AVS 47th International Symposium
    Photonics Tuesday Sessions
       Session PH-TuM

Paper PH-TuM3
Infrared Ellipsometry Characterization of Porous Silicon Bragg Reflectors

Tuesday, October 3, 2000, 9:00 am, Room 310

Session: Fundamental Properties and Applications of Photonic Materials
Presenter: J.A. Woollam, University of Nebraska, Lincoln
Authors: S. Zangooie, University of Nebraska, Lincoln
M. Schubert, University of Nebraska, Lincoln
C. Trimble, University of Nebraska, Lincoln
D.W. Thompson, University of Nebraska, Lincoln
J.A. Woollam, University of Nebraska, Lincoln
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Porous silicon (PS) has been the subject of intensive investigation for applications of the material in disciplines such as silicon based electronics, gas- and biosensor technologies, as well as optics. Industrial applications of PS demand fast and non-destructive determination of the material properties in terms of, e.g., thickness, porosity and surface chemical characteristics. In this work variable angle of incidence infrared spectroscopic ellipsometry is employed to simultaneously determine the real and imaginary parts of the dielectric function of the solid phase of the porous silicon Bragg reflectors without the necessity for additional measurements on reference samples, employment of the Kramers-Kronig technique, or extrapolation of experimental data beyond the measurement range. In addition to the thickness, volume porosity, inhomogeneity and optical anisotropy, properties of the solid part of the porous material are investigated in terms of the optical dielectric function and surface chemistry. The high sensitivity of the technique is employed to detect and identify infrared resonant absorptions related to different Si-H as well as Si-O-Si vibrational modes. Resonances due to Si@sub 2@-Si-H@sub 2@, Si@sub 3@-Si-H, Si@sub 2@-Si-H@sub 2@, Si-O-Si stretching modes, as well as Si@sub 2@-Si-H@sub 2@ scissor and Si@sub 2@-Si-H@sub 2@ wagging mode are revealed and characterized. A relatively large resonance at 626 cm@super -1@ is attributed to the Si-Si bond stretching caused by the asymmetry in surface related bonds creating an IR-active net dipole moment. The material is found to have positive birefringence. The electrical resistivity of the solid part of the porous material is determined to be 0.03 @ohm@ cm, and larger than the corresponding bulk value of 0.019 @ohm@ cm. Furthermore, the carrier concentration in the investigated porous material shows a decrease from 6.2 * 10@super 18@ cm@super -3@ to 4 * 10@super 18@ cm@super -3@.