Probing Hyperbolic Shear Polaritons in ß-Ga2O3 Nanostructures Using STEM-EELS.

Phonon polaritons, quasiparticles arising from strong coupling between electromagnetic waves and optical phonons, have potential for applications in subdiffraction imaging, sensing, thermal conduction enhancement, and spectroscopy signal enhancement. A new class of phonon polaritons in low-symmetry monoclinic crystals, hyperbolic shear polaritons (HShPs), have been verified recently in ß-Ga2O3 by free electron laser (FEL) measurements. However, detailed behaviors of HShPs in ß-Ga2O3 nanostructures still remain unknown. Here, by using monochromatic electron energy loss spectroscopy in conjunction with scanning transmission electron microscopy, the experimental observation of multiple HShPs in ß-Ga2O3 in the mid-infrared (MIR) and far-infrared (FIR) ranges is reported. HShPs in various ß-Ga2O3 nanorods and a ß-Ga2O3 nanodisk are excited. The frequency-dependent rotation and shear effect of HShPs reflect on the distribution of EELS signals. The propagation and reflection of HShPs in nanostructures are clarified by simulations of electric field distribution. These findings suggest that, with its tunable broad spectral HShPs, ß-Ga2O3 is an excellent candidate for nanophotonic applications.

Atomic-Scale Investigation of the Lattice-Asymmetry-Driven Anisotropic Sublimation in GaN.

Thermal sublimation, a specific method to fabricate semiconductor nanowires, is an effective way to understand growth behavior as well. Utilizing a high-resolution transmission electron microscope (TEM) with in situ heating capability, the lattice-asymmetry-driven anisotropic sublimation behavior is demonstrated of wurtzite GaN: sublimation preferentially occurs along the [ 000 1 ¯ $000\bar{1}$ ] and [0001] directions in both GaN thin films and nanowires. Hexagonal pyramidal nanostructures consisting of six semipolar { 1 1 ¯ 01 } $\{ {1\bar{1}01} \}$ planes and one (000 1 ¯ $\bar{1}$ ) plane with the apex pointing to the [0001] direction are generated as a sublimation-induced equilibrium crystal structure, which is consistent with the lattice-asymmetry-driven growth behaviors in wurtzite GaN. These findings offer a new insight into the thermal stability of wurtzite GaN and provide essential background for tailoring the structure of III-nitrides for atomic-scale manufacturing.

Lattice Polarity Manipulation of Quasi-vdW Epitaxial GaN Films on Graphene Through Interface Atomic Configuration.

Quasi van der Waals epitaxy, a pioneering epitaxy of sp3 -hybridized semiconductor films on sp2 -hybridized 2D materials, provides a way, in principle, to achieve single-crystal epilayers with preferred atom configurations that are free of substrate. Unfortunately, this has not been experimentally confirmed in the case of the hexagonal semiconductor III-nitride epilayer until now. Here, it is reported that the epitaxy of gallium nitride (GaN) on graphene can tune the atom arrangement (lattice polarity) through manipulation of the interface atomic configuration, where GaN films with gallium and nitrogen polarity are achieved by forming CONGa(3) or COGaN(3) configurations, respectively, on artificial CO surface dangling bonds by atomic oxygen pre-irradiation on trilayer graphene. Furthermore, an aluminum nitride buffer/interlayer leads to unique metal polarity due to the formation of an AlON thin layer in a growth environment containing trace amounts of oxygen, which explains the open question of why those reported wurtzite III-nitride films on 2D materials always exhibit metal polarity. The reported atomic modulation through interface manipulation provides an effective model for hexagonal nitride semiconductor layers grown on graphene, which definitely promotes the development of novel semiconductor devices.

Deep-Ultraviolet Micro-LEDs Exhibiting High Output Power and High Modulation Bandwidth Simultaneously.

Deep-ultraviolet (DUV) solar-blind communication (SBC) shows distinct advantages of non-line-of-sight propagation and background noise negligibility over conventional visible-light communication. AlGaN-based DUV micro-light-emitting diodes (µ-LEDs) are an excellent candidate for a DUV-SBC light source due to their small size, low power consumption, and high modulation bandwidth. A long-haul DUV-SBC system requires the light source exhibiting high output power, high modulation bandwidth, and high rate, simultaneously. Such a device is rarely reported. A parallel-arrayed planar (PAP) approach is here proposed to satisfy those requirements. By reducing the dimensions of the active emission mesa to micrometer scale, DUV µ-LEDs with ultrahigh power density are created due to their homogeneous injection current and enhanced planar isotropic light emission. Interconnected PAP µ-LEDs with a diameter of 25 µm are produced. This device has an output power of 83.5 mW with a density of 405 W cm-2 at 230 mA, a wall-plug efficiency (WPE) of 4.7% at 155 mA, and a high -3 dB modulation bandwidth of 380 MHz. The remarkable high output power and efficiency make those devices a reliable platform to develop high-modulation-bandwidth wireless communication and to meet the requirements for bio-elimination.