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.

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).

In this work, we develop an ultrathin epitaxial h-AlN as an interfacial layer (IL) between HfO2 and monolayer WS2 channel by atomic layer deposition (ALD). The growth of ultrathin dielectrics using ALD has been challenging due to the dangling-bond-free surface of 2D materials. By utilizing sub-1 nm h-AlN as an IL and depositing HfO2 high-k dielectric, we were able to form a uniform and atomically flat gate dielectric without voids. The resulting structure exhibits an equivalent oxide thickness as low as 1 nm and ultra-low leakage currents of ∼10−6 A/cm2. The fabricated top-gate WS2 transistors demonstrate on-off ratios of around 106 and subthreshold swing as low as 93 mV/dec. Furthermore, we have verified the feasibility of using h-AlN IL for a gate-all-around structure. Our work presents a CMOS-compatible low-temperature ALD process for integrating gate dielectrics, offering excellent thickness scalability and uniform coverage around monolayer WS2 nanosheets. The combination of high-quality two-dimensional dielectrics and semiconductors will contribute to the development of future high-performance and low-power electronic devices.

The integration of scalable, high-performance Hf0.5Zr0.5O2 (HZO) ferroelectric materials with high-mobility channels such as germanium is critical for next-generation logic and memory devices. However, maintaining robust ferroelectricity in films thinner than 5 nm remains a major challenge. In this study, we systematically investigate 3.7-nm HZO films incorporated into a titanium nitride/HZO/Ge capacitor structure. We demonstrate that a conventional single-step post-metallization annealing (PMA) process is insufficient for inducing strong ferroelectricity in ultrathin HZO films. To address this limitation, we formulate a synergistic strategy combining interface engineering with ZrO2-termination and a two-step annealing sequence comprising post-deposition annealing (PDA) followed by PMA. This approach stabilizes the ferroelectric orthorhombic o(020)/o(002) phase and produces robust remanent polarization and improved polarization–voltage loop (P–V loop) squareness, as reflected by an enhanced polarization ratio and (dP/dV)max. Consequently, polarization switching becomes more complete and the P–V loop becomes more rectangular. Comprehensive structural, spectroscopic, theoretical, and electrical analyses further confirm the effectiveness of this strategy. This study provides mechanistic insights into the integration of ultrathin ferroelectric HZO with Ge-based electronic platforms.