Repositório Institucional da Universidade de Aveiro
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Defect studies and optical activation of Yb doped GaN
No full textWide band-gap semiconductors, particularly III-nitrides, became one of the most studied materials during the last decades. These compounds are the base of a new generation of optoelectronic devices operating in the UV-Blue region of the electromagnetic spectrum. Incorporation of rare-earth (RE) ions into nitrides creates new routes to build all-nitride electroluminescent devices, using the sharp intra-4fn transitions of these elements. The introduction of the RE ions in the nitride lattice during the growth or by ion implantation creates defects which influence the optical behaviour of the doped region. In this work we report the results on Yb implanted GaN. A combination of techniques (Rutherford backscattering/Channeling and Photoluminescence) was used to assess the mechanisms responsible for the optical and structural behaviour of the doped materials. Lattice site location experiments showed that Yb is incorporated into positions slightly displaced from the Ga-site. Clearly the optical activity of the RE could be enhanced by orders of magnitude reducing the number of non-radiative recombination paths related with defectsFCT - PPCDT/FIS/57550/2004bilateral collaboration Portugal-Poland (GRICES)
Erbium-doped nanocrystalline silicon thin films produced by RF sputtering - Annealing effect on the Er emission
No full textPartilhar documento na coleção da comunidade Laboratório Associado I3NIn the present work, erbium doped nanocrystalline silicon thin films were produced by reactive magnetron sputtering on glass substrates under different conditions (substrate temperature and Er content). The film structure was studied using Raman spectroscopy. The chemical composition was determined using the RBS technique. All the samples show sharp 4I13/2 → 4I15/2 intra-4f11 emission of Er 3+ related centres with its maximum positioned at the1.54 μm. However, the intensity of this transition (strongly dependent on the chemical composition of the matrix where the nanocrystals are embedded in and also on the structure of the matrix) changes after thermal annealing treatment. For the less crystalline samples our results show an increase of the Er3+ PL intensity and for the highly crystalline ones the Er emission vanishes even at low temperature. This behaviour was studied and explained in this work, on the basis of energy transfer between Si and Er ions
High temperature annealing of rare earth implanted GaN films: Structural and optical properties
No full textGaN epilayers grown by MOCVD were implanted with Tm and Eu under different implantation conditions, in order to optimize the implantation parameters of fluence, implantation temperature and energy. Rutherford backscattering spectrometry in the channelling mode (RBS/C), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL) and room temperature cathodoluminescence (CL) were used to study structural and optical properties of the samples. Different annealing methods are compared. According to RBS/C measurements, Tm at low fluences is found to incorporate entirely on substitutional Ga-sites while for higher fluences the substitutional fraction decreases. Increasing the fluence leads to increasing damage levels and finally to the formation of a partially amorphous, nanocrystalline surface layer. The luminescence intensity increases with fluence at first. However, for fluences in excess of 2 × 1015 Tm/cm2 it decreases again, showing a strong correlation between structural and optical properties. Implanting at higher temperature inhibits the formation of the nanocrystalline layer and increases both the substitutional fraction and the luminescence intensity. We also found that the presence of a thin AlN cap protects the sample during post-implant high temperature annealing and prevents the formation of a nanocrystalline surface layer during the implantation even for high fluences. The intensity of Eu-related CL near 622 nm at room temperature increases by a factor of 40 within the studied annealing range from 1000 to 1300 °C.European Research Training Network project RENiBEL HPRNCT-2001-0029
Bandwidth boosting in transimpedance amplifiers for optical LANs, using delayed feedback
No full textDelayed-feedback amplifiers are composed of, a forward gain amplifier, a feedback network and a time-delay element. Recently published results [1] claim the possibility of bandwidth enhancement for this type of systems, simply by a judicious choice of loop-gain, open loop poles and the net time-delay. This paper describes the design steps for a shunt-shunt feedback amplifier with an active delay on the feedback loop. The designed circuit was optimized for Free-Space Optical (FSO) line-of-sight receivers. Achieved results show that the inclusion of the active delay, improve bandwidth in a maximum of 60%. A maximum of 1.5THzΩ. GBW (11kΩ. transimpedance gain) was achieved with a photodiode having intrinsic capacitance of 100pF
Novel cohesive beam model for the End-Notched Flexure (ENF) specimen
No full textThis paper presents a new Cohesive Beam Model (CBM) for the mode II interlaminar fracture End-Notched Flexure (ENF) specimen. The ideal linear elastic solution agrees with a recent analysis and is similar to the well-known Corrected Beam Theory. However, the cohesive zone is known to be particularly important for modelling the large mode II fracture process zone. CBM with a bilinear cohesive law agreed very well with finite element analyses. Comparison with experimental results for two carbon/epoxy laminates indicated that the bilinear cohesive law provides a fairly good conservative approximation
Influence of intralaminar cracking on the apparent interlaminar mode I fracture toughness of cross-ply laminates
No full textOne of the major difficulties in interlaminar fracture tests of multidirectional laminates is the high tendency for intralaminar cracking and the resulting wavy crack propagation. Experimental work showed that this occurred in double cantilever beam (DCB) tests of cross-ply laminates having a starter crack on a 0degrees/90degrees interface. Moreover, under steady-state propagation conditions, the apparent values of the critical strain energy release rate G(Ic) were two times higher than those of 0degrees/0degrees specimens. In this paper, a finite-element-based progressive damage model was used to simulate crack propagation in cross-ply specimens. The results showed that transverse cracking alone cannot be responsible for the above difference of G(Ic) values. Therefore, the higher propagation G(Ic) values for cross-plies must be attributed to the more extensive fibre bridging observed and to plastic deformations of the 90degrees interfacial ply.FCT - POCTI/EME/38731/2001, FEDER EU fun
Unsupervised feature extraction via kernel subspace techniques
No full textThis paper provides a new insight into unsupervised feature extraction techniques based on kernel subspace models. The data projected onto kernel subspace models are new data representations which might be better suited for classification. The kernel subspace models are always described exploiting the dual form for the basis vectors which requires that the training data must be available even during the test phase. By exploiting an incomplete Cholesky decomposition of the kernel matrix, a computationally less demanding implementation is proposed. Online benchmark data sets allow the evaluation of these feature extraction methods comparing the performance of two classifiers which both have as input either the raw data or the new representations.FCT - PhD Scholarship (SFRH/BD/28404/2006)Grants of DAA
Studies comparing surrogate measures for head posture in individuals with and without neck pain
No full textBackground: Head posture (HP) assessment of patients with neck pain (NP) is claimed to be useful in aiding diagnosis, determining treatment strategies and monitoring patient progress. It is assumed that patients with NP have poorer HP than asymptomatic individuals.
Objectives: To determine whether there are differences in angles or linear distances between anatomical landmarks used as surrogates for HP between individuals with and without NP.
Methods: Studies were sought from PubMed, CINAHL, Physiotherapy Evidence Database, Sports Discus, Web of Science, Academic Search Premier and The Cochrane Library. Two reviewers screened titles and abstracts, and assessed full reports for potentially eligible studies.
Data extraction and synthesis: Two reviewers independently extracted information on participants' characteristics, study methods and study quality. Discrepancies were resolved using a third reviewer as arbiter. Study heterogeneity prevented meta-analysis so a tally of study outcome was performed.
Results: Seven of 13 included studies found no statistically significant differences in measurements of components of HP between participants with NP and asymptomatic participants. When compared to asymptomatic individuals NP patients were found to have greater forward HP in 4 of 19 comparisons (11 studies), greater head extension/flexion in two of nine comparisons (six studies) and less side-flexion and less rotation in one of one comparison (one study).
Conclusion: There is insufficient good quality evidence to determine whether forward HP, head extension, side-flexion and rotation differ between participants with NP and asymptomatic participants.SFRH/BD/30735/200
Comparison of single and multi-user pre-filtering techniques using different chip mapping for DL MC-CDMA systems
No full textIn this paper we compare the performance of the joint space-frequency single and multi-user pre-filtering techniques that have been proposed recently for DL MC-CDMA systems. We consider the use of antenna arrays at the base station and a single antenna at the mobile terminal (MT). We investigate how the chip mapping strategy impacts on the performance of the pre-filtering approaches. Thus, practical combinations of prefiltering and chip mapping are discussed. The performance is evaluated for different chip mapping strategies, considering typical indoor and pedestrian scenarios and both uncoded data and channel turbo coding based on UMTS specifications. © 2007 IEEE.European WISQUAS Celtic project CP2-035FCT FFTXXI project POSI/CPS/4670/200
AIN content influence on the properties of Al x Ga1- x N doped with Pr ions
No full textThe purpose of this work is the study of the structural and optical properties of Alx Ga1- x N films grown on (0 0 0 1) sapphire substrates with different AlN molar fraction implanted with 150 keV of praseodymium ions with a fluence of 2.5 × 1014 cm-2. The main goal is to achieve the optical doping of Alx Ga1- x N with the rare earth Pr. Structural properties, damage accumulation and Pr lattice site location were studied combining Rutherford backscattering/channeling spectrometry and high resolution X-ray diffraction. The channeling data clearly indicate a higher resistance of the lattice to irradiation damage with the increase of the AlN content. Detailed angular scans reveal a fraction above 90% of Pr incorporated in near substitutional sites. A pronounced narrowing at the bottom of the Pr angular curve along the c-axis suggests the presence of a minor fraction with a higher displacement towards the center of the channel. The displacement is less pronounced with the increase of Ga content in the samples. Ionoluminescence with a 1H+ microbeam reveal an enhancement of the optical activity when the AlN content is in the intermediate range of concentrationsPTDC/CTM/100756/200