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Normal Incidence Option for the Picometer Ellipsometer

Setup of Normal Incidence Picoelli With the Normal Incidence Option, the Picometer Ellipsometer becomes a compact, highly sensitive and fast surface probe.

While isotropic materials have r_s = r_p at normal incidence, anisotropic materials have different reflectivities for light polarised along different axes of the material. Normal incidence measurements therefore can be used to study anisotropy. Isotropic crystals often show surface anisotropy due to surface reconstruction (Silicon and GaAs are examples). When the bulk of such a crystal is isotropic, an ellipsometer measuring at normal incidence will not detect any bulk signal, and will therefore be only sensitive to the surface anisotropy. The method has been termed Reflectance Anisotropy Spectroscopy (RAS) or Reflectance Difference Spectroscopy (RDS). It is capable of detecting differences in the surface reflectivities that are several orders of magnitude smaller than the bulk reflectivity. Since most RAS systems are based on a phase modulation setup, the high sensitivity and rapid response of the Picometer Ellipsometer make it an ideal instrument to study and monitor this anisotropy.

Adaptation to standard MBE system

The Normal Incidence Picometer Ellipsometer can be readily adapted to standard growth systems.

In the normal incidence configuration, the Picometer Ellipsometer is fitted with the spectroscopic option (Xe lamp and monochromator) for measurements in the photon energy range between 1.5 and 5.5 eV. The components are removed from their optical track mounts and mounted on a breadboard, providing a very compact setup that can be easily fitted to a vacuum chamber or other sample preparation systems. A Beam Splitter configuration with a laser light source for exact normal incidence is also available. Additional components to the Picometer Ellipsometer control software provide functions for baseline correction and automatic growth rate calculation. The Normal Incidence Picometer Ellipsometer is available as a stand-alone instrument or as an add-on to the standard Picometer Ellipsometer. In both cases, the instrument can be reassembled for oblique incidence ellipsometry measurements (in the standalone version, this requires the optional Picometer Ellipsometer Table).