Phase Transitions on Gan Surfaces
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Phase Transitions on Gan Surfaces C. Adelmann1, L. Lymperakis2, J. Brault1, G. Mula1, J. Neugebauer2, B. Daudin1 1
Equipe mixte CEA-CNRS-UJF Nanophysique et Semiconducteurs, Département de Recherche Fondamentale sur la Matière Condensée, SP2M/PSC, CEA-Grenoble, 17 rue des Martyrs, 38054Grenoble Cedex 9, France 2 Fritz-Haber-Institut, Faradayweg 4-6, 14195 Berlin, Germany ABSTRACT Recent experimental and theoretical studies highlighted the importance of the growing surface structure on the final morphology of GaN. Actually, optimum morphology is achieved by growth in presence of a Ga bilayer adsorbed on the GaN surface. The threshold fluxes limiting the region of the Ga bilayer adsorption have been measured as a function of the GaN substrate temperature, giving rise to a Ga adsorption phase diagram. The Ga flux limiting the regions in the adsorption phase diagram exhibit a linear behavior in an Arrhenius plot. However, both energy activation (about 5 eV) and prefactor (in the 1025 range) are surprinsingly high. These questions were adressed by studying the adsorption/desorption of Ga adatoms and small islands (consisting of 2 and 3 Ga adatoms) on the Ga bilayer surface employing first principle density functional theory calculations. We find a desorption barrier of 2.1 eV and a binding energy between two Ga atoms of approximately 0.3 eV. Using these numbers we derived a simple growth model (based on rate equations). An analysis of the experimental data with the model revealed the origin of the large difference in the activation energies and the unusually large prefactor. We find that the nucleation of the droplets cannot be described by a simple Arrhenius behavior (as commonly assumed to fit experimental data) but that the nucleation energy is temperature dependent. INTRODUCTION The potentialities of GaN-based devices as light emitters at wavelength ranging from visible to UV strongly depend on the ability to control growth in order to optimise both optical and structural properties of the material. This is particularly true for plasma assisted molecular beam epitaxy (PAMBE) of nitrides. The ability to control the growth of nitride heterostructures at the monolayer scale, which is a prominent advantage of molecular beam epitaxy (MBE), is somewhat balanced by a rather low growth temperature, compared to metalorganic chemical vapor deposition (MOCVD). Low growth temperature results in a relatively weak diffusion of adatoms which may be detrimental to the structural properties of nitride material. Alternately, the strong dependence of surface morphology to metal/N ratio value makes the optimisation of PAMBE of nitride materials delicate, as N-rich conditions lead to rough surface whereas metal-rich conditions lead, in the general case, to metal droplet accumulation. Surfactants provide solutions to overcome the difficulties mentioned above, which are partly inherent to the relatively low growth temperature in MBE. It has been demonstrated that In acts as a surfactant for growth of GaN and prevents Ga droplet fo
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