Under a Compulsory Course of "Materials Physics and Technology for Nanoelectronics" a team of BE Students of Nanotechnology, Nanoelectronics and Bionnotechnology prepared this seminar for Prof. Marc Heyns, marc.heyns@imec.be Kapeldreef 75, B-3001 Heverlee IMEC Building IV, room 2.33 Tel: 016 281 348

1. 1
Organic Charge Trapping Memory
Transistors
Dries Agten – Erik Bury – André Cardoso – Evelien Mathieu – Pieter Weckx

2. 2
Overview
 Introduction
o Memory types
o FET-based memories
 Working principle
o Charge trapping
o Band diagram analysis
o Programming sequence
 Comparison of two devices
o PVA & PCBM
o PαMS & Pentacene
 Conclusion

3. 3
Memory types
Q.D. Ling et al, Progr. Polym. Sci. 2008, 33, 92
Introduction

4. 4
FET-based memories
Floating Gate FerroelectricCharge Trapping
Cyferz, Wikipedia, 2007, Flash_cell_structure Cyferz, Wikipedia, 2008, 1T_FeRAM_cell_structure
Introduction

5. 5
SONOS/MONOS Dielectrics
Cyferz, Wikipedia, 2007, SONOS_cell_structure L. Forbes, free patents onl. 2005, 10/775908
Charge Trapping
Introduction

6. 6
Charge Trapping
Organic field-effect transistors with polarizable gate insulators
Howard E. Katz et al., Appl. Phys. 2002, 91, 1572–6
“There is growing appreciation of the capability to fabricate
various kinds of electronic circuits from organic materials.”
Introduction

7. 7
From MOS to FGMOS
 Based on MOSFET
transistor
 Gate is uncoupled, memory
is non-volatile
 Charges are trapped in the
floating gate
 Data sensed by Vth shift of
MOSFET
Data represented by
carriers stored in the
floating gate
J. D. Casperson et al., CIT, Journal of Applied Physics, 2009, 92, 261
Working principle

8. 8
Influences on Vth
Traps close to/in the
channel
Charge injection from
the semiconductor into
the dielectric
Slow reactions
of charge carriers in the
organic semiconductor
Mobile ions in the
semiconductor
Ferroelectric effect
Mobile ions in th
dielectric
Charge injection from the
gate electrode
M. Egginger et al., Monatsh Chem, 2009,140,735
Working principle

9. 9
Band diagram analysis (1)
Vgs=0V Vgs=-5V
-> accumulation-> depletion
Working principle

10. 10
Band diagram analysis (2)
Vgs=-5V Vgs=-10V
-> accumulation-> inversion
Working principle

11. 11
Working principle
 When applying an external voltage to the gate, charges
tunnel from the channel to the interface (~104
carriers to
represent 1 bit)
 Both electrons and holes
Programming sequence

12. 12
Measurement setup
S.Y. Chou, Princeton University, Publication on website
Working principle

13. 13
Measurements
Shift in threshold voltage
M. Debucquoy et al, Organic electronics 2009, 10, 1252
Working principle

14. 14
Ambipolar vs unipolar
 Ambipolar semiconductor: both p-type and n-type operations are
realised (eg. program by holes and erase by electrons)
→ Balanced mobility (n and p) and ON/OFF operation needed
Insufficient electron mobility
→ Trapped holes cannot be erased
Wide memory window
Working principle

15. 15
Stacking holes for memory
usage
 Example: electrons tunnel back too easy -> poor
retention time
 Negative part of memory window is useful
M. Debucquoy et al , Organic electronics 2009, 10, 1252
Working principle

16. 16
Search for improvement in terms of Materials and Scheme
Singh et al, 2004
Heremans et al, 2009
Comparison of two devicesComparison of
two devices

17. 17
Comparison - Structure
 PVA – polyvinylalcohol
 PCBM – methanofullerene
T.B. Singh et al, Appl. Phys. Lett. 2004,
85, 22, 5409
Organic components
Size: channel length W=1000um L=10um
Comparison of
two devices
 Pentacene – polyaromatic
hydrocarbon
 PαMS – polystyrene
 Thiol monolayer –
fluorinated thiols

18. 18
Comparison - Semiconductor
 PCBM
 p-type
 High mobility
 High ON/OFF current ratio
T.B. Singh et al, Appl. Phys. Lett.
2004, 85, 22, 5409
 Pentacene (~14 Å)
o 5 benzene rings
o Crystal Structure
 p-type
 High Mobility
 Reasonable ON/OFF current ratio
IBM Zurich,AFM Image
Penacene Aug 2009
Campbell, 1961
molecular packing
Prof. Takao Someya
Comparison of
two devices

19. 19
Comparison - Electret
 Charge trapping within bulk
or at interface
 Hydrophilic
T.B. Singh et al, Appl. Phys. Lett.
2004, 85, 22, 5409
 PαMS (polyalpha-methylstyrene)
 Hydrophobic
(Insulator coated with a very thin-layer)
Reduces: trapped electrons at the
interface between the pentacene
and the gate dielectric
To suppress the degradation of the
on–off ratio
High-quality, electron-trap free
surface allowing excellent
electron transport
Comparison of
two devices

20. 20
Comparison - Adjacent materials
 Cr for source & drain:
o No diffusion into PCBM
 ITO gate
 No insulator layer
 Heptadecafluoro-1-decanethiol
(Thiol monolayer)
 Other materials:
o Au ; SiO2 ; n++Si
Improved
interface:
Improves pentacene layer growth
Reduces interface states
 Mobility increases
 Threshold voltage approaches zero
Appl. Phys. Lett. 88, 222103 (2006)
Improve the interface
Contacts <-> Semiconductor layer
Comparison of
two devices

21. 21
Performance measurements (Singh)
T.B. Singh et al, Appl. Phys. Lett.
2004, 85, 22, 5409
T.B. Singh et al, Appl. Phys. Lett.
2004, 85, 22, 5409
Comparison of
two devices

22. 22
Performance Measurements (Herem.)
- pulse (write)
h trapped
Programming voltage Transistor’s mobility
e-
trapped
dielectric
ΔVon =2V
Gate
Decrease 1.9V->1.4V
h mobility decrease
Shift VON+ & VON-
Retention time
Memory Window
+pulse (erase)
e trapped
Saturation
Comparison of
two devices

23. 23
Performance Comparison
FLASH
[a]
Floating
Gate
[b]
Ferroelectric
[c]
PVA &
PCBM
[d]
PαMS &
pentacene
[e]
Retention time (h) ~ 3 years ~ 11 ~ 168 > 15 > 3 months
Programming time (s) 0.8 1 0.3e-3 500 1.5e-3
Programming/erasing
voltage (V)
+8/-8 +6/-6 +77.5/-77.5 +50/-50 -15/+15
[a] R. Bez et al, Proc. of the IEEE 2003, 91, 4, 489
[b] S. Kolliopoulou et al, Microel. Eng. 2004, 73-74, 725
[c] R.C.G. Naber et al, Nat. Mater. 2005, 4, 243
[d] T.B. Singh et al, Appl. Phys. Lett. 2004, 85, 22, 5409
[e] P. Heremans et al, Appl. Phys. Lett. 2009, 95, 103311
Comparison of
two devices

24. 24
Conclusion
 Conclusion from Singh organic memory:
o The combination of PVA & PCBM
does not make a good memory-element
 Conclusion from Heremans organic memory:
o It is possible to fabricate a device with
reprogrammable nonvolatile organic
memory usable in Plastic Logic.
“Organic Transistor/Memory devices will reach $21.6 Billion in 2015”, NanoMarkets (2001)
Conclusion

25. 25
References
 K.J. Baeg et al, Adv. Funct. Mater. 2008, 18, 3678-3685
 T.B. Singh et al, Appl. Phys. Lett. 2004, 85, 5409-5411
 K.J. Baeg et al, Adv. Mater. 2006, 18, 3179-3183
 Q.D. Ling et al, Progr. Polym. Sci. 2008, 33, 917-978
 R. Bez et al, Proc. of the IEEE 2003, 91, 4, 489-502
 S. Kolliopoulou et al, Microel. Eng. 2004, 73-74, 725-729
 R.C.G. Naber et al, Nat. Mater. 2005, 4, 243-248
 P. Heremans et al, Appl. Phys. Lett. 2009, 95, 103311
 J. Kang et al, J. Am. Chem. Soc., 2008, 130 (37), 12273–12275
 K. Myny et al, Appl. Phys. Lett. 2006, 88, 222103
 K. Asadi et al, Nature Materials 2008, 7, 547
 Forbes, free patents onl. 2005, 10/775908
 H.E. Katz et al, Appl. Phys. 2002, 91, 1572–6
 Cyferz, Wikipedia 2007, Flash_cell_structure
 Cyferz, Wikipedia 2008, 1T_FeRAM_cell_structure
 Cyferz, Wikipedia 2007, SONOS_cell_structure