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Tuning Work Func-on Of Halfnia-‐Gold Interface With Organophosphonates
Ma=hew Kwan Graduate Student
Department of Material Science and Engineering Rensselaer Polytechnic Ins:tute, Troy, NY, USA
2014 Rensselaer Nanotechnology Center Research Symposium Wednesday, October 29, 2014
Ganpa- Ramanath
P.H. Mu-n, R. Ramprasad,
Na-onal Science Founda-on
Enhancing the metal/oxide interface for electronic devices
• HfO2 is used as a high performance gate oxide in MOS devices • New gate interconnects must be engineered to adapt to HfO2 • Phosphonic acids can be used to modify metal-‐oxide interface
– Will allow more flexibility in selec-on of metals to be used with HfO2 by: • Improving adhesion • Improving thermal conduc-vity • tuning the work func-on
Source Drain
Substrate
Gate Metal
Gate Oxide
MNL
Organophosphonates used to modify metal-‐oxide interfaces
• 1-‐Dodecylphosphonic acid (DDPA)
• 12-‐Mercaptododecylphosphonic acid (MDPA)
• 3-‐Mercatopropylphosphonic acid (MPPA)
• 10-‐Carboxydecylphosphonic acid (CDPA)
• (12-‐phosphonododecyl)phosphonic acid (PDPA)
CH3
SH
1 mM ethanol or methanol solu-on for 24 hours
HfO2
wash and deposit metal
Previous work: Modifying Au-‐TiO2 interface to enhance thermal conduc-vity and adhesion
• MDPA (-‐SH) added to the Au-‐TiO2 interface – adhesion of monolayer > mul-layer > no MDPA – adhesion é due to S-‐Au and P-‐O-‐Ti covalent bonds
• ordered monolayer with both Au and Ti linkages is more effec-ve than disordered
– Thermal conductance é with adhesion – heat transport é when interface bonding é
from 1) P.K. Chow, et al., Appl. Phys. Le=. 102, 201605 (2013). from 2) P.J. O’Brien et al., Nature Mater. 12, 118 (2012).
Heating pulses
Probe Au
NML
TiO2 Si
How the work func-on shics are measured with UPS
• ΔEvac due to shics in the surface poten-al from bonding or electron distribu-on • EWF = hν - (ESEO – EF) • as ESEO é, Evac and EWF ê
EWF Kine
-c Ene
rgy (eV)
ESEO
Clean Substrate with MNL
HeI hν=21.22 eV
Evac
EF ELUMO
EHOMO
21.22
UPS data
EF
Inte
nsity
(a.u
.)esdNML
-‐ -‐ -‐ -‐ -‐ -‐ -‐ -‐ -‐
+ + + + + + +
Valence Ba
nd structure
0
Tuning work func-on of Au and Pt with organophosphonates
• MDPA and MPPA MNLs on Au and Pt surfaces • Effec-ve work func-on (φeff) ê as molecular length ê
– Contrary to MD calcula-on of monolayers on Au and Pt – We had mul-layered MNLs on Au and Pt – Δφeff for mul-layer/bilayer > monolayer
-5 0 5 10 15 20-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
PA-(CH2)
n-S/Au
PA-(CH2)
n-S/Pt
HS-(CH2)
3-PA:PA-(CH
2)
3-S/Au
6\
eff (eV
)
number of CH2 groups, n From 3) G. Ramanath, M. Kwan, et al., Appl. Phys. Le=.,
105, 081601 (2014).
Variable angle XPS indicates that MNLs are mul-layers on Au and Pt surface
• S 2p binding energy indicates two forms of S – -‐S-‐Au and –S-‐H
• as θsd ê -‐S-‐Au intensity ê while –S-‐H intensity é – a Au bound monolayer lies below the physisorbed mul-layer
Inte
nsity
(a.u
.)hi
esd
detector
NML
From 3) G. Ramanath, M. Kwan, et al., Appl. Phys. Le=., 105, 081601 (2014).
Mul-layer bonding between organophosphonates affect φeff
• The untethered moiety’s (-‐PO3) binding energy é as Δφeff ê – as hydrogen bonding é between -‐PO3 the binding energy é – The MNL’s first layer has ordered hydrogen bonding and makes Δφeff ê – intra-‐MNL hydrogen bonding |Δφeff| > MNL-‐metal covelent bonding |Δφeff|
From 3) G. Ramanath, M. Kwan, et al., Appl. Phys. Le=., 105, 081601 (2014).
Tuning work func-on of halfnia-‐gold interface with organophosphonates
• The Evac and φeff is measured on Au-‐HfO2 interface, HfO2 and Au surface – each substrate allows the MNL to be measure with different bonding and orienta-ons
• Can separate MNL’s contribu-ons to Evac and φeff into: – MNL-‐HfO2 bonding – MNL morphology and polarity – Au-‐MNL bonding
20 15 10 5 0
Inte
nsity
(a.u
.)
MDPA AuPDPA Au
CDPA AuDDPA Au
Au20 15 10 5 0
Inte
nsity
(a.u
.)
HfO2
DDPA HfO2
CDPA HfO2
PDPA HfO2
MDPA HfO2
20 15 10 5 0
Au on MDPA HfO2Au on PDPA HfO2
Au on CDPA HfO2
Au on DDPA HfO2
Inte
nsity
(a.u
.)
Au on HfO2
e-
hi=21.2 eV
-+-+
-+
HfO2
AuVarious dipole contributions to 6q
-+
-+
-
+-+
Binding Energy (eV)
Variable angle XPS shows all MNLs are monolayers tethered to HfO2 through PO3
• MDPA had the most complex orienta-on – P and SOx peak intensity decrease at lower angles – SOx a=enua-on is greater than P
• C and HS are at the surface of the MNL • Both P and SOx are near the interface, but HfO2 favors bonding to SOx over P
295 290 285 28025 20 15
Inte
nsity
(a.u
.)
540 535 530 525140 135 130 125
Binding Energy (eV)175 170 165 160 155
Hf 4f C 1sS 2p P 2p O 1s75o
30o
HfO2
-PO3
C-C
C-PO3
-SOx
-SH
MDPA CDPA
PDPADDPA
1.0 1.2 1.4 1.6 1.8 2.0
90.0o 56.5o 45.6o 38.7o 33.8o 30.0o
sin(e<�
Nor
mal
ized
Inte
nsity
(a.u
.)
S 2p (SOx)
S 2p (HS)
P 2p
C 1s
e
hi
esd
detector
NML
Organophosphates at Au-‐HfO2 interfaces have a broader range of ΔEvac than at HfO2 surfaces
-+-+
-+
-+
-+
• ΔEvac is constant on HfO2 • As binding energy é, bond strength é. As |ΔEvac| é, surface polarity/dipole é
– Bond strength and morphology between different MNLs varies, but surface polarity remains the same
• At the Au-‐HfO2 interface only MDPA (-‐SH) wilΔl covalently bond with Au – Covalent bond ê ΔEvac, while weaker interac-ons é ΔEvac – Aside from MDPA as P-‐O-‐Hf bond strength é, ΔEvacê
17
-0.5
0.0
0.5
HfO
2 6E va
c(eV
)
530 530.5
Au-MNL-HfO2
MNL-HfO2
133 133.5
DDPA
PDPAMDPACDPA
Hf 4f 7/2 (eV) P 2p (eV) HfO2, O 1s (eV)
6E va
c (eV
)
Thickness (nm)
17
-0.5
0.0
0.5
Au-
HfO
2 6
E vac(e
V)
530 530.5133 133.5Hf 4f 7/2 (eV) P 2p (eV) HfO2 O 1s (eV)
(b)
16.5
16.5
(a)
Au 6
E vac (
eV)
MDPA DDPA PDPA CDPA-1.0
-0.5
0.0
0.5
MNL/Au
Au/MNL-
Moiety polarity
Au-MNLcovalent bond
0.5 1.0 1.5
-0.5
0.0
0.5
DDPACDPAMDPAPDPA
ΔEvac of MNL on Au matches the extrapolated ΔEvac of Au on MNL
• (Au/MNL/HfO2 data) – (MNL/HfO2 data) gives the Au-‐MNL bond contribu-on to ΔEvac
• Except MDPA, MNLs on Au formed disordered physisorbed mul-layers – ΔEvac only depends on van der Waals interac-ons between MNL and Au surface – van der Waals interac-ons é when the non-‐phosphonic acid moiety’s polarity ê
• MDPA on Au forms a chemisorbed monolayer – ΔEvac depends on the covalent bond and the dipole of the molecule
+-+
-
+-+
17
-0.5
0.0
0.5
H
fO2 6
E vac(e
V)
530 530.5
Au-MNL-HfO2
MNL-HfO2
133 133.5
DDPA
PDPAMDPACDPA
Hf 4f 7/2 (eV) P 2p (eV) HfO2, O 1s (eV)
6E va
c (eV
)
Thickness (nm)
17
-0.5
0.0
0.5
Au-
HfO
2 6
E vac(e
V)
530 530.5133 133.5Hf 4f 7/2 (eV) P 2p (eV) HfO2 O 1s (eV)
(b)
16.5
16.5
(a)
Au 6
E vac (
eV)
MDPA DDPA PDPA CDPA-1.0
-0.5
0.0
0.5
MNL/Au
Au/MNL-
Moiety polarity
Au-MNLcovelent bond
0.5 1.0 1.5
-0.5
0.0
0.5 -
+-+
Acknowledgements
This work is support by the NSF ECCS 1002282/301 grant Thanks to the MNCR staff, and Rob Planty
References
1 P.K. Chow, Y. Cardona Quintero, P. O’Brien, P.H. Mu-n, M. Lane, R. Ramprasad, and G. Ramanath, Appl. Phys. Le=. 102, 201605 (2013). 2 P.J. O’Brien, S. Shenogin, J. Liu, P.K. Chow, D. Laurencin, P.H. Mu-n, M. Yamaguchi, P. Keblinski, and G. Ramanath, Nature Mater. 12, 118 (2012). 3 G. Ramanath, M. Kwan, P. K. Chow, Y. Cardona Quintero, P. H. Mu-n, and R. Ramprasad, Appl. Phys. Le=., 105, 081601 (2014).
