Direct Transition from Ultrathin Orthorhombic Dinickel Silicides to Epitaxial Nickel Disilicide Revealed by In Situ Synthesis and Analysis

Understanding phase transitions of ultrathin metal silicides is crucial for the development of nanoscale silicon devices. Here, the phase transition of ultrathin (3.6 nm) Ni silicides on Si(100) substrates is investigated using an in situ synthesis and characterization approach, supplemented with ex situ transmission electron microscopy and nano-beam electron diffraction. First, an ultrathin epitaxial layer and ordered structures at the interface are observed upon room-temperature deposition. At 290 degrees C, this structure is followed by formation of an orthorhombic delta-Ni2Si phase exhibiting long-range order and extending to the whole film thickness. An unprecedented direct transition from this delta-Ni2Si phase to the final NiSi2-x phase is observed at 290 degrees C, skipping the intermediate monosilicide phase. Additionally, the NiSi2-x phase is found epitaxial on the substrate. This transition process substantially differs from observations for thicker films. Furthermore, considering previous studies, the long-range ordered orthorhombic delta-Ni2Si phase is suggested to occur regardless of the initial Ni thickness. The thickness of this ordered delta-Ni2Si layer is, however, limited due to the competition of different orientations of the delta-Ni2Si crystal. Whether the formed delta-Ni2Si layer consumes all deposited nickel is expected to determine whether the monosilicide phase appears before the transition to the final NiSi2-x phase.

Automated summary

Understanding the phase transitions of ultrathin metal silicides is crucial for the development of nanoscale silicon devices. In this study, the phase transition of ultrathin Ni silicides on Si(100) substrates was investigated using in situ synthesis and characterization methods. The results showed the formation of an ultrathin epitaxial layer and ordered structures at the interface upon room-temperature deposition. At 290 degrees C, a direct transition from the orthorhombic delta-Ni2Si phase to the final NiSi2-x phase was observed, skipping the intermediate monosilicide phase. The thickness of the ordered delta-Ni2Si layer was found to be limited due to competition between different crystal orientations.