Jiangnan Liu

The increasing demand for data-centric and artificial intelligence workloads has intensified the need for device platforms that integrate logic, memory, and adaptive computing beyond conventional von Neumann architectures. Here, an aluminum scandium nitride (AlScN)-based nonvolatile four-mode reconfigurable field-effect transistor is demonstrated as a multifunctional device platform. By combining the large remanent polarization of AlScN with the ambipolar transport of the MoTe2 channel, the same transistor can be persistently programmed into logic nFET, logic pFET, memory-on, and memory-off states. These modes arise from polarization-controlled modulation of carrier injection and channel electrostatics, enabling a single device to function as a polarity-reconfigurable logic element, nonvolatile memory cell, and analog synaptic element. Programming is performed through voltage pulses applied to the source, drain, and back gate, while top-gate readout is decoupled from AlScN programming, enabling stable nonvolatile operation. Based on this device, a single-transistor ternary content-addressable memory cell concept and two-input reconfigurable multifunctional logic gates are demonstrated. The wide polarization-tunable memory window further supports multilevel cell memory and nearly linear long-term potentiation and depression for neuromorphic synaptic operation. These results establish ferroelectric AlScN as a robust platform for integrating reconfigurable logic, nonvolatile memory, associative search, and neuromorphic computing within a common device framework.

We report the molecular beam epitaxy and structural characterization of single-phase wurtzite LaxAl1−xN thin films, with a La composition as high as x=0.20. In situ reflection high-energy electron diffraction confirms epitaxial film formation, and atomic force microscopy reveals sub-nanometer surface roughness for all compositions investigated. High-resolution x-ray diffraction and scanning transmission electron microscopy further confirm c-axis orientation, high crystalline quality, and precise in-plane epitaxial alignment. Importantly, atomic-resolution structural analysis demonstrates a continuous decrease in the c/a lattice-parameter ratio with increasing La content. In addition, out-of-plane piezoresponse force microscopy measurements show an enhanced effective d33, increasing from 5.00 pm/V for AlN to 7.66 pm/V for La0.20Al0.80N. These results establish LaAlN as a promising new wurtzite nitride alloy platform for future wide bandgap electronics, optoelectronics, and quantum photonics.

Understanding strain relaxation during the initial stages of heteroepitaxy is essential for controlling interfacial structure and film quality, yet direct real-time quantification remains difficult. Here, we report on the demonstration of AutoRHEED, an AI-assisted in situ framework for tracking subnanometer strain relaxation during epitaxial growth. The method integrates image quality filtering, autonomous streak identification, and physics-guided geometric calibration to directly extract quantitative in-plane interplanar spacing (d spacing) evolution from raw reflection high-energy electron diffraction (RHEED) videos. We applied the framework to highly lattice-mismatched AlSb/Si(001) growth and demonstrated that AutoRHEED could capture the transient evolution from the initially strained state to full relaxation with monolayer-level sensitivity. Moreover, the in situ-derived relaxation trajectory shows excellent agreement with ex situ HAADF-STEM, confirming its structural fidelity and temporal reliability.

We report molecular beam epitaxy and ferroelectric properties of Al1-xYxN thin films with Y composition up to x = 41%. Structural characterization reveals that a wurtzite lattice structure is well preserved despite the large lattice distortions induced by increasing Y content. Both the coercive field and remanent polarization show a strong dependence on Y composition. The coercive field varies from 5.5 to 2.8 MV/cm with increasing Y compositions from 10% to 41%, whereas the remanent polarization is reduced from 140 to 70 μC/cm2. These properties are further compared with previously reported AlScN and GaScN. This study establishes AlYN as a tunable ferroelectric ultrawide-bandgap semiconductor for next-generation electronic, photonic, and electromechanical devices.

We report Sc0.15Al0.85N/AlN/GaN fin high-electron-mobility transistors (FinHEMTs) with stable operation up to 600 °C, enabled by a nearly lattice-matched Sc0.15Al0.85N barrier grown by molecular beam epitaxy. The fin sidewalls provide additional electrostatic control, strengthening gate modulation and thereby reducing off-state current (Ioff) and gate leakage (IG), improving subthreshold swing (SS), and shifting the threshold voltage (VT) positively relative to planar devices. At room temperature, the FinHEMTs with a fin width (Wch) of 0.4 μm achieve an on/off current (Ion/Ioff) ratio of ∼107, an SS of ∼105 mV/dec, and an on-state current (Ion) of ∼330 mA/mm. The devices also exhibit strong thermal robustness, maintaining Ion of 110 mA/mm at a gate–source voltage (VGS) of 0 V and an Ion/Ioff ratio of 890 at 600 °C. These results establish ScAlN/AlN/GaN FinHEMTs as a promising platform for extreme-temperature electronics.

Given its strong electro-optic and optical nonlinearities as well as CMOS compatibility, scandium-alloyed aluminum nitride (ScAlN) stands out as a material with transformative potential for next-generation integrated photonics. A comprehensive understanding of its nonlinear characteristics is critical for developing practical nonlinear optical devices and fully integrated photonic systems. In this work, we experimentally and computationally investigate the second-order nonlinear properties of epitaxial ScAlN thin films grown by molecular beam epitaxy across a range of scandium concentrations (0%–30%). We report a 17-fold increase in the d31 coefficient compared to aluminum nitride, reaching 2.59 pm/V at 30% Sc at 1266 nm while shorter wavelengths and larger Sc concentrations are needed to increase d33. Our two independent first-principles calculations verify these qualitative trends. With its enhanced second-order nonlinearities, the ScAlN material class holds promise for enabling efficient type-I/II nonlinear interactions in modulators, frequency converters, and entangled photon sources.

Mid and deep ultraviolet (UV) laser diodes remain among the least explored devices in semiconductor optoelectronics, despite their importance for spectroscopy, biochemical sensing, disinfection, and emerging quantum photonics. Here, we demonstrate an electrically pumped AlGaN-based laser diode operating in the UV-B band (280–315 nm). The device is grown by molecular beam epitaxy on a single-crystal AlN substrate and fabricated in a ridge-waveguide geometry. The laser diode operates at 298.5 nm and exhibits a relatively low threshold current density of 3.4 kA/cm2. Clear non-linear light–current characteristics and pronounced spectral narrowing with a full width at half maximum of 0.2 nm are measured above the threshold.

InGaN-based micro-light-emitting diodes (micro-LEDs) are emerging to revolutionize the display and lighting technologies, particularly for their excellent robustness, high brightness, high efficiency and small pixel size. Despite the success of blue LEDs, long-wavelength emission, particularly the red emission, has been a challenge for InGaN-based micro-LEDs. Overcoming the low quantum efficiency, color instability, and broad emission in the red wavelength regime are among the most urgent and critical problems that inhibit the commercial implementation of micro-LED technology. In this work, we utilize a nanowire photonic crystal (PhC) structure to reform the radiation behavior of red-emitting InGaN micro-LEDs. Through detailed optimization on the PhC design and device fabrication, we demonstrate red-emitting micro-LEDs with a peak wavelength at 617 nm and a full-width-at-half-maximum (FWHM) of 5 nm, which is about one order of magnitude narrower than previous reported values and is paramount for achieving high color purity. The chromaticity property is highly stable with varying injection currents due to the coupling of emission to photonic band edge mode. A high external quantum efficiency of over 10% was measured from micro-LEDs with a size of 1 µm ² . This work provides a vital strategy for high-performance red-emitting micro-LEDs and a potential pathway for full-color micro-LED technology by using all III-nitride semiconductors.