INTERFACE ROUGHNESS OF SHORT-PERIOD ALAS GAAS SUPERLATTICES STUDIED BY SPECTROSCOPIC ELLIPSOMETRY

Spectroscopic ellipsometry (SE) has been used to study the effects of interface roughness on the optical properties of ultrathin short-period 3 X 3 GaAs/AlAs superlattices grown by molecular-beam epitaxy (MBE). The complex dielectric function and thickness of the whole superlattice and the thickness of the native oxide overlayer were simultaneously determined by an inversion technique from data in the 1.5-5.0 eV region. The main optical critical points E0, E0 + DELTA0, E1, E1 + DELTA1, and E2 were deduced by line-shape fitting of the second derivative of the complex dielectric function of the superlattice to the analytical line-shape expression. The interface roughness is found to shift the optical transitions, except E2, to higher energy and broaden their line shapes. A simple interpretation of the shift and broadening is given. The interface roughness and layer thicknesses obtained by SE are found to be consistent with the results of x-ray diffraction and Raman scattering studies previously reported. The results in this study demonstrate the capability of the post-growth nondestructive characterization by SE to provide useful information about the interface quality of superlattice structures, and consequently to optimize the MBE growth conditions in order to achieve the desired structural parameters.