Controlling Mobility in Perovskite Oxides by Ferroelectric Modulation

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Controlling mobility in perovskite oxides by ferroelectric modulation of atomic-scale interface structure Andrei Malashevich, Matthew S.J. Marshall, Cristina Visani, Ankit S. Disa, Haichao Xu, Frederick Walker, Charles Ahn, and Sohrab Ismail-Beigi Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04715 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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Controlling mobility in perovskite oxides by ferroelectric modulation of atomic-scale interface structure Andrei Malashevich,†,‡ Matthew S. J. Marshall,†,‡ Cristina Visani,†,‡ Ankit S. Disa,† Haichao Xu,¶,†,‡ Frederick J. Walker,†,‡ Charles H. Ahn,†,‡,§,k and Sohrab Ismail-Beigi∗,†,‡,§,k † Center for Research on Interface Structures and Phenomena (CRISP), Yale University, New Haven, Connecticut 06520, USA ‡ Department of Applied Physics, Yale University, New Haven, Connecticut 06520, USA ¶ Advanced Materials Laboratory, Fudan University, Shanghai 200433, Peoples Republic of China § Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06520, USA k Department of Physics, Yale University, New Haven, Connecticut 06520, USA E-mail: [email protected]

Abstract Coherent and epitaxial interfaces permit the realization of electric field driven devices controlled by atomic-scale structural and electronic effects at interfaces. Compared to conventional field effect devices where channel conductivity is modulated by carrier density modification, the propagation of atomic-scale distortions across an interface can control the atomic scale bonding, interatomic electron tunneling rates and

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thus the mobility of the channel material. We use first principles theory to design an atomically abrupt epitaxial perovskite heterostructure involving an oxide ferroelectric (PbZr0.2 Ti0.8 O3 ) and conducting oxide channel (LaNiO3 ) where coupling of polar atomic motions to structural distortions can induce large, reversible changes in the channel mobility. We fabricate and characterize the heterostructure and measure record values, larger than 1000%, for the conductivity modulation. Our results describe how purely interfacial effects can be engineered to deliver unique electronic device properties and large responses to external fields.

The physics underlying the ubiquitous semiconductor field-effect transistor is the field effect: applying a field across an insulating dielectric gate oxide strongly changes the conductivity in the channel material by modulating the carrier density in the channel. 1,2 A significant departure from this paradigm, that involves different physics, is a heterostructure whose conductivity change is driven by interfacial mobility modulation. This requires control over the atomic-scale structure of the interface: recent work shows that the interface between a ferroelectric and a conducting channel can create reversible structural distortions in the channel region; 3–6 these ferroelectric-based distortions in turn modify interfacial electronic and magnetic properties. 1,2 Such a device requires a channel material exhibiting a strong coupling between structure and electronic transport. As an added benefit, the use of a ferroelectric creates a non-volatile device since the polarization is an order parameter of the ferroelectric. Rare-earth perovskite nickelates RNiO3 (R = rare earth atom) display precisely such a strong dependence of transport properties on structural parameters. 7 The governing structural parameters in nickelates are Ni–O bond lengths and Ni–O–Ni bond angles that link successive oxygen octahedra. 8 Electronic transport is controlled by the overlap of Ni 3d and O 2p orbitals, which in turn is determined by bond lengths and angles. 8–12 Static structural modifications affecting Ni–O bonding are achievable via strain and surface termination. 13–15 Regarding reversible modifications, a (001)-oriented heterostructure of NiO2 -

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terminated LaNiO3 and ferroelectric PbZr0.2 Ti0.8 O3 (PZT) exhibited a large change in conductivity upon switching the ferroelectric polarization state. 16 The conductivity change was not due to the canonical field effect (i.e., carrier concentration modulation) but by mobility modification: specifically, in one polarization state a high-mobility channel is created in the interfacial PbO layer of the ferroelectric PZT for the NiO2 /PbO interfacial structure. The differing structural distortions in the two polarization states should also play some role in modifying the mobility, but as the effect of the high-mobility channel is strong and competes with the expected effect of structural distortions, 16 it is not possible to separate the two competing effects and one ends up with a modest ∼30% conductivity modulation. Since structural perturbation of RNiO3 alone can lead to much larger changes in transport properties, one can hope that an interfacial nickelate system where only structural modification is at play would display superior properties. This is indeed the case, and spectacularly so, as we show below. In this Letter, we use first principles theory to understand the structural behavior of the LaNiO3 /PZT interface. This in turn allows us to theoretically design an interface with a large structural-distortion-based conductivity modulation. Again, we emphasize that the transport modulation in our oxide system is due to mobility modification and not the standard field effect operative in key works to date 17–23 (as our data show, the change in carrier density is too small to explain the measured conductivity change 24 ). Experimental fabrication and characterization of the system shows an order-of-magnitude enhancement of the on/off ratio resulting in one of the highest ratios (above 1000%) reported for this type of non-volatile oxide-based system. 25–28 This welcome and unexpectedly large on/off ratio has potentially exciting scientific and technological ramifications: future work on distortion-based interfacial systems could create novel interfacial materials with unusual properties that in turn could lead to enhanced electronic devices with high performance. Our ab initio calculations use density functional theory (DFT) within the DFT+U approach, 29 the local-density approximation (LDA), a plane-wave basis and ultrasoft pseudopo-

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tentials as implemented in the Quantum ESPRESSO software package. 24,30 We employ U = 8 eV for Ti 3d orbitals, while U = 0 for Ni 3d orbitals provides the best description for LaNiO3 . 24,31 The heterostructure is modeled via a (001) slab geometry containing a LaNiO3 film with four NiO2 layers and a PbTiO3 film with five TiO2 layers. (We use PbTiO3 as a proxy material for PZT in the theoretical work.) On the topmost layer of the PbTiO3 , several layers of Pt are placed to provide a fixed electron reservoir (electrode) for the overall system. Other methods of creating charge reservoirs on the PbTiO3 surface were explored and found to give very similar results. 24 For simplicity, the results reported here are based on 1×1 in-plane periodicity; more complex simulations using c(2 × 2) interfacial cells that include octahedral tilts and rotations are described in the Supporting Information. 24 The in-plane lattice parameters of the slabs are strained to the theoretical lattice constants of bulk LaAlO3 (LAO, 3.71 ˚ A) or SrTiO3 (STO, 3.85 ˚ A). Most calculations do not include the substrate explicitly, but explicit inclusion of a substrate below the LaNiO3 film show negligible effect on the interfacial properties. 24 Finally, as shown in Fig. 1, two choices of (001) interfaces are possible: NiO2 /PbO and LaO/TiO2 . Further technical details are found in the Supporting Information. 24 We begin with the structural behavior of the interfaces. Fig. 1 displays the predicted interfacial structures as a function of polarization direction and choice of interfacial atomic planes. The depletion state refers to the case when the polarization of the PbTiO3 points towards the interface: the majority carriers in LaNiO3 are holes 14,16,33 which become depleted from the interface for this choice of polarization when compensating electrons are drawn to the interfacial LaNiO3 region. The opposite polarization is termed the accumulation state. Fig. 1 shows obvious qualitative structural changes at the interface upon polarization reversal, the most significant being in the NiO2 layer closest to the interface. A key structural parameter is the Ni–O–Ni bond angle for this layer θ (defined in Fig. 1). For the NiO2 /PbO interface, θ = 176◦ is almost flat in depletion while θ = 162◦ is more buckled in accumulation. The opposite situation holds for the LaO/TiO2 interface: θ = 161◦ in depletion and θ = 174◦

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Figure 1: (Color online) Two types of perovskite LaNiO3 /PbTiO3 (001) interfaces can be constructed with LaNiO3 terminated with either (a, c) NiO2 or (b, d) LaO layers. For each termination, the ferroelectric PbTiO3 can be polarized in two states, (a, b) depletion (dipole moments point towards the interface) and (c, d) accumulation (dipole moments point away from the interface). The large blue arrows indicate the direction of polarity inside PbTiO3 . The smaller red arrows indicate the direction of LaNiO3 surface polarity. The Ni–O polar distortions are strongest when the two polarities agree and are weak when the polarities point in opposite directions. The visualization of atomic structures was done with the 32 XCrySDen program. in accumulation. The buckling quickly decays away within a few layers of the interface: this is a purely interfacial structural distortion. This pattern of distortions is understood by accounting for the direction of electrical polarity in both materials across the interface and the cation-ion displacements that reflect the polarity at the atomic scale. The polarity of the ferroelectric is straightforward: the cation-anion displacements in each atomic plane of PbTiO3 are controlled by the polarization state (i.e., cations above anions in Fig. 1 for accumulation and vice versa for depletion). The polarity of the LaNiO3 film is driven by the charge state of the constituent ions. Assuming standard formal valance states of La3+ , Ni3+ , and O2− , (001) LaNiO3 films terminated by NiO2 layers have a polarization vector pointing towards the PbTiO3 while LaO termination means polarization pointing away from the ferroelectric. This termination dependence of the polar direction of (001) LaNiO3 is known to control the cation-ion displacements in the terminating layers of the LaNiO3 : 15 for NiO2 termination, cations move above anions

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along (001) while the opposite happens for LaO termination. The choice of termination also controls the conductivity in ultrathin films below 5 uc thick, which may be metallic (LaO termination) or insulating (NiO2 termination). 14,15 The combination of the two polarities provides a simple mechanical explanation of the structures in Fig. 1: opposite polarities mean the interfacial NiO2 layer is being “pulled” in opposite directions resulting in a small final distortion; when polarities agree, the NiO2 layer is “pulled” in the same direction leading to a continuous polarity across the interface and a large final distortion. Having understood the atomic-scale structure of the interfaces, we use the known structureproperty relations in nickelates to understand how the conductivity should behave as a function of the polarization direction. The conductivity of LaNiO3 thin films is directly connected with the in-plane Ni–O–Ni bond angles. 14,15 When the Ni–O–Ni bond angle θ is flat, the overlap between the Ni 3d and O 2p orbitals is maximized, leading to higher conductivity. Conversely, as θ deviates from 180◦ , the overlap is reduced and the system becomes less conducting. Hence, we make some straightforward predictions: e.g., the conductivity of the LaO/TiO2 interface should be higher in accumulation than in depletion. We turn to examining the electronic structure in more detail, focusing on the more interesting LaO/TiO2 interface (key aspects of the NiO2 /PbO interface have been discussed previously 16 ). We expect the states at the Fermi level to be dominated by the LaNiO3 film, while the PbTiO3 should be insulating; the change of potential due to the ferroelectric field effect should move the Fermi level inside the PbTiO3 band gap. The layer-resolved density of states (DOS) for the LaO/TiO2 interfaces, shown in Fig. 2, confirms our expectations. We note that the DOS at the Fermi level is indeed dominated by the NiO2 layer, but the structure and shape of the DOS differs between the two polarizations. This means that the bands do not simply move rigidly as per the standard field effect; the structural distortions clearly modify the nature and energetic distribution of the interfacial electronic states at the Fermi level. (Band structures for the different polarization states are in the Supporting Information. 24 ) An equally important fact is that, for this LaO/TiO2 interface, there are

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no conducting channels in the ferroelectric itself (e.g., no conducting PbO layer), so the properties of the LNO channel are solely responsible for the mobility and conductivity.

Figure 2: (Color online) Layer-resolved density of states of the LaO-terminated LaNiO3 /PbTiO3 interface. The Fermi level is at 0 eV. We quantify the magnitude of the ferroelectric polarization-induced changes of electronic structure and transport for the LaO/TiO2 interfaces from first principles by using semiclassical Boltzmann theory for conductivity 24,34 with the relaxation time approximation based on the DFT band structure information. In our case, however, the states at the Fermi level stem not only from the LaNiO3 but also from the artificial Pt electrode reservoir, which is not part of the interfacial system. To disentangle the contamination of the Pt-derived states from the band structure, we perform a Wannier function analysis of our system to construct an ab initio tight-binding model describing the bands using Ni 3d, O 2p, and Pt 5d Wannier orbitals. This Wannier basis reproduces the states near the Fermi level. 24 We then remove the Pt Wannier orbitals from the tight binding model to recover only the bands of interest in the interfacial system. We then use the tight-binding model to sample the Brillouin zone very densely in order to calculate the conductivity. Assuming the electron relaxation time τ to be constant, the computed room temperature conductivities are σacc /τ = 1.29×1020 Ω−1 m−1 s−1 in the accumulation state and σdpl /τ = 0.69×1020 Ω−1 m−1 s−1 in the depletion state. Thus, in this semiclassical approximation, we quantitatively confirm

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our expectation that the LaO-terminated LaNiO3 /PbTiO3 interface is more conducting in the accumulation state. The computed on/off conductivity ratio is σacc /σdpl = 1.9. We note that this difference is obtained based on Ni–O–Ni bond angles that differ only by about 10◦ between the accumulation and depletion states, underscoring the exceptional sensitivity of the electronic structure nickelates to structural distortions. The final theoretical results concern slabs with STO substrates which are under tensile strain. The qualitative structural response to the ferroelectric switching is identical to the LAO substrate case. Quantitatively, the conductivities for accumulation and depletion states are computed to be σacc /τ = 1.24 × 1020 Ω−1 m−1 s−1 and σdpl /τ = 0.99 × 1020 Ω−1 m−1 s−1 , respectively, resulting in an on/off ratio σacc /σdpl = 1.25. The effect of tensile strain is to “pull” on the Ni–O bonds in the xy plane and thus to straighten out the Ni–O–Ni bond angles. Since in the accumulation state in the case of LAO strain the bond angle was already flat, it remains almost unchanged with tensile strain (θ = 178◦ ) and so the conductivity in this case did not change significantly. For depletion, the conductivity increases compared to the LAO case due to a larger Ni–O–Ni bond angle (θ = 175◦ ) compared to the LAO result. Motivated by these theoretical predictions, we experimentally fabricate devices based on the LaNiO3 /PZT interface with different interfacial terminations, shown schematically in Fig. 3(a). Thin films of NiO2 and LaO-terminated LaNiO3 are grown on SrTiO3 (001) using oxygen-plasma assisted molecular beam epitaxy (MBE). Using a TiO2 -terminated (001) SrTiO3 substrate, we grow three stoichiometric unit cells of LaNiO3 , leaving an NiO2 terminated surface. To achieve an LaO termination, an additional monolayer of LaO is deposited, for a total of 3.5 deposited unit cells of LaNiO3 . The complete growth of each layer is confirmed in situ by monitoring the oscillations of the reflection high energy electron diffraction intensity. The surface termination of films grown by this method has been verified using synchrotron X-ray diffraction. 15 Epitaxial films of PbZr0.2 Ti0.8 O3 are subsequently grown on top of the LaNiO3 using off-axis radio-frequency (RF) magnetron sputtering. Further details regarding growth, characterization and ferroelectric hysteresis loops are provided in the

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Figure 3: (Color online) (a) A cross-sectional schematic of heterostructure is shown where the resistance of the channel is measured in a four-point configuration. The resistivity as a function of temperature for the depletion and accumulation states for a 3.5 u.c. LaOterminated LaNiO3 film (b) and for a 4 u.c. thick NiO2 -terminated LaNiO3 film (c), both grown on SrTiO3 . The room-temperature resistivity of a 3.5 u.c. LaO-terminated LaNiO3 channel as a function of time is shown in (d) as a series of voltage pulses of 0.1 s duration are applied to switch the polarization state of the PZT.

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Supporting Information. 24 Figures 3(b) and (c) display the temperature-dependent changes in resistivity of the device when the polarity of ferroelectric PZT is switched. The low resistivity state is indeed the accumulation state, as predicted by theoretical calculations. For the LaO/TiO2 interface, the accumulation state (undistorted NiO2 layers) is quite metallic with a room temperature resistivity of ρ ∼ 5 mΩcm, while the depletion state (distorted NiO2 layers) has more than an order of magnitude higher ρ at room temperature and an insulating temperature dependence of ρ. The non-volatile nature of the ferroelectric-induced change in conductivity for the LaO/TiO2 interface is shown in Fig. 3(d). The NiO2 /PbO interface, in contrast to the LaO/TiO2 interface, has a higher room temperature resistivity in the accumulation state and shows a smaller change in resistivity between accumulation and depletion. This behavior agrees with our expectations and theoretical predictions that the on/off ratio would be enhanced in the LaO/TiO2 interface, where the competition between structural distortions and the ferroelectric conduction channel is eliminated. Since the on/off ratios in Fig. 3 are enhanced by lowering temperature, we can exclude the motion of defects (e.g., oxygen vacancies) as being primarily responsible for this effect as lowering temperature hinders their thermally activated motion. We point out that Hall measurements 24 show explicitly that it is the change of mobility, and not carrier density, upon ferroelectric switching that leads to the observed conductivity changes. The measured resistivity modulation is exceptionally large for the LaO/TiO2 interface: it is σacc /σdpl = 11.4 at room temperature and exceeds our theoretical predictions. There are a number of potential intrinsic and extrinsic reasons for the difference. Extrinsic effects include inhomogeneity of transport along the interface due to spatial fluctuations of ferroelectric polarization or domain formation, percolative behavior, intermixing of ions across the interface, localization due to disorder, etc. all of which depend on the details of materials fabrication and whose possible influence will require future work. While most of these imperfections should decrease the on/off ratio, some can enhance it: if Fermi level falls inside a mobility edge for the depletion state, this leads to insulating transport behavior and

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greatly enlarged on/off ratio, while the theoretical results are for perfect interfaces. Intrinsic effects we discuss briefly below include electron-phonon and electron-electron interactions. While electron-phonon interactions exist in all materials and can strongly affect transport, our system shows a stronger conductivity modulation at lower temperatures where phonons are less operative: thus even if electron-phonon scattering in our system is for some reason strongly polarization dependent, we do not expect it to be the dominant effect. In fact, low temperature transport data 24 show that electron-electron scattering dominates, so we focus on intrinsic electron interaction effects. Considering electron-electron interactions, DFT is known to not properly account for Mott-type strongly correlated behavior of electrons in LaNiO3 : 12,35–37 in fact, the DFT band structures are metallic for both polarization states (see Fig. 2 and the Supporting Information 24 ) whereas the experiment show insulating transport behavior for depletion. If this difference is intrinsic, better theoretical accounting for electronic interactions should increase the on/off ratio, especially at lower temperatures. This is because bulk nickelate systems have low-spin Ni3+ cations, which are quarter-filled (e1g ) 3d localized electronic systems. Introducing strong on-site electronic correlations will lead to a bandwidth reduction for either polarization states. However, the polarization state strongly dopes the interfacial NiO2 layer (the polarization screening charge is not uniformly distributed in the LNO but mostly concentrated at the interface): accumulation reduces the electron density while depletion increases it. The increased eg electron count in depletion may drive the system nearer to a half-filled eg system, which has a stronger propensity for correlation-induced insulating behavior. Incorporating electronic correlations in the theoretical modeling of such complex interfaces is an interesting and open problem for ferroelectric field effect devices involving correlated transition metal oxide channel materials. 24 In summary, we suggest a novel approach to create a non-volatile ferroelectric transistor device in which the changes in conductivity are mediated via the structural changes at the interface between the oxide metal and ferroelectric. Within this approach, we have designed an interface between a metallic LaNiO3 thin film and ferroelectric PbTiO3 displaying a

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large change in electrical conductivity upon ferroelectric switching. Using density-functional theory and semiclassical transport theory, we show that the changes in conductivity are indeed attributed to the structural modifications of the Ni–O–Ni bond angles and Ni–O bond lengths. We have also shown that by choosing the LaO termination of the LaNiO3 the ferroelectric switching of the conductivity can be greatly enhanced. Our results also show that the conductivity on/off ratio can be further tuned via the in-plane strain, which can be adjusted by choosing an appropriate substrate. Based on our predictions, we have fabricated the LaNiO3 /PbZr0.2 Ti0.8 O3 device with LaO/TiO2 termination, which shows dramatic changes of conductivity including an on/off ratio of 11.4 at room temperature. This result is an important milestone towards developing non-volatile ferroelectric field effect transistor electronics based on oxide materials.

Acknowledgement This work was supported by NSF MRSEC DMR 1119826 (CRISP) and ONR and FAME and by the facilities and staff of the Yale University Faculty of Arts and Sciences High Performance Computing Center. Additional computations used the NSF XSEDE resources via grant TG-MCA08X007.

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Controlling Mobility in Perovskite Oxides by Ferroelectric Modulation

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