2D WSe2 van der Waals Heterostructures - ACS

Aug 18, 2016 - Institute of Nanoscience and Nanotechnology, National Center for Scientific ... National Technical University of Athens, Department of ...
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Epitaxial 2D SnSe2/ 2D WSe2 van der Waals Heterostructures Kleopatra Emmanouil Aretouli,†,‡ Dimitra Tsoutsou,† Polychronis Tsipas,† Jose Marquez-Velasco,†,§ Sigiava Aminalragia Giamini,†,‡ Nicolaos Kelaidis,† Vassilis Psycharis,† and Athanasios Dimoulas*,† †

Institute of Nanoscience and Nanotechnology, National Center for Scientific Research “DEMOKRITOS”, 15310, Athens, Greece University of Athens, Department of Physics, Section of Solid State Physics, 15684 Athens, Greece § National Technical University of Athens, Department of Physics, 15780 Athens, Greece ‡

S Supporting Information *

ABSTRACT: van der Waals heterostructures of 2D semiconductor materials can be used to realize a number of (opto)electronic devices including tunneling field effect devices (TFETs). It is shown in this work that high quality SnSe2/WSe2 vdW heterostructure can be grown by molecular beam epitaxy on AlN(0001)/Si(111) substrates using a Bi2Se3 buffer layer. A valence band offset of 0.8 eV matches the energy gap of SnSe2 in such a way that the VB edge of WSe2 and the CB edge of SnSe2 are lined up, making this materials combination suitable for (nearly) broken gap TFETs. KEYWORDS: 2D semiconductors, SnSe2, WSe2, van der Waals heterostructures, molecular beam epitaxy, tunneling field effect transistors

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INTRODUCTION Two-dimensional materials of the MX2 class (M = Mo, W, V, Nb, Sn, Ti, Zr, Hf, Re and X = S, Se, Te) and their van der Waals (vdW) heterostructures have a great potential to impact diverse areas in (opto)nanoelectronics and spintronics due to the rich physical properties.1 The latter include metallic, semimetallic, and semiconductor behavior, superconductivity, and spin polarized transport. Most of the disulfides and diselenides are semiconductors with band gaps in the 1−2 eV range, offering great flexibility to design electronic and optoelectronic devices. The large variety of band gaps, workfunctions, and electron affinities offer an excellent opportunity to design vdW heterostructures (HS) with very different band alignments. Particularly interesting is the type-II band alignment which allows the fabrication of atomically thin p-n heterojunctions and optoelectronic devices,2,3 based on long-lived excitonic states due to large exciton binding energies and indirect (interlayer) excitons for electric field tunable light emission.4 Moreover, the lack of unsaturated (dangling) bonds at the interfaces allows uninhibited band to band tunneling through a very thin (∼3.3 Å) vdW gap when the valence band of one of the 2D semiconductor is aligned with the conduction band of the other in a type III HS configuration. This can lead to (nearly) broken gap 2D−2D vertical tunnel FETs (TFETs) with subthreshold slopes (SS) less than the thermal limit of 60 mV/dec and large current in the ON state making them suitable for low power electronics with little compromise in performance and with potential advantages over conventional homojunction TFETs5 or HS TFETs based on III−V materials.6 According to theoretical predictions,7 type III HS can be formed by combining group IVB-MX2 metal dichalcogenides (M = Hf, Zr; X = S, Se), with group VIB-MX2 (M = Mo, W; X © XXXX American Chemical Society

= Te, Se). The two classes of semiconductors possess a variety of band gaps and work functions which give us flexibility to design appropriate heterostructures with the desired band alignments. Experimental studies 8 of the HfSe 2 /MoSe 2 candidate combination, however, reveal type II HS with small (∼0.13 eV) valence band offset, which is attributed8 to the observed9,8 small HfSe2 WF of 5.5 eV8 and small WF difference between HfSe2 and MoSe2,8 and hence the electron affinity difference of the two materials is much less than theoretical estimations. An interesting alternative combination involving WSe2/SnSe2 vdW HS has been proposed10 on the basis of device modeling predicting ultrasmall SS ∼ 14 mV in a near broken-gap configuration supported by early experimental findings of molecular beam epitaxy (MBE)-grown SnSe2 on bulk WSe2.11 Recently it has been shown that tin diselenide (SnSe2) can be used for the fabrication of a number of devices such as Esaki tunnel diodes when combined with phosphorene,12 supercapacitor,13 photodetector14 and field effect transistor devices,15,16 all based on exfoliated flakes. While most of the research device work is based on small layers exfoliated from bulk, large area epitaxial growth (e.g., by MBE) is required for real world applications. Following early pioneering MBE work,17−19,11 more recently, the MBE growth of WSe220,21 and other 2D semiconductors (MoSe2,22−24 HfSe29) was reported on “friendly” 2D substrates like HPGO, graphene, and exfoliated MoS2 substrates. On the other hand, MBE growth on bulk (3D) substrates, although challenging, it is technologically very important. Wurtzite epi-AlN(0001)/ Si(111) substrates which are available in 300 mm wafer sizes at Received: March 9, 2016 Accepted: August 18, 2016

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DOI: 10.1021/acsami.6b02933 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ARPES measurements were carried out using a 2D-CCD detector and a He excitation source with He I radiation at 21.22 eV. The energy resolution of the system was better than 40 meV. Photoelectrons emitted by the samples are measured in the energy distribution curve (EDC) mode with a polar angle step of 1°. STM topographies were obtained in UHV (base pressure of low 10−9 mbar) at room temperature using an Omicron Large Sample SPM microscope with a freshly cut Pt/Ir tip. Data acquisition is made with the Omicron MATRIX control system regulating the tunnel current with an active distance feedback loop. SEM measurements were performed using 5 kV at low vacuum. The density functional theory (DFT) calculations were performed using the Vienna Ab Initio Simulation Package35 and projector-augmented waves. The generalized-gradient approximation (GGA)36 with Perdew−Burke−Ernzerhof (PBE)37 parametrization was used as exchange correlation functional. To include vdW corrections, the semiempirical DFT-D3 Grimme’s method36 was applied. Self-consistent force optimizations were performed until the Hellmann−Feynman force between the atoms converged to 5 × 10−5eV/Å. The energy cutoff was set at 500 eV, while the Monkhorst− Pack scheme38 with a k-point grid of 15 × 15 × 1 was chosen. Calculations of the band structure with and without SOC were performed, using for these calculations k-mesh 31 k-point per symmetric line along the KΓM direction.

low cost are suitable for upscaling growth to large area wafers allowing processing according to industry standards for volume production. Following our previous work on the MBE growth of MoSe2, HfSe2, ZrSe2, TaSe2, and their vdW HS on AlN/Si substrates,25−27 we extend our investigations here on the growth of WSe2 and SnSe2 and their vdW HS on the same substrates with and without a Bi2Se3 buffer layer. While epitaxial WSe2 has been studied to some extent,20,21 the epitaxy of SnSe2 is not known in sufficient detail. Unlike most transition metals which have only one oxidation state forming stoichiometric diselenide compounds, Sn has two oxidation states (+2 and +4), so except from the diselenide, the monoselide phase is also possible in several polytypes (orthorhombic Pnma and Cmcm and the rock-salt)28,29 which introduce complexity in the growth efforts and device design. While by using CVD methodologies30−33 it is possible to control the growth of the different Sn selenide phases and polytypes, it may not be easy to do the same with MBE. Moreover, SnSe2 has been reported in two different crystal structures, namely, the 2H with D6h (P63/mmc) symmetry and the CdI2-type 1T phase with D3d (P3̅m1) symmetry (Figure S1), although it is not clear which one is the most stable and most frequently observed. In the present paper optimal growth conditions and substrates are employed demonstrating high quality 1T-SnSe2 phase in combination with WSe2 in a vdW epitaxial configuration.

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RESULTS AND DISCUSSION Raman Scattering Results. Each of the tin and tungsten selenide materials is first grown directly on AlN/Si substrates. Using high Se overpressure such that the evaporation rate ratio is Se/Sn ∼ 15/1, a dominant SnSe2 phase is obtained at an optimum temperature of 225 °C, which is well below the temperature of 270 °C at which the material is found to be thermally unstable dissociating in UHV. On the other hand, the growth of WSe2 on AlN/Si proceeds in two steps. First, a 4 ML WSe2 is deposited at 490 °C followed by post-deposition annealing at 670 °C in order to improve the surface ordering and crystallinity. The Raman spectra of SnSe2 and WSe2 are shown in Figure 1a,b, respectively. In Figure 1a, two dominant peaks at 186.27 and 112 cm−1 agree well with the out-of-plane A1g and in-plane Eg components, respectively, as previously reported14,16 for 1T SnSe2 films with the D3d symmetry, exfoliated from bulk. A rich Raman spectrum is observed (Figure 1b) for WSe2 featuring two pronounced peaks at 249 and 259 cm−1 which can be assigned to E12g (in-plane) and A1g (out of plane) modes of 2H-WSe239 and a number of secondorder peaks in agreement with results reported for bulk chemical vapor transport (CVT)-grown 2H-WSe2.40 As we will discuss later the layers were also grown on Bi2Se3, which however makes the Raman inconclusive (Figure S4). Epitaxial Orientation Probed by Reflection High Energy Electron Diffraction. The RHEED patterns as recorded along the [112̅0] azimuth (Figure 2a) indicate that there is a preferential azimuthal alignment between the SnSe2 or WSe2 epilayers and the substrate. However, it can be inferred from the diffused diffraction streaks that the films are of inadequate crystalline quality, although well-resolved X-ray diffraction (XRD) peaks acquired in Bragg−Brentano mode (Figure S5) confirm an epitaxial alignment along the growth direction such that [001] WSe2(SnSe2)//[0001]AlN. Then, a good quality 5 QL Bi2Se3 buffer layer was first grown epitaxially on AlN41 followed by SnSe2 growth at 200 °C and a top layer WSe2 (Figure 2b) at T = 210 °C. Growth in reverse order is also performed where a WSe2 layer is first grown at T = 250 °C on Bi2Se3 buffer, followed by a top SnSe2 layer at T = 225 °C (Figure 2c). It should be noted that the growth is necessarily kept at low temperature because of the instability of

EXPERIMENTAL SECTION

The epitaxial growth of WSe2 and SnSe2 thin films performed by MBE on AlN/Si(111) substrates. Initially the substrates were chemically cleaned to remove residual surface organic contaminants and then an in situ degassing followed at 730 °C in the MBE chamber. High purity metals W and Sn were evaporated from an e-gun evaporator, whereas Bi and Se were supplied from k-effusion cells. A rate/thickness monitor is used in our chamber which measures the deposition rate in units of Å/s. In our WSe2 and SnSe2 growth experiments, the deposition rate is determined by the deposition rate of the metal which is 0.2 Å/s for both W and Sn (both evaporated using e-gun). The Se kcell is heated at 185 °C, generating a pressure of 5 × 10−7 Torr during the growth. This creates conditions of Se overpressure to ensure that Se is supplied in sufficient quantities so that it can be incorporated in the film efficiently to form the stoichiometric compound despite its low (≪1) sticking coefficient at growth temperatures higher than 220 °C. The evaporation rate ratio is Se/Sn(W) ∼ 15/1 in both cases. The thickness of the epilayers was determined using the rate monitor which was calibrated using thick samples (8 ML) directly grown on AlN/Si(111) substrates whose thickness was measured by X-ray reflectivity (XRR) using a Cross Beam Optics (CBO) unit and Cu Kα radiation. The analysis of measurements was performed using the Global Fit program.34 At the early stage of analysis the Global optimization method was used and the final models were obtained using the nonlinear least-squares fitting method (Figure S2). Finally, the thicknesses of the thin films (4 ML nominal) were measured by spectroscopic ellipsometry (SE) which is more sensitive compared to XRR at small thicknesses. It is confirmed that the thickness was close to the nominal within the uncertainties of the measurement (see Supporting Information Figure S3 and Table S1). Deposited samples were structurally and physically characterized by means of in situ reflection high energy electron diffraction (RHEED), Raman, X-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES) techniques, UHV-scanning tunneling spectroscopy (STM), and scanning electron microscopy (SEM). Raman spectroscopy was performed using a 514.5 nm laser beam with a power of ∼1.2 mW. XPS spectra were collected with a PHOIBOS 100 (SPECS) hemispherical analyzer, at a pass energy of 15 eV. The takeoff angle was set at 37° relative to the sample surface. Gaussian− Lorentzian shapes (Voigt functions) were used for deconvolution of the recorded spectra after standard Shirley background subtraction. B

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and hexagonal SnSe2 and WSe2.41−43 It can be seen from the same figures (Figure 2 and Figure S6) that all layers are azimuthally aligned despite their large lattice mismatch, indicating vdW epitaxial growth for all three layers independent of the growth order. It is also evident from the RHEED pattern that the tin selenide adopts a hexagonal structure which excludes the possibility to have a monoselenide (SnSe) phase with the orthorhombic crystal structure. Surface Morphology by Means of UHV-STM and SEM. The films’ microstructure was studied using UHV-STM at room temperature, probing areas from 120 × 120 nm2 to 1 × 1 μm2 (Figure 3) for the grown WSe2/Bi2Se3 and SnSe2/Bi2Se3 thin films. In the case of SnSe2, the bottom layer (darker contrast in Figure 3c) becomes complete before the next layer is formed on top indicating a layer by layer island growth. The top layer image shows that smaller 2D islands join to form bigger ones (joints shown by arrows) until they cover the full area. In Figure 3e, the profile along the line in Figure 3c is shown, reflecting a step of 6.1 Å from the top layer to the layer underneath, in line with the c lattice constant of SnSe2 crystal. The rms roughness is less than 0.4 nm in all magnifications. In the case of WSe2, grains are visible in the high magnification (200 × 200 nm2) which maintain the 2D character, although some defects, e.g., holes of 40 nm in diameter and 6 nm in depth, and some particles of unidentified origin are observed. The corresponding rms roughness is less than 0.61 nm. The surface morphology of SnSe2/WSe2/Bi2Se3 heterostructures in larger scale was studied using SEM, probing areas from 55 × 55 μm2 to 1500 × 1500 μm2 (Figure S7). It turns out that the films are continuous and crack-free all over the wafer. It should be noted that the defects could be more critical in the case of devices with lateral transport, while in the case of vertical transport tunneling devices such as TFETs considered here, their influence is less critical. Electronic Band Structure Imaging by ARPES. The electronic band structure of 4 ML SnSe2/Bi2Se3/AlN/Si is imaged by ARPES and compared with DFT band structure calculations of the CdI2−SnSe2 phase with D3d symmetry as shown in Figure 4. With the exception of minor details (see Discussion below) a very good agreement between the experiment (Figure 4a) and theory (Figure 4c) is observed along two different crystallographic directions of high symmetry (Figure 4b), indicating oriented films of good crystalline quality, confirming also the predominant growth of the 1TSnSe2 phase. It should be noted that the 2H-SnSe2 with D6h symmetry and the orthorhombic SnSe phase (Pnma) have distinctly different band structure, which does not match our findings (Figure 4a). In summary, the experimental results (RHEED, Raman, ARPES) indeed support the formation of a dominant 1T SnSe2 phase, even though the existence of other phases in much smaller quantities and beyond the detection limits of our techniques cannot be excluded. The conduction band (CB) is not observed in the image of Figure 4a; however, a closer look at the line scan (Figure 4d) obtained at a wavevector kx = 0.9A−1 corresponding to the Mpoint of the surface Brillouin Zone (BZ) reveals a weak peak near the Fermi energy, exactly where the CB minimum (CBM) is expected according to theory (Figure 4c). We attribute this weak peak to the onset of photoemission as the Fermi level crosses the bottom of the CB. Our claim is further supported by the enhanced contrast image presented in Supporting Information (Figure S9) showing a faint photoemission signal from the CB at the M-point. By comparing with the lines can at

Figure 1. Raman spectrum of the MBE deposited SnSe2 and WSe2samples. Part a shows the two characteristic peaks at 186.27 and 112 cm−1 corresponding to the A1g and Eg active modes of 1TSnSe2. Part b shows two pronounced peaks at 249 and 259 cm−1 which correspond to E12g (in-plane) and A1g (out of plane) modes of 2HWSe2 and a number of second-order (2M) peaks. A peak arising from the Si substrate is also visible.

Figure 2. RHEED patterns along [112̅0] azimuth of (a) 4 ML WSe2/ AlN(0001)/Si(111) (top) and 4 ML SnSe2/AlN/Si(111) (bottom) grown separately on AlN substrates (middle); (b) 5QL Bi2Se3/AlN (top), 4 ML SnSe2/Bi2Se3 (middle) showing improved crystal quality, and 4 ML WSe2/SnSe2 (bottom); (c) 5QL Bi2Se3/AlN, 4 ML WSe2/ Bi2Se3 (middle), and 4 ML SnSe2/WSe2 (bottom). Despite the large lattice mismatch between the different epitaxial layers and the substrate, all RHEED streaks are azimuthally aligned in the plane.

Bi2Se3 and SnSe2 layers at higher temperatures. The Bi2Se3 template is a layered metal selenide material, chemically and structurally compatible with SnSe2 and WSe2 epilayers. It is also lacking unsaturated (dangling) bonds, which promotes vdW epitaxy of other 2D materials. Thus, the epilayers grown on Bi2Se3 have substantially improved crystallinity compared to those directly deposited on AlN as can be seen from the sharper diffraction streaks in Figure 2b and c. The RHEED patterns recorded along the [101̅0] azimuth are provided in the SI (Figure S6). From the positions of the streaks relative to AlN substrate (a = 3.11 Å) we deduce the lattice constant values of the epilayers which are found to be a(Bi2Se3) = 4.12 Å, a(SnSe2) = 3.76 Å, and a(WSe2) = 3.25 Å, in very good agreement with the lattice constants of rhombohedral Bi2Se3 C

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Figure 3. UHV-STM imaging at room temperature of WSe2/Bi2Se3/AlN films probing areas (a) 200 × 200 nm2 and (b) 1 × 1 μm2, and SnSe2/ Bi2Se3/AlN films probing areas (c) 120 × 120 nm2 and (d) 1 × 1 μm2. In (e) the profile along the line in image (c) is depicted, reflecting a step of 6.1 Å (1 ML) from the top layer to the layer underneath, in line with the c lattice constant of SnSe2 crystal.

the kx = 0 (Γ-point), we infer an indirect energy gap Eg of ∼0.8 eV. It should be noted that the experimental valence band maximum (VBM) is found to be at the Γ point (Figure 4a,d) which differs from theoretical results according to which the VBM is predicted to be at a point between Γ and M, near Γ (Figure 4c and ref 13). This discrepancy may be explained considering the influence of the substrate on the SnSe2 epilayer. The experimental Eg value is substantially larger compared to theoretically predicted Eg ∼ 0.4 eV (Figure 4c) which is attributed to the well-known problem of band gap underestimation by DFT. The experimentally determined Eg in this work is also slightly smaller than the experimentally reported value of ∼1 eV44−47which may be attributed to different physical properties of our films grown by MBE. The Fermi level (Figure 4d and Figure S9) is located ator just abovethe bottom of the conduction band, indicating a strongly n-type material, which is in fair agreement with previous reports of unintentionally n-type doped SnSe215,44,47 although the origin of the unintentional doping is unknown. Band Lineups. The band alignments of SnSe2/WSe2 heterostructure on Bi2Se3/AlN/Si substrates were investigated

and found to be nearly independent of the growth order between the SnSe2 and WSe2 on Bi2Se3. We will present our band alignment analysis for the heterostructure which is grown in the order shown in Figure 2b (first SnSe2 grown on Bi2Se3, then WSe2 grown on top). The WSe2 is first grown thin (∼1 ML) and the thickness is sequentially increased up to 8 ML. The band alignment configuration does not change as the thickness increases. The energy position of the core levels and the valence band maxima obtained from photoelectron spectroscopy data (Figure 5a−c) are used to determine the HS valence and conduction band offsets (VBO, CBO) as in the case of other MX2/M′X′2 HS systems,48 according to Kraut’s method49 using the following expressions. VBO = (E W4f7/2 − VBM)WSe2 + (ESn3d5/2 − E W4f7/2)SnSe2/WSe2 − (ESn3d5/2 − VBM)SnSe2 D

(1)

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Figure 4. (a) ARPES valence band imaging using He I excitation of 21.22 eV along the high symmetry directions K/H-Γ/A-M/L of the Brillouin zone for a 4 ML SnSe2/Bi2Se3/AlN/Si(111) sample. The arrow indicates the indirect gap from Γ point (experimental maximum of the valence band) to M-point (minimum of conduction band), (b) Brillouin zone for hexagonal lattice system, (c) DFT band structure calculations of 4 ML SnSe2 freestanding slab with spin orbit coupling, and (d) line scans of the SnSe2/Bi2Se3/AlN/Si(111) sample, recorded at both the Γ (kx = 0) and M (kx = 0.9 Å−1) points of the surface BZ. Eg is the indirect energy gap, while CBM and VBM denote the conduction band minimum and valence band maximum, respectively.

CBO = (Eg )WSe2 − (Eg )SnSe2 + VBO

CBM in accordance with the observed VB edge (Figure 5a,b) and CB edge positions (Figure 5d). The vacuum level is positioned with respect to the Fermi level taking into account the measured workfunctions and is found to be nearly the same in both sides of the heterostructure, yielding electron affinities χ = 3.8−4.1 eV and 5.43−5.23 eV for WSe2 and SnSe2 close to previously published experimental values.11 The small difference δvac ∼ 0.11 V in the vacuum level indicates a nearly ideal heterostructure with little charge transfer and very small dipole moment built at the interface. In good agreement with predictions,10 WSe2/SnSe2 HS forms a broken or near-broken gap band alignment depending on the Eg(SnSe2) used, yielding ΔECV ∼ 0−0.2 eV (Figure 5e). It should be noted that the experimentally determined band alignment in the present work may have been influenced by elastic deformations due to lattice mismatch and weak vdW interactions as theoretically studied for different 2D HS systems.56 The (near) broken band alignment of WSe2/SnSe2HS could be an ideal situation for the TFET operation. Pending a verification on the basis of electrical evaluation, the band alignments allow for an increase of the WSe2 VB above the CB of SnSe2 with only a small negative gate bias applied on the WSe2 side, thus producing a large band to band tunneling current turning the TFET on. The complete device layer structure analyzed in this work and a device layout in cross section are schematically illustrated in Figure S10a,b. The processing of a TFET as in Figure S10b is a demanding task that requires additional research and development which is

(2)

From eq 1, the VBO is estimated to be 0.8 eV. Accordingly, from eq 2 the CBO value is calculated to be 1.6−1.3 eV, using WSe2 band gap energy values Eg in the range of 1.3−1.6 eV as reported in the literature10,50,51 and SnSe2 Eg of 0.8 eV, as measured in the present work (Figure 4d). Note that in WSe2 samples the Fermi level lies about 0.3 eV above the valence band maximum (Figure 5b), as deduced from XPS measurements for thick (8 ML) WSe2 samples, so by taking into account the reported band gap of WSe2, it turns out that the Fermi level lies closer to the valence band and therefore it has a p-type conductivity. This conclusion extracted from our data is supported by the experimental findings from other teams.52−54 The latter published works suggest that WSe2 comes out as unintentionally p-type doped semiconductor. The p-type nature of WSe2 has been previously related to intrinsic defects such as selenium interstitials.55 A slightly smaller CBO value of 1.4−1.1 eV is obtained, taking into account the literature reported band gap value Eg ∼ 1 eV for SnSe2.44−47 Finally, the workfunctions (WF) are measured from the low energy electron cutoff in the UPS spectra (Figure 5d) and found to be 5.35 and 5.1 eV for SnSe2 and WSe2, respectively. The constructed band alignment at the WSe2/SnSe2 heterointerface is given in Figure 5e. The Fermi level is well aligned through the heterointerface as expected for two layers in thermodynamic equilibrium, while it is found to be located about 0.3 eV above the WSe2 VBM and only about 0.07 eV above the SnSe2 E

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Figure 5. Band alignment of the WSe2/SnSe2 heterostructure (a) XPS of thick SnSe2, (b) XPS of thick WSe2, and (c) XPS of thin SnSe2/ WSe2samples. (d) Low-energy electron cutoff for the epitaxially grown SnSe2 and WSe2 samples, as deduced from UPS spectra. (e) Schematic of the WSe2/SnSe2 band alignments as reconstructed by XPS core level analysis. χ and EF denote the electron affinity and the Fermi energy, respectively. Eg is the energy gap while CBO and VBO denote the conduction and valence band offsets, respectively. The shaded areas denote the energy range of variation of the conduction band minimum in both materials due to the different reported Eg values in the literature.

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discussed in the SI. We mention here some of the biggest challenges. First, the top and bottom 2D semiconductor layers must be selectively etched, isolated by oxide depositions, and then contacted for source and drain definition. Second, processing of low resistivity contacts for such epitaxial layers is not established. Third, appropriate gate dielectric and deposition methodology has not been identified. Finally, the limited thermal and chemical stability of some of the layers could significantly complicate the transistor fabrication process. These processing difficulties, though, cannot be regarded as showstoppers and difficulties can be overcome in the future to obtain functional devices. As evident from the discussion above, the Bi2Se3 buffer layer is necessarily introduced here in order to ensure a good crystalline quality of the heterostructure. However, Bi2Se3 is conductive, since it is low gap (0.3 eV) material, and being a topological insulator, it possesses metallic surface states;41 therefore, Bi2Se3 could possibly serve as a bottom electrode in TFET device implementation (Figure S10), although other options can be identified based on intensive research efforts in the future.

CONCLUSIONS

In this work SnSe2/WSe2 van der Waals heterostructures are epitaxially grown by MBE on technologically important AlN(0001)/Si(111) substrates using a thin Bi2Se3 buffer layer to improve the crystal quality of the layers. Although tin selenide can be found in different crystal structures and stoichiometries, optimized growth conditions are employed in this work to obtain single phase material, namely, the 1TSnSe2polytype. This phase, as grown by MBE, has an indirect energy gap around 0.8 eV while the Fermi level is located at the conduction band edge indicating a strongly n-type unintentionally doped semiconductor. On the other hand, the MBEgrown 2H WSe2 has a Fermi level lying about 0.3 eV above the valence band maximum behaving as a p-type semiconductor. When the two materials are combined, the conduction and valence band edges of SnSe2 and WSe2, respectively, are lined up in a (nearly) broken gap configuration implying that band to band tunneling through an ultrathin van der Waals gap can be switched on and off easily upon application of a small bias across the interface. This defines a suitable device layer structure for the implementation of 2D−2D vertical TFETs. F

DOI: 10.1021/acsami.6b02933 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02933. XRR and SE measurements, Raman spectra, RHEED pattern recorded along [101̅0] azimuth, schematic representation of 1T-SnSe2 crystal structure, DFT band structure calculations, enhanced contrast ARPES image, illustration of the proposed device layer structure, and SEM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the ERC Advanced Grant SMARTGATE-291260. The authors thank Dr. G. Kordas for making the Raman set up available to us, and Mr. A. Zeniou and Dr. E. Gogolides for spectroscopic ellipsometry measurements. Finally, we acknowledge IMEC for providing the AlN/Si substrates.

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ABBREVIATIONS TFET, tunnel field-effect transistors; SS, subthreshold slope; HS, heterostructure; vdW, van der Waals; TMDs, transitionmetal dichalcogenides; BZ, Brillouin zone; VBO, valence band offset; CBO, conduction band offset; MBE, molecular beam epitaxy; vdW, van der Waals; XPS, X-ray photoelectron spectroscopy; XRR, X-ray reflectivity; RHEED, reflection high energy electron diffraction; ARPES, angle-resolved photoelectron spectroscopy; PBE, Perdew−Burke−Ernzerhof; GGA, generalized-gradient approximation; SOC, spin−orbit coupling

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DOI: 10.1021/acsami.6b02933 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX