Thermal reactions of phosphine with Si(100): A combined photoemission and scanning-tunneling-microscopy study

This study investigates the adsorption and thermal decomposition of phosphine (PH₃) on the Si(100)-(2×1) surface. The adsorption species, dissociation reactions, atomic ordering, and surface morphology of the phosphine/Si(100) surface at temperatures between 300 and 1060 K are examined by scanning tunneling microscopy (STM) and high-resolution core-level photoemission spectroscopy employing synchrotron radiation. The P 2p core level spectra clearly indicate that phosphine molecularly adsorbs at room temperature and partially dissociates into PH₂ and H on a time scale of minutes at low (<0.2 ML) coverages. An exposure of >15 Langmuirs (L, 1 Langmuir = 10^-6 Torr s^-1) of phosphine on the Si(100)-(2×1) surface at room temperature produces a saturated and disordered surface. The total amount of P on the saturated surface is ca 0.37 ML as calibrated by the P 2p photoemission intensity. Successive annealing of the saturated surface at higher temperatures converts PH₃ into PH₂, converts PH₂ to P-P dimers, and causes the desorption of PH₃. These processes become complete at approximately 700 K, and the resulting surface is a H/Si(100)-(2×1) surface interspersed with one-dimensional P-P islands. Desorption of hydrogen from that surface occurs at approximately 800 K, and is accompanied by partial displacement of P with Si atoms on the substrate. At 850 K, the Si(100) surface, interspersed with 0.22 ML of two-dimensional islands, is a random alloy of nominal 0.5 ML Si-P heterodimers and 0.5 ML Si-Si dimers.