Optical characterization of HfO2 thin films grown by atomic layer deposition

Optical absorption and photoluminescence of amorphous and crystalline HfO2 thin films grown by atomic layer deposition from HfCl4 and H2O were studied. Band-gap energy of (5.55 +/- 0.03) eV was determined for monoclinic HfO2 with mean crystallite sizes of 30-40 nm as well as for amorphous HfO2. Excitation in the range of intrinsic absorption resulted in emission that had maximum intensity at 3.2 eV in the case of amorphous films and at 2.6 or 4.4 eV in the case of monoclinic films. The emission intensity of crystalline films exceeded that of amorphous films by an order of magnitude at all temperatures studied. The main luminescence band at 4.4 eV was tentatively assigned to the emission of self-trapped excitons while the emission at lower photon energies was attributed to defects and impurities. With the increase of temperature from 10 to 295 K, the low-energy edges of excitation spectra shifted towards lower energies by 0.1 eV in the case of amorphous films and by 0.15 eV in the case of crystalline films, indicating corresponding changes in the band-gap energies. (C) 2004 Elsevier B.V. All rights reserved.