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The effects of hydrogen incorporation into beta-Ga2O3 thin films have been investigated by chemical, electrical and optical characterization techniques. Hydrogen incorporation was achieved by remote plasma doping without any structural alterations of the film; however, X-ray photoemission reveals major changes in the oxygen chemical environment. Depth-resolved cathodoluminescence (CL) reveals that the near-surface region of the H-doped Ga2O3 film exhibits a distinct red luminescence (RL) band at 1.9 eV. The emergence of the H-related RL band is accompanied by an enhancement in the electrical conductivity of the film by an order of magnitude. Temperature-resolved CL points to the formation of abundant H-related donors with a binding energy of 28 +/- 4 meV. The RL emission is attributed to shallow donor-deep acceptor pair recombination, where the acceptor is a VGa-H complex and the shallow donor is interstitial H. The binding energy of the VGa-H complex, based on our experimental considerations, is consistent with the computational results by Varley et al [J. Phys.: Condens. Matter, 23, 334212, 2011]., Comment: 3 figures

Large reflection losses at interfaces in light emitting semiconductor devices cause a significant reduction in their light emission and energy efficiencies. Metal nanoparticle (NP) surface coatings have been demonstrated to increase the light extraction efficiency from planar high refractive index semiconductor surfaces. This emission enhancement in Au NP-coated ZnO is widely attributed to involvement of a green (~ 2.5 eV) deep level ZnO defect exciting localized surface plasmons in the NPs that non-radiatively decay into hot electrons, which return to ZnO and radiatively recombine. In this work, we achieve a 6 times enhancement of the ultra-violet excitonic emission in ZnO nanorods that are coated with 5 nm Au NPs without the aid of ZnO defects. Cathodoluminescence (CL), photoluminescence (PL) and a novel concurrent CL-PL technique as well as time resolved PL spectroscopy revealed that the increase in UV emission is due to the formation of an additional fast excitonic relaxation pathway that increases the exciton spontaneous emission rate. Concurrent CL-PL measurements ruled out the presence of charge transfer mechanism in the emission enhancement process. While time resolved PL results confirmed the existence of a new excitonic recombination channel that is attributed to exciton relaxation via the plasmon-assisted excitation of rapid non-radiative Au interband transitions. Our results establish that ZnO defect levels ~ 2.5 eV are not required to facilitate Au NP induced enhancement of the ZnO UV emission.

The UV enhancement of Al-coated ZnO single crystals with a wide range of carrier densities is systematically studied using depth-resolved cathodoluminescence (CL) and photoluminescence (PL) as well as valence band X-ray photoemission spectroscopy (VB-XPS). AN up to 17-fold enhanced PL UV emission for Al-coated ZnO with the highest carrier density (2 x 10^17 cm^-3) was measured, which falls to a 12-fold increase for the lowest carrier density (3 x 10^13 cm^-3). Depth-resolved CL measurements confirm that the enhancement is strongest near the metal coating-ZnO interface consistent with an increased UV emission due to an exciton-localized surface plasmon coupling mechanism. Correlative CL, PL and VB-XPS studies reveal that a number of additional effects to the presence of the Al surface coating also contribute to the UV enhancement factor. These include increased UV enhancement due to the formation of a surface depletion layer induced by the metal surface coating, which also passivates competitive non-radiative surface recombination channels found in uncoated ZnO. Significantly, it was established that the magnitude of the emission enhancement factor can be raised in a controlled way by reducing the thickness of the depletion layer by increasing the carrier density. The contribution of these effects collectively provides an explanation for the large span of enhancement factors reported in the literature., Comment: 22 pages, 10 figures

Metal nanoparticle (NP) surface coatings are known to significantly enhance the ultra-violet luminescence intensity of ZnO. Although there is general agreement that resonantly excited Localized Surface Plasmons (LSPs) in metal NPs can directly couple to excitons in the semiconductor increasing their spontaneous emission rate, the exact mechanisms involved in this phenomenon are currently not fully understood. In this work, LSP-exciton coupling in a ZnO single crystal and ZnO nanorods coated with a 2 nm Al layer has been investigated using correlative photoluminescence and depth-resolved cathodoluminescence and time-resolved photoluminescence spectroscopy. Temperature-resolved cathodoluminescence and photoluminescence measurements from 10 K to 250 K show enhancement factors up to 12 times of the free exciton (FX) emission at 80 K. The FX couple more efficiently to the LSPs in Al compared to the localized donor-bound excitons. Furthermore, a strong polarization dependence of the LSPs with respect to the FX was observed with higher enhanced FX transitions polarized in the same direction as the electric field of the incident excitation. These results indicate that selective enhancement of the ultra-violet excitonic PL in ZnO can be achieved by careful alignment of the crystallographic axes of the ZnO relative to the electric vector of the excitation source., Comment: 20 pages, 9 figures

Online social media, periodically serves as a platform for cascading polarizing topics of conversation. The inherent community structure present in online social networks (homophily) and the advent of fringe outlets like Gab have created online "echo chambers" that amplify the effects of polarization, which fuels detrimental behavior. Recently, in October 2018, Gab made headlines when it was revealed that Robert Bowers, the individual behind the Pittsburgh Synagogue massacre, was an active member of this social media site and used it to express his anti-Semitic views and discuss conspiracy theories. Thus to address the need of automated data-driven analyses of such fringe outlets, this research proposes novel methods to discover topics that are prevalent in Gab and how they cascade within the network. Specifically, using approximately 34 million posts, and 3.7 million cascading conversation threads with close to 300k users; we demonstrate that there are essentially five cascading patterns that manifest in Gab and the most "viral" ones begin with an echo-chamber pattern and grow out to the entire network. Also, we empirically show, through two models viz. Susceptible-Infected and Bass, how the cascades structurally evolve from one of the five patterns to the other based on the topic of the conversation with upto 84% accuracy.