Use of spectroscopic ellipsometry for feedback control during the growth of thin AlAs layers

Spectroscopic ellipsometry (SE) is an accurate in situ method for determini ng the composition and thickness of III-V semiconductor layers during growt h. In order to achieve this control, an optical constant database is used t o compare the modeled and experimental ellipsometric data. This procedure i s very effective for controlling thickness and composition when the film ha s been growing for some time but is usually unreliable until several minute s into the growth (corresponding to similar to 50 nm). To use SE for the co ntrol of thinner layers (1-20 nm), a different approach has to be used. A n ew strategy is proposed which consists in looking at the raw SE signal in a limited wavelength range where the signal varies almost linearly with the film thickness. For AlAs grown on GaAs, it is found the phase part of the S E signal for light between 2.5 and 2.8 eV varies nearly linearly with the A lAs thickness. A series of AlAs/GaAs multiquantum well structures are grown and analyzed. The thickness determined by this use of the ellipsometric da ta are in close agreement with independent thickness measurements obtained from high resolution x-ray diffraction. (C) 1999 American Vacuum Society. [ S0734-211X(99)06003-5].