organic charge trapping memory transistors
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1
Organic Charge Trapping Memory Transistors
Dries Agten – Erik Bury – André Cardoso – Evelien Mathieu – Pieter Weckx
2
Overview
Introductiono Memory typeso FET-based memories
Working principleo Charge trappingo Band diagram analysiso Programming sequence
Comparison of two deviceso PVA & PCBMo PαMS & Pentacene
Conclusion
3
Memory types
Q.D. Ling et al, Progr. Polym. Sci. 2008, 33, 92
Introduction
4
FET-based memories
Floating Gate FerroelectricCharge Trapping
Cyferz, Wikipedia, 2007, Flash_cell_structure Cyferz, Wikipedia, 2008, 1T_FeRAM_cell_structure
Introduction
5
SONOS/MONOS Dielectrics
Cyferz, Wikipedia, 2007, SONOS_cell_structure L. Forbes, free patents onl. 2005, 10/775908
Charge TrappingIntroduction
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Charge Trapping
Organic field-effect transistors with polarizable gate insulatorsHoward E. Katz et al., Appl. Phys. 2002, 91, 1572–6
“There is growing appreciation of the capability to fabricatevarious kinds of electronic circuits from organic materials.”
Introduction
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
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Influences on Vth
Traps close to/in the channel
Charge injection from the semiconductor into the dielectric
Slow reactionsof charge carriers in the organic semiconductor
Mobile ions in the semiconductor
Ferroelectric effect
Mobile ions in the dielectric
Charge injection from the gate electrode
M. Egginger et al., Monatsh Chem, 2009,140,735
Working principle
9
Band diagram analysis (1)
Vgs=0V Vgs=-5V
-> accumulation-> depletion
Working principle
10
Band diagram analysis (2)
Vgs=-5V Vgs=-10V
-> accumulation-> inversion
Working principle
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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
Measurement setup
S.Y. Chou, Princeton University, Publication on website
Working principle
13
Measurements
Shift in threshold voltage
M. Debucquoy et al, Organic electronics 2009, 10, 1252
Working principle
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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
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
Search for improvement in terms of Materials and Scheme
Singh et al, 2004
Heremans et al, 2009
Comparison of two devicesComparison of two devices
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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
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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 ringso 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
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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
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 growthReduces 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
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
Performance Measurements (Herem.)
- pulse (write) h trapped
Programming voltage Transistor’s mobilitye-
trapped
dielectricΔVon =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
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
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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
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
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