INVESTIGATION OF THE STRUCTURAL PROPERTIES OF HAFNIUM ZIRCONIUM OXIDE THIN FILMS

Abstract

Hafnium-zirconium oxide (HfxZr1-xO2)-based dielectrics have received an abundance of attention recently, holding promise for use in CMOS-compatible ferroelectric memory technology. Unlike in conventional ferroelectrics, ferroelectricity in HfxZr1-xO2 is retained in ultrathin films of only a few nanometers thickness. The ultrathin nature of these structures enables ferroelectric tunnel junctions with higher readout currents than FeRAM, and with non-destructive read-out. Both of these properties are in favour of scaling. Ferroelectricity in HfxZr1-xO2 is attributed to the presence of the polar orthorhombic phase, formed in-situ by the transformation of the tetragonal phase. This transition is thought to occur only for crystallites exhibiting a specific orientation. There is some debate as to whether or not some proportion of the monoclinic and tetragonal phases are also required as to permit switching of the ferroelectric crystallites. This is complicated by the fact that these characteristics can only be controlled by film deposition conditions. Controlled growth of thin films using various deposition techniques capable of preparing films exhibiting these characteristics has been established. However, the quest for optimal preparation conditions calls for a routine technique able to assess the phase composition in ultrathin dielectric films, as required for the development of ferroelectric tunnel junction (FTJ) devices.

Several films consisting of Hf0.5Zr0.5O2 deposited on TiN and amorphous SiO2 were prepared using radio-frequency (RF) sputtering and hybrid RF-pulsed laser deposition. Analysis by synchrotron grazing incidence X-ray diffraction (GIXRD) revealed that most samples contained only the monoclinic phase with fiber texture along the [1-1-1]/[11-1] axis, with the SiO2-based samples exhibiting stronger preferred orientation. Two TiN and SiO2-based samples contained the tetragonal phase with fiber texture along the [011] axis. Complimentary transmission electron microscopy (TEM) experiments showed that crystallites are indeed preferentially oriented with some degree of mosaicity. The techniques described are suitable for the identification of texture in ultrathin HfxZr1-xO2 films, and are likely able to differentiate between the structurally similar tetragonal and ferroelectric orthorhombic phases.