Ashley Howkins

Nanocathodoluminescence Reveals Mitigation of the Stark Shift in InGaN Quantum Wells by Si Doping

Nanocathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation in the emission wavelength of each quantum well, in correlation with the Si dopant concentration in the quantum barriers. This is reproduced by band profile simulations, which reveal the reduction of the Stark shift in the quantum wells by Si doping. We demonstrate nanocathodoluminescence is a powerful technique to optimize doping in optoelectronic devices.

Nano-cathodoluminescence reveals the effect of electron damage on the optical properties of nitride optoelectronics and the damage threshold

This work was carried out with the support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Nos. EP/NO17927/1 and EP/J003603/1. R. Oliver acknowledges funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013) ERC grant agreement number 279361 (MACONS) and the from the Royal Academy of Engineers/Leverhulme Trust senior research fellowship. This research is conducted in part using the research computing facilities offered by Information Technology Services at the University of Hong Kong.

Structural impact on the nanoscale optical properties of InGaN core-shell nanorods

The authors would like to thank OSRAM Opto Semiconductors for the provision of the GaN/Silicon templates and acknowledge the financial support from the European Union FP7 under Contract Nos. 228999 (SMASH) and 279361 (MACONS) and the EPSRC, UK (EP/M015181/1 “Manufacturing of nanoengineered III Nitride semiconductors”).

Nanodiamonds for device applications: An investigation of the properties of boron-doped detonation nanodiamonds

The inclusion of boron within nanodiamonds to create semiconducting properties would create a new class of applications in the field of nanodiamond electronics. Theoretical studies have differed in their conclusions as to whether nm-scale NDs would support a stable substitutional boron state, or whether such a state would be unstable, with boron instead aggregating or attaching to edge structures. In the present study detonation-derived NDs with purposefully added boron during the detonation process have been studied with a wide range of experimental techniques. The DNDs are of ~4 nm in size, and have been studied with CL, PL, Raman and IR spectroscopies, AFM and HR-TEM and electrically measured with impedance spectroscopy; it is apparent that the B-DNDs studied here do indeed support substitutional boron species and hence will be acting as semiconducting diamond nanoparticles. Evidence for moderate doping levels in some particles (~1017 B cm−3), is found alongside the observation that some particles are heavily doped (~1020 B cm−3) and likely to be quasi-metallic in character. The current study has therefore shown that substitutional boron doping in nm NDs is in fact possible, opening-up the path to a whole host of new applications for this interesting class of nano-particles.

Investigating the Origin of Luminescence in Zinc Oxide Nanostructures With STEM-Cathodoluminescence

ZnO nanostructures display luminescence in the UV and across the visible spectrum, and show promise as future nanoscale electronic, optoelectronic, and sensing devices [1]. The visible luminescence arises from surface or bulk states at energies inside the ZnO bandgap, however, a fundamental understanding of the luminescent sources is still lacking. The assignment of particular defects to different visible emission peaks is a highly controversial and active area of current research [1,2]. Here, we perform the first spatially resolved scanning transmission electron microscopy cathodoluminescence (STEM-CL) measurements on ZnO nanostructures, and show the emergence of CL spectral peaks associated with morphological changes in ZnO nanorods. Further studies using parallel techniques in the TEM sensitive to intrinsic and extrinsic defects (e.g. HRTEM, atomic resolution HAADF-STEM, EELS, and EDS) will likely conclusively reveal the origin of emission in ZnO and other technologically relevant, luminescent nanostructures.

Secondary Phase Interaction at Interfaces of High-Strength Brazed Joints made using Liquid Phase Sintered Alumina Ceramics and Ag-Cu-Ti Braze Alloys

Alumina-to-alumina brazed joints were formed using 96.0 and 99.7 wt.% Al2O3 ceramics using 150 µm thick Ticusil® (68.8Ag-26.7Cu-4.7 wt.% Ti) braze preforms. Brazing was conducted in a vacuum of 1 × 10−5 mbar at 850 °C for 10 minutes. Joint strengths were evaluated using four-point bend testing and were compared to the monolithic flexural strengths of standard alumina test bars according to ASTM C1161-13. Brazed joints made using 96.0 wt.% Al2O3 consistently outperformed brazed joints made using 99.7 wt.% Al2O3, despite similarities in both the flexural strengths of the standard alumina test bars and the microstructures of brazed joints. Secondary phase interaction led to the formation of Ti5Si3 reaction products at locations where the triple pocket grain boundaries of the 96.0 wt.% Al2O3 surface intersected the Ti-rich reaction layers. It is proposed that due to this interaction, brazed joints made using 96.0 wt.% Al2O3, which were relatively cost-effective to produce, achieved higher strengths than brazed joints made using 99.7 wt.% Al2O3.