Resistive Switching at the Nanoscale in the Mott Insulator Compound

Letter pubs.acs.org/NanoLett

Resistive Switching at the Nanoscale in the Mott Insulator Compound GaTa4Se8 Vincent Dubost,† Tristan Cren,*,† Cristian Vaju,‡ Laurent Cario,*,‡ Benoît Corraze,‡ Etienne Janod,‡ François Debontridder,† and Dimitri Roditchev† †

Institut des Nanosciences de Paris, Université Pierre et Marie Curie, CNRS UMR 7588, 4 place Jussieu, F-75005 Paris, France Institut des Matériaux Jean Rouxel, CNRS Université de Nantes, UMR 6502, 2 rue de la Houssinière, BP32229, 44322 Nantes, France

‡

ABSTRACT: We study the Mott insulator compound GaTa4Se8 in which we previously discovered an electric-field-induced resistive transition. We show that the resistive switching is associated to the appearance of metallic and super-insulating nanodomains by means of scanning tunneling microscopy/ spectroscopy (STM/STS). Moreover, we show that local electronic transitions can be controlled at the nanoscale at room temperature using the electric field of the STM tip. This opens the way for possible applications in resistive random access memories (RRAM) devices. KEYWORDS: Resistive switches, Mott memories, electronic phase separation, STM lash memories are currently leading the field of nonvolatile memories. However, this technology will soon reach physical limits related to the decrease of bit size, which will prevent further improvement of integration density.1 Resistive random access memories (RRAM) are considered as challenging candidates to succeed to the flash technology in the coming years. In the RRAM technology, the “0” and “1” logical states correspond to two different resistance levels of a material, that is, a low resistance state and a high resistance state.2 The application of electric pulses enables switching back and forth between low and high resistance states. Several physical phenomena have led to various types of RRAM,2−5 and advances in this field are yearly published in the International Technology Roadmap for Semiconductors (ITRS).1 Interestingly, a new class of emerging research memory devices, mentioned as “Mott memories”, appears in the 2011 edition. This new class of memory devices exploits various kinds of insulator-to-metal transitions that occur in some Mott insulators to store information. VO2 is a good example of a first kind of Mott memories, where resistive switching is related to a temperature-driven insulator-to-metal transition (IMT).6,7 In this compound, the electrical pulse induces a Joule heating sufficient to overcome locally TIMT ≈ 340 K and to reach the metallic state. After the pulse, some metallic nanodomains are retained within the insulating matrix. This phenomenon explains the lowering of the resistance.8 Alternatively, a second type of Mott memories, which uses an electric-field-driven IMT, has recently emerged.9−11 It was indeed suggested that the electric field could create an electronic phase separation at the nanoscale in the Mott insulator compound GaTa4Se8.17 However, so far the nature of this electronic phase separation is unknown, and the question

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of the downscalability of a Mott memory based on this mechanism remains completely open.1 Here we show that a reversible resistive switching can be achieved at the nanoscopic scale in the Mott insulator GaTa4Se8 using an electric-field-driven IMT. Our scanning tunneling microscopy/spectroscopy (STM/STS) study of the Mott insulator GaTa4Se8 shows that an electric pulse generates, within the bulk, an electronic phase separation concomitant with a lattice deformation. In the transited state, some metallic and super-insulating nanodomains (30−50 nm) coexist within a semiconducting matrix that behaves as the pristine material. Our study also demonstrates that voltage pulses applied on the surface of GaTa4Se8 by the STM tip can drive a reversible resistive switching between these different electronic states at the nanoscale. This work gives hope that Mott memories based on an electric-field-driven IMT could be downscaled below 30 nm, which corresponds to an areal density of 0.7 Tbits/in2. The compound GaTa4Se8 adopts a deficient spinel structure of rocksalt-type packing of Ta4 Se 4 cubanes and GaSe 4 tetrahedra (Figure 1a).12,13 Compared to the regular spinel structure, the most interesting difference is the formation of tetrahedral Ta4 clusters (in blue in Figure 1a). The intracluster Ta−Ta distances are compatible with the formation of molecular bonds, while the larger intercluster distances prevent metal−metal bonding. With one unpaired electron in the highest molecular cluster orbitals, one expects this material to be metallic. But instead of being a metal, GaTa4Se8 behaves like an insulator, pointing out the role of electronic correlations. In Received: April 25, 2013 Revised: June 11, 2013

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Figure 1. (a) Representation of the crystal structure of GaTa4Se8. Ta atoms: blue, Se atoms: yellow, Ga-centered tetrahedra: green. (b) Temperature dependence of the resistance of one of the crystals used in this study, measured before (red curve) and after (blue curve) electric pulse. (c) Representative tunneling conductance spectrum of a pristine cleaved crystal. The gap EG ∼ 100−200 meV measured by optical and resistivity measurements is indicated by the threshold blue line. (d) Large-scale 3D STM topography image of the pristine crystal (VT = −600 mV, IT = 100 pA). (e) Small-scale topography (VT = −600 mV, IT = 300 pA) of the pristine crystal. (f) Conductance map measured at −200 mV of the area shown in (e that shows a homogeneous electronic state. (g) Large-scale 3D STM image of a cleaved transited crystal (VT = −600 mV, IT = 100 pA). (h) Small-scale topography (VT = 640 mV, IT = 260 pA) of the transited crystal. (i) Conductance map measured at −200 mV of the area shown in h that exhibits strong electronic inhomogeneities.

electronic phase separation we have engaged a detailed STM/ STS study of GaTa4Se8, both in the pristine state and after the electric-pulse-induced IMT. Figure 1 displays large scale topographic STM images for pristine (Figure 1d) and transited (Figure 1g) crystals, acquired on freshly cleaved samples. All crystals show atomic-flat terraces several hundred of nanometers wide. The step heights are 0.5 nm, or its multiple, which corresponds to the thickness of slabs of Ta4Se4 clusters and GaSe4 tetrahedra (Figure 1a). Besides these steps, the topographic images of pristine insulating GaTa4Se8 crystal remain structureless as seen in Figure 1e. Figure 1f presents a conductance map taken at −200 mV on the area of Figure 1e. This map reveals a spatially homogeneous insulating state. A typical conductance spectra of the pristine crystal is displayed in Figure 1c. It presents a strong suppression of spectral weight around zero bias, reflecting the gap around the Fermi level of the Mott insulating state. Since the STS experiments were conducted at room temperature, a thermal broadening of 3.5kBT ≃ 90 meV affects the tunneling characteristics and results in a smeared apparent gap and in a finite conductance at zero bias for the insulating pristine material. In that respect the conductance spectra of the pristine crystal is consistent with the results of optical conductivity and resistivity measurements that pointed out a gap EG ∼ 100−200 meV.23 In the transited crystals that shows a metallic-like electronic conductivity (see blue curve in Figure 1b), we find a very different picture. The surface of these GaTa4Se8 crystals reveals some very specific topographic features with typical lateral sizes of a few tens of nanometers (Figure 1h) which are absent in the

fact GaTa4Se8 belongs to the AM4Q8 (A = Ge, Ga; M = V, Nb, Ta; Q = S, Se, Te) series of compounds, a family of narrow gap Mott insulators very sensitive to external perturbations. For example, bandwidth-control IMT and superconductivity were reported in GaTa4Se8 under pressure.12,14 Moreover, filling control IMTs were achieved in the AM4Q8 compounds thanks to appropriate chemical substitution on the A or M sites.15,16 Recently, we have reported that an electric field is able to induce a resistive switching in these compounds, making these materials very promising for RRAM applications.9,11,17−19 The phenomenology of the resistive switching was described in detail in our previous publications.9,10,17,18,20 Above a threshold field of a few kV/cm, the whole family of AM4Q8 compounds undergoes a sudden decrease of the resistance during the pulse; after the pulse the resistance recovers its initial value. This volatile resistive switching was recently ascribed to an avalanche breakdown showing the same phenomenology in these Mott insulators as in classical semiconductors.20 For an electric field much larger than the threshold field, the resistive switching in AM4Q8 compounds turns nonvolatile; the sample indeed stays in its low resistance state after the voltage pulse. Figure 1b shows a typical example of the temperature dependence before and after a nonvolatile resistive switching. While pristine samples display an insulating behavior expected for a Mott insulator (see red curve), the transited crystals exhibit a “metallic-like” low resistance state (see blue curve). Our preliminary STM study have suggested that the low resistance nonvolatile state results from the creation of an electronic phase separation inside the material.17 To reveal the nature of the B

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Figure 2. (a−b) Conductance maps of the same area of a cleaved transited crystal acquired respectively at −200 mV and 0 mV (set point: VT = −500 mV and IT = 250 pA). (c) Tunneling spectra corresponding to zones A (green), B (blue−violet), and C (red) depicted on image b. The dI/dV spectra of the zone A (in green) are similar to the one of the insulating pristine samples; the spectra from zone B (blue−violet) are more insulating and hence are called super-insulating while the spectra from zone C (in red) are “metallic-like”. (d) Composite topography/spectroscopy image, with topography in 3D channel and conductance at −200 mV in color channels. These figures show that the “metallic” and super-insulating zones are topographically inflated.

density of states at the Fermi level; hence such a spectrum can be considered as “metallic-like”. Figure 2d displays a composite image where the relief represents the topography, while the color is given by the spectroscopic map measured at the Fermi level. This 3D image reveals a clear spatial correlation between the metallic (in red) and super-insulating (in blue) patches and the presence of topographic blobs: this demonstrates that local electronic switching is always concomitant to sample deformations. To get more insight on the evolution of the electronic structure after the resistive switching, we analyzed the spatial repartition of the local gap (see the Methods part for details). The inset of Figure 3a shows the map of local gap value for a pristine sample. This gap map was performed on the same area as the one shown in Figure 1e−f. As expected, the map is homogeneous, and the distribution of gap values (Figure 3a) corresponds to a peak centered around 200 meV. Conversely, the map of local gap for the transited sample, displayed in the inset of Figure 3b, is inhomogeneous and reveals that some gapless patches (in white) and large-gap patches (in black) are dispersed in an insulating matrix (orange). The main contribution in the gap distribution is, as for the pristine sample, a peak centered at 200 mV corresponding to undisturbed domains. In addition, Figure 3b shows that the super-insulating domains actually correspond to a continuum of larger gaps distributed almost homogeneously between 200 and 700 mV. As all of the gapless metallic spectra (above threshold) are peaked on EG = 0, the histogram was smoothed to better reveal their weight. It is worth noting that taking into account the appearance of these metallic grains we were able to model the macroscopic conductivity of the transited sample using a two-conductance model with two resistances in parallel, the

pristine samples (see Figure 1e). The surface appears covered with elongated blobs of 20−50 nm organized in filaments oriented along the direction of the electric field applied to switch the sample (see Figure 1h). The corresponding conductance map taken at −200 mV (Figure 1i) reveals a strongly inhomogeneous electronic background. The detailed analysis of the dI/dV(V) conductance maps taken in several locations evidences that the macroscopic resistive switching induces a complex electronic patchwork corresponding to an electronic phase separation at the nanoscale (Figure 2). The complex topographic/electronic patchwork is representative of the whole surface and has been encountered in numerous locations close or far from the macroscopic electrodes. This supports the conclusion drawn from resistivity measurements9,17 that the resistive switching in GaTa4Se8 is not related to any interface phenomenon occurring near the external electrodes used to switch the sample. As underlined in Figure 2a−b, the electronic patchwork is made of three typical zones, namely, A (green), B (violet), and C (red), that correspond to three different electronic states for which respective conductance spectra are displayed on Figure 2c. A conductance spectrum from the zone A is featured in green. It is insulating-like and very similar to the spectra observed in the pristine material, hence suggesting that the green zones were barely affected by the nonvolatile resistive switching. A typical spectrum from zone B, depicted in blueviolet, has no conductance at zero bias (dI/dV(0) = 0), and its corresponding gap of ∼600 meV is significantly larger than the gap of the pristine state. This larger gap reveals a pronounced insulating character compared to the pristine insulating medium; therefore we call these regions “super-insulating”. A spectrum from zone C, represented in red, presents a finite C

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On the other hand, the STM/STS analysis give strong indications that the electric field applied with macroscopic external electrodes creates an electronic phase separation concomitant to strong local sample deformations.22 The fact that the material inflates where new electronic patches appear indicates that a very strong electron−lattice coupling is involved in the electric pulse induced resistive switching. This strong electron−lattice coupling is reminiscent of the volume change encountered in canonical Mott insulators, such as V2O3, at the pressure-induced Mott IMT.24 In that respect one can get a first hint on the origin of the metallic and super-insulating domains in view of the temperature−pressure (T−P) phase diagram of the Mott insulator GaTa4Se8 shown in Figure 3c. The phase diagram suggests that the metallic and super-insulating domains correspond respectively to compressed and expanded domains. Indeed, in a Mott insulator, negative pressure (volume expansion) is expected to lead to a continuous increase of the Mott−Hubbard gap, similar to the one observed in Figure 3b. Conversely, a positive pressure (volume compression) can lead to a first-order transition from a Mott insulator to a correlated metal. The peak centered at 0 meV in Figure 3b could therefore correspond to the signature of such compressed metallic domains that have experienced this IMT. Note that the phase diagram of Figure 3c is constructed assuming a bulk material, with the constraint of volume conservation. However, at the surface, the constraint of volume conservation is relaxed. This could explain why, surprisingly, both metallic and superinsulating domains appear extended while one expects a volume contraction for metallic domains. We expect that compressed metallic domains may appear extended as they are pushed away by super-insulating domains located just beneath the surface. All our results suggest that the AM4Q8 compounds exhibit a new type of resistive switching related to an electric-fieldtriggered breakdown of the Mott insulating state at the nanoscale and a concomitant local deformation due to strong electron−lattice coupling. The control at the nanometer scale of this transition could lead to an important downscalability for Mott memory based on this mechanism. To tackle this issue we have used the strong electric field generated by the STM tip to induce electronic transitions at the local scale. Figure 4 shows successive topographic/spectroscopic maps measured on the same location of a transited crystal. The successive topography and conductance maps reveal that some zones switch back and forth among the super-insulating, insulating, and metallic states. These resistive switchings are observed during spectroscopic measurements using voltage sweeps (±800 mV) applied through the STM tip, while topographic measurements performed at 600 mV do not affect the electronic structure of the surface. Figure 4a−c shows a series of 3D spectroscopic maps of a small region where a metallic blob of about 30 nm appears and disappears during successive spectroscopic scans of the area. Some super-insulating blobs can also appear and disappear as depicted on Figure 4d−f. This demonstrates that a reversible resistive switching between at least three levels of resistance can be achieved at room temperature and at the nanoscale in the Mott insulator GaTa4Se8. This ability to write and erase down to 30 nm thus shows the potential of GaTa4Se8, and hence of the AM4Q8 family, for RRAM applications. The high sensitivity of these compounds to the electric field at the tip apex can be further exploited to pattern the electronic structure of the samples at the nanoscale. Figure 4g show a rectangular metallic patch of about 50 nm × 10 nm that was induced, at room temperature, by scanning the STM tip at 1.2

Figure 3. (a) Histogram of gap width for a pristine crystal, established from the gap-width map shown in the inset. The analyzed area is the same as in Figure 1e−f. Both the map and the histogram are representative of the homogeneous insulating background of pristine material. (b) Histogram of gap width for a transited crystal, established from the gap-width map shown in the inset. The area corresponds to the one shown in Figure 2. The metallic regions appear in white and the super-insulating regions in black. The histogram was smoothened to evidence the metallic peak at zero gap width. The gap widths were determined by thresholding the conductance curves as depicted by the blue line in Figure 2c. (c) Schematic temperature−pressure phase diagram of the Mott insulator GaTa4Se8 in its pristine state. For negative pressure (expansion), the Mott−Hubbard gap increases continuously. For positive pressure (compression), a discontinuous first order transition occurs at a critical pressure (≈ 6 GPa), and the compound undergoes a Mott IMT.23

first one accounting for the pristine-like material and the second one representing a granular metallic-like phase.18 So far, all reported nonvolatile RS mechanisms were based on chemical or structural changes,2,3 and it is therefore important to clarify if the metallic and super-insulating domains could be ascribed to a difference of composition or of structure. Transited AM4Q8 crystals were analyzed with EDXS21 and high-resolution TEM.9 Our results suggest that metallic and super-insulating domains do not present a different composition nor a structural change with symmetry breaking compared to the pristine material.9,17,18,21 D

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Figure 4. (a−c) Series of consecutive topographic/spectroscopic measurements showing a reversible insulating−metal (in red)−insulating transitions induced by the electric field of the STM tip. A metallic blob (in red) appears and disappears during successive spectroscopic scans of the same area. (d−f) Same with insulating−super-insulating (in blue)−insulating transition, obtained in a different area. In a−f, the 3D channel corresponds to topography, while the color channel corresponds to the conductance map at −200 mV. (g) Metallic patch induced by scanning the STM tip with a bias voltage of 1.2 V, clearly associated with a strong blow up of the surface. (h) Nanowriting of “IMN” by applying electric pulses on the surface.

corresponding to an electric field of about 10−30 kV/cm was applied to the samples at 60 K to induce a nonvolatile IMT. After the electric pulse, the transited crystals exhibit a low resistance state as shown in blue curve in Figure 1b. Pristine and transited crystals were cleaved, in air (crystals of Figures 1−3) or under ultra high vacuum (crystals of Figure 4), with a blade to obtain clean (100) surfaces. Sample prepared in air were immediately introduced to the ultrahigh vacuum STM chamber after the cleavage (base pressure of 3 × 10−11 mbar). Under these conditions our study did not reveal significant differences between crystals cleaved in air or under vacuum. The same conclusion was drawn from our preliminary ARPES measurements on cleaved crystals in air or in UHV. The STM/ STS experiments were provided in UHV, at room temperature. Electrochemically etched tungsten tips, thermally annealed under UHV by direct current heating, were used. The tunneling conductance dI/dV(V) spectra were obtained by direct numerical derivation of the raw I(V) spectra. The analysis of the gap distribution and its spatial repartition shown in Figure 3 was performed as follows. In the pristine homogeneous GaTa4Se8, a level of conductance is chosen (see blue line in Figure 1c) so that the crossing of this line with dI/ dV(V) defines two points separated by the gap value EG ∼ 100−200 mV determined by optical conductivity and transport measurements.23 A similar procedure is used in transited samples, but the choice of appropriate conductance level (horizontal blue line in Figure 2c) is then performed on spectra issued from undisturbed domains A (green curve in Figure 2c).

V on the surface of a weakly transited GaTa4Se8 crystal, cleaved in UHV. The metallic zone (in red) coincides with the concomitant blow-up of the surface. This way to pattern the electronic structure of the surface has been used for writing the logo of one of the laboratory (IMN) on the surface of the same sample (Figure 4h). These preliminary results suggests therefore that resistive switching in Mott insulators might be sufficiently controllable to be able to design nanoscale electric circuits and memory storage devices. In conclusion, the comparison of our STM/STS data acquired on pristine and transited crystals gives a direct experimental evidence that the electric-field-induced resistive switching recently observed in the Mott insulator GaTa4Se8 arises from the formation of a nanoscale electronic phase separation. The pristine crystals are homogeneous, while the transited crystals display a peculiar electronic patchwork with a filamentary structure, which consists of a metallic/superinsulating network embedded in a barely pristine insulating matrix. The induced electronic switching is associated with local topographic blow-up probably originating from a strong electron−lattice coupling. A write and erase operation using this new type of resistive switching can be induced at the nanometer scale and at room temperature by the electric field of a STM tip. All of these results therefore give hope that Mott memories based on an electric field triggered Mott IMT can be downscalable down to 30 nm. This phenomena might also be used to write and erase nanoscale electric circuits on the surface of Mott insulators. Methods. Single crystals of GaTa4Se8, of typical sizes around 300 μm, were obtained by the selenium transport method.17 Transited crystals used in this STM/STS study were prepared as follows. First, two electric contacts were glued onto the small GaTa4Se8 single crystals. Then a voltage pulse

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. E

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank J. Martial at IMN for her skilled help in sample preparation. This work was supported by Grants ANR05-JCJC-0123-01 (to L.C., B.C., and E.J.) and ANR-09-Blan0154-01 from the French Agence Nationale de la Recherche.

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REFERENCES

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