We report the demonstration of growing two-dimensional (2D) hexagonal-AlN ( h-AlN) on transition metal dichalcogenide (TMD) monolayers (MoS2, WS2, and WSe2) via van der Waals epitaxy by atomic layer deposition (ALD). Having atomically thin thickness and high theoretical carrier mobility, TMDs are attractive semiconductors for future dense and high-performance 3D IC, and 2D hexagonal boron nitride ( h-BN) as a gate dielectric is known to significantly improve TMD device performance. However, h-BN growth requires 1000 °C temperature that is not compatible with CMOS fabrication, and ALD deposition of any high-k 2D insulator on TMD continues to be an elusive goal. The epitaxial 2D layered h-AlN by low-temperature ALD is characterized by synchrotron-based grazing-incidence wide-angle x-ray scattering and high-resolution transmission electron microscopy. In addition, we demonstrate the feasibility of using layered h-AlN as an interfacial layer between WS2 and ALD HfO2. The significantly better uniformity and smoothness of HfO2 than that directly deposited on TMD are desirable characteristics for TMD transistor applications.

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Single-phase two-dimensional (2D) indium monoselenide (γ-InSe) film is successfully grown via solid phase epitaxy in the molecular beam epitaxy (MBE) system. Having high electron mobility and high photoresponsivity, ultrathin 2D γ-InSe semiconductors are attractive for future field-effect transistor and optoelectronic devices. However, growing single-phase γ-InSe film is a challenge due to the polymorphic nature of indium selenide (γ-InSe, α-In2Se3, β-In2Se3, γ-In2Se3, etc.). In this work, the 2D α-In2Se3 film was first grown on a sapphire substrate by MBE. Then, the high In/Se ratio sources were deposited on the α-In2Se3 surface, and an γ-InSe crystal emerged via solid-phase epitaxy. After 50 min of deposition, the initially 2D α-In2Se3 phase was also transformed into a 2D γ-InSe crystal. The phase transition from 2D α-In2Se3 to γ-InSe was confirmed by Raman, XRD, and TEM analysis. The structural ordering of 2D γ-InSe film was characterized by synchrotron-based grazing-incidence wide-angle x-ray scattering (GIWAXS).

It has been demonstrated that the WS2 monolayer is an excellent template for AlN epitaxy at 400 °C low temperature. Low-temperature AlN thin films exhibit much superior crystalline quality than those grown directly on sapphire substrates. In addition to the small lattice mismatch between AlN and WS2 monolayer, we proposed a growth mechanism to explain the excellent van der Waal epitaxy by looking at the initial growth. This growth model reveals that transition metal dichalcogenides (TMDCs) are promising buffer layers for the deposition of III-nitrides but also suggests the novel combination of AlN and TMDCs in the research of future 2D field-effect transistors due to the extremely low leakage current of high-quality AlN films.