RAS: in-situ analysis in MOVPE

An analytical technique to be used in MOVPE must be applicable at pressures from 20 hPa up to atmospheric pressure. This rules out the standard methods of electron diffraction (LEED, RHEED) used in MBE, since they only function in ultrahigh vacuum (approx. 10-10hPa).
Instead, optical methods suggest themselves. The methods of reflectometry are particularly suited for the requirements of MOVPE since, on the one hand, information can be obtained from the bulk such as layer thickness and composition. On the other hand, minor modifications also make processes on the surface accessible whose knowledge is necessary for the fundamental understanding of the growth processes.
The reflectivity of a sample depends on its dielectric properties which, in turn, depend on the type of atoms and their arrangement with respect to each other. In this way, on the one hand, material-specific data can be obtained, such as the composition of a layer. On the other hand, the dielectricity is also temperature-dependent so that e.g. the exact growth temperature can be directly measured in situ. If heterostructures are deposited, the layer thickness and growth rate can be determined from the interface effects (Fabry-Perot oscillations).
Reflective anisotropy spectroscopy (RAS) provides information from the dependence of the reflected light on the polarization. To this end, particular crystal properties of the III-V semiconductors are utilized: The classical III-V semiconductors are known to crystallize in the zinc blende structure, a cubic structure. In these semiconductors, the reflectivity of the bulk is isotropic, i.e. the same everywhere in the material, and not dependent on polarization. This does not apply to the surface of the crystal because reconstructions are formed here as a result of symmetry breaking


Consequently, the surface reflects polarized light anisotropically. The spectra thus generated are also specific so that the surface reconstruction can be determined from them. This surface reconstruction or its modification during epitaxy permits conclusions to be drawn concerning the processes taking place during layer growth. This is important for the understanding of MOVPE and it also enables process control since it is thus possible to supervise the quality of the surface or dopings.
A RAS spectrum consists of a number of normalized intensity differences as a function of the wavelength and energy of the reflected light. The reflectivity of two crystal directions perpendicular to each other (on (100) surfaces, for example, [011] and [01-1]), is measured and the difference in intensities is divided by the intensity averaged over the two directions.