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We present the first results for microelectromechanical (MEMS) resonators fabricated on epitaxial nitride semiconductors with thin buffers engineered for MEMS and NEMS applications. These results are used to assess the use of thin buffers for GaN MEMS fabrication. On a 700 nm thick AlGaN/GaN epilayer, a high tensile stress is observed to increase the resonant frequency. The electromechanical coupling efficiencies of integrated transducers are assessed and compared with previously obtained results on commercially available 2 µm thick epilayers used for power transistor applications. A 28 nm V−1 actuation efficiency is measured on the 700 nm thick structure which is slightly better than the one measured on the 2 µm buffer. The electrical response of a gateless detector designed as a piezoresistance is obtained and a gauge factor of 60 is estimated. These results show that material issues can be unlocked to exploit the potentialities of III-nitrides for NEMS applications. [ABSTRACT FROM AUTHOR]

In this paper, we present the preparation of few-layer MoS 2 films on single-crystal sapphire, as well as on heteroepitaxial GaN templates on sapphire substrates, using the pulsed laser deposition (PLD) technique. Detailed structural and chemical characterization of the films were performed using Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction measurements, and high-resolution transmission electron microscopy. According to X-ray diffraction studies, the films exhibit epitaxial growth, indicating a good in-plane alignment. Furthermore, the films demonstrate uniform thickness on large areas, as confirmed by Raman spectroscopy. The lateral electrical current transport of the MoS 2 grown on sapphire was investigated by temperature (T)-dependent sheet resistance and Hall effect measurements, showing a high n-type doping of the semiconducting films (n s from ~1 × 10 13 to ~3.4 × 10 13 cm -2 from T = 300 K to 500 K), with a donor ionization energy of E i = 93 ± 8 meV and a mobility decreasing with T. Finally, the vertical current injection across the MoS 2 /GaN heterojunction was investigated by means of conductive atomic force microscopy, showing the rectifying behavior of the I-V characteristics with a Schottky barrier height of ϕ B ≈ 0.36 eV. The obtained results pave the way for the scalable application of PLD-grown MoS 2 on GaN in electronics/optoelectronics.

Parameterized cellular microstructures allow for the development of efficient multiscale optimization strategies for the design of Functionally Graded Scaffolds (FGSs). This work assesses the biomimetic capabilities of the Voronoi-based cancelous bone microstructure introduced by Fantini et al. (2016) in terms of histomorphometric and elastic properties. Histomorphometric data of 23 bovine bone specimens and elastic data of 140 human bone specimens are used as reference. Based on the key findings that there exists a strong correspondence between the trabecular thickness and the solid volume fraction for natural cancelous bone, and that the stretching of the Voronoi microstructure is an effective means to induce anisotropy, the generative procedure by Fantini et al. (2016) is assessed and tuned to account for anisotropy and elastic properties. It is shown that the resulting mimetic microstructures have histomorphometric features and elastic properties that are in very good accordance to those of the natural samples. The outcomes of this work are a step forward towards the integration of the Voronoi-based microstructure into multiscale design tools., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Optically pumped whispering-gallery mode (WGM) lasing is observed from a thin-film GaN microdisk processed from GaN-on-Si InGaN/GaN multi-quantum well wafers by selective wet-etch removal of the substrate. Compared with thin-film microdisks processed from GaN-on-sapphire wafers through laser lift-off of the sapphire substrate, the exposed surface is significantly smoother as laser-induced damage is avoided, with a root-mean-square roughness of 1.3 nm compared with 5.8 nm of the latter wafer. The ∼8- μ m diameter microdisks, fabricated by pattern transfer from a silica microsphere and dry etching, benefit from the surface smoothness to offer superior optical confinement within the cavity. WGM lasing thresholds of ∼2.9 mJ/cm 2 and ∼3.5 mJ/cm 2 with quality (Q)-factors of ∼3100 and ∼1700 are observed at the peak lasing wavelengths of ∼453 nm and ∼532 nm, respectively, which are significantly better than thin-film microdisks processed from GaN-on-sapphire wafers despite lower internal quantum efficiency, highlighting the importance of surface smoothness in such optical cavities.

Microdisks fabricated with III-nitride materials grown on GaN substrates are demonstrated, taking advantage of the high material quality of homoepitaxial films and advanced micro-fabrication processes. The epitaxial structure consists of InGaN/GaN multi-quantum wells (MQWs) sandwiched between AlGaN/GaN and InAlN/GaN superlattices as cladding layers for optical confinement. Due to lattice-matched growth with low dislocations, an internal quantum efficiency of ∼40% is attained, while the sidewalls of the etched 8 µm-diameter microdisks patterned by microsphere lithography are optically smooth to promote the formation of whispering-gallery modes (WGMs) within the circular optical cavities. Optically pumped lasing with low threshold of ∼5.2 mJ/cm 2 and quality (Q) factor of ∼3000 at the dominant lasing wavelength of 436.8 nm has been observed. The microdisks also support electroluminescent operation, demonstrating WGMs consistent with the photoluminescence spectra and with finite-difference time-domain (FDTD) simulations.

It is shown that substrate pixelisation before epitaxial growth can significantly impact the emission color of semiconductor heterostructures. The wavelength emission from In x Ga 1-x N/GaN quantum wells can be shifted from blue to yellow simply by reducing the mesa size from 90 × 90 µm 2 to 10 × 10 µm 2 of the patterned silicon used as the substrate. This color shift is mainly attributed to an increase of the quantum well thickness when the mesa size decreases. The color is also affected, in a lesser extent, by the trench width between the mesas. Cathodoluminescence hyperspectral imaging is used to map the wavelength emission of the In x Ga 1-x N/GaN quantum wells. Whatever the mesa size is, the wavelength emission is red-shifted at the mesa edges due to a larger quantum well thickness and In composition.

AlN nucleation layers are the basement of GaN-on-Si structures grown for light-emitting diodes, high frequency telecommunication and power switching systems. In this context, our work aims to understand the origin of propagation losses in GaN-on-Si High Electron Mobility Transistors at microwaves frequencies, which are critical for efficient devices and circuits. AlN/Si structures are grown by Metalorganic Vapor Phase Epitaxy. Acceptor dopant in-diffusion (Al and Ga) into the Si substrate is studied by Secondary Ion Mass Spectroscopy and is mainly located in the first 200 nm beneath the interface. In this region, an acceptor concentration of a few 10 18  cm -3 is estimated from Capacitance-Voltage (C-V) measurements while the volume hole concentration of several 10 17  cm -3 is deduced from sheet resistance. Furthermore, the combination of scanning capacitance microscopy and scanning spreading resistance microscopy enables the 2D profiling of both the p-type conductive channel and the space charge region beneath the AlN/Si interface. We demonstrate that samples grown at lower temperature exhibit a p-doped conductive channel over a shallower depth which explains lower propagation losses in comparison with those synthesized at higher temperature. Our work highlights that this p-type channel can increase the propagation losses in the high-frequency devices but also that a memory effect associated with the previous sample growths with GaN can noticeably affect the physical properties in absence of proper reactor preparation. Hence, monitoring the acceptor dopant in-diffusion beneath the AlN/Si interface is crucial for achieving efficient GaN-on-Si microwave power devices.

Ultraviolet microdisk lasers are integrated monolithically into photonic circuits using a III-nitride-on-silicon platform with gallium nitride (GaN) as the main waveguide layer. The photonic circuits consist of a microdisk and a pulley waveguide, terminated by out-coupling gratings. In this Letter, we measure quality factors up to 3500 under continuous-wave excitation. Lasing is observed from 374 to 399 nm under pulsed excitation, achieving low-threshold energies of 0.14 m J / c m 2 per pulse (threshold peak powers of 35 k W / c m 2 ). A large peak-to-background dynamic of around 200 is observed at the out-coupling grating for small gaps of 50 nm between the disk and the waveguide. These devices operate at the limit of what can be achieved with GaN in terms of operation wavelength.

On-chip microlaser sources in the blue constitute an important building block for complex integrated photonic circuits on silicon. We have developed photonic circuits operating in the blue spectral range based on microdisks and bus waveguides in III-nitride on silicon. We report on the interplay between microdisk-waveguide coupling and its optical properties. We observe critical coupling and phase matching, i.e. the most efficient energy transfer scheme, for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the spontaneous emission regime. Whispering gallery mode lasing is demonstrated for a wide range of parameters with a strong dependence of the threshold on the loaded quality factor. We show the dependence and high sensitivity of the output signal on the coupling. Lastly, we observe the impact of processing on the tuning of mode resonances due to the very short coupling distances. Such small footprint on-chip integrated microlasers providing maximum energy transfer into a photonic circuit have important potential applications for visible-light communication and lab-on-chip bio-sensors.

Nanophotonic circuits using group III-nitrides on silicon are still lacking one key component: efficient electrical injection. In this paper we demonstrate an electrical injection scheme using a metal microbridge contact in thin III-nitride on silicon mushroom-type microrings that is compatible with integrated nanophotonic circuits with the goal of achieving electrically injected lasing. Using a central buried n-contact to bypass the insulating buffer layers, we are able to underetch the microring, which is essential for maintaining vertical confinement in a thin disk. We demonstrate direct current room-temperature electroluminescence with 440 mW/cm 2 output power density at 20 mA from such microrings with diameters of 30 to 50 μm. The first steps towards achieving an integrated photonic circuit are demonstrated.