Optical properties of amorphous hydrogenated carbon films: A spectroscopic ellipsometry study
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Optical properties of plasma-deposited amorphous hydrogenated carbon films were studied by spectroscopic ellipsometry. From the ellipsometry data, the real and imaginary parts, n and k, of the complex index of refraction of the film have been deduced for photon energies between 2.0 and 4.0 eV for as-grown as well as for thermally annealed films. Here n and k showed considerable variation with subsequent annealing, even under 400 °C. A tentative explanation of the results is proposed.
I. INTRODUCTION
The photometric ellipsometer is of the rotating analyzer type,9 the wavelengths being adjustable between 2700 and 6000 A continuously. The azimuth of the polarizer was set at 45°, measured from the plane of incidence. No quarter wave plate was used. The 1-6 relations were measured by rotating the analyzer, / being the signal value indicated by the photomultiplier amplifier system and 6 being the azimuth of the analyzer. The ellipsometric parameters xp and A could be calculated9 from the minimum position 6min, the maximum signal value 7 max , and the minimum signal value 7min of the 1-6 curve. Here ip is the relative amplitudes of the attenuation of the P wave and the 5 wave under reflection and A is the difference between the phase shifts of the P wave and the S wave, where the P wave is for the electric field vector in the plane of incidence and the S wave is for the electric field vector perpendicular to the plane of incidence. Any light beam consists of a P-wave component and an S-wave component. The same applies for the reflected beam. Measuring 7 max , 7 min , and 6min at various wavelengths determines the ellipsometric spectrum, i.e., the (ip,A)-E relationship. When the a-C:H film was thicker ( > several microns) and the photon energy E was higher, the following simple formulas could be used to calculate n and k:
Amorphous hydrogenated carbon films (a-C:H) are of considerable interest because of their outstanding properties, such as extreme hardness, chemical inertness, high electrical resistivity, and infrared transparency. A large number of analytical techniques have been employed to characterize electrical and optical properties of the films, including conductivity measurement, photoemission spectroscopy,' absorption and reflectivity measurement,2 Raman, 3 infrared,4 electron spin resonance (ESR), 5 photoluminescence,6 and electron energy loss spectroscopy.7 Very little work has been done on a-C:H films using spectroscopic ellipsometry.8 Absorption measurement can be used to determine the optical properties of the films, but are only possible for films on transparent substrates. In addition, there is an upper limit to the photon energy E for which this method is applicable. On the other hand, ellipsometry can measure films either on transparent or on opaque substrates. It also has the advantage of being a direct optical method in the evaluation of n and k, and theoretically there is no upper limit to the photon energy for which this method is applicable. In this paper the optical properties of a-C:H films me
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