1,720,971 research outputs found
Dislocations in III-nitrides investigated by atomic force microscopy
No full textResearch on III-nitride semiconductors is achieving new heights due their high potential applications in photonics and electronics. The wide range of variation of band gap of III-N alloys allows for interesting optoelectronic applications. Low dimensional structures with high band-offsets (AlN/GaN or InN/GaN) are the basis for high efficient Light Emitting devices. Moreover, the polarization –induced high electric field, strongly localized at the interface of these heterojunctions, induces a very good confinement of 2 dimensional electron gas, which has been applied to the realization of high mobility field effect transistors [1]. However, GaN-based semiconductors are mostly grown epitaxially on sapphire, and due to the large lattice mismatch and the differences in the thermal expansion coefficients, the structures usually contain a high density of threading dislocations (TDs). While growth procedures and structures of TDs are well known [2], their electronic properties are still debated. Dislocations in GaN are known to be negatively charged and to affect mobility and electrical conduction and leakage current of GaN based devices [3,4]. Alloying of GaN with In or Al led to even a more complex scenario, as In atoms easily segregate at dislocations due to its high surface diffusivity, changing their electronic properties.
In the present contribution we will show conductive AFM and phase contrast AFM studies of TDs in GaN andAl/In GaN ternary alloys to evidence the role of the strain and the composition on the electrical properties of dislocations in III-nitrides. Local I-V curves were measured at the dislocations. Metallorganic Chemical Vapor Deposition (MOCVD) grown GaN layers and heterostructures made of AlInN/AlN/GaN with different AlN thickness, InGaN/GaN with varying percentage of In, and AlGaN/GaN with varying percentage of Al were examined. The samples were obtained by different growers (AIXTRON, III-V Lab). Surface morphology, phase separation, defect structures and their effect on the electrical properties of TDs will be reported. The comparison between the results obtained in the different alloys allowed us to understand the role of In and Al on the TDs electrical properties.
[1] M. Gonschorek, J.-F. Carlin, E. Feltin, M. A. Py, N. Grandjean, V. Darakchieva, B. Monemar, M. Lorenz, and G. Ramm, J. Appl. Phys. 103, 093714 (2008).
[2] A. Mouti, J.-L. Rouvière, M. Cantoni, J.-F. Carlin, E. Feltin, N. Grandjean, and P. Stadelmann, Phys. Rev. B 83, 195309 (2011).
[3] D. C. Look and J. R. Sizelove, Phys. Rev. Lett. 82, 1237 (1999).
[4] P. J. Hansen,Y. E. Strausser, A. N. Erickson, E. J. Tarsa, P.Kozodoy, E. G. Brazel, J. P. Ibbetson, U. Mishra, V. Narayanamurti, S. P., DenBaars, and J. S. Speck, Appl. Phys. Lett. 72, 2247 (1998).
Acknowledgments This work was supported by the EU under Project No. PITN-GA-2008-213238 (RAINBOW). The Project partners (AIXTRON and III-V Lab) are gratefully acknowledged for providing samples
Electrical Properties of Quantum Wells in III-NITRIDE Alloys and the Role of Defects
Full text linkIII-nitrides (III-Ns) semiconductors and their alloys have shown in the last few years high potential for interesting applications in photonics and electronics. III-Ns based heterostructures (HS) have been under wide investigation for different applications such as high frequency transistors, ultraviolet photodetector, light emitters etc. In the present contribution a III-Ns based heterostructure, in particular the nearly lattice matched Al1-xInxN/AlN/GaN HS will be discussed. The formation of the two dimensional electron gas (2DEG), its origin, its electrical and optical properties, the confined subband states in the well and its effect on the conduction mechanisms have been studied. Moreover, extended defects and their effect on the degradation phenomena of the 2DEG have been analyzed
Optical and electrical characterization of ternary and quaternary gallium nitride based alloys
No full textProgress in epitaxial growth techniques in the last 20 years has led to the availability of high quality ternary (InGaN) and quaternary (AlInGaN) alloys for optical devices [1], high efficient solar cells, high electron mobility transistors, microwave power applications [2] and, recently, photo-electrochemical (PEC) devices for water splitting cells [3].
Their most interesting feature is the bandgap tunable with composition, covering the whole visible spectrum. Despite extensive research on In-based alloy systems, the dependence of the bandgap, polarization properties, role of dislocations and strain relaxation mechanism on the In-content is still debated, and the effect of crystal defects on optical and electrical properties of heterostructures based on InxGa1-xN and AlxInyGa1-x-yN alloys is not yet clear.
This contribution presents the results of electrical and optical characterization of InxGa1-xN/GaN and AlxInyGa1-x-yN/GaN structures by Surface Photovoltage (SPV) Spectroscopy and deep level transient spectroscopy (DLTS) in correlation with Transmission electron microscopy analysis (TEM) and light-assisted Kelvin probe force microscopy (KPFM). The influence of In content and Si-doping concentration on the misfit dislocations and electronic transitions in InxGa1-xN layers has been investigated has been discussed. Furthermore, the two-dimensional electron gas at the AlxInyGa1-x-yN/GaN heterointerfacehas been characterized by SPV and the main recombination centers in quaternary AlxInyGa1-x-yN/GaN alloys have been determined in terms of its activation enthalpy and capture cross-section. Furthermore, light-assisted KPFM reveals direct visualization of sites associated with recombination centers/traps, which includes threading dislocations, misfit dislocations and coalescence boundaries.
The present analysis has allowed us to clarify the role of In content and misfit dislocations on alloy disorder and to characterize the defects acting as strong recombination center in ternary and quaternary GaN based alloys.
[1] S. Nakamura et al., Proc. IEEE 101, 2211 (2013).
[2] N. M. Johnson et al., Phys Today 53, 31 (2000).
[3] J. Jia et al., Nat. Commun. 7, 13237 (2016)
Electrical properties of extended defects in III-nitrides
No full textElectrical and structural properties of extended defects including threading dislocations/V-defects and nanopipes in unintentionally doped GaN, InGaN (50 nm)/GaN and AlInN (33 nm)/AlN(1 nm)/GaN heterostructures have been investigated by means of various scanning probe (Kelvin
probe and conductive-Atomic Force Microscopy) and electron beam (electron beam induced current and transmission electron microscopy) microscopy
methods. Due to low energy measurements of Kelvin probe force microscopy, charge state of the dislocations have been correctly identified
where threading dislocations (TDs) with screw-component are negatively charged, while pure-edge type TDs are neutral in InGaN/GaN. It is
explained how various factors such as indium segregation, surface termination, presence of vacancies and/or impurities affect the electrical charge,
conductivity and recombination properties of the extended defects. They are found to be strongly correlated to the type of dislocations as identified from TEM
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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