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

Abstract

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 ⟨ε⟩ 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 Eg shifts from 5.56±0.05eV for HfO2 to 5.92±0.05eV 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 Eg to higher photon energies with increasing aluminum content indicates that intermixing of HfO2 and Al2O3 occurred during the ALCVD growth process. We found that Eg 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 ⟨ε1⟩ and imaginary ⟨ε2⟩ part of ⟨ε⟩ of HfAlO films with respect to those of HfO2 as the Al density increased. The absence of the ≈5.7eV peak in the ⟨ε⟩ spectrum, which was previously reported for polycrystalline HfO2 films, indicates that these films are amorphous as confirmed by their HRTEM images.