Two and three-dimensional (2D/3D) hybrid materials
Beyond two dimensional (2D) heterostructures, 2D/3D hybrids enable mixed dimensional van der Waals heterostructures and provide unique opportunities towards steep slope transistors, highly efficient p–n junction solar cells, light emitters and photovoltaics. Transition metal dichalcogenide (TMD) and nitride semiconducting heterostructures are especially interesting because of the wide variety of band alignments, and the possibility to convert the nitride to a 2D form. Recently, p–n diodes and room temperature Esaki diodes were demonstrated with transferred 2D layers. However, transfer-induced contamination and damage often degrades the performance of the devices. Additionally, the device size is limited by the exfoliated sample size (typically < 10μm), which is not compatible to the industrial process.
In his previous work, Randall M. Feenstra had shown the epitaxially growth of monolayer MoS2 on GaN which was electrically active in the vertical direction. However, the non-uniformity was still limits its applicability to large area synthetic 2D/ 3D heterostructures. This time, they have utilized metal organic chemical vapor deposition (MOCVD) to synthesize mono to few-layer MoS2 and WSe2 on p-type and n-type GaN, respectively, to probe the 2D/ 3D electrical properties. MOCVD enables large area, uniform TMDs films via layer-by-layer growth as identified by atomic force microscopy (AFM) and Raman spectroscopy, where the layer number is tuned via growth time. X-ray photoelectron spectroscopy (XPS) characterization confirms nearly stoichiometric MoS2 and WSe2 layers are grown with negligible metal–oxide bonding compared to powder vaporized TMDs. Furthermore, there is no structural change of the GaN substrate.
They have demonstrated that the vertical transport in the 2D/3D hybrids vary with 2D thickness and the choice of the heterostructure. Monolayer TMDs on GaN exhibit clear direct tunneling, while few layer (FL) (>2 layers) TMDs lead to p–n junction behavior. Although p–n diodes based on FL MoS2/p-GaN and WSe2/ n-GaN exhibit similar turn-on voltages under forward bias, FL MoS2/p-GaN exhibits 100× higher current under reversed bias due to strong charge transfer and an intrinsic dipole between the MoS2/p-GaN interface. As a result, they have hypothesized that few layer WSe2/n-GaN is the most appropriate choice for high quality synthetic 2D/3D p–n heterostructures. Finally, they have demonstrated MoS2/WSe2/n-GaN hybrids with atomically sharp interfaces. Unlike the resonant tunneling observed on MoS2/ WSe2/epitaxial graphene (EG) heterostructures, single layer MoS2/WSe2 on n-GaN exhibits Ohmic behavior, while FL MoS2/ WSe2 on n-GaN is dominated by Schottky barrier transport.
In conclusion, the work demonstrates the scalable growth of TMD/GaN hybrid structures, where film thickness is controlled by tuning the growth time. The process results in high quality MoS2 and WSe2, and enables a route to 2D heterostructures on GaN for next generation electronics. Transport vertically through the hybrid structures is dominated by the 2D layer thickness and materials choice, where monolayer films exhibit strong direct tunneling characteristics and few layer 2D enables p–n junction formation at the 2D/3D interface. Compared to FL WSe2, FL MoS2 films exhibit high leakage current in reversed bias. Using KPFM and LEEM/LEER characterization techniques, they reveal a counterintuitive work function difference between the MoS2 and p-GaN, potentially due to strong interface charge transfer and possible non-structural degradation (Mg passivation) in p-GaN. Furthermore, FL MoS2/FL WSe2 heterostructures on n-GaN exhibits rectifying behavior due to the presence of the metal/MoS2 Schottky barrier. In contrast, they find Ohmic behavior when the thickness of the film is reduced to one atomic layer. This study elucidates that the thickness and materials choice is critical towards developing high quality 2D/3D heterostructures.