Use of in situ spectroscopic ellipsometry during the molecular beam epitaxial growth of Hg 1-x Cd x Te Giacomo Badano 1* , James W. Garlad 2 , Hisham Abad 3 and Sivalignam Sivananthan 4 1 Currently with the French Atomic Energy Commission, CEA/LETI Minatec,17 rue des Martyrs, 38054 Grenoble Cedex, France 2 EPIR Ltl., 590 Territorial Dr, Unit B, Bolingbrook, IL , USA 3 Alfaisal University, P. O. Box 50927, Riyadh, 11533, Kingdom of Saudi Arabia 4 EPIR Ltl., 590 Territorial Dr, Unit B, Bolingbrook, I, USA and University of Illinois at Chicago, 845 W. Taylor St. Room 2236, Chicago IL 60607, USA * Correspondig author: giacomo.badano@cea.fr Due to its intrinsic accuracy and non-invasiveness, spectroscopic ellipsometry (SE) is widely used as an in situ tool to monitor, control and characterize the molecular beam epitaxial (MBE) growth of Hg 1-x Cd x Te alloys. The growth temperature, the alloy composition, and the presence of defects, overlayers or roughness at the surface in principle all are measurable by in situ SE, if the appropriate data analysis technique is used. Here, greatest attention is given to the use of SE to determine the temperature and surface roughness of substrates and the composition and surface roughness of growing Hg 1- x Cd x Te, and to the principal methods of analysis of SE data, pointing out the strengths and weaknesses of each method. Various specialized applications of SE in the study of the 1