Femtosecond Spectroscopy Group | Stepped Silicon Surfaces  
   
       
 
 
Optical second-harmonic generation (SHG) and reflectance anisotropy spectroscopy (RAS) of molecular adsorption at stepped Si(001) surfaces
 
Why stepped surfaces?
Controlled growth of nanoscale structures on Si substrates is a forefront challenge of microelectronics. In particular ordered organic-semiconductor interfaces are playing increasingly important roles in fields ranging from molecular electronics to biosensing. Successful integration of molecular electronic devices with conventional silicon microelectronic technology requires a detailed understanding and control of adsorption structure in order to achieve favorable electronic properties and ensure sufficient stability. One example, cyclopentene molecules on Si(001) are a prototypical organic-silicon system for which robust reversible negative differential resistance through single molecules have been reported. Applications in large scale circuit fabrication of silicon-based molecular electronics, require high quality crystals with electronic properties comparable to those of inorganic materials. Thin organic films however cannot easily be made by the vapor-deposition methods typically used in microelectronics fabrication. The self-assembled growth of organic layers via cycloaddition on vicinal Si(001) surfaces as templates offers a viable alternative.
While self-directed growth is more rapid than atom-by-atom assembly, it is also less controlled, and will thus rely on non-invasive, in-situ sensors with access to bonding configurations at the buried interface between silicon and adsorbed nanostructure to guide nanofabrication. Only optical techniques are able to perform this task as routine metrology tools.		  
 
Optical Techniques
Surface-specific optical spectroscopies such as second-harmonic generation (SHG) and reflectance-anisotropy spectroscopy (RAS) are attractive candidates for non-invasive, real-time monitoring of surface adsorption processes. SHG is sensitive to the surface discontinuity of centrosymmetric materials such as Si, to chemisorption of step-edges. It can probe charge transfer among surface atomic layers induced by chemisorption \cite{Lim00} and monitor surface chemistry during interface formation. Spectroscopic implementation of SHG, however, remains infrequent, and accurate first-principle calculations of surface SHG responses remain elusive. RAS is well suited to stepped surfaces because it distinguishes inherently anisotropic step-edges and single-domain terraces from the isotropic bulk, is sensitive to a fractional monolayer of adsorbates and can be modeled accurately by first-principles calculations.
 
Reflectance Anisotropy Spectroscopy (RAS) RAS probes the optical anisotropy of a material. For cubic materials like silicon the bulk dielectric tensor is isotropic. The RAS signal therefore originates in areas of reduced symmetry as reconstructed surfaces and interfaces. RAS is well suited to stepped surfaces because it distinguishes inherently anisotropic step-edges and single-domain terraces from the isotropic bulk. It is sensitive to a fractional monolayer of adsorbates and can be modeled accurately by first-principles calculations. RAS probes the difference in the reflectance between two perpendicular axes of a sample in normal incidence. The incoming light is linearly polarized with an angle of 45 degree towards the anisotropy axis as shown in Fig. With the setup utilizing two polarizers and a photo-elastic modulator, not only the real reflectance difference is measured but the anisotropy of the complex reflectivity. Both the real and imaginary part can be probed, the first by analyzing the signal at 2w, the latter at w the frequency of the polarization modulation of the PEM.
 
Second-Harmonic Generation (SHG)
 
Ultra-High Vacuum