Abstract
Since the first report of ferroelectricity in hafnium oxide (HfO2) doped with SiO2 in 2011, significant research efforts have been directed toward understanding the switchable spontaneous polarization in this material. HfO2 is chemically compatible with silicon, is currently used as a high-κ dielectric in complementary metal-oxide semiconductor (CMOS) devices, and in the ferroelectric phase, is not susceptible to the thickness scaling effects that impose application limitations on traditional ferroelectrics. Thus, this material presents opportunities for technological developments in devices, such as renewed scaling of ferroelectric random access memory (FeRAM), ferroelectric field effect transistors (FeFETS), and new devices such as ferroelectric tunnel junctions (FTJs) that previously required epitaxial growth.
The wide-scale adoption of ferroelectric HfO2 into devices is constrained, in part, by an inability to prepare phase-pure films. In equilibrium at room temperature and atmospheric pressure, HfO2 exists in the nonpolar monoclinic phase. Multiple metastable phases exist, including a polar orthorhombic phase, an antipolar orthorhombic phase, and a nonpolar tetragonal phase, each of which can be stabilized by various factors. Ferroelectricity in HfO2 has been attributed to the polar Pca21 orthorhombic phase. Several factors have been shown to impact phase constitution, including dopant type and concentration, biaxial stress, oxygen vacancies, and film thickness or grain size. The work in this dissertation will show the results of an evaluation of the use of deposition and processing parameters to tailor film properties to better understand and control the factors that stabilize the ferroelectric phase in this material.
First, a process for the deposition of ferroelectric HfO2 films using high-power impulse magnetron sputtering (HiPIMS) is developed. Since oxygen vacancies significantly affect the performance of ferroelectric HfO2-based thin films, the impact of plasma oxygen content during HiPIMS deposition of HfO2 films is investigated. It is shown that the oxygen content in the plasma directly relates to the oxygen content in the films, and this oxygen content has a strong influence on phase formation and ferroelectric performance. The oxygen vacancy concentration plays a larger role in phase stability than grain size at this approximately 20 nm size scale. Neutral oxygen vacancies, which are often overlooked in the literature, are also identified in crystalline HfO2 films. These results demonstrate that oxygen content can be used to dictate phase nucleation in HfO2 films.
Next, infrared (IR) spectroscopy is demonstrated as a means by which phases can be unambiguously assessed in this material system. While grazing-incidence X-ray diffraction (GIXRD) is commonly used to distinguish between phases, it has difficulty unambiguously differentiating between the ferroelectric orthorhombic phase and other metastable phases that may exist. Using three HfO2 films consisting primarily of the monoclinic, polar orthorhombic, and antipolar orthorhombic phases, respectively, the unique signatures of each phase are identified using synchrotron nano-Fourier transform infrared spectroscopy (nano-FTIR) measurements. Vibrational spectroscopy is demonstrated as a means to characterize phases present in this material.
Ion bombardment is assessed using two methods – during film growth via the HiPIMS pulse width and irradiation of HfO2 and hafnium zirconium oxide (HZO) films with 2.8 MeV Au2+. The HiPIMS pulse width, which affects the ionization fraction of the depositing species, is shown to alter nucleation behavior and the phases that form during crystallization. Similarly, heavy ion irradiation is demonstrated to affect nucleation and the phases that form in films grown by atomic layer deposition (ALD), although this method also results in the crystallization of the films.
One challenge facing this material is the so-called “wake-up” effect in which the remanent polarization increases with field cycling. Many mechanisms have been hypothesized to be responsible for this effect; using nano-FTIR measurements, it is proven here that phase transformations contribute to this effect. Further, the evolution of film stress during wake-up is quantified for the first time.
Ultimately, stabilization of the ferroelectric phase in HfO2-based thin films is necessary for the implementation of this material into devices; the findings presented here offer insight into the mechanisms of phase stability in this material and provide engineering strategies to produce ferroelectric HfO2-based devices with robust properties.
