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Effect of Residual Strain on the Electronic Structure of Alumina and Magnesia

The reflectance spectra of polycrystalline Al2O3 and singlecrystal MgO, with different surface treatments were measured using vacuum ultraviolet (VUV) spectroscopy to determine the effect of residual stresses and strains. The bulk stresses were estimated using the R-line flourescence of trace Cr3+ in Al2O3. No grain-size dependence was observed for the residual thermal expansion anisotropy stresses in polycrystalline Al2O3. The effects of mechanical-polishing-induced damage in Al2O3 shifted the band-to-band and exciton transitions to higher energies, compared with those of chemically polished Al2O3. There was little difference in peak positions between cleaved and chemically polished surfaces of MgO. However, mechanically polished MgO surfaces exhibited band-to-band and exciton-peak shifts to higher energies. The VUV technique of determining the exciton-peak and band-gap shifts was used with knowledge of the band-gap strain coefficient to estimate the magnitudes of residual surface stresses in the 1-μm surface layer probed. The polishing-damage-induced residual surface stress is 2.2 GPa for Al2O3 and 1.1 GPa for MgO. These values are based on band-gap strain coefficients of 700 meV/(lin% strain) derived for Al2O3 and 400 meV/(lin% strain) estimated for MgO.