Optical Study of the Electronic Structure of Diamond

In order to resolve some of the experimental discrepancies existing between various published optical studies of diamond, absolute near-normal incidence-reflectance measurements have been taken on a polished type-I and both cleaved and polished type-IIa diamond crystals. These measurements covered the range from 5.5 to 11.5 eV at liquid-nitrogen temperature and from 5.5 to 31 eV at room temperature. The Kramers-Kronig analysis used in this research to determine the phase angle differs from that used in previous work inasmuch as no extrapolation of data outside the measured high-energy limit was required. The dielectric response function ^ε was obtained over a broad energy range (5.5-31 eV). Structure in its imaginary part ε₂ occurring at 7.3, 7.8, 12.2, and 23 eV was observed and assigned to direct interband transitions on the basis of recent band calculations. Room-temperature data agreed with the latest previous measurement on type-I diamond over the common energy range (5.5-25 eV). The temperature insensitivity of the first peak suggests that the direct band edge is near 7.3 eV. The previously unobserved temperature-dependent 7.8-eV peak is probably caused by changes in the complex structure of the energy bands near the Γ point, although an exciton interpretation is not excluded. Evidence that the small 23-eV peak may be due to surface contamination is also presented.