Wide-Bandgap Semiconductors: Nanostructures, Defects, and Applications

International audienceNanostructured wide-bandgap semiconductors (NWS), such as III-nitrides, SiC, ZnO, TiO 2 , diamond, AlN, and BN, have attracted intensive research attention owing to prospective applications in solid-state lighting, solar cells, power electronics, sensors, spintronics, and MEMS/NEMS. These nanostructured semiconductors exhibit tremendous advantages in terms of power capability, energy conversion efficiency , optical properties, radiation strength, high temperature , and frequency operation. Although great progress has been achieved in the synthesis of the NWS materials and promising device applications have been demonstrated since the new century, much further research in the crystallinity improvement, electronic structure control, impurities doping , and devices design need to be carried out. The growth dynamics and the defect physics of NWS should be better understood to push forward their potential applications. This special issue is focused on recent research progress in wide-bandgap semiconductors materials including novel growth strategies of NWS materials, the electronic structure tailoring for functionalization, novel devices concepts, devices physics, and applications in various fields. By using first-principles calculations based on the density functional theory, D. Ma et al. investigated the defects in gallium arsenide. A deep donor level of 0.85 eV below the conduction band minimum on the gallium arsenide crystal surface was disclosed, while the lowest donor level of the defect inside the gallium arsenide bulk was 0.83 eV. The calculations also predicted that the formation energies of internal and surface defects were around 2.36 eV and 5.54 eV, respectively. They concluded that the formation of defect within the crystal was easier than that on surface. This work would assist in tailoring the electronic structures of gallium arsenide, thus favouring the development of high-performance electronic devices. Two papers on ZnO are contributed to this issue. One is on thin film ZnO and the other on 1-dimensional ZnO nanorods. L. Meng et al. report 2-dimensional electron-gas (2DEG) properties of a Zn polar ZnMgO/MgO/ZnO structure with low Mg composition layer (= 0.05) grown on a-plane (11-20) sapphire by radical-source laser molecular beam epitaxy. They observed that the insertion of a thin (1 nm) MgO layer between ZnMgO and ZnO layers in the ZnMgO/ZnO 2DEG structures resulted in an increase of the sheet density and affected the electron mobility slightly. The resultant carrier concentration was as high as 1.1 × 10 13 cm −2 and the Hall mobility was as high as 3090 cm 2 /Vs at 10 K and 332 cm 2 /Vs at RT. The authors also calculated the dependence of carrier sheet density of the 2DEG on ZnMg

Up-conversion hybrid nanomaterials for light- and heat-driven applications

Composites or hybrid materials offer diverse properties not achievable in pure materials. Here we critically review the interesting and controllable fluorescence and photothermal properties of diverse hybrid materials containing up-conversion nanoparticles (UCNPs). These hybrids couple plasmons, photonic crystals, bio-surfaces, and two dimensional (2D) materials to the UCNPs, offering optical non-linearity, and enable effective photo-electro-thermal control leading to new light and heat driven applications. Among the light driven applications, coupling of UCNPs with graphene and molybdenum disulfide (MoS2) enables photodetectors with better photoresponse, and broader spectral range not accessible to individual components. Irradiated MoS2 coupled-UCNPs is a new paradigm in resistive random access memory devices. Conjugation of graphene and perovskites, with the UCNPs, have led to novel optical limiting phenomenon and better solar cells. Examples of new opportunities offered by UCNPs in heat driven applications are photothermal water desalination using solar daylight and photothermal disintegration of fat droplets in obesity treatment. Phonons, manifesting as heat, can also be utilized to enhance fluorescence and translate to high sensitivity nanothermometers. This review covers fundamentals, and applications of the new UCNP-enabled class of hybrid materials in energy harnessing, light sources and detectors, memory devices, nanothermometers, desalination, intracellular pH sensing, and cancer theranostics.</p

Design for Approaching Cicada-Wing Reflectance in Low- and High-Index Biomimetic Nanostructures

Natural nanostructures in low refractive index Cicada wings demonstrate ≤1% reflectance over the visible spectrum. We provide design parameters for Cicada-wing-inspired nanotip arrays as efficient light harvesters over a 300–1000 nm spectrum and up to 60° angle of incidence in both low-index, such as silica and indium tin oxide, and high-index, such as silicon and germanium, photovoltaic materials. Biomimicry of the Cicada wing design, demonstrating gradient index, onto these material surfaces, either by real electron cyclotron resonance microwave plasma processing or by modeling, was carried out to achieve a target reflectance of ∼1%. Design parameters of spacing/wavelength and length/spacing fitted into a finite difference time domain model could simulate the experimental reflectance values observed in real silicon and germanium or in model silica and indium tin oxide nanotip arrays. A theoretical mapping of the length/spacing and spacing/wavelength space over varied refractive index materials predicts that lengths of ∼1.5 μm and spacings of ∼200 nm in high-index and lengths of ∼200–600 nm and spacings of ∼100–400 nm in low-index materials would exhibit ≤1% target reflectance and ∼99% optical absorption over the entire UV–vis region and angle of incidence up to 60°

Microplasma-Enabled Graphene Quantum Dot-Wrapped Gold Nanoparticles with Synergistic Enhancement for Broad Band Photodetection

Plasmonic nanostructure/semiconductor nanohybrids offer many opportunities for emerging electronic and optoelectronic device applications because of their unique geometries in the nanometer scale and material properties. However, the development of a simple and scalable synthesis of plasmonic nanostructure/semiconductor nanohybrids is still lacking. Here, we report a direct synthesis of colloidal gold nanoparticle/graphene quantum dot (Au@GQD) nanohybrids under ambient conditions using microplasmas and their application as photoabsorbers for broad band photodetectors (PDs). Due to the unique AuNP core and graphene shell nanostructures in the synthesized Au@GQD nanohybrids, the plasmonic absorption of the AuNP core extends the usable spectral range of the photodetectors. It is demonstrated that the Au@GQD-based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of ∼103 A/W, ultrahigh detectivity of 1013 Jones, and fast response time in the millisecond scale (65 ms rise time and 53 ms fall time). We suggest that the synergistic effect can be attributed to the strong fluorescence quenching in Au@GQD coupled with the two-dimensional graphene layer in the device. This work provides knowledge of tailoring the optical absorption in GQDs with plasmonic AuNPs and the corresponding photophysics for broad band response in PD-related devices

High <i>K</i> Nanophase Zinc Oxide on Biomimetic Silicon Nanotip Array as Supercapacitors

A 3D trenched-structure metal–insulator–metal (MIM) nanocapacitor array with an ultrahigh equivalent planar capacitance (EPC) of ∼300 μF cm^–2 is demonstrated. Zinc oxide (ZnO) and aluminum oxide (Al₂O₃) bilayer dielectric is deposited on 1 μm high biomimetic silicon nanotip (SiNT) substrate using the atomic layer deposition method. The large EPC is achieved by utilizing the large surface area of the densely packed SiNT (∼5 × 10¹⁰ cm^–2) coated conformally with an ultrahigh dielectric constant of ZnO. The EPC value is 30 times higher than those previously reported in metal–insulator–metal or metal–insulator–semiconductor nanocapacitors using similar porosity dimensions of the support materials