Silane-Induced N-Polarity in Wires Probed by a Synchrotron Nanobeam

Spatially and Time-Resolved Carrier Dynamics in Core-Shell InGaN/GaN Multiple-Quantum Wells on GaN Wire

Time-resolved cathodoluminescence is a key tool with high temporal and spatial resolution. However, optical spectroscopic information can be also extracted using synchrotron pulses in a hard X-ray nanoprobe, exploiting a phenomenon called X-ray excited optical luminescence. Here, with 20 ps time resolution and 80 nm lateral resolution, we applied this time-resolved X-ray microscopy technique to individual core-shell InGaN/GaN multiple quantum well heterostructures deposited on GaN wires. Our findings suggest that the m-plane related multiple quantum well states govern the carrier dynamics. Likewise, our observations support not only the influence of In incorporation in the recombination rates, but also carrier localization phenomena at the hexagon wire apex. In addition, our experiment calls for further investigations of the spatiotemporal domain on the underlying mechanisms of optoelectronic nanodevices. Its great potential becomes more valuable when time-resolved X-ray excited optical luminescence microscopy is used in operando with other methods, such as X-ray absorption spectroscopy.

Mapping Inversion Domain Boundaries along Single GaN Wires with Bragg Coherent X-ray Imaging

Gallium nitride (GaN) is of technological importance for a wide variety of optoelectronic applications. Defects in GaN, like inversion domain boundaries (IDBs), significantly affect the electrical and optical properties of the material. We report, here, on the structural configurations of planar inversion domain boundaries inside n-doped GaN wires measured by Bragg coherent X-ray diffraction imaging. Different complex domain configurations are revealed along the wires with a 9 nm in-plane spatial resolution. We demonstrate that the IDBs change their direction of propagation along the wires, promoting Ga-terminated domains and stabilizing into {11̅00}, that is, m-planes. The atomic phase shift between the Ga- and N-terminated domains was extracted using phase-retrieval algorithms, revealing an evolution of the out-of-plane displacement (∼5 pm, at maximum) between inversion domains along the wires. This work provides an accurate inner view of planar defects inside small crystals.

A helium mini-cryostat for the nanoprobe beamline ID16B at ESRF: characteristics and performance

A helium mini-cryostat has been developed for the hard X-ray nanoprobe ID16B of the European Synchrotron to collect X-ray excited optical luminescence and X-ray fluorescence at low temperature (<10 K). The mini-cryostat has been specifically designed to fit within the strong space restrictions and high-demanding mechanical constraints imposed by the beamline to provide vibration-free operation and maximal thermal stability. This paper reports the detailed design, architecture and technical requirements of the mini-cryostat, and presents the first experimental data measured using the cryogenic equipment. The resulting cryo-system features ultimate thermal stability, fast cool-down and ultra-low vibrations. The simultaneous X-ray fluorescence and X-ray excited optical luminescence data acquired from bulk GaN and core/shell InGaN/GaN multi-quantum wells validated the excellent performance of the cryostat with ultimate resolution, stability and sensitivity.

Capabilities of time-resolved X-ray excited optical luminescence of the Taiwan Photon Source 23A X-ray nanoprobe beamline

Time-resolved X-ray excited optical luminescence (TR-XEOL) was developed successfully for the 23A X-ray nanoprobe beamline located at the Taiwan Photon Source (TPS). The advantages of the TR-XEOL facility include (i) a nano-focused X-ray beam (<60 nm) with excellent spatial resolution and (ii) a streak camera that can simultaneously record the XEOL spectrum and decay time. Three time spans, including normal (30 ps to 2 ns), hybrid (30 ps to 310 ns) and single (30 ps to 1.72 µs) bunch modes, are available at the TPS, which can fulfil different experimental conditions involving samples with various lifetimes. It is anticipated that TR-XEOL at the TPS X-ray nanoprobe could provide great characterization capabilities for investigating the dynamics of photonic materials.

Determining GaN Nanowire Polarity and its Influence on Light Emission in the Scanning Electron Microscope

The crystal polarity of noncentrosymmetric wurtzite GaN nanowires is determined nondestructively in the scanning electron microscope using electron backscatter diffraction (EBSD). The impact of the nanowire polarity on light emission is then investigated using cathodoluminescence (CL) spectroscopy. EBSD can determine polarity of noncentrosymmetric crystals by interrogating differences in the intensity distribution of bands of the EBSD pattern associated with semipolar planes. Experimental EBSD patterns from an array of GaN nanowires are compared with theoretical patterns produced using dynamical electron simulations to reveal whether they are Ga- or N-polar or, as in several cases, of mixed polarity. CL spectroscopy demonstrates the effect of the polarity on light emission, with spectra obtained from nanowires of known polarity revealing a small but measurable shift (≈28 meV) in the GaN near band edge emission energy between those with Ga and N polarity. We attributed this energy shift to a difference in impurity incorporation in nanowires of different crystal polarity. This approach can be employed to nondestructively identify polarity in a wide range of noncentrosymmetric nanoscale material systems and provide direct comparison with their luminescence.

Investigation of Cavity Enhanced XEOL of a Single ZnO Microrod by Using Multifunctional Hard X-ray Nanoprobe

The multifunctional hard X-ray nanoprobe at Taiwan Photon Source (TPS) exhibits the excellent ability to simultaneously characterize the X-ray absorption, X-ray excited optical luminescence (XEOL) as well as the dynamics of XEOL of materials. Combining the scanning electron microscope (SEM) into the TPS 23A end-station, we can easily and quickly measure the optical properties to map out the morphology of a ZnO microrod. A special phenomenon has been observed that the oscillations in the XEOL associated with the confinement of the optical photons in the single ZnO microrod shows dramatical increase while the X-ray excitation energy is set across the Zn K-edge. Besides having the nano-scale spatial resolution, the synchrotron source also gives a good temporal domain measurement to investigate the luminescence dynamic process. The decay lifetimes of different emission wavelengths and can be simultaneously obtained from the streak image. Besides, SEM can provide the cathodoluminescence (CL) to be a complementary method to analyze the emission properties of materials, we anticipate that the X-ray nanoprobe will open new avenues with great characterization ability for developing nano/microsized optoelectronic devices.

Crystal engineering by tuning the growth kinetics of GaN 3-D microstructures in SAG-HVPE

The growth of GaN 3-D microstructures is investigated by SAG-HVPE.