Epitaxial growth and study of 2D Se-based ultrathin films: Bi2Se3, MoSe2, HfSe2 , ZrSe2

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Epitaxial growth and study of 2D Se-based
ultrathin films Bi2Se3, MoSe2, HfSe2 , ZrSe2

• Aretouli E. Kleopatra
• 20/2/15
• NCSR DEMOKRITOS, Athens, Greece

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Outline

• Topological insulators Bi2Se3
• MBE growth and structural characterization
• Semiconducting Transition metal dichalcogenides
  (TMDs)
• MoSe2
• HfSe2
• ZrSe2
• Conclusions/ Future work

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3D Topological Insulators
Bi2Se3 , Bi2Te3, Bi1-xSbx
Spin locked to orbital momentum
Gapless metallic surface states
Spin polarized (helical) Dirac cones
insulating bulk
relativistic movement of e- light-like
particles

• Topologically protected

spin
k
-k
k
Non-magnetic impurity
e-
-k
-k

• Backscattering is suppressed
• Novel switching mechanisms/functionalities

Y. Xia et al., Nat. Phys. 5, 398 (2009)
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Ultra high vacuum champers for growth and
structural characterization
XPS
RHEED
ARPES
STM
MBE
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HRTEM and XPS
3QL epitaxial Bi2Se3
Substrates 200 nm AlN(0001) /200 mm Si (111)
Bi2Se3/AlN No reaction sharp crystalline
interfaces
1 QL
11-20
P. Tsipas et al., ACS Nano, 8 (7), 6614 (2014)
High epitaxial quality and clean crystalline
interfaces
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Gapless surface states in ultrathin Bi2Se3
In-situ ARPES
Ultrathin films Hybridization-gap opening
Thick films (gt6QL exp.) Non-interacting
Gapless surface states
EF
3QL Thinnest Bi2Se3 with gapless surface states
(Dirac cone) ever reported experimentally !
Reduce surface to volume ratio - applications
in nanoelectronics
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Heterostructures with Chemically compatible
semiconductors Two Layer MoSe2 on Bi2Se3Template
RHEED
2ML MoSe2/5QL Bi2Se3
3QL Bi2Se3/2ML MoSe2/3QL Bi2Se3
aAlN3.11Å aBi2Se34.14Å mismatch of 33
5 QL
aBi2Se34.14Å aMoSe23.299 Å mismatch of 20
Perfect alignment of the 2 hexagonal lattices No
rotated domains-single crystal
11-20 MoSe2 //11-20 Bi2Se3 // 11-20 AlN
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Semiconducting 2D Transition Metals
Dichalcogenides (TMDs)
Layered TMDs crystals of the composition MX2
M transition metal (VIB Mo, W and IVB Zr,Hf
) X Chalcogen species (S, Se, Te)
Honeycomb like structures superior
properties to those of graphene ???
2H structure
1T structure
Se
Se
Hf
Indirect to direct band gap crossover when
thickness reduces to a single layer
Indirect band gap very close to Si
HfSe2 K.E. Aretouli et al. submitted 2015
MoSe2 E. Xenogiannopoulou et al. submitted 2014

• anisotropic mechanical optical and electrical
  properties
• Sizable band gap in the visible and NIR region of
  the solar spectrum

Applications in Optoelectronic devices(energy
conversion systems) and Field Effect
Transistors/ low power logic devices
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RHEED, TEM, STM of 3ML MoSe2/AlN(0001)
Two step growth process
d
AlN
d
AlN
vdW gap
e
b
MoSe2 350C
Bi2Se3 300C
MoSe2 350C
MoSe2 350C
f
MoSe2 300C
MoSe2 690C
MoSe2 690C
STM image honeycomb structure
estimated distance of 3.3 Å between Se-Se atoms
aMoSe23.299 Å
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Valence Band Imaging
He I
1st Brillouin zone MoSe2
Binding energy (eV)
GKMoSe2 1.274 Å-1
EF
He II

• Shift of VB at G-point to higher binding energy
• Indirect to direct band gap transition in the
  1ML limit

k//,y (Å-1)
RT measurements
E. Xenogiannopoulou et al. submitted 2014
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Raman and PL characterization of MoSe2 films at
ML-limit on AlN(0001)
Active modes of MoSe2 A1g at 240.8 cm-1 E2g at
288.5 cm-1 B2g at 352 cm-1 in few layer material
The direct band gap in single layers results in
intense room temperature photoluminescence (PL)
Applications from optoelectronics to energy
conversion
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HfSe2 and MoSe2 / HfSe2 films on AlN(0001)
6ML HfSe2
HfSe2 deposition at 570 oC Annealing at 810 oC
XPS
mismatch of 6
DFT calculations
mismatch of 15
Absence of strain aHfSe23.78Å v.d. Waals
heteroepitaxy
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• Conclusions
• Thinnest Bi2Se3 (3QL) with gapless surface
  states (Dirac cone) ever reported experimentally
  
• High structural quality MoSe2 and HfSe2 on AlN/Si
  substrates
• MoSe2/Bi2Se3 and MoSe2/HfSe2 multilayers can be
  produced
• Future work
• Exploring the semiconductors HfSe2, ZrSe2
• Electrical characterization of Bi2Se3, MoSe2
  HfSe2 , ZrSe2 and their heterostructures
• Magnetoresistance measurements/ Hall effect
  measurements

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Thank you for your attention