Jean Paul Salvestrini

We report broadband electroluminescence spanning from the infrared to the ultraviolet in a p-hexagonal boron nitride (hBN)/n-AlGaN tunneling heterojunction. By circumventing the need for n-type hBN, this heterostructure enables efficient electrical excitation of hBN defect states through resonant tunneling-mediated carrier injection. Bias-controlled alignment of defect levels leads to the progressive activation of multiple defect-related emission bands, while AlGaN band-to-band emission appears only at the highest forward biases. The results establish resonant tunneling as a robust mechanism for electrically driven, voltage-tunable light emission in hBN.

The formation of an ohmic p-contact represents a critical step in the processing of GaN-based devices such as light-emitting diodes (LEDs). Thermal annealing in an O2-containing atmosphere at high temperature of a GaN/Ni–Au contact is mostly used today, while nonannealed contacts have been reported to provide much lower electrical performances. We experimentally evidence that the quality of the contact before annealing strongly depends on the nature of the Ni/GaN interface. Electrical results are correlated to microstructural and physicochemical analysis using high-resolution transmission scanning electron microscopy coupled with energy dispersive x-ray spectroscopy for this purpose. Using an annealed contact formed in standard conditions as a reference, we evidence from in situ treatment of the interface before metal evaporation that interfacial Ga-Oxide plays a detrimental role. We show that the metal evaporation conditions may strongly impact the physicochemical composition of the interface before annealing. We obtain nonannealed contact with electrical characteristics close to that of an annealed, standard one, using a reduced evaporation rate, in our evaporation conditions. This result is of interest for the processing of p-GaN contacts at a low thermal budget.

We report flexible III-Nitride drum resonators in which actuation and detection are both performed piezoelectrically and integrated monolithically on the same released epitaxially grown film—to our knowledge, the first such fully integrated bidirectional piezoelectric transduction on a flexible III-Nitride film epitaxy platform. The present devices use either undoped AlGaN/GaN or GaN active layers and combine on-chip piezoelectric actuation with charge-based detection. Simultaneous electrical and optical characterization of the resonance modes yields electromechanical coupling factors up to 2.60 fC nm ⁻¹ and nanometer-scale displacements under sub-volt excitation. We show that AlGaN/GaN heterostructures outperform GaN-only devices and attribute this to the constructive addition of the same-sign AlGaN and GaN piezoelectric coefficients combined with increased geometric stiffness. Optical profilometry and finite-element modeling further identify residual compressive stress as a dominant factor accounting for the higher frequencies of AlGaN/GaN devices relative to GaN. Finally, as a proof of concept, we exploit the resonance frequency–temperature dependence to operate the devices as thermal sensors, achieving a temperature coefficient of frequency of ≈−1500 ppm °C ⁻¹ and a temperature resolution down to ∼0.015 °C (best device). These results establish integrated piezoelectric III-N resonators as building blocks for flexible sensing, actuation, and operation in harsh environments.

High-resolution transmission electron microscopy (TEM) coupled to energy dispersive X-ray spectroscopy (EDX) is used to clarify the exact role of Ni–Au–Ga interdiffusion mechanisms taking place during rapid thermal annealing under an oxygen atmosphere of a Ni–Au/p-GaN contact. It is shown that oxygen-assisted, Ni diffusion to the top surface of the metallic contact through the formation of a nickel oxide (NiOx) is accompanied by Au diffusion down to the GaN surface and by Ga out-diffusion through the GaN/metal interface. Electrical characterizations of the contact by a transmission line method show that an ohmic contact is obtained as soon as a thin, Au–Ga interfacial layer is formed, even after complete diffusion of Ni or NiOx to the top surface of the contact. Our results clarify that the presence of Ni or NiOx at the interface is not the main origin of the ohmic-like behavior in such contacts. Auto-cleaning of the interface during the interdiffusion process may play a role, but TEM-EDX analysis evidences that the creation of Ga vacancies associated with the formation of a Ga–Au interfacial layer is crucial for reducing the Schottky barrier height and maximizing the amount of current flowing through the contact.

We present a simple and efficient process for fabricating III-Nitride (III-N) mechanical resonators on flexible metal substrates. This method combines Van der Waals epitaxy of III-N epilayers with the deposition of a thick metal stressor atop the III-N layers. During thermal treatment, the 30 μm thick metal stressor deposited on a 300 nm AlGaN/500 nm GaN layer grown on a 3 nm two-dimensional hexagonal-Boron Nitride (2D h-BN) release layer, initiates a one-step Self-Lift-Off and Transfer (SLOT) process. This process effectively transfers the III-N heterostructure from the h-BN/Sapphire growth wafer to the flexible metal stressor substrate. Additional local etching of the metal stressor and deposition of front electrodes allow for releasing self-standing III-N layers with integrated actuation. Fabricated III-N MEMS drum resonators were analyzed using optical profilometry and laser Doppler vibrometer, enabling the observation of static deflections and distinct vibration modes. Finite element method (FEM) simulations were also performed to further understand experimental observations and assess the mechanical properties of the released III-N layers, particularly enabling the estimation of stress in the GaN and AlGaN released layers. This straightforward approach not only provides a practical solution for cost-effective III-N MEMS resonators but also ensures flexibility, and crack-free structures.

P-hBN/n-AlGaN heterojunctions were fabricated using a dry-selective lift-off/transfer of Mg-doped hexagonal boron nitride (hBN) layer on top of n-AlGaN. Electrical contacts were used as mechanical stressors to provide structural rigidity to hBN layers as well as enabling selective lift-off. These junctions exhibit a rectifying behavior with a rectification ratio of approximately 3 × 105 at 3 V. When junctions were forward biased, ultraviolet (UV) emission around 262 nm was measured. This emission corresponds to recombinations in the n-AlGaN layer, demonstrating good hole injection in the structure. Full light emitting diode (LED) structures were fabricated by integrating UV multi quantum wells (MQWs) into these junctions. Produced UV LEDs emit around 290 nm serving as a proof of concept for future layer transferred p-hBN/MQWs/n-AlGaN structures in which the Al content is increased to go toward deep ultraviolet (DUV) emission. The selective pick and place process used to build these LEDs has multiple advantages. First, it allows independent optimization of the p-side as well as of the n-side, which includes the quantum wells. Second, UV MQWs are protected from the high temperatures needed for high hBN material quality growth, and thus their thermal stability is not affected.

We present a simple and robust process for fabricating III-Nitride (III-N) mechanical resonators on flexible metal substrates. This method combines Van der Waals epitaxy of III-N epilayers with the deposition of a thick metal stressor atop the III-N layers. During thermal treatment, the 30 µm thick metal stressor deposited on a 300 nm AlGaN / 500 nm GaN layer grown on a 3 nm two-dimensional hexagonal-Boron Nitride (2D h-BN) release layer, initiates a one-step Self-Lift-Off and Transfer (SLOT) process. This process effectively transfers the III-N heterostructure from the h-BN/Sapphire growth wafer to the flexible metal stressor substrate. Additional local etching of the metal stressor and deposition of front electrodes allow for releasing self-standing III-N layers with integrated actuation. Fabricated III-N MEMS were analyzed using optical profilometry and laser Doppler vibrometer, enabling the observation of static deflections and distinct vibration modes. Finite element method (FEM) simulations were also performed to further understand experimental observations and assess the mechanical properties of the released III-N layers, particularly enabling the estimation of stress in the GaN and AlGaN released layers. This straightforward approach not only provides a practical solution for cost-effective III-N MEMS resonators but also ensures flexibility, and crack-free structures.

In recent years, the remarkable properties and potential applications of III-nitride (III-N) semiconductors have sparked a significant interest in the field of microelectromechanical systems (MEMS). Traditionally, III-N MEMS are fabricated through a process involving the epitaxial growth of III-N epilayers on a silicon substrate followed by etching the handle wafer to generate free-standing structures. In this study, we explore the potential of a relatively simple approach based on the two-dimensional (2D) material-based liftoff and transfer to fabricate III-N mechanical resonators. The methodology involves van der Waals epitaxy of III-N layers on 2D hexagonal-boron nitride (h-BN), which leverages the weak van der Waals adhesion between h-BN layers to lift off and transfer these layers from their original growth substrate to an alternative host substrate. The employed method is demonstrated by fabricating 600 nm thick GaN/AlGaN and 2.5 μm thick h-BN micro-resonators onto pre-patterned cavities etched in silicon substrates. These devices are characterized using laser Doppler vibrometry, enabling the observation of well-defined modes of vibration and resonant frequencies. Furthermore, finite element method simulations are performed to gain insights into the experimental observations and the mechanical properties of the transferred layers. This approach could be extended to transfer high-quality III-N MEMS devices onto various host substrates, including flexible substrates, and could be used to assess the mechanical properties of emerging III-N semiconductor materials.