Hard X-ray Photoelectron
Spectroscopy Research at
NIST beamline X24A
Joseph C. Woicik
NIST
Outline
• Introduction to hard x-ray photoelectron
spectroscopy (HAXPES)
• Semiconductor gate stacks on silicon
• Epitaxial, room temperature feroelectric
SrTiO3 on silicon
• Site Specific XPS and the nature of the solid
state chemical bond: Cu, GaAs, TiO2
• N doping of TiO2
Hard X-ray Photoelectron Spectroscopy (HAXPES)
Synchrotron Tool
Variable Kinetic Energy HAXPES
• Tune the HAXPES depth sensitivity
(photoelectron kinetic energy) by tuning the Xray excitation energy using a synchrotron X-ray
beamline
• The U7A and X24A endstations form a unique
measurement suite for surface to near bulk
HAXPES by spanning X-ray excitation energy
from 0.2 to 5 keV
Example: Si electron binding energy 2p ~99 eV
and 1s ~1840 eV, surface to bulk sensitivity
(Lab source has fixed energy Al K
Photoelectron Kinetic Energy = h – Binding Energy
Advantages of HAXPES
Advantages of HAXPES
• Study samples from air
Advantages of HAXPES
• Study samples from air; i.e., real samples!
Advantages of HAXPES
• Study samples from air; i.e., real samples!
• Tune λ for experimental system
Advantages of HAXPES
• Study samples from air; i.e., real samples!
• Tune λ for experimental system
• Variable kinetic energy XPS (VKE-XPS)
for depth profiling
Depth Sensitive VKE-XPS
N o r m a l i z e d I n t e n s i t y ( a r b . u n it s )
Si 1s
h
= 2100 eV
h
= 2500 eV
h
= 2600 eV
HfO2
SiO 2
h
Si
= 3000 eV
5 n m
more
oxidized
less
oxidized
"lab-like"
spectrum
Si 2p
-8
-6
-4
-2
Kinetic Energy (E-E
h
= 3500 eV
h
= 2100 eV
0
2
4
c)
P.S. Lysaght, J. Barnett, G.I. Bersuker, J.C. Woicik, D.A. Fischer, B. Foran, H.-H. Tseng, and R. Jammy
Advantages of HAXPES
• Study samples from air; i.e., real samples!
• Tune λ for experimental system
• Variable kinetic energy XPS (VKE-XPS)
for depth profiling
• Bulk and surface sensitive core lines
accessible for same element
Surface and Bulk Sensitive Core Lines
Si 2p
Si 1s
B.E. = 99 eV
B.E. = 1840 eV
h
h
2068
= 2180 eV
2072
2076
Kinetic Energy (eV)
2080
324
328
= 2180 eV
332
Kinetic Energy (eV)
336
338
Advantages of HAXPES
• Study samples from air; i.e., real samples!
• Tune λ for experimental system
• Variable kinetic energy XPS (VKE-XPS)
for depth profiling
• Bulk and surface sensitive core lines
accessible for same element
• Eliminate Auger interference
Auger Interference
10000
In te n s ity ( c o u n ts /s e c o n d )
Si KLL Auger
8000
6000
4000
O 1s
2000
0
1500
1550
1600
Kinetic Energy (eV)
1650
1700
Advantages of HAXPES
• Study samples from air; i.e., real samples!
• Tune λ for experimental system
• Variable kinetic energy XPS (VKE-XPS)
for depth profiling
• Bulk and surface sensitive core lines
accessible for same element
• Eliminate Auger interference
• Photoemission and other techniques
(SSXPS)
What is a “gate stack?”
Metal
SiO2
Si
What is a “gate stack?”
Metal
Metal
SiO2
Si
SiO2
Si
~ 2 nm
What is a “gate stack?”
Metal
C = εrεoA/d
~ 2 nm
HfO2
K ~ 20
Metal
~ 1 nm
SiO2
K~4
SiO2
Si
Si
~ 2 nm
SEMATECH: HfO2/SiO2/Si Gate Stacks
Intensity (a.u.)
hv = 2200 eV
NH3 postdeposition
anneal
Hf 4f
HfO2
HfO2
SiO 2
Hf-N
Si
5 n m
NH3 predeposition
anneal
HfO2
N 1s
Hf-N
hv = 2200 eV
Intensity (a.u.)
B
NH3 pre- & postdeposition anneal
HfO2
Hf-N
Intensity (a.u.)
Intensity (a.u.)
A
NH3 postdeposition
anneal
NH3 predeposition
anneal
A
Si-N
B
NH3 pre- &
post-deposition
anneal
C
C
1799
2181
2183
2185
Kinetic Energy (eV)
2187
1781
1783
1805
1807
1809
Kinetic Energy (eV)
P.S. Lysaght, J. Barnett, G.I. Bersuker, J.C. Woicik, D.A. Fischer, B. Foran, H.-H. Tseng, and R. Jammy
Motorola: 5 ML (20 Å) SrTiO3 films on Si(001)
In te n s ity ( a r b . u n its )
Si 2p
h
= 2210 eV
SiO
SrTiO
2104
2 /Si(001)
3 /Si(001)
2106
2108
Kinetic Energy (eV)
2110
2112
HfO2/SiGe/Si
2 nm
HfO2
30 nm
SiGe
Si
950˚C anneal
as deposited (300˚C)
SiGe
SiGe
Si 2p
Ge 3d
-8
-6
-4
-2
Kinetic Energy (E - E
0
2
c)
Si 1s
= 2140 eV
In te n s ity ( a r b . u n its )
= 2140 eV
In te n s ity ( a r b . u n its )
h
h
4
-12
Ge 2p
-8
-4
0
Kinetic Energy (E - E
4
c)
Hf 4f
h
= 2140 eV
In te n s ity ( a r b . u n its )
as deposited
950°C anneal
2116
2200
Kinetic Energy (eV)
2204
Si 2p
In te n s ity ( a r b . u n its )
h
= 2140 eV
950°C anneal
as deposited
SiGe
2030
2034
2038
Kinetic Energy (eV)
2042
Si 1s
h
= 2140 eV
In te n s ity ( a r b . u n its )
950°C anneal
as deposited
SiGe
288
292
296
Kinetic Energy (eV)
300
304
Ge 3d
= 2140 eV
In te n s ity ( a r b . u n its )
h
950°C anneal
as deposited
SiGe
2100
2104
2108
Kinetic Energy (eV)
2112
Ge 2p
= 2140 eV
In te n s ity ( a r b . u n its )
h
950°C anneal
as deposited
SiGe
914
918
922
Kinetic Energy (eV)
926
Advantages of HAXPES
• Study samples from air; i.e., real systems!
• Tune λ for experimental system
• Variable kinetic energy XPS (VKE-XPS)
for depth profiling
• Bulk and surface sensitive core lines
accessible for same element
• Eliminate Auger interference
• Photoemission and other techniques
(SSXPS)
Electronic Structure of GaAs
J. Chelikowsky, D.J. Chadi, and M.L. Cohen
X-ray Standing Waves
Bragg's Law:
H
H=k
h
-k
ko
o
kh
Electric Field:
E(r,t) = [
^
.
E o e ik o r +
^
.
E h e ik h r ] e
-i
t
Electric Field Intensity:
.
I(r) = E E* = |E
where E
o|
h
2
[ 1 + R + 2¦R cos(
/ E o = ¦R e
i
and 0 <
+ H .r ) ]
<
Bragg Diffraction:
= 2d sin(
d
Reflectivity
R
EB
XSW Yield
Y
EB
)
In te n s i ty ( a r b . u n i ts )
2
1
0
Ga
As
d 111
200
As 3d
GaAs
I n t e n s ity ( c n t s . / s e c o n d )
160
120
Ga 3d
80
40
Valence Band
0
-40
-30
-20
Kinetic Energy (eV) (E - E
-10
)
0
VBM
T h e o ry
V a le n c e B a n d
In te n s ity ( a r b itr a r y u n its )
E x p e r im e n t
Ga
As
-1 6
-1 2
-8
-4
K in e tic E n e r g y ( E - E
0
VBM
)
Electronic Structure of Cu
J. Friis, B. Jiang, K. Marthinsen, and R. Holmestad
I n t e n s it y ( a r b . u n it s )
Cu(11-1)
Cu (valence)
Cu 3p (core)
2970
2972
2974
Photon Energy (eV)
2976
2978
Cu(11-1)
I n t e n s it y ( a r b . u n its )
valence spectra
2954
2958
2962
Kinetic Energy (eV)
2966
2970
N o r m a liz e d I n t e n s it y ( a r b . u n it s )
What happened to the
“free-electron” theory of
metals?
Cu(11-1)
valence spectra
-8
-6
-4
-2
Kinetic Energy (eV) (E - E
0
2
f
)
Photoemission process Feynman diagram:
hole
What is left
behind.
photon
What goes in
the crystal.
photoelectron
What comes out
of the crystal.
Electron/hole propagators are
shown by solid lines. This process
is forbidden for free electrons
because of energy/momentum
conservation.
E.L. Shirley
Photoemission process Feynman diagram:
hole
What is left
behind.
photoelectron
What comes out
of the crystal.
Phonon
lattice recoil
photon
What goes in
the crystal.
Electron/hole propagators are
shown by solid lines. This process
is forbidden for free electrons
because of energy/momentum
conservation.
E.L. Shirley
Recoil Effects of Photoelectrons in a Solid
Y. Takata, Y. Kayanuma, M. Yabashi, K. Tamasaku, Y. Nishino, D. Miwa, Y. Harada, K. Horiba, S. Shin, S. Tanaka, E. Ikenaga, K. Kobayashi, Y. Senba, H. Ohashi, and T. Ishikawa
Electronic Structure of Rutile TiO2
I n t e n s ity ( a r b . u n it s )
DOS
theory
expt.
c-theory
-8
-6
-4
Energy (eV) (E - E
0
-2
)
VBM
2
O
c
Ti
b
a
I n t e n s ity ( a r b . u n it s )
TiO 2(110)
Ti 3p
O 2s
VB
-50
-40
-30
-20
Energy (eV) (E - E
-10
)
VBM
0
I n t e n s ity ( a r b . u n it s )
(a)
DOS
theory
expt.
(b)
pDOS
Ti
expt.
x4
O
expt.
-8
-6
-4
Energy (eV) (E - E
0
-2
)
VBM
2
Chemical Hybridization
I n t e n s ity ( a r b . u n it s )
(a)
DOS
theory
expt.
(b)
pDOS
Ti
π
σ
expt.
O
x4
O-π
expt.
-8
-6
-4
Energy (eV) (E - E
0
-2
)
VBM
2
(a)
pDOS
I n t e n s it y ( a r b . u n it s )
Ti
theory
expt.
(b)
pDOS
O
theory
expt.
-8
-6
-4
Energy (eV) (E - E
0
-2
)
VBM
2
lpDOS
x 1.0
I n t e n s ity ( a r b . u n it s )
O 2p
x 110
O 2s
Ti 3d
-8
x 1.8
Ti 4p
x 11
Ti 4s
x 15
-6
-4
Energy (eV) (E - E
0
-2
)
VBM
2
Theoretical Atomic Cross Sections
σTi
σTi
4s
≈ 30 !
3d
σO
σO
2s
≈ 10 !
2p
XPS favors states with smaller values of angular momentum!
I(E,ħω) α ∑i,l ρi,l(E)
σi,l(E,ħω)
(a)
pDOS
I n t e n s it y ( a r b . u n it s )
Ti
c-theory
expt.
(b)
pDOS
O
c-theory
expt.
-8
-6
-4
Energy (eV) (E - E
0
-2
)
VBM
2
Electronic Structure of Rutile TiO2
I n t e n s ity ( a r b . u n it s )
DOS
theory
expt.
c-theory
-8
-6
-4
Energy (eV) (E - E
0
-2
)
VBM
2
I n t e n s ity ( a r b . u n it s )
TiO 2(110)
Ti 3p
O 2s
VB
-50
-40
-30
-20
Energy (eV) (E - E
-10
)
VBM
0
N doped TiO2 (anatase)
Ti 2p
lpDOS
Electronic Structure of TiO2 (anatase)
lpDOS
Electronic Structure of N doped TiO2 (anatase)
Many Thanks……
•
•
•
•
Daniel Fischer (NIST) and Barry Karlin (NIST)
Pat Lysaght (SEMATECH)
Hao Li (Motorola Labs)
Erik Nelson (SSRL), David Heskett (URI), and
Lonny Berman (NSLS)
• Leeor Kronik (Weizmann) and Jim Chelikowsky
(Austin)
• Abdul Rumaiz (NSLS) and Eric Cockayne
(NIST)
30
In te n s ity ( c n ts ./s e c )
Ga
20
As
10
to ta l
0
-1 6
-1 2
-8
K in e tic E n e r g y ( E - E
-4
VBM
0
) eV
d
d
Ho =
ik o . r
|<f|e
i
2
. p)|i >|
(
p i 2 /2m + V(
r)
H ' = A o. p
E kin = h
[ p ,H] = -i
d
(h
b
ik f. r
|f>=e
d
-E
h
|<f|
>> E
(k f >> k
b)
o)
V( r )
.
V( r ) | i > |
2
V a le n c e P h o to e m is s io n
| f >
| i >
V( r )
d
d
| < f | e
ik o . r
(
^
.p) | i > |
2
Chemical Hybridization
+
+
-
=
λ2s
2p
2s
+
+
λ2s + 2pz
λ2s - 2pz
λ2s + 2pz
+
2pz
-
+
-
-
Hi,j = ∫ ψi H ψj dV
“s-p hybrid”
Stronger
Chemical
Bond!