Mixed Al and Si doping in ferroelectric HfO₂ thin films

The emergence of ferroelectricity in HfO₂-based thin films is remarkable due to the advancements it may enable for developing and emerging memory technologies such as ferroelectric random access memory (FRAM) and ferroelectric field effect transistors (FeFETs).¹ Ferroelectric behavior in HfO₂-based thin films can be generated through a suitable combination of doping and annealing conditions.² Various dopants have promoted ferroelectricity in HfO₂ thin films including Si, Al, Y, Gd, Sr, La, and Zr.^2–9 Atomic layer deposition (ALD) is most frequently used to deposit HfO₂ thin films because of its ability to tailor precise doping profiles¹⁰ and its highly conformal growth process which can enable high aspect ratio device structures.¹¹ Ferroelectric HfO₂-based thin films are typically grown in an amorphous state and then crystallized during a rapid thermal anneal (RTA) with a capping electrode. A capping electrode during rapid thermal annealing enhances and often facilitates ferroelectric behavior in HfO₂-based thin films.^3–5 

The combination of chemical doping and a mechanically constraining top electrode is postulated to encourage a tetragonal to orthorhombic (t → o) phase transformation during the crystallization anneal.² A noncentrosymmetric, polar Pca2₁ orthorhombic phase is the candidate source for ferroelectricity in HfO₂ thin films. Atomic imaging has provided some structural evidence for the existence of this crystal phase,¹² although a mixture of phases may be present in polycrystalline HfO₂ thin films after rapid thermal annealing.¹³ The coexistence of the monoclinic, tetragonal, orthorhombic, and cubic phases may thus become manifest in doped polycrystalline HfO₂ thin films. In order to maximize the remanent polarization (P_r) of ferroelectric HfO₂, the fraction of the polar orthorhombic crystal phase should be increased within the thin films to enlarge the magnitude of switched charge.

While ferroelectric Hf_0.5Zr_0.5O₂ is attractive due its simple binary composition and low thermal budget,¹³ more flexibility in engineering the properties of HfO₂-based ferroelectrics may be achieved with other dopants. For instance, adjusting the Si-dopant layering distribution in HfO₂ thin films integrated into the metal-ferroelectric-insulator-semiconductor (MFIS) structure illustrated how the P_r could be lowered by increasing the distance between the Si layers.¹⁰ FeFETs may benefit from the engineering of the ferroelectric for lower P_r's because of the consequent reduction in the depolarization field.¹⁴ Furthermore, the small quantity of Al and Si dopants required to induce ferroelectricity in HfO₂ thin films opens up the possibility of placing the dopants within the HfO₂ stack simultaneously. In this letter, we show that mixed Al and Si-doped HfO₂ thin films exhibit a wide range of ferroelectric properties through the placement of the dopant layers within the HfO₂ film.

ALD TiN bottom electrodes 10 nm thick were deposited at 200 °C on highly doped (0.001–0.005 Ω cm) (100) p + Si wafers. Tetrakis(dimethylamido)titanium was used as the Ti precursor and exposed to N₂ plasma during TiN growth. HfO₂ thin films doped with both Al and Si were grown by thermal ALD at 200 °C. The Hf, Al, and Si precursors used were tetrakis(dimethylamido)hafnium, trimethylaluminum (TMA), and tris(dimethylamino)silane, respectively. A 24:1 Hf:(Al, Si) pulse ratio was maintained where a total of 4 dopant layers were incorporated into the 10 nm thick HfO₂ thin films. Three different distributions of the Al and Si dopant layers within the film stack were fabricated. The three different dopant distributions may be denoted as Si-Al-Al-Si (0.87 mol. % Al, 0.49 mol. % Si), Al-Si-Si-Al (1.08 mol. % Al, 0.75 mol. % Si), and Al-Si-Al-Si (1.12 mol. % Al, 0.68 mol. % Si), which represent the placement of the dopant layers from the bottom-up within the HfO₂ film. Time of flight secondary ion mass spectrometry (TOF-SIMS) was employed to quantify the Al and Si content within the HfO₂ thin films. Approximately 2.4 nm of HfO₂ are in between each dopant layer in the as-grown films. ALD TiN top electrodes 10 nm thick were deposited at 200 °C. In previous work, we had observed ferroelectric behavior in Si-doped HfO₂ with a 24:1 HfO₂:SiO₂ ALD cycle ratio.¹⁵ Thus, the 24:1 ALD cycle ratio was chosen to investigate mixed Al and Si doping in HfO₂ thin films. The entire TiN metal-ferroelectric-metal (MFM) stack was grown within the ALD chamber at 200 °C to prevent oxidation of the HfO₂/electrode interface.¹⁵ The MFM stacks were annealed at 700 °C–900 °C for 20 s in N₂. 50 nm thick Pt contacts were deposited in a liftoff process and used as a hard mask in an SC1 wet etch for capacitor geometry formation. The capacitor area was 3600 μm².

The hysteresis characteristics, also referred to as polarization vs. electric field (P–E) plots, of the mixed Al and Si doped HfO₂ thin films were extracted from a Sawyer-Tower circuit with a 220 nF linear capacitor and a 1 kHz triangle wave. Virgin device characteristics were extracted from the average of the first 50 hysteresis measurements. Endurance cycling was performed at 1 MHz with bipolar square waves and a 50 Ω resistor in series with the ferroelectric capacitor during cycling. Grazing incidence x-ray diffraction (GIXRD) was performed with a Rigaku SmartLab X-Ray Diffractometer with a grazing incidence angle of 0.5°, a 0.05° step size, and a 4 s count time per step.

Simultaneously doping HfO₂ thin films with spatially segregated Al and Si dopant layers may influence the internal strain of the ferroelectric. The strain in ferroelectric HfO₂ impacts the formation of the ferroelectric o-phase¹⁶ and could alter the magnitude of the coercive field.¹⁷ Internal strain within HfO₂ can be expected to arise from mixed doping in view of the experimentally extracted differences in the HfO₂ lattice volume between pure Al (127.88 ų) and Si (129.57 ų) doped HfO₂ thin films.¹⁷ In-plane tensile stress is thought to promote the favorable t → o phase transformation¹⁶ and enhance the P_r for ferroelectric HfO₂ thin films.

The hysteresis characteristics of the mixed Al and Si doped HfO₂ thin films reveal significant differences in the P_r and overall shape of the P-E loop before and after electric field cycling at anneal temperatures of 800 °C and 900 °C, Figs. 1(a)–1(d). The virgin characteristics of the Si-Al-Al-Si doped HfO₂ thin films exhibit a P_r of 19 μC/cm² which increases by ∼1 μC/cm² after cycling. The P-E loops of both the Al-Si-Si-Al and the Al-Si-Al-Si doped HfO₂ thin films look distorted horizontally in their virgin state with a P_r of 13 μC/cm² and 7 μC/cm², respectively. Cycling subsequently increases the P_r in the two films by 2 μC/cm² and 5 μC/cm², respectively. Electric field cycling, sometimes referred to as the “wake-up” effect, has been postulated to increase P_r and decrease the asymmetry in ferroelectric Si-doped HfO₂ through the redistribution of mobile defect charge (i.e., oxygen vacancies) originating from the electrode interfaces.^15,18 Since the mixed Al and Si doped HfO₂ thin films show unequal enhancements of P_r with electric field cycling, lattice relaxations associated with the promotion of the o-phase may occur at different rates within the differently doped films. At 900 °C, the P_r trends amongst the three doping layering distributions remain similar, although a significant increase in the coercive field occurs for all of the films. The remanent polarization value of ∼20 μC/cm² found in these mixed Al and Si doped HfO₂ thin films are in good agreement with theoretical simulations of the spontaneous polarization predicted for the Pca2₁ phase of ∼52 μC/cm², which is expected to be double the P_r found experimentally.^17,19 The mixed Al and Si doped HfO₂ thin films exhibit remanent polarization values comparable to Si-, Al-, and Y-doped HfO₂, although higher P_r values have been reported for Gd and La-doped HfO₂ thin films.⁸ 

GIXRD was used to confirm that all of the mixed Al and Si doped HfO₂ thin films show the possible existence of the Pca2₁ orthorhombic phase, Fig. 2. GIXRD indicates the coexistence of multiple phases within the doped HfO₂ thin films. The full width at half maximum (FWHM) of the t(111)/o(211) diffraction peak around 30° is largest in the Si-Al-Al-Si and smallest in the Al-Si-Al-Si doped HfO₂ thin films. Peak width broadening of the mixed doped HfO₂ thin films supports the hypothesis that the doping distribution of the Al and Si layers may be influencing the strain of the HfO₂ thin film, although changes in grain size cannot be ruled out. Since the largest peak width broadening was found in the Si-Al-Al-Si doped HfO₂ thin films, these films may have the greatest amount of microstrain. The sharp reflections in the GIXRD plots at ∼52° are attributed to the signal from the single crystal quartz sample holder.

The way in which the different dopants are layered within the HfO₂ thin films affects the formation of the orthorhombic phase through the induced strain and competing phase transformations. Since the Si-Al-Al-Si doped HfO₂ thin films exhibit the highest remanent polarization and the most significant peak broadening, it is likely that strain is aiding in the stabilization of the orthorhombic phase. Simulations have shown that the tetragonal P4₂/nmc phase may transition into the polar orthorhombic Pca2₁ through a suitable combination of stress and/or electric field.¹⁹ In particular, the preferential formation of the o-phase in the Si-Al-Al-Si distribution may be generated through a large in-plane tensile strain due to the smaller Al-doped HfO₂ lattice volume compared to the larger Si-doped HfO₂ lattice volume.^16,17 The doping distribution of Al and Si in the annealed HfO₂ thin films was confirmed with TOF-SIMS, Fig. 3. The Gaussian shape of the Si and Al distributions is caused by the limited depth resolution of TOF-SIMS. The dopant layers are consistent with a monolayer or sub-monolayer of Al or Si.

High resolution cross-sectional transmission electron microscopy (HR-TEM) confirmed that the Al and Si doped HfO₂ thin films were crystallized after annealing, Fig. 4. The P_r dependence on annealing temperature and electric field cycling are shown for the mixed Al and Si-doped HfO₂ thin films in Figs. 5(a)–5(d). The Al-Si-Al-Si doped HfO₂ thin films favor a lower anneal temperature and reach a maximum virgin P_r of ∼9 μC/cm² at 700 °C. The Al-Si-Si-Al doped HfO₂ thin films reach a maximum virgin P_r of ∼14 μC/cm² at 850 °C. An anneal temperature range from 800 °C to 900 °C yields the highest P_r values on the order of ∼20 μC/cm² in the Si-Al-Al-Si doped HfO₂ thin films with the “wake-up” effect being noticeably absent for the 800 °C and 900 °C anneals. These trends indicate that the proper selection of annealing conditions to maximize P_r is highly dependent on the doping distributions employed in HfO₂-based thin films. Endurance cycling characteristics of the Si-Al-Al-Si doped HfO₂ thin films annealed at 800 °C for 20 s are shown in Fig. 5(d). At 3.5 MV/cm, a P_r greater than 15 μC/cm² is observed up to 10⁸ until breakdown. Breakdown is accelerated at 4 MV/cm and occurs after 10⁶ cycles. Cycling the mixed Al and Si doped HfO₂ thin film at 3 MV/cm delays breakdown until 10⁹ cycles at the cost of a lower P_r and the acceleration of fatigue. The cycling characteristics are promising for memory applications such as FRAM, which require a very high number of read and write cycles. Further optimization of the processing conditions, including the doping and annealing conditions, is likely to provide greater improvements in the maximum number of cycles HfO₂-based ferroelectrics can withstand before breakdown.

In summary, HfO₂ thin films 10 nm thick were simultaneously doped with Al and Si with different dopant layering distributions. The resulting ferroelectric properties in the mixed Al and Si doped HfO₂ thin films were dependent on the layering distribution and the anneal temperature. The influence of strain on the formation of the ferroelectric o-phase may be supported by differences in peak broadening measured from GIXRD, which was found to be dependent on the Al and Si doping distribution. The Al and Si doped HfO₂ thin films were cycled up to 10⁸ times with an applied electric field strength of 3.5 MV/cm before breakdown occurred. The ability to engineer the ferroelectric properties of HfO₂ thin films by simultaneously doping HfO₂ thin films with multiple dopant species is thus presented as an accessible route for enhancing HfO₂-based ferroelectrics for a variety of applications.

This work was supported in part by the NSF I/UCRC on Multi-functional Integrated System Technology (MIST Center) IIP-1439644. P.L. is the recipient of a SRC Graduate Fellowship, and J.J. acknowledges support from NSF under DMR-1207293 (J.J., C.Z.). The authors acknowledge the use of the Analytical Instrumentation Facility (AIF) at North Carolina State University, which was supported by the State of North Carolina and the National Science Foundation.