In this work, the reaction of yttrium (Y) on Si(001) is investigated using in-situ X-ray photoelectron spectroscopy (XPS) for different coverage of Y with low temperature annealing. We report on the 3 general reaction phases of yttrium on Si that are coverage dependent. Firstly, for 1ML of Y coverage, a strong Y-Si bonding is formed at room temperature. The strong bonding could be seen from the significant dipole induced band bending effects evidenced by Si2p and Y3d core-level binding energy shift. The bonding stability is manifest itself as being resistant to both oxidation in ultra-high-vacuum (UHV) or any further Si interdiffusion after a 300ºC annealing. For higher coverage of 2-4ML, spontaneous room-temperature mixing of Y-Si is observed. This is consistent with most rare-earth metal's reaction with Si at these coverage regimes. Upon a 300ºC annealing in UHV, selective oxidation of Y in the 2-4ML of film is observed while no significant diffusion of Si occurs. Finally, for coverage of >4ML, pure metallic Y forms during room temperature deposition. Subsequent annealing at 300ºC shows substantial Si-diffusion accompanied by a reduction in the intake of oxygen. The diffusion is attributed to a metal-weakened Si bonding effect and the competition of Si with O for the metallic Y limits the amount of oxygen intake in UHV.

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