Giant reversible nanoscale piezoresistance at room temperature in Sr2IrO4 thin films

Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin-orbit coupling, which opens up a narrow gap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, and the giant and fully reversible perpendicular piezoresistance makes iridates a promising material for room temperature piezotronic devices.


ABSTRACT:
Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin-orbit coupling, which opens up a narrow gap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr 2 IrO 4 . The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, and the giant and fully reversible perpendicular piezoresistance makes iridates a promising material for room temperature piezotronic devices.
Piezoresistance defines the change in electrical resistance of a material as a function of a mechanically induced deformation. Originally, its main use was in deformation sensors (strain gauges), but it is now moving into the center stage of the electronic industry as the basis for new transistor concepts such as Piezoelectronic Transistors (or PET), 1, 2 aiming to circumvent the gate-voltage bottleneck in transistor miniaturization. 3 The term "giant piezoresistance" was used by He and Yang 4 to describe the 40 times bigger piezoresistance of Si nanowires compared to bulk silicon. [5][6][7] While the nature and origin of the effect in Si nanowires still remains controversial, 8 it is generally accepted that the increase in conductivity of semiconductors such as silicon or germanium under strain is due to an enhanced mobility of their carriers. 9,10 An alternative route towards giant piezoresistance does not change the mobility of the carriers but their number density in materials where the band gap is strain dependent. Archetypal examples of giant piezoresistance through this mechanism are rare-earth selenides, 1 for which the proximity to a metal-insulator transition renders resistivity extremely sensitive to mechanical deformations.
Oxides are a fertile ground in the search for novel or enhanced electronic phenomena, with notable examples including high temperature superconductivity, colossal magnetoresistance, and room temperature magnetoelectric multiferroicity. 11 Some perovskite oxides, such as manganites and vanadates, 12 are also known to exhibit very large piezoresistance, 13, 14 but unfortunately their features are usually optimal only in the immediate vicinity of temperature-driven metal-insulator transitions. In this context, strontium iridate Sr 2 IrO 4 (SIO) appears as a promising material where the strong competition of spin-orbit coupling, Coulomb on-site repulsion, and crystal field opens a conduction gap in a material that would otherwise be a metal. 12,[15][16][17][18] Chemical substitutions, 19 tuning of the Ruddlesden-Popper series' dimensionality, 20 interface exchange-coupling, 21 magnetic fields, 19 etc., can all unbalance the subtle equilibrium of competing interactions and therefore alter the electronic transport. SIO has been realized as epitaxial thin film recently, [21][22][23] and substrate-induced strain has been observed to cause a substantial modification of its transport activation energy 22 and optical bandgap, 24 which suggests that SIO might be a good piezoresistive material. In this paper, we report five orders of magnitude and fully reversible piezoresistive change at room temperature in thin films of SIO induced by pressure perpendicular to the surface.
Piezoresistive characterization is typically performed by applying uniform strain to the samples fixing their extremes in microelectromechanical systems. 25 Here we propose a different method, based on the use of Atomic Force Microscopy, which not only allows us to deliver high pressures with tiny forces, but moreover enables high frequency sampling and measurement of local piezoresistance with nanoscopic lateral resolutions. The pressure is applied by controlling the load exerted by a conductive tip of an atomic force microscope (AFM) with nanometric radius, whilst simultaneously measuring the current across the film by connecting the tip to an amplifier. The tip thus acts as both anvil cell and top electrode, and the small contact area enables delivering very high pressures in the GPa range using only modest forces of the order of N, making this a very convenient bench-top tool for high pressure experiments. The measured piezoresistance (relative increase in resistance divided by applied pressure) is 10000 10 -11 Pa -1 ; this is larger than the giant piezoresistance of silicon nanowires 4 and comparable to that of rare earth selenides, 1 marking the emergence of Sr 2 IrO 4 as a viable oxide for piezotronics.

RESULTS AND DISCUSSION
The sample under study is a SIO 6 nm-thick film, grown on 12 nm-thick La 0.7 Sr 0. Time-dependent current data, collected at a constant force and bias voltage, showed no evidence of transient behavior, and the surface topography was intact after measurements, indicating that increase in current is not due to piercing of the SIO film. Figure 2b shows the total effective resistance (R) as a function of applied load (F) obtained from the slope of the I(V bias ) in the linear regime, i.e., at low voltages, for different sets of experiments. All the data obtained fall into a single curve displaying an exponential decay of the resistance (note the logarithmic R scale) as a function of the applied force and independently of the surface point or the environmental conditions. Beyond F*~10 µN and up to our maximum recorded force of 25 µN, the resistance reaches a saturation value becoming force independent.
The 3D representation of the electronic current as a function of both, voltage and applied force, I(V bias ,F), is depicted in Figure 2c (see 3D modes description in Experimental Section).
The increase in conductance for increasing load is evident from the gap reduction seen as a narrowing of the nearly flat region (intermediate color in the I scale) at low V bias . The use of mechanically robust and stiff diamond-coated tips with k = 40 N/m allows us modeling the elastic deformation of the film under the tip, and thus converting resistance into resistivity.
The tip radius determination and its mechanical stability were verified using high resolution SEM images that show no apex deterioration and a radius r tip = 150 nm both before and after the experiment (inset in Figure 2d Where E* is the Young elastic modulus and r tip is taken from the SEM images. With these values, we can determine the mean contact pressure exerted on a column of SIO, as: and estimate an effective resistivity ( eff ) for different loading conditions on the basis of the normalization of the total resistance (R) by the effective tip-sample contact area: where we also consider the reduction on the film thickness d due to the deformation depth = 2 . The obtained effective resistivity as a function of the mean contact pressure is plotted in Figure 2d and show an outstanding five orders-of-magnitude decrease in resistivity.
Notice that these calculations already take into account the tip-sample contact area and thus the increase in conductivity is not due to any increase of contact area or any other geometric effect. We also notice that the high-pressure resistivity is much smaller than the 4-probe resistivity of bare SIO thin films (10 3 ·cm at room temperature). 28 This indicates that the piezoresistive effects cannot be attributed to changes in the tip, because otherwise the resistivity would saturate at the resistivity of the highest-resistance element in the circuit, which in the absence of piezoresistance would be 10 3 ·cm. Moreover, it shows that the perpendicular deformation induced by the tip is more effective at modulating resistivity than the in-plane deformation induced by epitaxy.  N respectively), yielding a value G = 25000, which is bigger than the giant piezoresistive ratio of oxide cobalates 29 (G = 7000) and bigger even than that of graphene (G=18000). 30,31 The piezoresistive and gauge coefficients or SIO are therefore comparable to or exceeding those of the best piezoresistive materials. In addition, as oxides that can be grown epitaxially on perovskites, iridates are naturally compatible with the piezoelectric elements in piezoelectronic transistors. 32 We now examine the origin of the enormous change in electrical resistivity. The elastic calculations indicate that ~10 µN forces translate into pressures of 10 GPa. Under hydrostatic conditions, such pressures are sufficient to cause a substantial closure of the conduction band gap. 15 Hydrostatic pressure experiments reduce all inter-atomic distances, which should naturally increase bandwidth and decrease bandgap. However, due to the higher compressibility of the in-plane bond, hydrostatic pressure also results in an increased tetragonality of SIO. 15 In such experiments, the bandgap decreases from 60 to 30 meV at a pressure of c.a. 15 GPa, but saturates thereafter and metallic state is never reached even up to 100 GPa. Meanwhile, epitaxial tensile strain expands the in-plane lattice parameter and, via Poisson's ratio, contracts the out-of-plane lattice parameter, resulting in a decrease of the tetragonality of thin films. Experimentally, a 1% decrease in tetragonality is correlated with a reduction in the transport gap from 200 meV to 50 meV. 22 This result already indicates that anisotropic deformations are more efficient than hydrostatic pressure in modulating the bandgap. As a matter of fact, electric transport in iridates is known to be highly anisotropic: single crystal experiments show that resistivity along the c-axis is at least two orders of magnitude larger than along the a and b axis 19 , consistent with the larger Ir-Ir distance brought about by the rocksalt-like SrO intercalated layers sandwitched between perovskitelike layers in the Ruddlesen-Popper structure of SIO. It is therefore natural to expect that a reduction of out-of-plane interlayer distances will reduce the large out-of-plane resistivity.
However, ascribing the enormous change in electrical resistivity solely to a reduction of the interplanar distances can't be straightforwardly reconciled with the fact that, if the in-plane resistivity marks the lower limit, then one would expect one two orders of magnitude change resistance at most, instead of the observed 5 orders. It is therefore plausible that, together with As oxides that can be epitaxially grown on perovskites, iridates are also naturally compatible with the piezoelectric ceramics used as actuator elements in piezoelectronic transistors, 32 making them very attractive for such devices. In addition, spin may also be exploited separately to manipulate the transport properties below the Néel temperature, 19,33 while strain manages stronger variations of the bandwidth and thus larger resistance changes at room temperature.

EXPERIMENTAL SECTION:
Samples preparation and characterization: The heterostructure is deposited by pulsed laser deposition (PLD) assisted by reflection high-energy electron diffraction (RHEED) as detailed in previous work. 21 LSMO and SIO were grown epitaxial on the STO substrates from the stoichiometric targets at a laser energy density of ~1.5 J/cm 2 and a repetition rate of 1 Hz.
LSMO is grown at 700 °C and 150 mTorr partial oxygen pressure while SIO is grown at 800