In Situ Spectroscopic Ellipsometry for Real Time Composition Control of Hg 1-x Cd x Te Grown by Molecular Beam Epitaxy
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R. DAT, F. AQARIDEN,' W.M. DUNCAN, D. CHANDRA, and H. D. SHIH * Raytheon TI Systems, Sensors and Infrared Laboratory, P.O Box. 655936, MS 150, Dallas, TX 75265, U.S.A ** Permanent address: Microphysics Laboratory, Department of Physics, M/C 273, University of Illinois at Chicago, 845 W Taylor #2236SES, Chicago, IL 60607, U. S. A. *** Texas Instruments Incorporated, Components and Materials Research Center, P. 0. Box 655936, MS 147, Dallas, TX 75265, U.S.A ABSTRACT Spectral ellipsometry (SE) was applied to in situ composition control of HgixCdxTe grown by molecular beam epitaxy (MBE), and the impact of surface topography of the Hg,_ Cd.Te layers on the accuracy of SE was investigated. Of particular importance is the presence of surface defects, such as voids in MBE- Hg1 ,CdTe layers. While dislocations do not have any significant impact on the dielectric functions, the experimental data in this work show that MBE- Hg1 ,CdTe samples having the same composition, but different void densities, have different effective dielectric functions. INTRODUCTION Accuracy of the composition control during epitaxial growth is always desirable. Spectral ellipsometry (SE) offers several advantages as an in situ, feedback sensor for molecular beam epitaxial (MBE) growth. The technique is non-intrusive and non-destructive. It offers reproducible growth and can provide real-time feedback when integrated with a fast data processor. It also exhibits high sensitivity to layer thickness and complex index of refraction (i.e., n-ik). Hence, it is ideal for sensing applications. This work is on the application of SE to composition control during MBE growth of Hg,-,Cd.Te on Cd, -ZnTe(21 1)B. The issue of real-time computation with spectral ellipsometry is especially challenging since numerical solution methods depend not only upon the reflection equations, but also upon several other factors which strongly influence numerical solution. Some of the key factors include surface morphology of the starting substrate and the evolving thin film; library dielectric functions relevant at the deposition temperature; angle and plane of incidence at the substrate surface at the time the spectral data are acquired; and surface condition and bulk properties of the view ports. A poor surface morphology increases the scattering and depolarization of the incident radiation, degrades the signal-to-noise ratio of the spectral information and, consequently, leads to erroneous data to the feedback loop. In such a situation, the feedback becomes unstable and real-time control is lost. The second factor, library dielectric function, is
377 Mat. Res. Soc. Symp. Proc. Vol. 484 01998 Materials Research Society
very sensitive to the quality of the MBE film. Defects such as voids and hillocks [1,2] need to be minimized in films that are specifically grown for the purpose of providing library functions. The last two points have been discussed in an earlier report [3]. Preliminary results show that MBE- Hgl-.CdTe films with the same composition but different de
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