OPTICAL-PROPERTIES OF ALUMINUM-OXIDE - DETERMINED FROM VACUUM-ULTRAVIOLET AND ELECTRON-ENERGY-LOSS SPECTROSCOPIES

The optical properties of alpha-Al2O3 have been determined by two inde pendent methods, vacuum ultraviolet (VUV) spectroscopy and electron en ergy-loss spectroscopy (EELS) over the energy range from 6 to 142 eV, For each experimental method two sets of high-quality data have been m easured and analyzed from alpha-Al2O3. The evolution of the difference s between the data for each experimental method and its associated ana lytical method indicate the reliability of the results, and the compar ison between the methods emphasizes the relative advantages of each. V UV spectroscopy offers a higher energy resolution, resolving sharper f eatures in the spectra, whereas EELS significantly extends the energy range, which has been exploited here to 140 eV but can be extended eve n further, Good overall consistency is found, but there are pronounced differences in the valence region from 10 to 30 eV, where strong abso rptions are present, increasing the relative variability of the analys is. This demonstrates that accuracy estimates of the optical propertie s are meaningful only when supplied as a function of energy, because b oth strong intrinsic absorptions and weak experimental signals make ac curate determinations difficult. Moreover, variations in the optical p roperties are observed most easily in the interband transition strengt h, J(cv), and are less evident when the optical properties are represe nted as the complex index of refraction, Because the optical propertie s can change with specimen origin and preparation and because of the d etails of data acquisition and numerical analysis procedures, assessin g the contributions of each of these methods to our accurate knowledge of the optical constants is essential.