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Gate Carrier Injection and NC-Non-
Volatile Memories
Jean-Pierre Leburton
Department of Electrical and Computer
Engineering and Beckman Institute
University of Illinois at Urbana-Champaign
Urbana, IL 61801, USA
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Hot Carrier Effects in MOSFETs
High-field/non-linear transport
vx
kBT
c
Fx
Long tail energy
distribution
E =(1 / 2)m*v2
*
*After R.S. Muller and T.I Kamins, DEIC, Wiley, 3d ed.
f(v)
J.P. Leburton, IWSG-2009, IITB, India
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Hot Carrier Effects: Substrate Current*
Ec
EG
Ev
SS
SS
S
S
SS
SE>EG
e
ee
h+
Impact Ionization
I-V characteristics
*After R.S. Muller and T.I Kamins, DEIC, Wiley, 3d ed.J.P. Leburton, IWSG-2009, IITB, India
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Tunneling Injection into the Gate*
Direct Tunneling
Fowler-Nordheim TunnelingTrap-Assisted Tunneling
Electron trapping in SiO2 Hole trapping in SiO2
*After Y.Taur and T.H. Ning, FMVD, Cambridge, 2d ed.
JFN !exp(!4 2qm *
3"Fox"ox
3/ 2 )
Fox
!ox
VT- Degradation
Dissipation (gate leakage)
JTun
= JTun
(tox
,!ox)
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Injection into Floating Gates
n-channel
p-channel
Drain-avalanche
(impact ionization)
Injection by channel hot electrons (CHE)
No CHEbecause largeroxide barrier
J.P. Leburton, IWSG-2009, IITB, India
After R.S. Muller and T.I Kamins, DEIC, Wiley, 3d ed.
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Flash Memory Device: Basic Operation*
ETOX: Hot electron-tunneling combined VT-shift
But leakage throughdefects!!!
* A. Thean and J.P.Leburton, IEEE Potentials, 21(4) 35, (2002)
!FG electrically disconnected
!Data stored in form of charge packages
!Transport mechanisms (CHE)!FN tunnelling (oxide damage)
!Memory cells altered individually
!Data storage sensed by conductance
!Non-volatile storage
!Down scaling X retention time
J.P. Leburton, IWSG-2009, IITB, India
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Nanocrystal Memories*
* S. Tiwari et al. IEDM Tech Dig., 521, Dec. 1995.
**
** Courtesy Motorola Inc.
Single electron charging***
"VG=e/C; C:NC capacitance
NC memory operation principle*
NC memory device
structure
"E
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NC Memory Device: QM Modeling*
Schroedinger Equation (effective mass approx.)
Simulated structure Crystallographic orientations
!!2
2("x ,"y ,"z )
mxx!1("r ) mxy
!1("r ) mxz
!1("r )
myx!1("r ) myy
!1("r ) myz!1("r )
mzx!1("r ) mzy
!1("r ) mzz
!1("r )
#
$%%%
&
'(((
"x"y"z
#
$
%%%
&
'
(((+V("r )
)
*
+++
,
-
..
./v,n ("r ) = Ev,n/v,n("r )
Mv,T
!1=
"T
!1 Mv
!1"T with M
v
!1=
m1!1
(!
r ) 0 0
0 m2!1
(!
r ) 0
0 0 m3!1
(!
r )
#
$
%%%
&
'
(((
Rotation matrix*J.S. de Sousa et al., APL 82, 2685 (2003)J.P. Leburton, IWSG-2009, IITB, India
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Energy Spectra: Effective Mass Anisotropy
Spherical nanocrystal
D = 10 nm
1 /miso =2 / (3m
t) +1 / (3m
l)
J.P. Leburton, IWSG-2009, IITB, India
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Energy Spectra: Size and Shape Effects *
Spherical Quantum Dots Truncated Nanocrystals
! Degeneracy among energy valleys
! Orbitals orientation follow the rotation of
the effective mass tensor
! Lifting of energy valleys degeneracy
! Accidental degeneracies
J.P. Leburton, IWSG-2009, IITB, India
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Crystallographic Orientation Effects*
! Different crystalline orientations are
responsible for accidental degeneracy
! !En< kBT (room temperature) for the [010]orientation
! Minibands appear for the [110] orientation
! Despite of the non-symetrical shape, energy
valleys degeneracy is recovered for the
[111] orientationJ.P. Leburton, IWSG-2009, IITB, India
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Data Operation Modeling: Dynamics*
Data programming
Data erase and retention
groundstate
Bardeen Hamiltonian approach
J.P. Leburton, IWSG-2009, IITB, India *J. S. de Sousa et al, J. Appl. Phys. 92, 6182 (2002)*J. S. de Sousa et al, Appl. Phys. Lett. 82, 2685 (2003)
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Charging Time Dynamics*
!Practical programming times ("100 ns) are only achieved by combining very thinoxide barriers ("20) and VG>2.0V (consistent with experiment)!Correlation between the average charging time and the number of electrons in thechannel
*J. S. de Sousa et al, J. Appl. Phys. 92, 6182 (2002)
Tunneling barrierthickness
D=7nm
J.P. Leburton, IWSG-2009, IITB, India
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*
1st consequence: redistribution of the
electrostatic potential (EP) across thedevice
EP drop in the oxide layer islarger forSiO2than for HfO2
Smaller HfO2!ECmay favor FNtunneling through the gate
compromising data write and retentionfor VG>2.5V. Thus, TCmust be
increased (>20nm)
Concerns on the dielectric
breakdown: F(HfO2)=10MV/cm
and F(SiO2)=20 MV/cm. Quality of
the oxide becomes crucial !!
High-K Oxides: Electrostatics
J.P. Leburton, IWSG-2009, IITB, India
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High-k materials increases writeperformance, but also decreaseretention time (device reliability).
Strategy: increase tunneling oxidethickness !
Main advantage: we can increase
the tunneling oxide and still obtaingood performances because of the
smaller !EC!
High-K Oxides: Programming
J.P. Leburton, IWSG-2009, IITB, India
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10 YearsSphere
3 MonthsTrunc. Sphere
11 DaysHemisphere
Retention TimeShapes
D= 70 TOX= 35
A. Thean et al., Proc. Nonvolatile Memory Technology Symp., 2000, pp.1621.
VG= 2.0V TOX= 15
The faster data are written ...
... the faster they are lost !
J. S. de Sousa et al, J. Appl. Phys. 92, 6182 (2002)J. S. de Sousa et al, Appl. Phys. Lett. 82, 2685 (2003)
A tough problem: Data retention
J.P. Leburton, IWSG-2009, IITB, India
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Optical Programming*
*J. S. de Sousa et al., Appl. Phys. Lett. 92, 103508 (2008)J.P. Leburton, IWSG-2009, IITB, India
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