Optical band gaps and composition dependence of hafnium-aluminate thin films grown by atomic layer chemical vapor deposition

We report the optical properties of unannealed hafnium-aluminate (HfAlO) films grown by atomic layer chemical vapor deposition (ALCVD) and correlate them with the aluminum contents in the films. Vacuum ultraviolet spectroscopic ellipsometry (VUV-SE), high-resolution transmission electron microscopy (HRTEM), channeling Rutherford backscattering spectrometry (RBS), and resonant nuclear reaction analysis (NRA) were employed to characterize these films. In the analyses of ellipsometry data, a double Tauc-Lorentz dispersion produces a best fit to the experimental VUV-SE data. As a result, the determined complex pseudodielectric <epsilon > functions of the films clearly exhibit a dependency on the aluminum densities measured by RBS and NRA, We show that the optical fundamental band gap E-g shifts from 5.56 +/- 0.05 eV for HfO2 to 5.92 +/- 0.05 eV for HfAlO. The latter was grown by using an equal number of pulses of H2O/HfCl4 and H2O/TMA (trimethylaluminum) precursors in each deposition cycle for HfO2 and Al2O3, respectively. The shift of E-g to higher photon energies with increasing aluminum content indicates that intermixing of HfO2 and Al2O3 occurred during the ALCVD growth process. We found that E-g varies linearly with the mole fraction x of Al2O3 in the alloy (HfO2)(x)(Al2O3)(1-x), but has a parabolic dependency with the aluminum density. We also observed a consistent decrease in the magnitudes of the real <epsilon(1)> and imaginary <epsilon(2)> part of <epsilon > of HfAlO films with respect to those of HfO2 as the Al density increased. The absence of the approximate to 5.7 eV peak in the <epsilon > spectrum, which was previously reported for polycrystalline HfO2 films, indicates that these films are amorphous as confirmed by their HRTEM images. (c) 2005 American Vacuum Society.