MBE Growth Of GaN Films In Presence Of Surfactants: The Effect Of Mg And Si
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TRACT We present here a description and an analysis of the modifications in the growth behaviour of GaN induced by the presence of foreign species. The particular cases of Mg and Si are analysed. Profound changes, both in microscopic and macroscopic scales, occur in presence of Mg, even for fluxes of about 1/1000th of the Ga flux. The growth rate can be increased by almost 50%, depending of the III/V ratio and on the amount of Mg. A theoretical model is proposed to describe the observed effect. It is found that Mg induces changes in the Ga and N diffusion barriers and acts as a surfactant. The effect is stronger on the α-GaN than on the β-GaN, where N is more tightly bonded. The effect of Si is by far less pronounced, probably because it is more easily incorporated than Mg, and its effect on the surface kinetics is then strongly reduced. INTRODUCTION The understanding of the kinetics of the growing species on the GaN surface is of paramount interest for optimal epitaxial growth. In particular, the growth by Molecular Beam Epitaxy (MBE) is still far from perfection, partially due to the strong dependence of the surface morphology on the III/V ratio. This situation is especially true for plasmasource MBE, where the growth in Ga-rich regime leads to a metal accumulation that eventually blocks the growth process, while the N-rich regime leads to rough surfaces. Therefore, the use of surfactants to modify the surface mobility of the chemical species can be a useful step in the search for optimisation. In fact, the determination of less critical growth conditions, where the surface can be flat and free of Ga droplets, should lead to better optical and structural properties. A few possible candidates for surfactants have already been examined. Theoretical [1] and experimental [2] reports show that As presence on the β-GaN surface leads to preferential growth of the cubic phase and to flatter surfaces. The use of Mg and Si during metal-organic chemical vapour deposition (MOCVD) growth [3] is shown to improve structural quality by reducing the defect density. The addition of In also shows significant surfactant effect [4], increasing the surface mobility. Si seems to have an “anti-surfactant” effect on MOCVD-grown α-GaN [5]. What can be extracted from these results is that the GaN growth seems to be extremely dependent on the presence of foreign species. Mg and Si are the most commonly used materials for doping, and the knowledge of their influence on the growth parameter is clearly fundamental. We will
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present here a study on Mg- and Si-induced modifications of the growth kinetics at αand β-GaN surfaces. A phenomenological model is proposed for the α-GaN phase. EXPERIMENTAL DETAILS All samples were grown by plasma-assisted molecular beam epitaxy (MBE) in a MECA2000 growth cham
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