Memory effects in nanolaminates of hafnium and iron oxide films structured by atomic layer deposition

Citation: Kalam, K.; Otsus, M.; Kozlova, J.; Tarre, A.; Kasikov, A.; Rammula, R.; Link, J.; Stern, R.; Vinuesa, G.; Lendínez, J.M.; et al. Memory Effects in Nanolaminates of Hafnium and Iron Oxide Films Structured by Atomic Layer Deposition. Nanomaterials 2022,12, 2593. https://doi.org/10.3390/ nano12152593 Academic Editors: Patrick Fiorenza, Raffaella Lo Nigro, Béla Pécz and Jens Eriksson Received: 5 July 2022 Accepted: 25 July 2022 Published: 28 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Memory Effects in Nanolaminates of Hafnium and Iron Oxide Films Structured by Atomic Layer Deposition Kristjan Kalam 1,*, Markus Otsus 1, Jekaterina Kozlova 1, Aivar Tarre 1, Aarne Kasikov 1, Raul Rammula 1, Joosep Link 2, Raivo Stern 2, Guillermo Vinuesa 3, JoséMiguel Lendínez 3, Salvador Dueñas 3, Helena Castán3, Aile Tamm 1and Kaupo Kukli 1 1Institute of Physics, University of Tartu, W. Ostwaldi 1, 50411 Tartu, Estonia; [email protected] (M.O.); [email protected] (J.K.); aivar[email protected] (A.T.); [email protected] (A.K.); [email protected] (R.R.); [email protected] (A.T.); [email protected] (K.K.) 2Laboratory of Chemical Physics, National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia; [email protected] (J.L.); [email protected] (R.S.) 3Department of Electronics, University of Valladolid, Paseo Belén 15, 47011 Valladolid, Spain; [email protected] (G.V.); [email protected] (J.M.L.); [email protected] (S.D.); [email protected] (H.C.) *Correspondence: [email protected] Abstract: HfO 2 and Fe 2 O 3 thin films and laminated stacks were grown by atomic layer deposition at 350 ◦ C from hafnium tetrachloride, ferrocene, and ozone. Nonlinear, saturating, and hysteretic magnetization was recorded in the films. Magnetization was expectedly dominated by increasing the content of Fe 2 O 3 . However, coercive force could also be enhanced by the choice of appropriate ratios of HfO 2 and Fe 2 O 3 in nanolaminated structures. Saturation magnetization was observed in the measurement temperature range of 5–350 K, decreasing towards higher temperatures and increasing with the films’ thicknesses and crystal growth. Coercive force tended to increase with a decrease in the thickness of crystallized layers. The films containing insulating HfO 2 layers grown alternately with magnetic Fe 2 O 3 exhibited abilities to both switch resistively and magnetize at room temperature. Resistive switching was unipolar in all the oxides mounted between Ti and TiN electrodes. Keywords: multilayers; atomic layer deposition; hafnium oxide; iron oxide; ferromagnetism; resistive switching; nanolaminates 1. Introduction Prospective applications to magnetoresistive [ 1 ] and resistive [ 2 ] memory effects, which were notified few decades ago, have extended the search for potentially multiferroic materials, whereby the list of such materials has continuously been updated. Studies on layered compound materials, concurrently exhibiting resistive switching (RS) and ferromagnetic (FM) characteristics, have been conducted. Both RS and FM properties could have simultaneously been registered in devices built on thin films of few different compounds, e.g., ZnO:Co [ 3 ], ZnO:Co/SiO 2 :Co [ 4 ], copper oxides [ 5 ], or HfO 2 [ 6 ]. Aside from nonlinear, saturative, and hysteretic magnetization behavior, bipolar resistive switching behavior was recorded in devices built on these compounds, expressed by two distinct conduction current (resistivity) states stabilized during consecutive programming pulses upon changes in voltage polarity. The films referred to above were formed by physical vapor deposition (PVD) techniques. The exploitation of PVD allows the formation of solid films with maximum chemical purity, as the purity of a deposited material is determined by that of the precursor sublimed. At the same time, PVD techniques may face challenges before the uniform deposition of thin films over substrates of arbitrary area and shape. Chemical vapor deposition routes to the multifunctional ferroic films are thus additionally sought in order to open complementary perspectives to tailor materials possessing useful, and differently manifested, Nanomaterials 2022,12, 2593. https://doi.org/10.3390/nano12152593 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022,12, 2593 2 of 18 physical properties. Amongst several materials that exhibit magnetoresistive performance, HfO 2 can be regarded as a material feasibly grown over large area substrates if appropriate routes, such as atomic layer deposition (ALD), were used. HfO 2 might further stand out as a compound exhibiting resistive switching behavior together with magnetic susceptibility, especially when supported by an additive or dopant-enhancing internal magnetization, particularly Fe2O3. HfO 2 , together with Ta 2 O 5 , has been considered one of the most intensely investigated thin film materials for application in nonvolatile, resistively switching (memristive) memories, which are based on the migration of either cations or oxygen vacancies into the lattice of a switching medium [ 7 ]. The resistive switching (RS) effect is a reversible and non-volatile change in the resistance of a material. By applying a certain electric field, a conductive filament (CF) is formed through the oxide, connecting the metals that surround it. Once the filament is formed for the first time (electroforming process), different electric field values will allow for repetitively disrupting (RESET process) and forming (SET process) the CF reversibly [ 2 , 8 , 9 ]. If the SET and RESET processes occur at different voltage polarities, the effect is known as bipolar resistive switching (BRS), which is usually due to oxygen anion migration and electron hopping through oxygen vacancies in the oxide media (valence change mechanism or VCM) whereby the metal electrodes are inert metals [ 2 , 8 , 10 – 13 ]. BRS can also be produced by the formation of a metallic filament (electrochemical metallization mechanism or ECM, also known as conductive bridge RAM or CBRAM). In the latter case, an electrochemically active metal electrode is needed as a source of the metal cations that will diffuse through the oxide, creating the CF [ 2 , 8 , 10 – 13 ]. On the other hand, unipolar resistive switching (URS) is attributed to thermochemical processes dominating over electrochemical ones (thermochemical memory or TCM) [ 14 , 15 ]. Thus, temperature gradients produced by Joule heating lead to redox reactions and local changes in the material (oxide) stoichiometry, which results in a change of the conductivity [ 10 – 13 ]. The switching mechanism in oxide film media, including HfO 2 , has largely been described as that based on filamentary conduction [ 16 ]. Nonetheless, switching in HfO 2 has also been found to be dependent on electrode metals, whereby both bipolar and unipolar switching could be initiated [ 17 ]. Resistively switching HfO 2 films can be synthesized using different techniques, including metalorganic chemical vapor deposition [ 17 ] and atomic layer deposition (ALD) [18]. The application of external magnetic fields can influence the resistive switching performance of ALD-processed HfO 2 based cells [ 18 ]. Further, internal magnetization in HfO 2 films alone has been possible due to the presence of significant amounts of oxygen vacancies [ 19 , 20 ], which may also be related to the formation of metastable HfO 2 polymorphs. In general, the achievement and appearance of ferromagnetic-like behavior has been regarded as an inherent, although sometimes unexpected, property of nanocrystalline materials [ 21 ]. Nonlinear hysteretic magnetization is in such cases caused by defective crystallite boundaries involving vacancies and unsaturated coordination of metal atoms, i.e., the factors generally causing leakage and increasing conductivity. Thereby, the magnetization should take place in a medium that, at the same time, should also switch resistively. The prerequisite for the resistive switching process is insulation in the virgin, although defective, state of the medium. Thus, the demands for materials demonstrating both magnetization and resistive switching may appear controversial. Such materials should in principle simultaneously perform as wide-band-gap dielectrics and electrically rather conductive magnetic materials, which may be regarded as contradicting properties. Therefore, a tradeoff between insulating dielectric properties and the ability to hysteretically magnetize upon choosing the constituent materials is necessary. Engineering combinations of reliably resistively switching material layers, such as HfO 2 , with complementary compounds reliably magnetizing and also growing, such as Fe 2 O 3 [ 22 – 24 ], is justified and is to be purposefully examined in terms of both memory effects. Quite naturally, before the examination of coupling between hysteretic magnetization and electrical switching, it would be reasonable to examine the appearance of both effects separately, in order to attempt Nanomaterials 2022,12, 2593 3 of 18 optimization of the deposition process and the forming structures. This is essentially the aim of the present study. Studies on composites or solid solutions based on iron and hafnium oxides are scarce. Crystallization has been investigated in the HfO 2 -Fe 2 O 3 system [ 25 ] at temperatures more than two times higher than those applied in the present study. The enhancement of magnetization has been observed in Fe-doped HfO 2 [ 26 ] due to the phase segregation and formation of Fe 2 O 3 . Composite materials, especially in the form of functional thin films consisting of iron and hafnium oxides, are thus not quite explored yet. Regarding layered materials containing either HfO 2 or Fe 2 O 3 , we have earlier observed both saturative hysteretic magnetization and bipolar resistive switching behavior in HfO 2 - Al 2 O 3 [ 27 ], HfO 2 -ZrO 2 [ 28 , 29 ], and SiO 2 -Fe 2 O 3 [ 30 ] multilayers grown by atomic layer deposition. In the present study, nonlinear saturative and hysteretic magnetization in HfO 2 -based thin solid films grown by ALD, enhanced by the contribution from Fe 2 O 3 to HfO 2 -based multistoried films, was examined. Here, HfO 2 and Fe 2 O 3 layers were grown sequentially into stacks to tailor magnetic and insulating materials. The goal was to ensure nonlinear hysteretic magnetization as well as resistive switching in the same materials, yet without a detailed investigation of the coupling effects that remain beyond the scope of the present study. The objective was to examine whether it is possible to observe reliable switching behavior and hysteretic magnetization at room temperature in HfO 2 -Fe 2 O 3 nanolaminates grown using the same deposition cycle sequences. 2. Materials and Methods The films studied in this work were grown in a low-pressure flow-type ALD reactor [ 31 ]. Hafnium tetrachloride (HfCl 4 , 99.9%, Sigma Aldrich, Burlington, MA, USA), used as the hafnium precursor, and ferrocene (Fe(C 5 H 5 ) 2 , 99.5%, Alfa Aesar, Word Hill, MA, USA), used as an iron precursor, were evaporated at 160 and 83 ◦ C, respectively, from a half-open glass boat inside the reactor. Nitrogen (N 2 , 99.999%, AS Linde Gas, Tallinn, Estonia) was applied as the carrier and purging gas. Ozone produced from O 2 (99.999% purity, AS Linde Gas) was used as an oxidizer. The ALD reactions were carried out at 350 ◦ C. Cycle times for Fe 2 O 3 deposition were 5-5-5-5 s for the sequence: metal precursor pulse—N 2 purge—O 3 pulse—N 2 purge, respectively. Cycle times for HfO 2 were 5-2-5-5 s for an analogous sequence. Single HfO 2 and Fe 2 O 3 films were grown to thicknesses ranging from 20 to 60 nm in order to acquire the reference data from composition analysis as well as resistive switching or magnetizing media. Further, a double-layered Fe 2 O 3 -HfO 2 stack as well as nanolaminates of HfO 2 and Fe 2 O 3 were deposited, aiming at the formation of a series of stacks consisting of both iron-rich and hafnium-rich solid media, as presented in Table 1. Nanomaterials 2022, 12, x FOR PEER REVIEW 4 of 20 500 × Fe2O3 0 37 0 0 The films were grown on undoped Si(100) and, for the electrical evaluation, also on highly doped conductive Si substrates covered by a 10 nm TiN film. The conductive Si wafers were boron-doped to concentrations of 5 × 1018 − 1 × 1019 cm–3 and coated with crystalline TiN layer by pulsed chemical vapor deposition using a batch TiCl4/NH3 process [32,33] at temperatures of 450–500 °C in an ASM A412 Large Batch 300 mm reactor at Fraunhofer IPMS-CNT. The films, which were deposited on TiN substrates for electrical measurements, were also supplied with Ti/Au electron-beam evaporated electrodes on top of the films, with the Ti layer directly contacting the switching oxide medium and Au deposited in order to provide non-oxidizing electrical contact to the measurement circuit. The crystal structure of the films was evaluated by grazing incidence X-ray diffraction (GIXRD) method using a SmartLab (Rigaku, Tokyo, Japan) X-ray diffractometer and the CuKα radiation with a wavelength of 0.15406 nm. The same apparatus was exploited to determine the thickness, density, and roughness of the films by X-ray reflectometry (XRR). Energy dispersive X-ray spectrometry (EDS) measurements were carried out at an accelerating voltage of 15 kV with a current of 0.69 nA using an INCA Energy 350 EDS spectrometer (Oxford Instruments, Abingdon, Oxfordshire, UK) connected to a Helios Nanolab 600 (FEI) scanning electron microscope. Scanning transmission electron microscopy (STEM) and elemental mapping of the films in cross-sectional orientation were performed in a Cs-corrected Titan Themis 200 microscope (FEI, Hillsboro, OR, USA). EDS maps were acquired using Esprit software version 1.9 (Bruker, Billerica, MA, USA). Thin cross-sectional samples for STEM observations were prepared using the in situ lift-out technique using a Helios Nanolab 600 scanning electron microscope/focused ion beam system (FEI, Hillsboro, OR, USA), equipped with a super-X EDX system (FEI/Bruker). In order to protect the surface from ion milling during the preparation of STEM samples, the area of interest was covered with a platinum protection layer. An X-ray fluorescence (XRF) analyzer ZSX400 (Rigaku) was complementarily used for the elemental composition analysis. Considering the feasibility of the measurements, the composition analysis was conducted on Fe2O3 and HfO2 reference films grown to somewhat higher thicknesses using 500 ALD cycles, whereas thinner films grown using 200 cycles to thicknesses similar to those of nanolaminates were further subjected to electrical and magnetic measurements. Figure 1. Contents of elements (a) and residual impurities (b) measured by XRF, constituting the HfO2-Fe2O3 films, expressed in atomic% against relative amount of HfO2 deposition cycles. The elements are indicated in the legends. Polynomial lines are guides for the eye. Electrical measurements were carried out in a probe station using a Keithley 4200- SCS semiconductor analyzer (Keysight Technologies, Cleveland, OH, USA). In the DC measurements, the bias voltage was applied to the top electrode, and the bottom electrode remained grounded. To initiate RS, every sample required an electroforming procedure that was carried out as a voltage sweep with positive bias and a current compliance of 10 0.0 0.2 0.4 0.6 0.8 1.0 0 15 30 45 60 75 Content, atomic per cents HfO2/(HfO2+Fe2O3) cycle ratio O, at.% Fe, at.% Hf, at.% (a) 0.0 0.2 0.4 0.6 0.8 1.0 0.01 0.1 1 Chlorine content, atomic per cents HfO2/(HfO2+Fe2O3) cycle ratio (b) Figure 1. Contents of elements ( a ) and residual impurities ( b ) measured by XRF, constituting the HfO 2 -Fe 2 O 3 films, expressed in atomic% against relative amount of HfO 2 deposition cycles. The elements are indicated in the legends. Polynomial lines are guides for the eye. Nanomaterials 2022,12, 2593 4 of 18 Table 1. List of HfO 2 , Fe 2 O 3 , and HfO 2 -Fe 2 O 3 samples revealing the deposition cycle sequences, relative HfO 2 /(HfO 2 + Fe 2 O 3 ) deposition cycle ratio, total film thickness in accord with XRR, and relative Hf/(Hf + Fe) cation ratio measured by XRF and EDS. The samples are presented in the order of decreasing relative cycle ratio for HfO2and, concurrently, descending cation ratio for Hf, measured by XRF. For the contents of residual chlorine, also measured by XRF, see Figure 1. Cycle Sequence Cycle Ratio ttotal, nm Hf/(Hf + Fe) by XRF Hf/(Hf + Fe) by EDS 500 ×HfO21 54 1 1 2×(150 ×HfO2+50 ×Fe2O3) 0.75 23 0.96 (0.03) 0.98 100 × HfO 2 + 100 × Fe 2 O 3 + 100 × HfO 20.67 27 0.91 (0.04) 0.95 2×(100 ×HfO2+100 ×Fe2O3) 0.5 26 0.88 (0.02) 0.91 2×(50 ×HfO2+150 ×Fe2O3) 0.25 29 0.51 (0.01) 0.49 400 ×Fe2O3+100 ×HfO20.2 63 0.21 (0.01) 0.24 500 ×Fe2O30 37 0 0 The films were grown on undoped Si(100) and, for the electrical evaluation, also on highly doped conductive Si substrates covered by a 10 nm TiN film. The conductive Si wafers were boron-doped to concentrations of 5 × 10 18 − 1 × 10 19 cm –3 and coated with crystalline TiN layer by pulsed chemical vapor deposition using a batch TiCl 4 /NH 3 process [ 32 , 33 ] at temperatures of 450–500 ◦ C in an ASM A412 Large Batch 300 mm reactor at Fraunhofer IPMS-CNT. The films, which were deposited on TiN substrates for electrical measurements, were also supplied with Ti/Au electron-beam evaporated electrodes on top of the films, with the Ti layer directly contacting the switching oxide medium and Au deposited in order to provide non-oxidizing electrical contact to the measurement circuit. The crystal structure of the films was evaluated by grazing incidence X-ray diffraction (GIXRD) method using a SmartLab (Rigaku, Tokyo, Japan) X-ray diffractometer and the CuK α radiation with a wavelength of 0.15406 nm. The same apparatus was exploited to determine the thickness, density, and roughness of the films by X-ray reflectometry (XRR). Energy dispersive X-ray spectrometry (EDS) measurements were carried out at an accelerating voltage of 15 kV with a current of 0.69 nA using an INCA Energy 350 EDS spectrometer (Oxford Instruments, Abingdon, Oxfordshire, UK) connected to a Helios Nanolab 600 (FEI) scanning electron microscope. Scanning transmission electron microscopy (STEM) and elemental mapping of the films in cross-sectional orientation were performed in a Cs-corrected Titan Themis 200 microscope (FEI, Hillsboro, OR, USA). EDS maps were acquired using Esprit software version 1.9 (Bruker, Billerica, MA, USA). Thin cross-sectional samples for STEM observations were prepared using the in situ lift-out technique using a Helios Nanolab 600 scanning electron microscope/focused ion beam system (FEI, Hillsboro, OR, USA), equipped with a super-X EDX system (FEI/Bruker). In order to protect the surface from ion milling during the preparation of STEM samples, the area of interest was covered with a platinum protection layer. An X-ray fluorescence (XRF) analyzer ZSX400 (Rigaku) was complementarily used for the elemental composition analysis. Considering the feasibility of the measurements, the composition analysis was conducted on Fe 2 O 3 and HfO 2 reference films grown to somewhat higher thicknesses using 500 ALD cycles, whereas thinner films grown using 200 cycles to thicknesses similar to those of nanolaminates were further subjected to electrical and magnetic measurements. Electrical measurements were carried out in a probe station using a Keithley 4200- SCS semiconductor analyzer (Keysight Technologies, Cleveland, OH, USA). In the DC measurements, the bias voltage was applied to the top electrode, and the bottom electrode remained grounded. To initiate RS, every sample required an electroforming procedure that was carried out as a voltage sweep with positive bias and a current compliance of 10 µ A to avoid irreversibly breaking the device. Capacitance measurements were performed by applying an AC signal of 30 mV along with a DC bias of 0.1 V in the Nanomaterials 2022,12, 2593 5 of 18 frequency range 1–1000 kHz. Magnetic measurements were performed using the Vibrating Sample Magnetometer (VSM) option of the Physical Property Measurement System 14T (Quantum Design, San Diego, CA, USA) by scanning the magnetic field from − 1.5 to 1.5 T parallel to the film surface in the temperature range of 5–350 K. 3. Results and Discussion 3.1. Growth and Structure The metal oxide films constituting the nanolaminates were grown in processes under the same reactor conditions and exploiting the same precursor chemistry as those earlier suited to the growth of HfO2[34] and Fe2O3[35] films. In the present study, the thickness of multilayered HfO 2 -Fe 2 O 3 films could be appreciably well controlled by adjusting the amounts of the growth cycles for constituent oxide layers (Table 1). In addition, the presence of iron and hafnium in the films was proven by both XRF and EDS analysis, whereby the relative contents of both metals were correlated to the relative amounts of their growth cycles applied (Table 1, Figure 1a). Further, chlorine and carbon were detected as impurities present in the films (Figure 1b). Both impurities can be regarded as natural residues arising from the ligands to the metal precursors. Within the accuracy limits of the analysis method, the content of Cl and C was not systematically dependent on the relative deposition cycle ratio. The HfO 2 -Fe 2 O 3 multilayers were truly formed as nanolaminates, as proven by XRR results depicted in Figure 2. The X-ray reflection intensity curves allowed one to fit the measured data, in a good approximation, with the predicted thicknesses of the constituent layers of both metal oxides, roughly correlated to the amounts of deposition cycles applied in the case of both HfO 2 and Fe 2 O 3 . It is well known that in the case of ALD, the growth rates of materials on substrates of foreign composition may markedly differ at different growth stages, generally being essentially lower at the early stages of growth, i.e., at low thicknesses. The influence of the nucleation rate and the length of the so-called incubation period depends on temperature, substrate, and growing material, and should be explored separately, if required. In the present study, the growth rate of HfO 2 on Fe 2 O 3 noticeably exceeded the growth rate of Fe 2 O 3 on HfO 2 . In the case of the growth of HfO 2 on crystallized Fe 2 O 3 (Figure 2c), the growth rate of HfO 2 could reach as high as 0.18 nm/cycle, possibly supported by a larger specific surface area of underlying polycrystalline iron oxide layer. STEM studies revealed sequential deposition of the hafnium and iron oxide layers, distinct in terms of structure (Figure 3) and elemental distribution (Figure 4), thus supporting the XRR results. One can see that the layers were crystallized throughout the film thickness (Figure 3a,b) without structurally sharp interfaces between the HfO 2 and Fe 2 O 3 constituent layers, still enabling the distinction between metal oxides due to the differences in atomic numbers. At the same time, the interface between HfO 2 and the amorphous SiO 2 top layer on the Si substrate was sharply formed and distinct (Figure 3c). It is, however, to be noted that the growth rates and resulting thicknesses of component layers in nanolaminate structures are not to be compared to those of reference films. It is well-known that the films grown by ALD require an incubation time before achieving stable growth and structural formation. The length of the incubation period may vary considerably, depending on the material to be grown as well as the substrate material and structure. Nucleation at early stages, i.e., growth of HfO 2 on Fe 2 O 3 and vice versa would require a separate study. Compositionally, the constituent layers became clearly distinguishable (Figure 4), allowing one to rely on the formation of nanomaterial composed of physically and chemically different oxides, further enabling the appearance of both magnetic and insulating materials’ properties. Nanomaterials 2022,12, 2593 6 of 18 Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 20 Figure 2. X-ray reflectivity results for selected stacks of Fe2O3 and HfO2 layers grown on Si, denoted by the labels revealing the amounts of ALD cycles used for the deposition of constituent layers. The thicknesses of constituent layers as the results of the curve fittings are also given by labels. The curves with fitting results are presented for the four-layer laminate grown using equal amounts of cycles for both constituent oxides (a), the three-layer laminate grown using equal amounts of cycles for both constituent oxides (b), the double-layer consisting of relatively thicker Fe2O3 and thinner HfO2 films (c), and the four-layer laminate containing Fe2O3 layers relatively thicker compared to the HfO2 component (d). STEM studies revealed sequential deposition of the hafnium and iron oxide layers, distinct in terms of structure (Figure 3) and elemental distribution (Figure 4), thus supporting the XRR results. One can see that the layers were crystallized throughout the film thickness (Figure 3a,b) without structurally sharp interfaces between the HfO2 and Fe2O3 constituent layers, still enabling the distinction between metal oxides due to the differences in atomic numbers. At the same time, the interface between HfO2 and the amorphous SiO2 top layer on the Si substrate was sharply formed and distinct (Figure 3c). It is, however, to be noted that the growth rates and resulting thicknesses of component layers in nanolaminate structures are not to be compared to those of reference films. It is wellknown that the films grown by ALD require an incubation time before achieving stable growth and structural formation. The length of the incubation period may vary considerably, depending on the material to be grown as well as the substrate material and structure. Nucleation at early stages, i.e., growth of HfO2 on Fe2O3 and vice versa would require a separate study. Compositionally, the constituent layers became clearly distinguishable (Figure 4), allowing one to rely on the formation of nanomaterial composed of physically and chemically different oxides, further enabling the appearance of both magnetic and insulating materials’ properties. 1 2 3 4 5 6 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1002 x( 100x HfO2 + 100 x Fe2O3) 10.2 nm HfO2 + 2.6 nm Fe2O3 + 10.3 nm HfO2 + 2.9 nm Fe2O3 intensity, a.u. degree (a) 1 2 3 4 5 6 10-6 10-5 10-4 10-3 10-2 10-1 100100x HfO2 + 100 x Fe2O3 + 100x HfO2 Intensity, a. u. degree 11.1 nm HfO2 + 4.2 nm Fe2O3 + 11.7 nm HfO2 (b) 1 2 3 4 5 6 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 400 x Fe2O3 + 100 x HfO2 45.1 nm Fe2O3 + 18.3 nm HfO2 intensity, a.u. degree (c) 1 2 3 4 5 6 10-6 10-5 10-4 10-3 10-2 10-1 1002 x( 50x HfO2 + 150 x Fe2O3) 4.4 nm HfO2 + 13.7 nm Fe2O3 + 1.3 nm HfO2 + 9.6 nm Fe2O3 intensity, a.u. degree (d) Figure 2. X-ray reflectivity results for selected stacks of Fe 2 O 3 and HfO 2 layers grown on Si, denoted by the labels revealing the amounts of ALD cycles used for the deposition of constituent layers. The thicknesses of constituent layers as the results of the curve fittings are also given by labels. The curves with fitting results are presented for the four-layer laminate grown using equal amounts of cycles for both constituent oxides ( a ), the three-layer laminate grown using equal amounts of cycles for both constituent oxides ( b ), the double-layer consisting of relatively thicker Fe 2 O 3 and thinner HfO 2 films ( c ), and the four-layer laminate containing Fe 2 O 3 layers relatively thicker compared to the HfO 2 component (d). Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 19 Figure 3. Bright field STEM images of HfO2-Fe2O3-HfO2 nanolaminate grown using 100 ALD cycles for each constituent layer, taken under different magnifications (a,b), and an image of the interface between silicon substrate and the first HfO2 layer in the same laminate (c). Figure 4. Elemental mapping for iron (a), hafnium (b), and oxygen (c) in the HfO2-Fe2O3-HfO2 nanolaminate grown using 100 ALD cycles for each constituent layer. The Fe2O3 films grown alone without alternate layering with HfO2 were moderately crystallized in their as-deposited states (Figure 5, the bottom pattern). Two weak but still distinct reflection peaks at 33.6 and 56.5° could be attributed to the 104 and 116 reflections of rhombohedral Fe2O3, that is, the hematite phase. At the same time, the HfO2 films grown alone without alternate layering with Fe2O3 were relatively more crystallized in their as-deposited states (Figure 5, the 2nd pattern from bottom). The HfO2 film grown using 200 deposition cycles could be described as a multiphase solid medium consisting of a stable monoclinic phase of HfO2 and a metastable, quite likely tetragonal, polymorph of HfO2. Whereas most of the reflection peaks remained very weak, the most distinct reflections unambiguously belonging to the −111 and 111 of monoclinic HfO2 peaked at 28.3 and 31.5°, respectively. Between the latter reflections, a peak assigned as 101 of tetragonal HfO2 was clearly detected at 30.3°. These three neighboring reflections can be regarded as proof of multiphase composition. Further and notably, after 500 ALD cycles, the metastable phases, if initially formed and present in HfO2 films, were already almost insignificant in the diffraction patterns. The diffractogram from the HfO2 film grown using 500 cycles comprised reflections attributed exclusively to monoclinic HfO2 (Figure 5, the 3rd pattern from bottom). Figure 3. Bright field STEM images of HfO 2 -Fe 2 O 3 -HfO 2 nanolaminate grown using 100 ALD cycles for each constituent layer, taken under different magnifications ( a , b ), and an image of the interface between silicon substrate and the first HfO2layer in the same laminate (c). Nanomaterials 2022,12, 2593 7 of 18 Nanomaterials 2022, 12, x FOR PEER REVIEW 7 of 19 Figure 3. Bright field STEM images of HfO2-Fe2O3-HfO2 nanolaminate grown using 100 ALD cycles for each constituent layer, taken under different magnifications (a,b), and an image of the interface between silicon substrate and the first HfO2 layer in the same laminate (c). Figure 4. Elemental mapping for iron (a), hafnium (b), and oxygen (c) in the HfO2-Fe2O3-HfO2 nanolaminate grown using 100 ALD cycles for each constituent layer. The Fe2O3 films grown alone without alternate layering with HfO2 were moderately crystallized in their as-deposited states (Figure 5, the bottom pattern). Two weak but still distinct reflection peaks at 33.6 and 56.5° could be attributed to the 104 and 116 reflections of rhombohedral Fe2O3, that is, the hematite phase. At the same time, the HfO2 films grown alone without alternate layering with Fe2O3 were relatively more crystallized in their as-deposited states (Figure 5, the 2nd pattern from bottom). The HfO2 film grown using 200 deposition cycles could be described as a multiphase solid medium consisting of a stable monoclinic phase of HfO2 and a metastable, quite likely tetragonal, polymorph of HfO2. Whereas most of the reflection peaks remained very weak, the most distinct reflections unambiguously belonging to the −111 and 111 of monoclinic HfO2 peaked at 28.3 and 31.5°, respectively. Between the latter reflections, a peak assigned as 101 of tetragonal HfO2 was clearly detected at 30.3°. These three neighboring reflections can be regarded as proof of multiphase composition. Further and notably, after 500 ALD cycles, the metastable phases, if initially formed and present in HfO2 films, were already almost insignificant in the diffraction patterns. The diffractogram from the HfO2 film grown using 500 cycles comprised reflections attributed exclusively to monoclinic HfO2 (Figure 5, the 3rd pattern from bottom). Figure 4. Elemental mapping for iron ( a ), hafnium ( b ), and oxygen ( c ) in the HfO 2 -Fe 2 O 3 -HfO 2 nanolaminate grown using 100 ALD cycles for each constituent layer. The Fe 2 O 3 films grown alone without alternate layering with HfO 2 were moderately crystallized in their as-deposited states (Figure 5, the bottom pattern). Two weak but still distinct reflection peaks at 33.6 and 56.5 ◦ could be attributed to the 104 and 116 reflections of rhombohedral Fe 2 O 3 , that is, the hematite phase. At the same time, the HfO 2 films grown alone without alternate layering with Fe 2 O 3 were relatively more crystallized in their as-deposited states (Figure 5, the 2nd pattern from bottom). The HfO 2 film grown using 200 deposition cycles could be described as a multiphase solid medium consisting of a stable monoclinic phase of HfO 2 and a metastable, quite likely tetragonal, polymorph of HfO 2 . Whereas most of the reflection peaks remained very weak, the most distinct reflections unambiguously belonging to the − 111 and 111 of monoclinic HfO 2 peaked at 28.3 and 31.5 ◦ , respectively. Between the latter reflections, a peak assigned as 101 of tetragonal HfO 2 was clearly detected at 30.3 ◦ . These three neighboring reflections can be regarded as proof of multiphase composition. Further and notably, after 500 ALD cycles, the metastable phases, if initially formed and present in HfO 2 films, were already almost insignificant in the diffraction patterns. The diffractogram from the HfO 2 film grown using 500 cycles comprised reflections attributed exclusively to monoclinic HfO 2 (Figure 5, the 3rd pattern from bottom). Nanomaterials 2022, 12, x FOR PEER REVIEW 8 of 19 15 20 25 30 35 40 45 50 55 60 65 70 110 M 102 M 130 M 200 T -102 M 500 x HfO 2 116 R 100 x HfO 2 +100 x Fe 2 O 3 +100 x HfO 2 200 x HfO 2 Intensity, a.u. 2θ, degree 200 x Fe 2 O 3 400 x Fe 2 O 3 +100 x HfO 2 2 x (150 x HfO 2 + 50 x Fe 2 O 3 ) 2 x (50 x HfO 2 +150 x Fe 2 O 3 ) 2 x (100 x HfO 2 +100 x Fe 2 O 3 ) -111 M 111 M 101 T 104 R 020 M 002 T 112 T Figure 5. Grazing incidence X-ray diffraction patterns of Fe2O3-, HfO2-, and HfO2-Fe2O3-laminated films. The growth cycle sequences are denoted by labels. The reflections supplied with Miller indexes are assigned as those belonging to either monoclinic (M, ICDD PDF-2 card no 43-1017) or tetragonal (T, card 01-078-5756) HfO2, whereby reflections from rhombohedral hematite Fe2O3 are denoted by R (card 01-1053). Somewhat surprisingly, in the diffractograms taken from most of the films consisting of stacked HfO2 and Fe2O3 layers, no reflections attributable to any of the known iron oxide phases could be recognized. At the same time, crystallization in the films was obvious and due to the crystal growth in the HfO2 corresponding to the stacks or multilayers. With regard to the reflections characteristic of HfO2, the significance of the ones attributable to −111 and 111 of monoclinic HfO2, peaking at 28.3 and 31.5°, respectively, tended to increase with the relative amounts of HfO2 deposition cycles (Figure 5). The relative significance of the reflection assigned as 101 of tetragonal HfO2, at 30.3°, quite expectedly increased with the decrease in the relative amount of the HfO2 deposition cycles (Figure 5). Notably, reflections characteristic of Fe2O3 did not appear in the stacked HfO2 and Fe2O3 layers, with an exception of the sample grown using the cycle sequence of 400 × Fe2O3 + 100 × HfO2, where a weak 104 peak of rhombohedral hematite phase could be recognized at 33.5°. 3.2. Magnetization Behavior The reference Fe2O3 film grown on diamagnetic Si substrates expectedly demonstrated ferromagnetic-like magnetization behavior (Figure 6a) with the coercive force measured as strong as 2272 Oe at 5 K. The thinnest 24 nm-thick HfO2 films grown in the present study using 200 ALD cycles were magnetized nonlinearly and saturatively in external fields (Figure 6a). Both the saturation magnetization and the coercive field remained low, although clearly measurable, in the 24 nm-thick HfO2 film compared to those of the Fe2O3 films. At the same time, the saturation magnetization and coercivity in the 54 nm-thick HfO2, grown using 500 cycles, became suppressed almost entirely. This is plausibly due to the obvious difference between phase compositions of 24 and 54 nm-thick HfO2 films (Figure 5). The thinner HfO2 film contained, besides stoichiometric monoclinic HfO2, probably also oxygen-deficient metastable either tetragonal or cubic HfO2, which would give rise to the magnetization. Upon an increase in the film thickness and crystal growth, the formation of dominant monoclinic dioxide caused a suppression of the magnetization in the solid material. At room temperature, the saturation magnetization values were not significantly decreased, differently from coercitivities. Upon increasing the measurement temperatures from 5 to 300 K, the coercivities were decreased nearly 10, 4, Figure 5. Grazing incidence X-ray diffraction patterns of Fe 2 O 3- , HfO 2- , and HfO 2 -Fe 2 O 3 -laminated films. The growth cycle sequences are denoted by labels. The reflections supplied with Miller indexes are assigned as those belonging to either monoclinic (M, ICDD PDF-2 card no 43-1017) or tetragonal (T, card 01-078-5756) HfO 2 , whereby reflections from rhombohedral hematite Fe 2 O 3 are denoted by R (card 01-1053). Nanomaterials 2022,12, 2593 8 of 18 Somewhat surprisingly, in the diffractograms taken from most of the films consisting of stacked HfO 2 and Fe 2 O 3 layers, no reflections attributable to any of the known iron oxide phases could be recognized. At the same time, crystallization in the films was obvious and due to the crystal growth in the HfO 2 corresponding to the stacks or multilayers. With regard to the reflections characteristic of HfO 2 , the significance of the ones attributable to − 111 and 111 of monoclinic HfO 2 , peaking at 28.3 and 31.5 ◦ , respectively, tended to increase with the relative amounts of HfO 2 deposition cycles (Figure 5). The relative significance of the reflection assigned as 101 of tetragonal HfO 2 , at 30.3 ◦ , quite expectedly increased with the decrease in the relative amount of the HfO 2 deposition cycles (Figure 5). Notably, reflections characteristic of Fe 2 O 3 did not appear in the stacked HfO 2 and Fe 2 O 3 layers, with an exception of the sample grown using the cycle sequence of 400 ×Fe2O3+ 100 ×HfO2, where a weak 104 peak of rhombohedral hematite phase could be recognized at 33.5◦. 3.2. Magnetization Behavior The reference Fe 2 O 3 film grown on diamagnetic Si substrates expectedly demonstrated ferromagnetic-like magnetization behavior (Figure 6a) with the coercive force measured as strong as 2272 Oe at 5 K. The thinnest 24 nm-thick HfO 2 films grown in the present study using 200 ALD cycles were magnetized nonlinearly and saturatively in external fields (Figure 6a). Both the saturation magnetization and the coercive field remained low, although clearly measurable, in the 24 nm-thick HfO 2 film compared to those of the Fe 2 O 3 films. At the same time, the saturation magnetization and coercivity in the 54 nm-thick HfO 2 , grown using 500 cycles, became suppressed almost entirely. This is plausibly due to the obvious difference between phase compositions of 24 and 54 nm-thick HfO 2 films (Figure 5). The thinner HfO 2 film contained, besides stoichiometric monoclinic HfO 2 , probably also oxygen-deficient metastable either tetragonal or cubic HfO 2 , which would give rise to the magnetization. Upon an increase in the film thickness and crystal growth, the formation of dominant monoclinic dioxide caused a suppression of the magnetization in the solid material. At room temperature, the saturation magnetization values were not significantly decreased, differently from coercitivities. Upon increasing the measurement temperatures from 5 to 300 K, the coercivities were decreased nearly 10, 4, and 2 times in the Fe 2 O 3 film grown using 200 and 500 cycles, and in HfO 2 film grown using 200 cycles, respectively (Figure 6b) With regard to the HfO2films, undoped and crystallized hafnium dioxide is not supposed to magnetize in its bulk and stoichiometric form, and contamination by handling with stainless-steel tweezers may sometimes have led to measurable magnetic signals [ 36 ]. However, magnetization earlier unexpectedly detected in HfO 2 films [ 37 ] can intentionally be induced, as supported by the presence of defects, in the first place oxygen vacancies [19,20]. Oxygen vacancies are inevitable constituents in the metal oxide lattices, also considered as a cause of the filamentary switching mechanism [38–40] in the device cells. In the samples in which Fe 2 O 3 and HfO 2 films were grown into double, triple, or fourlayered stacks, the saturation magnetization values obtained at both 5 and 300 K tended, somewhat expectedly, to be the highest in the samples where the amount of sequential Fe 2 O 3 growth cycles exceeded those applied for the HfO 2 by 3–5 times (Figure 6c). However, in the case of laminated films, one should take into account that the microstructure of the individual Fe 2 O 3 layers, most strongly affecting the magnetic properties, also varies due to the thickness variation (Figure 2). For instance, in the double-layered (400 × Fe 2 O 3 + 100 × HfO 2 ) sample, the Fe 2 O 3 was moderately crystallized, whereas in all other laminated samples, Fe 2 O 3 was X-ray-amorphous (Figure 5). Notably, the thickness of Fe 2 O 3 in the double layer was nearly 10 times higher compared to that in the three- and four-layer samples when 100 cycles of Fe 2 O 3 was applied (see Figure 2a,b). Apparently, in the latter samples, smaller Fe 2 O 3 nanocrystals could form and possibly agglomerate. The coercivity value measured at 5 K for the film consisting of two double layers of HfO 2 and Fe 2 O 3 grown using 50 and 150 cycles, respectively, exceeded 1100 Oe. Coercivity in the double layer consisting of a Fe 2 O 3 film grown at first using 400 cycles, followed by HfO 2 grown using Nanomaterials 2022,12, 2593 9 of 18 100 cycles, reached nearly 1500 Oe. At room temperature, these values were decreased down to 65 and 271 Oe, respectively. Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 20 Aa Figure 6. Magnetization-field curves from reference HfO2 and Fe2O3 films measured at 5 K (a) and at 300 K (b), as compared to the curves from HfO-Fe2O3-laminated structures measured at 5 K (c) and 300 K (d). The films were grown on SiO2/Si substrates using cycle sequences represented by labels. With regard to the HfO2 films, undoped and crystallized hafnium dioxide is not supposed to magnetize in its bulk and stoichiometric form, and contamination by handling with stainless-steel tweezers may sometimes have led to measurable magnetic signals [36]. However, magnetization earlier unexpectedly detected in HfO2 films [37] can intentionally be induced, as supported by the presence of defects, in the first place oxygen vacancies [19,20]. Oxygen vacancies are inevitable constituents in the metal oxide lattices, also considered as a cause of the filamentary switching mechanism [38–40] in the device cells. In the samples in which Fe2O3 and HfO2 films were grown into double, triple, or fourlayered stacks, the saturation magnetization values obtained at both 5 and 300 K tended, somewhat expectedly, to be the highest in the samples where the amount of sequential Fe2O3 growth cycles exceeded those applied for the HfO2 by 3–5 times (Figure 6c). However, in the case of laminated films, one should take into account that the microstructure of the individual Fe2O3 layers, most strongly affecting the magnetic properties, also varies due to the thickness variation (Figure 2). For instance, in the double-layered (400 × Fe2O3 + 100 × HfO2) sample, the Fe2O3 was moderately crystallized, whereas in all other laminated samples, Fe2O3 was X-ray-amorphous (Figure 5). Notably, the thickness of Fe2O3 in the double layer was nearly 10 times higher compared to that in the three- and four-layer samples when 100 cycles of Fe2O3 was applied (see Figure 2a,b). Apparently, in the latter samples, smaller Fe2O3 nanocrystals could form and possibly agglomerate. The coercivity value measured at 5 K for the film consisting of two double layers of HfO2 and Fe2O3 grown using 50 and 150 cycles, respectively, exceeded 1100 Oe. Coercivity in the double layer consisting of a Fe2O3 film grown at first using 400 cycles, followed by HfO2 grown using 100 cycles, reached nearly 1500 Oe. At room temperature, these values were decreased down to 65 and 271 Oe, respectively. -10000 0 10000 -0.0025 0.0000 0.0025 200 x Fe2O3 200 x HfO2 at 5 K Moment, emu/g Magnetic field, Oe 500 x HfO2 (a) -10000 0 10000 -0.0025 0.0000 0.0025 -2000 -1000 0 1000 2000 -0.0015 -0.0010 -0.0005 0.0000 0.0005 0.0010 0.0015 Moment, emu/g Magnetic field, Oe 200 x HfO2 Moment, emu/g Magnetic field, Oe at 300 K 200 x Fe2O3 500 x HfO2 200 x HfO2 (b) -10000 0 10000 -0.005 0.000 0.005 2 x (150 x HfO2 100 x HfO2 +100 x HfO2 + 150 x Fe2O3) + 50 x Fe2O3) + 100 x Fe2O3) 2 x (100 x HfO2 +100 x Fe2O3 at 5 K Moment, emu/g Magnetic field, Oe 400 x Fe2O3 + 100 x HfO2 2 x (50 x HfO2 (c) -10000 0 10000 -0.005 0.000 0.005 + 150 x Fe2O3) + 50 x Fe2O3) + 100 x Fe2O3) + 100 x HfO2 + 100 x HfO2 Moment, emu/g Magnetic field, Oe at 300 K 2 x (100 x HfO2 100x HfO2+100x Fe2O3 2 x (150 x HfO2 400 x Fe2O3 2 x (50 x HfO2 -600-400 -200 0 200 400 600 -0.0006 -0.0004 -0.0002 0.0000 0.0002 0.0004 0.0006 2 x (100 x HfO2 + 100 x Fe2O3) moment, emu/g magnetic field, Oe at 300 K (d) Figure 6. Magnetization-field curves from reference HfO 2 and Fe 2 O 3 films measured at 5 K ( a ) and at 300 K (b), as compared to the curves from HfO-Fe2O3-laminated structures measured at 5 K (c) and 300 K (d). The films were grown on SiO2/Si substrates using cycle sequences represented by labels. Despite the ability of nanocrystalline HfO 2 to moderately magnetize, also revealing hysteretic performance, the magnetization in terms of both saturation and coercive force was quite naturally dominated by Fe 2 O 3 constituting the nanolaminates. Higher amounts of iron in the iron-hafnium oxide multilayers certainly caused increments in both saturation magnetization (Figure 7a) and coercivity (Figure 7b). Interestingly, relatively strong coercivities among nanolaminate samples, reaching nearly 1850 Oe at 5 K and remaining below 50 Oe at 300 K, were obtained in the triple HfO 2 -Fe 2 O 3 -HfO 2 layer and in four-layer film consisting of two HfO 2 -Fe 2 O 3 double layers (Figure 6c). These samples were characterized by Hf/(Hf + Fe) ratios of 0.91 and 0.88, respectively (Figure 7b, Table 1). In the latter two samples, all the constituent metal oxide layers were grown using 100 ALD cycles. At the same time, the saturation magnetization in the same samples was decreased about five times below the values characterizing the films containing Fe 2 O 3 layers grown using 150 and 400 cycles, described above. It is thus possible that despite the relatively low Fe 2 O 3 amount in such laminates and accompanying weak saturation magnetization, the growth of constituent oxides in such nanolaminates after application of sufficient amounts of deposition cycles has enabled the ordering and growth of nanocrystals enhancing structure or shape anisotropy and a simultaneous increment in coercivity. Further and more detailed studies including parametrization and scaling up the process would be required in order to clarify the interdependencies between deposition cycle numbers, crystallographic orientation, and magnetic performance. Nanomaterials 2022,12, 2593 16 of 18 Author Contributions: Conceptualization, K.K. (Kristjan Kalam) and K.K. (Kaupo Kukli); methodology, K.K. (Kristjan Kalam); software, G.V., J.L., J.M.L., R.R. and M.O.; formal analysis, G.V., J.L., R.R. and A.T. (Aivar Tarre); investigation, R.R., G.V., J.M.L., M.O., A.K. and J.K.; resources, R.S., S.D., H.C., A.T. (Aile Tamm) and K.K. (Kaupo Kukli); writing—original draft preparation, K.K. (Kristjan Kalam), K.K. (Kaupo Kukli), G.V. and J.L.; visualization, G.V. and A.T. (Aivar Tarre); project administration, K.K. (Kaupo Kukli), A.T. (Aile Tamm), S.D. and H.C.; funding acquisition, K.K. (Kaupo Kukli), A.T. (Aile Tamm), R.S. and H.C. All authors have read and agreed to the published version of the manuscript. Funding: The present study was partially supported by the European Regional Development Fund project “Emerging orders in quantum and nanomaterials” (TK134), the Spanish Ministry of Economy and Competitiveness (TEC2017-84321-C4-2-R) with the support of Feder Funds, and the Estonian Research Agency (PRG753, PRG4). Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. References 1. Wolf, S.A.; Awschalom, D.D.; Buhrman, R.A.; Daughton, J.M.; von Molnár, S.; Roukes, M.L.; Chtchelkanova, A.Y.; Treger, D.M. Spintronics: A spin-based electronics vision for the future. Science 2001,294, 1488–1495. [CrossRef] [PubMed] 2. Waser, R.; Aono, M. Nanoionics-based resistive switching memories. Nat. Mater. 2007,6, 833–840. [CrossRef] [PubMed] 3. 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