The carrier dynamics in InGaN/GaN LEDs are directly tied to their efficiency and maximum modulation speed, which are important metrics for solid-state lighting, displays, and optical communication. We measure the carrier dynamics of nonpolar, semipolar, and commercial c-plane InGaN/GaN LEDs using small-signal electroluminescence methods. Rate equations and a small-signal circuit are used to model the carrier dynamics. The model is fit to the optical frequency response and input impedance of the LEDs to extract the carrier lifetimes and the recombination rates. The results offer insight into the underlying causes of efficiency droop and the green gap, and inform device design strategies.
The increasing demand for wireless data communication and popularity of solid-state lighting has prompted research into visible-light communication (VLC) systems using GaN-based light-emitting diodes (LEDs). VLC is a promising candidate for next-generation (5G and beyond) network systems. To support multi-Gb/s data rates, VLC systems will require efficient LEDs with large modulation bandwidths. Conventional lighting-class LEDs cannot achieve high-speed operation due to their large chip size, large active region volume, and phosphor-converted output. Conversely, micro-scale LEDs (micro-LEDs) offer a viable path to high-speed operation. Furthermore, conventional c-plane LEDs suffer from polarization-related electric fields, which reduce the overlap between the electron and hole wave functions and lower the carrier recombination rate. Since modulation bandwidth is proportional to the carrier recombination rate, the overlap between the wave functions should be maximized for high-speed operation. Nonpolar and semipolar orientations have significantly reduced polarization effects and wave function overlaps approaching unity. These orientations can enable high-efficiency LEDs with simultaneously large modulation bandwidths. In this work, we introduce VLC and discuss progress on the growth, fabrication, and characterization of high-speed micro-LEDs. Polar (0001), nonpolar (10-10), and semipolar (20-2-1) InGaN/GaN micro-LEDs on free-standing GaN substrates are investigated for their small-signal modulation characteristics as a function of current density, temperature, device area, and active region design. Record modulation bandwidths above 1 GHz are achieved for the nonpolar and semipolar orientations. We also present a small-signal method for determining the RC characteristics, differential carrier lifetime, carrier escape lifetime, and injection efficiency of the LEDs under electrical injection.
In this study we introduce Gallium Nitride (GaN) nanowire (NW) as high aspect ratio tip with excellent durability for nano-scale metrology. GaN NWs have superior mechanical property and young modulus compare to commercial Si and Carbon tips which results in having less bending issue during measurement. The GaN NWs are prepared via two different methods: i) Catalyst-free selected area growth, using Metal Organic Chemical Vapor Deposition (MOCVD), ii) top-down approach by employing Au nanoparticles as the mask material in dry-etch process. To achieve small diameter tips, the semipolar planes of the NWs grown by MOCVD are etched using AZ400k. The diameter of the NWs fabricated using the top down process is controlled by using different size of nanoparticles and by Inductively Coupled Plasma etching. NWs with various diameters were manipulated on Si cantilevers using Focus Ion Beam (FIB) to make tips for AFM measurement. A Si (110) substrate containing nano-scale grooves with vertical 900 walls were used as a sample for inspection. AFM measurements were carried out in tapping modes for both types of nanowires (top-down and bottom-up grown nanowires) and results are compared with conventional Si and carbon nanotube tips. It is shown our fabricated tips are robust and have improved edge resolution over conventional Si tips. GaN tips made with NW’s fabricated using our top down method are also shown to retain the gold nanoparticle at tip, which showed enhanced field effects in Raman spectroscopy.
The growth of ordered arrays of group III-nitride nanostructures on c-plane gallium nitride (GaN) on sapphire using selective-area metal organic chemical vapor deposition (MOCVD) is presented. The growth of these nanostructures promotes strain relaxation that allows the combination of high indium content active regions with very low dislocation densities and also gives access to nonpolar and semipolar crystallographic orientations of GaN. The influence of the starting template and the growth conditions on the growth rate and morphology is discussed. The growth of indium gallium nitride (InGaN) active region shells on these nanostructures is discussed and the stability of various crystallographic orientations under typical growth conditions is studied. Finally, the effect of the growth conditions on the morphology of pyramidal stripe LEDs is discussed and preliminary results on electrical injection of these LEDs are presented.
Metallic metamaterial structures are used in nanophotonics applications in order to localize and enhance an incident
electromagnetic field. We have theoretically and experimentally studied resonant coupling between plasmonic modes of
an SRR array and a quantum dot-in-a-well (DWELL) heterostructure. The near-field distribution from the SRRs on the
GaAs substrate was first modeled by electromagnetic simulations and optimized SRR dimensions for maximum nearfield
coupling at the peak absorption were extracted. The DWELL sample with a ground state emission peak at 1240 nm
was grown by molecular beam epitaxy on a semi-insulating GaAs substrate. The sample was uniformly covered with an
array of SRRs, and patterned by standard electron-beam-lithography. In order to study the near field coupling of the
plasmonic structure into the DWELL, optical characterization was performed on the SRR-DWELL heterostructure,
including room temperature photoluminescence, and transmission measurement.
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