In situ monitoring of CdTe nucleation on GaAs (100) using spectroscopic ellipsometry

In situ spectroscopic ellipsometry was used to monitor the nucleation behavior of CdTe grown on vicinal GaAs (100) substrates by organometallic vapor phase epitaxy. CdTe was grown on GaAs (100) substrates of exact and 2° off towards 110 orientations. A spectroscopic ellipsometer was used to collect in situ data at 44 wavelengths from 4000–7000 Å. The Bruggeman's effective medium approximation was employed to determine the variation of the epilayer volume fraction with thickness, which was an indirect way of monitoring the expected island growth behavior. The Stranski-Krastonov (layer plus island) mode of growth was clearly observed for CdTe growth. The growth on the 2° off substrate was also “denser” than that on exact (100), which implied that coalescence of the islands occurred at lower thickness. This was expected since island nucleation is most favored along the ledges on the surface whose spacing decreases with increasing misorientation. A simple nucleation model, assuming cylinder-like islands, was able to fit the experimental data quite well, lending support to the island growth model.