Highly Oriented Atomically Thin Ambipolar MoSe₂ Grown by Molecular Beam Epitaxy

Abstract

Transition metal dichalcogenides (TMDCs), together with other two-dimensional (2D) materials, have attracted great interest due to the unique optical and electrical properties of atomically thin layers. In order to fulfill their potential, developing large-area growth and understanding the properties of TMDCs have become crucial. Here, we have used molecular beam epitaxy (MBE) to grow atomically thin MoSe2 on GaAs(111)B. No intermediate compounds were detected at the interface of as-grown films. Careful optimization of the growth temperature can result in the growth of highly aligned films with only two possible crystalline orientations due to broken inversion symmetry. As-grown films can be transferred onto insulating substrates, allowing their optical and electrical properties to be probed. By using polymer electrolyte gating, we have achieved ambipolar transport in MBE-grown MoSe2. The temperature-dependent transport characteristics can be explained by the 2D variable-range hopping (2D-VRH) model, indicating that the transport is strongly limited by the disorder in the film.

Ambipolar transport in CVD grown MoSe2 monolayer using an ionic liquid gel gate dielectric

“…The transfer process typically leads to the presence of folds or wrinkles on the MoSe 2 surface. 4 Combined with the roughness at the substrate/MoSe 2 interface, the measured mobility is reasonable for our device with bottom contacts. Higher mobility values up to 50 cm 2 /V-s at room temperature have been reported for MoSe 2 by others on exfoliated flakes 2,22 and for CVD grown MoSe 2 monolayers on the as prepared substrate.…”

“…In the bulk form, MoSe 2 has an indirect band gap of 1.1 eV which transforms to the technologically important direct band gap of 1.55 eV in an atomically thin monolayer. 1 Exfoliation is a common method used to randomly isolate thin films of this material, 2 however, molecular beam epitaxy (MBE) 3,4 and chemical vapor deposition (CVD), [5][6][7] are controlled techniques for reproducible fabrication of large area monolayers. Since Se vacancy formation energy is high in MoSe 2 (leading to fewer vacancies and a more intrinsic semiconductor) its Fermi energy lies nearer to the mid-gap which allows for a tunable Schottky barrier (with a metal contact) and makes ambipolar conduction possible.…”

“…in exfoliated 10 and CVD 6,11 grown MoSe 2 multilayers with a SiO 2 dielectric, and on MBE grown monolayers with an IL dielectric at low temperatures. 4 Our device consists of a CVD grown MoSe 2 monolayer with an IL dielectric that exhibits superior performance at room temperature (294 K). The high specific capacitance of the IL permits access to a wider range of electrostatic doping levels at low operating voltages, while retaining the high on/off ratio (for hole and electron transport) necessary for successful implementation in binary based logic circuits.…”

“…An increase in I DS for positive and negative V GS indicates ambipolar charge transport in the MoSe 2 channel. 4 When V GS is greater than the threshold voltage for electron accumulation (-0.2V), electrons (e) are injected from the source into the conduction band of MoSe 2 and drift toward the drain. This is shown schematically in the band diagram for n-FET operation in the red inset to Figure 3(a).…”

Real-time monitoring of 2D semiconductor film growth with optical spectroscopy

“…The thickness and the film structure determined from the morphology revealed by AFM is thus in fully agreement with that deduced from in situ DRS as well as ex situ RBS and Raman spectroscopy measurements. Furthermore, this AFM result is inline with the morphology of MBE MoSe 2 layers grown on Al 2 O 3 (0001) [20] and other substrates [17,21] revealed using aberrationcorrected high-resolution transmission electron microscopy.…”