which aligns with the characteristic trap-controlled space-charge-limited current (SCLC) behavior []. At low bias (0—0.1 V)， consistent with Child’s law behavior， ]. Among the metastable orthorhombic structures， thereby creating oxygen vacancies near the interface. While the filament will be ruptured near the AZO bottom electrode since these ions recombine with vacancies near the interface. This rupture increases the device resistance (shown in Fig. (h))， further confirm the epitaxial nature of the AZO layer. From the QZ position of the AZO 002 reflection， and O2− ionic states in the as-grown samples. The optical band gap of the thin films was found to reduce with the reduction in film thickness. The o-HZO was found to exhibit weak ferroelectric polarization with 2Pr value of 1.6 μC/cm2 with Ec of 1.17MV/cm。
further depositions at different numbers of laser pulses were carried out at 100 mTorr. Figure (b) displays the XRD profile of HZO thin films at 33 nm，， including W。
whereas the stable monoclinic phase is non-ferroelectric. Various thermodynamic factors such as surface energy， confocal Raman spectroscopy was performed in the samples with 532 nm laser excitation [see Supplementary Information Fig. ] with 50 × optical zoom. Data were acquired for 2 s exposure for 20 accumulations with 100% open neutral density filter (NDF) for all the samples. All the samples were found to show strong vibration at 140 cm−1 and 385 cm−1 representing the Ag mode of the monoclinic phase [， leading to ferroelectricity。
with RMS roughness of ~ 0.5 nm. Further， consistent with previously reported values []. Within the temperature range of 300–473 K， the resistive switching is forming-free， nanoscale ZnO incorporation into HfO2-based memristors facilitated conductive filament formation， HZO deposited on AZO/ c-Al2O3 exhibited a major orthorhombic phase with a minor tetragonal phase contribution. Notably， recorded under a compliance current (CC) of 50 μA during the set operation. The device demonstrates bipolar resistance switching (RS) behavior。
which in turn destabilize the monoclinic phase []. Beyond their role in phase stabilization。
17 nm。
which is shown in Fig. (b). Ferroelectricity was observed in the as-grown film stack. The 2Pr value of 1.6 μC/cm2 was obtained with coercive field (Ec) of 1.17MV/cm. The polarization value of the stacks is comparatively small due to the high leakage current and the coexistence of non- centrosymmetric phases. The current–voltage characteristics of the Au/o-HZO/AZO device， respectively). Structural characterization of the thin films was performed using PANalytical Empyrean X-ray diffractometer. Confocal Raman spectroscopy was performed to correlate the structural features， and charge trapping that accelerate fatigue and degrade endurance， using the Pseudo-Voigt peak profile function with the background variation described by linear interpolation between the set of points with refinable heights. Figure (a) shows the XRD profile of 56 nm HZO thin films deposited at 2 mTorr and 100 mTorr. At low oxygen partial pressure， molecular beam epitaxy (MBE)， driving the system into the low-resistance state (LRS). The resistive switching in the Al/m-HZO/AZO device is governed by the formation and rupture of conductive filaments composed of oxygen vacancies. Under a negative bias applied to the top Al electrode (during the set process)， and 0.821° for the samples with thickness 33 nm， confirming the crystallization of the ceramic into a monoclinic phase with space group \(P{2}_{1}/c\) . The refinement was carried out for monoclinic HZO， reveals consistent and degradation-free operation across 104 sweep cycles [Fig. (c)]. The device exhibits a memory window of 1.6， where the current increases linearly with applied voltage， strain engineering， affecting the relative energies of competing polymorphs []. Partial substitution of Zr in HfO2 results in a stable f–phase at room temperature， resulting in reduced operating voltage and enhanced endurance []. Building on these insights， pushing oxygen ions back toward the Al top electrode/switching interface。
laser energy of 120mW。
revealing the local piezoresponse at a diverse combination of bottom electrodes and substrates [。
FEI. Inc.). High-resolution spectra were taken at a step size of 0.05 eV. Atomic force microscopy of the samples was performed using the Asylum MFP 3D scanning force microscope. The current–voltage characteristics of the MIM devices were measured using a Keysight B1500A semiconductor parameter analyzer， and a repetition rate of 3 Hz. 1500 pulses of epitaxial 2wt% Al:ZnO layer were deposited on c–Al2O3 substrates keeping the substrate–target distance of 4 cm at substrate temperature of 600 °C with laser energy of 100 mW and repetition rate of 6 Hz. Oxygen partial pressure was maintained at 100 mTorr. 1000 pulses (10 nm) of HZO are grown on AZO layer using aforementioned conditions. The thickness of the as-grown films was measured using Dektak XT stylus profilometer consisting of diamond tip stylus of 2.5 μm radius with an uncertainty of ± 10% in the measurements (5000， the preferred orientation was observed to change to \(\overline{1}11\) of the monoclinic m phase with a minor orthorhombic O 111 phase which is consistent with previous reports []. Therefore， conducted by tracking the resistance values of the high-resistance state (HRS) and low-resistance state (LRS) at a read voltage of 0.1 V， the current rises precipitously， La0.67Sr0.33MnO3 (LSMO)， the low-resistance state shows linear ohmic behavior。
and 10 nm， ]. One more split was observed in the sample with 10 nm thickness。
1500， the conduction mechanism transitions， including atomic layer deposition (ALD)， HZO thin film was deposited on the AZO layer. The θ–2θ XRD profile of monoclinic HZO (m-HZO) and orthorhombic HZO (o-HZO) on AZO/c- Al2O3 is shown in Fig. (c). It has been noted that the o-HZO layer grown over AZO consists mainly of O 111 polar phase， the poor complementary metal oxide (CMOS) compatibility and limited scalability have been the major drawbacks of perovskite-based devices. The discovery of ferroelectricity in lead-free HfO2–ZrO2 (HZO)-based solid solutions has revived the interest in ferroelectric memory technology due to its CMOS compatibility and robust electric dipoles at nanoscale thicknesses []. Due to the characteristics of the wide band gap， temperature-dependent resistivity (ρ) measurements were carried out， the device exhibits ohmic conduction， Zr4+， with a negative voltage triggering the set process and a positive voltage initiating the reset. Importantly， confirming the existence of recommended Hf4+ ionic state as shown in Fig. (a) []. Zr–O binding structures are confirmed by a Zr3d5/2 and Zr3d3/2 single doublet located at 181.9 eV and 184.4 eV， attributing to the surface OH groups [， etc. []. The similarity in the crystal structure and properties of HfO2 and ZrO2 allows their total solubility in the whole range of chemical composition. HfO2 exhibits a high-temperature fluorite-type cubic phase ( \(Fm\overline{3 }m\) ) which transforms to a tetragonal ( \(P{4}_{2}/nmc)\) upon cooling and further into a monoclinic phase ( \(P{2}_{1}/c)\) which is stable at ambient temperature and pressure. In addition to this， both of which contribute to the mosaic character of the films. To assess the applicability of AZO as an electrode material， etc. [， respectively. To analyze the ferroelectric properties of Au/o-HZO/AZO device， a progressive decrease in ρ was observed with increasing temperature， ]， and 10 nm thickness at wavelength range of 200–600 nm. The reflectance of all the samples was found to be lower in the visible region， metastable phases with orthorhombic symmetry such as \(Pbca ， \(Pca{2}_{1}\) (O–phase or f–phase) has been observed to be ferroelectric， the out-of-plane lattice constant was determined to be c = 5.185 Å. Compared to bulk ZnO (c = 5.206 Å)。
the stochasticity in the set and reset voltages of the RRAM device is measured for 100 cycles by the ratio of standard deviation to the median value (σ/μ)[]. The σ/μ values for set and reset voltages were found to be 6.5% and 12.6% indicating the long-term reliability in set-reset operation. The memristor maintains stable switching performance over 100 consecutive DC cycles， and 1000 pulses optimized for approximate thickness of 56 nm， respectively [] [see Fig. (b)]. The 1:1 ratio of Zr and Hf is confirmed from the Hf and Zr core levels. Deconvolution of O1s core level in samples with 3000 and 1500 pulses show two components out of which the O1s peak located at 530 eV corresponds to the oxygen in the orthorhombic lattice of HZO，。
pulsed laser deposition (PLD)， the optical band gap of the HZO thin films was calculated from the Tauc plot using the reflectivity data. Figure (b) shows the reflectivity of HZO samples deposited at 33 nm， the XRD profiles of both films were deconvoluted using Gaussian fitting。
thereby suggesting the out-of-plane compressive stress with reduced thickness []. Further，。
high dielectric constant， while the RRAM devices fabricated based on m-HZO were found to exhibit a stable bipolar resistive switching over 104 sweep cycles. Experimental details The HZO thin films were grown using Pulsed Laser Deposition (PLD) (Excel instruments with Coherent 248 nm KrF excimer laser). A PLD target of Hf0.5Zr0.5O2 was synthesized using conventional solid-state reaction HfO2 and ZrO2 powders (purity—99.9%). The pellets were sintered at 1200 °C for 8 h followed by air cooling. PLD of Hf0.5Zr0.5O2 on c–Al2O3 substrates was carried out by varying oxygen partial pressure (2 mTorr and 100 mTorr) and the film thickness. The substrate–target distance was fixed to 4 cm with the substrate temperature at 750 °C。
and lattice strain contribute to stabilizing these metastable phases， to confirm the increase in oxygen vacancies。
17 nm， the present study explores the phase evolution on c- sapphire (c – Al2O3) substrates. As ZnO can promote the polar phases of HZO， whereas the m-HZO consists of m \(\overline{1}11\) phase. To compare the growth behavior of o-HZO films deposited on bare c-Al2O3 and on AZO/c-Al2O3 at identical thicknesses， Hf0.5Zr0.5O2 (HZO) thin films have been grown by a variety of deposition methods， and superior chemical and thermal stability， 3000， confirming the typical non-degenerate semiconducting behavior arising from the thermal activation of n-type charge carriers. Figure 2 Full size image (a) θ–2θ XRD profile of AZO layer with inset showing the rocking (ω) scan (b) out-of-plane reciprocal space map of AZO layer on c-Al2O3 (c) orthorhombic and monoclinic phased HZO grown over AZO layer on c-Al2O3 substrates. Subsequently， indicative of space-charge-limited conduction (SCLC) in the presence of traps. With further increase in bias beyond 1 V。
oxygen vacancies， interface，， as presented in Supplementary Fig.. At room temperature， Pt，，， thereby limiting the robustness of the switching process []. In the case of Al/m-HZO/AZO devices， using a laser excitation of 532 nm with 2 s of exposure at 50 × optical zoom. The optical properties of the films were analyzed using UV–Vis spectrophotometry in diffuse reflectance mode at a wavelength range of 200–800 nm. Ionic states in the as-grown films were investigated via X-ray photoelectron spectroscopy (PHI 5000 Versa Prob II， whereas the mode could not be seen in samples deposited with 10 nm thickness. A small peak observed at 550 cm−1 can be attributed to the orthorhombic phase， and 10 nm thickness in which the major polar orthorhombic phase O \(\overline{1}11\) was found to be stabilized. At 33 nm， and the plotted Tauc’s relation for a direct band gap systems suggests the optical band gap to lie across 5.1 eV which is comparable to the previous reports as shown in Fig. (c) [， reflecting carrier transport dominated by thermally generated free carriers. As the voltage is raised， minor m \(\overline{1}11\) as well as m 111 phases were found to coexist with O 111 [， whereas the highest reflectivity is observed in the UV region. A monotonic lower shift in maximum reflectance is evident as the thickness is reduced， the incorporation of an AZO layer plays a crucial role in regulating the distribution and density of oxygen ions and vacancies. It also adjusts local barriers and enables forming-free or lower-voltage switching in HZO/AZO bilayers. This enhanced control over the conductive pathways contributes to improved resistive switching performance and greater device stability. Conclusions In summary， the m 111 phase was completely suppressed， m 111 as well as m \(\overline{1}11\) phases begin to suppress. Further decreasing the thickness to 10 nm。
presented in Supplementary Fig. ， Zr， presented in Fig. (b)， HfO2-based materials are largely used in micro-nanoelectronics industries. These materials find promising roles in ferroelectric memories (FeRAM)， have been used in crystallizing ferroelectric HZO thin films [， indicating the coexistence of all the polymorphs in all the samples []. The surface morphology of o-HZO was visualized using atomic force microscopy as shown in Fig. (a)。
are further increased while the electric field drives oxygen ions (O2−) toward the bottom electrode (AZO). This movement of ions creates oxygen vacancies near the AZO bottom electrode， the luminescence intensity is found to decrease considerably. This confirms the increase in the oxygen vacancies as the thickness is reduced as reported previously [， such as elemental doping [， Raman spectroscopy was performed. The tetragonal (t) phase generally coexists with the polar phase []. To detect the existence of t phase and other polymorphs，]. Several strategies， confirming reliable state distinction and robust non-volatile behavior. Figure 5 Full size image (a) Current–Voltage (I-V) characteristics of Al/m-HZO/AZO RRAM device on c-Al2O3 substrate (b) Polarization–electric field (P-E) curves of Au/o-HZO/AZO device on c-Al2O3 substrate. (c) Endurance of the RRAM device for over 104 cycles. (d) Cumulative probability of set and reset voltages with average value (μ) and standard deviation (σ). and (e) the I-V curves of set and reset cycles plotted in log scale。
while those with orthorhombic HZO displayed room-temperature ferroelectricity. Results and discussion Polar HZO thin films were deposited on AZO buffered c-sapphire substrate， SrRuO3 (SRO)。
17 nm。
the resistivity was measured to be 8.42 × 10⁻3 Ω·cm， ferroelectric tunnel junctions (FTJs)，， 33 nm， as shown in Fig. (d). 5000 pulses of HZO were deposited at oxygen partial pressure of 2 mTorr as well as 100 mTorr to observe the possible crystallization state on c–Al2O3. Figure depicts the Rietveld refined XRD profile of the polycrystalline HZO target， (a) Hf 4f core level (b) Zr3d core level with respective spin–orbit splits deconvoluted using Lorentzian and Gaussian peak profiles. (c)， and Nb:STO， and 10 nm， which enabled the formation of high-quality interfaces for charge transfer in two-terminal devices []. Similarly。
our work has clearly shown the phase evolution of HZO on c-cut sapphire substrates and has studied the electromechanical properties of HZO/AZO/c-Al2O3 structure. After optimizing the oxygen pressure to 100 mTorr， referring to Fig. (d)。
]. Several reports have witnessed the switching of the ferroelectric domains via piezoresponse force microscopy (PFM)， have been shown to enhance ferroelectricity via improving O–phase volume ratio in HZO. A variety of bottom electrodes， the full width at half maxima (FWHM) of the O-phase was found to be 0.780°， restoring the high-resistance state (HRS) []. Here， the I–V characteristics are replotted on a double-logarithmic scale as shown in Fig. (e) and (f). In all the DC cycles。
thereby favoring the stabilization of the polar orthorhombic phase. The reduction of non-polar-phase fractions highlights AZO as an effective bottom electrode for promoting stable o-HZO growth. The schematic of the as-grown layers is depicted in Fig. (d). X-ray photoelectron spectroscopy was performed on the sample deposited at different thicknesses to confirm the ionic states and to derive oxygen vacancy distributions. The spectra were calibrated as per the C1s peak located at a binding energy of 284.6 eV. The Hf，， (d) and (e) O 1 s core level of HZO at different film thicknesses deconvoluted using Lorentzian and Gaussian peak profiles. In order to study the phase evolution with thickness， ]. Stabilization of O–phase HZO on hexagonal sapphire (0006) via PLD has been discussed previously []. Zheng et al. [] has stabilized polar rhombohedral phase of HZO on ZnO substrates via domain matching epitaxial growth. The incorporation of doubly charged oxygen vacancies plays a crucial role in shifting the energy balance between competing phases， whereas the peak at 531.4 eV suggests the existence of non-latticed oxygen [， polarization–electric field (P-E) loop measurements were done at room temperatures， photoluminescence spectroscopy was also done at 325 nm and 488 nm excitations [shown in Fig. Supplementary materials]. As the number of pulses is reduced。
oxygen vacancies critically govern the electrical performance and reliability of ferroelectric HZO. Excessive vacancy concentration promotes domain-wall pinning， The demand for data storage has been continuously increasing in modern micro and nanoelectronics systems，。
]， imprint， which are already present near the Al electrode due to its electrochemically active nature。
especially the memories based on ferroelectric materials due to their durability and low power consumption. Recently， 2 wt%) was deposited on c- Al2O3， ]. Figure 4 Full size image (a) AFM micrographs of 10 nm o-HZO on AZO/c-Al2O3 (b) UV- Vis Optical reflectivity spectra of HZO samples (c) (F(R)hν)2 versus photon energy plot of HZO samples grown at different film thickness. Figure (a) illustrates the current–voltage (I–V) characteristics of the Al/m-HZO/AZO memristor， ferroelectric field-effect transistors (Fe-FETs)。
]. Recently。
]. Raman modes at 640 cm−1 are observed in the samples deposited 33 nm and 17 nm thicknesses which can be attributed to the minor tetragonal phase in the sample， a conductive AZO layer can serve as an effective interface to boost memristive performance while simultaneously acting as a template for stabilizing polymorphic HZO. However， serving both as a bottom electrode and a seed layer for HZO growth. The XRD profile in Fig. (a) confirms the epitaxial orientation of AZO along the 002 plane， Pnma\) and \(Pca{2}_{1}\) have also been synthesized at high pressures [。
respectively. The decrease in FWHM with the thickness is partially due to the decrease in the crystallite size and increased crystallite strain with reduced number of pulses []. Overall， TaN， ]. As the thickness is reduced to 17 nm， with electrical contacts established via two-point gold-coated tungsten probes (15 μm tip) integrated into an optical microscope probe station. ， which is attributed to the asymmetrical position of four out of eight oxygen ions within the symmetric cation sublattice. The formation energy of oxygen vacancies is thermodynamically higher in the orthorhombic phase compared to the non-polar monoclinic phase. An increased concentration of oxygen vacancies ( \({V}_{O}^{2+}\) ) induces local structural distortions， resistive switching is typically governed by polarization modulation at the interfaces rather than conventional filamentary conduction. However， RF sputtering。
the contribution of non-polar monoclinic phases found to reduce at lower thicknesses. Figure 1 Full size image θ—2θ XRD profile of (a) HZO samples with a film thickness of 56 nm grown at 2 mTorr and 100 mTorr PO2 (b) HZO samples grown with different film thickness at 100 mTorr pO2. Further， the influence of the bottom electrode on the phase evolution of HZO thin films was investigated. Al-doped ZnO (AZO。
various ferroelectric memories based on perovskites have been designed. However， thereby forming a connected chain of oxygen vacancies (Vo++) across the HZO layer， and defect engineering [， ]. It is evident from Fig. (c)–(e) that the latticed oxygen content is decreasing with a decrease in the number of pulses indicating the enhancement in the oxygen vacancies contributing to the stabilized orthorhombic phase [，]. Furthermore， indicative of mosaic spread arising from the lattice mismatch between AZO and Al2O3. Reciprocal space maps (RSM) across the out-of-plane AZO 002 and Al2O3 (0006) reflections， 0.792°， a highly conductive Al-doped ZnO (2wt%) was used as an epitaxial bottom electrode to help stabilize the polar O–phase via PLD. Orthorhombic and monoclinic HZO phases were employed as active layers in the MIM device structure. Devices incorporating monoclinic HZO exhibited resistive switching behavior characteristic of RRAM， alongside the dominant orthorhombic o \(\overline{1}11\) phase. In contrast， the evolution of polymorphs on sapphire continues to be an active area of study. As there are a limited number of studies on the phase evolution of HZO， thereby stabilizing the metastable polar phase. Earlier efforts focused on fabricating buried HfO2/ZnO heterostructures through internal oxidation， the high-resistance curves reveal three sequential regions: an initial ohmic response (I ∝ V)， confirming the formation of conducting filaments within the HZO films during the set process. In contrast， the electric field direction reverses。
and the slope escalates to values exceeding 12.4. This abrupt surge in current signifies the onset of a highly conductive pathway， the major O-phased HZO was successfully stabilized with coexisting m \(\overline{1}\) 11 and m 111 reflections. Reducing the film thickness resulted in a reduction in the coexisting out-of-plane monoclinic phases. AZO was found to promote the growth of highly crystalline O-phase at a film thickness of 10 nm. XPS confirmed the existence of recommended Hf4+。
2Ɵ peak at 34° corresponds to the monoclinic/ tetragonal phase of the (002) plane. As the oxygen partial pressure was increased to 100 mTorr， large breakdown field， TiN， respectively， respectively. (f) Schematic representation of (g) SET process (h) RESET process. To elucidate the underlying switching mechanisms， and the slope of the I–V curve increases to approximately 2.1， when a positive bias is applied to the Al electrode (during the reset process)。
]. Figure 3 Full size image High-resolution XPS spectra for different film thicknesses of HZO， m \(\overline{1}11\) peak was further suppressed to have a major O 111 with minor m \(\overline{1}11\) reflections. A higher angle shift of O 111 observed in the sample with 10 nm thickness indicates the reduction in the out-of-plane lattice parameter， and finally a sharp current increase， followed by a Child’s law regime (I ∝ V2)， stabilizing the conductive path and achieving the low-resistance state (LRS) as shown in Fig. (g). Conversely， the AZO film exhibits an out-of-plane compressive strain of approximately –0.4%， which is typically accompanied by an in-plane tensile strain. The broadening observed along QX reflects the lattice mismatch and elevated strain， the large-scale ionic migration required to establish a stable conductive filament is significantly hindered， point defects， under conditions of tight structural confinement at low thicknesses。
the diffraction pattern revealed minor contributions from the monoclinic m \(\overline{1}11\) and m 111 phases。
as shown in Supplementary Fig.. For HZO grown directly on c-Al2O3， LaNiO3 (LNO)， with a characteristic 2θ peak at 34.6°. The rocking curve (ω-scan) of the AZO film [shown in inset of Fig. (a)] exhibits a full width at half maximum (FWHM) of 1.72°， underscoring its potential for low-power memory applications. The switching ratio of LRS to HRS under DC cycle was found to be ≈56. Endurance analysis， a crossover of the reflectance curves is evident. The 10 nm HZO film on AZO was found to have the roughness of ~ 0.5 nm. The low roughness helps in maintaining the highest specular reflectance compared to the thicker films of 17 nm and 33 nm. Similar trends have previously been reported in HfO2-based thin films []. The reflectivity is converted into Kubelka–Munk function (F(R))。
thereby requiring a careful control of the oxygen vacancy density to balance polar-phase stabilization against long-term device reliability [。
suggesting a slight enhancement in the optical band gap with increase in orthorhombic nature. The normalized reflectance is observed to decrease slightly and monotonically in the wavelength region below 350 nm. At longer wavelengths。
with set and reset voltages observed at –1.198 V and + 1.168 V。
reveal a weak resistive switching behavior with reduced stability compared to the Al/m-HZO/AZO counterpart. In polar HZO systems。
and O core levels are deconvoluted and plotted in Fig. . The Hf core-level spectra were found to be single doublet of Hf4f7/2 and Hf4f5/2 positioned at 16.7 eV and 18.3 eV， 17 nm， HfO2/ZnO combinations have been employed as charge-trap layers in three-terminal memory devices []. In another study。