aug1 07 presentation

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1 August 2007 J. F. Wager 1

Transparent Electronics:

An Enabling DisplayTechnology?SID Pacific Northwest Chapter

Beaverton, OR

1 August 2007

J. F. Wager

OREGON STATE UNIVERSITY

School of Electrical Engineering & Computer ScienceCorvallis, OR 97331-5501

U. S. A.




Transparent Electronics & Displays?

• Transparent electronics

– OverviewOverview• Display applications

– AMLCD transparent switchAMLCD transparent switch – – AMOLED backplanesAMOLED backplanes

– – Spatial light modulator projection displaySpatial light modulator projection display

– – Transparent displayTransparent display




Transparent Electronics?

• Electronic devices which are

optically transparent – see-through

– invisible

– “transparent in the visible portion of

the electromagnetic spectrum”




OSUA WORLD LEADER IN TRANSPARENT ELECTRONICS

• First fully-transparent thin-film transistor (TTFT TTFT ) (2003)

• First ZnO (2003), SnO2 (2004), zinc tin oxide (2005), zinc

indium oxide (2005), indium gallium oxide (2006) TTFTs

• First spin-coat synthesized channel layer TTFT (2003)

• First transparent circuits; inverters + ring oscillators(2006)

Transparent Electronics @ OSU




TTFTs

ITOITO

Channel

Glass

ITOInsulator

SOURCE DRAIN

GATE




Sputtered Sputtered ZnO TTFTs

Patterned ZnO transparent transistor test structures

are evident in the upper portion of the glass substrate,

which sits on a penny.

J. F. Wager, Science 300, 1245-1246 (2003).




Sputtered ZnO TTFTs




SpinSpin- - Coated Coated ZnO TTFTs

56 patterned ZnO TTFTs and 24 contact resistance test

structures are present inside the red box.

B. J. Norris et al., J. Phys. D. 36, L105 (2003).




SnO2 TTFTs VGS

40 V

25 V

30 V

35 V

15 V

20 V

10 V

R. E. Presley et al., J. Phys. D. 37, 2810 (2004).




SnO2 TTFTs

~105





SnO2 TTFTs

RAW

CORRECTED [T/(1-R)]





TTFTs

ZnO

SnO2

ZTO!!!!

ITOITO

Channel

Glass

ITOInsulator

SOURCE DRAIN

GATE




ZTO TTFTs

H. Q. Chiang et al., Appl. Phys. Lett. 86, 013503 (2005).

~108

VGS = 15 V (top) → 0 V (3 V steps)Enhancement-mode (E-mode)

600°C ANNEAL




ZTO TTFTs

ZnSnO3

Zn2SnO4

100,000 X

ZTO

200 nm

SCHERRER < ~ 5 nm

600°C ANNEAL

H. Q. Chiang et al., Appl. Phys. Lett. 86, 013503 (2005).




M. S. Grover et al., J. Phys. D 40, 1335 (2007).

ZITO TTFTs

Polycrystalline

Amorphous




ZTO TTFTs

ZTO attractive attributes: Mobility ~10-30 cm2V-1s-1

Device stability = excellent (using SiO2+T)

Chemical stability (oxidation & etching)

Physically robust (scratch resistant)

Low cost

Amorphous Amorphous (extremely smooth surfaces)

Easy to integrate & manufacture

Superior reliability (no grain boundaries)

Enhanced performance (compared to poly)




SURFACE & INTERFACE ROUGHNESS

• Mobility reduction• Integration challenges

• Reliability issues

NEG Glass

ITO

ATO

ITO ITO

CHANNEL

Polycrystalline Channel TFT & TTFTs

ZnO, SnO2

A h id i d t




29

Cu63.54

47

Ag107.87

48

Cd112.40

79Au196.97

80Hg200.59

49

In114.82

30

Zn65.37

50

Sn118.69

81Tl204.37

82Pb207.19

83Bi208.98

51

Sb121.75

31

Ga69.72

32

Ge72.59

33

As74.92

11 1312 14 15

4

5

6

H. Hosono et al., J. Non-Crystalline Solids 203, 334 (1996).

Amorphous oxide semiconductors

(AOS)




AOS

small metal cation large metal cation

O

MM

O

M M

O

MM


AOS




AOS


crystalline

amorphous

O

MM

O

MM

O

MM

OO

O

MM

O

MM

OM

MO

O

4s,5s,6s

AOS





29

Cu63.54

47

Ag107.87

48

Cd112.40

79

Au196.97

80

Hg200.59

49

In114.82

30

Zn65.37

50

Sn118.69

81

Tl204.37

82

Pb207.19

83

Bi208.98

51

Sb121.75

31

Ga69.72

32

Ge72.59

33

As74.92

11

III

1312 14 15

4

VIV

5

6

X

X XX

X

X

X

AOS

AOS




• Total # of binary combinations = 105• # of restricted binary combinations = 28

CuZnOCuGaO

CuGeO

CuInOCuSnO

CuSbO

CuBiO

ZnGaOZnGeO

ZnInO

ZnSnOZnSbO

ZnBiO

GaGeOGaInO

GaSnO

GaSbOGaBiO

GeInOGeSnO

GeSbO

GeBiO

InSnOInSbO

InBiO

SnSbOSnBiO

SbBiO

AOS




ZTO & ZIO

ZnO

hexagonal

wurtziteSnO2

tetragonal

rutile

In203

cubic

bixebyte

Transparent Circuits





0.0

5.0

10.0

15.0

20.0

25.0

V o u t ( V

)

-20 -10 0 10 20 30 40

Vin (V)

INVERTER






RING OSCILLATOR

9

10

11

12

13

14

15

16

V o u t

( V )

0.0000 0.0002 0.0004 0.0006 0.0008 0.0010

Time ( s)

0.000670.00032

f = 1/(0.00067-0.00032)f = 2.8 kHz

V bias

Vdd

Vout

Applications




Applications• Printed electronics

• Low-cost electronics• Disposable electronics

• Large-area electronics

• Macroelectronics• Flexible electronics

• Wearable electronics

Better performance than organics & polymers

• mobility (theoretical limit ~ 10 cm2V-1s-1)

• chemical stability

• physical durability

• manufacturability

Transparency not required.

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• Transparent electronics

– OverviewOverview

• Display applications

– AMLCD transparent switchAMLCD transparent switch

– – AMOLED backplanesAMOLED backplanes – – Spatial light modulator projection displaySpatial light modulator projection display

– – Transparent displayTransparent display

AMLCD Fla t -Panel Displays





TFT

Select line

Data line Storage capacitor

LC pixel

Common

TFT

Select line

Data line Storage capacitor

LC pixel

Common

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Superior performanceSuperior performance:

• mobility (~X10-30)

• device stability• + turn-on voltage

• transparency (a bonus)

Simpler to manufactureSimpler to manufacture:

• no source-drain doping

• sputtering vs CVD

• hazard, toxicity issues

TTFTs are an alternative to

amorphous silicon TFT technology!





Switching transistor Switching transistor :• on-to-off ratio ≥ 106

• manufacturability

• voltage-control • mobility (…)

• stability (…)

AMLCD Fl t P l Di l





aa- - Si Si advantagesadvantages:

• adequate solution

• proven technology

• industrially well established• immense capital investment

• “Why fix it, if it’s not broken?”

It is UNLIKELY UNLIKELY that TTFTs will soon

displace amorphous silicon TFT

technology in mainstream AMLCDapplications!

AMOLED Fla t Pane l Disp lays




AMOLED Fla t -Pane l Disp lays

OLED

T1

Vdd

CS

T2 T4T3

Select line

D a t a l i n e


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h ν

1 TFT addressing 1 TFT addressing :

• ‘smart pixel’• light emitted only when

addressing

• 1/N = fraction of frame

period during which light

emission occurs (N =

number of display rows)

• impractical impractical






h ν

2 TFT addressing 2 TFT addressing :• T1 = ‘select’ transistor

• T2 = ‘drive’ transistor

• CS = storage capacitor • store charge on CS

during addressing

• voltage on CS keeps T2turned on after

addressing (T2 = current

source)

• OLED brightnessOLED brightness

depends ondepends on μ μ & V & V T T

uniformity and stability uniformity and stability






4 TFT addressing 4 TFT addressing :

• T1,T2 = ‘select’ transistors• T3,T4 = current mirror

• CS = storage capacitor

• compensated for μ & VT

non-uniformity & instability

• minimumminimum- - complexity complexity

practical OLED circuit practical OLED circuit

(using a(using a- - Si Si TFTsTFTs ) )

h ν

C. Church and A. Nathan, Information Display 3&4/05, 22 (2005).

K. Sakariya et al., IEEE Trans. Electron Devices 51, 2019 (2005).






4 TFT operation4 TFT operation:

• ADDRESSING: Select line = on

CS charges thru T2

current flow thru T1,

T3, OLED, to ground

• voltage programming of

Cs does not depend on VT

of T3 or T4

h ν

+

+

+VCS






4 TFT operation4 TFT operation:

• AFTER AFTER ADDRESSING: Select line = off

T1, T2 = off

current thru T4,

OLED, to ground

• T3,T4 = current mirror

• current programming

does not depend on VT’s

h ν

0

0

+VCS

+





TTFTs = alternative to a-Si TFT technology!Superior performanceSuperior performance:

• mobility (~X10-30) (!)

• device stability (!?!)

• transparency (a bonus)

Simpler to manufactureSimpler to manufacture:

• no source-drain doping• sputtering vs CVD

• hazard, toxicity issues

AMOLED applications AMOLED applications:• 2-TFT circuit?

• peripheral drive circuitry?

• aperature ratio…







PropertyProperty AmorphousAmorphoussilicon (asilicon (a--SiSi)) PolycrystallinePolycrystallinesiliconsilicon TransparentTransparentelectronicselectronics

Manufacturability,

infrastructure

mature, proven emerging early stage R&D

Grain size amorphous 0.5-5 μm amorphous

Threshold voltage

uniformity

good fair ?

Threshold voltagestability

poor good good…

Mobility <1 100-500 ~10-30

Mobility uniformity good fair ?

Device type NMOS CMOS NMOS

Transparency opaque,

light-sensitive

opaque,

light-sensitive

transparent

Am orphous s i l ic on TFTs




M. J. Powell et al., Phys. Rev. B 45, 4160 (1992).


a-Si is a metastable material!VT(t)







Amorphous silicon is a metastable material!







Nature of aNature of a- - Si Si metastability metastability :

• 2 types of charged dangling bonds

[Si4]o(weak bond) + electrons- → [Si3]-(dangling bond)

[Si4]o(weak bond) + holes+ → [Si3]

+(dangling bond)

• Formation energy of dangling bonds depends on the

Fermi level position

Si Si

Si

SiSi Si

Si

Si

Si

[Si4]o [Si3]

o





p

EC

EF

EV=0

-

o A = [Si3]-

EA

ΔE(A-) = ΔE(Ao) – (EF - EA)Si Si

Si

Si

[Si3]-






p

EC

EFEV=0

o+ D = [Si3]

+ED

ΔE(D+) = ΔE(Do) – (ED - EF)Si Si

Si

Si

[Si3]

+


Defect ‘pool’,

rather than a

discrete state.





p


Defect ‘pool’

[Si3]+

Si Si

Si

Si

[Si3]o

Si Si

Si

Si

[Si3]-

Si Si

Si

Si






p

Why use nitride with aWhy use nitride with a- - Si Si TFTsTFTs? ?

• More positive fixed charge (VG-adjustable)

• Electron accumulation @ zero bias

• Fermi level position ↓[Si3]+ and ↑ [Si3]-

Good Good for a large dynamic range switch!

Bad Bad for enhancement-mode operation…





C. Van Berkel, in Amorphous & Microcrystalline Semiconductor Devices II (1992).

pBefore stress After stress

EC

EF

EV

EC

EF

EV

State

creation[Si3]-

--

-

-

-

-

EC

EF

EV

Charge

trapping--





aa- - Si Si TFT SUMMARY:TFT SUMMARY:

• a-Si = inherently metastable

state creation instability instability

• ‘Fix’ this problem using a nitride gate

charge trapping instability instability

• Works for voltage-controlled switching!

• What about current-controlled applications?

Stability + mobility Stability + mobility

Peripheral drive circuitry…

ZIO + Therm al SiO2 TTFTs




J. F. Wager et al., Transparent ElectronicsTransparent Electronics (Springer 2007).

28 hour aging @ +30 V

(aging = reversible, due to electron trappingat or the near channel/insulator interface.)

200 ºC 400 ºC



Light Valve Pro jec t or Disp lays




g

l

a

s

s

PC LC

I

T

O

g

l

a

s

s

I

T

O

Vapplied

~

modulated UV, absorbed in PC

incident & modulated visible light

C. Spiegelberg, Fury Technologies, Corp, Vancouver, WA (2007).

AOS’s!




•Transparent electronics – aa--TMOS channel layersTMOS channel layers

•Display applications – –AMLCD transparent switchAMLCD transparent switch -- nono

– – AMOLED backplanesAMOLED backplanes – – hmmmmhmmmm…… – – Spatial light modulator projection displaySpatial light modulator projection display ??

– – Transparent displayTransparent display – – difficult (requiresdifficult (requiresglobal , rather than, rather than local transparency)transparency)

•An enabling display technology – YES!YES!

aug1 07 presentation

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