Broadband ultraviolet plasmonic enhanced AlGaN/GaN heterojunction photodetectors with close-packed Al nanoparticle arrays.

Plasmonic metallic nanostructures could concentrate optical fields into nanoscale volumes and support efficient light scattering and absorption, which therefore stimulates the continuing development of advanced plasmonic-assisted semiconductor photodetectors. In this work, by fabricating Al nanoparticle (NP) arrays in AlGaN surface using the AAO template transferring method, significant broadband ultraviolet (UV) photoresponse enhancement was demonstrated on AlGaN/GaN heterojunction photodetectors. By deliberately designing the close-packed Al NP arrays, the broadband UV plasmonic resonance with large optical field absorption and strong interface field enhancement are enabled, hence, the highest responsivity exceeding 8.1 A W-1 and maximum external quantum efficiency of 3500% was obtained at the resonance wavelength 292 nm, revealing more than 80 times the excellent enhancement in responsivity. Specifically, owing to coupling among NPs at the Al/AlGaN interface, the smaller size Al NP array exhibits an excellent photoresponse enhancement encompassing the entire UV band compared to the relatively larger size Al NP array. In addition, different photoresponse enhancements depending on the applied bias were observed. The Al NPs detector also demonstrates a fast photoresponse with a rise time of around 60 ms and a relatively long fall time of 1.42 s. This work could be of great significance for gaining a low and efficient approach to achieve plasmonic-empowered heterojunction broadband UV detectors.

Narrow-Linewidth GaN-on-Si Laser Diode with Slot Gratings.

This letter reports room-temperature electrically pumped narrow-linewidth GaN-on-Si laser diodes. Unlike conventional distributed Bragg feedback laser diodes with hundreds of gratings, we employed only a few precisely defined slot gratings to narrow the linewidth and mitigate the negative effects of grating fabrication on the device performance. The slot gratings were incorporated into the ridge of conventional Fabry-Pérot cavity laser diodes. A subsequent wet etching in a tetramethyl ammonium hydroxide solution not only effectively removed the damages induced by the dry etching, but also converted the rough and tilted slot sidewalls into smooth and vertical ones. As a result, the threshold current was reduced by over 20%, and the reverse leakage current was decreased by over three orders of magnitude. Therefore, the room-temperature electrically pumped narrow-linewidth GaN-on-Si laser diode has been successfully demonstrated.

Asymmetric angular dependence for multicolor display based on plasmonic inclined-nanopillar array.

Asymmetric multicolor displays have unique and fascinating applications in the field of artificial color engineering. However, the realization of such multicolor displays still faces challenges, due to limitations associated with nanofabrication techniques. In this work, asymmetric photonic structures were realized through inclined 2D aluminum nanopillar arrays, which demonstrate asymmetric angle-dependence as multicolor displays. It was numerically and experimentally demonstrated that the distinctive symmetry breaking leads to the plasmonic coupling effect with angle-dependence and reflection differences with the opposite observing angle. Based on this concept, several color printings were designed as prototypes, which prove the utility of the controlled asymmetric color display with varied observing angles. Our results demonstrate a simple and efficient platform for asymmetric plasmonic nanostructures, which paves the way for further study and designation in artificial color engineering.

Transmission electron microscopy sample preparation method for micrometer-sized powder particles using focused ion beam.

A TEM sample preparation technique for micrometer-sized powder particles in the 1-10 µm size range is proposed, using a focused ion beam (FIB) system. It is useful for characterizing elemental distributions across an entire cross-section of a particle. It is a simple and universal method without using any embedding agent, enabling the powder particles with different size, shape or orientation to be easily selected based on the SEM observations. The suitable particle is covered with Pt coating layers through an ion-beam-assisted deposition. The Pt coating layers provide sufficient support for the TEM lamella. A small piece of tungsten needle is used as a support under the particle by taking a series of operations using a micromanipulator. The particle can be precisely thinned by the ion beam to be suitable for both TEM observation and EDX elemental mapping. This novel technique reduces the TEM sample preparation time to a few hours, allowing much higher efficiency compared to complicated and time-consuming embedding methods.

Epitaxial nucleation and lateral growth of high-crystalline black phosphorus films on silicon.

Black phosphorus (BP) is a promising two-dimensional layered semiconductor material for next-generation electronics and optoelectronics, with a thickness-dependent tunable direct bandgap and high carrier mobility. Though great research advantages have been achieved on BP, lateral synthesis of high quality BP films still remains a great challenge. Here, we report the direct growth of large-scale crystalline BP films on insulating silicon substrates by a gas-phase growth strategy with an epitaxial nucleation design and a further lateral growth control. The optimized lateral size of the achieved BP films can reach up to millimeters, with the ability to modulate thickness from a few to hundreds of nanometers. The as-grown BP films exhibit excellent electrical properties, with a field-effect and Hall mobility of over 1200 cm2V-1s-1 and 1400 cm2V-1s-1 at room temperature, respectively, comparable to those exfoliated from BP bulk crystals. Our work opens the door for broad applications with BP in scalable electronic and optoelectronic devices.

Precise determination of surface band bending in Ga-polar n-GaN films by angular dependent X-Ray photoemission spectroscopy.

We present a systematic study of surface band bending in Ga-polar n-GaN with different Si doping concentrations by angular dependent X-ray photoelectron spectroscopy (ADXPS). The binding energies of Ga 3d and N 1 s core levels in n-GaN films increase with increasing the emission angle, i. e., the probing depth, suggesting an upward surface band bending. By fitting the Ga 3d core level spectra at different emission angles and considering the integrated effect of electrostatic potential, the core level energy at the topmost surface layer is well corrected, therefore, the surface band bending is precisely evaluated. For moderately doped GaN, the electrostatic potential can be reflected by the simply linear potential approximation. However, for highly doped GaN samples, in which the photoelectron depth is comparable to the width of the space charge region, quadratic depletion approximation was used for the electrostatic potential to better understand the surface band bending effect. Our work improves the knowledge of surface band bending determination by ADXPS and also paves the way for studying the band bending effect in the interface of GaN based heterostructures.

Scale Effect of a Fluorescent Waveguide in Organic Micromaterials: A Case Study Based on Coumarin Microfibers.

The classical method for evaluating the waveguide ability only focuses on the optical loss coefficient. However, for the micro- or submicroscale, an organic waveguide is demonstrated by the present study whose scale effect should not be neglected. We found that the optical loss coefficient increased remarkably when decreasing the sectional size of the microfibers. Furthermore, simulations based on Finite-Difference Time-Domain also demonstrated the size-dependent effect of the waveguide. Both the experimental and simulating results showed that the optical loss coefficient converges to a certain value, which means that the scale effect can be neglected as the sectional size is large enough. On the basis of the present study, we suggest that the scale-dependent effect on the sectional size of the waveguide should be investigated by evaluating the waveguide ability by the optical loss coefficient.

Ion Sputter Induced Interfacial Reaction in Prototypical Metal-GaN System.

Contact property is now becoming to be a key factor for achieving high performance and high reliability in GaN-based III-V semiconductor devices. Energetic ion sputter, as an effective interface probe, is widely used to profile the metal/GaN contacts for interfacial analysis and process optimization. However, the details of ion-induced interfacial reaction, as well as the formation of sputter by-products at the interfaces are still unclear. Here by combining state-of-the-art Ar+ ion sputter with in-situ X-ray photoelectron spectroscopy (XPS) and ex-situ high resolution transmission electron microscopy (HRTEM), we have observed clearly not only the ion-induced chemical state changes at interface, but also the by-products at the prototypical Ti/GaN system. For the first time, we identified the formation of a metallic Ga layer at the GaOx/GaN interface. At the Ti/GaOx interface, TiCx components were also detected due to the reaction between metal Ti and surface-adsorbed C species. Our study reveals that the corresponding core level binding energy and peak intensity obtained from ion sputter depth profile should be treated with much caution, since they will be changed due to ion-induced interface reactions and formation of by-products during ion bombardment.

Constant current etching of gold tips suitable for tip-enhanced Raman spectroscopy.

We introduce a setup and method to produce gold tips that are suitable for tip-enhanced Raman spectroscopy by using a single step constant current electrochemical etch. The etching process is fully automated with only three preset parameters: the etching current, the reference voltage and the immersed length of gold wires. By optimizing these parameters, reproducible high quality tips with smooth surface and a radius curvature of about 20 nm can be formed. Tips prepared with this method were examined by tip-enhanced Raman spectroscopy experiments on the samples of single-wall carbon nanotube, p-aminothiophenol, and graphene. In the Raman mapping of single-wall carbon nanotubes, the spatial resolution is about 15 nm.

Polarized GaN-based LED with an integrated multi-layer subwavelength structure.

A novel type of GaN-based LED with a highly polarized output using an integrated multi-layer subwavelength grating structure is proposed. Characteristics of both optical transmission and polarization extinction ratio of the polarized GaN-based LED with three different multi-layer subwavelength structures are investigated. It is found that both TM transmission (T(TM)) and the extinction ratio(ER) of the LED output can be effectively enhanced by incorporating a dielectric transition layer between the metal grating and GaN substrate with a lower refractive index than that of the GaN substrate. Flat sensitivity of the T(TM) on the period, duty cycle of the metallic grating, and the wide range of operating wavelength have been achieved in contrast to the conventional sensitive behavior in single-layer metallic grating. Up to 0.75 high duty cycle of the metallic grating can be employed to achieve >60dB ER while T(TM) maintains higher than ~90%, which breaks the conventional limit of T(TM) and ER being always a pair of trade-off parameters. Typical optimized multilayer structures in terms of material, thickness, grating periods and duty cycle using MgF(2) and ZnS, respectively, as the transition layers are obtained. The results provide guidance in designing, optimizing and fabricating the novel integrated GaN-based and polarized photonic devices.