reference textbook - sinica
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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