In-situ control of temperature and alloy composition of Cd1-xZnxTe grown by molecular beam epitaxy

In this work in-situ spectroscopic ellipsometry (SE) has been applied for the simultaneous determination of the growth temperature and alloy composition for the epitaxial Cd1-xZnxTe(211)/Si(211) structure. The optical dielectric functions of CdTe and Cd0.96Zn0.04Te (CZT) epilayers were studied as a function of temperature both ex-situ and in-situ in the range from 1.6 eV to 4.5 eV. We employed parametric models for the simulation of the optical properties of CZT at and between the critical points (CP) E-0, E-0 + Delta(0), E-1, E-1 + Delta(1), E-2(Sigma) and E-2(Sigma). Critical point energies and line widths for Cd1-xZnxTe were obtained through the fitting process, which included both zero order and higher order derivatives of the SE pseudo dielectric function. The dependence of the different critical points on Zn concentration x is discussed. It has been demonstrated that the energy of the weak E-0 + Delta(0) transition can be used to measure composition, while the E-1 energy can be used as a real-time temperature measure. The model parameters were optimized through the simultaneous analysis of multiple data sets, and the temperature dependent model was developed for in-situ application. Our analysis is estimated to produce uncertainties of only +/- 0.5 degrees C in measuring the temperature and +/- 0.5% in measuring the composition if only the zero order dielectric function is being fitted. The effects of a surface overlayer, of reflected beam deflections, and of other experimental problems on the overall accuracy, are discussed as well as ways to improve the in-situ SE data quality.