Doped Hf_0.5Zr_0.5O₂ for high efficiency integrated supercapacitors

First reported in 2011,¹ ferroelectricity in HfO₂-based thin films has gained considerable interest for memory applications as its ease of manufacturability^2,3 and wide-range of fabrication conditions^4–13 have become apparent. While many reports have viewed ferroelectric HfO₂ through the lens of incorporating the films into various memory technologies, such as ferroelectric random access memory (FRAM) or ferroelectric field effect transistors (FeFETs),^14–18 its use for energy storage applications remains an intriguing alternative application.¹⁹ By shifting the ferroelectric film properties into an antiferroelectric-like mode of behavior through a suitable combination of doping and annealing conditions, HfO₂-based supercapacitors with high efficiencies and energy storage densities have been demonstrated.^19–21 The origins of ferroelectricity in HfO₂ have been shown to come from a polar orthorhombic Pca2₁ phase,²² whereas the antiferroelectric behavior is postulated to be due to a field-induced, reversible phase transition between the tetragonal (t) phase and the orthorhombic (o) phase.²³ While many different dopants can be used to stabilize ferroelectric and antiferroelectric behavior in HfO₂, the Hf_xZr_x-1O₂ (HZO) compositional system is particularly relevant for applications due to its low anneal temperature,²⁴ robustness to H-annealing,²⁵ and fully miscible compositional range.²⁶ 

The antiferroelectric properties of Hf_0.3Zr_0.7O₂ showed a promising energy storage density (ESD) of 30 J/cm³ with a 50% efficiency over a wide temperature range.¹⁹ The greater amount of Zr in the Hf_xZr_1-xO₂ composition leads to a more tetragonal-rich polycrystalline film,²⁶ which is likely responsible for facilitating a t → o phase transition. Since it has been found that Si, and Al to a possibly lesser extent, can encourage the formation of the tetragonal phase,^1,4 doping Hf_xZr_1-xO₂ with Al or Si could be a viable route towards improving the antiferroelectric properties of the thin films. It was shown that antiferroelectric Si-doped HfO₂ thin films exhibited up to a 40 J/cm³ energy storage density with an efficiency of 80%.²¹ The potential for using antiferroelectric Si-doped HfO₂ thin films for supercapacitors or infrared sensors was demonstrated, although the large 1000 °C anneal temperature may prove prohibitive for integrated applications.²¹ As will be shown in this work, doping Hf_xZr_1-xO₂ with Al or Si can enhance the energy storage density and efficiency without increasing the thermal budget of HZO.

Atomic layer deposition (ALD) was used to deposit a TiN/doped HZO/TiN capacitor stack at 200 °C on a highly doped p+ (0.001–0.005 Ω cm) Si wafer (100). The ALD precursors for Ti, Zr, Hf, Si, and Al were tetrakis(dimethylamido)titanium, tetrakis(dimethylamino)zirconium, tetrakis(dimethylamido)hafnium, tris(dimethylamino)silane, and trimethylaluminum. Thin film growth during ALD occurs as a sequence of precursor pulses which deposit an atomic layer of material per pulse. Doping is achieved by a ratio of the evenly split Hf/Zr pulses to the dopant precursor. The ALD cycle ratio of Hf_0.5Zr_0.5O₂:Si was 48:1 and 24:1 which led to the incorporation of 2 and 4 layers of Si, respectively. Thus, for 24:1 ratio films, the sequence of ALD was performed with alternating pulses of Hf and Zr to a total of 24 combined pulses, then a dopant pulse would occur. The sequence was repeated to achieve a film thickness of 10 nm. Al-doped HZO films were deposited with a 24:1 ALD ratio. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to measure the trace doping of Si and Al in the Hf_0.5Zr_0.5O₂ thin films. Films were annealed at temperatures between 500 °C and 900 °C. Hysteresis loops were extracted at 1 kHz from a Sawyer-Tower circuit with a 220 nF linear capacitor in series with the ferroelectric capacitor. Dynamic hysteresis currents were measured by replacing the 220 nF capacitor with a 50 Ω resistor in the Sawyer-Tower circuit. Endurance cycling was performed at 1 MHz with bipolar square waves at temperature on a hot plate unless otherwise mentioned. The capacitor areas were 3600 μm² and 14 400 μm².

The binary Hf_0.5Zr_0.5O₂ thin film composition has previously been found to be optimal for ferroelectric behavior while more Zr-rich compositions lead to antiferroelectric-like characteristics.²⁶ As shown in Figure 1(a), 10 nm thick Hf_0.5Zr_0.5O₂ capacitors exhibit good ferroelectric properties with a remanent polarization of ∼24 μC/cm² after cycling. The slight pinching of the virgin Hf_0.5Zr_0.5O₂ hysteresis loop may be due to a tetragonal → orthorhombic phase transition or a redistribution of charged defects with cycling.²⁷ Investigations into the effects of cycling on HfO₂-based ferroelectrics have been an active area of research.^27–32 Dynamic hysteresis currents show four switching current peaks in the virgin Hf_0.5Zr_0.5O₂ films which merge into two homogeneous switching peaks after cycling. Incorporating two monolayers of Si into Hf_0.5Zr_0.5O₂ leads to a pronounced pinching of the virgin polarization–electric field (P–E) loop in the 48:1 Hf_0.5Zr_0.5O₂:Si when compared to the binary composition [Fig. 1(b)]. Moreover, the four switching current peaks in the 48:1 Si-doped Hf_0.5Zr_0.5O₂ thin films move to higher electric fields. While cycling leads to an opening of the hysteresis loop, the resulting two switching current peaks are inhomogeneous and broad, indicating that there is a wide variation in the nucleation times for polarization reversal in the 48:1 Si-doped Hf_0.5Zr_0.5O₂ thin films.

Surprisingly, adding only two more monolayers of Si into Hf_0.5Zr_0.5O₂ thin films produces strongly antiferroelectric-like capacitors [Fig. 1(c)]. The antiferroelectric behavior of the 24:1 Si-doped Hf_0.5Zr_0.5O₂ thin films remains stable with cycling while the field induced phase transitions shift to lower electric fields. The energy storage density (ESD) is the amount of energy stored in the volume of the oxide and is extracted as shown in Fig. 1(c). The amount of energy lost is calculated from the inner area of the hysteresis loops, and the efficiency is calculated by ESD/(ESD + loss).¹⁹ Doping Hf_0.5Zr_0.5O₂ thin films with Al has a similar effect on Hf_0.5Zr_0.5O₂ thin films [Fig. 1(d)], although the field induced phase transitions occur at higher electric fields than the Si-doped Hf_0.5Zr_0.5O₂ thin films. In all the capacitors, cycling has the tendency to reduce the distance between the two pairs of positive and negative switching current peaks. These trends indicate that the predicted field-induced reversible t → o transition, which is believed to be the origin of antiferroelectric behavior in Hf_0.5Zr_0.5O₂ thin films,²³ requires less energy as the capacitors are cycled. Local conversion of the tetragonal phase to the orthorhombic phase, possibly as a result of a change in the charge distribution and local coercive fields within the thin films,²⁷ could be responsible for the cycling behavior shown in Figs. 1(a) and 1(b). The ferroelectric hysteresis loop, however, is not stabilized with cycling in Figs. 1(c) and 1(d), although the field induced t → o phase transition requires lower electric fields to occur after cycling. For this case, it seems highly likely that charged defects are being redistributed within the thin films which changes the local electric field in such a way as to make the t → o phase transition more favorable with cycling. Local charged defects could act as pinning centers which inhibit the t → o phase transition and alter the nucleation characteristics of the polar orthorhombic phase. Conversely, it is possible that enough local charged defects could lead to oppositely biased regions within a ferroelectric thin film, whereby the ferroelectric switching characteristics would mimic antiferroelectric behavior without an actual field-induced phase transition taking place.³³ However, charged defects alone cannot adequately describe why trace doping has such a large impact on antiferroelectric behavior, whereas it has been well-established that trace dopants can aid in stabilizing the tetragonal phase.^34–36 Thus, there seems to be an interplay between the crystal phases, dopants, and charged defects which coexist within the polycrystalline Hf_0.5Zr_0.5O₂ thin films that give rise to the observed cycling phenomena.^27–32 The leakage current density of the binary HZO and Si-doped HZO films annealed at 500 °C ranged from 10⁻⁵ to 10⁻⁴ A/cm², while the Al-doped HZO films were one order of magnitude lower at an applied field of 3.5 MV/cm. The breakdown fields for all of the films were found to be in the range of 4 MV/cm–5.5 MV/cm, similar to the ranges found in Si-doped HfO₂.³⁷ The breakdown fields were the largest in the Al-doped HZO films. Figures 2(a) and 2(b) show TEM cross-sections of the Hf_0.5Zr_0.5O₂ and 24:1 Al-doped Hf_0.5Zr_0.5O₂ thin films, verifying that both films are approximately 10 nm and polycrystalline. Grazing incidence x-ray diffraction (GI-XRD) of all three film compositions shows a mixture of the orthorhombic phase with the tetragonal phase, but no monoclinic phase is detected [Fig. 2(c)]. It should be noted that a high symmetry cubic phase also fits well with most of the GI-XRD patterns except the peaks located at around 42.5° and 55°. Thus, small fractions of the cubic phase may also exist in the films.

Due to the smaller inner area of the hysteresis loops in the 24:1 Al and Si-doped Hf_0.5Zr_0.5O₂ thin films, they may be well-suited for high efficiency energy storage capacitors. Figure 3 shows the energy storage density and efficiency of the 24:1 Si and Al-doped Hf_0.5Zr_0.5O₂ thin films. Antiferroelectric behavior is exhibited in the doped Hf_0.5Zr_0.5O₂ thin films in the anneal temperature range from 500 °C to 700 °C. Anneal temperatures greater than 700 °C led to capacitors which exhibited ferroelectric characteristics, as shown by the decrease in the efficiency which is due to the large internal area of the ferroelectric hysteresis loop. The Si-doped Hf_0.5Zr_0.5O₂ thin films have a large energy storage density from ∼40 to 50 J/cm³ with efficiency from 85 to 80% in the 500 °C–700 °C anneal temperature range. The energy storage characteristics surpass antiferroelectric Si-doped HfO₂ thin films while utilizing an anneal temperature that is ∼500 °C lower,²¹ making Si-doped Hf_0.5Zr_0.5O₂ thin films a more viable candidate for integrated supercapacitor applications. Hf_0.3Zr_0.7O₂ films 9.2 nm thick were also observed to have a large ESD of greater than 40 J/cm³, although the efficiency was less than 60%.¹⁹ Al-doped Hf_0.5Zr_0.5O₂ thin films exhibit a lower energy storage density of ∼22–32 J/cm³ with a slightly higher efficiency of 93%–87%. The switching current peaks of Al-doped films require higher electric fields which causes a lower energy storage density when equivalent electric fields are applied to the two differently doped Hf_0.5Zr_0.5O₂ films.

For practical applications, the doped Hf_0.5Zr_0.5O₂ thin film capacitors will have to operate at elevated temperatures and exhibit resistance to degradation with repeated cycling. The energy storage density and efficiency are shown as a function of electric field and number of cycles at room temperature for Si and Al-doped Hf_0.5Zr_0.5O₂ thin films in Fig. 4. The ESD of the Si-doped Hf_0.5Zr_0.5O₂ thin films increases from ∼38 J/cm³ to ∼53 J/cm³ when the electric field is increased from 4 MV/cm to 4.5 MV/cm [Fig. 4(a)]. In contrast, the Al-doped Hf_0.5Zr_0.5O₂ thin films can only achieve comparable ESD values at electric fields from 4.5 MV/cm to 5 MV/cm which would likely prove to be a reliability concern [Fig. 4(b)]. The cycling characteristics of the Si-doped Hf_0.5Zr_0.5O₂ thin films at elevated temperatures are shown in Fig. 5(a). At an electric field of 4 MV/cm, the antiferroelectric Si-doped Hf_0.5Zr_0.5O₂ capacitors exhibit an ESD of larger than 30 J/cm³ with an efficiency of greater than 80% up to 125 °C and 10⁹ cycles. The higher 600 °C anneal temperature yields larger energy storage densities with a slight sacrifice in efficiency, but has similar reliability characteristics as the Si-doped Hf_0.5Zr_0.5O₂ thin films annealed at 500 °C. The excellent cycling characteristics of the antiferroelectric Si-doped Hf_0.5Zr_0.5O₂ thin films make them excellent candidates for supercapacitors and possibly dynamic random access memory applications where charge storage is of paramount importance. The cycling characteristics of the ESD and efficiency at elevated temperatures for the Al-doped Hf_0.5Zr_0.5O₂ thin film capacitors are shown in Fig. 5(c). The energy storage density is significantly smaller for the Al-doped Hf_0.5Zr_0.5O₂ thin films and electric fields greater than 4 MV/cm led to high leakage currents at elevated temperatures.

Incorporating Al and Si dopants into binary hafnium-zirconium oxide thin films transformed the nominally ferroelectric Hf_0.5Zr_0.5O₂ thin film material into one with pronounced antiferroelectric characteristics. From the electrical characteristics, it was deduced that trace-doping HZO films with Al or Si may favor greater stabilization of the tetragonal phase over the orthorhombic phase, but GI-XRD was unable to unambiguously detect differences in the ratio of the tetragonal and orthorhombic phases in the Al or Si doped HZO films compared to the undoped HZO films. The stronger antiferroelectric-like behavior as observed in the hysteresis of the Al or Si doped HZO thin films supports the theory of a reversible, field-induced tetragonal-to-orthorhombic phase transformation.

This work was supported by the NSF I/UCRC of the Multi-functional Integrated System Technology (MIST Center) IIP-1439644. P.L. was a recipient of the Semiconductor Research Corporation Graduate Fellowship award. T.N. and P.L. acknowledge helpful discussions with Saeed Moghaddam and Qanit Takmeel. The authors acknowledge the use of the Nanoscale Research Facility (NRF) in the Nanoscience Institute for Medical and Engineering Technology at the University of Florida. This work was performed in part at the Analytical Instrumentation Facility (AIF) at the North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (Award No. ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). J.L.J. acknowledges the support from the Army Research Office through Grant No. W911NF-15-1-0593.