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Electromechanical couplings and growth instabilities in semiconductors

Abstract : In the last decades, solid mechanics has gone beyond its original issues of mechanical properties of materials and structures to embrace problems coming from other scientific fields and in particular physics. Semiconductors, the base materials of all electronic devices, are a prime example where crystalline solids show multiphysics couplings. Indeed, mechanics plays there an important role both in the fabrication process and in the operation of electronic devices. In this work, we examine these two aspects by studying first the couplings between electronic transport phenomena and mechanical deformations and second the morphological instabilities that develop in semiconductor epitaxial growth.First, developing a fully-coupled theory of deformable semiconductors that includes mechanical, electrical and electronic fields, we show for the first time the existence of an electronic contribution to mechanical stress. While for crystalline semiconductors this contribution is weak, the effect of strains on electronic transport remains significant through their modification on band energy levels, density of states and mobility of electrons and holes. Considering the advent of new technologies of flexible electronics, we apply the general theory to compute through asymptotic expansions, the effect of bending -causing non-uniform strains- on the current-voltage characteristic of a p-n junction, the basic device of solar cells. To complete this picture, we measure the changes induced by uniaxial stresses on the electronic characteristic of a silicon heterojunction solar cell.In the second part of this work, going down to the atomic scale, we consider the problem of epitaxial growth on vicinal surfaces. On these surfaces, the crystal grows through the propagation of the atomic steps, which may develop step bunching, an instability whereby the regular step spacing breaks down, resulting in an alternating pattern of wide atomic terraces and step bunches. Through a comprehensive linear stability analysis of the step dynamics governing equations, we discuss the influence of each physical mechanism on the step bunching instability. In particular, we clarify the impact on stability of the dynamics, of the recently pointed out adatom jump effect, and of elasticity, beyond the assumption of nearest-neighbor interactions. In addition, we show that our general stability results, i.e., obtained without neglecting the dynamics terms, are significantly different from those derived with the quasistatic approximation, even in regimes of slow deposition or evaporation where the latter was considered sufficient. Not only valuable from a theoretical prospective, these new results provide possible explanations for some cases of step bunching observed in silicon and gallium arsenide. In view of these new aspects, we reexamine the problem of step bunching under electromigration and show that the adatom jump and dynamics effects do not affect the stability dependence on the direction of the electromigration current.Finally, we investigate the mechanical properties at the atomic scale of another crystalline material with semiconducting properties, polycrystalline graphene. Using molecular dynamic simulation, we develop a cohesive zone model for fracture along grain boundaries.
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Laurent Guin. Electromechanical couplings and growth instabilities in semiconductors. Mechanics of the solides [physics.class-ph]. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLX105⟩. ⟨tel-02124459⟩

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