Electric Field Effect in Correlated Oxide Systems

progress Electric ﬁeld effect in correlated oxide systems

C. H. Ahn1, J.-M. Triscone2 & J. Mannhart3

1Department of Applied Physics, Yale University, PO Box 208284, New Haven, Connecticut 06520-8284, USA 2Condensed Matter Physics Department, University of Geneva, 24 quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland 3Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany ......

Semiconducting field-effect transistors are the workhorses of the modern electronics era. Recently, application of the field-effect approach to compounds other than semiconductors has created opportunities to electrostatically modulate types of correlated electron behaviour—including high-temperature superconductivity and colossal magnetoresistance—and potentially tune the phase transitions in such systems. Here we provide an overview of the achievements in this field and discuss the opportunities brought by the field-effect approach.

Our daily life is permeated by semiconducting field-effect transis- large part of this group’s claims7, however, may unfortunately and tors (FETs). About 1018 of these microscopic electronic switches are wrongly give the impression that the electric field effect in such produced every year to run the tools that are indispensable to us, materials does not exist. Here we provide an overview of the including our cars, computers, cellular phones and kitchen appli- electrostatic field effect in complex oxides, one of the newest classes ances. These devices work by modulating the electrical charge of materials to which the field effect has been applied. We consider carrier density, and hence electrical resistance, of a thin semicon- the role of electronic correlations, discuss examples in superconducting channel through the application of an electric field. This ductivity and magnetism, and outline future directions, comment- remarkably simple and very successful principle provides new ing on the technical challenges of this approach. opportunities for basic science and innovative device applications when applied to novel correlated electron systems whose properties Strongly correlated systems depend strongly on the carrier concentration. Excited by the In materials ranging from ionic insulators and elemental semicon- opportunities of the field effect in such materials—including ductors to simple metals, the band structure model and associated organic conductors1,2, high-temperature superconductors3,4 and Fermi liquid description provide a deep understanding of their colossal magnetoresistance compounds5—dozens of groups have fundamental physical properties. In this framework, insulators and worked on this approach, yielding a panoply of striking results6. The intrinsic semiconductors have only filled and empty bands, whereas burgeoning field of plastic electronics is a notable example that has metals have at least one partially filled band. These concepts reach reached technological fruition2. their limits, however, as the interactions between electrons become These opportunities probably served as a motivation for the strong or the dimensionality of the system is reduced. When the research directions of the group at Bell Laboratories7. Several of this electrostatic Coulomb interactions become dominant, localization group’s articles in Nature and Science, for example, addressed occurs for commensurate filling (for example, 1/4 or 1/2 filling), electric-field-induced superconductivity in new materials. The resulting in a special type of insulator, frequently called the Mott public debate about the scientific misconduct that underlies a insulator8. One-dimensional (1D) GaAs quantum wires, organic

Figure 1 Illustration of the zero-temperature behaviour of various correlated materials drawn to illustrate schematically the richness of materials available for ﬁeld-effect

as a function of sheet charge density (n 2D). Silicon is shown as a reference. The tuning and the spectrum of their phases. AF, antiferromagnetic; FM, ferromagnetic; examples for high-T c superconductors and for colossal magnetoresistive (CMR) I, insulator; M, metal; SC, superconductor; FQHE, fractional quantum Hall effect; manganites reﬂect YBa2Cu3O72d and (La,Sr)MnO3, respectively. The top bar has been Wigner, Wigner crystal.

Bechgaard salts, and undoped two-dimensional (2D) high-tem- In addition to this stringent requirement on the sustainable perature superconductors are notable examples of such insula- polarization, leakage currents through the insulator must be tors1,9,10. Disorder in these correlated electron systems also has an much smaller than the DS current, and the density of localized important role. At low densities, depending on the balance between interface states must be small compared with the modulated carrier disorder and the strength of the electronic correlations, one expects density. In conventional Si FETs, the universal gate dielectric is SiO2, to have Anderson localization (in 1D or 2D) or a crystallization of an excellent insulator with low leakage that is chemically stable and the electrons into a Wigner crystal; both schemes lead to an virtually free of interface states. These outstanding properties of 11,12 insulating state . Recent work on 2D, high-mobility Si field-effect SiO2 are a primary reason for the success of Si-based electronics. For structures proposes that extremely strong electron interactions may oxide FETs, however, SiO2 is unsuitable because its breakdown field 21 overcome the disorder that leads to localization, resulting in a E b ¼ 10 MV cm and dielectric constant e r ¼ 3.9 lead to polariz- metallic ground state13. ations of 3 mCcm22 (2 £ 1013 charges cm22) at best. In the case of Interactions combined with low dimensionalities thus lead to novel plastic electronics, this relatively small polarization does not pre- 14 phases, which are strongly influenced by the carrier concentration . clude the use of SiO2, although issues related to traps, surface states, Figure 1 illustrates several of the properties of the materials discussed degradation of the channel, and leakage had to be mastered. An above as a function of their sheet charge density; silicon is shown as a additional difficulty for organic FETs is that the design and reference. Because the carrier concentration is a crucial parameter fabrication of the contacts to the DS channels are non-trivial. that governs the properties of these systems, the electrostatic field- Because of these technical challenges, chemical doping, which can effect approach is an ideal tool to investigate the physics of correlated achieve metallic carrier densities, is by far the most commonly used electron systems, allowing controlled and reversible changes of the method to manipulate the carrier densities of complex oxides. carrier concentration without altering the disorder. The fractional Largely owing to advances in physical vapour deposition tech- quantum Hall effect, exhibited in GaAs/AlGaAs heterostructures, is a niques, it is only in recent years that doping of complex oxides by striking example of correlations in a clean, 2D system, in which the the electric field effect has become possible. It is now routine to field effect has been used effectively to tune the carrier density15. fabricate epitaxial high-er complex oxide dielectrics, such as SrTiO3, Other methods to change the carrier density without affecting the 16 and ferroelectric oxides, such as Pb(Zr,Ti)O3 (PZT), that can disorder include the application of pressure and photon irradiation . achieve polarizations in the range of 10–40 mCcm22. In situ multi- One challenge that is apparent from Fig. 1, however, is that much of layer growth of the oxide dielectric and DS channels has led to the novel behaviour beyond the realm of the fractional quantum Hall improved structural and electronic properties of the DS channel effect and Wigner crystallization occurs at relatively high carrier with acceptable interface trap densities6,17,18. densities, requiring the use of extremely large polarization fields for Interestingly, these advances in complex oxide gate dielectrics the field effect, as described below. have led the semiconductor industry to consider replacing SiO2 Experimental parameters with high-er complex oxide materials. As the thickness of the SiO2 layer approaches one nanometre, below which leakage from tunnel Field-effect experiments are conceptually elegant, but pose signifi- currents becomes unacceptably large, alternative gate dielectrics will cant challenges in practice because the underlying physics become a necessity19. The possibility of using thicker gate dielectrics requires structures that are difficult to achieve. Applying an external with larger values of e is driving the exploration of complex oxide electric field to a material attracts or repels charge carriers, creating a r gate dielectrics, as well as the development of the methodologies to thin charge accumulation or depletion layer at the surface that deposit these materials directly on Si. As with complex oxide FETs, modifies the electrical conductivity between a source (S) and a drain challenges related to the density of interface states and the chemical (D) contact (Fig. 2). The field is applied across a gate insulator using and thermodynamic compatibility of the gate insulator with the a gate electrode (G). The characteristic width of the accumulation or DS-channel material are being addressed20–22. depletion layer is given by the electrostatic screening length l , el Despite the technical challenges of fabricating complex oxide FETs, which in the semiclassical, metallic limit is the Thomas–Fermi the notion of switching exotic behaviour (such as superconductivity) length. In standard metallic systems, l el is extremely short, a fraction of an atomic diameter, and thus negligible field effects are on and off just by turning the knob on a power supply is so tempting found. In low-carrier-density systems, larger screening lengths and that dozens of groups have addressed these obstacles, with experi- field effects are expected, which are reasons why standard transistors ments starting as early as 1960 (for example, refs 23–34; for an are made with semiconductors rather than metals. extended list, see http://www.physik.uni-augsburg.de/exp6/field- Many of the interesting physical properties described in Fig. 1 occur effect/). It was in 1991, working with high superconducting tran- at carrier densities in the range of 1019–1022 carriers cm23,inter- sition temperature (high T c) copper oxide superconductors, that mediate between those of conventional metals and semiconductors Mannhart and colleagues at the IBM Research Laboratory in such as Si or GaAs. To achieve substantial carrier modulation at these Ru¨schlikon observed electric-field-induced shifts of T c of several 25 densities, two approaches are followed. The first is to use ultrathin K in epitaxial heterostructures (Fig. 3a) . Subsequent work using drain–source (DS) channels, which are only a few nanometres thick, the ferroelectric field effect has also allowed reversible modulation 18 so that the absolute number of carriers is small, ,1014 cm22, allowing of superconductivity to be induced in these materials (Fig. 3b, c). large relative changes in their total number. Because the carrier modulation occurs in an interfacial layer, it is also advantageous to use ultrathin channels to avoid shunting effects. The second means of achieving large modulation of the carrier density is to apply extremely large electric fields across insulators that are tailored for large breakdown field strengths (E b) and dielectric constants (er). The polarization (j), or areal charge density (charge cm22), that can be induced at a given gate voltageÐ VG applied across an insulator of ¼ VG ð Þ thickness t is given by j 0 101r V =t dV, where VG , Ebt.To change the carrier density by 50% in a 1-nm-thick oxide superconductor with 4 £ 1021 holes cm23, for example, requires sheet charge Figure 2 Cross-section of a typical sample geometry used for field-effect studies. densities or polarizations of 30 mCcm22, an order of magnitude S, source; G, gate; D, drain. larger than what can be achieved in standard FETs.

Figure 3 Field effects in superconducting ﬁlms. In each case, the blue curve GdBa2Cu3O72d ﬁlm induced by a 300-nm-thick PZT layer acting as ferroelectric gate. corresponds to depletion of the carrier density, and the red curve corresponds to The two curves have been normalized in the normal state. c, Resistance change of a

enhancement of the carrier density in the DS (drain–source) channel. a, Change of the ,2-nm-thick GdBa2Cu3O72d ﬁlm whose doping level has been chosen to be close to DS resistance of an ,8-nm-thick YBa2Cu3O72d channel with a ,300-nm-thick the superconductor–insulator transition, induced by a 300-nm-thick PZT layer acting Ba0.15Sr0.85TiO3 gate insulator. The scatter in the data results from the noise of the as ferroelectric gate. These data have been measured at 1 T (from ref. 18). measurement system (from ref. 6). b, Resistance change of a ,2-nm-thick

Correlated oxide superconductivity establishment of long-range phase coherence and should be pro- We now turn to the role of charge in the physics of the high-T c portional to the superfluid density n s. This relationship has been copper oxide superconductors. In transition metal oxides with a observed experimentally in muon spin resonance experiments on partially filled d-electron band, the electronic correlations lead to chemically doped bulk samples40. Recent ferroelectric field-effect the opening of a direct gap in the 3d band (Mott–Hubbard experiments on underdoped thin films of NdBa2Cu3O72d have also insulators) or an indirect gap between the oxygen 2p band and correlated quantitatively changes in T c with changes in the carrier the upper Hubbard band (charge transfer insulators); both cases concentration produced by the field effect, in agreement with the 41 result in an insulating state at half filling. The high-T c supercon- phase fluctuation model . In contrast to chemical doping, the field- ductors are doped charge transfer insulators, and the role of carriers effect experiments only modify the charge, revealing directly the is depicted in the temperature–doping (T–x) phase diagram of relationship between the carrier density and T c. Fig. 4. The parameter x stands for the doping level, which is usually Electrostatic modulation of the carrier density has also been used changed by non-isovalent chemical substitution of the insulating to study the normal state of the copper oxide superconductors above x ¼ 0 parent compound, and is roughly proportional to the number Tc. Besides the opening of a gap in the electronic spectrum (called the of ‘free’ electrical carriers in the system. At low doping levels, the pseudogap) at a temperature T* (Fig. 4), it was recognized early on system is an antiferromagnetic insulator owing to strong electronic that the normal-state transport properties are anomalous, in par- correlations. As the doping level is increased, superconductivity ticular the temperature dependence of the resistivity and Hall appears, with the transition temperature T c following a roughly constant RH. In ferroelectric/superconducting heterostructures, ana- parabolic dependence on x. lyses of the normal-state temperature dependence of the resistivity At low doping levels, near the superconductor–insulator phase and Hall constant as a function of the ferroelectric polarization show boundary, field-effect tuning of the carrier density should induce a that both these quantities can be rescaled over the entire measured reversible insulator–superconductor quantum phase transition, an temperature range42. These results point to a temperature-indepen- active area of research in condensed matter physics35,36. Such superconducting switching behaviour is appealing from both a fundamental perspective and for potential applications. Figure 3c shows ferroelectric field-effect experiments on underdoped, ultrathin ˚ (20 A) GdBa2Cu3O7 films, in which non-volatile, reversible switching between superconducting and insulating behaviour could be achieved by modulating the polarization state of the ferroelectric gate18. This switching occurs at a very large value of the sheet resistance (about 40 kQ per square), in agreement with values obtained by chemical doping and disorder-induced superconductor–insulator transitions in high-temperature superconductors, but significantly larger than the values observed in low-temperature superconductors. On the insulating side, analysis of the transport properties reveals a resistivity described by r / ln(1/T). This peculiar temperature dependence was also observed in very-high- magnetic-field (60 T) experiments that allowed superconductivity to 37 be suppressed in underdoped La22xSrxCuO4 and other compounds . These results suggest that the physics of the insulating state reached by high electric or magnetic fields is different from that obtained by chemical doping, which may be related to the fact that even as the superfluid density is modulated by the applied electric or magnetic field, the microstructure and disorder remain fixed. In addition, in the underdoped region of the T–x phase diagram, Figure 4 Phase diagram of the high-T superconductors. The electron-doped material the copper oxide superconductors have a relatively low carrier c represents Nd22xCexCuO42y, and the hole-doped system represents YBa2Cu3O7-d. density, resulting in a low superfluid density and weak phase The insulating, antiferromagnetic phases (AFM-I) are drawn in yellow, and the stiffness. It has thus been proposed that the T c in this region is superconducting phases (SC) are in orange. 38,39 controlled by phase fluctuations . As a result, T c only reflects the

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