imaging the local transport field of a bi se surface , a
TRANSCRIPT
Imaging the local transport fieldof a Bi Se surface2 3
Acknowledgement
References Contact
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Sebastian Bauer+49 203 379 [email protected]
Dr. Christian Bobisch+49 203 379 2558
Prof. Dr. Rolf Möller+49 203 379 [email protected]
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
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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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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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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
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� 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)
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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)
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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
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-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