Structural, morphological, optical and photoluminescence properties of HfO2 thin films

Nanocrystalline monoclinic HfO2 films with an average crystal size of 4.2-14.8nm were sputter deposited under controlled temperatures and their structural characteristics and optical and photoluminescence properties have been evaluated. Structural investigations indicate that monoclinic HfO2 films grown at higher temperatures above 400 degree C are highly oriented along the (-111) direction. The lattice expansion increases with diminishing HfO2 crystalline size below 6.8nm while maximum lattice expansion occurs with highly oriented monoclinic HfO2 of crystalline size about 14.8nm. The analysis of atomic force microscopy shows that the film growth at 600 degree C can be attributed to the surface-diffusion-dominated growth. The intensity of the shoulderlike band that initiates at ~5.7eV and saturates at 5.94eV shows continued increase with increasing crystalline size, which is intrinsic to nanocrystalline monoclinic HfO2 films. Optical band gap varies in the range 5.40 plus or minus 0.03-5.60 plus or minus 0.03eV and is slightly decreased with the increase in crystalline size. The luminescence band at 4.0eV of HfO2 films grown at room temperature can be ascribed to the vibronic transition of excited OH [radicaldot] radical while the emission at 3.2-3.3eV for the films grown at all temperatures was attributed to the radiative recombination at impurity and/or defect centers.