Spektroskopia fotoelektronów w zastosowaniu do izolatorow topologicznych

Jacek Szade Instytut Fizyki im. A. Chełkowskiego,

Śląskie Międzyuczelniane Centrum Edukacji i Badań Interdyscyplinarnych

Uniwersytet Śląski w Katowicach

Spektroskopia fotoelektronów – krótkie wprowadzenie Wzrost cienkich warstw BixTey

Polikrystaliczne warstwy BixTey na Si(100) Monokrystaliczne warstwy Bi2Te3 na mice Generacja fononów w cienkich warstwach BixTey

Podsumowanie

Photons:

- energy: few eV-103 eV

- polarisation: linear, circular

- UV lamp, x-ray lamp

- synchrotron: VUV, x-ray

sample

Photoelectrons:

detection of intensity in function of:

- kinetic energy

- polarisation of light

- angle (ARUPS, ARXPS)

- spin polarization (SRXPS)

- photon energy (CIS, CFS)

solid, gas in special chambers a spectrum is formed, usually in

function of binding energy

- calculated with the use of work function of the spectrometer

UPS – Ultraviolet Photoelectron Spectroscopy

XPS – X-ray Photoelectron Spectroscopy

ESCA – Electron Spectroscopy for Chemical Analysis

RESPE – Resonant Photoelectron Spectroscopy

Golden Fermi rule

photoelectric current

S

ifiPESpp hHhJ )(2

)(2

,

pAcm

eH

e

vol

PE

dipole interaction

one of the final electronic states with free electron of momentum p and N-1 electrons in atom

initial electronic state set of final possible

quantum states

energy conservation

frozen orbitals approximation

11 NN if

Koopmans theorem

Binding energy derived from the PE spectrum is equal to the initial energy in a non-perturbed atom

S. Hüfner „Photoelectron spectroscopy”

One step model

Koopmans theorem – binding energy is equal to the energy of an orbital

but

one has to take into account that an excited state is measured (photoelectron + photo-hole)

peak position may be different from the orbital energy (relaxation)

additional lines – satellites are present

background

for standard XPS (h up to 1500 eV) mean free path of photoelectrons is less than 2-3 nm

Photoelectron spectroscopy is a surface sensitive technique

photoionisation cross section depends on h and particular atomic orbital

spectrum is different for various excitation energies

XPS spectra can be used for determination of chemical composition

integration of photoemission lines plus photoionization cross sections plus spectrometer transmition function

all elements can be detected except H and He

S. Hüfner „Photoelectron spectroscopy”

Czułość powierzchniowa – zalety:

reakcje na powierzchni reakcje pomiędzy warstwami jeżeli grubość < 3-4 nm możliwość profilowania wgłębnego z dobrą rozdzielczością powierzchni

wady:

reakcja z gazami resztkowymi nawet w UHV konieczność usunięcia zanieczyszczeń z powierzchni:

trawienie jonowe łamanie w próżni drapanie, piłowanie w UHV

Zmiana czułości powierzchniowej:

Zmiana geometrii

Zmiana energii kinetycznej fotoelektronów przez: analizę innej linii fotoemisyjnej o innej energii zmianę energii fotonów

Surface Physics Laboratory at the Silesian Center for Education and

Interdisciplinary Research in Chorzów

Multitechnique UHV (ultra-high vacuum) system XPS – X-ray photoelectron spectoscopy PHI UPS - UV photoelectron spectoscopy PHI AES – Auger electron spectroscopy PHI SEM - Scanning electron microscopy SPM – Scanning probe microscopy (AFM, STM, MFM….) RHK/Prevac MBE - Molecular beam epitaxy (4 efusion cells, 2 electron beam evaporators) Prevac Electron diffractometers (RHEED and LEED) Steib, OCI Surface preparation facilities, cooling and heating in-situ

ToF SIMS – Time of Flight Secondary Ion Mass Spectrometer ION TOF

XPS, UPS – Prevac/VG Scienta

Topological insulator Bi2Te3

Bulk:

- Energy gap – insulator (semiconductor)

- Gap of about 150-170 meV

- Large thermoelectric (Seebeck) coefficient

Surface:

- Dirac states –

- Spin orbit coupling (SOC) drives a band inversion transition at the point

- Topologically protected surface state consisting of a single massless Dirac fermion

•

According to Zhang et al. 2009 for Bi2Se3, point

Chemical bonding

Crystal field

Spin-orbit

ARPES – Angle Resolved Photoemission

Spin Resolved ARPES

According to Alpichshev et al. 2010

Photoelectron spectroscopy Scanning tunelling spectroscopy

Studies of BixTey in the Institute of Physics, University of Silesia

Growth of thin films on Si (100) Different conditions and thickness

Growth of thin films on mica

• Growth by the MBE (Molecular Beam epitaxy)

– Substrate preparation, temperature of the substrate, flux proprtion of the components,…

– Characterization in-situ without the contact of the film with air electron diffraction RHEED (reflective high energy) and LEED (Low energy)

X-ray and ultraviolet photoelectron spectroscopy

AFM microscopy

LC AFM – Local conductivity AFM

Ex-situ characterization

XRD

XRR

Magnetometry (SQUID)

ToF SIMS

Ultrafast optical spectroscopy – le Mans, France

Surface Physics Laboratory at the Silesian Center for Education and

Interdisciplinary Research in Chorzów

Multitechnique UHV (ultra-high vacuum) system XPS – X-ray photoelectron spectoscopy PHI UPS - UV photoelectron spectoscopy PHI AES – Auger electron spectroscopy PHI SEM - Scanning electron microscopy SPM – Scanning probe microscopy (AFM, STM, MFM….) RHK/Prevac MBE - Molecular beam epitaxy (4 efusion cells, 2 electron beam evaporators) Prevac Electron diffractometers (RHEED and LEED) Steib, OCI Surface preparation facilities, cooling and heating in-situ

ToF SIMS – Time of Flight Secondary Ion Mass Spectrometer ION TOF

XPS, UPS – Prevac/VG Scienta

LABORATORIES – SURFACE PHYSICS LABORATORY

SURFACE PHYSICS LABORATORIES @ Chorzów ŚMCEBI, IF, UŚ

MBE – Molecular Beam Epitaxy

Vacuum conditions during the growth process:

UHV ~10-9 - 10-10mbar

Monitored by Residual Gas Analyzer (RGA)

Manipulator:

2-axis manipulator (tilt and rotation)

Shutter for the growth of wedge samples

Possibility of heating and cooling sample

(-120° – 1500ºC)

Preparation and Characterization:

Ar ion gun

RHEED

4-point Resistivity

2 electron beam evaporators (~2000ºC)

4 effusion cells (~1200ºC)

Thickness monitor

Software for the control of the growth process

What we usually grow: Bi, Te, Eu, Mn, Fe, Mo, Cr, Au, Ag, Ta

Typical growth rate 0.01 – 0.03Å/s, typical thicknesses 1-50nm

MULTILAYER DEPOSITION

CO-DEPOSITION

LABORATORIES – SURFACE PHYSICS LABORATORY

SURFACE PHYSICS LABORATORIES @ Chorzów ŚMCEBI, IF, UŚ

XPS (and UPS) – X-Ray (and Ultraviolet) Photoelectron Spectroscopy

CasaXP S (Thi s st ring can be edit ed in CasaXPS.DEF/P rintFootNote.txt)

Eu3d

1190 1180 1170 1160 1150 1140 1130 1120 1110

Bindi ng E nergy (eV)

x 104

6

8

10

12

14

16

18

20

22

24

CPS

Reversible valance transitions of europium

EU 3+ EU 2+

(1) Physical Electronics PHI 5700/660

(2) VG Scienta & Prevac

Monochromatic X-ray, Al, Mg anodes

Several apertures - probe size:

(1) 75 mm, 0.8-2mm

(2) 40-600mm x 4mm

5-axis manipulator (X, Y, Z, tilt, rotation)

Chemical state identification on surfaces

Identification of all elements except for H and He

Quantitative analysis, including chemical state differences

Depth profiling, line scans, chemical mapping

Applicable for a wide variety of materials

Detection limits typically ~ 0.1 at%, down to 100 ppm

Excellent surface sensitivity (~ 3-4 nm information depth)

Eu 3d

Chemical state identification → Evolution of chemical state → Depth profiling and atomic concentration calculations (during occurring processes)

NANOPARTICLES FOR CATALYTIC APPLICATIONS EU-MN THIN FILMS - MULTILAYERED SYSTEM

EU-MN THIN FILMS - MULTILAYERED SYSTEM

Eu-Mn thin

films

grown on Mo

Total thickness

of Eu-Mn layer

~20nm

Grazing angle XPS increase of the surface

sensitivity

λ

e-

λ

analyser

λ - IMFP

XPS regime

- VB states → about 30 Å

UPS regime → about 1-2 Å

TPP2M Quases by S. Tougaard

BixTey on Si (100) Thickness 16-23 nm

Roughness 1-3 nm

RHEED pattern

AFM

XPS analysis

Bi and Te core levels

VB

R. Rapacz, K. Balin, A. Nowak, J. Szade, J. of Cryst. Growth 401, 567-572 (2014)

XPS analysis of the Te 3d i Bi 4f photoemission lines Two chemical states of Te i Bi Grazing angle analysis gives information on localization of additional layers

Superstructure phases of Bi solid solutions in Bi2Te3 Metallic Te forms the layers on the surface

R.J. Cava, J. Huiwen , M.K. Fuccillo, Q.D. Gibson, Y.S. Hor, J. Mater. Chem. C 1 (2013) 3176-3189

Bi2Te3 on mica

Thickness 5-30 nm

Muscovite (KAl2(OH, F)2AlSi3O10)

LEED and AFM in lateral force mode

Single crystalline films starting from 5 nm thickness

Bi2Te3

AFM in UHV

Film 15 nm thick single crystal Stranskii – Krastanov mode of growth Screw dislocation driven growth Precipitations of unknown character

Morphology and local conductance of single crystalline Bi2Te3 thin films on mica

R. Rapacz, K. Balin, M. Wojtyniak and J. Szade

Nanoscale, 2015,7, 16034

Local conductivity AFM

Topography Current Iav = 18.41 nA ; V=5 mV

All values less than 57 % were cut o (the 35.03 nA value). Measured current value did not depend on the height of terraces, but was constant (within 47 % of scale).

Ultrafast spectroscpy for selected films A – BT on Si 10 nm B- BT on Si 15 nm

C – BT on mica 15 nm

Generation of coherent optical phonon A 1g and acoustic phonons

Pump 830 nm Probe 582 nm Transient reflectivity

Acoustic phonons derived from transient reflectivity

Ultrafast light-induced coherent optical and acoustic phonons in few quintuple layers of the topological insulator Bi2Te3 M. Weis, K. Balin, R. Rapacz, A. Nowak, M. Lejman, J. Szade, and P. Ruello Phys. Rev. B 92, 014301 – Published 14 July 2015

1551561571581591600

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Binding Energy (eV)

No

rma

lize

d I

nte

nsity

Fe/Bi2Te3/mika warstwa 0.5nm

Bi2Te3

Fe/Bi2Te3

Fe/Bi2Te3pochylona

5685695705715725735745755765775780

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Binding Energy (eV)

No

rma

lize

d I

nte

nsity

Fe/Bi2Te3/mika warstwa 0.5nm, linia Te 3d5/2

Bi2Te3

Fe/Bi2Te3

Fe/Bi2Te3pochylona

7027047067087107120

0.2

0.4

0.6

0.8

1

1.2

1.4

Binding Energy (eV)

No

rma

lize

d I

nte

nsity

Fe/Bi2Te3/mika warstwa 0.5nm, Fe 2p3/2

Bi2Te3

Fe/Bi2Te3

Fe/Bi2Te3pochylona

Film 0.5 nm Fe/Bi2Te3

Reaction Fe-Te at RT Bi metal layer is formed underneath FeTe

Bi2Te3 Bi2Te3

Fe FeTe Bi

M/BT reactions

3D Render of 129.69 () 3D Render of 208.69 () 3D Render of 152.68 () 3D Render of 152.88 ()3D Render of 129.69 () 3D Render of 208.69 () 3D Render of 152.68 () 3D Render of 152.88 ()3D Render of 129.69 () 3D Render of 208.69 () 3D Render of 152.68 () 3D Render of 152.88 ()

Eu+ Te- Bi- ~1

9n

m

ToFSIMS Time Of Flight Secondary Ion Mass Spectroscopy @Bi+, 30kV, 0.1pA, Fast Imaging Mode, Depth profiling - Cs sputtering at 250V

0 250 500 750 1000 1250 1500 1750 2000

100

1000

10000

100000

Inte

nsity

Sputter time [s]

Eu+

Te-

Bi-Mica

Eu1 63

x 102

25

35

45

55

65

CPS

1140 1130 1120

Bindi ng E nergy (eV)

Bi1 1

x 102

10

15

20

25

30

35

40

CPS

168 166 164 162 160 158 156 154

Bindi ng E nergy (eV)

Te1 32

x 102

20

30

40

50

60

70

80

90

CPS

580 570

Bindi ng E nergy (eV)

Bi4f Eu3d Te3d

Proces utleniania warstwy Eu/BT/mika

czas

Wnioski

Metoda MBE pozwala na uzyskanie dobrej jakości poli- i mononkrystalicznych warstw BixTey w zależności od podłoża

W warstwach osadzanych na Si (100) stwierdzono fazę Bi2Te3 oraz metaliczny Te lub superstrukturę Bi/BiTe w zależności od składu

Warstwy osadzane na mice są monokrystaliczne nawet dla grubości 5 nm

Spektroskopia fotoelektronów wykonana in-situ pozwala rozpoznać stany chemiczne pierwiastków i reakcje zachodzące na granicy warstw

Metoda lokalnego przewodnictwa AFM pozwoliła rozpoznać korelacje morfologii powierzchni i właściwośći elektrycznych

Metoda pump-probe femtosekundowej spektroskopii optycznej wykazała generację spójnych podłużnych fononów optycznych A1g(I) w warstwach zawierających tylko 10 QL

Współpraca

• Pascal Ruello – le Mans, France

• Mariusz Lejman – le Mans, France

• Katarzyna Balin – IF UŚ Chorzów

• Rafał Rapacz – IF UŚ Chorzów

• Marcin Wojtyniak - IF UŚ Chorzów

• Mateusz Weis – IF UŚ Chorzów

• Anna Nowak – IF UŚ Chorzów

• Bartosz Wilk – IF UŚ Chorzów

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