Artificial two-dimensional polar metal by charge transfer to a ferroelectric insulator

Integrating multiple properties in a single system is crucial for the continuous developments in electronic devices. However, some physical properties are mutually exclusive in nature. Here, we report the coexistence of two seemingly mutually exclusive properties-polarity and two-dimensional conductivity-in ferroelectric Ba$_{0.2}$Sr$_{0.8}$TiO$_3$ thin films at the LaAlO$_3$/Ba$_{0.2}$Sr$_{0.8}$TiO$_3$ interface at room temperature. The polarity of a ~3.2 nm Ba$_{0.2}$Sr$_{0.8}$TiO$_3$ thin film is preserved with a two-dimensional mobile carrier density of ~0.05 electron per unit cell. We show that the electronic reconstruction resulting from the competition between the built-in electric field of LaAlO$_3$ and the polarization of Ba$_{0.2}$Sr$_{0.8}$TiO$_3$ is responsible for this unusual two-dimensional conducting polar phase. The general concept of exploiting mutually exclusive properties at oxide interfaces via electronic reconstruction may be applicable to other strongly-correlated oxide interfaces, thus opening windows to new functional nanoscale materials for applications in novel nanoelectronics.


3
Oxide interfaces provide a fertile ground for multifunctional integration because the delicate balance between spin, orbit, charge and lattice degrees of freedom in oxides can be easily destabilized with relatively small stimuli [1][2][3][4] . Ground-breaking examples are the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 (LAO/STO) interface although ferromagnetism is expected to destroy the pairing interaction responsible for superconductivity [5][6] , and the coexistence of ferromagnetism and ferroelectricity at Fe/BaTiO3 interface although ferroelectricity normally requires empty d orbitals while ferromagnetism is usually a result of partially filled d orbitals 3,7 . Recently, polar metals, which possess another pair of seemingly mutually exclusive properties, ferroelectric-like polarity and conductivity, has attracted a flurry of interest. Traditionally, it was considered that introducing itinerant electrons into a ferroelectric can eliminate ferroelectricity and associated polarity as the electrons screen the long-range Coulomb forces, which favor a polar structure [8][9][10][11] . However, recent first-principles calculations show that the charge rearrangements associated to electrostatic screening induces local lattice response, which favors polar distortions and that local off-centering can be sustained up to ~0.1 electron per unit cell (e/uc) [11][12][13] , suggesting that there is no fundamental incompatibility between metallicity and polar distortions.
In recent decades, the research on polar metals has been mainly on three routes: (i) Searching for native polar metals, such as LiOsO3 14 , Bi5Ti5O17 15 , Cd2Re2O7 16 and WTe2 17 ; (ii) Doping charges into ferroelectric insulators, such as BaTiO3-  and CaTiO3- 12 ; and (iii) Stabilizing a polar phase in an otherwise non-polar metal (NdNiO3) by deliberate geometric design 18 .
However, most of the research was focused on bulk polar metals and the question of whether such coexistence can occur in a two-dimensional system remains largely unexplored. In this report, we directly show that polarity and two-dimensional electron gas (2DEG) can coexist in a single phase by charge transfer doping to a ferroelectric insulator. 4 A promising candidate to study this problem is the interface between a "polar" oxide (LAO) and a ferroelectric (Ba0.2Sr0.8TiO3). In essence, ferroelectricity arises from the charge separation between positive and negative ions, which create a spontaneous polarization and an internal electric field. Similar to the charge separation in ferroelectrics, in the "polar" oxide LAO, the physically separated alternating stacking of charge-positive (LaO) + and chargenegative (AlO2)layers also creates a built-in electric field. The possible charge transfer generated by the potential across LaAlO3/Ba0.2Sr0.8TiO3 (LAO/BST) interface created by the difference between the electric fields in LAO and BST can thus serve as a useful method to dope electrons into ferroelectric BST. This charge transfer is possible because the polardiscontinuity, which generates the 2DEG at the LAO/STO interface is also present at the LAO/BST interface as BST have a stacking of charge-neutral Ti 4+ O2 2and Sr 2+ O 2-(or Ba 2+ O 2-) layers similar to that of STO 19,20 . We use ferroelectric BST thin films instead of the conventional BaTiO3 (BTO) used in previous studies 10, 11 as we found that the Coulomb forces in BTO strongly localize electrons (see supplementary note 1 for more details), while doping Sr in BTO is known to weaken the ferroelectricity and hence improving electron mobility.

Results
Ferroelectricity of BST thin films on conducting Nb-doped STO substrate. We first check the ferroelectricity of the BST thin films by performing hysteresis loops, domain writing and reading experiments on a 10 uc BST/Nb:STO (001) sample using piezoresponse force microscopy at room temperature (Fig. 1a). In its bulk form, the Curie temperature of BST is 105 K 21 . However, we find that the as-grown BST film is ferroelectric at room temperature in a single-domain state with a downward spontaneous polarization (pointing from BST to Nb:STO). This large Curie temperature enhancement is probably a result of strain 22 and dimensional effects 23 . The 180 phase change between 4 and -7 V (Fig. 1b,c) and the good 5 domain writing and reading with both positive and negative voltages (Fig. 1d,e) indicate robust ferroelectricity in the BST thin film.
The -1.5 V voltage shift in the phase and amplitude hysteresis loops is due to the well-known imprint effect as commonly observed in other ferroelectric thin films 24-26 . This could be collectively caused by the following mechanisms: (i) Strain effect imposed on BST thin film by Nb:STO substrate 24 , (ii) Asymmetric electrostatic boundary condition caused by the different work functions between the two electrodes 25 , (iii) Defect dipole accumulation at the BST/Nb:STO interface due to interfacial diffusion of chemical species 26 . Note that in Fig. 1e, some partial relaxation is observed, which is attributed to polarization relaxation, as is commonly observed in other ferroelectric ultrathin films, such as BaTiO3 27 .
Ferroelectricity modulated electronic transport properties of the 2DEG. Having observed room temperature ferroelectricity in BST thin film, we grew BST and LAO successively on atomically flat TiO2-terminated STO (001) substrates by pulsed laser deposition to form a LAO/BST/STO heterostructure (see supplementary notes 2-4 for more details), where a 2DEG is observed at the LAO/BST interface. On one hand, the itinerant electrons introduced by the polar discontinuity at the LAO/BST interface tend to destroy the ferroelectricity and associated polarity of BST. On the other hand, the downward polarization of BST creates a downward electric field in LAO, which obstructs the polar-discontinuity-induced charge transfer from the LAO surface to the LAO/BST interface. The competition between the built-in electric field of LAO and the polarization of BST is exploited by varying the magnitude of the polarization of BST with different thicknesses. Figure 2 shows the electronic transport properties of a set of samples with fixed LAO thickness (15 uc) and different BST thicknesses. Figure 2a shows temperature dependent sheet resistance (Rs-T) for these metallic samples. It should be noted that a single layer of BST with varying thicknesses deposited on STO substrate is insulating, indicating that a LAO layer is required to obtain conductivity in LAO/BST/STO 6 heterostructure. Above 100 K, sheet resistance gradually increases with increasing BST thickness. However, a crossover in the Rs-T curves can be observed around 20-50 K and the sheet resistance dependence on BST thickness is reversed at low temperatures (2-20 K).
Carrier density gradually decreases with increasing BST thickness as the magnitude of the polarization increases (Fig. 2b) 27 . Another feature in Fig. 2b is that the temperature dependent carrier density (n-T) curves become less temperature dependent with increasing BST thickness and finally turns into (nearly) temperature independent when BST exceeds 8 uc (see supplementary notes 5-6 for more details). The decreasing carrier density with decreasing temperature in STO is attributed to freeze-out of oxygen-vacancy-induced carriers 20 . The decreasing carrier density dependence on temperature indicates that the oxygen vacancies in our samples decrease with increasing BST thickness. This is confirmed by the decreasing photoluminescence (PL) intensity with increasing BST thickness in our PL measurements (see supplementary note 7 for more details), where the PL intensity is proportional to the oxygen vacancy concentration in STO. In addition, theoretical calculations show that under reducing conditions, the oxygen vacancy formation energy in ferroelectric BTO is higher than that in STO 28 . Hence, we conclude that the oxygen vacancy formation energy in ferroelectric BST is higher than that in paraelectric STO. When the BST thickness is below 8 uc, a 2DEG resides in both STO and BST, and a large number of oxygen-vacancy-induced carriers are present in STO. In this case, the decreasing carrier density with increasing BST thickness is a result of both the electric field effect and reduced oxygen vacancy concentration. When the BST thickness exceeds 8 uc, the 2DEG lies only in the BST and few oxygen-vacancy-induced carriers are present, leading to observation of temperature-independent carrier density. In this case, the transport properties are influenced only by the competition between the built-in electric field of LAO and the polarization of BST without the influence of oxygen vacancies, signifying the coexistence of polarity and the 2DEG above 8 uc of BST. The carrier density of 7 the 8 uc sample is ~3×10 13 cm -2 , which corresponds to ~0.05 e/uc. Here, we estimate the thickness of the 2DEG to be 8 uc (~3.2 nm). This is in good agreement with previous observations, which showed that the thickness of the 2DEG at the LAO/STO interface is around 2-4 nm 29,30 . As discussed above, both carrier density and oxygen vacancies decrease with increasing BST thickness, leading to less electron-electron scattering and defect scattering, which consequently account for the mobility enhancement with increasing BST thickness (Fig.   2c,d).
We note the results in Fig. 2 are strikingly similar to the electric field gating experiments of LAO/STO interface 31,32 , suggesting that the 2DEG is modulated by the electric field provided by the ferroelectric BST. We also note that the upturn of the Rs-T curve and the corresponding decreasing carrier density with decreasing temperature of the 15 uc BST sample suggest carrier localization and is consistent with Mott variable-range hopping model (see supplementary note 8 for more details). This behavior could be due to the strong expulsion of electrons as the BST becomes thicker, as well as increasing interfacial disorder with increasing film thickness and strain effects introduced by substrate misfit as observed in epitaxial LAO/STO systems 33 . fitting of the experimental spectra (see supplementary note 10 for more details) 38 . Another way to trace the valence state of Ti is from the O K edge, which also shows significant changes when the Ti oxidation state changes across the interface (Fig. 3f) 37 . The energy difference between peak A and C (E) has been recognized as an accurate indicator of valence change in perovskites 37 , the decrease of E from BST to LAO/BST interface is evident in Fig. 3f-g and is related to a decrease in the Ti valence. These results unambiguously reflect a decrease of Ti valence from Ti 4+ in BST to Ti 3+ at the LAO/BST interface, indicating that the 2DEG lies at the LAO/BST interface and the carrier density of the 2DEG decreases away from the LAO/BST interface into the BST (Fig. 3g). The STEM and EELS results directly show that the polar displacements and Ti 3+ (i.e. excess electrons) coexist at the LAO/BST interface in a single phase without any detectable phase separation. Our first-principles calculations were performed using density-functional theory based Vienna ab initio Simulation Package (VASP) 39,40 with the local density approximation (LDA) for the exchange-correlation functional 41 and the frozen-core all-electron projector-augmented wave (PAW) method 42,43 . The cutoff energy for the plane wave expansion is set to 400 eV.
The electrons mainly locate at Ti atoms in both Ba0.5Sr0.5TiO3 and SrTiO3 with the largest charge density at LaAlO3/Ba0.5Sr0.5TiO3 interface (0.21 e/uc), and decays gradually from this interface into SrTiO3 (Fig. 4a). This result agrees both qualitatively and quantitatively with the Ti 3+ fraction analysis in Fig. 3f. The B-OII and A-OI displacements, which are in downward polarization (see supplementary note 11 for more details), are largest at the LaAlO3/Ba0.5Sr0.5TiO3 interface and gradually decreases into LaAlO3 and SrTiO3 (Fig 4b). The trend of the calculated layer-dependent displacements well reproduces the STEM results in Fig.   3b. We note that the absolute values of the calculated displacements are around one half of those from experimental observations. This may be attributed to the underestimation of lattice constant and polarization by LDA functional, possible defects in the samples and/or measurement errors in STEM-extracted displacements. Figure 4c shows the structural guide of the supercell used in the calculations.

Discussion
Several mechanisms may contribute to the behaviour of the layer-dependent displacements observed in Fig. 3  compressive strain, which has been reported to be a useful way to increase the critical electron density in BTO 44 . We also note that the carrier density calculated from DFT and that extracted 11 from EELS measurement are larger than the experimental observations (0.05 e/uc). This is probably because that some of the charges are localized in the first few layers of BST at LAO/BST interface as commonly observed at LAO/STO interface 29,30 . To disentangle these inter-correlated mechanisms, more detailed experimental and theoretical work needs to be done.
Nevertheless, the overall effect in Fig. 3 and 4 clearly suggests the coexistence of 2DEG and polarity in a single phase.
Our discoveries demonstrate a route to create a two-dimensional polar metal at oxide interface through interfacial electronic reconstruction, which is achieved by deliberately engineering the competition of the electric fields between a ferroelectric and a "polar" oxide. As coexistence of ferromagnetism and superconductivity has already been demonstrated at the LAO/STO interface 5,6 , the integration of polarity further expands the functionality of this interface and offers new opportunities for future multifunctional devices. Moreover, the ferroelectric soft phonons could be utilized to stabilize the superconducting phase at elevated temperatures 45 .
We notice that during the review process of our work, a similar work by Cao et al. reported the coexistence of polarity and 2DEG in a tri-color BaTiO3/SrTiO3/LaTiO3 heterostructure and we highly recommend this work to the readers of our work 46 . Finally, we note that a recent publication reported an electrically switchable ferroelectric topological semimetal WTe2 17 .
Nevertheless, the switchability remains elusive in doped complex-oxide-based ferroelectrics, despite a previous theoretical report proposed a promising candidate Bi5Ti5O17 15 . Scanning transmission electron microscopy. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) studies were conducted using a JEOL ARM200F atomic resolution analytical electron microscope equipped with a cold fieldemission gun and a new ASCOR 5th order aberration corrector and Gatan Quantum ER spectrometer.

Data availability
The data that support the findings of this study are available from the corresponding author on request.

45.
Yildirim, T. Ferroelectric soft phonons, charge density wave instability, and strong electron-phonon coupling in BiS2 layered superconductors: A first-principles study. prepared the manuscript. All authors discussed the results and commented on the manuscript.

Competing interests:
The authors declare no competing interests.  Fig. 1a). At 300 K, the 2DEG has a carrier density n2D ≈ 8 × 10 13 cm -2 with a mobility of 0.6 cm 2 V -1 s -1 (supplementary Fig. 1b). On the other side, a LaAlO3/SrTiO3 (LAO/STO) sample prepared under similar conditions has n2D ≈ 1 × 10 14 cm -2 with a mobility of 5-6 cm 2 V -1 s -1 (see main text Fig. 3, the 15 unit cells LAO/STO sample). Hence, one can clearly see that the 2DEG density in BTO is not only smaller as compared to that observed with angle-resolved photoemission spectroscopy (ARPES) in supplementary Reference 1, but also smaller than that in LAO/STO. Furthermore, the mobility of the 2DEG in BTO is one order of magnitude smaller than that of LAO/STO, indicating strong localization of electrons in BTO due to strong Coulomb forces imposed by ferroelectricity.
In addition, the a-LAO/BTO samples show very non-uniform conductivity, which could be because that the ferroelectric ordering and metallic conduction occur in two distinct 2 regions. These results suggest at BTO is not an ideal candidate to study the coexistence of ferroelectricity and 2DEG.  Due to the electric field produced by BST, the critical thickness of LAO for insulator-metal transition should be larger than that of LAO/STO heterostructure. Experimentally, the LAO critical thickness was found to be 5-8 uc, which is larger than that of LAO/STO (4 uc). As shown in supplementary Fig. 6b, the conductance of the 2DEG increases by five orders from 5 to 8 uc of LAO. Here, conductance is defined as Rs. Supplementary Fig. 6c  As explained in the main text, carrier density gradually decreases with increasing BST thickness as the magnitude of the polarization increases (Fig. 2b,d) 8 , which obstructs the charge transfer from LAO surface to LAO/BST interface. When BST is 15 uc, the polarization becomes strong enough to deplete most of the mobile electrons and the remaining electrons lie below the mobility edge. These remaining electrons can be excited by thermal energy at high temperature and become localized at low temperature. This explains the upturn of the Rs-T curve and the corresponding decreasing carrier density (n) with decreasing temperature.

Supplementary
We also note, that the electron localization of epitaxial LaAlO3/SrTiO3 interface has been studied in various systems such as LaAlO3/SrTiO3/NdGaO3 (LAO/STO/NGO) 9