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-perio d 3 X 3 GaAs/AlAs superlattices grown by molecular-beam epitaxy (MBE). The complex dielectric function and thickness of the whole superlatti ce and the thickness of the native oxide overlayer were simultaneously determined by an inversion technique from data in the 1.5-5.0 eV regi on. 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 lin e shapes. A simple interpretation of the shift and broadening is given . The interface roughness and layer thicknesses obtained by SE are fou nd to be consistent with the results of x-ray diffraction and Raman sc attering studies previously reported. The results in this study demons trate the capability of the post-growth nondestructive characterizatio n by SE to provide useful information about the interface quality of s uperlattice structures, and consequently to optimize the MBE growth co nditions in order to achieve the desired structural parameters.