Noncontact atomic force microscopy and density functional theory studies of the (2×2) reconstructions of the polar AlN(0001) surface

Combined experimental and theoretical studies permit us to determine new protocols for growing by molecular beam epitaxy the technologically interesting N-rich aluminum nitride (AlN) surfaces. This is achieved by dosing the precursor gases at unusually low rates. With the help of calculated structures by using density functional theory and Boltzmann distribution of the reconstructed cells, we proposed to assign the measured surface obtained with a growth rate of 10 nm/h to a (2×2) reconstructed surface involving one additional N atom per unit cell. These N-rich AlN surfaces could open new routes to dope AlN layers with important implications in high-power and temperature technological applications

Theoretical study of the reactivity of the (2X2) reconstruction of AIN(0001)

L'utilisation de systèmes moléculaires individuels pouvant jouer le rôle de composants avec des fonctions électroniques ou logiques requiert des interfaces parfaitement contrôlées. Plus précisément, le support sur lequel ces systèmes sont déposés et les électrodes métalliques qui permettent de contacter une molécule individuelle, sont des interfaces qui nécessitent un soin d'élaboration particulier. La croissance d'îlots bidimensionnels (2d) de métaux sur un isolant monocristallin permet de générer des nano-plots 2d pouvant servir de réservoirs d'électrons en minimisant les courants de fuite en surface. Ainsi, il apparaît capital de bien comprendre les modes de croissance des systèmes métal/isolant qui sont à l'heure actuelle mal connus. Ce travail de thèse s'attache à décrire et à expliquer la croissance de différents métaux sur la surface de l'AlN(0001) polaire, qui est un composé nitrure à grand gap, par des calculs basés sur la théorie de la fonctionnelle de la densité (DFT).Dans un premier temps, une description approfondie des différentes surfaces du nitrure d'aluminium est présentée. Des calculs DFT ont permis de rationaliser les reconstructions en fonction des conditions expérimentales. La reconstruction (2 x 2)-Nad est discutée, puisqu'elle a été observée en microscopie à force atomique. Dans un deuxième temps, le cas du dépôt d'atomes d'or est abordé en connexion avec des résultats expérimentaux. Les calculs DFT donnent un aperçu des mécanismes qui conduisent à la stabilisation d'îlots 2d sur l'AlN. L'adsorption d'or s'accompagne, d'une part, d'un transfert de charge vertical provenant du substrat d'AlN, ce qui satisfait au critère de stabilité électrostatique pour un matériau polaire et, deuxièmement, par des transferts de charges horizontaux reliés aux propriétés acido-basiques locales de la reconstruction (2 x 2)-Nad. Enfin, des calculs effectués sur deux autres métaux, le magnésium et l'argent, sont exposés. Ces résultats ouvrent la voie à de nouvelles stratégies utilisant des substrats polaires pour développer des monocouches métalliques sur des substrats isolants.The use of individual molecular systems that can act as components with electronic or logical functions requires perfectly controlled interfaces. More precisely, the support on which these systems are deposited and the metal electrodes that allow to contact an individual molecule, are interfaces that require special care in preparation. The growth of two-dimensional (2d) islands of metals on a monocrystalline insulator allows to generate 2d nano-pads that can be used as electron reservoirs by minimizing surface leakage currents. Thus, it is essential to understand the growth modes of metal/insulating systems which are at present poorly known. This work aims at describing and explaining the growth of different metals on the surface of the polar AlN (0001), which is a large gap nitride compound, by calculations based on density functional theory (DFT). In a first step, a detailed description of the various surfaces of the aluminum nitride is presented. DFT calculations permit to rationalize the reconstructions according to the experimental conditions. The (2 x 2)-Nad reconstruction is discussed, since it was observed by atomic force microscopy. In a second step, the case of the deposit of gold atoms is tackled in connection with experimental results. DFT calculations give an overview of the mechanisms that lead to the stabilization of 2d islands on AlN. The adsorption of gold is accompanied, on the one hand, by a vertical charge transfer from the AlN substrate, which satisfies the electrostatic stability criterion for a polar material and, on the other hand, by horizontal charge transfers related to the local acid-base properties of the (2 x 2)-Nad reconstruction. Finally, calculations made on two other metals, magnesium and silver, are exposed. These results open the way to new strategies using polar substrates to develop metallic monolayers on insulating substrates

Etude théorique de la réactivité de la reconstruction (2x2) de l'AlN(0001)

The use of individual molecular systems that can act as components with electronic or logical functions requiresperfectly controlled interfaces. More precisely, the support on which these systems are deposited and the metalelectrodes that allow to contact an individual molecule, are interfaces that require special care in preparation.The growth of two-dimensional (2d) islands of metals on a monocrystalline insulator allows to generate 2dnano-pads that can be used as electron reservoirs by minimizing surface leakage currents. Thus, it is essentialto understand the growth modes of metal/insulating systems which are at present poorly known. This workaims at describing and explaining the growth of di↵erent metals on the surface of the polar AlN (0001), whichis a large gap nitride compound, by calculations based on density functional theory (DFT). In a first step,a detailed description of the various surfaces of the aluminum nitride is presented. DFT calculations permitto rationalize the reconstructions according to the experimental conditions. The (2 x 2)-N ad reconstruction isdiscussed, since it was observed by atomic force microscopy. In a second step, the case of the deposit of goldatoms is tackled in connection with experimental results. DFT calculations give an overview of the mechanismsthat lead to the stabilization of 2d islands on AlN. The adsorption of gold is accompanied, on the one hand,by a vertical charge transfer from the AlN substrate, which satisfies the electrostatic stability criterion for apolar material and, on the other hand, by horizontal charge transfers related to the local acid-base propertiesof the (2 x 2)-N ad reconstruction. Finally, calculations made on two other metals, magnesium and silver, areexposed. These results open the way to new strategies using polar substrates to develop metallic monolayerson insulating substrates.L’utilisation de systèmes moléculaires individuels pouvant jouer le rôle de composants avec des fonctions électroniques ou logiques requiert des interfaces parfaitement contrôlées. Plus précisément, le support sur lequelces systèmes sont déposés et les électrodes métalliques qui permettent de contacter une molécule individuelle,sont des interfaces qui nécessitent un soin d’élaboration particulier. La croissance d’îlots bidimensionnels (2d)de métaux sur un isolant monocristallin permet de générer des nano-plots 2d pouvant servir de réservoirsd’électrons en minimisant les courants de fuite en surface. Ainsi, il apparaît capital de bien comprendre lesmodes de croissance des systèmes métal/isolant qui sont à l’heure actuelle mal connus. Ce travail de thèses’attache à décrire et expliquer la croissance de différents métaux sur la surface de l’AlN(0001) polaire, qui estun composé nitrure à grand gap, par des calculs basés sur la théorie de la fonctionnelle de la densité (DFT). Dansun premier temps, une description approfondie des différentes surfaces du nitrure d’aluminium est présentée.Des calculs DFT ont permis de rationaliser les reconstructions en fonction des conditions expérimentales. Lareconstruction (2 x 2)-N ad est discutée, puisqu’elle a été observée en microscopie à force atomique. Dans undeuxième temps, le cas du dépôt d’atomes d’or est abordé en connexion avec des résultats expérimentaux.Les calculs DFT donnent un aperçu des mécanismes qui conduisent à la stabilisation d’îlots 2d sur l’AlN.L’adsorption d’or s’accompagne, d’une part, d’un transfert de charge vertical provenant du substrat d’AlN, cequi satisfait au critère de stabilité électrostatique pour un matériau polaire et, deuxièmement, par des transfertsde charges horizontaux reliés aux propriétés acido-basiques locales de la reconstruction (2 x 2)-N ad . Enfin, descalculs effectués sur deux autres métaux, le magnésium et l’argent, sont exposés. Ces résultats ouvrent la voieà de nouvelles stratégies utilisant des substrats polaires pour développer des monocouches métalliques sur dessubstrats isolants

Stabilization of Au Monatomic-High Islands on the (2×2)- Nad Reconstructed Surface of Wurtzite AlN(0001)

cited By 0International audienceNoncontact atomic force microscopy images show that gold grows on the (2×2)−Nad reconstructed polar (0001) surface of AlN insulating films, in the form of large monatomic islands. High-resolution images and in situ reflection high-energy electron diffraction spectra reveal two moiré patterns from which an atomic model can be built. Density functional theory calculations confirm this model and give insight into the mechanisms that lead to the stabilization of the monolayer. Gold adsorption is accompanied, first, by a global vertical charge transfer from the AlN substrate that fulfills the electrostatic stability criterion for a polar material, and second, by lateral charge transfers that are driven by the local chemical properties of the (2×2)−Nad reconstruction. These results present alternative strategies to grow metal electrodes onto nitride compounds with a better controlled interface, a crucial issue for applications

Noncontact atomic force microscopy and density functional theory studies of the (2×2) reconstructions of the polar AlN(0001) surface

Combined experimental and theoretical studies permit us to determine new protocols for growing by molecular beam epitaxy the technologically interesting N-rich aluminum nitride (AlN) surfaces. This is achieved by dosing the precursor gases at unusually low rates. With the help of calculated structures by using density functional theory and Boltzmann distribution of the reconstructed cells, we proposed to assign the measured surface obtained with a growth rate of 10 nm/h to a (2×2) reconstructed surface involving one additional N atom per unit cell. These N-rich AlN surfaces could open new routes to dope AlN layers with important implications in high-power and temperature technological applications.This work is supported by the European Commission within the projects AtMol (Contract No. ICT-270028) and PAMS (Contract No. ICT-610446). Part of the calculation was performed using High Performance Computing resources from the Calcul in Midi-Pyrenees (CALMIP) facilities (Grant No. 2011-[P0832]).Peer Reviewe

Noncontact atomic force microscopy and density functional theory studies of the (2×2) reconstructions of the polar AlN(0001) surface

Combined experimental and theoretical studies permit us to determine new protocols for growing by molecular beam epitaxy the technologically interesting N-rich aluminum nitride (AlN) surfaces. This is achieved by dosing the precursor gases at unusually low rates. With the help of calculated structures by using density functional theory and Boltzmann distribution of the reconstructed cells, we proposed to assign the measured surface obtained with a growth rate of 10 nm/h to a (2×2) reconstructed surface involving one additional N atom per unit cell. These N-rich AlN surfaces could open new routes to dope AlN layers with important implications in high-power and temperature technological applications