Electronic States in Tetrahedral Carbon Films

Abstract

Disordered tetrahedral sp3-carbon films of different allotropic forms are grown by ion beam deposition at ≈ 120 to 200 eV carbon ion energies. The delocalized and localized electronic states of films on KCl and Al2O3 substrates are investigated by absorption and secondary radiation under N2 laser or Ar laser excitation at 90 and 300 K. A strong maximum at 6.15 to 5.80 eV (E) and a threshold at 4.8 to 4.5 eV are resolved in intrinsic absorption spectra of films deposited at high currents which are tentatively attributed to direct (Γ−Γ) and indirect (Γ−K2) transitions in partially-ordered lonsdaleite carbon films. The disordered cubic sp3-carbon phases obtained at low ion currents show maxima at 5.45 to 5.50 eV (Γ−ΔX, indirect transition) and at about 7.0 eV (Γ−Γ15, direct transition). All spectra contain low-energy Urbach tails of different threshold energies, intensities, and slopes. A two-component luminescence band 2.407 and 2.680 eV at 90 K under UV-laser excitation for films with strong Urbach tails is resolved. A sample dependent “blue” emission including four well-shaped maxima 3.031, 2.849, 2.600, and 2.480 eV at 90 K is discovered. At 90 K under strong 488.0 and 514.5 nm laser excitation of disordered carbon and polycrystalline CVD diamond films luminescence response above 4.0 eV is found.