1

ZnO based transparent

electronics

Jidong Jin

Research Associate, Dept. of EEE

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Outline

1.Transparent electronic devices

• ZnO based TFTs

• ZnO based Schottky diode

• ZnO based MESFETs

• ZnO based planar nano-devices

2. New applications

• Display technology

• Transparent integrated circuit

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Speed is relative

material cost

sp

ee

d

Plastic

electronics

Amorphous silicon

Metal oxides

Single crystal

silicon

III-V

semiconductors

e.g., GaAs

Material and applications dictate what is “fast” and what is “slow”

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Why ZnO Thin Films ?

• Intensively studied only since 2003

• Still needs a lot of research and development

• Wide band gap (3.4 eV) - visual transparent.

• High electron mobility - high performance.

• Low cost, easily fabricated at room temperature.

• Large area and flexibility.

Zinc oxide applications? • Transparent electrodes

• Light-emitting diode

• Driving circuitry for OLED display

• Solar cells

• Flexible electronics

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Part 1

ZnO-Based TFTs

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Main landmarks achieved with TFTs

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Oxide TFTs related papers

In the legend S means “solution processed”

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ZnO TFT Applications

ZnO TFT based OLED panel

A Ring oscillator on a glass substrate

University of Manchester

Fully Transparent TFT

An inverter on a flexible substrate

University of Manchester

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• Conventional TFTs are fabricated on Si substrate

using metal contacts and undoped ZnO active layers.

• Transparent TFTs are fabricated on glass substrates

using doped ZnO contacts and undoped ZnO active

layers.

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Metal Oxide based TFTs for OLED technology

Display technology

• Liquid crystal display (LCD)

• Organic light emitting diode (OLED)

Why OLED ?

• Self emitting – Does not require back lighting

• Fast response – Fast video applications

• Very thin – Thin and light weight display

• Flexible substrate – Flexible display

Metal oxide thin films for OLED technology

• IGZO – It is amorphous and suitable for flexible substrate

mobility: 10 – 20 cm2/Vs

• ZnO – It is usually polycrystalline and suitable high speed application

mobility: over 30 cm2/Vs is possible

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Pixel circuit

OLED LCD

• LCD – normally off state is important for TFTs

• OLED –on and off states are both important for TFTs

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Required carrier mobility for future displays

~1 cm2/Vs

~5 cm2/Vs

~40 cm2/Vs

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SEL introduces 3-fold 8.7-inch AMOLED display

At the Display Innovation 2014 trade show in Yokohoma City, Japan,

Semiconductor Energy Laboratory (SEL) introduced an 8.7" Super

AMOLED display, which can fold in three. It sports 1920 x 1080 pixel

resolution resulting in a pixel density of 254 ppi.

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Sputtered ZnO Thin Film Transistors with Carrier

Mobility over 50 cm2/Vs*

ZnO TFT structure

• Saturation mobility ~103 cm2/Vs

• VT =1.3 V

• On/off ratio: 4.1×105

• S=0.29 V/decade

• RF sputtering was used to deposit both

ZnO and Ta2O5 gate insulator

To our knowledge, the obtained mobility is one of the highest values in

sputtered ZnO TFTs

ZnO TFT characteristics

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Tuning the Electrical Properties of ZnO Thin-

Film Transistors*

• Very high conductivity in as-deposited films, typical σSD ~ 11300 S/m.

• Little field effect observed in as-deposited films.

• Good transistor behaviour observed when annealing at 220 oC in air.

• The experiments show that annealing in air increases the threshold

voltage of the TFTs, while annealing in nitrogen gas reduces it.

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Tuning the Electrical Properties of ZnO Thin-

Film Transistors

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Part 2

ZnO-Based MESFETs & Schottky

diode

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ZnO based MESFET

ZnO TFT ZnO MESFET

• In 1966 Carver Mead made first MESFET [1]

• MESFET exhibits much lower operating voltage than TFT

• A higher channel mobility than TFT [2]

[1] C. Mead, Proceedings of the institute of Electrical and

Electronics Engineers, vol. 54, pp. 307-308, 1966,

[2] Frenzel.et.al, Appl.Phys.Lett. Vol. 92, p19, 2008

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Depletion and Enhancement mode MESFET

Threshold voltage VT is given by

for the uniformly doped case.

Where Vbi is Schottky barrier

buit in potential.

Depletion Enhancement

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Logic Circuit Design (Schottky-diode FET-

Logic Inverter & NOR gate)

10 µm

20 µm 1520 µm

1490 µm

20 µm SDFL inverter NOR gate

Characteristics of a SDFL inverter

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ZnO based Schottky diode (Future work)

Al

Silver oxide

• Substrate: Glass

• ZnO: RF sputtering or ALD

• Al: thermal evaporation

• Silver oxide: RF sputtering via shadow mask

(radius – 50 µm)

ZnO

1expexp2*

Tnk

IRVq

Tk

qTAAI

B

S

B

B

Parameter Symbol

Barrier height 𝜙𝐵

Series resistance 𝑅𝑆

Ideality factor 𝑛

Richardson constant 𝐴∗

Area of the diode 𝐴

Objective

• n < 1.5

• Frequency response: ~ 800 MHz

Rectifier schematic

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Schottky contacts on ZnO

1. Frenzel et al. Thin Solid Films, vol. 518, pp. 1119-1123, 2009.

2. Weichsel et al. Semi. Sci. and Tech., vol. 20, pp. 840-843, 2005

3. Aydogan et al. J. Alloys Compounds, vol. 476, pp. 913-918, 2009.

4. Krajewski, et al. Acta Physica Polonica A, vol. 120, pp. A17-A21, 2011.

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Part 3

ZnO-Based Nano-Devices

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SGT and SSD

Source

Drain

Gate

Gate

Semiconductor

Insulating trenches

Anode

CathodeSemiconductor

Insulating trenches

Side-Gated Transistor (SGT)

Self-Switching Diode (SSD)

Conventional TFTs:

- Require multi-layer stack structures.

- Exact alignment required.

- Difficult to maintain alignment over large

area on flexible substrate.

Planar nanodevices:

- Simpler structure than conventional TFTs.

- single layer.

- Nanometre-size allowing ultra-high speed.

- Suitable for one-step nanoimprint.

- Low printing cost. Conventional TFTs

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Self switching diode (SSD) Etched trenches

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Self switching diode (SSD)

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Side-gated transistor (SGT)

drain

source

gate

gate

0.0 0.5 1.0 1.5 2.00.0

0.5

1.0

1.5

2.0

2.5

Dra

in C

urr

ent (

A)

Drain Voltage (V)

2.0 V

1.5 V

1.0 V

0.5 V

0 V

-1.0 V

-1.5 V

• The charge in the nanochannel is controlled by two lateral electrodes.

• The transistor threshold depends on the geometry, NOT the material.

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EBL & Wet-etching

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One-step process (direct embossing)

Thermal indentation (imprint) using semiconductor deposited

on top of a polymer buffer layer

SEM image of the device

Yield is not high enough

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Multi-step process (Nano imprint & RIE)

ZnO

Substrate

shim

PMMA ZnO

Substrate

O2 RIE

ZnO

Substrate

CH4+H2 RIE

EVG 520

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ZnO based SGT

• The transfer and output curve for the planar ZnO SGT fabricated by EBL

and wet-etching process.

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ZnO based SSD

• I-V curve for the planar ZnO thin film nano-diode fabricated by EBL and wet-

etching process.

• Separate experiments showed 50MHz high speed.

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ZnO based planar nanodiode operating at 50 MHz*

Optical image AFM image Frequency response

• A parallel array of 50 SSDs fabricated by EBL and wet-etching

• Input – a sinusoidal voltage supply of 4V (RMS value)

• Separate experiment – ZnO TFTs mobility 0.1 to 0.3 cm2/Vs.

• If ZnO films with higher mobilities are used, frequency response can be

up to a few GHz.

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ZnO based planar inverter

Channel Length

(µm)

Channel Width

(nm)

SSD 2 500

SGT 2 450

• All terminals on the same layers.

• No need of interconnect layers.

• Circuits are fabricated by “writing” lines on the substrate.

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Circuits applications

NOR NAND

1 1

1 0 1

0 A

B 1 0

A

B Out

NAND 1 0

0 0 1

0 A

B 1 0

A

B Out

NOR

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Novel technology for planar ultra-fast devices

One lithography step

No mask

alignment nanoimprinting

Easier

interconnect

layers

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Low parasitics = high speed

RFID tagging Fast logics THz technology

ZnO GaAs

Novel technology for planar ultra-fast devices

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Thank you !