Hydrogen-induced crystallization of amorphous Si thin films. II. Mechanisms and energetics of hydrogen insertion into Si–Si bonds

Abstract

We report a detailed study of the mechanisms and energetics of hydrogen (H) insertion into strained Si–Si bonds during H-induced crystallization of hydrogenated amorphous Si (a-Si:H) thin films. Our analysis is based on molecular-dynamics (MD) simulations of exposure of a-Si:H films to H atoms from a H2 plasma through repeated impingement of H atoms. Hydrogen atoms insert into Si–Si bonds as they diffuse through the a-Si:H film. Detailed analyses of the evolution of Si–Si and Si–H bond lengths from the MD trajectories show that diffusing H atoms bond to one of the Si atoms of the strained Si–Si bond prior to insertion; upon insertion, a bridging configuration is formed with the H atom bonded to both Si atoms, which remain bonded to each other. After the H atom leaves the bridging configuration, the Si–Si bond is either further strained, or broken, or relaxed, restoring the Si–Si bond length closer to the equilibrium bond length in crystalline Si. In some cases, during its diffusion in the a-Si:H film, the H atom occupies a bond-center position between two Si atoms that are not bonded to each other; after the H diffuses away from this bond-center position, a Si–Si bond is formed between these previously nonbonded Si atoms. The activation energy barrier for the H insertion reaction depends linearly on both the initial strain in the corresponding Si–Si bond and a strain factor that takes into account the additional stretching of the Si–Si bond in the transition-state configuration. The role of the H insertion reactions in the structural relaxation of the a-Si:H network that results in disorder-to-order transitions is discussed.