Reference Textbook:Solid State Physics, Gerald BurnsSolid State Physics, Ashcroft and MerminPhysics at Surfaces, Andrew ZangwillModern techniques of surface science, D. P. Woodruff and T. A. DelcharIntroduction to Synchrotron Radiation, G. Margaritondo

What makes surface different?

3D vs 2DSurface is what we usually “see”.Technology:

SemiconductorsCatalysis

Surface Structures

Surface Reconstruction and Relaxation

2D crystalline structureAtoms on surfacetruncation

Surface CrystalographyNotationsLEEDRHEED

LEED

RHEED

Surface electronic structuresWork functionThermionic EmissionSurface StateTangential Surface Transport

Work FunctionDifference in potential energy of an electron between the vacuum level and the Fermi level.Work functions depends on the orientation of the exposed crystal face.Photoemission

Thermionic EmissionRate of thermionic emission depends exponentially on the work functionRchardson-Dushman Equation

Surface States

Surface States

Tangential Surface Transport

Surface preparationUltrahigh VacuumSurface cleaningCleaving crystals to expose fresh surfaceIn vacuum deposition

Surface CharacterizationPhotonElectronSPMIon

p-n JUNCTIONSRectificationSolar Cells and Photovoltaic DetectorSchottky Barrier

p-n JunctionsDonor and acceptor

Rectification

Schottky BarrierFermi Level alignmentBand Bending

Solar Cells and Photovoltaic Detectors

Photovoltaic effectThe basic requirements

the absorption of photons through the creation of electron-hole pairs; the separation of the electron and hole so that their recombination is inhibited and the electric field within the semiconductor is alteredthe collection of the electrons and holes, separately, by each of two current-collecting electrodes so that current can be induced to flow in a circuit external to the semiconductor itself.

HETEROSTRUCTURESn-N Heterojunction

SEMICONDUCTOR LASERS

LIGHT-EMITTING DIODES

Optical Absorption ProcessUV, Soft-x-rayX-ray

Fermi’s Golden Rule

∑ +•−=i

Ri

iI A

cmeH )]()([ 2

iiiR rprA

( )if EE +−=Γ ωδπ 2int2,

2 iHffih

Optical Absorption Process in Atoms--Transitions for the H atom

1. to an unbound state in the continuum above the ionization edge. Creates H+ produces photon absorption in a spectral continuum, at photon energies above the threshold.

2. brings the electron to a bound excited state. produces a discrete line in the Rydbergseries.

a(H)

1 2

a(H)

1 2

Other Atomic processesThe bound states of the Rydberg series which accompanies a given continuum can overlap the unbound states of another continuum. This is the case of the final state of process 4.

• 1, 5 and 3 are ionization processes, which produce two different kinds of ion states. 3 produce a different continuum than 1 and 5.

• 2 and 4 are excitations to two of the discrete bound states corresponding to these continua. They produce lines in the Rydberg series.

• Note that, after process 4 has taken place, the system can further evolve from the bound excited state to an ionization state without changing its energy. The combination of process and the transition to the ionized state is an example of an autoionization process.

1 2 5 4

b

*bA

+aA

+bA

1 2 5 4

b

*bA

+aA

+bA

Continuum

A B

Core

Valence

RydbergShape Resonance

Electronic States of Molecules

Bound Molecular OrbitalsCoreValenceRydberg

Unbound Molecular OrbitalsShape resonanceContinuum orbitals

Optical transition in Metal and Insulator

M I

CB

VB

CL

c ccc

a b

Optical Absorption SpectraAtoms and Molecules

Discrete states: Rydberg StatesNo EXAFS oscillation in XAS

SolidsBand StructureEXAFS oscillation

Core-Leve Absorption from Atom

10 200

AB

SO

RP

TIO

N C

OE

FFIC

IEN

T

Ar 1s

h (keV)ν

h (keV)ν2 4 6 8

Example of a Example of a corecore--level optical level optical absorption threshold, absorption threshold, corresponding to the corresponding to the excitation of the is excitation of the is level in level in ArAr. The inset . The inset shows the fine shows the fine structure near this structure near this threshold due to its threshold due to its RydbergRydberg series.series.

The Delay OnsetA

BS

OR

PTI

ON

CO

EFF

ICIE

NT

Xe4d

h (eV)ν70 100 130

The delayed onset for the excitation of Xeelectrons, due to the “centrifugal barrier”which tends to exclude the electrons from the core region.

αC

-1

6

3

0-8 -4 0 4 8ε

q=

-2.5-1

0ABSO

RPTI

ON C

OEFFIC

IEN

T

N25dσ

6sσ

h (eV)ν18.318.1 18.2

Abs

orpt

ion

Coe

ffic

ient Ta L edge3

h (eV)ν-20 0 20 40

Opt

ical

Abs

orpt

ion

RbClRb 4p

A

B

C

D E F

h (eV)ν15 17 19

Photoemission SpectroscopyGeneral description“Electronic” Structure and PropertiesGas phase—Atomic and Molecular processSolid state

3 step descriptionRetrieving “initial” state information from final “free”electron state

Available information in the final free electron state

Plan wave final state:

Kinetic Energy, Ek (Ek=p2/2m)Momentum, p=hk0

Spin

riVf eA ⋅= 0k

0ψ

Electron Energy Analyzer

OUTICOC

MP'P

S

Cylindrical Mirror Analyzer

Hemispherical Analyzer

PhotoelectronEnergySpectrum

FERMI LEVEL

CORELEVEL

VACUUMLEVEL

PHOTOELECTRONENERGYSPECTRUM

E

hν

ADSORBATELEVEL

metal vacuum

hν

Ek

e

φW

VL

EF

Photoemission Process

a

b

hνe

eθ

Direction ofPhoton Polarization

x φθ

Atom

Solid

Angular dependence in PES

NeonPhotoionizationCross section

0.2 1 1.8h (keV)ν

2p

2s

1sMb

10-3

10

TOTAL

Total Photoionization Cross Section

Xenon 5sPhotoionizationCross section

30 40 60h (eV)ν

Mb

0

0.8

50

Cooper Minimum

1 2

2 EdEk

Ei

hνEN

ER

GY

0

Photoemission Resonance

Ground State

N O2N O A2 Σ++ 2

N O X2 Π++ 2

200 100 000

13 .6 1 3.9 14 .2h (ke V)ν

β2

13.6 13.9 14.2h (keV)ν

β

N O X (100) Resonance2 Π++ 2

33 39 45E (eV)k

61

ArgonEDC'sh =77.2eVν 3s

3p

-32 -28 -26ENERGY(eV)

hν=

BiIBi5d EDC's

3

40.5eV

49eV

PES in SolidExtreme Surface SensitivityEDC (Energy Distribution Curve) vs. DOS (Density of States)Bulk SolidsSurface & InterfacesPhotoelectron diffraction (PhD) and photoelectron holography

10 102

E (eV)k

103

10

20

30

λ m(A

)

-101 -99ENERGY(eV)

-97

Si(111) + 4 A Au

Si2p EDC's

X 0.75

110

120

130a

b

c

c-b

X3

hνEN

ER

GY

-8 -4 0=EF

ENERGY(eV)

24

hν=

GaSeEDC's

20eV

30

-12 -4 0=EF

ENERGY(eV)-8

bZnSeEDC

aPt EDC

-6 -2 0=EF-4

-15 -10 0=EF

ENERGY(eV)

DOS

SnSe2

EDC

-5

-4 0=EF

ENERGY(eV)-8

a Si(111)2x1 EDC

CleavedSi+ClEDC

-8 0=EF-4ENERGY(eV)

-6 -4 0ENERGY(eV)

ZnSe+Ge EDC's

-2

8A4A

∆EV

0A

-8 -4 0=EFENERGY(eV)

Pd Si EDC's2

hν=80

130eV

Band Structure MappingARPES (ARUPS) is the ONLY method to map the band structure in SolidThe tunability of SR UV reduced the ambiguity of ARUPS in determining the band structureHigh energy and angular (momentum space) resolution has enabled the determination of quasi-particle electronic structure of highly correlated electronic system

hν

PO

TEN

TIA

LE

NE

RG

Y

Wi

K

KK

e K┴

K║ K

Kº┴

Kº║

Kº

DirectInterbandTransition

ELE

CTR

ON

EN

ER

GY

Ei

Ef

E

Ev

k

GaAs(110) NORMAL EMISSION SPECTRA

(eV)

25.0

27.530.032.535.037.540

42.545.0

47.550.055.0

60.065.070.075.080.0

859095100

A'

A

INITIAL STATE ENERGY Ei (eV)

EF-5-10-15

Band dispersionGaAs(110)Normal emission

Γ M Γ

GaSe

EN

ER

GY

(eV

)0

-2

-4

-6

Γ M

-4 -2 0=EF

ENERGY(eV)

GaAs(110)+Al

h =120eVν

θ20

1

0.05

x3

x50

Al 2p EDC's

21

3 4

a

b

θ

-41 -40 -19.5 -18.5ENERGY(eV)

AsGa

(110)

GaAs EDC'sGa3dAs 3d

s

Taking Advantage of The Tunability

Constant Initial State PESConstant Final State PESResonance PES

DOSE

NE

RG

Y

CFS CIS

VL

EF hν2hν1 hν 2' hν 1'

E1i

E2i

E1k

E2i

-6 60=EVENERGY(eV)

SnS2

12

EDC CIS Curve

Spin-Polarized PESEnergy bands in ferromagnetic metals are split by the exchange interaction into the

up-spin (majority spin) andDown-spin (minority spin) band

First experimentally observed by PES in NiSpin polarized detectorsTotal PES

Spin-Resolved Electron Detectors

e

A

LD

RD

BM

MottDetector

Magnetic bands

0 1 2 kΓ ∆ H

-4 -2 0=EF

ENERGY(eV)

T/T =0.3c

Fe(001)Spin-polarizedEDC'sSpin-polarized

photoelectron EDC'scorresponding to the spin-up and spin-down polarizations

T is the Curie of Iron

Recent developmentsComplete photoemission experiment: Very high energy (< 5meV) and angular (<0.5°) resolutionStrongly correlated electronic systems

High Tc SupercondutorsHeavy Fermion systemQuantum Well State

PhD and Photoelectron Holography

Fermi edges

NiobiumFermi edge

2080

150235

DOS (T=0)Fermi-Dirac

∆E=6meV

-100 -50 0 50 100Energy (meV)

20 -50 0 50Energy (meV)

HTC Gap Spectroscopy

Pseudo-GapSpecial to HTSC. It persists at temperatures up to three times as high as the limit of superconductivity, in some cases even at room temperature. The properties of the pseudo-gap were determined using the highly energy-resolved SR-ARUPS. It was found that the pseudo-gap and the superconducting gap have the same angular symmetry, suggesting they are related. The temperature dependence of the pseudo-gap was determined as well.

Temperature regions where the gap and the pseudo-gap exist. The maximum temperature where a pseudo-gap is still observed (T*) can be much higher than the maximum temperature for superconductivity (Tc) and as high as room temperature (about 290 K).

The results are reported by H. Ding The results are reported by H. Ding and coworkers in Nature and coworkers in Nature 382382, 51 , 51 (1996) and in Physical Review (1996) and in Physical Review Letters Letters 7878, 2628 (1997). Highlights , 2628 (1997). Highlights can be found in Physics Today, June can be found in Physics Today, June 1996, p. 17. 1996, p. 17.

Symmetry of the pseudo-gap when the sample is rotated. The characteristic minimum at 45 degrees is observed for the superconducting gap and the pseudo-gap, suggesting they are related.

Quantum Well State

Quantum WellStates

Further readings:Review of magnetic nanostructures: Himpsel et al., Advances in Physics 47, 511 (1998). Basic principles of magnetic recording: Grochowski and Thompson, IEEE Trans. Magn. 30, 3797 (1994) Measurement of magnetic quantum well states: Ortega et al., Phys. Rev. Lett. 69, 844 (1992) and Phys. Rev. B 47, 1540 (1993). Himpsel, J. Phys. Cond. Matter 11, 9483 (1999) and Science 283, 1655 (1999).

Diffraction and Holography

Ni

C

O

Ni Ni Ni

C

O

NiNi

40 100 130ENERGY(eV)

Ni(001)+S PhD Curve

70

THEORY

EXPERIMENT

Photoelectron HolographyProduces 3-D pictures by superimposing two beams, one coming directly from a laser,the other reflected off the three-dimensional object. Using electrons instead of light one can produce three-dimensional pictures of very tiny objects, because the wavelength of electrons is much shorter that that of light.

A 3-D picture of atoms at a silicon surface that was obtained using electrons emitted from the surface under irradiation with SR.The analog to the direct laser beam are electrons emitted directly from the red atom, the analog of the reflected light are electrons scattered by neighbor atoms (green).

H. Wu and G.J. Lapeyre, Phys. Rev. B 51, 14549 (1995)

References:Books on photoelectron spectroscopy

Cardona M. and Ley L., editors, Photoemission in Solids I & II, Springer Verlag, Berlin 1978Kevan S.D, editor, Angle-resolved Photoemission -Theory and current application, Elsevier, Amsterdam 1992