imaging the local transport field of a bi se surface , a

Imaging the local transport fieldof a Bi Se surface2 3

Acknowledgement

References Contact

·

·

·

Sebastian Bauer+49 203 379 2558sebastian.bauer.1988@uni-due.de

Dr. Christian Bobisch+49 203 379 2558

Prof. Dr. Rolf Möller+49 203 379 4220rolf.moeller@uni-due.de

christian.bobisch@uni-due.de

S. Bauer, A. M. Bernhart, M. R. Kaspers, R. Möller, and C. A. Bobisch

University of Duisburg-Essen, Faculty of Physics, Lotharstraße 1, Duisburg, Germany

a)

Motivation: transport in topological insulating surface states

sketch of the STP experiment

Chemical structure of Bi Se [4]2 3

line scan throughthe Bi Se film2 3

Theoretical band structure of the bulk and of the surface inBi Se (calculations by Y. Xia et al. )2 3 [1]

Results

O58.25

Preparation of the Bi Se sample2 3

Scanning tunneling potentiometry (STP)[2]

Outlook

Vq

-+

It

STM tip

potentiometer

contact tip contact tip

Vmod

1

3

2

Iq

surface conductor

bulk

�

�

�

�

two contact tips apply lateral current to thesurface

adjusting the potential between STM tip andsample

I = 0

(dc-component vanishes)

control of the tunneling distance using the accomponent of I by modulating the tunneling

voltage

STP maps topography and potentialsimultaneously

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t

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chemical structure of Bi Se films

rhombohedral structure, can be referred to a hexagonal basis

consists of quintuple layers, which are bond covalently withinthemselves and by van der Waals forces between each other

preparation procedure according to Zhang et al. [3]

first step: preparation of the Si(111)-( )R30°-Bi reconstruction

Si(111)-(7 7) substrat (figure (a))

3.6 nm Bi deposited at room temperature

annealed to 450°C (ca. 20°C/min)

Si(111)-( )R30°-Bi (figure (b))

second step: preparation of the Bi Se film

coadsorption of Bi and Se with a deposition rate of0.21 nm/min (Bi) and 0.31 nm/min (Se)

the rates of Bi and Se are conform to an atomic ratio of 1:2(excess of Se as compared to Bi Se bulk)

film annealed to 120°C for 5 min

Bi Se film (analyzed by LEED and STM) (figure (c)-(f))

nominal film thickness: 4.5 nm (4-5 quintuple layers)

high film quality but unknown doping level

lattice constant of Bi Se films: 0.41 nm (LEED)

(reference [3]: 0.42 nm)

height of the quintuple layer: 0.96 0.05 nm (STM)(reference [3]: 0.95 nm)

substrate induced steps, Bi Se islands and domain boundaries

2 3

2 3

2 3

2 3

2 3

2 3

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Ö3´Ö3® ´®®

Þ Ö3´Ö3

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®

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topological insulators represent a new classof materials: surface state protected frombackscattering

scanning tunneling microscopy (STM) toanalyze the topographic and electronicstructure of Bi Se on the atomic scale

analysis of the local transport field withnm spatial resolution by scanning tunnelingpotentiometry (STP)

current through surface states

surface vs. bulk conductivity

impact of surface defects to thelateral potential variation

scattering of electrons at (surface)defects and adsorbates

2 3

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Au tips as contact probes

resistance scales logarithmic vs. tip distance:

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topography dominated by substrate inducedsteps

applied current of 0.65 mA at 9.6 V

electrochemical potential (local transport field)exhibits a gradient of about 43 V/cm (here)

additional thermovoltage effects

evaluation of the average current density(applied current of 0.54 mA at 9.1 V)

electric field current density

measurement of the electric field along thedashed line (see SEM image)

integral of the current density along the lineis equal to the total current

average current density of 0.3 A/m

® µ®

®

®

� current-to-voltage (IV) characteristic betweenthe contact tips as a function of

tip material

contact geometry (e.g. metallic islandsbetween contact tip and the Bi Se film)

®®

2 3

� variation of the film thickness / analysis of

wedges bulk vs. surface conductivity

finite size effects

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Resistance of the surfaceBi Se2 3

Potential imaging of the surfaceBi Se2 3

Analysis of transport properties

Variation of the sample geometry

Scatterers at the surfaceBi Se2 3

LEED pattern for the preparation steps

Bi Se film2 3

Si(111)-( )R30°-BiÖ3 Ö3´

30 eV

Si(111)-(7 7)´

(a) (f)

(c)

(e)(d)

(b)

30 eV

30 eV

[1][2][3][4] H

Y. Xia, et al., Nat. Phys. , 398 (2009)P. Muralt and D. Pohl, Appl. Phys. Lett. , 514, (1986)G. Zhang, et al., Appl. Phys. Lett. , 053114 (2009)

. Zhang, et al., Nat. Phys. , 438 (2009)[5] F. Yang et al., Phys. Rev. Lett. , 016801 (2012)

548

955

109

Financial support is granted by the Deutsche Forschungsgemeinschaft(DFG) through the SFB 616 „Energy dissipation at surfaces“

230 nm 32 nm

topography at room temperature,I = 5 pA, U = 1V, 1.1x1.1 m²μ

topography closeup at room temperatureI = 5 pA, U = 1V, 160x160 nm²

domain boundary

140120100806040200

1.2

1

0.8

0.6

0.4

0.2

0

distance (nm)

z (

nm

)

quintuplestep

tip 1 (Au)

tip2

(Au)

24 µm 54 µm 464 µm221 µm105 µm

0 100 200 300 400 500

9

10

11

12

13

14

resis

tance (

k)

W

probe spacing (µm)

logarithmic fit

280 nm 280 nm

topography electrochemical potential

19 mV 0 mV

two dimensional conductivity

conducting sheet is thinner than probe spacing

®®

-600 -400 -200 0 200 400 60010

20

30

40

50

60

distance (µm)

Lorentzian fit

tip 2

tip 1

STP tip

ele

ctr

ic fie

ld (

V/c

m)

1900µm

50 nm 50 nm

2 2 21

Bi Se2 3 bulk

2

Iq

Bi Se2 3 surface

Vq

21

Bi Se2 3 bulk

Iq

Bi Se2 3 surface

Vq

epitaxialmetallicislands

substrate

thin filmBi Se2 3

Bi Se bulk2 3 substrate

thin filmBi Se2 3

decorated stepnon-magnetic atom

surfacedefects

(magnetic) organic molecules

Bi layer [5]

e-

e-

e-

e-

pote

ntial (µ

V) distance (µm)

magnetic atom

vacancysub-surfaceimpurities

sub-surfacedefects

domainboundary

ternarylayer

laterally resolvedpotential /local transportfield:

? ? ?

e-e

-

e-

substrate

substrateinduced

stepsurfacestep

Bi Se2 3 material

Bi Se2 3

http://www.exp.physik.uni-duisburg-essen.de/moeller/uploads/media/

DPG_2013_Poster_BiSe.pdf

averaged profile

1.41.210.80.60.40.20

6

4

2

0

distance (µm)

pote

ntial (m

V)

http://www.exp.physik.uni-duisburg-essen.de/moeller/index.html