5 research outputs found
Gallium Nitride Based Logpile Photonic Crystals
We demonstrate a nine-layer logpile three-dimensional photonic crystal (3DPC) composed of single crystalline gallium nitride (GaN) nanorods, ā¼100 nm in size with lattice constants of 260, 280, and 300 nm with photonic band gap in the visible region. This unique GaN structure is created through a combined approach of a layer-by-layer template fabrication technique and selective metal organic chemical vapor deposition (MOCVD). These GaN 3DPC exhibit a stacking direction band gap characterized by strong optical reflectance between 380 and 500 nm. By introducing a āline-defectā cavity in the fifth (middle) layer of the 3DPC, a localized transmission mode with a quality factor of 25ā30 is also observed within the photonic band gap. The realization of a group III nitride 3DPC with uniform features and a band gap at wavelengths in the visible region is an important step toward realizing complete control of the electromagnetic environment for group III nitride based optoelectronic devices
Atom Probe Tomography of <i>a</i>-Axis GaN Nanowires: Analysis of Nonstoichiometric Evaporation Behavior
GaN nanowires oriented along the nonpolar <i>a</i>-axis were analyzed using pulsed laser atom probe tomography (APT). Stoichiometric mass spectra were achieved by optimizing the temperature, applied dc voltage, and laser pulse energy. Local variations in the measured stoichiometry were observed and correlated with facet polarity using scanning electron microscopy. Fewer N atoms were detected from nonpolar and Ga-polar surfaces due to uncorrelated evaporation of N<sub>2</sub> ions following N adatom diffusion. The observed differences in Ga and N ion evaporation behaviors are considered in detail to understand the influence of intrinsic materials characteristics on the reliability of atom probe tomography analysis. We find that while reliable analysis of IIIāN alloys is possible, the standard APT procedure of empirically adjusting analysis conditions to obtain stoichiometric detection of Ga and N is not necessarily the best approach for this materials system
Three-Dimensional Mapping of Quantum Wells in a GaN/InGaN CoreāShell Nanowire Light-Emitting Diode Array
Correlated atom probe tomography,
cross-sectional scanning transmission
electron microscopy, and cathodoluminescence spectroscopy are used
to analyze InGaN/GaN multiquantum wells (QWs) in nanowire array light-emitting
diodes (LEDs). Tomographic analysis of the In distribution, interface
morphology, and dopant clustering reveals material quality comparable
to that of planar LED QWs. The position-dependent CL emission wavelength
of the nonpolar side-facet QWs and semipolar top QWs is correlated
with In composition
Spatial Mapping of Efficiency of GaN/InGaN Nanowire Array Solar Cells Using Scanning Photocurrent Microscopy
GaNāInGaN coreāshell
nanowire array devices are characterized
by spectrally resolved scanning photocurrent microscopy (SPCM). The
spatially resolved external quantum efficiency is correlated with
structure and composition inferred from atomic force microscope (AFM)
topography, scanning transmission electron microscope (STEM) imaging,
Raman microspectroscopy, and scanning photocurrent microscopy (SPCM)
maps of the effective absorption edge. The experimental analyses are
coupled with finite difference time domain simulations to provide
mechanistic understanding of spatial variations in carrier generation
and collection, which is essential to the development of heterogeneous
novel architecture solar cell devices
Nonpolar InGaN/GaN CoreāShell Single Nanowire Lasers
We report lasing from nonpolar p-i-n
InGaN/GaN multi-quantum well coreāshell single-nanowire lasers
by optical pumping at room temperature. The nanowire lasers were fabricated
using a hybrid approach consisting of a top-down two-step etch process
followed by a bottom-up regrowth process, enabling precise geometrical
control and high material gain and optical confinement. The modal
gain spectra and the gain curves of the coreāshell nanowire
lasers were measured using micro-photoluminescence and analyzed using
the Hakki-Paoli method. Significantly lower lasing thresholds due
to high optical gain were measured compared to previously reported
semipolar InGaN/GaN coreāshell nanowires, despite significantly
shorter cavity lengths and reduced active region volume. Mode simulations
show that due to the coreāshell architecture, annular-shaped
modes have higher optical confinement than solid transverse modes.
The results show the viability of this p-i-n nonpolar coreāshell
nanowire architecture, previously investigated for next-generation
light-emitting diodes, as low-threshold, coherent UVāvisible
nanoscale light emitters, and open a route toward monolithic, integrable,
electrically injected single-nanowire lasers operating at room temperature