V)
Threshold voltage -6.5V Max. Transconductance 0.55 mS/mm
On/Off ratio > 105
Field effect mobility 4.5 cm2V-1S-1
Gate Dielectric Leakage
5x10
-10
4x10
-10
3x10
-10
2x10
-10
1x10
-10
Gate W/L: 100 m X 36 m
Gate Current (A)
Gate dielectric: SiO2 50 nm
0
-3
-2
-1
0
1
2
Gate Voltage (V)
Gate leakage current ~ 10-10 A
3
Small Gate length Depletion Mode TFTs
Au 80nm
Au 80nm
Ti 20nm
Au 80nm
Pt 20nm
SiNx 12.5nm
Ti 20nm
IZO 50nm
Glass
IZO film : Carrier concentration ~1018 cm-3
Gate W/L =200um / 1um
Gate dielectric : SiNx 12.5 nm
Drain-Source Current (mA)
Small Gate length Depletion Mode TFT
Performance
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
Gate W/L=200 m/1 m
Vg= 0V
Step= -0.5V
1
2
3
4
Drain-Source Voltage (V)
5
1/2
Drain-Source Current (A )
Gate W/L= 200m/1m
0.05
10
0.04
8
I ds
0.03
6
gm
0.02
4
0.01
0.00
-6
2
-5
-4
-3
-2
-1
0
1
0
Extr. Transconductance (mS/mm)
Small Gate length TFT Performance
Gate Voltage (V)
Threshold voltage -2.5V
Max. Transconductance 7.5 mS/mm
On/Off ratio > 105
Field effect mobility 14.5 cm2V-1S-1
Small Gate Length TFTs : Cut-off frequency
and Maximum Oscillation Frequency
S-parameters
12
Gain (dB)
8
fmax 155 MHz
Sufficient for display
applications
h21
6
4
2
fT 180 MHz
h21
U
U
10
0
10
fmax
100
Frequency (MHz)
fT
Small Gate length Enhancement Mode TFTs
IZO film : Carrier concentration ~1x1016 cm-3
Gate W/L =100um / 1um
Gate dielectric : SiO2 100 nm
Enhancement Mode TFT : Ids vs. Vds
Enhancement Mode TFT
Performance
Threshold voltage 0.5V
On/Off ratio ~
105
Max. Transconductance ~10 mS/mm
Sub-threshold voltage swing ~0.135 V/decade
Summary
Very high performance depletion mode and enhancement
mode TFTs were achieved on glass substrates.
Very good frequency response from a depletion mode TFT
which is very sufficient for display applications.
IZO and IGZO will be used as channel layers to fabricate
depletion mode and enhancement mode TFTs and ring
oscillators on glass and flexible transparent substrate (PET).
The SiO2-In2O3-ZnO system and N2 plasma incorporated
IZO film will be grown to get a better controllability of the
carrier concentration.