Y. RosenwaksDepartment of Physical Electronics, Faculty of
Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel
[email protected]://www.eng.tau.ac.il/~yossir/rosenwaks/html
T e l A v iv U n iv e rs ity
Nanoscale Measurement of Surface States (and something different…)
C-V: Measure the high and low frequency capacitance of
a MIS structure as a function of metal biasSurface photovoltage spectroscopy (SPS)
Derivative sub-bandgap SPS with a tunable laser as
the illumination sourceUPS and angle-resolved UPS
Sensitive to occupies states, poor energy resolution and
sensitivity
Measuring Surface Band Bending Dependence on the Fermi-
level Position
How to Measure Surface Band Bending and surface states energy distribution
Some of Yoram’s works on surface states (from 1993)I. I. Shalish, Y. Shapira, L. Burstein, J. Salzman
"Surface states and surface oxide in GaN layers"J. Appl. Phys., 89, 390, 2001.
II. L. Kronik, Y. Shapira "Surface photovoltage phenomena: Theory, experiment and applications"Surface Science Reports, 37, 1-206, 1999.
III. O. B. Aphek, L. Kronik, M. Leibovitch, Y. Shapira "Quantitative assessment of the photosaturation technique" Surface Science, 409, 485, 1998.
IV. E. Fefer, Y. Shapira, I. Balberg"Direct determination of the bandgap states in a-Si:H using surface photovoltage spectroscopy"Appl. Phys. Lett., 67, 371, 1995.
V. L. Kronik, L. Burstein, Y. Shapira, M. Oron"Laser surface photovoltage spectroscopy: A new tool for determination of surface state distributions"Appl. Phys. Lett., 63, 60, 1993.
Some other works
• Electronic states and effective negative electron affinity at the cesiated p-GaNsurface, C.I. Wu and A. Kahn, J. Appl. Phys. 86, 3209 (1999)
• C.M. Aldao and J.H. Weaver "Atomic-Scale Chemistry of Metal-Semiconductor Interfaces," Chapter 7 in Contacts to Semiconductor Surfaces, edited by L.J. Brillson (Noyes Publication, New Jersey, 1993) pp. 465-555.
The Photovoltage Saturation (L.J. Brillson 1981)
How to Measure Absolute Semiconductors Surface Band Bending
pn diode p++n diode
p n
Measuring the Surface Band Bending
n p++n p
Contact Potential Difference: Definition and Measurement
εF
EVAC
ΦPΦS
VCPD
εF
EVAC
+ -VCPD
Semiconductor
Neutral
Neutral
+
-VCPD no force
ΦSEF
EVAC
EF
EVAC
ΦP
VCPD
Semiconductor
Negative
Positive
force
Semiconductor
Neutral
Neutral
EF
EVAC
EF
EVAC
ΦPΦS
VCPD
Electron Current to equalizethe Fermi levels
LT-UHV KPFM
AFM
KPFMelectronics
Samplecleaver
Glove-box KPFM
0.0 0.5 1.0 1.5 2.0 2.5 3.0
1E13
1E14
1E15
1E16
1E17
1E18
1E19
1E20 I10 (p++n) I12 (n++p)
|N
D-N
a| Cm
-3
Position (µm)
Dopant Distributions of p++n & n++p Si Junctions
• The doping was measured by SIMS (secondary ion mass spectroscopy)
Simulation
S.C. : ( )DVnVpqVs
+−⋅−=∇ )()(2
ε
Interface :
⋅
⋅−⋅+−+
⋅−==⋅−⋅
TkVqVqEE
NqQEESjuncbft
tSSairCSs
0
*0.. )(
exp1εε
Air : 02 =∇ V
p n
Surface
Air
x
z
* For an acceptor state
Air
p nSurface
x
z
Real Structure 2D Simulation
⇒
Where VS0 is the surface band bending, Vjunc is the junction built in
voltage in the bulk and Nt is the number of states per unit area.
0 1 2 3
-0.6
-0.4
-0.2
0.0
-6.0x10-8
-4.0x10-8
-2.0x10-8
0.0
2.0x10-8
Position (µm)
CPD
(V)
n++
p
Sur
face
Cha
rge
(C*c
m-2
)
Surface Charge of a Cleaved n++p Si Diode n++ p
0 1 2 3 4
-0.6
-0.4
-0.2
0.0
0.2
0.4
-0.4
-0.2
0.0
0.2
0.4Bulk Potential
-Pot
entia
l (V)
Position (µm)
p
n++
Surface Potential
Sur
face
Ban
d B
endi
ng (V
)
Surface Band Bending of a Cleaved n++p Si Diode
n++p (I12) Band Structure
p++n Diode Band Structure
Measuring the Surface States Energy Distribution or Density of States Function
The idea-’Scanning’ the Fermi Level Across the Bandgap
Principle
1. Changing the Temperature - Equilibrium
3. Scanning a pn Junction - Equilibrium2. Illumination - Nonequilibrium
Junction
0 1 2 3 40.0
0.5
1.0
1.5
Ef
EV
Ene
rgy
(eV
)
Position (µm)
EC
p n
Obtaining the Surface States Energy Distribution - Principle
[ ] ;)()()(1)(
⋅⋅−−⋅⋅⋅= ∫∫∞
∞−
∞
∞−ttt
ASS
Atttt
DSS
DtSS dEEfENNdEEfENNqQ
+−−+
=
kTqVqVEE
EfjuncSft
t
exp1
1)(
2
exp1
exp
+−−+
+−−
=
+−−
kTqVqVEE
kTqVqVEE
kTqVqVEE
SjuncSft
juncSft
juncSft
dxdQ
dxdQ SCSS −=
−⋅
⋅⋅
−=⋅+⋅
dxdV
dxdV
VTkq
dxdQ
ENNENNSjunc
t
SC
ASS
At
DSS
Dt )()(
Obtaining the Surface States Energy Distribution – Formulation
+−−⋅+
+−−⋅⋅
−⋅= ∫∫
∞
∞−
∞
∞−t
juncSftt
ASS
Att
juncSftt
DSS
Dt
Sjunc
t
SS dEkT
qVqVEESENNdE
kTqVqVEE
SENNdx
dVdx
dVVq
dxdQ )()(
0 1 2 30.1
0.2
0.3
0.4
0.5
n
CPD
(V)
Position (µm)
p++
Obtaining the Energy Distribution of Oxidized Si (110)
p++ n
EV 0.2 0.4 E0 = 0.61x109
1x1010
1x1011
1x1012
Donor
S.
S. D
ensi
ty (e
V-1
cm-2
)
Energy (eV)
Acceptor
EfNt = 7.7x1011cm-2
Surface States Energy Distribution of a polished Si (110)
-1 0 1 2 30.1
0.2
0.3
0.4
0.5
0.0 0.1 0.2 0.3 0.4 0.5
0
1x1012
2x1012
3x1012
0.00 0.15 0.30 0.45 0.60 0.75
0
1x1012
2x1012
3x1012
C
PD (V
)
Position (µm)
Measured CPD
Initial Calc.
Final Calc.
Energy (eV)
S. S
. Den
sity
(eV
-1*c
m-2)
Energy (eV)
S. S
. Den
sity
(eV
-1*c
m-2)
Si-p++n (Polished)
0 1 2 3
0.2
0.3
0.4
0.5
- Sur
face
Pot
entia
l (V
)
Position (µm)
Sensitivity
Nt = 7.7x1011cm-2, E0 = 0.6eV above EV
Nt = 7.7x1011cm-2, E0 = 0.5eV above EV
Nt = 7x1011cm-2, E0 = 0.6eV above EV
EV 0.3 0.6 0.9 EC
1x1010
1x1011
1x1012
PH
S. S
. Den
sity
(cm
-2 e
V-1
)
Energy (eV)
UM
PL
Model of the Si/SiO2 Interface States*
*H. Flietner, Surface Science, 200, 463 (1988)
PL & PH – Donor type states
Upper (lower) part of UM is acceptor (donor) type states
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.0
5.0x1011
1.0x1012
1.5x1012
2.0x1012
2.5x1012
3.0x1012
3.5x10
"Correct" Distribution E0=0.5eV E0=0.7eV
S. S
. Den
sity
(eV
*cm
)
Energy (eV)
Ef
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.1
0.2
0.3
0.4
0.5
- Sur
face
Pot
entia
l (V)
Position (µm)
Nt = 7.7e11, E0 = 0.6eV Nt = 7.7e11, E0 = 0.5eV Nt = 7.7e11, E0 = 0.7eV
0.0 0.2 0.4 0.6 0.8
0.0
5.0x1011
1.0x1012
1.5x1012
2.0x1012
2.5x1012
3.0x1012
3.5x1012
S. S
. Den
sity
(eV
-1*c
m-2)
Energy (eV)
"Correct" Distribution Nt = 7e11 Nt = 9e11
Ef
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.1
0.2
0.3
0.4
0.5
Nt = 9e11, E0 = 0.6eV
- Sur
face
Pot
entia
l (V)
P iti ( )
Nt = 7e11, E0 = 0.6eV
Nt = 7.7e11, E0 = 0.6eV
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.00E+000
2.00E+012
4.00E+012
6.00E+012
8.00E+012
1.00E+013
Calculate Assumed
Donors
NSS
(eV-1
*cm
-2)
Energy (eV)
EHO
Acceptors
Entire Energy Distribution of I12 (n++p)
0 1 2 3 4
-0.2
0.0
0.2
0.4
0.6
0.8
1E16 1E17 1E18 1E19 1E20 1E21
0
50
100
150
200
With BGN Without BGN- S
urfa
ce P
oten
tial (
V)
Position (µm)
BG
N (m
V)
Doping (cm-3)
S.C. Jain & D. J. Roulston , Solid State Electronics, 34, (1991), 453-465.
Here the donors are in the lower part of the distribution and the acceptorsare on the upper part.
0 1 2 3 4
-0.2
0.0
0.2
0.4
0.6
- S
urfa
ce P
oten
tial (
V)
Position (µm)
"Correct Distribution" "Inverted Distribution"
Preliminary Results-I12 Surface States Energy Distribution
0.55 0.60 0.65 0.70 0.750.00E+000
2.00E+012
4.00E+012
6.00E+012
8.00E+012
1.00E+013
N
ss(e
V-1*c
m-2)
Energy (eV)
AcknowledgementsCoworkers:S. Saraf, M. Molotskii, A. Schwarzman,Y. Devash.
Collaboration:P. Eyben, C. Trudo, and W. Vandervost, IMEC, Belgium.
Funding:Herculas (5th European program)Israel Science Foundation
Now, something different….
Yoram Between Meals....
What is the bike for ...
Riding to the Wolfson materials center
Yoram after a paper has been accepted
And after a proposal has been accepted......
Yoram (and another PI) after a bi-nationalproposal has been accepted
And after receiving a grant from the Turkish government
Yoram is a very optimistic guy
And a very organized one !!!!
And see you all in the 70th birthday