Composition modulation and local structure in strained diluted GaInNAs nitride alloy thin layers on GaAs substrates
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Composition modulation and local structure in strained diluted GaInNAs nitride alloy thin layers on GaAs substrates. Arnaud Metsue and Catherine Priester IEMN/ISEN, CNRS-UMR 8520, BP 60069 F-59652, Villeneuve d'Ascq Cedex, FRANCE. ABSTRACT The theoretical study reported here is devoted to diluted GaInNAs nitride alloys, and focuses on correlation between local chemical atomic neighborhood and atomic distances. The model used is Valence Force Field approximation, and we model a periodic Ga1-xInxNyAs1-y alloy film deposited on a GaAs substrate. The surface is dimerised (for simplicity we consider 2x1 anion rich surfaces). First starting from a random film, first and second nearest neighbor distances are calculated, and the corresponding histogram drawn. Then a pseudo-annealing process is simulated by allowing the N atoms to choose their optimal location. This pseudo-annealing strongly enhances the number of In-N bonds, in agreement to experimental studies. From this statistical study, fine structure of each peak of the histograms is shown to be not directly related to a given surrounding chemical distribution up to 3rd nearest neighbors. The positions (in distance) of the peaks appear not to be modified by alloy composition nor alloy segregation, which only alter relative intensities and peak shapes. Last, we consider stripes of In-rich and In-poor zones: calculated energy variations show a strong tendency for N atoms to completely desert In-poor zones. INTRODUCTION GaInNAs quantum wells on GaAs substrates appear to be very attractive for promising applications such as lasers for telecommunication or solar cells, but composition modulation may strongly alter the optical or electronic properties of these heterostructures [1,3]. Thanks to the large difference in lattice parameters between the binaries involved in this quaternary alloy, their local structure can be experimentally investigated by making use of EXAFS, for example [4-7]. In the theoretical work reported here, we focus on correlation between local chemical atomic neighborhood and atomic distances. Using a Valence Force Field description, we model a periodic Ga1-xInx NyAs1-y alloy film deposited on a GaAs substrate. Then we calculate the distance between In atoms and their first and second nearest neighbors for several alloy concentrations either in random or “pseudo-annealed” alloy films. The thus obtained distances histograms are correlated with local chemical arrangement. Last, guided by experimental observation [2], we investigate, using a simple In-rich and Inpoor striped model, the chances for such an alloy film to demix, and look for a signature of such a decomposition in nearest neighbor atomic distances. MODEL For describing Ga1-xInx NyAs1-y alloys we have chosen to use Valence Force Field approximation, in its Keating's formulation, as successfully used for studying alloy partial decomposition in other III-V epitaxial layers [8-9]. We model a periodic Ga1-xInx NyAs1-y alloy (period of 8 or 32 atomic rows) film (typically 24 atomic pl
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