Y. RosenwaksDepartment of Physical Electronics, Faculty of

Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel

yossir@eng.tau.ac.ilhttp://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