XPEEM with energy filter

S. Heun, Laboratorio TASC-INFM, Trieste, Italy,

A. Locatelli and M. Kiskinova,Sincrotrone Trieste, Basovizza, Trieste, Italy

MotivationWhy XPS?

chemical state informationsurface sensitiveease of quantificationnondestructive

Why spectromicroscopy ?(semicond.) nanostructures: self-organization, lithographydevicesdiffusion, segregationalloying (silicide formation)chemical reactionssurface magnetism (XMCD)

Scanning vs. direct imaging type

Photon optics is demagnifying the beam:Scanning Instrument

Whole power of XPS in a small spot mode.Flexibility for adding different detectors.Rough surfaces can be measured.Limited use for fast dynamic processes.Lower lateral resolution than imaging instruments

Electron optics to magnify irradiated area:Imaging Instrument

High lateral resolution (20 nm).Multi-method instrument (XPEEM/PED).Excellent for monitoring dynamic processes.Poorer spectroscopic ability.Sensitive to rough surfaces.

Monochromaticlight

Sample(scanned)

Photo-emissiondetector

Detector hv

Object plane

Object lensBackfocal planeStigmator

Intermediate imageIntermediatelens

Projectivelens

Image planeMCPScreen CCD camera

Sample

Intermediate image

e-

PEEM: instrumentation

PEEM without energy filterPEEM with imaging retarding field filterPEEM with area selective spectroscopyPEEM with full spectromicroscopic capabilities

PEEM with sector field (LEEM)PEEM with aberration correction

PEEM with retarding field filter 1Sample

Imaging DeviceImaging Device

Objective lensObjective lens

Contrast aperture

Stigmator/Deflector

First Projective lens

Multi Channel PlateFluorescent Screen

Iris ApertureIris Aperture

Second Projective lens

GmbH

Double Grid System

Retard LensImaging

Energy Filter

Photons (UV..VUV..X rays)

retard lensdouble grid

MultiMulti--channelplatechannelplate

Fluorescent Screen

divergent rays

parallel rays

E0E1< < E2

PEEM with retarding field filter 2

Counting mode:Screen as collector for integral electron flux Numerical derivation delivers N(E) MicrospectroscopyFOV > 0.5µm and dE > 0.5eVAlternative: Modulation technique

Gm

bH

Imaging mode:High pass filtered image differences:Spectromicroscopy

PEEM with retarding field filter 3

M. Merkel et al.: Surf. Sci. 480 (2001) 196

Raw data 1st derivative

Micro-XPS using soft X-ray synchrotron radiation,Bessy I, hv = 100 eV, W(110),

spectra taken with retarding field filterin counting mode,

field of view: 100 µm, 109 ph/s

PEEM with band pass filter

R. Wichtendahl et al.: Surf. Rev. Lett. 5 (1998) 1249Y. Sakai et al.: Surf. Rev. Lett. 5 (1998) 1199G. Schönhense et al.: Surf. Sci. 480 (2001) 180

Time Of FlightWien FilterHemispherical AnalyzerDouble Hemispherical AnalyzerOmega Filter

The SPELEEM

Spectroscopic Photoemission and Low Energy Electron MicoscopePEEM with full spectromicroscopic capabilitiesPEEM with magnetic sector field in combination with MEM and LEEMNo aberration correction yetModes of operation:

Imaging XPEEM (spectromicroscopy)Dispersive plane (microspectroscopy)Diffraction mode (PED)

The SPELEEMSpectroscopic photoemission and low energy electron microscope

The SPELEEM at ELETTRA

The energy filter

R = 100 mmPass Energy = 900 V

Modes of Operation

XPEEMLEEM

PESELS

PEDLEED

XPEEM: Spectroscopic Microscopy

Images from a Field Effect Transistor (FET) at different binding energies.Photon energy 131.3 eV.

XPEEM: Core Level Spectroscopy

Imaging of Dispersive Plane

W{110} clean surfaceW 4f core level hv = 98 eVResolution 210 meV

(1) Riffe et al., PRL 63 (1989) 1976.(2) Webelements

SCLS (eV)

W7/2-W5/2 BE diff. (eV)

Gauss. Broad. (eV)

Alpha S

Gamma S (eV)

Alpha B

Gamma B (eV)

parameter

0.3040.321

2.142.2 (2)

0.210.04

Fixed0.063

Fixed0.084

Fixed0.035

Fixed0.06

Our fitRef. (1)

Lateral resolution

C 1s image (hv= 350 eV, KE = 62 eV)

Lateral resolution: 32 nm

inte

nsity

(a.u

.)100806040200

distance (pixel)

lateral resolution:0.7 * (4 pixel /435 pixel ) * 5000 (nm)= 32 nm acquisition time: 1min

275

270

265

260

255

250

Inte

nsity

(arb

. uni

ts)

10008006004002000Length (nm)

C 1s光電子像

∼ 43 nm

光電子強度プロファイル

放射光

62.5 eV

ラインパタンパタンの影

SWNT（束）

Satoru Suzuki, Yoshio Watanabe and Yoshikazu HommaStefan Heun and Andrea Locatelli

AFM NanolithographyStudied by Spectromicroscopy

S. Heun, M. Lazzarino, G. Mori, D. Ercolani, B. Ressel, and L. Sorba,Laboratorio TASC-INFM, S.S. 14, km 163.5, I-34012 Trieste, Italy,

A. Locatelli, S. Cherifi, A. Ballestrazzi, and K. C. Prince,Sincrotrone Trieste, S.S. 14, km 163.5, I-34012 Trieste, Italy,

P. Pingue,NEST-INFM and Scuola Normale Superiore, I-56012 Pisa, Italy.

Local Anodic Oxidation (LAO)

Water electrolysis H2O → H+ + OH-.OH- groups migrate towards the sample.Oxide penetration induced by the intense local electric field.

Versatile tool at relatively low costHigh lateral resolution but small area

Si Oxide: Image Contrast at Si 2p

Field of view 12 µm, hv = 132.5 eV, energy resolution: 1 eV

M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.

Binding energy 104.7 eV

a-14V

-12V

-10V

-8V

Binding energy 102.3 eV

b

Si Oxide: Spectroscopy at Si 2p

Spectra show high unformity of each stripeBulk silicon peak: position and width constantNative oxide: SiO2 peak (3.9 eV shifted)AFM oxide shows higher binding energyThere is no evidence of broadeningSpectra explain contrast inversion

M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.

104.7eV

102.3eV

ba

110 109 108 107 106 105 104 103 102 101 100 99 98 97 96 950.0

0.2

0.4

0.6

0.8

1.0

native oxideAFM oxide

Inte

nsity

(a.u

.)

binding energy (eV)

Si Oxide: Writing Voltage Effect

14V14V

nativenative

12V12V

10V10V

8V8V

1.0

0.8

0.6

0.4

0.2Nor

mal

ized

Pho

toem

issi

on In

tens

ity

110 105 100 95Binding Energy (eV)

: native oxide: 14 V: 12 V: 10 V: 8 V

Si 2p

siliconoxide

bulksilicon

M. Lazzarino, S. Heun, B. Ressel, K. C. Prince, P. Pingue, and C. Ascoli: Appl. Phys. Lett. 81 (2002) 2842.

ΔE = 3.97 eV (native)ΔE = 4.62 eV (U = 14V)ΔE = 4.46 eV (U = 12V)ΔE = 4. 41 eV (U = 10V)ΔE = 4.39 eV (U = 8V)

Shift (Sibulk - SiOx) increases with increasing writing voltage (oxide thickness).

Si Oxide: Charging Effects

H. Kobayashi, T. Kubota, H. Kawa, Y. Nakato, and M. Nishiyama: Appl. Phys. Lett. 73 (1998) 933.

+ +

+ +

+ +

Intensity decreases due to escape depth

effectCharged (yellow)

layer contribution is more important

Squared points: our data

Line: Kobayashi model

GaAs Oxide: Photon Exposure

AFM after: height 13 nm

AFM before: height 18 nm

Images taken with secondary electronsPhoton energy: 125 eVKinetic energy: 4 eVField of view: 10 µmOne image every 2 sec

Height reduction vs. exposure time

D. Ercolani et al.: Advanced functional materials, submitted.

Spectra From AFM GaAs Oxide

Sample S03BHole (3,2)Writing voltage 15 VStructure height 3 nmImage taken with secondary electrons:

Photon energy: 130 eVKinetic energy: 0.3 eVField of view: 10 µm

Time resolved spectroscopy with SPELEEM using Dispersive Plane (hv = 130 eV)

Spectra From AFM GaAs Oxide

Time resolved spectroscopy with SPELEEM using Dispersive Plane (hv = 130 eV)

2500

2000

1500

1000

Inte

nsity

(a.

u.)

12 10 8 6 4 2Relative Binding Energy (eV)

Exposure Time 3 min 5 min 7 min 13 min 20 min 28 min 43 min 63 min

As 3dhv = 130 eV

1000

800

600

400

200

Inte

nsity

(a.

u.)

12 10 8 6 4 2Relative Binding Energy (eV)

Exposure Time 4 min 6 min 10 min 17 min 25 min 54 min 74 min

Ga 3dhv = 130 eV

As oxide GaAsO 2s

Ga oxides+ GaAs

The As-oxide signal decreases and disappears.The Ga-oxide signal remains unchanged (early stage of exposure).

Spectra From AFM GaAs Oxide

4000

3500

3000

2500

2000

Inte

nsity

(a.

u.)

888684828078Kinetic energy (eV)

As 3dhv = 130 eV

GaAs

AsO

20x103

15

10

5

0

Inte

nsity

(a.

u.)

110108106104102100Kinetic Energy (eV)

Ga 3dhv = 130 eV

GaAs

Ga2O

Ga2O3

O 2s

After about 1 hour of exposure, only traces of As-oxides are observed in the As 3d core level. The AFM-oxide is mainly composed of Ga2O.

The Knotek-Feibelman mechanism

The valence electrons are mainly localized at O atoms.This Auger decay leads to a final state with two vacancies in valence band weakening the bond between Ga and O.

Valence electrons

E

3d

GaInitial state

Valence electrons

E

3d

Ga

Valence electrons

E

3d

Ga

Valence electrons

E

3d

GaFinal state

Summary

Si oxide:The AFM induced oxidation produces chemically uniform, stoichiometric SiO2 with dielectric properties comparable to those of thermal SiO2.

GaAs oxide:Photon assisted partial desorption of the AFM-grown oxide was observed.All As oxides are desorbed.The AFM-oxide is mainly composed of Ga2O.We proposed a simple model for the dynamics of the desorption.

Ge/Si concentration in Ge/Si(111)

Fulvio Ratto, Federico RoseiNFL, INRS–EMT, Université du Québec, 1650 Boul. Lionel Boulet, J3X 1S2 Varennes (QC) Canada

Andrea Locatelli, Salia Cherifi, Stefano Fontana, Stefan HeunSincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, 34012 Trieste (Italy)

Pierre-David Szkutnik, Anna Sgarlata, Maurizio De CrescenziDip. di Fisica and Unità INFM, Univ. di Roma II, Via della Ricerca Scientifica n.1, 00133 Roma (Italy)

Nunzio MottaDip. di Fisica and Unità INFM, Univ. di Roma TRE, Via della Vasca Navale 84, 00100 Roma (Italy)

Motivation

Goal: integrate optoelectronics and microelectronics on the same Si wafer3D islands could act as quantum dots (QD)Critical Issues in the self–assembly of QD:

Size and shape uniformity of the islandsControlling the nucleation sitesStability of the islandsComposition of 3D structures => determines structural and electronic properties

XPEEM

“top view” mapping of island concentration

X-Ray excited photoelectrons are energy filtered and focused on a

screen, creating a real space image with chemical contrast

Previous work on intermixing:

Cross sectional techniques yielding a “side view”concentration mapping

Our approach:X-Ray photoemission electron microscopyPhotoelectron spectra with high lateral resolution (~30 nm)

Chemical contrast

F. Ratto et al., APL 84 (2004) 4526

10 ML Ge on Si(111), T = 560 °C

Ge segregates in the 3D islands

island

partially shadowed region

Intensity contour plots

Concentration profile in 3D islands

Diffusion mechanisms

Island centre is richer in Si than the borders

95% 100%

island

partially shadowed region

T = 530 °CF. Ratto et al., APL 84 (2004) 4526

Concentration vs. Base area *

F. Ratto et al., APL 84 (2004) 4526

Si concentration increases with island’s base area, up to ~40%

Deposition temperature determines the relation between Si surface content and base area

*Boscherini et al., APL 76 (2000) 682

Diffusion mechanisms

Intermixing may be simply due to surface mobility

Borders richer in Si than centre

Thermodynamically favoured configurations need atomic exchange

Centre richer in Si than borders

Conclusions

XPEEM allows a top view mapping of surface Si concentration

Island’s center is richer in Si than the borders

Si surface content in individual islands increases with their base area

The deposition temperature determines the Si concentration-base area relation

AcknowledgementsXPEEM instrumentation

T. Schmidt, S. Günther (Sincrotrone Trieste)C. Koziol (Elmitec)J. Westermann, B. Holmes (Omicron)

AFM NanolithographyM. Lazzarino, G. Mori, D. Ercolani, B. Ressel, L. Sorba (TASC-INFM, Trieste)S. Cherifi, A. Ballestrazzi, K. C. Prince (Sincrotrone Trieste)P. Pingue (NEST-INFM, Pisa)

Ge / Si islandsF. Ratto, F. Rosei (Université du Québec)S. Cherifi, S. Fontana (Sincrotrone Trieste)P.-D. Szkutnik, A. Sgarlata, M. De Crescenzi (Università di Roma II)N. Motta (Università di Roma TRE)

Postdoc wanted !!!

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http://www.elettra.trieste.it

and look under employment opportunities