Adsorption and desorption of hydrogen at nonpolar GaN(1-100) surfaces: Kinetics and impact on surface vibrational and electronic properties

The adsorption of hydrogen at nonpolar GaN(1-100) surfaces and its impact on the electronic and vibrational properties is investigated using surface electron spectroscopy in combination with density functional theory (DFT) calculations. For the surface mediated dissociation of H2 and the subsequent adsorption of H, an energy barrier of 0.55 eV has to be overcome. The calculated kinetic surface phase diagram indicates that the reaction is kinetically hindered at low pressures and low temperatures. At higher temperatures ab-initio thermodynamics show, that the H-free surface is energetically favored. To validate these theoretical predictions experiments at room temperature and under ultrahigh vacuum conditions were performed. They reveal that molecular hydrogen does not dissociatively adsorb at the GaN(1-100) surface. Only activated atomic hydrogen atoms attach to the surface. At temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface atoms and result in an inversion of the Ga-N surface dimer buckling. The signatures of the Ga-H and N-H vibrational modes on the H-covered surface have experimentally been identified and are in good agreement with the DFT calculations of the surface phonon modes. Both theory and experiment show that H adsorption results in a removal of occupied and unoccupied intragap electron states of the clean GaN(1-100) surface and a reduction of the surface upward band bending by 0.4 eV. The latter mechanism largely reduces surface electron depletion

MEAM interatomic potentials of Ni, Re, and Ni–Re alloys for atomistic fracture simulations

Second nearest neighbor modified embedded atom method (2NN-MEAM) interatomic potentials are developed for the Ni, Re, and Ni–Re binaries. To construct the potentials, density functional theory (DFT) calculations have been employed to calculate fundamental physical properties that play a dominant role in fracture. The potentials are validated to accurately reproduce material properties that correlate with material’s fracture behavior. The thus constructed potentials were applied to perform large scale simulations of mode I fracture in Ni and Ni–Re binaries with low Re content. Substitutional Re did not alter the ductile nature of crack propagation, though it resulted in a monotonous increase of the critical stress intensity factor with Re content

Κοινή Πολιτική Ασφάλειας και Άμυνας της Ευρωπαϊκής Ένωσης : Η περίπτωση των πολιτικών και στρατιωτικών επιχειρήσεων

Το τέλος του ψυχρού πολέμου είχε σημαντικές επιπτώσεις στο γνωστικό αντικείμενο των Διεθνών Σχέσεων. Κατά συνέπεια, η διεθνής κοινότητα βρέθηκε αντιμέτωπη με μια σειρά διλημμάτων πολιτικής, ηθικής και νομικής φύσης. Οι αλλαγές αυτές και τα γεγονότα που τις προκάλεσαν οδήγησαν στη διαμόρφωση ενός νέου κώδικα αξιών και τάσεων που προσδιόρισε την συμπεριφορά των κρατών απέναντι στις προκλήσεις που χτυπάγανε τη πόρτα τους. Η δημιουργία ενός νέου συστήματος Διεθνών Σχέσεων έχει σημασία από πρακτική αλλά και από θεωρητική σκοπιά εφόσον εμπεριέχει αρχές και αξίες που επηρεάζουν τη λήψη αποφάσεων εξωτερικής πολιτικής. «Έχουμε εισέλθει σε μια μεταβατική εποχή, … όπου αναφύονται νέες σφοδρές διεκδικήσεις, εθνικιστικού και κυριαρχικού χαρακτήρα με αποτέλεσμα η συνοχή των κρατών να απειλείται από έντονες εθνικιστικές, θρησκευτικές, κοινωνικές, πολιτιστικές και γλωσσικές διαμάχες.» B.B. Ghali. Εν μέσω μεταπολεμικής περιόδου διατυπώνεται η ιδέα της ευρωπαϊκής ολοκλήρωσης και ενότητας. Η ανάγκη υλοποίησης της γίνεται ολοένα και μεγαλύτερη μέσα από την εξεύρεση τρόπου αντιμετώπισης ενός φάσματος δυσεπίλυτων προβλημάτων και προκλήσεων, όπως ο οικονομικός όλεθρος, το «γερμανικό πρόβλημα», η αυγή της πρωτοκαθεδρίας των ΗΠΑ με τη ταυτόχρονη διάλυση της ΕΣΣΔ και οι εσωτερικές διαμάχες. Μέσα από διαβουλεύσεις και σωρεία συσκέψεων και διαπραγματεύσεων αρχίζει να διαφαίνεται η ευρωπαϊκή πολιτική ενοποίηση, ιδιαίτερα μετά το τέλος του Ψυχρού Πολέμου. Στη διαδικασία αυτή –της ευρωπαϊκής ολοκλήρωσης– που χαρακτηρίστηκε από την έντονη διαμάχη μεταξύ των υποστηρικτών της φεντεραλιστικής προσέγγισης και εκείνων του λειτουργισμού, σαφώς εμπεριέχονται και τα πεδία της εξωτερικής πολιτικής, άμυνας και ασφάλειας, με την έννοια του συγκερασμού των διπλωματικών, στρατηγικών και αμυντικών συμφερόντων των κρατών-μελών ενός υπερεθνικού οργανισμού, η οποία ωστόσο συνάντησε και εξακολουθεί να συναντά αρκετές δυσχέρειες και αντιστάσεις για να περάσει από το πεδίο της διακυβερνητικής συνεργασίας σ’ εκείνο της πολιτικής, υπερεθνικής ενοποίησης. Η θέσπιση, μέσα σε αυτό το πλαίσιο, της ΚΕΠΠΑ και κατεπέκταση της ΚΠΑΑ (ως βραχίονάς της, που αποτελεί αναπόσπαστο κομμάτι της), είναι ίσως το σημαντικότερο βήμα για την ιδέα της κοινής άμυνας και ασφάλειας. Είναι σημαντικός πυλώνας της Ευρωπαϊκής Ένωσης σε μια χρονική περίοδο που διανύουμε στην οποία οι παγκόσμιες εξελίξεις επιτάσσουν περισσότερο από ποτέ την ανάγκη μιας ‘δυνατής’ σε πολλά επίπεδα Ένωσης. Το κοινό όραμα κρατών και προσώπων παίρνει σάρκα και οστά μετά από αρκετά χρόνια λόγω της δυσκολίας των διαδικασιών που την αποτελούν, τη δημιουργία των καταλλήλων κέντρων Διοικήσεως και Ελέγχου καθώς και των θεσμικών οργάνων που θα ασκούν την οποιαδήποτε εξουσία που απορρέει από τις νομικές επιτάξεις. Με κέντρο τη Συνθήκη της Λισσαβόνας οι ΕΕ αλλάζει και οι επιχειρήσεις που απορρέουν από αυτή, χαρακτηρίζονται από μεγαλύτερη επίτευξη στόχων και η αποκόμιση της εμπειρίας συνδράμει στην εξέλιξη της ΚΠΑΑ.The end of the Cold War had a major impact on the field of International Relations. As a result, the international community has faced a number of political, moral and legal dilemmas. These changes and the events that led to them have led to the development of a new code of values and trends that defined the behavior of states towards the challenges that hit their door. Creating a new system of international relations is both practical and theoretical in that it contains principles and values that influence foreign policy-making. “We have entered a transitional period,… where new vigorous claims of a nationalist and sovereign nature arise, with the result that states' cohesion is threatened by intense nationalist, religious, social, cultural and linguistic conflicts.” B.B. Ghali. The idea of European integration and unity was formulated during the post-war period. The need to implement it is becoming more and more evident by finding ways to deal with a range of unsolvable problems and challenges, such as economic collapse, the "German problem", the dawn of US primacy with the simultaneous dissolution of the USSR, and internal conflicts. European political unification is beginning to emerge through consultations and a host of meetings and negotiations, especially after the end of the Cold War. This process - of European integration - marked by the fierce conflict between supporters of the federalist approach and those of the functionary, clearly encompasses the fields of foreign policy, defense and security, in the sense of a combination of diplomatic, strategic and defense interests. Member States of a transnational organization, which has nevertheless encountered and continues to encounter many difficulties and obstacles to move beyond the field of intergovernmental cooperation at that policy, supranational integration. The adoption, within this framework, of the CFSP and by extension of the CSDP (as its arm, which forms an integral part of it), is perhaps the most important step for the idea of common defence and security. It is an important pillar of the European Union at a time when world developments are more than ever in need of a 'strong' multi-level Union. The common vision of states and persons is flesh and bone after many years because of the difficulty of the procedures that constitute it, the creation of appropriate Command and Control Centers, and the institutions exercising any power arising from legal requirements. At the heart of the Lisbon Treaty, the EU is changing, and the resulting operations are characterized by greater achievement of objectives and the experience gained contributes to the development of the CSDP

Al_5+αSi_5+δN₁₂, a new Nitride compound

The family of III-Nitride semiconductors has been under intensive research for almost 30 years and has revolutionized lighting applications at the dawn of the 21st century. However, besides the developments and applications achieved, nitride alloys continue to fuel the quest for novel materials and applications. We report on the synthesis of a new nitride-based compound by using annealing of AlN heteroepitaxial layers under a Si-atmosphere at temperatures between 1350 °C and 1550 °C. The structure and stoichiometry of this compound are investigated by high resolution transmission electron microscopy (TEM) techniques and energy dispersive X-Ray (EDX) spectroscopy. Results are supported by density functional theory (DFT) calculations. The identified structure is a derivative of the parent wurtzite AlN crystal where the anion sublattice is fully occupied by N atoms and the cation sublattice is the stacking of 2 different planes along lt;0001gt;: The first one exhibits a ×3 periodicity along lt;11–20gt; with 1/3 of the sites being vacant. The rest of the sites in the cation sublattice are occupied by an equal number of Si and Al atoms. Assuming a semiconducting alloy, a range of stoichiometries is proposed, Al5+αSi5+δN12 with α being between −2/3 and 1/4 and δ between 0 and 3/4. © 2019, The Author(s)

Role of hole confinement in the recombination properties of InGaN quantum structures

We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions

Band offsets at zincblende-wurtzite GaAs nanowire sidewall surfaces

The band structure and the Fermi level pinning at clean and well-ordered sidewall surfaces of zincblende (ZB)-wurtzite (WZ) GaAs nanowires are investigated by scanning tunneling spectroscopy and density functional theory calculations. The WZ-ZB phase transition in GaAs nanowires introduces p-i junctions at the sidewall surfaces. This is caused by the presence of numerous steps, which induce a Fermi level pinning at different energies on the non-polar WZ and ZB sidewall facets.This study was financially supported by the EQUIPEX program Excelsior, the European Community’s Seventh Framework Program (Grant No. PITN-GA-2012- 316751, “Nanoembrace” Project) and the Impuls- und Vernetzungsfonds of the Helmholtz-Gemeinschaft Deutscher Forschungszentren under Grant No. HIRG-0014. T. Xu acknowledges the support from the National Natural Science Foundation of China (Grant No. 61204014)

Efficient electronic passivation scheme for computing low-symmetry compound semiconductor surfaces in density-functional theory slab calculations

Removing artificial bands from the back side of surface slabs with pseudohydrogen atoms has become the method of choice to boost the convergence of density-functional theory (DFT) surface calculation with respect to slab thickness. In this paper we apply this approach to semipolar compound semiconductor surfaces, which have recently become attractive for device applications. We show that approaches employing saturation of dangling bonds by pseudohydrogen atoms alone are inadequate to properly passivate the surfaces, remove spurious surface states from the fundamental band gap, and achieve flat band conditions in the slab. We propose and successfully apply to technologically interesting semipolar wurtzite surfaces of III-N, III-V, and II-VI semiconductors a reconstruction-inspired passivation scheme that utilizes native anions to passivate cation dangling bonds and pseudohydrogen atoms to obey the electron counting rule and compensate for polarization-induced surface-bound charges. This scheme is generic and robust and can be straightforwardly implemented in DFT investigations of low-symmetry surfaces as well as in high-throughput and machine learning studies. © 2021 authors

Ab-initio study of boron incorporation and compositional limits at GaN and AlN (0001) surfaces

Density functional theory calculations are employed to investigate B incorporation at the GaN(0001) and AlN(0001) surfaces. It is found that under typical metal-organic chemical vapor deposition (MOCVD) and metal rich molecular beam epitaxy (MBE) conditions, the maximum B contents at the surfaces are in the order of 3% for GaN and 15% for AlN. Under MBE N-rich growth conditions the calculations reveal a rehybridization enhanced solubility mechanism that dominates at the surface. This mechanism offers a promising route to kinetically stabilize B contents above the bulk solubility limit and as high as 25%