Role of oxygen vacancies in ferroelectric or resistive switching hafnium oxide

Abstract

HfO₂ shows promise for emerging ferroelectric and resistive switching (RS) memory devices owing to its excellent electrical properties and compatibility with complementary metal oxide semiconductor technology based on mature fabrication processes such as atomic layer deposition. Oxygen vacancy (V_o), which is the most frequently observed intrinsic defect in HfO₂-based films, determines the physical/electrical properties and device performance. V_o influences the polymorphism and the resulting ferroelectric properties of HfO₂. Moreover, the switching speed and endurance of ferroelectric memories are strongly correlated to the V_o concentration and redistribution. They also strongly influence the device-to-device and cycle-to-cycle variability of integrated circuits based on ferroelectric memories. The concentration, migration, and agglomeration of V_o form the main mechanism behind the RS behavior observed in HfO₂, suggesting that the device performance and reliability in terms of the operating voltage, switching speed, on/off ratio, analog conductance modulation, endurance, and retention are sensitive to V_o. Therefore, the mechanism of V_o formation and its effects on the chemical, physical, and electrical properties in ferroelectric and RS HfO₂ should be understood. This study comprehensively reviews the literature on V_o in HfO₂ from the formation and influencing mechanism to material properties and device performance. This review contributes to the synergetic advances of current knowledge and technology in emerging HfO₂-based semiconductor devices.

Keywords: Ferroelectricity; HfO2; Memory device; Resistive switching; Semiconductor.

Fig. 1

Frequently observed crystallographic phase of HfO₂, presenting the lattice parameters, unit cell volume, and coordination number of Hf and oxygen ions in each phase

Fig. 2

a Time-of-flight secondary ion mass spectroscopy (ToF–SIMS) depth profile and high-angle annular bright-field (HAABF) TEM image of TiN/Hf_0.5Zr_0.5O₂/TiN capacitor using H₂O and O₂ plasma (O₂^*) reactant. b Intensity fraction of Hf 4f oxide (blue), sub-oxide (red) peak depending on the deposition temperature. c V_o formation energy with various dopants. Dopants are classified by chemical group. d, e XPS spectra of 2-nm Hf_0.5Zr_0.5O₂ on Mo and TiN electrodes, respectively. Stoichiometric (HfO₂) and non-stoichiometric (HfO_2–x) peaks are deconvoluted from the Hf 4f spectrum. a reproduced with permission from [39]. b data from [42]. c data from [57]. d, e data from [66]

Fig. 3

Integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) image of rhombohedral Hf_0.5Zr_0.5O₂ film under a 0 V (left), 4 V (right). b Out-of-plane displacement of V_o under a positive voltage. Inset shows the in-plane and out-of-plane displacements. c High-angle annular dark-field scanning transmission electron microscopy image overlaid with O K line profile and EDS mapping of O K edges of oxygen-deficient channel on the HfO₂ film. a, b reproduced with permission from [71]. c reproduced with permission from [75]

Fig. 4

a Bandgap of HfO₂ with defect states depending on the charge state of V_o. b Reconstructed 3D C-AFM images of V_o evolution and migration in HfO_x-based RS device. c Atomic structure of monoclinic HfO₂ with GB at the center (dashed line). The 4-Å-wide shadow region near the GB is a favorable region for V_o aggregation. d Current scan map using C-AFM of polycrystalline HfO₂. e Topography (blue) and current (red) data along the green solid line in d. Schematic of conduction mechanism in HfO₂ f before and g after breakdown. h Schematic and HRTEM of a complete CF resulting from V_o aggregation in an LRS device with a typical polymorphous HfO_x region, namely h-Hf₆O and m-HfO₂ regions a reproduced with permission from [76]. b reproduced with permission from [80]. c reproduced with permission from [83]. d, e reproduced with permission from [184]. h reproduced with permission from [79]

Fig. 5

Total energies of the various crystal phases of HfO₂ with different V_o concentrations of the a bulk and b interface region. c Energies of the orthorhombic Pca2₁ phase of HfO₂ with different V_o states of charge and relaxation conditions relative to the monoclinic phase (ΔE) as a function of V_o concentration. Phase content of 10-nm HZO samples as a function of the d ozone dose time during the ALD process and e oxygen flow during the sputtering. The phase portions are extracted and calculated by the GIXRD pattern. f Polarization–electric field curves of the 10-nm samples vs. ozone dose time during ALD process. a, b reproduced with permission from [23]. c reproduced with permission from [94]. d, e reproduced with permission from [227]. f reproduced with permission from [37]

Fig. 6

a Polarization–electric field and current–electric field curves of a HfO₂ ferroelectric thin film for pristine, wake-up, and fatigue states. Evolution of the Preisach/switching density (ρ) of the HfO₂ thin film after b ~ 1, c ~ 10³, and d ~ 10.⁶ cycles corresponding to the pristine, wake-up, and fatigue states, respectively. e Evolution of the 2P_r, leakage current, defect concentration, and E_bias through repeated electric-field cycling. b-e reproduced with permission from [100]

Fig. 7

a EDS depth profile, b Evolution of P_r and leakage current as functions of number of cycles for TiN/Hf_0.5Zr_0.5O₂/TiN capacitors with and without NH₃ plasma treatment. c Polarization–voltage curves. d Endurance test of Cu/VO_x/Hf_0.5Zr_0.5O₂/TiN capacitor with various sol–gel solutions concentration. e Transient currents during constant voltage stress. f Endurance test of TiN/Hf_0.5Zr_0.5O₂/TiN capacitor annealed with 1- 2-step RTA processes. a, b reproduced with permission from [137]. c, d reproduced with permission from [138]. e, f reproduced with permission from [145]

Fig. 8

a Schematic of the working principle of a VCM device. b Various strategies to control the effect of V_o properties on the metrics of the RS device performance

Fig. 9

Low forming voltage exhibited on low-energy GBs in HfO₂-based RS devices. a Forming process in annealed polycrystalline HfO₂ at specific locations. b Typical bipolar RS behavior observed at the low voltage forming site (GB location). c Current map measurement in a write–read–erase–read cycle, which allows the locations to recover their insulating properties. a, b, c reproduced with permission from [184]. d, e reproduced with permission from [187]

Fig. 10

Reduced forming voltage in oxygen stoichiometry-engineered HfO₂-based RS devices. a Forming process in annealed polycrystalline HfO₂ according to the specific location. Low variation of forming voltage with thickness for oxygen deficient HfO_2–x film. b Model for the thickness dependence of forming voltage in stoichiometric (left) and oxygen deficient HfO_2–x (right). c Switching characteristics tuned in oxygen deficient t-HfO_2–x. Schematic model of filament formation in oxygen engineered HfO_x-based RS devices. a, b reproduced with permission from [228]. c reproduced with permission from [12]

Fig. 11

Doping and interfacial engineering of HfO₂-based RS devices. a Mg doping to modify V_o migration kinetics in HfO₂. The multilevel I–V cycles with varying reset voltages of TiN/Mg:HfO_x/Pt devices. b Improvement in performance of HfO_x-based RS device using doping. c Enhanced switching characteristics for TiN/HfO₂/Ti/HfO₂/Pt/Ti stack RS devices, which use Ti as the interlayer. d Forming voltage distribution for PVD-TiN (left) devices and ALD-TiN (right) devices. e Structural modulation of HfO₂ switching matrix as nanorod structures for exploiting the environment as the V_o reservoir. a reproduced with permission from [195]. b reproduced with permission from [199]. c reproduced with permission from [213]. d reproduced with permission from [215]. e reproduced with permission from [217]

Fig. 12

Comprehensive schematic of the ferroelectric and RS HfO₂ memory device and the effect of V_o for their performances. a Ferroelectric switching mechanism based on the movement of oxygen atoms (up) and V_o-mediated RS mechanism (down). b Effects of V_o and their roles in ferroelectric and resistive memory performance. c Key parameters affecting the concentration and motion of V_o in the film