Electrical, Optical, Mechanical Study of Differences in ZnO Based TCOs and ITO

Thin Films

Jing YangSCI Engineered Materials, Inc

The Science of Engineered MaterialsTM

History of SCI

Founded in 1987 by Prof. Funk ( Ohio State University) as Superconductive Components, Inc. in Columbus, OH. Changed name to SCI Engineered Materials, Inc. in 2007

Initially focused on R&D with high temperature superconducting materialsand devices

Developed manufacturing capabilities to produce advanced ceramiccompositions for sputtering targets

Manufacture products for diverse global markets

Continue to leverage manufacturing capabilities, intellectual property andproprietary knowledge into complementary growth markets

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Basic Properties of ZnO Based TCO vs. ITO

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ITO ZnO based TCO

90%In2O3/10%SnO2 ChemistryZnO: Al (AZO);

ZnO:Ga (GZO) and others

Rocksalt(Cubic based)

Crystal structure

Wurtzite(Hexagonal based)

PVD (sputtering)Mass

productiondeposition

PVD (sputtering)

Sn doping and Ovacancies

Conductivemechanism

Defects, vacancies (O deficiencies)

1-2x10-4ohm.cmpost anneal

Thin filmresistivity

Thickness dependant3x10-4 ohm.cm for more than 400nm

High Visible light transmission High

Partial pressure of O2 is very critical

Sputteringcondition Ar only

Materion Technical Paper

TCO Application – DisplayAZO (GZO) and ITO as electrode in four modes:

SCI sponsored research successfully replace ITO with AZO/GZO in all four LC modes (Kent State Liquid Crystal Institute)

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Twisted Nematic (TN) Vertical Aligned (VA): In plane Switch (IPS) Fringe Field Switch (FFS)

Contrast Ratio 1801:1 1812:1 3000:1 3000:1 1300:1 1100:1 1500:1 500:1

turn on/ms 6.2 6.1 400 440 20.7 17.5 28.9 28.3

Und

er P

OM

Illustration Credit to: FFS Technology

TCO Application - PDLC ElectrodePDLC- Polymer Dispersed Liquid Crystal:

Application: Smart curtains

We used AZO to displace ITO as electrode in PDLC cell

AZO: ~200ohm/sqr, 100nm

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Optimize Sputtering Condition for TCO Thin Film Sputtering AZO and ITO in-house: In-house DC sputtering: MRC, 5E-6Torr

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Time(s)

Thickness(nm)

Rs(Ω/)

ρ(Ω.cm)

53 120 145 1.7 x10-3

70 170 95 1.5 x10-3

106 250 57 1.4 x10-3

Ar+5%O2

Thickness(nm)

Rs(Ω/)

ρ(Ω.cm)

14 sccm 150 45 6.8x10-4

18 sccm 150 42 6.3x10-4

22 sccm 150 49 7.3 x10-4

AZO thin film Resistivity with different thickness3kw, Ar 9E-3Torr

ITO thin film Resistivity with different O2 flow 1kw, Ar 34 sccm

Z Ma et al, Nanophotonics 2015

Post Annealing for AZO and GZO:

Annealing Temperature for ITO: 220-260oC Significant lower annealing T for AZO than ITO Little anneal needed for GZO thin film to achieve low resistance AZO and GZO is more compatible with low temperature process

Low Annealing T for AZO and GZO

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AZO annealing study GZO annealing study

GZO before anneal

0 20 40 60 80 100 120 140 160 180 200 220-40

-30

-20

-10

0

10

20

30

Decr

ease

per

cent

age

(%)

Annealing Temp (C)

Rs

160nm 64Ω/

Optimized in-house ITO annealing T

210nm 63Ω/

Optical Comparison between AZO and ITO

Visible (400nm-700nm) – Display Industry:

• The transmission of AZO in visible range is very high but needs optimization due to thickness fringes. (~130nm film renders highest visible transmittance)

• To achieve same visible transmittance, ITO needs be significant thinner than AZO.Proprietary to SCI Engineered Materials

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AZO visible T% vs. film thickness

Sample Thickness Rs T%(400-700nm)

AZO 130nm 70Ω/sq >96%ITO* 30nm 80Ω/sq 96.5%

* Commercially available ITO thin film on glass

300 400 500 600 700 800 900 10000

20

40

60

80

100

ZnO-3(115nm) ZnO-4(120nm)

Tran

smitt

ance

(%)

Wavelength (nm)

Optical Comparison between AZO and ITO

Visible to near IR – Solar Industry :

• Depends on the specific solar cell, when low temperature process needed, AZO has higher transmission in the visible to near IR wavelength.

IR:

• AZO has lower plasma wavelength than ITO due to a high carrier concentration thus reflecting more in the IR wavelength

• It is specifically beneficial for low E glass industry

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Sample Thickness R T%(400-1200nm)

AZO 250nm 56Ω/sq 87.3%AZO-a 250nm 42Ω/sq 90.4%

ITO 185nm 40Ω/sq 88.9%

Wavelength (nm)Tr

ansm

ittan

ce ()

% ITO

AZO

AZO annealed

Annealed AZO vs. ITO with same Rs

UV:

AZO absorb more UV light than ITO due to a smaller band gap which is tunable with different doping level.

It is attractive to protect some organic device from UV damage.

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300 400 500 600 700 800 900 10000

20

40

60

80

100

ITO 30nm AZO 115nm AZO 210nm AZO 310nm

Tran

smitt

ance

(%)

Wavelength (nm) Eg1%AZO 3.55eV

1.5%AZO 3.58eV

2%AZO 3.60eV

y = 0.0127x - 0.0457

Tran

smitt

ance

%

Wavelength (nm)

Optical Comparison between AZO and ITO

Resistance changes of thin film after bending (tensile)

The resistance of AZO/Ag is stable on PEN

Ag is very thin (<5nm) as oxygen/humidity barrier

AZO is mechanically more stable under bending test.

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Mechanical Property Comparison: Bending test

G. Torrisi et. al. Solar Energy Materials and Solar cell, 2017

R. Liu et al, Solar Materials and Solar Cell, 2019

Coalesced film

Columnar film

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SummaryProperties ITO Metal doped ZnOResistivity Low Higher

Annealing T High Low

Thickness to achieve similar Rs Thin Thick (x2)

Transmittance in visible High High

Transmittance in IR High Low

Cut off in UV Lower wavelength Higher wavelength

Mechanical stability Low High

Cost/unit weight $$$ $

Raw material availability Low High

AZO STD- 94%TD AZO HD – 98%TD

Special thanks to:

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Any questions?

Stability study of ITO and AZO on flexible thin film:

The resistance of AZO on PET increases 100% within 2 weeks of exposure in air

The resistance of AZO on PEN is better but still increased 40% after 60 days.

ITO on PET is more stable than AZO on PET and increases less than 10% after 5 months.

AZO/Ag is very stable on PEN.

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Thin Film Properties Comparison between AZO and ITO

R. Liu et al, Solar Materials and Solar Cell, 2019

Coalesced film

Columnar film

Oxygen, Humidity

Oxygen, Humidity when on flexible substrate such as PET and PEN

AZO/GZO on PET in house:

The Rs is largely depend on how flat the PET surface is.

With edge taping to the backing plate, it doesn’t render uniform nor low resistance comparing to that on glass substrate. Proprietary to SCI Engineered Materials

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8”*12”

AZO/GZO on PET R2R with collaborators:

Collaborator 1: - ~160nm, Rs~300Ω/sq GZO on PET and glass.

- ρ: 4.8*10-3Ω-cm.(~310nm, Rs~125Ω/sq, ρ: 3.9*10-3ohm-cm)- T:88-90%- Base pressure ~1x10-5torr

We think the high Rs is due to high base pressure degas of PET roll.

Collaborator 2: Power: 3kw DC & DC pulse; Base pressure: 3-

4x10-6torr Substrates: slide & film ST505 (DuPont 5mil PET)Film: 200nm 2%AZOOn glass: 1.3-1.6 x 10-3 Ω-cmOn PET: 1.2-1.9x on glass

Thin Film Properties Comparison between AZO and ITO