Near-ideal van der Waals rectifiers based on all-two-dimensional Schottky junctions

The applications of any two-dimensional (2D) semiconductor devices cannot bypass the control of metal-semiconductor interfaces, which can be severely affected by complex Fermi pinning effects and defect states. Here, we report a near-ideal rectifier in the all-2D Schottky junctions composed of the 2D metal 1 T′-MoTe2 and the semiconducting monolayer MoS2. We show that the van der Waals integration of the two 2D materials can efficiently address the severe Fermi pinning effect generated by conventional metals, leading to increased Schottky barrier height. Furthermore, by healing original atom-vacancies and reducing the intrinsic defect doping in MoS2, the Schottky barrier width can be effectively enlarged by 59%. The 1 T′-MoTe2/healed-MoS2 rectifier exhibits a near-unity ideality factor of ~1.6, a rectifying ratio of >5 × 105, and high external quantum efficiency exceeding 20%. Finally, we generalize the barrier optimization strategy to other Schottky junctions, defining an alternative solution to enhance the performance of 2D-material-based electronic devices.

Raman spectroscopy was employed to characterize the components and interface coupling quality of the 1T′-MoTe2/MoS2 Schottky junctions in Supplementary Fig. 2b. Firstly, the frequency difference between A1g and E 1 2g modes in Supplementary Fig. 2b is ~17.5 cm -1 , which also proves the MoS2 film is a single layer 1,2 . Secondly, compared to the isolated MoS2, the E 1 2g and A1g peaks of the MoS2 in the overlapped region have been softened and stiffen to varying degrees. This feature indicates that there is a strong interlayer coupling effect at the 1T′-MoTe2/1H-MoS2 interface. Otherwise, these peaks won't shift 3 . The out-of-plane E 1 2g peak in the overlapped region exhibited a prominent redshift (~0.7 cm -1 ) relative to that of the isolated MoS2, which can be ascribed to the thermal lattice mismatch of the strong interlayer coupling effect (or vdWs force) between the top and bottom layers 4 . The blue-shift of ~0.5 cm -1 of the in-plane vibrational mode A1g is attributed to the occupation of anti-bonding states in the conduction band of MoS2 by the electron concentration transfer from MoS2 to 1T′-MoTe2 5 .
The occupation of anti-bonding states reduces the total electronic energy of the system, enhancing the Mo-S bonds and eventually stiffening the Raman mode. To further characterize the electronic structure of the 1T′-MoTe2, a KPFM was employed to characterize the surface potential in Supplementary Fig. 3c. The contact potential difference (CPD) between the AFM tip (Pt/Ir coated tips) and the sample is defined as 1, 6 : where (5.2-5.6 eV), , and are the work functions of the tip and sample, and the elementary charge, respectively. Bases on the work function of 5.10 eV of Au substrate, the work function of other materials can be calculated according to the following formula: The work function of 1T′-MoTe2 film is extracted as approximately 4.83 eV. Besides, the surface potential of the metallic 1T′-MoTe2 is independent of the film thickness in Supplementary Fig. 3c, which is completely different from semiconducting TMDCs 7 . The principle of the SVSH is that the SVs are healed spontaneously by the sulfur adatom clusters on the monolayer MoS2 surface through an acid-induced hydrogenation process in Supplementary Fig. 5a. To clarify the atomic structure variation, STEM was employed to determine the defect concentration variation of monolayer MoS2 before and after healing at the atomic scale. As the intensity of STEM images is directly related to the atomic number (Zcontrast), SVs (1S) and sulfur adatom clusters can be visualized and differentiated from the three-fold coordinated two sulfur atoms (Supplementary Fig. 5b and 5c). Besides, to quickly and efficiently distinguish these lattice defects, the interference of Mo atoms was filtered in filtered STEM images without Mo atoms in Supplementary Fig. 5b and S5c bottom. Disordered regions, in which contrast is significantly lower than the nearest six S atom sites, can be considered as SVs. While disordered regions, which contrast is significantly higher than the nearest six S atom sites, refer to as sulfur clusters. This contrast fluctuation was also confirmed by the extracted Z-value mapping in Supplementary Fig. 5d.
Compared with the as-prepared MoS2, both the SV and sulfur cluster concentrations of the healed MoS2 showed a significant decrease in Supplementary Fig. 5b and S5c. Thus, the reduction of SV concentration in healed MoS2 can be attributed to the acid-induced SVSH effect of the PEDOT:PSS solution 6 . According to a large number of data statistics, the S/Mo ratios in monolayer MoS2 before and after treatment are ~1.85 and ~1.92, respectively. While the 1T′-MoTe2/MoS2 metal-semiconductor was being constructed, the 1T′-MoTe2 and MoS2 field-effect transistors were also being simultaneously constructed to remove the competing interferences from other electrode contacts in Supplementary Fig. 7a. The linear current-voltage relationships in Supplementary Fig. 7b-7c also show that the Cr electrodes and MoS2 can remain Ohmic contacts both before and after the SVSH. The removal of SVs makes the threshold voltage of the MoS2 FET close to zero in Supplementary Fig. 7d, illustrating the electron concentration is significantly lowered. The following formula is used to quantitatively calculate the electron concentration N2D of monolayer MoS2 10 : where Ci =1.15×10 -4 Fm -2 is the gate capacitance of the 300 nm SiO2 dielectric layer, VG is the gate voltage, VTH is the threshold voltage, and q is the elementary charge. With the gate voltage sweeping from positive to negative, the 1T′-MoTe2/as-prepared MoS2 Schottky junction will transform the behavior from Ohmic to rectifying in Supplementary Fig. 8a. However, the 1T′-MoTe2/healed MoS2 Schottky junction shows obvious rectifying behavior in the full gate voltage regime between -60 and 60 V in Supplementary Fig. 8b, after the SVs of the CVD-grown MoS2 healed by the SVSH. The discrepancy of the reverse-biased currents between the 1T′-MoTe2/as-prepared MoS2 and 1T′-MoTe2/healed MoS2 Schottky junctions is significantly large in Supplementary Fig. 8c left.
Under reverse bias, enlarging the Schottky barrier width can transform the charge injection style from thermionic emission to thermionic field emission (also call thermally assisted tunneling) in Fig. 3g. While the discrepancy of the forward-biased currents is very small in Supplementary Fig. 8c right. The gate-tunable variation trends of both the forward and reverse currents extracted from the output curves are similar to that of the transfer curves in Supplementary Fig. 8d, indicating that the rectifying behaviors are not measured accidentally but reliable. More detailed explanations for the discrepancy can be obtained in Fig. 3f-g. The specific experiment in Supplementary Fig. 10 shows that even if the PEDOT:PSS treatment heals the SVs of the covered MoS2 of the 1T′-MoTe2/MoS2 diode, the enhancement behavior of the rectifying performance won't be greatly changed. Since the rectifying ratio of ~39 of the as-prepared (A1-A2) diodes at VD= ±2 V and VG= 0 V is smaller than that of ~370 of the healed (B1-B2) diodes in Supplementary Fig. 10g. Furthermore, by the secondary PEDOT:PSS treatment, the rectifying ratio of the as-prepared (A1-A2) diode was increased from ~39 to ~279, which is comparable to that of ~370 of the healed (B1-B2) diode ( Supplementary Fig. 10h).
The possible reason why this contact fluctuation of the covered MoS2 is independent of the rectifying performance is that, in such Schottky junctions with a large Schottky barrier height of ~0.5 eV, the electron concentration of the covered monolayer MoS2 won't be affected by the PSS-induced SVSH effect but is mainly determined by the work function of the metal electrode 1T′-MoTe2 (Fig. 3f-g). In general, the thickness of 0.85 nm of monolayer MoS2 is much less than the depletion region (>2.9 nm) width of the 1T′-MoTe2/MoS2 Schottky junctions 14 . Next, the PL spectrum was also characterized to reconfirm the decrease in the electron concentration of MoS2 in the overlapped depletion region. Compared to the monolayer MoS2 supported on the SiO2 insulating substrate, the PL spectrum intensity and peak position of the overlapped region is substantially reduced and blue-shifted of ~20 meV in Supplementary Fig. 14a.
This blue-shift in PL peak position is mainly due to the obvious difference in electron concentration between MoS2 in the depletion region and MoS2 on the insulating substrate.
Whether supported by SiO2 or MoTe2, the PL spectra of monolayer MoS2 can be broken down into B excitons, intrinsic excitons (X), and trions (X -) by peak fitting in Supplementary Fig.   14b. A trion in n-type monolayer MoS2 is mainly composed of two electrons and one hole, and its component is largely positive with the degree of the electron concentration 10,11 . Different