High-Mobility Two-Dimensional Electron Gas at InGaN/InN Heterointerface Grown by Molecular Beam Epitaxy

ε-Ga2 O3 : An Emerging Wide Bandgap Piezoelectric Semiconductor for Application in Radio Frequency Resonators

The explosion of mobile data from the internet of things (IoT) is leading to the emergence of 5G technology with dramatic frequency band expansion and efficient band allocations. Along with this, the demand for high-performance filters for 5G radio frequency (RF) front-ends keeps growing. The most popular 5G filters are constructed by piezoelectric resonators based on AlN semiconductor. However, AlN possesses a piezoelectric constant d₃₃ lower than 5 pm V^-1 and it becomes necessary to develop novel semiconductors with larger piezoelectric constant. In this work, it is shown that strong piezoelectricity exists in ε-Ga₂ O₃ . High-quality phase-pure ε-Ga₂ O₃ thin films with a relatively low residual stress are prepared. A switching spectroscopy piezoelectric force microscope (SS-PFM) measurement is carried out and the piezoelectric constant d₃₃ of ε-Ga₂ O₃ is determined to be ≈10.8-11.2 pm V^-1 , which is twice as large as that of AlN. For the first time, surface acoustic wave (SAW) resonators are demonstrated on the ε-Ga₂ O₃ thin films and different vibration modes resonating in the GHz range are observed. The results suggest that ε-Ga₂ O₃ is a great material candidate for application in piezoelectric devices, thanks to its wide bandgap, strong piezoelectric property, small acoustic impedance, and low residual stress.

Correlation of Morphology Evolution with Carrier Dynamics in InN Films Heteroepitaxially Grown by MOMBE

In this study, we report the catalyst-free growth of n-type wurtzite InN, along with its optical properties and carrier dynamics of different surface dimensionalities. The self-catalyzed epitaxial growth of InN nanorods grown by metal–organic molecular-beam epitaxy on GaN/Al2O3(0001) substrates has been demonstrated. The substrate temperature is dominant in controlling the growth of nanorods. A dramatic morphological change from 2D-like to 1D nanorods occurs with decreasing growth temperature. The InN nanorods have a low dislocation density and good crystalline quality, compared with InN films. In terms of optical properties, the nanorod structure exhibits strong recombination of Mahan excitons in luminescence, and an obvious spatial correlation effect in phonon dispersion. The downward band structure at the nanorod surface leads to the photon energy-dependent lifetime being upshifted to the high-energy side.

Optical magnetic lens: towards actively tunable terahertz optics

As we read this text, our eyes dynamically adjust the focal length to keep the image in focus on the retina. Similarly, in many optics applications the focal length must be dynamically tunable. In the quest for compactness and tunability, flat lenses based on metasurfaces were introduced. However, their dynamic tunability is still limited because their functionality mostly relies upon fixed geometry. In contrast, we put forward an original concept of a tunable Optical Magnetic Lens (OML) that focuses photon beams using a subwavelength-thin layer of a magneto-optical material in a non-uniform magnetic field. We applied the OML concept to a wide range of materials and found out that the effect of OML is present in a broad frequency range from microwaves to visible light. For terahertz light, OML can allow 50% relative tunability of the focal length on the picosecond time scale, which is of practical interest for ultrafast shaping of electron beams in microscopy. The OML based on magneto-optical natural bulk and 2D materials may find broad use in technologies such as 3D optical microscopy and acceleration of charged particle beams by THz beams.

Robust Piezo-Phototronic Effect in Multilayer γ-InSe for High-Performance Self-Powered Flexible Photodetectors

The piezo-phototronic effect has been promising as an effective means to improve the performance of two-dimensional (2D) semiconductor based optoelectronic devices. However, the current reported monolayer 2D semiconductors are not regarded as suitable for actual flexible piezotronic photodetectors due to their insufficient optical absorption and mechanical durability, although they possess strong piezoelectricity. In this work, we demonstrate that, unlike 2H-phase transition-metal dichalcogenides, γ-phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers and has a strong second-harmonic generation effect. Moreover, driven by the piezo-phototronic effect, a flexible self-powered photodetector based on multilayer γ-InSe, which can work without any energy supply, is proposed. The device exhibited ultrahigh photon responsivity of 824 mA/W under light illuminations of 400 nm (0.368 mW/cm²). Moreover, the responsivity and response speed of this photodetector were enhanced further by as much as 696% and 1010%, respectively, when a 0.62% uniaxial tensile strain was applied. Our devices exhibit high reliability and stability during a 6 month test time. These significant findings offer a promising pathway to construct high-performance flexible piezo-phototronic photodetectors based on multilayer 2D semiconductors.