Abstract

Optical second-harmonic generation (SHG) has proved to be a sensitive probe of the first few atomic layers of a surface. The technique appears to be applicable to the broad class of interfaces between two centrosymmetric media, since within the bulk of such materials the process of SHG is forbidden. Measurements have exploited the surface sensitivity of the SH signal in obtaining information on the surface coverage, orientation, and spectroscopy of adsorbed molecular monolayers.¹ Previous studies of crystalline materials exposed to air have suggested that SHG should also provide a tool for probing the ordering and symmetry of the atoms of a well-defined surface layer.² In this paper we present results for Si(111) surfaces cleaved in ultrahigh vacuum. It is found that the SH signal, which is dominated by the surface contribution, exhibits a distinctive symmetry associated with the 2×1 surface reconstruction. The signals arising from amorphous and 7×7 reconstructed surfaces also display definite symmetry properties. Using the probe of SHG, we have been able to monitor in real time the phase transformations between the different surface structures. Preliminary results have also been obtained for surface reactions leading to oxide and metal silicide formation.

© 1984 Optical Society of America