SUPPLEMENTAL MATERIAL
Two-dimensional semiconductor HfSe2 and MoSe2/HfSe2 van der Waals
heterostructures by molecular beam epitaxy
K. E. Aretouli, P. Tsipas, D. Tsoutsou, J. Marquez-Velasco, E. Xenogiannopoulou,
S. A. Giamini, E. Vassalou, N. Kelaidis, A. Dimoulas*
Institute of Nanoscience and Nanotechnology, National Center for Scientific Research
“Demokritos”, 15310, Aghia Paraskevi, Athens, Greece
1. Substrates and Growth
The epitaxial growth of HfSe2 thin films on AlN/Si(111) substrates was performed by
Molecular Beam Epitaxy (MBE). The 200 nm unintentionally n-type doped AlN(0001) layers
(fabricated by IMEC) were grown by MOCVD on B-doped p-type 1150 μm Si(111) substrates,
with a resistivity > 1 Ohm-cm. Initially the substrates were chemically cleaned to remove
residual surface organic contaminants. This process includes sequential treatment of the substrate
in organic solvents such as trichloroethylene, acetone and isopropyl alcohol. In order to reduce
the surface oxide the substrates were then immersed in HF solution (1:10 HF:H2O). Finally, they
were rinsed thoroughly in deionized water and were dried. The chemical cleaning process was
followed by an in situ degassing at 730°C in the MBE chamber. In situ x-ray photoelectron
spectroscopy (XPS) verified that the AlN surface was free of C while negligible quantities of O
contaminants were present.
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High purity Hf (99.95%) and Se (99.999%) were evaporated from an electron-gun
evaporator and a Knudsen cell respectively, under Se-rich conditions with Hf/Se flux ratio of
~1:15 and growth rate of 1 ML/min. To ensure efficient incorporation of Se, first, HfSe2 is coevaporated at low temperature of 570 oC exhibiting a disordered surface, followed by a UHV
post-deposition annealing (PDA) step at 810 oC to improve the crystalline quality. MoSe2 is
grown using a two–step process. Initially, MoSe2 is deposited on AlN/Si(111) substrates
at the temperature of 350 oC. High purity Mo (99.95%) and Se (99.999%) are evaporated from
the electron-gun evaporator and the K-cell respectively, under Se-rich conditions with Mo/Se
flux ratio of ~1:10 and a MoSe2 growth rate of 3 ML/min. In the second step, a postdeposition annealing in UHV is performed at 690 oC.
2. Details of the DFT calculations
The Density–Functional Theory (DFT) calculations were performed using the Vienna Ab
Initio Simulation Package [1] and projector-augmented waves [2]. The generalized-gradient
approximation (GGA) [3] with Perdew–Burke–Ernzerhof (PBE) parameterization was used as
exchange correlation functional. To include van der Waals corrections, the semi-empirical DFTD3 Grimme’s method [4] was applied. Self-consistent force optimizations were performed until
the energy and the force were converged to 5x10-5 eV and 1x10-4 eV/Å respectively. In order to
minimize interlayer atomic interactions, a 20 Å vacuum was applied above the HfSe2 layer. The
energy cutoff was set at 500 eV, while the Monkhorst-Pack scheme with a k-point grid of
19x19x1 was chosen. Calculations of the band structure with and without SOC were performed,
where a k-mesh 25 k-point per symmetric line along KΓΜ direction was used.
2
Plane average electrostatic potential calculations were performed in the DFT framework
for a system consisting of 6 ML HfSe2 slab with and without Se adatom layer, where a 40 Å
vacuum was applied (results are shown in Fig. 5(b)). The work functions WF were derived from
these calculations taking into account the energy location of the Fermi level EF: 𝑊𝐹 = 𝑉𝑚 − 𝐸𝐹 ,
where Vm is the maximum potential in vacuum. Dipole correction was used in order to take into
account the influence of the bottom surface.
3. XPS data and description of Kraut’s method
In-situ XPS and UPS measurements (Fig. S1) were performed in order to study the
stoichiometry, the workfunctions and the band offsets at the HfSe2/AlN and MoSe2/HfSe2
interfaces. For thick HfSe2 films, the energy position and the lineshapes of the Hf 4f (Fig.S1(a))
and Se 3d (Fig. S1(b)) peaks indicate Hf-Se bonds only, precluding reaction with the AlN
substrate. Moreover, they can be satisfactorily fitted with only one doublet each, yielding slightly
over-stoichiometric material with Se/Hf ratio around 2.1/1 as estimated using literature
sensitivity values [5]. It should be noted that the Se 3d linewidths are 0.95-1 eV which are near
the energy resolution of our XPS system (no monochromator is used) and for this reason it is not
possible to resolve any Se-Se bonds that might be present. Similarly, in the case of the (1ML)
MoSe2/HfSe2/AlN heterostructure, the Mo 3d5/2 peak position indicates the formation of Mo-Se
bonds (Fig. S1(c)), while the Se 3d peak has been fitted using both Se-Mo and Se-Hf
contributions (Fig. S1(d)), yielding to stoichiometries close to 2 for both HfSe2 and MoSe2
layers.
The experiments are performed in situ in a controlled oxygen environment. Nevertheless,
oxygen is always present in small quantities on the AlN substrate due to high affinity of Al in
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oxygen. A very small oxygen peak (Fig. S1(e)) indicates the presence of oxygen “buried” at the
bottom HfSe2/AlN interface. Although the possibility to have an even smaller amount of oxygen
on the top surface may not be excluded, the predicted effect is not in line with observations about
the surface structure and the workfunction lowering. More specifically, using DFT, we have
found that oxygen penetrates deeper compared to the case of Se (Fig.3) and severely distorts the
lattice giving no resemblance with STM observations of a hexagonal lattice. Moreover,
calculations of the electrostatic potential as in Fig. 5 (b) predict that the presence of oxygen
adsorbants increases the workfunction from 5.59 eV to 5.81 eV and then to 6.3 eV when adding
one and two oxygen adatoms, respectively, instead of decreasing it as observed in experiments.
Based on the above, we conclude that possible presence of oxygen adsorbant on the surface is
not in good agreement either with STM observations or with the observed workfunction
reduction.
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FIGURE S1: Core levels measured by XPS: (a) Hf 4f (b) Se 3d for 6 ML HfSe2,
(c) Mo 3d and (d) Se 3d of 1 ML MoSe2 grown on top of 6 ML HfSe2 and (e) O
1s for 6 ML HfSe2.
The band offsets of the MoSe2/HfSe2 heterojunction were derived from XPS
measurements according to Kraut’s method [6]. Specifically the valence and conduction band
offsets (VBO, CBO) for the heterojunction were calculated using the following equations:
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VBO   EMo3d 5/2  VBM MoSe 2  ( EMo3d 5/2  EHf 4 f 7/2 ) HfSe 2/ MoSe 2   EHf 4 f 7/2  VBM 
CBO   Eg 
where EMo
3d5/2
MoSe 2
  Eg 
and EHf
HfSe 2
4f7/2
HfSe 2
 VBO
are the core level energies, whereas VBM are the corresponding
valence band maxima. The energy differences between the selected core levels and VBM are
measured in bulk samples, as shown in Fig. S2. A VBO value of 0.13 eV was determined for the
HfSe2/MoSe2 heterostructure. Accordingly, the CBO value has been calculated to be 0.61 eV,
using band gap energies (Eg) for MoSe2 (1.58 eV) and HfSe2 (1.1 eV).
FIGURE S2: Energy separations between core levels and VBM as
estimated from XPS spectra, are used to calculate band offsets by
Kraut’s method
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4. Large area STM images
The morphology of HfSe2 films grown on AlN/Si(111) was studied by in situ STM microscopy.
Figure S3 shows large area STM images of 6ML HfSe2 on AlN indicating a smooth surface with
RMS roughness of 0.45 nm for (5x5) μm2 area.
FIGURE S3: STM images of 6ML HfSe2/AlN sample, of 5x5 μm2.
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