The structure, optical behavior, and thermal stability of sputter-deposited hafnium dioxide single layer and hafnium dioxide-aluminum oxide nanolaminate thin-films

Hafnium dioxide (HfO2, hafnia) is a refractory material that has gained importance in thin films for wide band gap and optical coating applications. The goal of this work is to examine the crystallization behavior, optical absorption, and thermal stability of single layer HfO2 and nanolaminate HfO2-Al2O3 films, and to investigate the nucleation behavior of nanocrystalline HfO2 in ultrathin layers.;Films were sputter deposited using rf-excited Ar/O2 discharges on unheated Si and SiO2 substrates. Combinations of cathode voltage and sputtering gas O2 content were used to grow ∼250 nm-thick single layer HfO2 films. Nanolaminate films of HfO2 and Al2O3 multilayer stacks (∼250 nm) with several architectures were sputtered from Hf and Al targets in 80% Ar - 20% O 2 discharges.;Films were annealed in air for 1 hr to 24 hr, cooled to room temperature in air, and examined using double angle x-ray diffraction (XRD) and ultraviolet-visible-(very) near infrared spectrophotometry. High resolution transmission electron spectroscopy (HRTEM) was carried out on selected films.;XRD results showed that all as-grown single layer films are nanocrystalline with diffraction solely from monoclinic (m-) HfO2, the standard-temperature-and pressure (STP) phase. For a fixed cathode voltage, the m-HfO2 crystallite orientation with respect to the substrate, i.e., film texture, is dependent upon the gas O2 content. At high gas O2 content, the (11-1) m-HfO2 "lowest energy" orientation with respect to the substrate is observed, i.e., the films have a (11-1) texture. Other orientations of m-HfO2 nanocrystallites occur when films were grown at low sputtering gas O2 content and low applied voltage. Annealing all the single layer films produced m-HfO2 with a strong (111) texture.;All films are transparent in the visible spectral region. The ultraviolet fundamental optical absorption edge (FOAE) characteristics of the single layer HfO2 films are independent of nanocrystal orientation. A distinct low energy band on the FOAE (denoted the "pre-gap" band) is observed. The intensity, or level of absorption, of this band increases with increased annealing time and temperature, and consequent crystallization, although its spectral position does not change. This band is proposed to be intrinsic to the short range order of the monoclinic HfO2 crystal structure.;With respect to the nanolaminates, HfO2 is nanocrystalline and Al2O3 is amorphous in the as-grown and annealed films. The m-HfO2 phase prevails with increasing HfO2 layer thickness. However, HRTEM shows that as-grown nanolaminates with HfO 2 layers < 10 nm contain two non-STP polymorphs: tetragonal (t) HfO 2, a high temperature phase, and orthorhombic (o) HfO2, a high pressure phase. These metastable phases are termed "initial growth" phases, presumed to be formed by a finite crystal size effect.;All nanolaminate architectures are thermally stable up to 700°C. However, after a 900°C anneal for 1 h, XRD data shows evidence of interfacial mixing in the form of a t-HfxAlyO2-z structure. Continued annealing at 900°C for up to 33 hr results in phase separation into m-HfO2 and t-HfxAlyO2-z, with the amount of Al in the t-HfO2 lattice decreasing over time. At 1000°C, t-HfxAlyO2-Z disappears and HfO2 layers consist entirely of m-HfO2.;Spectrophotometry of the annealed nanolaminates shows that the pre-gap band mentioned above is suppressed in films whose HfO2 layers consist largely of the initial growth phases. Heat treatment at 1000°C for as little as 15 min produces a well crystallized m HfO2 structure with its characteristic pre-gap optical absorption band.