Effect of Buffer Layer and III/V Ratio on the Surface Morphology of GaN Grown by MBE

The surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and "loop" defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than 1 nm RMS

Structure of CdTe/ZnTe superlattices

The structure of CdTe/ZnTe superlattices has been analyzed through θ/2θ x‐ray diffraction, photoluminescence, and in situ reflection high‐energy electron diffraction (RHEED) measurements. Samples are found to break away from Cd_(x)Zn_(1−x)Te buffer layers as a consequence of the 6% lattice mismatch in this system. However, defect densities in these superlattices are seen to drop dramatically away from the buffer layer interface, accounting for the intense photoluminescence and high‐average strain fields seen in each of our samples. Observed variations in residual strains suggest that growth conditions play a role in forming misfit defects. This could explain discrepancies with calculated values of critical thickness based on models which neglect growth conditions. Photoluminescence spectra reveal that layer‐to‐layer growth proceeded with single monolayer uniformity, suggesting highly reproducible growth. Our results give hope for relatively defect‐free Cd_(x)Zn_(1−x)Te/Cd_(y)Zn_(1−y)Te superlattices with the potential for applications to optoelectronics offered by intense visible light emitters

Accommodation of lattice mismatch in Ge_(x)Si_(1−x)/Si superlattices

We present evidence that the critical thickness for the appearance of misfit defects in a given material and heteroepitaxial structure is not simply a function of lattice mismatch. We report substantial differences in the relaxation of mismatch stress in Ge_(0.5)Si_(0.5)/Si superlattices grown at different temperatures on (100) Si substrates. Samples have been analyzed by x‐ray diffraction, channeled Rutherford backscattering, and transmission electron microscopy. While a superlattice grown at 365 °C demonstrates a high degree of elastic strain, with a dislocation density <10^5 cm^(−2) , structures grown at higher temperatures show increasing numbers of structural defects, with densities reaching 2×10^(10) cm^(−2) at a growth temperature of 530 °C. Our results suggest that it is possible to freeze a lattice‐mismatched structure in a highly strained metastable state. Thus it is not surprising that experimentally observed critical thicknesses are rarely in agreement with those predicted by equilibrium theories

Electrical determination of the valence-band discontinuity in HgTe-CdTe heterojunctions

Current-voltage behavior is studied experimentally in a Hg0.78Cd0.22Te-CdTe-Hg0.78Cd0.22Te heterostructure grown by molecular beam epitaxy. At temperatures above 160 K, energy-band diagrams suggest that the dominant low-bias current is thermionic hole emission across the CdTe barrier layer. This interpretation yields a direct determination of 390±75 meV for the HgTe-CdTe valence-band discontinuity at 300 K. Similar analyses of current-voltage data taken at 190–300 K suggest that the valence-band offset decreases at low temperatures in this heterojunction

Statistical Arbitrage Mining for Display Advertising

We study and formulate arbitrage in display advertising. Real-Time Bidding (RTB) mimics stock spot exchanges and utilises computers to algorithmically buy display ads per impression via a real-time auction. Despite the new automation, the ad markets are still informationally inefficient due to the heavily fragmented marketplaces. Two display impressions with similar or identical effectiveness (e.g., measured by conversion or click-through rates for a targeted audience) may sell for quite different prices at different market segments or pricing schemes. In this paper, we propose a novel data mining paradigm called Statistical Arbitrage Mining (SAM) focusing on mining and exploiting price discrepancies between two pricing schemes. In essence, our SAMer is a meta-bidder that hedges advertisers' risk between CPA (cost per action)-based campaigns and CPM (cost per mille impressions)-based ad inventories; it statistically assesses the potential profit and cost for an incoming CPM bid request against a portfolio of CPA campaigns based on the estimated conversion rate, bid landscape and other statistics learned from historical data. In SAM, (i) functional optimisation is utilised to seek for optimal bidding to maximise the expected arbitrage net profit, and (ii) a portfolio-based risk management solution is leveraged to reallocate bid volume and budget across the set of campaigns to make a risk and return trade-off. We propose to jointly optimise both components in an EM fashion with high efficiency to help the meta-bidder successfully catch the transient statistical arbitrage opportunities in RTB. Both the offline experiments on a real-world large-scale dataset and online A/B tests on a commercial platform demonstrate the effectiveness of our proposed solution in exploiting arbitrage in various model settings and market environments.Comment: In the proceedings of the 21st ACM SIGKDD international conference on Knowledge discovery and data mining (KDD 2015

The topographies and operant functions of challenging behaviours in Fragile X Syndrome: a systematic review and analysis of existing data

Background: Challenging behaviour, such as self-injury and physical aggression, is an issue of concern regarding a high proportion of individuals with fragile X syndrome. The aim of this review was to provide a comprehensive overview of the topographies and operant functions of challenging behaviours within the syndrome. Method: Five electronic databases were searched, identifying 18 manuscripts. Overall proportions of individuals with particular topographies of behaviour, or behaviour serving different functions, were calculated. Results: Across all participants, biting was the most common form of self-injury for males but not females. A pattern of behavioural function was observed, characterised by high levels of social-negative reinforcement, such as escape from demands. Conclusion: The existence of within-syndrome biases in the manifestation of behavioural challenges is supported by our revie

Large-Scale Correlations in the Lyman-alpha Forest at z = 3-4

We present a study of the spatial coherence of the intergalactic medium toward two pairs of high-redshift quasars with moderate angular separations observed with Keck/ESI, Q1422+2309A/Q1424+2255 (z_em = 3.63, theta = 39") and Q1439-0034A/B (z_em = 4.25, theta = 33"). The crosscorrelation of transmitted flux in the Lyman-alpha forest shows a 5-7 sigma peak at zero velocity lag for both pairs. This strongly suggests that at least some of the absorbing structures span the 230-300/h_70 proper kpc transverse separation between sightlines. We also statistically examine the similarity between paired spectra as a function of transmitted flux, a measure which may be useful for comparison with numerical simulations. In investigating the dependence of the correlation functions on spectral characteristics, we find that photon noise has little impact for S/N >~ 10 per resolution element. However, the agreement between the autocorrelation along the line sight and the crosscorrelation between sightlines, a potential test of cosmological geometry, depends significantly on instrumental resolution. Finally, we present an inventory of metal lines. These include a a pair of strong C IV systems at z ~ 3.4 appearing only toward Q1439B, and a Mg II + Fe II system present toward Q1439 A and B at z = 1.68.Comment: 33 pages, 13 figures, submitted to Ap

Drivers of intrapopulation variation in resource use in a generalist predator, the macaroni penguin

Intrapopulation variation in resource use occurs in many populations of generalist predators with important community and evolutionary implications. One of the hypothesised mechanisms for such widespread variation is ecological opportunity, i.e. resource availability determined by intrinsic constraints and extrinsic conditions. We combined tracking data and stable isotope analysis to examine how breeding constraints and prey conditions influenced intrapopulation variation in resource use among macaroni penguins Eudyptes chrysolophus. Isotopic variation was also examined as a function of breeding success, individual traits and individual specialisation. Variation in isotope ratios was greatest across multiple tissue types when birds were able to undertake mid-range foraging trips (i.e. during incubation and pre-moult). This variation was highly consistent between years that spanned a 3-fold difference in local krill Euphausia superba density and was also highly consistent at the individual level between 2 years that had similar krill densities. However, by comparing our results with previous work on the same population, it appeared that a decrease in local prey availability can increase intrapopulation variation in resource use during periods with more restricted foraging ranges (i.e. during brood-guard and crèche). This study highlights the importance of considering ecological interactions that operate on multiple spatio-temporal scales when examining the drivers of resource use in populations of generalist predators

Biochemical assessment of patients following ketogenic diets for epilepsy : current practice in the UK and Ireland

Biochemical assessment is recommended for patients prior to initiating and following a ketogenic diet (KD). There is no published literature regarding current practice in the UK and Ireland. We aimed to explore practice in comparison with international guidelines, determine approximate costs of biochemical testing in KD patients across the UK and Ireland, and promote greater consistency in KD services nationally. A survey was designed to determine the biochemical tests requested for patients at baseline, 3, 6, 12, 18, and 24 months + on KD. The survey was circulated to 39 centers across the UK and Ireland. Sixteen centers completed the survey. Full blood count, electrolytes, calcium, liver function tests (LFTs), lipid profile, and vitamin D were requested at all centers at baseline, in keeping with international guidelines. Bicarbonate, total protein, and urinalysis were less consistently requested. Magnesium and zinc were requested by all centers, despite not being specifically recommended for pre-diet evaluation in guidelines. Urea and electrolyte profiles and some LFTs were consistently requested at follow-up, in accordance with guidelines. Other LFTs and renal tests, full blood count, lipid profile, acylcarnitine profile, selenium, vitamin D, and urinalysis were less consistently requested at follow-up. The mean costs of the lowest and highest number of tests requested at baseline in our participating centers were £167.54 and £501.93; the mean costs of the lowest and highest number of tests requested at 3-month follow-up were £19.17 and £450.06. Biochemical monitoring of KD patients varies widely across the UK and Ireland and does not fully correspond to international best practice guidelines. With an ongoing drive for cost-effectiveness within health care, further work is needed to streamline practice while ensuring patient safety. [Abstract copyright: © 2019 The Authors. Epilepsia Open published by Wiley Periodicals Inc. on behalf of International League Against Epilepsy.