Atomic-resolution study of overlayer formation and interfacial mixing in the interaction of phosphorus with Si(001)

Scanning tunneling microscopy (STM), tunneling spectroscopy, and Auger-electron spectroscopy have been used to study the formation of phosphorus-terminated silicon (001) surfaces by the thermal decomposition of phosphine (PH₃). The STM images show that surface phosphorus atoms readily displace Si from the substrate, dramatically changing the overall surface morphology through the formation of large numbers of islands and an extreme roughening of step edges. The surfaces are terminated with P-P dimers, but also contain large numbers of line defects which act as a strain relief mechanism. STM images are used to determine the symmetry of these line defects and are compared with model defect structures. Line defects in the phosphorus-terminated surface both nucleate islands and also constrain their growth in one dimension, such that the phosphorus-terminated surfaces contain large numbers of extremely anisotropic islands and very rough step edges. At lower phosphorus coverage, the STM experiments are able to identify Si and P atoms individually, revealing the formation of large numbers of Si-P heterodimers and the simultaneous disappearance of the strain-induced line defects. Counting statistics are used to study the equilibrium between Si=Si, Si-P, and P-P dimers, showing that the surface is a nearly random alloy with a slight nonstatistical preference for formation of the Si-P heterodimer. The geometric and electronic properties of Si=Si, Si-P, and P-P dimers on Si(001) are discussed on the basis of tunneling microscopy and tunneling spectroscopy measurements.