Selection of Combinations of Semiconductor/Dielectric Metal Oxides Deposited … Event/5....

ELECTRONIC DEVICES & MATERIALS GROUP

Selection of Combinations of Semiconductor/Dielectric Metal Oxides Deposited without Substrate Heating for Transparent Thin Film Transistors A. J. Flewitt, F. M. Li, M. Mann, R. Waddingham, A. Kiani, N. S. Staack, S. M-L. Pfaendler and W. I. Milne

Electrical Engineering Division, Cambridge University

CIKC CAMBRIDGE INTEGRATED KNOWLEDGE CENTRE


Outline

• Technology Selection

• Materials vs. patterning vs. deposition

• HiTUS Deposition Technology

• Materials

• Dielectrics and n- and p-type semiconductors

• TFTs with Varying Materials Combinations

• Metrics for selection

• Conclusions


Large Area Electronic Materials

AMORPHOUS SILICON

Metal Oxides

Other Silicons

Organic


Understanding the Incumbent a-Si:H Technology

Known technology

• Silicon is a known technology that is reproducible, physically (if not electronically) stable and non-toxic

• Range of dielectrics available

Amorphous structure

• Excellent homogeneity over 10 m2 areas

Low temperature processing

• Reasonable performance as low as ~150 °C

Low carrier mobility

• a-Si:H TFTs cannot be used for drive circuitry

Metastable

• Defects are created when a gate bias is applied to a TFT

Patterning costs

• Photolithography and etching used for patterning


Understanding Metal Oxide Technology

Unknown technology

• Metal oxides are difficult with developing consensus on the physics

• Chemical stability, electrical stability and reproducibility are all challenging

Amorphous structure

• Excellent homogeneity possible

Low temperature processing

• Evidence of excellent device performance with processing < 100 °C

High carrier mobility

• 10 cm2 V-1 s-1 is a conservative estimate of what should be possible

? Stable

• Evidence of instability under certain conditions, but management may be possible

? Patterning costs

• Photolithography and etching used for patterning, but alternatives are possible due to low deposition temperature


Material Class Material Sets

N-type Semiconductors

• ZnO, IZO, GIZO, HIZO

• SnO2

• ZTO

P-type semiconductors

• Cu2O

• SnO

• Acceptor-doped ZnO

Dielectrics

• Si3N4, SiO2

• HfO2, Al2O3

• HfxSiyO

Material Technologies


Deposition Methods

Features

ALD

Excellent quality films

Slow deposition rates (0.1 – 1 nm/min)

High-temperature deposition or annealing

RF Magnetron Sputtering

Up to 5 nm/min at high RF powers

…but ion-induced damage

Targets need frequent changing and systems frequent cleaning

Sputtering systems seen in standard manufacturing lines (e.g. ITO layers)

PLD

Very good quality films

Relatively slower deposition rates (~1 nm/min)

High-temperature deposition or annealing

Deposition Technologies


Patterning Technologies

• Photolithography is the dominant patterning technology

• High resolution

• High cost

• Wet chemical etching of metal oxides can be difficult

• Lift-off processing possible but reduced yield

• Low deposition temperature does allow for alternative patterning technologies • Printing-based patterning

• Alternative masking

• Marrying a high-rate, reproducible, low cost vacuum deposition technology with a low cost patterning technology is key


Technology Interdependence

Material technology

Deposition technology

Patterning technology


Transparent Electronics

5

12

3

4

1

2

3

4Corning 7059 glass substrate

ITO gate

Hafnium oxide gate insulator

Zinc oxide channel

ITO source/drain metallisation5

Transparent TFT


HiTUS Sputtering System

High target utilisation

High deposition rate

Independent control of plasma density and sputtering ion energy Excellent control of material

properties including stress

Sample removed from sputtering plasma Reduced sample ion

bombardment

Ar Gas99.999%

O Gas99.999%

2

5 6

7

78

8

8

9

10

1

3

3

8 8

2

4

11

12

13

14

1

2

3

4

5

6

Sputter target

Earth shield

Gas ring

Rotating sample stage

13.56 MHz rf power supply

Matching network

Mass flow controllers

Valves

7

8

9

10

Turbo pump

11

12

13

14

15

15 Shutter


Sputtering Processes

• Deposition of metal species

• Deposition of metal oxide species

• Oxidation

• Gas phase oxidation and thermalisation due to collisions

• Sputtering of substrate material by Ar+ ions

• Oxidation of target surface (poisoning)

Ar Gas99.999%

O Gas99.999%

2

5 6

7

78

8

8

9

10

1

3

3

8 8

2

4

11

12

13

14

1

2

3

4

5

6

Sputter target

Earth shield

Gas ring

Rotating sample stage

13.56 MHz rf power supply

Matching network

Mass flow controllers

Valves

7

8

9

10

Turbo pump

11

12

13

14

15

15 Shutter


Polycrystalline Zinc Oxide

• A sharp transition occurs as a function of O2 flow rate between highly resistive and highly conductive material

• Deposition rate ~50 nm min-1

• Polycrystalline structure


InZnO

• Sputtering a binary oxide is more difficult due to additional stoichiometry control

• InZn (50%:50%) alloy target Polycrystalline structure

• Zn target with an In well

Polycrystalline and segregated structure

• InZn (10%:90%) alloy target Amorphous structure

Wide variation in conductivity

105 -1 m-1 down to ~ 1 -1 m-1

Indium target depletion

10 20 30 40 50 60 70

ZnO

In2O

3 (222)

IZO

From literature:

IZO: 2 ~= 32o

ZnO (pure): 2 = 34.4o

In2O

3:

2 = 30.6° (222)

2 = 35.5° (400)

2 = 63.6° (444)

IZO Sample 19-02-09-7 (Mixed Phase InZn Target):

Launch Power =900W, Target Power = 800W, O2 Flow = 45sccm, 5 min., t

f ~50nm

Co

un

ts [

a.u

.]

2 [Degrees]

smoothed with baseline substraction In2O

3

29.51

33.0901

32.71

10 20 30 40 50 60 70

IZO

In2O

3 (222)

ZnO (002)

smoothed with baseline substraction

IZO Sample 2009-08-12-1 (Zn Target with In-Well):

Launch Power =600W, Target Power = 500W, O2 Flow = 40sccm, 10min., t

f ~ 128nm

Co

un

ts [

a.u

.]

2 [Degrees]

29.55

33.97

34.21

35.77

33.0503

Literature Values:

IZO: 2 ~= 32o

ZnO (pure): 2 = 34.4o

In2O

3: 2 = 30.6° (222)

2 = 35.5° (400)

2 = 63.6° (444)

In2O

3 (400)

31.59

In2O

3ZnO


ZnSnO

• ZnSnO sputtered from metallic Zn:Sn (90%:10%) target

• Tuneable range: 10-1 to 108 -cm

• Hall mobility measurement:

• N-type; for = 5.7x104 -cm, hall = 49 cm2/V-s

• Optically transparent in visible

• EG= 2.8-3.0 eV

• Deposition rate: 70 nm min-1


Cuprous Oxide

• Deposition from metallic Cu target

• Deposition rates ~ 20 nm min-1

• Resistivity ~10-1 to 104 cm

• Hall measurements

• Confirm majority hole carriers (p-type)

• Hole mobility approaching 10 cm2 V-1 s-1

• Optical Tauc Gap: EG = 1.0 to 2.0 eV

• Narrow operating window to get Cu2O (vs. CuO)

1 2 3

0

5

10

15

20

25

30

O2 (

sccm

)

Lp/Tp

CuO/Cu2O

Cu/Cu2O

Cu2O

Opaque

Metallic

Conductive

Semi-transparent

Intrinsic

Conductive

p-type

Semi-transparent


Amorphous Aluminium Oxide

• Resistivity up to 1016 cm

• Breakdown ~ 4 - 6 MV cm-1

• Refractive index ~ 1.45

• 90% visible range transmission

• Amorphous

• AlxOy, y/x ~ 1.7

• Band gap ~ 8.8 eV

• Dielectric constant ~ 9.1


• IZO channel on a 2 µm SiO2

• µFE = 10 cm2 V-1 s-1

• VT = -10 V

• Sub-threshold slope 1.4 V dec-1

• Switching ratio > 106

-0.1 0.0 0.110

-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

VDS

= 0.2 V

VDS

= 1.0 V

VDS

= 2.0 V

VDS

[V]

I DS [

mA

]

TDP00C_a

I DS [

A]

EG [MV cm

-1]

-5 0 5 10 15 20 25 30

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6 VGS

= -2.5 V

VGS

= 0.0 V

VGS

= 2.5 V

VGS

= 5.0 V

VGS

= 7.5 V

VGS

= 10.0 V

IZO/SiO2 TFT Transfer Characteristics


IZO/Al2O3 TFT Transfer Characteristics

• On-off ratio ~ 105

• Mobility ~ 0.1 cm2 V-1 s-1 • Sub-threshold slope 3 V dec-1

• Threshold ~ 6.3 V • Contact resistance: relatively

high • Large hysteresis

• Electron trapping -20 -10 0 10 20

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

VDS

= 1 V

VDS

= 2 V

VDS

= 3 V

VDS

= 4 V

VDS

= 5 V

I DS [

A]

VGS

[V]


IZO/Al2O3 TFT Transfer Characteristics – post-annealing

• On-off ratio ~ 107

• Mobility ~ 2.2 cm2 V-1 s-1

• Sub-threshold ~ 4 V dec-1

• Threshold ~ 11 V

• Significantly reduced hysteresis • Ion migration

-20 0 2010

-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

VDS

= 1 V

VDS

= 2 V

VDS

= 3 V

VDS

= 4 V

VDS

= 5 V

I DS [

A]

VGS

[V]


• HfOx deposited by HiTUS

Dep. Rate ~ 25 nm/min

Resistivity ~ 1.4 x 1014 cm

Breakdown ~ 3 MV/cm

Band-gap ~ 5.9 eV

XRD: amorphous film

IZO TFT with HfO2 Dielectric

TFTs broke down at 100 kV/cm

• Silicon nitride (Si3N4) by rf-PECVD

Resistivity = 1014 -cm

Breakdown >3 MV/cm

-20 -10 0 10 20 30 4010

-12

10-11

10-10

10-9

10-8

VDS

= 5V

VDS

= 10V

VDS

= 15V

VDS

= 20V

I DS [

A]

VGS

[V]

Before Annealing IZO TFTs with SiNx Dielectric:

Before annealing: - small ION/IOFF ~103

- large hysteresis (e- trapping)

After annealing: IZO channel became conductive!

Chemical reaction? [H] diffusion & doping?

IZO/HfOx & IZO/SiNx TFT


ZnO/Al2O3 TFTs

• Four mask process

• Lift-off/selective etch with no vacuum break at dielectric/channel interface

• Post-fabrication annealing

• Mobility ~0.4 cm2 V-1 s-1

-10 0 10 20 3010

-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

VDS

= 4 V

VDS

= 7 V

VDS

= 10 V

I DS [

A]

VGS

[V]


P-type Cu2O TFTs

Bottom-gate top-contact Cu2O TFTs

• Thin channel layer (30 nm)

• Depletion mode

• Low off state current

• μFE = 0.01 cm2V-1s-1

• On/off ratio = 2.7

• No saturation

0 10 20 30 40 50 600.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-60 -40 -20 0 20 40 60

0.0

0.5

1.0

1.5

2.0

Id (A

)

Vd (V)

Vg (V)

-60

-40

-20

0

20

40

60

Ig (n

A)

Id (A

)

Vg (V)

Vd = 60 V

-4

-2

0

2

4

6

8

10

12

b)a)

Requires further optimisation of carrier concentration & defect density


Conclusions

• HiTUS deposition allows sputtering of high quality metal oxides over large areas without substrate heating at high rates

• Indium mobility presents a problem at low T

• Silicon dioxide is consistently the best dielectric

• Hydrogen in silicon nitride presents a problem at low T

• Hafnium oxide does not present a good electron barrier to IZO

• Aluminium oxide causes collapse of high mobility in ZTO

• Work ongoing on hafnium oxide with ZTO and on p-type materials


Acknowledgements

• Stuart Speakman

• 3T Technologies

• EPSRC & TSB

• Grant No. EP/F063865/1

• Grant No. TS/G001960/1

• Grant No. TS/I001158/1

• CAPE

• Cambridge IKC

• Plasma Quest Ltd.

• The EDM Group is a member of

CIKC CAMBRIDGE INTEGRATED KNOWLEDGE CENTRE

Selection of Combinations of Semiconductor/Dielectric Metal Oxides Deposited … Event/5....

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