Yu-Lin Wang

Department of Materials Science and Engineering , University of Florida

Oxide-based Electric Devices :Oxide-based Electric Devices :

Flexible Transparent Thin Film TransistorsFlexible Transparent Thin Film Transistors

Background

Soldiers can use flexible display computers on the battlefield for communication and information access.

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.

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

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

Advantages of the IZO Thin Film Transistors

Conduction behavior of transparent conductive oxides (TCOs)

Hosono et. al., Nature, 432, 488 (2004)

SiPost-transition metal oxide (n 4) Ex. ≧Indium oxide, or Tin oxide

Requirements for TCOs in flexible transparent TFTs applications

• 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

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).

Currently, most TFTs use α-Si or organic TFTs (OTFTs). Two methods are used to fabricate these TFTs on flexible substrates.

InGaZnO TFTs on PETPET : 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)

By choosing different oxygen ratio, we can choose what carrier concentration we need

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.51x1013

1x1015

1x1017

1x1019

1x1021

1x1023

C

arri

er C

once

ntr

atio

n (

cm-3)

% O2 in O

2 + Ar [%]

Carrier concentration

0

10

20

30

40

50

60R

esistivity (ohm

-cm)

125W-ZnO 150W-In2O

3

5 mTorr chamber pressure

Carrier mobility

Carrier M

obility (cm

2/V-sec.)

Resistivity

IZO Film : Oxygen Ratio Dependence

Depletion mode TFTs

Fabricated

Channel : IZO

Gate dielectrics : SiO2, SiNx

Plan to do

Channel : IZO or IGZO or SIZO

Gate dielectrics : SiO2, SiNx, Sc2O3

Process sequences

Enhancement mode TFTs

Fabricated

Channel : IZO

Gate dielectrics : SiO2

Plan to do

Channel : IZO or IGZO or SIZO

Gate dielectrics : SiO2, SiNx, Sc2O3

Large Area Depletion Mode TFTs

IZO film : Carrier concentration ~1018 cm-3

Gate Width/Length =100um / 36umG

S

D

Source

Gate

Drain

SiO2 SiO2

Glass Substrate

SiO2 50nm

IZO 50 nm

Pt 20nm

Au 80nm

Au 80nmTi 20nm Ti 20nm

Au 80nm

0 1 2 3 4 50.00

0.05

0.10

0.15

0.20

0.25

Step= -1V

Vg= 0V

Gate W/L: 100 m / 36 m

Dra

in-S

ourc

e C

urr

ent

(mA

)

Drain-Source Voltage (V)

Large Area Depletion Mode TFT Performance

-9 -8 -7 -6 -5 -4 -3 -2 -1 00.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

dsI

Gate W/L: 100 m / 36 m

Dra

in-S

ourc

e C

urr

ent

(A1/

2 )

Gate Voltage (V)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

mg

Ext

r. T

ran

scon

du

ctan

ce (

mS

/mm

)

Max. Transconductance 0.55 mS/mm

Field effect mobility 4.5 cm2V-1S-1

Threshold voltage -6.5V

On/Off ratio > 105

Large Area TFT Performance

-3 -2 -1 0 1 2 30

1x10-10

2x10-10

3x10-10

4x10-10

5x10-10Gate W/L: 100 m X 36 m

Gat

e C

urr

ent

(A)

Gate Voltage (V)

Gate dielectric: SiO2 50 nm

Gate Dielectric Leakage

Gate leakage current ~ 10-10 A

Small Gate length Depletion Mode TFTs

IZO film : Carrier concentration ~1018 cm-3

Gate W/L =200um / 1um

Gate dielectric : SiNx 12.5 nm

Pt 20nmAu 80nm

IZO 50nm

Au 80nm

Ti 20nm Ti 20nm

Au 80nm

SiNx 12.5nm

Glass

0 1 2 3 4 50.00.20.40.60.81.01.21.41.61.8

Gate W/L=200 m/1 m

Vg= 0V

Step= -0.5V

Drain-Source Voltage (V)

Dra

in-S

ourc

e C

urr

ent

(mA

)

Small Gate length Depletion Mode TFT Performance

Max. Transconductance 7.5 mS/mm

Field effect mobility 14.5 cm2V-1S-1

Threshold voltage -2.5V

On/Off ratio > 105

-6 -5 -4 -3 -2 -1 0 10.00

0.01

0.02

0.03

0.04

0.05Gate W/L= 200m/1m

mgdsI

Dra

in-S

ourc

e C

urr

ent

(A1/

2 )

Gate Voltage (V)

0

2

4

6

8

10

Ext

r. T

ran

scon

du

ctan

ce (

mS

/mm

)

Small Gate length TFT Performance

10 1000

2

4

6

8

10

12

fT

fmax

Gai

n (

dB

)

Frequency (MHz)

U

h21

h21

U

Small Gate Length TFTs : Cut-off frequency and Maximum Oscillation Frequency

fT 180 MHz

fmax 155 MHz

Sufficient for display applications

S-parameters

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 ~ 105Max. Transconductance ~10 mS/mm

Sub-threshold voltage swing ~0.135 V/decade

SummaryVery 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.