Gradients in Three-Dimensional Core-Shell GaN/InGaN Structures: Optimization and Physical Limitations.

Three-dimensional InGaN/GaN nano- and microstructures with high aspect ratios and large active sidewall areas are still of great interest in the field of optoelectronics. However, when grown by metalorganic chemical vapor deposition (MOCVD), their optical performance can be negatively affected by gradients in thickness and peak emission wavelength along their sidewalls, which is still a key obstacle for using such structures in commercial products. In this work, we present a detailed study on the different mechanisms causing this gradient, as well as means to alleviate it. Gas-phase mass transport and surface diffusion are found to be the two main processes governing the shell growth, and the predominance of one process over the other is varying with the geometry of the 3D structures as well as the spacing between them. Consequently, variations in temperature, which mainly affect surface diffusion, will have a stronger impact on structures with small separation between them rather than larger ones. On the other hand, variations in pressure modify gas-phase diffusion, and thus, structures with a large spacing will be more strongly affected. A proper design of the dimensions of 3D architectures as well as the separation between them may improve the gradient along the sidewalls, but a tradeoff with the active area per wafer footprint is inevitable.

Size-Dependent Electroluminescence and Current-Voltage Measurements of Blue InGaN/GaN µLEDs down to the Submicron Scale.

Besides high-power light-emitting diodes (LEDs) with dimensions in the range of mm, micro-LEDs (µLEDs) are increasingly gaining interest today, motivated by the future applications of µLEDs in augmented reality displays or for nanometrology and sensor technology. A key aspect of this miniaturization is the influence of the structure size on the electrical and optical properties of µLEDs. Thus, in this article, investigations of the size dependence of the electro-optical properties of µLEDs, with diameters in the range of 20 to 0.65 µm, by current-voltage and electroluminescence measurements are described. The measurements indicated that with decreasing size leakage currents in the forward direction decrease. To take advantage of these benefits, the surface has to be treated properly, as otherwise sidewall damages induced by dry etching will impair the optical properties. A possible countermeasure is surface treatment with a potassium hydroxide based solution that can reduce such defects.

Directly addressable GaN-based nano-LED arrays: fabrication and electro-optical characterization.

The rapid development of display technologies has raised interest in arrays of self-emitting, individually controlled light sources atthe microscale. Gallium nitride (GaN) micro-light-emitting diode (LED) technology meets this demand. However, the current technology is not suitable for the fabrication of arrays of submicron light sources that can be controlled individually. Our approach is based on nanoLED arrays that can directly address each array element and a self-pitch with dimensions below the wavelength of light. The design and fabrication processes are explained in detail and possess two geometries: a 6 × 6 array with 400 nm LEDs and a 2 × 32 line array with 200 nm LEDs. These nanoLEDs are developed as core elements of a novel on-chip super-resolution microscope. GaN technology, based on its physical properties, is an ideal platform for such nanoLEDs.

Toward three-dimensional hybrid inorganic/organic optoelectronics based on GaN/oCVD-PEDOT structures.

The combination of inorganic semiconductors with organic thin films promises new strategies for the realization of complex hybrid optoelectronic devices. Oxidative chemical vapor deposition (oCVD) of conductive polymers offers a flexible and scalable path towards high-quality three-dimensional inorganic/organic optoelectronic structures. Here, hole-conductive poly(3,4-ethylenedioxythiophene) (PEDOT) grown by oxidative chemical vapor deposition is used to fabricate transparent and conformal wrap-around p-type contacts on three-dimensional microLEDs with large aspect ratios, a yet unsolved challenge in three-dimensional gallium nitride technology. The electrical characteristics of two-dimensional reference structures confirm the quasi-metallic state of the polymer, show high rectification ratios, and exhibit excellent thermal and temporal stability. We analyze the electroluminescence from a three-dimensional hybrid microrod/polymer LED array and demonstrate its improved optical properties compared with a purely inorganic microrod LED. The findings highlight a way towards the fabrication of hybrid three-dimensional optoelectronics on the sub-micron scale.

Preparation of silver nanoparticles coated ZnO/Fe3O4 composites using chemical reduction method for sensitive detection of uric acid via surface-enhanced Raman spectroscopy.

In this study, silver nanostructures decorated magnetic nanoparticles for surface-enhanced Raman scattering (SERS) measurements were prepared via heat induced catalytic activity of ZnO nanostructures. The ZnO/Fe3O4 composite was first prepared by dispersing pre-formed magnetic nanoparticles into alkaline zinc nitrate solutions. After annealing of the precipitates, the formed ZnO/Fe3O4 composites were successfully decorated with silver nanostructures by dispersing the composites into silver nitrate/ethylene glycol solution at 95 °C in water bath. To find the optimal condition when preparing Ag/ZnO/Fe3O4 composites for SERS measurements, factors such as reaction time and concentration of silver nitrate were studied. Results indicated that the formation of silver nanoparticles (AgNPs) on ZnO/Fe3O4 was significantly improved with the assistance of ZnO. The concentration of silver nitrate and reaction time affected the morphologies and sizes of the formed composites and optimal condition in preparation of the composites for SERS measurement was found using 100 mM of silver nitrate with a reaction time of 20 min. Under optimized conditions, the obtained SERS intensities were highly reproducible. The substrates were applied for quantitative analysis of uric acid in aqueous solution and a linear response for concentrations up to 10 µM was obtained. Successful application of these prepared composites to determine uric acid in urine sample without any pretreatment of the urine sample was done.