Search

We report a hexagonal GaN nanorod-based two-dimensional photonic crystal (PhC) slab for phosphor-conversion white light emitting devices analyzed by three-dimensional finite-difference time-domain simulation; this slab has a broad reflection band at yellow wavelength with low Fabry-Pérot background at normal incidence. For practical use as a wavelength-selective reflector, a buffer layer under the PhC slab is employed to sustain the nanorods in the PhC slab. However, we observed that the buffer layer placed below the slab destroys the broad reflection band due to evanescent coupling of electromagnetic field in the slab and the buffer layer. By introducing small-sized base pillars between the slab and the buffer layer, we could decouple the interaction between the slab and the buffer layer and maintain the broad reflection band without any unexpected dips. Since this GaN nanorod-based PhC slab is designed for practical light emitting devices by considering dielectric and transparent conducting layers, this structure is directly applicable for developing hybrid white light emitting devices having both an (active) blue-color-emitting nanorod emitters and a (passive) normal reflector of phosphor emission.

Future technologies require faster data transfer and processing with lower loss. A photonic diode could be an attractive alternative to the present Si-based electronic diode for rapid optical signal processing and communication. Here, we report highly asymmetric photonic diode behavior with low scattering loss, from tapered core-shell quantum well semiconductor rods that were fabricated to have a large gradient in their bandgap energy along their growth direction. Local laser illumination of the core-shell quantum well rods yielded a huge contrast in light output intensities from opposite ends of the rod.

Luminescence and carrier dynamics of GaN/InGaN multi-quantum-well coaxial nanorods (MCNRs) were studied by means of photoluminescence (PL), cathodoluminescence (CL) and time-resolved PL (TRPL). The PL of as-grown MCNRs showed an intense blue emission together with broad emission at longer wavelengths. CL measurements of several single MCNRs attributed the broad emission to the wetting layer frequently observed in vapor–liquid–solid-grown nanorods. Non-single exponential intensity decays were observed by TRPL, which were ascribed to the In fluctuation in the InGaN alloy. Radiative and non-radiative lifetimes were then calculated via a stretched exponential model. An ultrafast carrier lifetime in the range of a few tens of picoseconds along with a high internal quantum efficiency (IQE) of about 59% resulted. The ultrafast carrier lifetimes were attributed to the improvement in the carrier collection efficiency due to the radial heterostructuring of GaN with InGaN shells, while the high IQE implied that carriers were mostly recombined radiatively. This study reveals that the coaxial growth of InGaN with GaN nanorods resulted in an ultrafast carrier lifetime and a luminescence efficiency that could be controlled by adjusting the growth temperature gap between the GaN and the InGaN.

White light emitting InGaN nanostructures hold a key position in future solid-state lighting applications. Although many suggested approaches to form group III-nitride vertical structures have been reported, more practical and cost effective methods are still needed. Here, we present a new approach to GaN/InGaN core-shell nanostructures at a wafer level formed by chemical vapor-phase etching and metal-organic chemical vapor deposition. Without a patterning process, we successfully obtained high quality and polarization field minimized In-rich GaN/InGaN core-shell nanostructures. The various quantum well thicknesses and the multi-facets of the obelisk-shaped core-shell nanostructures provide a broad spectrum of the entire visible range without changing the InGaN growth temperature. Due to their high crystal quality and polarization field reduction, the core-shell InGaN quantum wells show an ultrafast radiative recombination time of less than 200 ps and uniformly high internal quantum efficiency in the broad spectral range. We also investigated the important role of polarization fields in the complex recombination dynamics in InGaN quantum wells.

A dislocation-eliminating chemical control method for high-quality GaN nanostructures together with various types of InGaN quantum well structures are demonstrated using a chemical vapor-phase etching technique. Unlike chemical wet etching, chemical vapor-phase etching could efficiently control the GaN and form various shapes of dislocation-free and strain-relaxed GaN nanostructures. The chemically controlled GaN nanostructures showed improved crystal quality due to the selective etching of defects and revealed various facets with reduced residual strain via the facet-selective etching mechanism. These structural properties derived excellent optical performance of the GaN nanostructures. The chemical vapor-phase etching method also showed possibilities of the fascinating applications for high-efficiency InGaN quantum well structures, such as InGaN quantum well layer on void embedded GaN layer, InGaN quantum well embedded GaN nanostructure, and InGaN/GaN core/shell nanostructure.

Electrically driven quantum dot, wire, and well hybrid light-emitting diodes are demonstrated by using nanometer-sized pyramid structures of GaN. InGaN quantum dots, wires, and wells are formed at the tops, edges, and sidewalls of pyramids, respectively. The hybrid light-emitting diodes containing low-dimensional quantum structures are good candidates for broad-band highly efficient visible lighting sources.

The hybrid nature of exciton polaritons opens up possibilities for developing a new concept nonlinear photonic device (e.g., polariton condensation, switching, and transistor) with great potential for controllability. Here, we proposed a novel type of polariton system resulting from strong coupling between a two-dimensional exciton and whispering gallery mode photon using a core–shell GaN/InGaN hexagonal wire. High quality, nonpolar InGaN multiple-quantum wells (MQWs) were conformally formed on a GaN core nanowire, which was spatially well matched with whispering gallery modes inside the wire. Both high longitudinal-transverse splitting of nonpolar MQWs and high spatial overlap with whispering gallery modes lead to unprecedented large Rabi splitting energy of ∼180 meV. This structure provides a robust polariton effect with a small footprint; thus, it could be utilized for a wide range of interesting applications.

We present group III-nitride semiconductor nanostructures grown on various types of GaN-based pyramid, annular, columnar, and tapered structures, together with their applications in broadband light emitting devices, unidirectional photonic diodes, and single photon sources.

We report the measurement of the (r13, r33) Pockels electro-optic coefficients in a GaN thin film grown on a Si(111) substrate. The converse piezoelectric (d33) and electro-absorptive coefficients are simultaneously determined. Single crystalline GaN epitaxial layers were grown with a AlGaN buffer layer by metal organic chemical vapor deposition, and their structural and optical properties were systematically investigated. The electro-optic, converse piezoelectric, and electro-absorptive coefficients of the GaN layer are determined using an original method. A semi-transparent gold electrode is deposited on the top of the GaN layer, and an alternating voltage is applied between top and bottom electrodes. The coefficients are simultaneously and analytically determined from the measurement of the electric-field-induced variation ΔR(θ) in the reflectivity of the Au/GaN/buffer/Si stack, versus incident angle and light polarization. The method also enables to determine the GaN layer polarity. The results obtained for a Ga-face [0001] GaN layer when using a modulation frequency of 230 Hz are for the electro-optic coefficients r13 = +1.00 ± 0.02 pm/V, r33 = +1.60 ± 0.05 pm/V at 633 nm, and for the converse piezoelectric coefficient d33 = +4.59 ± 0.03 pm/V. The value measured for the electro-absorptive variation at 633 nm is Δko/ΔE = +0.77 ± 0.05 pm/V.

A key issue in a single photon source is fast and efficient generation of a single photon flux with high light extraction efficiency. Significant progress toward high-efficiency single photon sources has been demonstrated by semiconductor quantum dots, especially using narrow bandgap materials. Meanwhile, there are many obstacles, which restrict the use of wide bandgap semiconductor quantum dots as practical single photon sources in ultraviolet-visible region, despite offering free space communication and miniaturized quantum information circuits. Here we demonstrate a single InGaN quantum dot embedded in an obelisk-shaped GaN nanostructure. The nano-obelisk plays an important role in eliminating dislocations, increasing light extraction, and minimizing a built-in electric field. Based on the nano-obelisks, we observed nonconventional narrow quantum dot emission and positive biexciton binding energy, which are signatures of negligible built-in field in single InGaN quantum dots. This results in efficient and ultrafast single photon generation in the violet color region.

International audience; We report here the experimental results on a field effect refractive index change into gallium nitride (GaN) structures. This effect is characterized through the common prism-coupling technique with the application of a vertical direct-current electric field. Surface plasmon propagation was used to increase the sensitivity of the electro-optic measurements. We have obtained a large refractive index variation for GaN epilayer, around 1.4×10−2 at 1.55μm wavelength. In order to understand the origin of the index modulation, we have conducted a scanning transmission electron microscopy analysis and discussed the influence of threading dislocations density acting as traps and thermo-optic effect. According to recent works, we observed experimentally the optical response of a non-linear electro-optic effect on GaN on Si(111) substrate and estimated a Kerr coefficient of about 2.14×10−16m2V−2.

A good justification for gallium nitride on silicone is the potential for integrated optoelectronic circuits and for the low cost bring by growth of GaN on a large size wafers. Actually, the application interest for GaN/Si is power electronics. This work focused on the optimization of the growth process for GaN/Si and the relation between the structure and the optical properties. Using the guided wave prism coupling technique, we have fully established the index dispersion of GaN at room temperature and its temperature dependence in the wavelength range 0.4 to 1.5µm. We report a slightly low temperature dependence. Results demonstrated excellent waveguide properties of GaN on silicon with optical propagation loss below 1dB/cm. We compared trhe results on Si with those on sapphire. This opens a real opportunity of future device using this technology.