Gigahertz Nano-Optomechanical Resonances in a Dielectric SiC-Membrane Metasurface Array.

Optically and vibrationally resonant nanophotonic devices are of particular importance for their ability to enhance optomechanical interactions, with applications in nanometrology, sensing, nano-optical control of light, and optomechanics. Here, the optically resonant excitation and detection of gigahertz vibrational modes are demonstrated in a nanoscale metasurface array fabricated on a suspended SiC membrane. With the design of the main optical and vibrational modes to be those of the individual metamolecules, resonant excitation and detection are achieved by making use of direct mechanisms for optomechanical coupling. Ultrafast optical pump-probe studies reveal a multimodal gigahertz vibrational response corresponding to the mechanical modes of the suspended nanoresonators. Wavelength and polarization dependent studies reveal that the excitation and detection of vibrations takes place through the metasurface optical modes. The dielectric metasurface pushes the modulation speed of optomechanical structures closer to their theoretical limits and presents a potential for compact and easily fabricable optical components for photonic applications.

Quantifying the Transverse-Electric-Dominant 260 nm Emission from Molecular Beam Epitaxy-Grown GaN-Quantum-Disks Embedded in AlN Nanowires: A Comprehensive Optical and Morphological Characterization.

There has been a relentless pursuit of transverse electric (TE)-dominant deep ultraviolet (UV) optoelectronic devices for efficient surface emitters to replace the environmentally unfriendly mercury lamps. To date, the use of the ternary AlGaN alloy inevitably has led to transverse magnetic (TM)-dominant emission, an approach that is facing a roadblock. Here, we take an entirely different approach of utilizing a binary GaN compound semiconductor in conjunction with ultrathin quantum disks (QDisks) embedded in AlN nanowires (NWs). The growth of GaN QDisks is realized on a scalable and low-cost Si substrate using plasma-assisted molecular beam epitaxy as a highly controllable monolayer growth platform. We estimated an internal quantum efficiency of ∼81% in a wavelength regime of ∼260 nm for these nanostructures. Additionally, strain mapping obtained by high-angle annular dark-field scanning transmission electron microscopy is studied in conjunction with the TE and TM modes of the carrier recombination. Moreover, for the first time, we quantify the TE and TM modes of the PL emitted by GaN QDisks for deep-UV emitters. We observed nearly pure TE-polarized photoluminescence emission at a polarization angle of ∼5°. This work proposes highly quantum-confined ultrathin GaN QDisks as a promising candidate for deep-UV vertical emitters.

Catalyst-Free Vertical ZnO-Nanotube Array Grown on p-GaN for UV-Light-Emitting Devices.

One-dimensional (1D) structures-based UV-light-emitting diode (LED) has immense potential for next-generation applications. However, several issues related to such devices must be resolved first, such as expensive material and growth methods, complicated fabrication process, efficiency droop, and unavoidable metal contamination due to metal catalyst that reduces device efficiency. To overcome these obstacles, we have developed a novel growth method for obtaining a high-quality hexagonal, well-defined, and vertical 1D Gd-doped n-ZnO nanotube (NT) array deposited on p-GaN films and other substrates by pulsed laser deposition. By adopting this approach, the desired high optical and structural quality is achieved without utilizing metal catalyst. Transmission electron microscopy measurements confirm that gadolinium dopants in the target form a transparent in situ interface layer to assist in vertical NT formation. Microphotoluminescence (PL) measurements of the NTs reveal an intense ZnO band edge emission without a defect band, indicating high quality. Carrier dynamic analysis via time-resolved PL confirms that the emission of n-ZnO NTs/p-GaN LED structure is dominated significantly by the radiative recombination process without efficiency droop when high carrier density is injected optically. We developed an electrically pumped UV Gd-doped ZnO NTs/GaN LED as a proof of concept, demonstrating its high internal quantum efficiency (>65%). The demonstrated performance of this cost-effective UV LED suggests its potential application in large-scale device production.

A Photodetector Based on p-Si/n-ZnO Nanotube Heterojunctions with High Ultraviolet Responsivity.

Enhanced ultraviolet (UV) photodetectors (PDs) with high responsivity comparable to that of visible and infrared photodetectors are needed for commercial applications. n-Type ZnO nanotubes (NTs) with high-quality optical, structural, and electrical properties on a p-type Si(100) substrate are successfully fabricated by pulsed laser deposition (PLD) to produce a UV PD with high responsivity, for the first time. We measure the current-voltage characteristics of the device under dark and illuminated conditions and demonstrated the high stability and responsivity (that reaches ∼101.2 A W-1) of the fabricated UV PD. Time-resolved spectroscopy is employed to identify exciton confinement, indicating that the high PD performance is due to optical confinement, the high surface-to-volume ratio, the high structural quality of the NTs, and the high photoinduced carrier density. The superior detectivity and responsivity of our NT-based PD clearly demonstrate that fabrication of high-performance UV detection devices for commercial applications is possible.

Double Charged Surface Layers in Lead Halide Perovskite Crystals.

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

Droop-Free, Reliable, and High-Power InGaN/GaN Nanowire Light-Emitting Diodes for Monolithic Metal-Optoelectronics.

A droop-free nitride light-emitting diode (LED) with the capacity to operate beyond the "green gap" has been a subject of intense scientific and engineering interest. While several properties of nanowires on silicon make them promising for use in LED development, the high aspect ratio of individual nanowires and their laterally discontinuous features limit phonon transport and device performance. Here, we report on the monolithic integration of metal heat-sink and droop-free InGaN/GaN quantum-disks-in-nanowire LEDs emitting at ∼710 nm. The reliable operation of our uncooled nanowire-LEDs (NW-LEDs) epitaxially grown on molybdenum was evident in the constant-current soft burn-in performed on a 380 µm × 380 µm LED. The square LED sustained 600 mA electrical stress over an 8 h period, providing stable light output at maturity without catastrophic failure. The absence of carrier and phonon transport barriers in NW-LEDs was further inferred from current-dependent Raman measurements (up to 700 mA), which revealed the low self-heating. The radiative recombination rates of NW-LEDs between room temperature and 40 °C was not limited by Shockley-Read-Hall recombination, Auger recombination, or carrier leakage mechanisms, thus realizing droop-free operation. The discovery of reliable, droop-free devices constitutes significant progress toward the development of nanowires for practical applications. Our monolithic approach realized a high-performance device that will revolutionize the way high power, low-junction-temperature LED lamps are manufactured for solid-state lighting and for applications in high-temperature harsh environment.