Origin of Ferroelectric Phase in Undoped HfO₂ Films Deposited by Sputtering

Abstract

the formation of a non-centrosymmetric Pca2 1 orthorhombic phase (o-phase). [1][2][3][4][5][6][7] For increasing doping concentrations, ALD HfO 2 films undergo a phase transition from a non-ferroelectric m-phase to ferroelectric orthorhombic phase and for higher concentrations to the tetragonal phase (t-phase; space group: P4 2 /nmc) if the dopants are smaller than Hf like Si and Al, or to the cubic phase if the dopants are larger than Hf like Gd, La, Sr, and Y. [8] Besides the influence of doping, four other factors are known to promote the stabilization of the ferroelectric phase: surface or interface/grain boundary energy, film stress, and the presence of oxygen vacancies. [9][10][11][12][13] Oxygen vacancies and the related defect states play an important role in the so-called wake-up effect. [14] Wake-up describes the increase of the remanent polarization during electrical field cycling with opening of an initially pinched polarization-voltage hysteresis. [11] In Hf 1−x Zr x O 2 films, Materlik et al. suggested that the bulk and surface free energy of the o-phase is located between those of the m-phase and t-phase. As a result, the o-phase is stabilized in a specific film thickness and grain size region. This suggestion matches well Thin film metal-insulator-metal capacitors with undoped HfO 2 as the insulator are fabricated by sputtering from ceramic targets and subsequently annealed. The influence of film thickness and annealing temperature is characterized by electrical and structural methods. After annealing, the films show distinct ferroelectric properties. Grazing incidence X-ray diffraction measurements reveal a dominant ferroelectric orthorhombic phase for thicknesses in the 10-50 nm range and a negligible non-ferroelectric monoclinic phase fraction. Sputtering HfO 2 with additional oxygen during the deposition decreases the remanent polarization. Overall, the impact of oxygen vacancies and interstitials in the HfO 2 film during deposition and annealing is correlated to the phase formation process.

An Overview of Ferroelectric Hafnia and Epitaxial Growth

“…[41] Density functional theory (DFT) calculations also verified this correlation that tetragonal and orthorhombic phase formation is suppressed for higher oxygen concentrations in HfO 2 , and monoclinic phase nuclei are formed. [41,99] In addition, the phonon vibrations upon the application of voltage bias would decrease with the increase in oxygen vacancy concentration. In this sense, the ferroelectric behavior such as fatigue and imprint field would degrade under high oxygen vacancies.…”

“…[45] In addition, the multigrain nature of the polycrystalline hafnia thin films poses another requirement on a "wake-up" process, [56] which is not favorable for ferroelectric memory devices. Other techniques have also been used for the growth of hafnia thin films, such as sputtering, [41,69,70] metal-organic chemical vapor deposition (MOCVD), [71] or chemical solution deposition (CSD), [72] almost all resulting in polycrystalline films.…”

Metal Nitride Electrode Stress and Chemistry Effects on Phase and Polarization Response in Ferroelectric Hf0.5Zr0.5O2 Thin Films

“…[32,33] Similar observations of larger remanent polarizations in films containing greater quantities of oxygen vacancies have been made experimentally, [12,13,34] leading to further development of HfO 2 deposition processes designed to enhance the concentrations of these defects. Such process modifications have included alteration of the oxygen content in the process gas during sputtering [12,13] or duration of the ozone dose during ALD. [34] Aside from HfO 2 processing conditions, studies have observed varied ferroelectric performance dependent upon the selected electrode material.…”

Impact of Iridium Oxide Electrodes on the Ferroelectric Phase of Thin Hf0.5Zr0.5O2 Films

“…[18] As PZT and HfO 2 show analogous performance instabilities and the presence of oxygen vacancies in HfO 2 has been widely observed, the current study adopts IrO 2 as an electrode material for HfO 2 -based ferroelectric materials. [11,[19][20][21][22] For pure HfO 2 , the amount of oxygen and oxygen vacancies within the film can significantly influence the stabilization of the crystalline phases. [21,23] It has been shown that oxygen-rich Thin film metal-ferroelectric-metal capacitors with an equal mixture of hafnium oxide and zirconium oxide as the ferroelectric material are fabricated using iridium oxide as the electrode material.…”

“…[11,[19][20][21][22] For pure HfO 2 , the amount of oxygen and oxygen vacancies within the film can significantly influence the stabilization of the crystalline phases. [21,23] It has been shown that oxygen-rich Thin film metal-ferroelectric-metal capacitors with an equal mixture of hafnium oxide and zirconium oxide as the ferroelectric material are fabricated using iridium oxide as the electrode material. The influence of the oxygen concentration in the electrodes during crystallization anneal on the ferroelectric properties is characterized by electrical, chemical, and structural methods.…”