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.
