Optical characterization of InGaAsN / GaAs quantum wells: Effects of annealing and determination of the band offsets

In the last decade great attention has been given to the characteristics of dilute nitrides. Both their peculiar physical properties and their wide range of possible applications have attracted the interest of many experimental and theoretical groups. In this thesis work some open questions about the fundamental properties of dilute nitrides have been answered. Two important topics have been investigated: the correlation between the optical and morphological properties of InGaAsN / GaAs single quantum well samples (SQWs) and a quantitative, model-independent determination of the band offsets for the same types of structures. In chapter 3, a combined study of photoluminescence (PL) measurements and transmission electron microscopy (TEM) analysis has allowed to find a direct correlation between the degree of carrier localization in the sample and the homogeneity of the material. In particular, the degree of localization increases with increasing the inhomogeneity of the QW layer. On the basis of that, it has been found that the growth temperature (Tg) and the indium content strongly influence the morphology of the InGaAsN QW samples. With increasing Tg or with increasing [In], the inhomogeneity of the sample increases. The growth temperature affects also the optical properties of InGaAsN SQWs. By raising Tg, the PL intensity degrades and the peak emission energy red shifts. On the other hand, the indium content does not remarkably influence the PL efficiency of the QW. The only exception is for very high indium contents ([In] > 34%). In this case, dislocations due to strain relaxation and / or other types of non-radiative recombination centres are created causing a drastic decrease of the PL intensity. After annealing both the morphological and the optical properties are modified. Most notably, by employing samples grown in the range of temperatures between 360 °C and 480 °C annealed in different environments, two important conclusions have been found. First of all, morphology and PL efficiency are not always correlated and secondly, the PL efficiency of a QW directly depends on the density of non-radiative centres. Annealing samples in different atmospheres is a novelty in the literature and it has been the key-point to reach these findings. The first conclusion has been obtained by performing photoluminescence measurements on samples annealed in hydrogen and argon environment, and comparing the results with those of as-grown samples. It has been shown that while the PL intensity of H2-annealed samples is maximum for low values of Tg (400 °C) and minimum for high Tg (450 °C), the PL intensity of the Ar-annealed samples is maximum for high values of Tg (450 °C) and minimum for low Tg (400 °C). In contrast, the degree of localization and the TEM images have shown the same Tg-behaviour, independently of the annealing environment. The second conclusion has been reached by performing time resolved photoluminescence measurements on the same series of samples. It has been shown that whilst the radiative decay time varies with Tg in the same manner for the two annealing atmospheres, i.e. it increases with increasing Tg, the non-radiative decay time varies with the growth temperature in a different way for different annealing environments. In particular, the non-radiative decay time decreases with increasing Tg for H2-annealed samples and increases with increasing Tg for Ar-annealed samples. This behaviour correlates in both cases with the dependence of the PL intensity on the growth temperature. In addition to that, by performing power dependent PL measurements, it has been verified that changes of degree of localization after annealing are only due to morphological modification of the sample. By comparing the results obtained performing PL measurements on GaAsN / GaAs, InGaAs / GaAs, and InGaAsN / GaAs SQW samples, it has been shown that at least two different type of defects are created during the growth of InGaAsN SQWs. One type of defect is related to the presence of nitrogen. The density of these defects increases with Tg and decreases by annealing. Defects of another type are related to the simultaneous presence of indium and nitrogen. They are created at low Tg and tend to agglomerate under annealing. These two types of defects have been employed in a simple model in order to justify the main results obtained in this chapter. In chapter 4, a much debated topic has been analysed: the evolution of the band offsets of InGaAsN / GaAs structures with varying QW parameters. The chapter has been initially focussed on the refinement of the information which can be obtained employing an experimental method developed at Infineon Technologies based on surface photovoltage (SPV) measurements. With this method it is possible to identify optical transitions involving bound states and extended states in a QW sample. In particular, in addition to the bound-to-bound transitions, also the indirect transition from the extended state of the valence band to the first confined state of the conduction band can be identified. This allows the easy determination of the practical band offsets of the QW. These quantities represent the energy values of the conduction (valence) band offset of the heterostructures without the value of the first quantized state of the electrons (holes). For the design of a device, the practical band offsets are fundamental quantities because they quantify the real confinement of the carriers in the well. SPV measurements have been performed on several dedicated series of samples. The results have been compared with those obtained employing other optical techniques and performing theoretical simulations. It has been shown that by using this method, it is possible to gather comprehensive information about a single quantum well which otherwise could be obtained only by combining different experimental techniques and theoretical calculations. With this method transitions related to the ground states of the QW involving both the heavy and light holes states can be detected. Also, the excited states can be identified. As a main condition, it has been shown that only bound-to-bound transitions having the same parity can generate a step in the spectra. This method has been employed to investigate the band states of dilute nitrides SQWs. In particular, the effect of varying nitrogen and indium content on the practical band offsets of InxGa1-xAs1-yNy /GaAs SQW samples has been analysed. As a main result, it has been found that with increasing nitrogen content, the conduction band offset strongly increases (with a rate of about 100 meV / [N]), while the valence band offset is almost unchanged. Moreover, with increasing indium concentration both the conduction and the valence band offsets are modified. In particular, the conduction band offset varies with indium content as in the case of N-free samples. These results represent the first quantitative analysis which directly, i.e. independently of any model, determines the band offsets in dilute nitrides quantum wells. More importantly, it allows to analyse the effect of nitrogen and indium on the conduction and valence band states separately. The practical band offsets are highly important parameters in the design of many devices. Therefore, in the end of this thesis, it has been shown that the SPV method can be employed to determine the practical band offsets of real device structures. In particular, the practical conduction and valence band offsets of lasers emitting at 1.3 µm and 1.5 µm have been determined from the SPV spectra

Optical characterization of InGaAsN / GaAs quantum wells: Effects of annealing and determination of the band offsets

In the last decade great attention has been given to the characteristics of dilute nitrides. Both their peculiar physical properties and their wide range of possible applications have attracted the interest of many experimental and theoretical groups. In this thesis work some open questions about the fundamental properties of dilute nitrides have been answered. Two important topics have been investigated: the correlation between the optical and morphological properties of InGaAsN / GaAs single quantum well samples (SQWs) and a quantitative, model-independent determination of the band offsets for the same types of structures. In chapter 3, a combined study of photoluminescence (PL) measurements and transmission electron microscopy (TEM) analysis has allowed to find a direct correlation between the degree of carrier localization in the sample and the homogeneity of the material. In particular, the degree of localization increases with increasing the inhomogeneity of the QW layer. On the basis of that, it has been found that the growth temperature (Tg) and the indium content strongly influence the morphology of the InGaAsN QW samples. With increasing Tg or with increasing [In], the inhomogeneity of the sample increases. The growth temperature affects also the optical properties of InGaAsN SQWs. By raising Tg, the PL intensity degrades and the peak emission energy red shifts. On the other hand, the indium content does not remarkably influence the PL efficiency of the QW. The only exception is for very high indium contents ([In] > 34%). In this case, dislocations due to strain relaxation and / or other types of non-radiative recombination centres are created causing a drastic decrease of the PL intensity. After annealing both the morphological and the optical properties are modified. Most notably, by employing samples grown in the range of temperatures between 360 °C and 480 °C annealed in different environments, two important conclusions have been found. First of all, morphology and PL efficiency are not always correlated and secondly, the PL efficiency of a QW directly depends on the density of non-radiative centres. Annealing samples in different atmospheres is a novelty in the literature and it has been the key-point to reach these findings. The first conclusion has been obtained by performing photoluminescence measurements on samples annealed in hydrogen and argon environment, and comparing the results with those of as-grown samples. It has been shown that while the PL intensity of H2-annealed samples is maximum for low values of Tg (400 °C) and minimum for high Tg (450 °C), the PL intensity of the Ar-annealed samples is maximum for high values of Tg (450 °C) and minimum for low Tg (400 °C). In contrast, the degree of localization and the TEM images have shown the same Tg-behaviour, independently of the annealing environment. The second conclusion has been reached by performing time resolved photoluminescence measurements on the same series of samples. It has been shown that whilst the radiative decay time varies with Tg in the same manner for the two annealing atmospheres, i.e. it increases with increasing Tg, the non-radiative decay time varies with the growth temperature in a different way for different annealing environments. In particular, the non-radiative decay time decreases with increasing Tg for H2-annealed samples and increases with increasing Tg for Ar-annealed samples. This behaviour correlates in both cases with the dependence of the PL intensity on the growth temperature. In addition to that, by performing power dependent PL measurements, it has been verified that changes of degree of localization after annealing are only due to morphological modification of the sample. By comparing the results obtained performing PL measurements on GaAsN / GaAs, InGaAs / GaAs, and InGaAsN / GaAs SQW samples, it has been shown that at least two different type of defects are created during the growth of InGaAsN SQWs. One type of defect is related to the presence of nitrogen. The density of these defects increases with Tg and decreases by annealing. Defects of another type are related to the simultaneous presence of indium and nitrogen. They are created at low Tg and tend to agglomerate under annealing. These two types of defects have been employed in a simple model in order to justify the main results obtained in this chapter. In chapter 4, a much debated topic has been analysed: the evolution of the band offsets of InGaAsN / GaAs structures with varying QW parameters. The chapter has been initially focussed on the refinement of the information which can be obtained employing an experimental method developed at Infineon Technologies based on surface photovoltage (SPV) measurements. With this method it is possible to identify optical transitions involving bound states and extended states in a QW sample. In particular, in addition to the bound-to-bound transitions, also the indirect transition from the extended state of the valence band to the first confined state of the conduction band can be identified. This allows the easy determination of the practical band offsets of the QW. These quantities represent the energy values of the conduction (valence) band offset of the heterostructures without the value of the first quantized state of the electrons (holes). For the design of a device, the practical band offsets are fundamental quantities because they quantify the real confinement of the carriers in the well. SPV measurements have been performed on several dedicated series of samples. The results have been compared with those obtained employing other optical techniques and performing theoretical simulations. It has been shown that by using this method, it is possible to gather comprehensive information about a single quantum well which otherwise could be obtained only by combining different experimental techniques and theoretical calculations. With this method transitions related to the ground states of the QW involving both the heavy and light holes states can be detected. Also, the excited states can be identified. As a main condition, it has been shown that only bound-to-bound transitions having the same parity can generate a step in the spectra. This method has been employed to investigate the band states of dilute nitrides SQWs. In particular, the effect of varying nitrogen and indium content on the practical band offsets of InxGa1-xAs1-yNy /GaAs SQW samples has been analysed. As a main result, it has been found that with increasing nitrogen content, the conduction band offset strongly increases (with a rate of about 100 meV / [N]), while the valence band offset is almost unchanged. Moreover, with increasing indium concentration both the conduction and the valence band offsets are modified. In particular, the conduction band offset varies with indium content as in the case of N-free samples. These results represent the first quantitative analysis which directly, i.e. independently of any model, determines the band offsets in dilute nitrides quantum wells. More importantly, it allows to analyse the effect of nitrogen and indium on the conduction and valence band states separately. The practical band offsets are highly important parameters in the design of many devices. Therefore, in the end of this thesis, it has been shown that the SPV method can be employed to determine the practical band offsets of real device structures. In particular, the practical conduction and valence band offsets of lasers emitting at 1.3 µm and 1.5 µm have been determined from the SPV spectra

Public Education – Information and Precedents: Effects of Deer Overabundance on Plant Communities

Final project for PLSC480: Urban Ecology, Management of Urban Forest Edges (Spring 2016). Department of Plant Science and Landscape Architecture, University of Maryland, College Park.White-tailed deer (Odocoileus virginianus) are present in overwhelming numbers in urban and suburban areas. Forest fragmentation supports populations of the species, which thrives in wooded edges, and those populations drive significant changes to the local plant communities. While researchers and managers know a great deal about these interactions, the general public is not always aware of the nature and severity of the effects of deer on plants. Public desire drives how money for management is applied, so poor communication of research results can reduce the effectiveness of management efforts. Communicating the negative impacts of deer on the environment is necessary to encourage public support for management.Howard Count

High brightness, highly directional organic light-emitting diodes as light sources for future light-amplifying prosthetics in the optogenetic management of vision loss

Funding: Engineering and Physical Sciences Research Council (Grant Number(s): EP/R010595/1). National Science Foundation (Grant Number(s): 1706207). Defense Sciences Office, DARPA (Grant Number(s): N66001-17-C-4012). Leverhulme Trust (Grant Number(s): RPG-2017-231). Alexander von Humboldt-Stiftung (Grant Number(s): Humboldt Professur). National Research Foundation of Korea (GrantNumber(s): 2017R1A6A3A03012331). China Sponsorship Council.Optogenetic control of retinal cells transduced with light-sensitive channelrhodopsins can enable restoration of visual perception in patients with vision loss. However, a light intensity orders of magnitude higher than ambient light conditions is required to achieve robust cell activation. Relatively bulky wearable light amplifiers are currently used to deliver sufficient photon flux (>1016 photons/cm2/s in a ±10° emission cone) at a suitable wavelength (e.g., 600 nm for channelrhodopsin ChrimsonR). Here, ultrahigh brightness organic light-emitting diodes (OLEDs) with highly directional emission are developed, with the ultimate aim of providing high-resolution optogenetic control of thousands of retinal cells in parallel from a compact device. The orange-emitting phosphorescent OLEDs use doped charge transport layers, generate narrowband emission peaking at 600 nm, and achieve a luminance of 684 000 cd m–2 at 15 V forward bias. In addition, tandem-stack OLEDs with a luminance of 1 152 000 cd m–2 and doubled quantum efficiency are demonstrated, which greatly reduces electrical and thermal stress in these devices. At the photon flux required to trigger robust neuron firing in genetically modified retinal cells and when using heat sinking and realistic duty cycles (20% at 12.5 Hz), the tandem-stack OLEDs therefore show a greatly improved half-brightness lifetime of 800 h.Publisher PDFPeer reviewe

Performance Comparison of Time-Step-Driven versus Event-Driven Neural State Update Approaches in SpiNNaker

The SpiNNaker chip is a multi-core processor optimized for neuromorphic applications. Many SpiNNaker chips are assembled to make a highly parallel million core platform. This system can be used for simulation of a large number of neurons in real-time. SpiNNaker is using a general purpose ARM processor that gives a high amount of flexibility to implement different methods for processing spikes. Various libraries and packages are provided to translate a high-level description of Spiking Neural Networks (SNN) to low-level machine language that can be used in the ARM processors. In this paper, we introduce and compare three different methods to implement this intermediate layer of abstraction. We have examined the advantages of each method by various criteria, which can be useful for professional users to choose between them. All the codes that are used in this paper are available for academic propose.EU H2020 grant 644096 ECOMODEEU H2020 grant 687299 NEURAM3Ministry of Economy and Competitivity (Spain) / European Regional Development Fund TEC2015-63884-C2-1-P (COGNET

Engine knock margin control using in-cylinder pressure data: preliminary results

Knock is an undesired phenomenon occurring in spark ignited engines and is controlled acting on the spark timing. This paper presents a closed-loop architecture that makes possible to address the knock control problem with a standard model-based design approach. An engine knock margin estimate is feedback controlled through a PI regulator and its target value is computed starting from the desired knock probability. A black-box modelling approach is used to identify the dynamics between the spark timing and the knock margin and a traditional model-based controller synthesis is performed. Experimental results at the test bench show that, compared to a conventional strategy, the proposed approach allows for a better compromise between the controller speed and the variability of the spark timing. Moreover, another advantage w.r.t. the conventional strategies is that closed-loop performance prove to be constant for different reference probabilities, leading to a more regular engine behaviour

First-in-human, double-blind, placebo-controlled, randomized, dose-escalation study of BG00010, a glial cell line-derived neurotrophic factor family member, in subjects with unilateral sciatica

OBJECTIVE: To evaluate the safety, tolerability, and pharmacokinetics of single doses of BG00010 (neublastin, artemin, enovin) in subjects with unilateral sciatica. METHODS: This was a single-center, blinded, placebo-controlled, randomized Phase 1 sequential-cohort, dose-escalation study (ClinicalTrials.gov identifier NCT00961766; funded by Biogen Idec). Adults with unilateral sciatica were enrolled at The Royal Adelaide Hospital, Australia. Four subjects were assigned to each of eleven cohorts (intravenous BG00010 0.3, 1, 3, 10, 25, 50, 100, 200, 400, or 800 μg/kg, or subcutaneous BG00010 50 μg/kg) and were randomized 3:1 to receive a single dose of BG00010 or placebo. The primary safety and tolerability assessments were: adverse events; clinical laboratory parameters and vital signs; pain as measured by a Likert rating scale; intra-epidermal nerve fiber density; and longitudinal assessment of quantitative sensory test parameters. Blood, serum, and plasma samples were collected for pharmacokinetic and pharmacodynamic assessments. Subjects were blinded to treatment assignment throughout the study. The investigator was blinded to treatment assignment until the Data Safety Review Committee review of unblinded data, which occurred after day 28. RESULTS: Beyond the planned enrollment of 44 subjects, four additional subjects were enrolled into to the intravenous BG00010 200 μg/kg cohort after one original subject experienced mild generalized pruritus. Therefore, a total of 48 subjects were enrolled between August 2009 and December 2011; all were included in the safety analyses. BG00010 was generally well tolerated: in primary analyses, the most common treatment-emergent adverse events were changes in temperature perception, pruritus, rash, or headache; no trends were observed in clinical laboratory parameters, vital signs, intra-epidermal nerve fiber density, or quantitative sensory testing. BG00010 was not associated with any clear, dose-dependent trends in Likert pain scores. BG00010 was rapidly distributed, with a prolonged terminal elimination phase. CONCLUSIONS: These data support the development of BG00010 for the treatment of neuropathic pain. TRIAL REGISTRATION: ClinicalTrials.gov NCT00961766.Paul E. Rolan, Gilmore O, Neill, Eve Versage, Jitesh Rana, Yongqiang Tang, Gerald Galluppi, Ernesto Aycard

Comparing Neuromorphic Solutions in Action : Implementing a Bio-Inspired Solution to a Benchmark Classification Task on Three Parallel-Computing Platforms

Copyright © 2016 Diamond, Nowotny and Schmuker. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.Neuromorphic computing employs models of neuronal circuits to solve computing problems. Neuromorphic hardware systems are now becoming more widely available and "neuromorphic algorithms" are being developed. As they are maturing toward deployment in general research environments, it becomes important to assess and compare them in the context of the applications they are meant to solve. This should encompass not just task performance, but also ease of implementation, speed of processing, scalability, and power efficiency. Here, we report our practical experience of implementing a bio-inspired, spiking network for multivariate classification on three different platforms: the hybrid digital/analog Spikey system, the digital spike-based SpiNNaker system, and GeNN, a meta-compiler for parallel GPU hardware. We assess performance using a standard hand-written digit classification task. We found that whilst a different implementation approach was required for each platform, classification performances remained in line. This suggests that all three implementations were able to exercise the model's ability to solve the task rather than exposing inherent platform limits, although differences emerged when capacity was approached. With respect to execution speed and power consumption, we found that for each platform a large fraction of the computing time was spent outside of the neuromorphic device, on the host machine. Time was spent in a range of combinations of preparing the model, encoding suitable input spiking data, shifting data, and decoding spike-encoded results. This is also where a large proportion of the total power was consumed, most markedly for the SpiNNaker and Spikey systems. We conclude that the simulation efficiency advantage of the assessed specialized hardware systems is easily lost in excessive host-device communication, or non-neuronal parts of the computation. These results emphasize the need to optimize the host-device communication architecture for scalability, maximum throughput, and minimum latency. Moreover, our results indicate that special attention should be paid to minimize host-device communication when designing and implementing networks for efficient neuromorphic computing.Peer reviewe