Layered two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors are promising materials due to their optoelectronic properties and weak inter-layer coupling, which facilitates heteromaterial integration and miniaturization. Applications include stacked nanosheet field effect transistors, lateral heterojunction diodes, and photodetectors, all requiring high-quality 2D-TMDCs with tunable electronic properties. Significant efforts focus on high-quality scalable production, controlled doping, and optimized electronic contacts. Epitaxial growth of wafer-scale substitutionally doped semiconducting 2D-TMDCs like MoS₂, WS₂, and WSe₂ has been achieved using metalorganic chemical vapor deposition (MOCVD). Doping with nonisovalent transition metals (V, Nb, Re, Mn) yielded expected p-type or n-type behavior, but dopant ionization energies are higher in the monolayer limit due to strong defect state localization. Metal-TMDC contacts suffer from Fermi level pinning due to metal-induced gap states (MIGS) or disorder-induced gap states, leading to high contact resistance and limiting device performance. Van der Waals semimetals like graphene effectively suppress MIGS and reduce interface disorder. This research investigates the layer-dependent valence and conduction band onsets of a semimetal-TMDC contact between multilayer WSe₂ grown on quasi-freestanding epitaxial graphene on 6H-SiC(0001) (QFEG/SiC), using scanning tunneling microscopy and spectroscopy (STM/S) to compare band onset evolution from 1 to 7 monolayers (ML) between p-type V-doped (0.44%, 4.7 × 10¹² cm⁻²) and nominally undoped WSe₂.

The literature extensively covers the properties and applications of 2D TMDCs, highlighting their potential for advanced electronics and optoelectronics. Studies on the growth and doping of TMDCs, especially using MOCVD, have shown progress in achieving controlled material properties. However, challenges related to contact resistance and Fermi level pinning at metal-semiconductor interfaces have been a major focus. Several studies have explored the use of van der Waals semimetals, such as graphene, to mitigate these issues by reducing interface defects and suppressing MIGS. Previous work on the layer-dependent properties of TMDCs has shown that the bandgap and electronic structure can change significantly with thickness due to interlayer coupling. This study builds on these previous investigations by focusing specifically on the layer-dependent Schottky contact behavior in a V-doped WSe₂/graphene system.

Undoped and V-doped WSe₂ samples were synthesized using a custom-designed vertical cold wall gas-source CVD reactor. Tungsten hexacarbonyl (W(CO)₆), hydrogen selenide (H₂Se), and vanadium (V(C₅H₅)₂) were used as precursors. The growth process involved three steps: nucleation, ripening, and lateral growth on c-plane sapphire. For V-doped WSe₂, V(C₅H₅)₂ was added to the precursors during nucleation and lateral growth. The vanadium density was determined from STM measurements. Undoped and V-doped WSe₂ were prepared ex situ on QFEG on SiC substrates, followed by annealing. STM/STS measurements were performed with a commercial LT STM at 5 K and pressures below 2 × 10⁻¹⁰ mbar. A tungsten tip was prepared on a clean Au(111) surface. STM topographic measurements were conducted in constant current mode, and STS measurements used a lock-in amplifier at 860 Hz with a 10 mV modulation amplitude. The band gap was determined by fitting the valence and conduction band edge and band gap intensities of the logarithmic STS data with a line.

STM/STS measurements revealed a distinct layer dependence in the electronic structure of both pristine and p-type V-doped WSe₂ on QFEG/SiC. In undoped WSe₂, the band gap narrowed symmetrically with increasing layers, with the Fermi level remaining near the center of the band gap. Each additional WSe₂ layer introduced a new subband in the valence band spectrum, consistent with a tight-binding model of weakly coupled 2D quantum wells. V-doped WSe₂ showed a Schottky contact behavior. In monolayer V-doped WSe₂, the Fermi level resided near the band gap center. However, beyond two layers, the Fermi level pinned to the highest dopant state. This indicated a depletion region of approximately 1.6 nm, with V dopants ionized in the first two layers and charge neutral in subsequent layers. The STM images showed a change in the appearance of V dopants with increasing layers, transitioning from circular depressions to hexagonal shapes beyond three layers. The dopant states progressively shifted toward the Fermi level with increasing layers, indicating a weakening of the defect state binding energy.

The findings demonstrate a layer-dependent Schottky contact at the van der Waals interface between V-doped WSe₂ and QFEG/SiC. The observed pinning of the Fermi level in thicker layers is consistent with the formation of a depletion region due to the ionization of V dopants. The transition from ionized to neutral dopants occurs within a relatively small thickness range (1.6 nm), which is crucial for device design. The symmetric band gap narrowing in undoped WSe₂ reflects the weak interlayer coupling, while the asymmetric behavior in doped WSe₂ highlights the influence of dopant states and charge transfer at the interface. This study provides valuable insights into the electronic structure of 2D TMDC/graphene heterostructures, contributing to the development of high-performance devices based on these materials.

This work demonstrates a clear layer-dependent electronic structure for both pristine and V-doped WSe₂ grown on QFEG/SiC. The observed Schottky contact behavior and the determination of the depletion depth are significant contributions for designing and optimizing 2D semiconductor devices. Future work could investigate other dopants and 2D materials to further explore the tunability of Schottky barriers in van der Waals heterostructures. Investigating the impact of substrate effects and interface engineering on the depletion depth could also be a fruitful research direction.

The study focused on a specific dopant (vanadium) and a particular substrate (QFEG/SiC). The generalizability of the findings to other dopants and substrates warrants further investigation. The STM/STS measurements were performed at cryogenic temperatures (5 K). The behavior at higher temperatures, relevant for device operation, might differ. The relatively high dopant concentration (0.44%) might also influence the observed behavior. Future studies with lower doping levels are needed to explore this aspect further.