This document discusses transparent electronic devices based on zinc oxide (ZnO), including ZnO thin-film transistors (TFTs), Schottky diodes, and metal-semiconductor field-effect transistors (MESFETs). It also describes new applications using these devices, such as for display technology and transparent integrated circuits. Finally, it presents ZnO-based planar nano-devices fabricated using electron beam lithography and wet etching, including self-switching diodes and side-gated transistors that could enable ultra-fast logic circuits.

1. 1
ZnO based transparent electronics
Jidong Jin Research Associate, Dept. of EEE

2. 2
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

3. 3
Speed is relative
material cost
speed
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”

4. 4
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

5. 5
Part 1
ZnO-Based TFTs

6. 6
Main landmarks achieved with TFTs

7. 7
Oxide TFTs related papers
In the legend S means “solution processed”

8. 8
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

9. 9
•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.

10. 10
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

11. 11
Pixel circuit
OLED
LCD
•LCD – normally off state is important for TFTs
•OLED –on and off states are both important for TFTs

12. 12
Required carrier mobility for future displays
~1 cm2/Vs
~5 cm2/Vs
~40 cm2/Vs

13. 13
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.

14. 14
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

15. 15
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.

16. 16
Tuning the Electrical Properties of ZnO Thin- Film Transistors

17. 17

18. 18
Part 2
ZnO-Based MESFETs & Schottky diode

19. 19
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

22. 22
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  






 


 
 


 

 
 exp exp 1 * 2
nk T
q V IR
k T
q
I AA T
B
S
B
B 
Parameter Symbol
Barrier height 휙퐵
Series resistance 푅푆
Ideality factor 푛
Richardson constant 퐴∗
Area of the diode 퐴
Objective
• n < 1.5
• Frequency response: ~ 800 MHz
Rectifier schematic

23. 23
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
Cathode
Semiconductor
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

27. 27
Self switching diode (SSD)

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Side-gated transistor (SGT)
drain
source
gate
gate
0.0 0.5 1.0 1.5 2.0
0.0
0.5
1.0
1.5
2.0
2.5
Drain Current (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

30. 30
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.

33. 33
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.

34. 34
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

38. 38
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 !