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Evidence for Highly p-type doping and type II band alignment in large scale monolayer WSe2 /Se-terminated GaAs heterojunction grown by Molecular beam epitaxy

Two-dimensional materials (2D) arranged in hybrid van der Waals (vdW) heterostructures provide a route toward the assembly of 2D and conventional III-V semiconductors. Here, we report the structural and electronic properties of single layer WSe2 grown by molecular beam epitaxy on Se-terminated GaAs(111)B. Reflection high-energy electron diffraction images exhibit sharp streaky features indicative of a high-quality WSe2 layer produced via vdW epitaxy. This is confirmed by in-plane x-ray diffraction. The single layer of WSe2 and the absence of interdiffusion at the interface are confirmed by high resolution X-ray photoemission spectroscopy and high-resolution transmission microscopy. Angle-resolved photoemission investigation revealed a well-defined WSe2 band dispersion and a high p-doping coming from the charge transfer between the WSe2 monolayer and the Se-terminated GaAs substrate. By comparing our results with local and hybrid functionals theoretical calculation, we find that the top of the valence band of the experimental heterostructure is close to the calculations for free standing single layer WSe2. Our experiments demonstrate that the proximity of the Se-terminated GaAs substrate can significantly tune the electronic properties of WSe2. The valence band maximum (VBM, located at the K point of the Brillouin zone) presents an upshifts of about 0.56 eV toward the Fermi level with respect to the VBM of WSe2 on graphene layer, which is indicative of high p-type doping and a key feature for applications in nanoelectronics and optoelectronics.

Two dimensional (2D) materials have attracted much attention thanks to their tunable electronic properties [1][2][3] . So far, a large body of research has been devoted to evolution of the quasiparticle bandgap as a function of the number of layers, doping and strain in the 2D transition metal dichalcogenides (TMDs) 4-6 . Despite promising physical properties, their integration in optoelectronics devices can be difficult because the fabrication of standard p-n junction requires the control of the carrier density, which is difficult to achieve by substitutional doping with electron acceptors or donors in 2D materials obtained by direct growth 7-10 . The presence of defects, such as chalcogen vacancies, may induce gap states which are responsible for the "as-grown" 2D material doping (i.e., "defect doping"). For example, S vacancies have been reported to act as electron donors in MoS 2 and WS 2 , resulting in a n-type character of the "as-grown" material 11,12 . Similar n-type doping is present in PtSe 2 and WSe 2 due to the presence of Se vacancies 13,14 . The manipulation of the chalcogen vacancies, via hydrogen passivation, provides a route to modulate the carrier concentration of such chalcogen-based monolayers 15, 16 . Other attempts to convert n-type "as-grown" TMDs into p-type semiconductors have been reported, via the creation of cation vacancies (e.g. Mo or W vacancies) 17 or their substitution with other metal atoms (e.g. Nb atoms or V) 18,19 . Alternatively, p-type doping in TMDs can be achieved using surface adsorption or intercalation of electron-accepting atoms or molecules [20][21][22] . However, all these techniques present several limitations related to the operational stability of these 2D materials in real devices 23 .
Interestingly, the growth of 2D materials on conventional 3D semiconductors results in hybrid 2D/3D heterostructures, which can bring some advantages of the more established 3D semiconductors while retaining some specificities of the 2D materials. An interesting combination is that of GaAs, a mature 3D semiconductor with applications spanning from light emitting diodes to high power electronics 24 , with 2D materials such as transition metal mono-and di-chalcogenides 25-27 28 . Although several research works have focused on the growth and fabrication of such heterostructures [29][30][31][32] , most of the resulting interfacial and electronic properties have not been specifically investigated. Thus, we dedicate this paper to the investigation of the electronic properties of 2D TMD WSe 2 and 3D GaAs, with a 2D/3D heterostructure formed at the GaAs(111)B surface 33 .
In this work, we report high p-type doping stabilized in the monolayer WSe 2 via an electron transfer from the WSe 2 to the Se-terminated GaAs substrate. A large area high quality crystalline single layer WSe 2 film was grown by molecular beam epitaxy (MBE) on Se-terminated GaAs(111)B substrate. The thickness of the WSe 2 layer was controlled thanks to a quartz crystal microbalance, by monitoring the amount of deposited W. Highresolution x-ray photoemission spectroscopy (HR-XPS) and high resolution scanning transmission electron microscopy (HR-STEM) investigations indicate that the Se-termination of the GaAs surface weakens the interaction between the TMD film and the substrate 32 , resulting in a van der Waals (vdW) epitaxy of WSe 2 in absence of any secondary compound detected at the 2D/3D interface. We find that WSe 2 grown on Seterminated GaAs has the same structural properties than monolayer WSe 2 obtained through other means.
However, we notice a significant upshift of the WSe 2 valence band maximum (i.e. p-type doping), compared to n-type WSe 2 obtained on graphene substrate. Moreover, ARPES measurements reveal that the WSe 2 /Seterminated GaAs heterostructure has a type-II band alignment, which is particularly interesting for applications in optoelectronic devices. This experimental study is crucial for understanding such heterostructures in a prospect of real applications, knowing that scaling up devices and processes based on mechanically exfoliated micrometer-sized flakes remains challenging.

Results and discussions:
A monolayer thick WSe 2 film was grown on a GaAs(111)B substrate by MBE. Prior to the WSe 2 growth, the GaAs(111)B surface was passivated with Se in the MBE reactor. During this surface passivation step, Se atoms substitute surface As atoms, bonding covalently with the underlying Ga atoms to form a top uniform Se layer with no dangling bonds 34,35 . The atomic structure of the Se-passivated GaAs(111)B surface and the grown WSe 2 layer are presented in figures 1(a) 36,37 . In figure 1 (b), we can show the typical RHEED patterns obtained after the WSe 2 growth on the Se-terminated GaAs(111)B surface along two azimuth. The persistence of streaky RHEED patterns after the WSe 2 growth indicates a well-ordered and flat epitaxy on GaAs, with a uniform WSe 2 layer. The RHEED patterns further indicate that the hexagonal cell of WSe 2 aligns with the three-fold surface  figure S1, the orientation distribution of WSe 2 and X layers reach 3.3° and 2.7° respectively, confirming the good epitaxy.
A macroscopic analysis of the chemical composition of the WSe 2 /Se-terminated GaAs heterostructure was performed using HR-XPS at the synchrotron facility (TEMPO beamline, SOLEIL, France). HR-XPS spectra recorded at a photon energy hv = 120 eV for Se 3p, As 3d, W 4f and Ga 3d are shown in Figure 1 respectively. The spectra were analyzed using the curve fitting procedure described in the Methods section. The experimental data are displayed as black circles. The solid blue line is the envelop of the fitted components. The Ga 3d peak (Figure 1(g)) presents a sharp doublet, characterized by Ga 3d 5/2 and Ga 3d 3/2 peaks with a spin orbit splitting (SO) of 0.44 eV and a branching ratio of 1.5. The Ga 3d 5/2 peak located at a binding energy (BE) of 19.0 eV corresponds to the Ga-As bond in GaAs. The other broad doublet with the Ga 3d 5/2 peak at BE = 19.6 eV is related to the Ga-Se bond 34 at the interface of the heterostructure. The As 3d peak (Figure 1(e)) is composed of a single doublet (As 3d 5/2 and As 3d 3/2 with SO = 0.7 eV and branching ratio of 1.5) at BE As 3d5/2 = 40.9 eV attributed to As bounded to Ga 38 . In the Se 3d core level (Figure 1(d)), we observe two doublets (Se 3d 5/2 and Se 3d 3/2 with SO = 0.86 eV and branching ratio of 1.5). The high intensity doublet Se 3d 5/2 at BE= 54.1 eV corresponds to the Se atoms embedded in the WSe 2 monolayer 39,40 , while the second Se 3d 5/2 at lower BE= 53.4 eV corresponds to the Se bounded to Ga at the interface 41 . The W 4f peak shown in Figure 1(f) presents a main doublet (W 4f 7/2 and W 4f 5/2 with a SO= 2.15 eV and branching ratio of 1.3) at BE = 31.8 eV, corresponding to stoichiometric WSe 2

40
. We find no additional signature from other compound; i.e. no carbon or oxygen related bonds 42,43 , which indicates that the surface and the heterostructure are free from contamination. In particular, the absence of covalent As-Se bound or W-Ga bounds points toward a clean and sharp vdW interface between the Se-terminated GaAs(111)B surface and the WSe 2 monolayer. bottom Se sub-layer of WSe 2 and the top Se-terminated GaAs layer is 3.35 Å, which is very close to the standard vdW gap between consecutive layers in bulk WSe 2 (3.37 Å) 44 . Combined with the absence of covalent bond from the XPS data, the spacing from the HR-STEM image points to a vdW bonding at the interface. Therefore, the WSe 2 /Se-terminated GaAs(111)B heterostructure is indeed a mixed 2D/3D vdW heterostructure. We also observed terraces in the underlying GaAs (111)B surface with clear individual steps (Figure 2(b)). Nevertheless, WSe 2 above these steps is continuous and retains a thickness of one monolayer (Figure 2(b)). The atomic structure is resolved on both upper and lower terraces, showing that the WSe 2 lattice orientation remains unaffected by the stepped surface. Such arrangement has already been observed in other 2D/3D material heterostructures, such as graphene layers obtained on vicinal SiC(0001) substrates 45 . The persistence of the WSe 2 lattice orientation and thickness over a surface which is not perfectly flat indicates the possibility of uniform large scale WSe 2 growth using MBE. Such atomically resolved HR-STEM also gives a direct evidence of the "quasi"-vdW gap formed between the Se-terminated GaAs crystal and the 2D layered material. Insight into the WSe 2 electronic structure was obtained using angle resolved photoemission spectroscopy (ARPES) at the TEMPO beamlines (SOLEIL synchrotron facility, France). In Figure 3  Combined with the good agreement with the DFT, we can conclude that the WSe 2 band structure in both heterostructures (with VBM WSe2 at K) is the same than the one of pristine direct-bandgap monolayer WSe 2 . transfer operates at the interface, its creates a permanent interface dipole, which is characterized by a potential step 52 . This potential step can be estimated as the difference between the work functions of the pristine Seterminated GaAs(111)B substrate [53][54][55][56][57] and of the heterostructure. The latter is experimentally determined via the measurement of the secondary electron (SE) edge 4.90 ± 0.05 eV ( Figure S5). We obtain a value of = 0.30± 0.15 eV, indicating the presence of an electric dipole. On the basis of these results, we are able to determine all the respective band offsets and we can draw the complete electrical schematic of WSe 2 /Se-terminated GaAs(111)B heterostructure in Figure 5.
Considering the known values of the quasiparticle bandgaps for WSe 2 (Eg WSe2 = 2.00±0.05 eV where the error bar takes into account the variation of the quasiparticle band gap and exciton binding energy induced by different environmental dielectric constants, i.e., different substrates) [58][59][60][61] and GaAs (E GaAs = 1.50±0.02 eV, for GaAs we considered the band gap value at low temperature T = 50K) 62,63 , we obtain a conduction band discontinuity ∆Ec = ∆Ev + (Eg WSe2 -Eg GaAs ) = 0.60 ± 0.09 eV, where ∆Ev = 0.10 ±0.02 eV is the valence band discontinuity. As shown in Figure 5, the final arrangement of the WSe 2 /Se-terminated GaAs(111)B heterojunction exhibits a staggered gap between WSe 2 and GaAs, and qualifies as a type-II band alignment.

Conclusion:
In summary, we have demonstrated the growth of single layer WSe 2 on Se-terminated GaAs(111)B using MBE to obtain a mixed vdW 2D/3D heterostructure. The WSe 2 monolayer exhibits continuity on terraces and step edges, indicating the possibility of growing large scale TMDs on GaAs using MBE. ARPES measurements revealed that the WSe 2 monolayer exhibits a high p-type doping with a VBM located at the K point. We directly observe a strong spin-orbit splitting at the K point of 0.45±0.02 eV, in excellent agreement with the DFT calculations on freestanding material. The ARPES measurements on the heterostructure shows that the WSe 2 related bands largely retain their original electronic structure and that freestanding WSe 2 monolayer is a good approximation of the electronic properties close to the top of the valence band. In our 2D/3D heterostructure with a sharp van der Waals interface, the majority carriers in the 2D material mostly depends on the respective material band offset. Compared to other fabrication techniques, such a mixed 2D/3D heterostructure offers a natural solution to tailor the majority carrier type and density in the 2D layer. Here, we are able to induce a substantial p-type doping in WSe 2 , via charge transfer to GaAs across the WSe 2 /Se-terminated GaAs(111)B interface. This first experimental realization of stable and highly doped p-type WSe 2 monolayer on a large-area epitaxial GaAs(111)B substrate is likely to apply to other TMDs and 2D materials, to broaden their use in future nanoelectronic and optoelectronic applications in a scalable fashion.

Work Function measurements:
The work function (WF) of the sample was determined by the measurement of the secondary electron (SE) edge with a photon energy of 120 eV. In order to ensure that the SE has a kinetic energy (KE) higher than the analyser vacuum level, the sample is negatively biased (-18 V) with respect to the analyser. The secondary electron energy distribution curve is shown in Figure S5 as a function of the kinetic energy referenced at the Fermi level. Consequenly, the secondary electron cut-off (KE cutoff ), measured by extrapolating the edge of the peak to the zero baseline ( Figure S5) gives directly the value of the workfunction, which is found to be 4.90 ± 0.05 eV. Figure S5: Secondary electron intensity as a function of the kinetic energy above the Fermi level for the WSe 2 / Se-terminated GaAs heterostructure measured with hν = 120 eV.