Periodic Reporting for period 1 - MODERN (Multiferroic Oxide 2DEgs for Reconfigurable nonvolatile memory Nanodevices (MODERN))
Reporting period: 2023-10-01 to 2025-09-30
The main idea of Multiferroic Oxide 2DEgs for Reconfigurable nonvolatile memory Nanodevices (MODERN) project is the realization of novel Ferroelectric (FE) and multiferroic oxide two-dimensional electron gases (2DEGs) characterized by a metal-insulating transition as a function of the FE-polarization direction. By locally switching-on and off the 2DEG using a scanning probe, devices with arbitrary nanoscale geometry will be demonstrated and sketched in a non-volatile manner, to realize prototype FE field effect transistors (FET), spin-FET and spin-charge conversion circuits for the generation and detection of spin-currents, all operating at ultra-low power energy density.
To achieve the above objectives, two heterostructures platform will be studied and realized:
1. K0.5Na0.5NbO3 (KNN)/KTaO3 2DEG, where KNN is a well-known lead-free FE layer and KTaO3 is an oxide characterized by a large spin-orbit coupling (up to 0.4 eV), that develops an interfacial 2DEG. Here KNN can both act as FE-layer with large polarization (up to 30 μC/cm2) and as interfacial donor of carriers by Nb-doping at the interface.
2. Delta-doped LAO/SrVO3 (down to 2 unit cells (UCs))/SrTiO3. Here, two UCs of VO2-terminated SrVO3 have been theoretically predicted to become a 2D multiferroic, with in plane FE order and an antiferromagnetic magnetic order.
The current project has enriched of understanding the physical control mechanisms like interface engineering, spin-orbit coupling (SOC) etc. Furthermore, the project directly supports to establish 2D oxide materials as an important promising candidate for quantum oxide electronics. 2DEG-oxides could become an important brick in future electronics and Europe is in a position to retain its leadership in this field. For this purpose, the MODERN project will empower the EU investments in high-level education of high skilled, young researchers working in the oxide electronics, so that this expertise will become more appealing to the market and help to generate new kind of technological job-profiles. At the end of the project, we could realize the prototype FE field effect transistors (FET) for low power, faster read/write and high endurance non-volatile memory nanodevices.
To achieve the above objectives, two heterostructures platform will be studied and realized:
1. K0.5Na0.5NbO3 (KNN)/KTaO3 2DEG, where KNN is a well-known lead-free FE layer and KTaO3 is an oxide characterized by a large spin-orbit coupling (up to 0.4 eV), that develops an interfacial 2DEG. Here KNN can both act as FE-layer with large polarization (up to 30 μC/cm2) and as interfacial donor of carriers by Nb-doping at the interface.
2. Delta-doped LAO/SrVO3 (down to 2 unit cells (UCs))/SrTiO3. Here, two UCs of VO2-terminated SrVO3 have been theoretically predicted to become a 2D multiferroic, with in plane FE order and an antiferromagnetic magnetic order.
The current project has enriched of understanding the physical control mechanisms like interface engineering, spin-orbit coupling (SOC) etc. Furthermore, the project directly supports to establish 2D oxide materials as an important promising candidate for quantum oxide electronics. 2DEG-oxides could become an important brick in future electronics and Europe is in a position to retain its leadership in this field. For this purpose, the MODERN project will empower the EU investments in high-level education of high skilled, young researchers working in the oxide electronics, so that this expertise will become more appealing to the market and help to generate new kind of technological job-profiles. At the end of the project, we could realize the prototype FE field effect transistors (FET) for low power, faster read/write and high endurance non-volatile memory nanodevices.
As of the first year of the project, the main goal of Work Package 1 is the realization and optimization of novel FE-based 2DEGs. Here, we have optimized high-quality ferroelectric ultra-thin K0.5Na0.5NbO3 (KNN) films on two different substrates such as SrTiO3 (STO) (001) and KTaO3 (111) single crystal substrates. In this project, the above samples are grown using the pulsed laser deposition (PLD) technique.
KNN is composed by alternating (NbO2)1+ and (K(Na)O)1- planes along the [001] direction. According to theory, in KNbO3/ STO (001) heterostructures a 2DEG is realized only at the (NbO2)+/(SrO)0 interface at variance with respect to the LaAlO3 (LAO)/SrTiO3 (STO) system where a 2DEG forms at the (LaO)1+/(TiO2)0 interface. In the present study, we have checked the formation of 2DEGs on two terminated i.e. TiO2 and SrO terminated STO (001) substrates. According to the previous report, it has shown that the growth of 5-10 uc of buffer homo epitaxial STO layer on TiO2 terminated STO substrates turns to SrO terminated STO substrates. During the deposition, we have grown KNN thin films from a polycrystalline ceramic target on both TiO2 and SrO terminated STO (001) substrates and tested their structural and electrical properties.
The substrate temperature, laser energy density and pulse repetition rate were optimized at 650 °C, 1.35 J/cm2 and 1 Hz, respectively for KNN thin films and for homo epitaxial STO layer on STO, the temperature was set to 750 oC. During deposition, oxygen (O2) partial pressure of 2x10-2 mbar for STO layer and 5x10-2 mbar for KNN thin films grown on TiO2 terminated STO (001) was maintained inside the chamber. Before cooling down to room temperature, a few u.cs of LAO layer, which acts as a capping layer was grown at 500 oC with oxygen (O2) partial pressure of 1x10-5 mbar on KNN thin films.
The surface topography and electronic states of Nb cation in KNN are investigated by atomic force microscopy (AFM), X-ray absorption spectroscopy (XAS), respectively. Furthermore, the transport properties of KNN films on two terminated STO substrates are also carried out. Finally, the Nanoscale ferroelectric polarization switching of KNN thin films is explored using piezo force microscopy. Once we realised FE 2DEGs in KNN/STO heterostructures, we have also grown KNN thin films on single crystal KTO (111) substrate.
The main achievements of the last one year of the project:
1. 10 unit cells of homo epitaxial STO layer on TiO2 terminated STO substrates turns to SrO terminated one, which is confirmed by AFM and the presence of double peaks in the RHEED oscillation evolution during the growth.
2. XAS absorption spectrum of M3 t2g peak shifts by an energy of 0.3 eV to the higher energy for horizontally polarized light compared to vertically one, indicating the reduction of oxidation states from Nb5+ to the occupied Nb4+ 4d1 states in KNN/STO (001) heterostructures.
3. From electrical transport properties, the sample with buffer STO shows a metallic interface i.e. (NbO2)+/(SrO)0 interface and a semiconducting nature of KNN sample without buffered STO on TiO2 terminated STO substrate.
4. Nanoscale ferroelectric polarization switching of KNN thin films is confirmed using Piezo Force Microscopy
5. In situ x-ray photoemission (XPS) measurements show the reduction of Ta5+ into Ta4+, which indicates the formation of 2DEGs at the epitaxial KNN/KTO (111) interface.
KNN is composed by alternating (NbO2)1+ and (K(Na)O)1- planes along the [001] direction. According to theory, in KNbO3/ STO (001) heterostructures a 2DEG is realized only at the (NbO2)+/(SrO)0 interface at variance with respect to the LaAlO3 (LAO)/SrTiO3 (STO) system where a 2DEG forms at the (LaO)1+/(TiO2)0 interface. In the present study, we have checked the formation of 2DEGs on two terminated i.e. TiO2 and SrO terminated STO (001) substrates. According to the previous report, it has shown that the growth of 5-10 uc of buffer homo epitaxial STO layer on TiO2 terminated STO substrates turns to SrO terminated STO substrates. During the deposition, we have grown KNN thin films from a polycrystalline ceramic target on both TiO2 and SrO terminated STO (001) substrates and tested their structural and electrical properties.
The substrate temperature, laser energy density and pulse repetition rate were optimized at 650 °C, 1.35 J/cm2 and 1 Hz, respectively for KNN thin films and for homo epitaxial STO layer on STO, the temperature was set to 750 oC. During deposition, oxygen (O2) partial pressure of 2x10-2 mbar for STO layer and 5x10-2 mbar for KNN thin films grown on TiO2 terminated STO (001) was maintained inside the chamber. Before cooling down to room temperature, a few u.cs of LAO layer, which acts as a capping layer was grown at 500 oC with oxygen (O2) partial pressure of 1x10-5 mbar on KNN thin films.
The surface topography and electronic states of Nb cation in KNN are investigated by atomic force microscopy (AFM), X-ray absorption spectroscopy (XAS), respectively. Furthermore, the transport properties of KNN films on two terminated STO substrates are also carried out. Finally, the Nanoscale ferroelectric polarization switching of KNN thin films is explored using piezo force microscopy. Once we realised FE 2DEGs in KNN/STO heterostructures, we have also grown KNN thin films on single crystal KTO (111) substrate.
The main achievements of the last one year of the project:
1. 10 unit cells of homo epitaxial STO layer on TiO2 terminated STO substrates turns to SrO terminated one, which is confirmed by AFM and the presence of double peaks in the RHEED oscillation evolution during the growth.
2. XAS absorption spectrum of M3 t2g peak shifts by an energy of 0.3 eV to the higher energy for horizontally polarized light compared to vertically one, indicating the reduction of oxidation states from Nb5+ to the occupied Nb4+ 4d1 states in KNN/STO (001) heterostructures.
3. From electrical transport properties, the sample with buffer STO shows a metallic interface i.e. (NbO2)+/(SrO)0 interface and a semiconducting nature of KNN sample without buffered STO on TiO2 terminated STO substrate.
4. Nanoscale ferroelectric polarization switching of KNN thin films is confirmed using Piezo Force Microscopy
5. In situ x-ray photoemission (XPS) measurements show the reduction of Ta5+ into Ta4+, which indicates the formation of 2DEGs at the epitaxial KNN/KTO (111) interface.
In the first year of the MODERN project, we are able to realize a 2DEG at the KNN/STO (001) interface by using a buffer homoepitaxial SrTiO3 layer deposited on TiO2 terminated STO (001), and a LAO capping protecting the KNN surface. The buffer homoepitaxial SrTiO3 layer, deposited at 750°C, is mainly SrO terminated, thus allowing the creation of a (NbO2)+/(SrO)0 interface. Interestingly, without the buffer STO film, the system does not develop a 2DEG, as expected from theory. The growth of 10 unit cells of homo epitaxial STO layer on TiO2 terminated STO substrates turns to SrO terminated one, which is confirmed by double peaks in the RHEED oscillation evolution during the growth and AFM measurements [as shown in Fig. 1&2]. Electrical transport measurements (300-10 K) of the KNN sample grown on STO buffered TiO2 terminated STO substrates show metallic/free charge carriers at the (NbO2)+/(SrO)0 interface in the heterostructures. While, the samples grown on the other STO substrates do not have free charge ions at the (NbO2)+/(TiO2)0 interface and exhibit semiconductor behaviour [as shown in Fig. 3]. We have performed total electron yield (TEY) x-ray absorption spectroscopy (XAS) and x-ray linear dichroism (XLD) at the Ti L3,2 (Ti 2p → 3d transitions, 450-470 eV) and Nb M3,2 (Nb 3p → 4d transitions, 350-400 eV) -edges to establish the oxidation state and the crystal-field splitting of the 2DEG formed at the KNN/STO interface. XAS absorption spectrum of M3 t2g peak shifts by an energy of 0.3 eV to the higher energy for horizontally polarized light compared to vertically one, indicating the reduction of oxidation states from Nb5+ to the occupied Nb4+ 4d1 states in KNN/STO heterostructures [as shown in Fig. 4]. Finally, we have carried out the local polarization switching of KNN/STO heterostructures with and without STO buffered layer using piezo force microscopy (PFM) technique to confirm their ferroelectricity at the nanoscale [as shown in Fig. 5]. In addition, 6 unit cells of KNN thin films can grow epitaxially on (111) KTO substrate and realise a 2DEG at the KNN/KTO interface with a few layers of LAO as a capping layer to protect the KNN surface [as shown in Fig. 6]. Our initial, in situ x-ray photoemission (XPS) measurements indicate the reduction of Ta5+ into Ta4+, which directs the formation of 2DEGs at the KNN/KTO (111) interface [as shown in Fig. 7].
In the second/ final year, we target the realization of switchable 2DEG devices characterized by on/off FE polarization directions and the realization of devices using PFM. The nanodevices will be tested are single- and multi-channels side gate. Micro-bridges will be realized by standard optical lithography and ion-beam etching. To realize nanochannels and side-gates, we will use the PFM writing. By applying an opportune bias to the AFM-tip, we will locally switch-off the 2DEG, thus realizing nm-size channels (down to 100 nm). In addition, as back-plan and risk mitigation, we will also apply a fabrication-method based by standard e beam lithography technique.
The success of the current project will open the road towards an oxide electronics and will establish 2D FE oxide materials as an important platform, complementary to graphene and non-oxide 2D systems already included in the European technology roadmap. Furthermore, the unique ability to tune the polarization directions with an electric field, making them suitable for low power consumption, and high endurance next-generation non-volatile memory devices like memristors.
In the second/ final year, we target the realization of switchable 2DEG devices characterized by on/off FE polarization directions and the realization of devices using PFM. The nanodevices will be tested are single- and multi-channels side gate. Micro-bridges will be realized by standard optical lithography and ion-beam etching. To realize nanochannels and side-gates, we will use the PFM writing. By applying an opportune bias to the AFM-tip, we will locally switch-off the 2DEG, thus realizing nm-size channels (down to 100 nm). In addition, as back-plan and risk mitigation, we will also apply a fabrication-method based by standard e beam lithography technique.
The success of the current project will open the road towards an oxide electronics and will establish 2D FE oxide materials as an important platform, complementary to graphene and non-oxide 2D systems already included in the European technology roadmap. Furthermore, the unique ability to tune the polarization directions with an electric field, making them suitable for low power consumption, and high endurance next-generation non-volatile memory devices like memristors.