Stabilizing Antiferroelectric‐Like Aluminum‐Doped Hafnium Oxide for Energy Storage Capacitors

Herein, a systematic study of aluminum‐doped hafnium oxide to utilize its antiferroelectric‐like (AFE) properties for energy storage applications is done. The doping concentration of aluminum is optimized to obtain the AFE‐like phase. In addition, the impact of the postmetallization annealing temperature on the energy storage properties of the materials is studied. Metal–insulator–metal capacitors are fabricated by varying the doping concentration of the Al in HfO2 from 1.9 to 6.2 at% with a constant thickness of 10 nm by atomic layer deposition. The devices are rapid thermal annealed by varying the annealing temperature from 650 to 800 °C for 20 s. Polarization measurements indicate a clear phase transformation from ferroelectric (FE) to AFE to paraelectric phase with the increase of doping concentration in the polarization measurements. The planar antiferroelectric devices have an energy storage density of 30 J cm−3 with 76% efficiency after 105 cycles. The storage density can be further increased by a factor of 16.5 using area‐enhanced substrates to 500 J cm− 3 at 73% efficiency. The endurance characteristics are studied for both planar and 3D capacitors which are found to be stable up to 108 cycles.


Introduction
Hafnium oxide-based ferroelectric thin films have ignited a spate of new research in the field of ferroelectrics due to their nontoxic properties and scalable complementary metal-oxide semiconductor (CMOS) manufacturing compatibility. [1,2]Various deposition techniques can be employed to create these films, with atomic layer deposition (ALD) emerging as the preferred method in device manufacturing.5] Ferroelectric thin films have been fabricated by the introduction of certain dopants such as Si, Al, Gd, Y, La, Sr, and Zr in hafnium oxide and hafniumzirconium oxides. [6]The material can be optimized to realize ferroelectric and antiferroelectric properties by adjusting various parameters such as dopant concentration, mechanical strain, and annealing temperature. [7]In comparison to titanate-based perovskite ferroelectrics, hafnium oxidebased ferroelectrics can be scaled to a few nanometers and be fabricated into novel devices like ferroelectric tunnel junctions (FTJ), ferroelectric random access memory (FeRAM), ferroelectric field effect transistors (FeFET), [3] and 3D trench capacitors [8,9] Although there are several systematic and in-depth studies in the fabrication and characterization of hafnium-zirconium oxide (HZO) and silicon-doped hafnium oxide (HSO) for ferroelectric and antiferroelectric properties, there have been no systematic studies on the fabrication and characterization of aluminumdoped hafnium oxide for the antiferroelectric properties that can be used as energy storage capacitors.This article attempts to fill that gap and aims to show a systematic study of aluminum doping of hafnium oxide (HAO), the effect of annealing temperature over the crystallinity and phase of the oxide, the thickness dependency for the polarization characteristics, the energy storage capabilities, and the endurance characteristics of AFE thin films.While hafnium oxide-based films do not exhibit the antiferroelectric effects observed in perovskites, as discussed in the study of Kittel, [10] they do possess antiferroelectric-like properties that can be effectively utilized in diverse applications.In this article, we employ the terms "antiferroelectric-like" and "antiferroelectric" interchangeably to refer to the antiferroelectric behavior.
Antiferroelectric HZO films for energy storage was first reported by Park et al. in 2014, [11] which showed a stored energy density of 45 J cm À3 and an efficiency of 51%.Later, Ali et al. showed antiferroelectric silicon-doped hafnium oxide with energy storage of 61.2 J cm À3 with 65% efficiency. [12]Kuehnel et al. worked on optimizing the antiferroelectric HSO onto 3D trench capacitors and achieved energy storage density of 450 μJ cm À2 with 67% efficiency. [9]This article shows the optimization of Al-doped HfO 2 in planar thin films and the optimized thin films engineered onto 3D capacitors maintaining the necessary stoichiometry for the AFE phase while enhancing the storage capacity and preserving the efficiency of the capacitors.

Material Characterization
Grazing angle X-ray diffraction patterns for increasing aluminum concentration in HfO 2 in the planar configuration are shown in Figure 1a.The samples were annealed at different temperatures at a fixed time of 20 s in N 2 and diffraction patterns for the samples annealed at 700 °C are shown.The undoped HfO 2 (annealed at 650 °C) has peaks corresponding to monoclinic phase at 17.3, 28.5, and 36.5 degrees, indicated with the miller indices in the plot.It can be seen that as the aluminum concentration is introduced and increased, the peak at the 30.5°indexed as o (111)/t(11) appears and grows stronger.This suggests that the film has the orthorhombic or tetragonal phase which cannot be distinguished in these diffraction patterns.The orthorhombic phase (Pca2 1 ) has been attributed to the ferroelectricity in hafnium oxide. [13]The tetragonal phase is attributed to the antiferroelectric-like phase. [14,15]While the peak at 28.5°is attributed to monoclinic phase, m(11), we also have the presence of polysilicon which has peaks at 28.3, 46.5, 55.2°referenced by the green dotted lines in the plot. [16]The polysilicon acts as an electrically conducting top electrode.It is possible that there is a small contribution of the monoclinic phase of HfO 2 present, which could lead to nonoptimal ferroelectric properties.The crystalline top TiN peak is also visible at 36.7 (111) and 42.6 (200) degrees referenced by the blue dotted lines.The shoulder peak at %26°could be the TiO 2 rutile 110 plane due to partial oxidation at the electrodes. [17]The highest concentration of Al does not have any characteristic crystalline peak, and is amorphous.Figure 1b shows the diffraction patterns of the 4.2% Al-doped samples with increasing thickness from 11 to 24 nm and annealed at 700 °C.The diffraction peak intensity at 30.5°, commonly attributed to the orthorhombic/tetragonal phase, increases with increasing thickness.The increasing intensity of the 30.5°peak, can be attributed to thicker films, and also a change in the orthorhombic component from amorphous, monoclinic, or tetragonal portions of the layer.Increasing the film thickness lowers the crystallization temperature and hence affects the in situ crystallization process during the deposition, which later acts as nucleation points for the crystallization during the high-temperature rapid thermal annealing process.The crystallization starts with nuclei seeds of the tetragonal phase, which is the most favorable phase given the temperature, strain, doping, and lowered ratio of bulk to interface free energy conditions.As the grain grows, the impact of the interface energy is reduced making it more favorable for the o-phase to be stabilized. [18]ence, careful tuning of the ALD parameters is essential while scaling up the thickness of the Al-doped hafnium oxide to meet the crystallinity conditions in achieving an antiferroelectric film.

Electrical Characterization
The polarization-electric field (P-E) measurements were performed on all sample variations as mentioned in the experimental section.Figure 2a shows the polarization hysteresis loops measured at 3 MV cm À1 for aluminum doping concentration varying from 1.9% to 6.2% at the optimum annealing temperature of 700 °C.The results displayed are after cycling the capacitor for 10 5 cycles, so that the ferroelectric devices are "woken up".A clear transition from ferroelectric to antiferroelectric to paraelectric phase is visible in the P-E loops as the Al concentration is increased.While the 1.9% Al-doped sample is ferroelectric, the 2.9% is a partially ferroelectric/antiferroelectric transitional material.The 4.2% Al doping has an antiferroelectric-like phase with the pinched hysteresis.At higher aluminum concentrations of 5.1 and 6.2%, the films are predominantly paraelectric.This trend has been seen in earlier articles available in the literature, however, the exact concentration of aluminum seems to vary.For example, Mueller et al. showed AFE behavior at 8.5 mol% of aluminum, however, the film thickness was higher at 16 nm and the annealing temperature was at 1,000 °C. [19]Payne demonstrated AFE-like hysteresis loops for both 4% Al and 8% Al doping in hafnium oxide in different thicknesses. [20]igure 2b shows the I-E plots of 4.2% Al at the different annealing temperatures, measured after 10 5 cycles.At lower annealing temperatures of 600 and 650 °C, the films behave like a dielectric and are not polarized evidenced by the ideal rectangle shape.The XRD diffraction spectrum (not shown here) at those annealing temperatures only shows a monoclinic phase and no sign of the tetragonal or orthorhombic phase.At 700 °C and above, the four switching peaks that are characteristic of AFE materials appear.The peaks in quadrant 1 and 3 represents the critical field-induced transition from tetragonal to orthorhombic and the peaks in quadrant 2 and 4 represent the critical fieldinduced transition from orthorhombic to tetragonal. [21]The broad peaks indicate that the films are polycrystalline with different coercive fields spread out over the large device area.The increasing leakage current with increasing annealing temperature is also visible with the tilts in the current at higher electric fields.
The measured remnant polarization P r was plotted across aluminum doping concentrations and the annealing temperature of the capacitors.In Figure 3a, it can be seen that as the aluminum concentration increases, the P r reduces, as the films transform from ferroelectric phase to antiferroelectric-like phase to paraelectric phase.This transformation is a function of annealing temperature, as it can be seen that at annealing temperature below 700 °C, the films are amorphous.At lower aluminum doping levels of 1.9% Al and annealing temperature of 650 °C, there is partial crystallization of the films such that we see an increase in P r .As the annealing temperature further increases, the leakage current increases such that we see the paraelectric films at 5.1% and 6.2% have a nonzero P r which indicates a leaky dielectric film.
The impact of the crystallization of the films can also be evidenced through the relative permittivity (ε r ), also known as the dielectric constant (κ).It is known that the amorphous hafnium oxide films have a dielectric constant of about 20 and the crystalline phases such as tetragonal phases have a higher dielectric constant that is greater than 40.This transition from amorphous to crystallization in the tetragonal phase is visible in Figure 3b, which plots the ε r of the 4.2% Al for the different annealing temperatures.There is a sharp increase in the κ value from 650 to 700 °C, which is supported by the XRD diffraction patterns and the hysteresis loops.Further increase in annealing temperature does not change the κ value anymore, as it is stabilized in the tetragonal phase. [22,23]The film thickness was varied for 4.2% Al which showed the AFE-like phase, by increasing the number of cycles during the deposition process.Figure 4a shows the hysteresis loops for the different thicknesses targeting 10, 15, and 20 nm, increasing polarization saturation with increasing thickness as expected.The corresponding I-E curves are shown in Figure 4b.Postdeposition thickness was measured by ellipsometer and verified by SEM that the actual thicknesses were 11.0, 15.5, and 24.3 nm for the target thickness of 10, 15, and 20 nm, respectively.The 24.3 nm sample has an additional switching peak, possibly that it is not fully saturated or "woken up" indicating a higher ferroelectric component.This is visible in the XRD spectrum, as seen in Figure 1b, which has a higher intensity peak at 30.5°indicating a higher amount in the orthorhombic phase.Further optimization is necessary to target the tetragonal/AFE phase for an increasing film thickness, which could lead to higher energy storage.It was shown earlier that introducing an Al 2 O 3 layer mid-thickness for Si:HfO 2 , helped in maintaining the correct phase for FE and AFE.The Al 2 O 3 layer helps in stopping early nucleation of crystallization as ALD tends to be a long process at elevated temperatures. [9,24]

Endurance Characterization
The AFE-like samples corresponding to 4.2% aluminum doping concentration were stressed with bipolar field cycling of 3 MV cm À1 at 10 kHz for 10 8 cycles.The samples annealed at 700 °C were chosen as optimal samples.The P-E hysteresis loops from pristine state to 10 8 cycles are shown in Figure 5a.At the pristine state, the hysteresis loop is pinched as expected, and there is an opening of the hysteresis loop as the number of cycles increases.In an ideal AFE sample, the P r would be zero and would be constant over multiple cycles which is often seen in perovskite materials. [25]However, in fluorite-based antiferroelectric materials, a nonzero P r is observed.This is attributed to domain pinning and depinning, defects at the electrode interface, and the existence of some orthorhombic phase responsible for the ferroelectric domains. [26]Since our samples are not phase pure and are polycrystalline in nature and without a clear distinction in orthorhombic/tetragonal phases, this could be significant for the increase in P r through increasing cycling.The change of the P r with respect to the number of switching cycles under increasing bipolar electric field stress is shown in Figure 5b.
It can be seen that there is an increase in P r values with an increase in electric field magnitude.The higher electric field results in a faster breakdown within fewer switching cycles.
The increase in P r with increase in field magnitude is attributed to higher pinned domains and distribution of charge carriers.The rapid increase in the P r beyond a certain switching cycle indicates that the device is entering into a fatigued state.

Energy Storage
Antiferroelectric materials are known to be suited for energy storage applications.AFE capacitors have low polarization losses compared to FE samples and a larger maximum polarization (P max ) as compared to dielectric capacitors.Ali et al., showed the energy storage properties of a dielectric Al 2 O 3 capacitor in comparison to an AFE HSO capacitor. [12]It can be seen that there is 6 times increase in stored energy from dielectric Al 2 O 3 to the polarized HSO at similar electrical operation conditions.The total energy (W total ) stored in an AFE capacitor is given by The recoverable storage energy which is the energy that can be used after subtracting the losses, is given by Equation (2) upon discharge.
Another important metric to characterize the capacitors is the efficiency of the storage density.The efficiency (η) of storage in an AFE capacitor is calculated as In Figure 6a, we can see the dependency of the recoverable energy storage density W rec and efficiency over 10 8 cycles at different annealing temperatures.As the annealing temperatures increase, the recoverable energy density increases, due to more domains being activated or polarized.However, as we see inefficiency plots in Figure 6b, there is a reverse trend wherein as the annealing temperature increases, the efficiency decreases.This is caused by higher leakage current at higher annealing temperature which leads to higher losses thereby decreasing efficiency and causing an early breakdown.
The 4.2% Al-doped HfO 2 annealed at 700 °C has a high storage density of 24 J cm À3 and 76% after 10 8 cycles, when measured at 2.8 MV cm À1 .The higher annealing temperatures of 750 and 800 °C have higher storage density, but lower efficiency has a faster breakdown.
The electric field also plays an important role in the amount of energy stored in the capacitor.As seen in Figure 7a, with increasing electric field, the energy storage density also increases, however, the efficiency decreases.An optimal condition is highlighted at 3 MV cm À1 at the maximum stored energy of 35 J cm À2 at a high efficiency of 76% in the pristine condition, without field cycling.Further increase in the field magnitude results in higher energy storage as well as higher energy losses, which is reflected in the lower efficiency.The internal area of the P-E hysteresis loop is considered to be the energy loss.The losses are more clearly visible in Figure 7b of the P-E hysteresis loops at electric fields greater than 3 MV cm À1 .It can also be seen that by increasing the electric field, the polarization increases significantly as more domains are activated or contribute to the total polarization.The measurements here refer to a single dynamic hysteresis measurement at 1 kHz and without any pre-cycling.

Effect of Temperature on the Polarization of Planar AFE Capacitors
The planar AFE capacitors were tested at different temperature levels from 25 to 150 °C in 25 °C steps at 2.8 MV cm À1 .Figure 8 shows the P-E hysteresis curves for the 10 nm 4.2% Al-doped HfO 2 measured after 10 5 cycles.There is an increase in the P r from 1.7 μC cm À2 at 25 °C to 3.3 μC cm À2 at 150 °C.This change results in an 11% variation in energy stored from 25 to 150 °C and an 18% reduction in the efficiency of the capacitors.Compared to Si-doped HfO 2 , where the P r reduces as the temperature is increased (P-E hysteresis becomes more pinched), the opposite is seen in Al-doped HfO 2 .With increasing temperature, there is further unpinching of the hysteresis curve, which leads to higher leakage and reduction in the energy storage and efficiency of the storage capacitor.

Scaling to 3D Capacitors
The Al-doped HfO 2 thin films were deposited into trench capacitors while maintaining the stoichiometry of the films.
The deposition process was carefully optimized such that the depths of trenches were uniformly coated with the oxide layer.Figure 9 depicts the trench images from scanning electron microscopy (SEM) for the entire trench structure and a magnified image of the bottom of the trench using transmission electron microscopy (TEM).
The bottom of the trench shows the conformal ALD coating of TiN/HAO/TiN of 10 nm each.The target thickness was maintained at the top of the trench, the bottom had reduced thickness to 8 nm for the aluminum-doped hafnium oxide.The key challenge is maintaining the stoichiometry through the trench depths.As an indication of the success of this approach, this can be seen Figure 10a, where the PE loops for the different aluminum doping concentration shows a very similar trend to the planar capacitors.As aluminum concentration increases the polarization loops transform from ferroelectric to antiferroelectric to paraelectric similar to the planar capacitor results.The thickness was varied for the 4.2% aluminum doping which was considered as the antiferroelectric-like phase and the P-E hysteresis loops are shown in Figure 10b.While the lower thicknesses targeted at 10 nm and 15 nm are still AFE, the higher thickness of 24 nm is paraelectric.This could be caused due to higher doping concentration than required or the 700 °C was insufficient for optimal crystallization.The material in the trenches at different thicknesses would have to be studied in more detail to understand the role of crystallization and its effects on the antiferroelectric properties.The energy storage properties of these samples were tested similarly to the planar capacitors and the results are shown in Figures 11 and 12.
Similar to the planar capacitor results in Figure 7, the effect of the electric field on 10 nm AFE capacitors was measured and plotted in Figure 11.There is a linear increase in the recoverable energy up to 4 MV cm À1 , beyond which the recoverable energy stored drops due to the high leakage and losses.A steady decrease in efficiency can be seen in Figure 11a.At an operational field of 3 MV cm À1 , an efficiency value of 80% is achieved.However, at higher field values, more energy can be stored, but at lower efficiencies.Hence, it is possible to tune the operational parameters based on the application requirements.At lower electric fields, higher efficiencies, and relative to planar capacitors, larger energy storage is possible.Figure 11b plots the P-E hysteresis loops, where the increase in the losses at higher fields is clearly visible.
When measured across the whole wafer, the 10 nm sample had a median energy storage of 387 J cm À3 and the representative endurance measurement had 500 J cm À3 stored energy measured at 3 MV cm À1 .For the 15 nm sample, the median energy stored 368 J cm À3 was measured at a slightly lower electric field of 2.8 MV cm À1 .However, there was a large variation across the wafer, and the energy storage of 700 J cm À3 which was measured at 3 MV cm À1 is shown in Figure 12c.
The energy storage density of 3D capacitors is 16.5 times higher compared to planar capacitors with an equivalent projected planar surface.This substantial increase is attributed to the augmented surface area achieved through the inclusion of trenches.The efficiency through the long cycling of 10 8 cycles is visible in Figure 12c and has been maintained in the 3D capacitors is noteworthy.Figure 13 shows the temperature-dependent P-E hysteresis loops for 3D AFE capacitors.The capacitors were tested from 25 to 150 °C in 25 °C increments.All capacitors were cycled upto 10 5 cycles at 3 MV cm À1 .Increasing temperature increases the leakage current, which can be seen by the unpinched hysteresis loops and the sharp increase in the current in the I-E characteristics at 3M cm À1 .The P r increases from 20 μC cm À2 to 95 μC cm À2 .The increase in leakage current could be due to the decreasing thickness at the bottom of the trench, which could cause higher fields and early breakdowns.

Benchmark
The capacitors in this work have been benchmarked with similar hafnium oxide-based antiferroelectric capacitors published in literature in Table 1.The HAO capacitors have shown high efficiency (73%-76%) for similar thicknesses of doped hafnium oxide in both planar and in 3 D capacitors.

Conclusion
An extensive study of aluminum-doped hafnium oxide has been presented in this article.The effect of doping concentration and annealing temperature on the material properties has been studied.Careful tuning of the doping concentration and annealing conditions is needed to achieve an antiferroelectric material that can be used for energy storage devices.In planar capacitors, energy density of 30 J cm À3 with an efficiency of 76% after 10 5 cycles, measured at electric field of 3 MV cm À1 , has been achieved.These optimized materials have been further scaled with increasing thickness and deposited on area-enhanced substrates of 3D capacitors.This scaling resulted in 16.5 times the planar storage density of 500 J cm À3 for 10 nm oxide thickness.This is the highest reported storage density and efficiency for aluminum-doped hafnium oxide in area-enhanced substrates.The improved storage density with a good efficiency rate, which endures long cycling times up to 10 8 , makes aluminumdoped hafnium oxide a valuable choice for a CMOS-compatible antiferroelectric energy storage capacitors.

Experimental Section
Sample Preparation: Metal-insulator-metal capacitors were fabricated on highly doped 300 mm Si (100) substrates.The bottom titanium nitride (TiN) layer of 10 nm was deposited using atomic layer deposition (ALD).Tetrakis(ethylmethylamido)hafnium (TEMAHf ) and trimethylaluminum (TMA) were used as precursors for hafnium and aluminum oxides, respectively.Ozone was used as the oxidizing coreactant and argon as the purge gas.The number of cycles of the hafnium and aluminum precursors were varied in a cycle-to-cycle manner, to control the concentration of aluminum in hafnium oxide in the layer.The deposition temperature for planar samples was maintained at 283 °C.The top TiN layer of 10 nm was deposited by chemical vapor deposition (CVD) and a subsequent layer of doped polycrystalline silicon was deposited at 520 °C which acts as electrical contact electrode.The capacitors were then annealed with rapid thermal processing (RTP) varying from 650 to 800 °C for a constant duration of 20 s in nitrogen ambience.Additional samples of target thickness 15 and 20 nm were prepared by increasing the number of ALD cycles.Planar devices were tested using capacitors of surface area of 15 000 μm 2 .For the 3D capacitors, a similar fabrication process is followed.The trenches of 8.5 μm were etched in silicon.The TiN, hafnium oxide, and TiN depositions are similar to the planar capacitors with increased purge duration to fill the trench depth and an increased deposition temperature for HfO 2 at 323 °C.The polysilicon is filled inside the trenches and act as electrical contacts.The annealing is optimized for 700 °C.The tested samples had a lateral surface area of 15,000 μm 2 .
Characterization: The thickness of the thin films was measured using an ellipsometer and confirmed by scanning electron microscopy.The thin films are characterized by X-ray photoelectroscopy (XPS) after the oxide deposition in ReVera Veraflex with analyzer pass energy of 141.2 eV, corresponding to Al Kα.The concentration of aluminum was calculated from the XPS results.The Al concentration has been calculated based on the area under Al peak, as a percentage of the entire area of Hf and O peaks.The grazing angle XRD was performed on Bruker with 0.5°grazing angle.Polarization measurements were performed using Aixxact TF3000, at 1 kHz triangular waves and 10 kHz square wave fatigue frequencies.The electric field applied was in the range of 2.8-3 MV cm À1 .The samples were probed by applying a voltage to the silicon substrate and measured at the top contact.All results are shown for samples after 10 5 cycles unless specified otherwise.For the endurance characteristics, 10 8 cycles were used, with similar frequencies as mentioned above.

Figure 1 .
Figure 1.GIXRD data for different doping concentration of Al-doped HfO 2 a) annealed at 700 °C with undoped HfO 2 annealed at 650 °C and b) for different thicknesses of 4.2%Al (AFE) samples also annealed at 700 °C.Dotted green lines represent polysilicon and dotted blue lines represent TiN peaks.

Figure 2 .Figure 3 .
Figure2.a) Polarization-electric field (P-E) hysteresis loops of capacitors at optimal annealing temperature of 700 °C.b) Switching current-electric-field characteristics of 4.2% Al at different annealing temperature conditions.Beyond 700 °C, the curves tilt at higher fields indicating the higher leakage current.

Figure 4 .Figure 5 .
Figure 4. a) P-E and b) I-E curves for increasing thickness of 4.2% Al-doped HfO 2 corresponding to AFE.

Figure 6 .Figure 7 .
Figure 6.a) Energy storage density (W rec ) and b) efficiency for 4.2% Al at different annealing temperature measured at 2.8 MV cm.

Figure 12 .Figure 13 .
Figure 12.Endurance characteristics for 4.2% Al-doped HfO 2 (AFE) for 3D capacitors, cycled for 10 8 cycles.a) P-E hysteresis loops for 10 nm thick films.b) I-E characteristics.c) Remnant polarization for 10 nm and 15 nm thick films.d) Energy storage density and efficiency for 10 and 15 nm at applied electric field of 3 MV cm À1 .