730
J. Phys. Chem. 1983, 87,730-731
Polarization-Modulated Attenuated Total-Internal Reflection Infrared Spectroscopic Study of Surfaces: An Application to the Aging of Oxide Films on Silicon Plates Kerln Scanlon, Richard J. Kvltek, Suelien F. Schulthesr, John F. Evans, and John Overend' Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received: December 9, 1982)
A technique of polarization-modulatedATR infrared spectroscopy has been developed for the study of surface species. This technique, which appears to be more sensitive than conventional ATR techniques, has been used
to study the aging of oxide films on single-crystal silicon surfaces.
I t is already well established that infrared spectra of adsorbates on low-area reflecting surfaces may conveniently be measured by using polarization-modulation techniques.l In principle, the same modulation scheme may be applied to the spectroscopy of preferentially oriented weakly absorbing samples and we now report the development of an experiment for the study of molecular species on silicon surfaces using polarization-modulated attenuated total-internal reflection (ATR) spectroscopy. The basic design of the apparatus is similar to that described in ref 1 except that the reflecting sample is replaced by a polished ATR plate of dimensions 2 X 20 X 50 mm with the ends bevelled at 45". This geometry gives 25 internal reflections within the ATR plate. The plates we have used were cut from single-crystal p-type silicon with a resistivity of 14 R cm. The orientation of the plate was as follows: main face (loo), edges (110) and bevelled edges (111). As shown in Figure 1,the signal was doubly modulated, first with a conventional chopper and second with a photoelastic modulator. The polarization modulation was set up so as to compare the transmitted radiation with its electric vector parallel to the plane of incidence, E,, with that of the radiation polarized perpendicular to the plane of incidence, E,. The probability of absorption of radiation by a molecule at the surface of the ATR plate is proportional to the square of the scalar product of the electric field and the electric dipole transition moment and we may expect the absorption of the p-polarized radiation to differ from that of the s-polarized radiation, either if there is a preferential orientation of the surface molecules in the p or s directions or if there is a difference in the E , and E, electric fields.2 The enhancement in sensitivity achieved through use of the polarization-modulation technique is nicely illustrated by the following experiment: a thermal oxide was grown on a freshly cleaned3 silicon ATR plate by heating the plate at 900 "C for 4 h in wet ~ x y g e n .The ~ average thickness of the silicon oxide layer was determined from ellipsometric measurment to be 800 A. After some aging and recleaning, the ATR plate was mounted in the spectrometer and a spectrum was scanned. The plate was then heated in a nitrogen atmosphere for 2 h at 750 "C and for 1 h at 900 "C and the spectrum rescanned. The first (1) Golden, W. G.; Dunn, D. S.; Overend, J. J. Catal. 1981, 71 395. (2) At high angles of incidence at a metal surface, E,
