This is about Si for thin film transistors, direct deposition of si on plastic, different types of modified si materilas used for TFT.

1. SILICON FOR THIN-FILM
TRANSISTORS
Presented by
Gawtham G Dath
AM.EN.P2VLD16025

2. OVERVIEW
• Introduction
• Silicon for thin-film transistors
• Direct deposition of silicon on plastic
• Amorphous silicon
• Nanocrystalline silicon
• Microcrystalline silicon
• Conclusion

3. Introduction
• Industrialization of flexible thin-film transistor (TFT)
backplanes.
• Two important research directions for the TFTs are:
– processability on flexible substrates.
– sufficient field-effect mobilities of electrons and
holes.
• Most important group of TFT capable
semiconductors are
– amorphous
– nanocrystalline
– microcrystalline

4. Contd…..
• Considerable research on TFTs which function as the
pixel switches in AM-LCDs.
• TFT channels made of
– Silicon,polymer organics,compound
semiconductors like CdSe.
• Amorphous Si TFTs are the current industrial
standard.
• Low power operations are out of reach.
• Nanocrystalline(nc) and microcrystalline (mc) silicon
have higher filed effect mobilities

5. Contd….
– high level integration raises yield and reduces cost.
– capable of driving high-current loads such as
organic light emitting diodes (OLEDs).
• Next innovation flexible displays fabricated on
flexible foil substrate.
– light weight and ruggedness

6. SI for TFT
• Two important research directions for TFTs are high
mobility and flexible substrates.
• TFT applications benefit from a high ‘on’ current,
which is proportional to carrier mobility.
• Switches and on-pixel circuits can be implemented
with n channel TFTs.
• P channel TFT used for reduction of power
consumption.

7. Contd..

8. Contd…
• Glass substrates invention of low temperature processes
for the crystallization(600◦C).
• Steel substrates permit temperatures upto 1000◦C.
• Organic polymers(plastics) have low temperature
stability.
• a-Si TFTs may be brought to plastic by careful adaptation
of the established 250–350◦C processes.
• Challenges to fabrication on plastic
– high-mobility TFTs made from directly deposited nc-
Si or from laser crystallized mc-Si.

9. Contd…
• TFT graded si made on foil substrate in three ways
– deposition of a precursor film followed by
crystallization
– direct deposition of the channel semiconductor
– physical transfer of separately fabricated circuits.

10. Contd..

11. Direct Deposition of Silicon on Plastic
• Deposition of device-grade silicon onto plastic
brings up many substrate issue. Desirable features:
– thermal and chemical stability
– low coefficients of thermal and humidity
expansion
– low permeability by water and oxygen
– good film adhesion
– Low chemical and mechanical inhomogenity
– Low surface roghness.(Kapton E used)

12. Contd…
• Plastic substrates selected by following features:
– High thermal stability
– Low coefficient of thermal expansion
– Film adhesion

13. Amorphous Silicon
• Deposition of both a-Si:H and nc-Si:H PE-CVD is
compatible with plastic substrates adapted to
T=150◦C.
• Advatage- it requires only an extension instead of a
new deposition technique.
• Deposition quality deteriorates with decresing
deposition temperature.
• Overcomed by diluting source gases with hydrogen
• Electronic properties comparable to those grown at
optimum temperature

14. Contd…

15. Nanocrystalline silicon
• Evolution of nc-Si has two consequences
– rise in the carrier mobilities in the top layer of the
film.
– the electrical activation of impurities that are
inactive in a-Si:H.

19. Microcrystalline silicon
• Different technologies for preparation of mc-Si are:
– PECVD
– HWCVD
• The best performance of μc-Si:H solar cells is always
observed near the transition to amorphous growth

20. Conclusion
• Silicon films are leading candidates for flexible and
conformal TFT backplanes.
• nc-Si and mc-Si both are capable of CMOS
operation-high mobility
• Reasearches are done on Laser crystallization of a-Si
on plastic substrates .