Transcript Epitaxial growth and study of 2D Se

Epitaxial growth and study of 2D
Se-based ultrathin films: Bi2Se3,
MoSe2, HfSe2 , ZrSe2
Aretouli E. Kleopatra
20/2/15
NCSR DEMOKRITOS, Athens, Greece
Outline
• Topological insulators: Bi2Se3
MBE growth and structural characterization
• Semiconducting Transition metal dichalcogenides
(TMDs)
MoSe2
HfSe2
ZrSe2
• Conclusions/ Future work
3D Topological Insulators
Bi2Se3 , Bi2Te3, Bi1-xSbx
Spin locked to orbital momentum
Gapless metallic surface states
Spin polarized (helical) Dirac cones
relativistic movement of e- :
light-like particles
“insulating” bulk
 Topologically protected
spin
k
-k
k
e-
-k
Non-magnetic
impurity
-k
 Backscattering is suppressed
 Novel switching mechanisms/functionalities
Y. Xia et al., Nat. Phys. 5, 398 (2009)
Ultra high vacuum champers for growth and structural
characterization
XPS
RHEED
STM
ARPES
MBE
HRTEM and XPS
3QL epitaxial Bi2Se3
Substrates : 200 nm AlN(0001) /200 mm Si (111)
Intensity (arb. units)
Thick ~ 20 Quintuple Layers (QL)
Experiment
Se3d
Fitting curve Bi Se /AlN
2
3
Background
3d5/2 (53.7 eV)
3d3/2 (54.5 eV)
Bi2Se3/AlN
No reaction –sharp
crystalline interfaces
57
1 QL ~ 1 nm
[11-20]
56
55
54
53
52
Binding energy (eV)
51
Se
Bi
*P. Tsipas et al., ACS Nano, 8 (7), 6614 (2014)
High epitaxial quality and “clean” crystalline interfaces
1 QL
Gapless surface states in ultrathin Bi2Se3
In-situ ARPES
Ultrathin films: Hybridization-gap opening
Thick films (>6QL exp.): Non-interacting
3 QL Bi2Se3/AlN(0001)
Μ
Γ
Μ
2nd derivative
EF
Gapless
surface
states
surface
states
EB (eV)
EB (eV)
CB
Μ
5 QL Bi2Se3/AlN(0001)
Γ
Μ
2nd derivative
Gapless
EF
0.47 eV
VB
k//,y (Å-1)
k //,y (Å-1)
k //,y (Å-1)
k //,y (Å-1)
3QL: Thinnest Bi2Se3 with gapless surface states (Dirac cone) ever reported
experimentally !
Reduce surface to volume ratio - applications in nanoelectronics
Heterostructures with Chemically compatible semiconductors
Two Layer MoSe2 on Bi2Se3Template
RHEED
2ML MoSe2 at 300
oC
2ML MoSe2/5QL Bi2Se3
5QL Bi2Se3 at 300 oC
AlN [11-20] azimuth
3QL Bi2Se3/2ML MoSe2/3QL Bi2Se3
aAlN=3.11Å
aBi2Se3=4.14Å
mismatch of~ 33%
5 QL
aBi2Se3=4.14Å
aMoSe2=3.299 Å
mismatch of ~20%
Perfect alignment of the 2 hexagonal lattices
[11-20] MoSe2 //[11-20] Bi2Se3 // [11-20] AlN
No rotated domains-single crystal
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
Mo
6.5Å
z
y
Hf
z
x
x
y
Indirect to direct band gap
crossover when thickness reduces to
a single layer
MoSe2 : E. Xenogiannopoulou et al. submitted 2014
Indirect band gap
very close to Si
HfSe2 : K.E. Aretouli et al. submitted 2015
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
RHEED, TEM, STM of 3ML MoSe2/AlN(0001)
Two step growth process
d
d
AlN
AlN
AlN
vdW gap
e
b
MoSe2
MoSe2
MoSe
MoSe
2 2
350°C
350°C
350°C
350°C
f
MoSe
MoSe2 2
690°C
690°C
690°C
MoSe2
Bi2Se3
300°C
MoSe2
300°C
STM image: honeycomb structure
Line 1
3.3Å
2
Se
Se
Se
1
Line 2
2Å
Mo
Se
Mo
estimated distance of 3.3 Å between Se-Se atoms ~ aMoSe2=3.299 Å
Valence Band Imaging
Γ
Binding energy (eV)
L
H
K
M
ΓKMoSe2 =1.274
Å-1
He II
 Shift of VB at Γ-point
to higher binding energy
 Indirect to direct band
gap transition in the
1ML limit
(a)
Γ/Α
6ML
Κ/Η
(c)
(b)
He I
4
2
0
-2
-4
Γ
EF
Binding energy (eV)
A
EF
3 ML
Γ/Α
Κ/Η
1ML MoSe2
Γ
Κ
Binding Energy (eV)
He I
1st Brillouin
zone
MoSe2
(e)
(d)
KM
(f)
He II
k// ,y(Å-1)
E. Xenogiannopoulou et al. submitted 2014
0
k//,y (Å-1)
0
k//,y (Å-1)
RT measurements
Γ
Raman and PL characterization of MoSe2 films at
ML-limit on AlN(0001)
-1
295 K
log(Intensity) (a.u.)
Intensity (a.u.)
Si 521cm
=532 nm
2 mW
MoSe2
A1g 240.8 cm
-1
Si peak
200
400
600
800
1000
A (1.55eV)
1.6
1.8
E12g
2.0
Si
Low
B12g
Breathing
mode
250
300
A B
Κ
350
-1
400
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
190 meV
B (1.75eV)
Energy (eV)
High
Raman shift (cm)
1200
Γ
1.4
A1g
200
295 K
PL Intensity (a.u)
λ=532 nm
2
I=2mW/μm
1.2
-1
Raman Shift (cm )
0.3cm
0.5cm
0.7cm
0.9cm
1.1cm
1.3cm
Μ
The direct band gap in single layers results in
intense room temperature photoluminescence (PL)
Applications from optoelectronics to energy
conversion
HfSe2 and MoSe2 / HfSe2 films on AlN(0001)
6ML HfSe2
HfSe2 deposition at 570 oC
(a) M
Annealing at 810 oC
Absence of strain
aHfSe2=3.78Å
v.d. Waals heteroepitaxy
Γ/Α
Μ/L
Γ
Binding Energy EB (eV)
XPS
kx (Å-1)
(b)
DFT
calculations
2
Energy (eV)
AlN
1ML HfSe2
6ML HfSe2
3ML MoSe2
mismatch of ~15%
mismatch of ~6%
[11-20] azimuth
K/H
1
Ef
0
-1
-2
-3
-4
-5
Κ
Γ
Μ
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