Michael N. Fairchild

Effect of threading defects on InGaN∕GaN multiple quantum well light emitting diodes

Photoelectrochemical etching was used to measure the threading defect (TD) density in InGaN multiple quantum well light-emitting diodes (LEDs) fabricated from commercial quality epitaxial wafers. The TD density was measured in the LED active region and then correlated with the previously measured characteristics of these LEDs. It was found that the reverse leakage current increased exponentially with TD density. The temperature dependence of this dislocation-related leakage current was consistent with a hopping mechanism at low reverse-bias voltage and Poole-Frenkel emission at higher reverse-bias voltage. The peak intensity and spectral width of the LED electroluminescence were found to be only weakly dependent on TD density for the measured TD range of 1×107–2×108cm−2.

Fabrication of GaN nanowire arrays by confined epitaxy

The authors report the fabrication of GaN nanowire arrays inside a thick SiNx, selective growth mask that was patterned by interferometric lithography and dry etching. The GaN nanowires are molded by the apertures in the selective growth mask and the growth is epitaxial with respect to the underlying GaN layer. The precise location and diameter of each nanowire in the array are controlled by the growth mask patterning, and the resulting array has a long-range order that is compatible with photonic crystal applications. This process uses conventional metal organic precursors and does not require any additional metal catalysts.

Defect reduction mechanisms in the nanoheteroepitaxy of GaN on SiC

This article describes defect reduction mechanisms that are active during the growth of GaN by nanoheteroepitaxy on (0001) 6H SiC. Nanoheteroepitaxial (NHE) and planar GaN epitaxial films were grown and compared using transmission electron microscopy, photoluminescence, x-ray diffraction, and time resolved photoluminescence. It was found that in addition to the previously reported defect reduction mechanism that results from the high compliance of nanoscale nuclei, other independent defect reduction mechanisms are also active during NHE including: (i) filtering of substrate defects, (ii) improved coalescence at the nanoscale, and (iii) defect termination at local free surfaces. Also, it was found that the biaxial strain in the GaN film could be significantly reduced by using a “grouped” NHE pattern geometry. Time resolved photoluminescence measurements on NHE GaN samples with this geometry showed a more than tenfold increase in carrier lifetime compared to GaN grown on planar SiC.

Nanoheteroepitaxial growth of GaN on Si nanopillar arrays

Nanoheteroepitaxial growth of GaN by metal-organic chemical-vapor deposition on dense arrays of (111) Si nanopillars has been investigated. Scanning electron microscopy, cross-sectional transmission electron microscopy, and electron-diffraction analysis of 0.15-μm-thick GaN layers indicate single-crystal films. Most of the mismatch defects were in-plane stacking faults and the threading dislocation concentration was <108cm−2 at the interface and decreased away from the interface. High-resolution transmission electron microscopy indicated that grain-boundary defects could heal and were followed by high quality, single-crystal GaN. Facetted voids were also present at the GaN∕Si interface and are believed to be an additional strain-energy reduction mechanism. The unusual defect behavior in these samples appears to be related to the high compliance of the nanopillar silicon substrate.

Using Electrons As a High-Resolution Probe of Optical Modes in Individual Nanowires

While nanowires show increasing promise for optoelectronic applications, probing the subwavelength details of their optical modes has been a challenge with light-based techniques. Here we report the excitation of dielectric optical waveguide modes in a single GaN nanowire using transition radiation generated by a 1 nm diameter electron beam. This spatially resolved study opens important gateways to probing the optical modes of more complex nanostructures, fundamental for optimization of optoelectronic device performance.

Photoelectrochemical etching measurement of defect density in GaN grown by nanoheteroepitaxy

The density of dislocations in n-type GaN was measured by photoelectrochemical etching. A 10× reduction in dislocation density was observed compared to planar GaN grown at the same time. Cross-sectional transmission electron microscopy studies indicate that defect reduction is due to the mutual cancellation of dislocations with equal and opposite Burger’s vectors. The nanoheteroepitaxy sample exhibited significantly higher photoluminescence intensity and higher electron mobility than the planar reference sample.

Controlled Growth of Ordered III-Nitride Core–Shell Nanostructure Arrays for Visible Optoelectronic Devices

We demonstrate the growth of ordered arrays of nonpolar

Ordered arrays of bottom-up III-nitride core-shell nanostructures

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