synchrotron radiation core level photoemission...

Synchrotron Radiation Core Level Photoemission Spectroscopy

R. CiminoINFN-LNF, Frascati

• Generalities: photoemission; experimental apparatus• Core Levels : - position - width - shape - intensity• Examples: - Clean semiconductor's surfaces

- Interface formations: chemical reactions and electronic properties

• Conclusion and open problems

Roma, 14-10-2005 Roberto Cimino-LNF

Photoemission The experimental discovery of the photoelectric effect (Hertz 1887) prepared the ground for Einstein's formulation, in 1905, of the photoelectric effect.

Photoelectron spectroscopies are all based on the physical fact that, when illuminated by photons, matters may produce a photocurrent.

The analysis of photoelectrons require UHV and photoemission, as a spectroscopic technique, was developed only after the fifties.


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Photoemission from a solid with a DOS.



Experimental Apparatus :Hemispherical Analyzer

UHP

U+HS

U-HS

UL

SAMPLE

UHV

Preamplifier

UC

Channeltron

hν

collecting lens

counting stage.

dispersive electron-optic

ESCA-XPS

Hv= 14



Hv= 14

ESCA-XPS

ESCA-XPS


What do we measure?Our photoemission spectrometer measure the Kinetic Energy (EK) of the outgoing electrons.

This is related to the Core level binding energy (EB) by:

EB = hν EK - e Φanalyser

Where : hν is the photon energy� e Φanalyser is the analyser Work function



The Kinetic energy of what we measure is referenced to the analyser Work function.No matter what is the Work function of the sample, photoelectrons from its Fermi level will always appear, for a given hν at the same kinetic energy.

Changing the Work function of a surface, does not shifts the energy (Ek) observed by the spectrometer.

Binding Energy

The binding energy is defined as the total energy difference between the final core-hole state, with N-1 electrons, and the unperturbed initial N electron state:

EB = ET(N-1) - ET(N)


Binding Energy


Let us first assume that, when ejecting one electron from an N-electron system, the (N-1) remaining electrons are unaffected by the photo-event. In this approximation, (Koopmans' theorem) the CL binding energy is opposite in magnitude to the one-electron energy of the orbital.

EB = �lWithin this approximation, it is possible to

identify, by measuring EK, the atomic constituents of our material.

Binding energy shiftsRather than its absolute energy position, we analyse the differences in energy of the core levels of a given element in different environments, ∆EB.We can schematically assume that ∆ EB is formed by two separate contributions: - one keeps track of the differences in the

ground state ET(N); - the other considers the differences in the core-hole final state ET(N-1).

∆ EB = ∆ init + ∆ fin


Initial state effects∆init = ∆ conf + ∆ charge + ∆ Madelung

∆conf represents the change in core level BE due to a change in the valence electronic configuration of the considered atoms (i.e. going from free atoms to surfaces or solid state).∆ charge considers the effects associated with a modification in the valence charge density surrounding an atom when this atom is included in a chemical bond (i.e. semiconductor surfaces reconstruction).∆ Madelung counts for the effects of charge density variation occurring at atoms surrounding the emitter.


Initial state effects∆init = ∆ conf + ∆ charge + ∆ Madelung

In case of semiconductors, "band bending", can also be considered as an initial state effect.

Note that such initial state shifts (also called "chemical shifts") are roughly similar for all the core levels of the same atoms and do not depends on l.


Roma, 14-10-2005 Roberto Cimino-LNFHollingher and Himpsel, Phys. Rev. B (1993)

Chemical shift Si 2p and Oxygen

SiSi

Si

SiSi∆EB=0 eV

OSi

SiSi

Si∆EB=0.9 eV

OO

SiSi

Si∆EB~1.8 eV

Chemical shift Si 2p + Oxigen

OO

OSi

Si

∆EB=2.6 eV

OO

OSi

O∆EB=3.5 eV SiO2

Hollingher and Himpsel, Phys. Rev. B (1993)


Final State effects• This term is taken to be zero at the lowest

level of the fully independent electron approximation.

• More in general, we will have:

∆final = ∆ relaxation = ∆ intra + ∆ extra


∆final = ∆ relaxation = ∆ intra + ∆ extra∆ intra considers the effect of intra-atomic relaxation; it is always present; it is large (in absolute value) but, since it is an atomic phenomena, does not play a major role in binding energy shifts.

∆ extra takes into account the variation of the surroundings of the excited atom due to the rearrangements of the most external charges to screen the core-hole. Such term is fundamental to understand core level shifts in metallic systems, where conduction electrons strongly relax to screen a core-hole.

• In semiconductors with high dielectric costant (εSi=12) one assumes that the core-hole is fully screened ===> ∆ extra =0

• In metals this approximation lose its validity.

Final State effects


The width of a core level line

( σ = σAnalyser2 +σmono

2 )

• The width of a core level line in photoemission is the convolution of different contributions:

• Broadening caused by the limited resolution of the experimental apparatus.

• electron-phonon broadening• Core level finite life-time.• Structural disorder and/or Inhomogeneities


Broadening induced by electron-phonon

couplingFranck-Condon principle:shows how the crystal lattice oscillations at T>0 can affect the width of a core level line. Small bond length variations around its equilibrium value can, in fact, result in appreciable changes of the transition energy [final state - initial state] due to the different number of phonons excited in the final state.


Broadening induced by the finite core hole life-time.

Heisenberg uncertainty principle states that the smallest is the decay time of a core hole the biggest will be the uncertainity of its energy position (hence the biggest its line width). This broadening have a Lorentzian shape:

I(E) = I(E0).�L/[(E-E0)2 + �L2 ]

where: I(E) is the intensity at energy EE0 is the energy at the peak centre.�L is the broadening directly related to the decay time of the core hole. that is:

�L =1/2 FWHM = h/� = 6.58 . 10-16 /� eV, where: FWHM is the full width half maximum of the peak and �is the hole life time.




How does a core-hole decay ?

E f

K.E.

B.E.

hv

a)

E f

hv

b)

E f

c)

a) photoemission from a core levelb) decay of the core-hole by fluorescence ( Z 4)c) core hole Auger decay ( Z 2)


How does a core-hole decay ?

In addition, for L1 edges, we have together with the Auger process (i.e. LMM) the so called Coster-Kronigtransitions (one of the two holes in the final state has the same quantum number of the initial core-hole: i.e L1L2M) and the Super-Coster-Kronig (both final state holes have the same quantum number of the initial core-hole ; i. e. L1L2L3). Such extra processes will quicken the decay process increasing accordingly the line broadening



Structural disorderIn an amorphous solid, the static fluctuations in valence charges brought about by bond-length and bond-angle variations causes a homogeneous broadening of core level lines.[as shown by L. Ley et al. PRL (82) in case of c-Si compared to a-Si 2p]

AmorphousSi

CristallineSi

Core level line intensity:It is proportional to the number of atoms which are present in the material and at its surface.

1. Reflects the high surface sensitivity of thetechnique.

2. It depends on the photoemitted electron atomicsubshell photoionization cross section.

3. It is connected to the structure of the analysed solid, if it is a single crystal, causing the so called "photoelectron diffraction".

P(E,z) ~ exp[-z λ(E)]



C. l. intensity: surface sensitivity λ(E) is the meen free path


Cross section effects


Photoelectron diffraction






In case of GaAs(110) clean surface



The GaAs(110) clean surface


The GaAs(110) clean surface Ga 3d Core Level Line:

1)We expect two components: a 3d5/2 and a 3d3/2 line splitted in energy by the spin-orbit interaction and with a relative intensity roughly proportional to their occupation probability (for a 3d orbital:(2j(3/2)+1)/(2j(5/2)+1) =0.66).

∆s.o.



2) We expect the presence of a doublet photoemitted from surface atoms. (Similar to the one emitted by volume atoms but centered at a slightly different energy position and with different intensity. In simple terms, we expect the Ga 3d surface peak at higher BE (SCLS >0) due to the "missing" of an As, which, in the bond, results negatively charged. A similar reasoning suggests for the As 3d peak a negative SCLS).


∆scls


3) Each of these lines will have alorentzian broadening (which, given the high angular momentum and the low binding energy of the 3d level, can be expected to be small) convoluted with a gaussian(∆Eexp=80meV).


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The GaAs(110) clean surface Ga 3d Core Level Line: Experimental data



The GaAs(110) clean surface As 3d Core Level Line: Experimental data

The GaAs(110) clean surface Ga 3db and As 3d Core Level Line: FIT

One can FIT the experimental data to obtain a quantitative estimate of the relevant physical quantities.

Ga 3d and As 3d in GaAs(110)


Eb

(eV)SCLS(eV)

∆b

(eV)∆s

(eV)Γb

(eV)Γs

(eV)BR ∆s-o

(eV)Is/Ib

Ga 3d

18.67 0.28 0.17 0.25 0.09 0.08 0.65 0.45 0.61

As 3d

40.45 -0.40 0.24 0.27 0.09 0.09 0.70 0.70 0.63

± 0.02 0.01 0.05 0.05 0.05 0.05 0.03 0.01 0.05


Given the well resolved surface component: check surface sensitivity

ab

Surface sensitivity


(Dependence of the relative intensity Is / Iv on the photon energy)

Photoemission spectra from Ga 3d and As 3d core level from GaAs(110).[From: Eastman et. al. PRL 45, 656 (1980)]

a

b

Surface sensitivity We can than confirm the "universal" dependence of the photoelectron mean free path versus kinetic energies.

In first approximation, the ratio between the emission intensity of the volume atoms and the surface atoms is given by:

where: d is the distance between two lattice planes iz the distance between the atom emitting the p.el and the surface.λ is the mean free path

than: λ = d [ln (Is/Ib + 1)]-1.being d = (21/2/4) a0 for the (110) surface, with a0, the GaAs lattice constant, equal to 5.65 Å, we obtain a mean free path :

λ ~ 4 Å.


Core Level Lines FIT

WARNING:

Often such fitting procedures are overrated, especially when it is not possible to individuate by eye-inspection or by means of a sound physical prediction, the different components which shouldform the signal.

It is than fundamental, before proceeding to a CL fit, to reduce at minimum the free parameters and to individuate the various components in order to obtain physically significant information.



Wrong Core Level Lines FIT

Huttel et al Surf. Rev. and Lett. (1995)

Metal-semiconductor interfaces

Due to the high surface sensitivity, core level photoemission is one of the most powerful technique to study metal semiconductors interfaces. We will see how it is possible to gain information on:

a) Chemical reactions occurring during the early stages of interface formation;b) Growth morphology; c) Electronic properties of the system.


Some examples:Chemistry and morphology at

Fe/GaAs(110):a reactive interface.

a) - Exchange reaction b) Interface chemistryc) Importance of experimental resolution



Fe/GaAs(110):

ReactedComponents

Free metallic Ga


Fe/GaAs(110):

Same exp. at low

resolutionBy: Ruckman et al

PRB 1986.

Some examples:Chemistry and morphology at In/GaP (110) and

In/GaAs(110):two non-reactive interfaces.

a) Morphology versus Temperatureb) Cross section effectsc) Chemistry at the interface



In/GaP (110):

What do we learn from The Core level analysis As a function of Temperature:Different growing mode!

Interface Growing Modes:





In/GaP (110):



TIF

From the attenuation curves

of the CL reacted/Surface component we can understand

the growing mode!

L.T. R.T.

Cross section Vs. Photon energy



Playing around with such cross section effects, we can better understand the chemistry at the

interface.

Bulk Ga 3d Interfacial

Ga 3d

Metallic In 4d


In/GaAs(110)

In/GaAs(110) similar to Ag/GaAs(110): a look at the Valence Band gives…


Metal-semicondutor interfaces: Schottky Barrier formation

(Macroscopic property of great techonological interest)

What is a Schottky barrier ?A metal-semiconductor interface is called a Schottky barrier in honor of W. Schottky who first described the rectifying property of such system in 1938.From then on a lot of work has been done on the subject. Up to now a unique and complete theory capable of explaining all the experimental findings is still lacking.


Schottky Barrier formationWork function model :

Φ Bn =S (Φ m −X s ) S = 1 in the Schottky model

where :

Φ Bn = SB Hight for an n-type semiconductor

Φ m = Metal work function

Xs =Semiconductor electron affinity.

Different models have been proposed. Among them:


Schottky Barrier formationModel based on interface states (defects or MIGS)

(Bardeen derived model)

ΦBn =S (Φm−X s ) - ∆

where:

∆ = interface dipole


Measuring a Schottky Barrier using Photoemission:

What is a band bending experiment? In most photoemission spectrometers the B.E. is referenced to the sample Fermi Energy.

During the formation of a Schottky barrier we will observe band bending, that is a rigid shift of the measured B.E. compared with flat band emission.(Assuming the very often valid condition: Photoelectron escape depth << Band bending depth λ<<W )

EB1 -EB2 = Band bending contribution to the induced barrier height.




In practice the substrate core level spectra are measured versus metal coverage. The determination of a rigid core level shift will give us the band bending of our system.


In the previous case we have (M. Prietsch et al. Europh. Lett. 6, 451 (1988)) :



The general observation is that a Schottky barrier is well established after metallic coverages as small as one Monolayer.

Hence the importance of studying SB at the very early stage of interface formation in order to gain an understanding of the mechanisms controlling the Fermi level pinning.Photoemission, due to its surface sensitivity, is considered as a optimal tool for those experiments.



Photoemission can now benefit from:

Higher photon flux

Better energy resolution

Better Instrumentation

BUT:

Flux and T- an interesting historical case:

•Between 1988 and 1990, using high flux beamlines and Low temperature manipulators, photoemission studies performed at <60 °K seemed to give new insight on the role played by the doping and by the temperature itself in determining the coverage-dependent Fermi level movement at the metal-semiconductor interface.

•Those data suggested that Fermi energy movements are controlled by the dynamic-coupling between atom induced states and substrate states. This coupling seems to have a strong dependence on the bulk doping of the semiconductor and on the temperature. [Aldao et al. P. R.B.39, 12977 (1989), Vitomirov et al. P.R. B 40, 3483 (1989)].


Flux and T- an interesting historical case:


Also on GaP:Photoemission studies performed by depositing different metal on GaP(110) (a larger gap semiconductor than GaAs) seem to indicate that this system reveals an almost ideal Schottky-like behavior. This is in contrast with the GaAs case, where no dependence on the metal work function was found for the Schottky barrier height. [Chiaradia et al. J.V.S.T. B5 1075 (1987) and B7, 195 (1989)]



GaP


Also on GaP:


Than in 1990: Ag/n,p-GaP(110)

M. Alonso, R. Cimino K. Horn Phy. Rev. Lett. 64 1947 (90)

Than in 1990:

Ag/n,p-GaP(110)

M. Alonso, R. Cimino K. Horn Phy. Rev.Lett. 64 1947 (90)


looking also at the V.B.

Surface Photovoltage effect

The possibility that photoemission data may be affected by photon-induced electron-hole pair creation and transport processes leading to a non-equilibrium charge distribution, has been largely overlooked in this studies, in spite of observations that such effects may occur on clean as well as on metal-covered semiconductor surfaces [Demuth et al. P.R.L. 56, 1408 (1986)].


Surface Photovoltage effect



Ag/n,p-GaP(110)

Referring each Core level to its real Fermi level (when measurable):

M. Alonso, R. Cimino K. Horn Phy. Rev. Lett. 64 1947 (90)

SPV effect

metal

conduction band

valenceband

w

hω

J leak

J

J th

tu

JrecJ pc

EF

SPVV

Jph= Jth + Jtu + Jrec + Jleak

Jph= The photo-generated electron hole pairs are separated by the built in potential in the depletion layer giving raise to the photo-induced currentJth = Thermionic excitation of electrons over the barrierJtu = carriers tunneling through the barrierJrec= recombination of electron hole pairs in the deplationJleak=leakage either through the semiconductor or along conducting paths on the crystal sides.

Where:



A. Bauer, et al. Europhys lett (1990)M. H. Hecht Phys. Rev. B 41, 7918(1990) and J.Vac. Sci. Technol. B 8, 1018 (1990).


Higher photon flux is welcome provided that one takes care of:

Charging effectsSurface photovoltageTemperature effectsNon-linear phenomena etc….

Those items will be essential if one consider future experiments using Free-Electron-Lasers in the x-ray region with peak brilliancevalues of 1034 (ph/sec·mrad2·0.1%bw) !!!!!

One more example: Line shape analysisand surface preparation.

Si 2p = ( Si 2p1/2 + Si 2p3/2 ) bulk +( Si 2p1/2 + Si 2p3/2 ) surface

Each doublet is formed by 2 spin-orbit splitted lorentzian lines convoluted with a gaussian (Phonon broadening & experimental resolution).

The number of surface peaks and their energy position depends on the surface reconstruction



Si (111) 7x7

Karlson et al PRB (90)


By a fitting procedure one can obtain information on the parameters defining the core level shape.

Si 2p bulkparameters:

From ref:

Back to III-V: STM-STS evidences

A density of different surface defects of about 1011- 5 1012 atoms/cm2 is typically present on a III-V (110) cleaved surface.

Some of this defects are electrically charged inducing a local band bending which roughly extends over the doping dependent Debye length.

See:Ebert et al PRL 94 and Stroscio et al. PRL 87


See the case of As Vacancies on GaAs(110) Lenghel et al PRL 94



What is the role played by any inhomogeneity of the surface in

broadening a core level line?• Defects, steps, etc, can:Change the atomic position around them

Create locally differently shifted components

At semiconductor surfaces, act as pinning centers

Form around them a region with non zero barrier hight

A)

B)


The core level photoemission signal is calculated as the sum of a discrete contributions from different atoms at different distances x from the pinned region, shifted according to the particular band bending ratio


How can we single out the effect on core level line shape of the presence of pinned regions on an otherwise unpinned surface?

By artificially flatten the bands with the help of Surface photovoltage effect.


The existence of such barrier height inhomogeneity results in an extra broadening due to an averaging over differently pinned regions. This effect need to be taken into account in order to correctly analyze semiconductors core level line shapes. (Cimino et al. Europhys. Lett (95))

RT

LT


Better resolution requires careful and exact knowledge of what we

are measuring.

Detailed sample preparation and characterisation is required !!!!


One more instructive example

Multi atom resonant photoemission


Single atom resonant photoemission

Multi atom resonant photoemission

LATER A REVISION APPEARED:



A CAREFULL DETECTOR CALIBRATION GAVE:


After atom B excitation: increase of backgroundOff resonance lower background


NEW AND HIGLY SOPHYSTICATED TECHNOLOGY IS NECESSARY BUT

REQUIRES CAREFULL KNOWLEDGE.

IT CAN NOT BE USED AS A IT CAN NOT BE USED AS A BLACK BOX!BLACK BOX!

THE SAME APPLYES TO NEW AND MORE SOPHISTICATED CALCULATIONS AND THEORIES.


PHOTOEMISSION IS AN EXTREEMELY POWERFOOL

TOOL AND A WELL CONSOLIDATED TECHNIQUE.

STILL THERE ARE PIONIRING APPLICATIONS WHICH PRESERVES THE CHARM FOR DISCOVERY……... AND THE RISK OF UNSUCCESS……

synchrotron radiation core level photoemission...

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