Undoped HfO2 Films Research Articles

Ferroelectric and electric field cycling properties of un-doped HfO2 films

As the most promising candidate material for nonvolatile memories, premature occurrence of fatigue phenomenon for ferroelectric HfO2 film has no appropriate solution method. In this paper, HfO2/Y/HfO2 films were deposited with reactive magnetron sputtering and realized ferroelectric property after wake-up process even if without doping and post-metallization annealing (PMA). Different with uniform or symmetrical distribution, oxygen vacancies concentrated in the film middle, resulting in a longer migration path and tardy fatigue. Fatigue appeared after the electric field cycling of 109 in HfO2 film inserted with 0.6 nm metal Y, which is obviously higher than films deposited by other methods. The remanent polarization of un-doped HfO2 films inserted with 0.6 and 1.5 nm metal Y interlayer is 11.8 and 14.8 μC/cm2, respectively. The results provide a method for to improve cycling property and confirm the important role of oxygen vacancies in wake-up and fatigue phenomenon for HfO2 films.

Impact of high-power impulse magnetron sputtering pulse width on the nucleation, crystallization, microstructure, and ferroelectric properties of hafnium oxide thin films

The impact of the high-power impulse magnetron sputtering (HiPIMS) pulse width on the crystallization, microstructure, and ferroelectric properties of undoped HfO2 films is investigated. HfO2 films were sputtered from a hafnium metal target in an Ar/O2 atmosphere, varying the instantaneous power density by changing the HiPIMS pulse width with fixed time-averaged power and pulse frequency. The pulse width is shown to affect the ion-to-neutral ratio in the depositing species with the shortest pulse durations leading to the highest ion fraction. In situ x-ray diffraction measurements during crystallization demonstrate that the HiPIMS pulse width impacts nucleation and phase formation, with an intermediate pulse width of 110 μs stabilizing the ferroelectric phase over the widest temperature range. Although the pulse width impacts the grain size with the lowest pulse width resulting in the largest grain size, the grain size does not strongly correlate with the phase content or ferroelectric behavior in these films. These results suggest that precise control over the energetics of the depositing species may be beneficial for forming the ferroelectric phase in this material.

Structure and Electrical Behavior of Hafnium-Praseodymium Oxide Thin Films Grown by Atomic Layer Deposition.

Crystal structure and electrical properties of hafnium-praseodymium oxide thin films grown by atomic layer deposition on ruthenium substrate electrodes were characterized and compared with those of undoped HfO2 films. The HfO2 reference films crystallized in the stable monoclinic phase of HfO2. Mixing HfO2 and PrOx resulted in the growth of nanocrystalline metastable tetragonal HfO2. The highest relative permittivities reaching 37–40 were measured for the films with tetragonal structures that were grown using HfO2:PrOx cycle ratio of 5:1 and possessed Pr/(Pr + Hf) atomic ratios of 0.09–0.10. All the HfO2:PrOx films exhibited resistive switching behavior. Lower commutation voltages and current values, promising in terms of reduced power consumption, were achieved for the films grown with HfO2:PrOx cycle ratios of 3:1 and 2:1 and showing Pr/(Pr + Hf) atomic ratios of 0.16–0.23. Differently from the undoped HfO2 films, the Pr-doped films showed low variability of resistance state currents and stable endurance behavior, extending over 104 switching cycles.

Influence of orientation on the electro-optic effect in epitaxial Y-doped HfO2 ferroelectric thin films

Recently, we reported linear electro-optic (EO) effects in (100)-epitaxial yttrium-doped hafnium dioxide (Y-HfO2) ferroelectric thin films. In this study, we have investigated the influence of orientation on the EO effect in Y-HfO2 thin-film. (111)-epitaxial undoped HfO2 and Y-HfO2 films were deposited on Sn-doped In2O3/yttria-stabilized zirconia (111) substrates at room temperature through radiofrequency magnetron sputtering. Although the undoped HfO2 film showed typical paraelectric characteristics, ferroelectricity was observed in the (111)-Y-HfO2 film. Remnant polarization in the (111)-Y-HfO2 film was higher than that in the (100)-Y-HfO2 film. The (111)-Y-HfO2 film exhibited a linear EO effect based on ferroelectricity, which is consistent with that of the (100)-Y-HfO2 film. The average EO coefficient r c of the (111)-Y-HfO2 film was 0.67 pm V−1, which is higher than that of the (100)-Y-HfO2 film. This result is reasonable considering the difference in remnant polarization between the (100)-Y-HfO2 and (111)-Y-HfO2 films.

Origin of ferroelectric phase in undoped HfO2 films deposited by sputtering

Origin of ferroelectric phase in undoped HfO2 films deposited by sputtering

Fabrication of ferroelectric Fe doped HfO2 epitaxial thin films by ion-beam sputtering method and their characterization

In this study, the effect of Fe doping on the crystal structure and electrical properties in 20-nm-thick HfO2 epitaxial thin films were systematically investigated. X-ray diffraction measurements revealed that undoped HfO2 films were composed of a paraelectric monoclinic phase. On the other hand, the formation of non-centrosymmetric orthorhombic phase was observed in Fe doped HfO2 films and was most promoted at an optimum doping concentration. In addition, high-temperature X-ray diffraction measurements showed that an orthorhombic phase to a highly symmetric phase transition occurs between 500 and 600 °C. Microstructural analysis using scanning transmission electron microscopy revealed multidomain structure consisting of orthorhombic and monoclinic phases. Ferroelectricity depended on Fe doping concentration, and the maximum remanent polarization value was 8.8 µC/cm2. These results indicate that Fe doped HfO2 thin films can be applied as nanoscale ferroelectrics.

The improved resistive switching of HfO2:Cu film with multilevel storage

The Cu-doped and undoped HfO2 films were fabricated by magnetron sputtering. The effect of Cu doping on resistive switching (RS) of HfO2 film was demonstrated. Improvements in ON/OFF ratio and switching parameters were observed. The multilevel behavior of Cu/HfO2:Cu/Si sample was also investigated by controlling the current compliance during set process and at least three low resistance states (LRSs) for data storage could be obtained. The three LRSs and the high resistance state can be distinguished in a range of 103 and the HfO2:Cu sample exhibited good reliability. The RS behavior of HfO2:Cu sample can be well interpreted by the proposed filamentary model. The oxygen vacancies and Cu ions in the film are both responsible for the RS behaviors.

The effect of Cu doping concentration on resistive switching of HfO2 film

The Cu-doped and undoped HfO2 films were fabricated and the effect of Cu doping concentration on resistive switching (RS) of HfO2 film was demonstrated. The X-ray photoelectron spectroscopy (XPS) was carried out to investigate the chemical bonding states of Cu in HfO2:Cu film. The improved RS behaviors in terms of ON/OFF ratio and switching parameters were observed for Cu-doped HfO2 film with bipolar resistive switching (BRS) behavior. With the increase of Cu doping concentration, the 9.7% Cu-doped HfO2 film showed both BRS and unipolar resistive switching (URS) behaviors with large operating voltages. The space charge limited current (SCLC) effect was proposed to interpret the switching mechanism of HfO2:Cu films with BRS behavior and the URS behavior can be explained by the migration of Cu ions.

High-magnetic field annealing effect on room-temperature ferromagnetism enhancement of un-doped HfO2 thin films

Monoclinic HfO2 thin films with intrinsic defects were prepared by annealing in flowing argon. The behavior of the HfO2 films could be manipulated by applying an external magnetic field during annealing processing. Room-temperature ferromagnetism and visible photoluminescence were observed in these un-doped HfO2 films due to the involvement of oxygen vacancies. The results demonstrated that the density of oxygen vacancies in HfO2 films could be controlled by varying the intensity of the magnetic field. This study could facilitate understanding of ferromagnetism origin in un-doped oxides, also suggests an effective way to alter the intrinsic defects in HfO2 to improve its performance.

Atomic layer deposition of Ti-HfO2 dielectrics

Titanium-doped hafnium oxide films, TixHf1−xO2−δ, have been deposited with a Ti content of x = 0.1 and x = 0.5, by atomic layer deposition. The TixHf1−xO2−δ growth rate is lower compared with the growth rates of the individual binary oxides; however, the composition of the films is unaffected by the reduced growth rate. An 850 °C spike anneal and a 500 °C 30 min furnace anneal were performed, and the resulting film composition and structure was determined using medium energy ion scattering, x-ray diffraction, and transmission electron microscopy. The Ti0.1Hf0.9O2−δ films readily crystallize into a monoclinic phase during both types of annealing. By contrast, the Ti0.5Hf0.5O2−δ films remain amorphous during both annealing processes. Electrical characterization of the as-deposited Ti0.1Hf0.9O2−δ films yielded a dielectric constant of 20, which is slightly higher than undoped HfO2 films. The as-deposited Ti0.5Hf0.5O2−δ films showed a significant increase in dielectric constant up to 35. After a 500 °C 30 min anneal, the dielectric constant reduced slightly to 27. The leakage current density of the amorphous film remains relatively unaffected at 8.7×10−7 A/cm2 at −1 MV/cm, suggesting this composition/heat treatment is a candidate for future device dielectrics.

Evidence for magnetism due to oxygen vacancies in Fe-doped HfO2 thin films

Fe-doped HfO2 thin films are room temperature ferromagnetic. In comparison with results of the undoped HfO2 films, it seems that the Fe doping is not the main cause for the ferromagnetism but only acts as a catalyst. Experimental results of oxygen annealing and vacuum heat treatments have proven that in this family of compounds, magnetism might originate from oxygen vacancies or defects. Removing oxygen enhances the magnetic moment, while reversibly filling up oxygen vacancies can destroy the ferromagnetic ordering of the system.

Search for ferromagnetism in undoped and cobalt-doped HfO2−δ

We report on the search for ferromagnetism in undoped and cobalt-doped high-k dielectric HfO2 films. Over a broad range of growth conditions, we do not observe ferromagnetism in undoped HfO2 films. On the other hand, we do observe room temperature ferromagnetism in dilutely Co-doped HfO2 films, but the origin of the same appears extrinsic (a Co rich surface layer) at least for the regime of growth conditions explored.