Mackillo Kira

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.

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.

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.

The advent of 2D materials has revolutionized condensed matter physics and materials science, offering unprecedented opportunities to explore exotic physical phenomena, engineer novel functionalities, and address critical technological challenges across diverse fields. Over the past two decades, the exploration of 2D materials has expanded beyond graphene, encompassing a vast library of atomically thin crystals and their heterostructures. These materials exhibit extraordinary electronic, optical, thermal, mechanical, and chemical properties, and hold promise for breakthroughs in electronics, optoelectronics, quantum technologies, energy storage, catalysis, thermal management, filtration and separation, and beyond. Many exciting new physics and phenomena continue to emerge, while select 2D materials, such as graphene, h-BN, and the semiconducting transition metal dichalcogenides (TMDCs), are transitioning from laboratory-scale demonstrations to industrial applications. In this context, a holistic understanding of synthesis, structure-property relationships, integration, and performance optimization is essential. This roadmap reviews the multifaceted challenges and opportunities in 2D materials research, focusing on the synthesis, properties and applications of representative systems including graphene and its derivatives, TMDCs, MXenes as well as their heterostructures and moiré systems.

Many surprising properties of quantum materials result from Coulomb correlations defining electronic quasiparticles and their interaction chains. In van der Waals layered crystals, enhanced correlations have been tailored in reduced dimensions, enabling excitons with giant binding energies and emergent phases including ferroelectric, ferromagnetic and multiferroic orders. Yet, correlation design has primarily relied on structural engineering. Here we present quantitative experiment–theory proof that excitonic correlations can be switched through magnetic order. By probing internal Rydberg-like transitions of excitons in the magnetic semiconductor CrSBr, we reveal their binding energy and a dramatic anisotropy of their quasi-one-dimensional orbitals manifesting in strong fine-structure splitting. We switch the internal structure from strongly bound, monolayer-localized states to weakly bound, interlayer-delocalized states by pushing the system from antiferromagnetic to paramagnetic phases. Our analysis connects this transition to the exciton’s spin-controlled effective quantum confinement, supported by the exciton’s dynamics. In future applications, excitons or even condensates may be interfaced with spintronics; extrinsically switchable Coulomb correlations could shape phase transitions on demand.

Both two-dimensional (2D) transitional metal dichalcogenides (TMDs) and III–V semiconductors have been considered as potential platforms for quantum technology. While 2D TMDs exhibit a large exciton binding energy, and their quantum properties can be tailored via heterostructure stacking, TMD technology is currently limited by the incompatibility with existing industrial processes. Conversely, III-nitrides have been widely used in light-emitting devices and power electronics but not leveraging excitonic quantum aspects. Recent demonstrations of 2D III-nitrides have introduced exciton binding energies rivaling TMDs, promising the possibility to achieve room-temperature quantum technologies also with III-nitrides. Here, we discuss recent advancements in the synthesis and characterizations of 2D III-nitrides with a focus on 2D free-standing structures and embedded ultrathin quantum wells. We overview the main obstacles in the material synthesis, vital solutions, and the exquisite optical properties of 2D III-nitrides that enable excitonic and quantum-light emitters.

Intense phase-locked terahertz (THz) pulses are the bedrock of THz lightwave electronics, where the carrier field creates a transient bias to control electrons on sub-cycle time scales. Key applications such as THz scanning tunnelling microscopy or electronic devices operating at optical clock rates call for ultimately short, almost unipolar waveforms, at megahertz (MHz) repetition rates. Here, we present a flexible and scalable scheme for the generation of strong phase-locked THz pulses based on shift currents in type-II-aligned epitaxial semiconductor heterostructures. The measured THz waveforms exhibit only 0.45 optical cycles at their centre frequency within the full width at half maximum of the intensity envelope, peak fields above 1.1 kV cm⁻¹ and spectral components up to the mid-infrared, at a repetition rate of 4 MHz. The only positive half-cycle of this waveform exceeds all negative half-cycles by almost four times, which is unexpected from shift currents alone. Our detailed analysis reveals that local charging dynamics induces the pronounced positive THz-emission peak as electrons and holes approach charge neutrality after separation by the optical pump pulse, also enabling ultrabroadband operation. Our unipolar emitters mark a milestone for flexibly scalable, next-generation high-repetition-rate sources of intense and strongly asymmetric electric field transients.