Multilevel recording in Bi-deficient Pt/BFO/SRO heterostructures based on ferroelectric resistive switching targeting high-density information storage in nonvolatile memories

We demonstrate the feasibility of multilevel recording in Pt/Bi(1-d)FeO3/SrRuO3 capacitors using the ferroelectric resistive switching phenomenon exhibited by the Pt/Bi(1-d)FeO3 interface. A tunable population of up and down ferroelectric domains able to modulate the Schottky barrier height at the Pt/Bi(1-d)FeO3 interface can be achieved by means of either a collection of SET/RESET voltages or current compliances. This programming scheme gives rise to well defined resistance states, which form the basis for a multilevel storage nonvolatile memory.

Ferroelectric materials with their electrically switchable spontaneous polarization have proven to be very effective in tuning the charge transport properties at a ferroelectric/metal interface [1][2][3][4][5][6][7][8][9][10][11][12][13][14] . A potential advantage over other mechanisms triggering the resistive switching (RS) is related to an intrinsic property of the material (ferroelectricity) rather than to extrinsic defects such as oxygen vacancies, whose concentration strongly depends on the particular features of the material. In addition, ferroelectric RS-based memories allow the possibility of non-destructive polarization state reading as compared with conventional ferroelectric RAMs (Fe-RAM) for which the read-out process deletes the stored logic state.
Multiferroic BiFeO 3 (BFO) displays interesting properties such as robust ferroelectricity 15 and small band gap 16 in comparison with other ferroelectric materials, ranging from 2.2 to 2.8 eV.
The narrow band gap provides a relatively small Schottky barrier height at the metal/BFO contact that can be easily modulated by the large polarization charge of BFO (around 100 C/cm^2 at room temperature along the [111] direction). Remarkably, this property has been used in Au/BFO/Pt capacitors, which show robust multilevel resistance states after irradiation of BFO with low-energy Ar + ions 17 . Recently, it has been reported that the performance of RS devices could be improved by controlling the amount of defect charges in BFO films.
Specifically, the control of electronic transport by polarization reversal in a Bi-deficient Pt/BFO/SrRuO 3 (SRO) heterostructure epitaxially grown on a SrTiO 3 (STO) substrate was demonstrated 18,19 , showing endurance of >10 5 cycles and data retention of >10 5 s at room temperature 18 . The Bi deficiency δ increases the valence of Fe ions and confers a p-type character to the Bi 1- FeO 3 films. Our recent study on piezoresponse force microscope (PFM) in the Bideficient BFO devices demonstrated that the devices show the PFM phase hysteresis loops which well agree with the pulsed-voltage-induced RS hysteresis loops 20 . This result clearly indicates the ferroelectric origin of the RS, although the possibility of oxygen vacancies cannot be absolutely excluded.
In this paper, we further report on the tunability of the ferroelectric RS effect observed at a rectifying Pt/Bi 1- FeO 3 interface. Here, the Bi 1- FeO 3 film behaves as a p-type semiconductor acting as the switching element. Its conductivity, as well as the depletion layer width associated with the formation of a Schottky-like barrier, can be controlled by changing the Bi deficiency concentration δ. Remarkably, this structure does not require electroforming, which is advantageous respect to conventional RS in metal oxides.
The samples consisted in Au/Pt/BFO/SrRuO 3 (SRO) layered structures on SrTiO 3 (001) singlecrystal substrates (Fig. 1a). A 50 nm-thick SRO bottom electrode was grown on the substrate prior to a 100 nm-thick BFO layer obtained by pulsed laser deposition (PLD). As revealed by Xray diffraction measurements, both BFO and SRO layers were confirmed to be epitaxially grown on the SrTiO 3 substrates. An Au(100 nm)/Pt(10 nm) top electrode was deposited on the BFO layer through a shadow mask (pad size of 100 μm × 100 μm) by using electron-beam evaporation. In order to control the Bi content, the BFO films were deposited from source targets with controlled Bi/Fe ratio. In our experiments we focused on devices with the Bi/Fe Asymmetric bipolar-type switching with zero-crossing hysteretic characteristics are obtained.
This behavior is ascribed to the resistive switching of a ferroelectric diode, where a Schottkylike potential barrier forms at the interface between the metal electrode and the conductive BFO. The potential profile of the barrier is reversibly modified by the polarization reversal 18 .
Interestingly, the rectifying I-V characteristic inherently includes the functionality of the selector, i.e., a suitable rectifying element for avoiding crosstalk in crossbar patterned memory arrays 21,22 . I-V measurements are shown to be repeatable upon several cycles without significant modification (Fig. 1b, blue line). Measurements under illumination conditions were also performed. The increase of the measured current under light conditions respect to dark conditions, especially in the low-current regime at negative bias, corroborates the semiconductive nature of the BFO film (Fig. 2).
Next, we report on the RS tunability of the devices. First, we performed a series of doublevoltage sweep ramps as -3 -> SET, where the SET voltage has been progressively increased from 2 to 3.5V. As seen in the left inset of Fig. 1b Tunability of resistance states is also possible by defining a set of compliance currents (Fig. 6), allowing to induce different amount of polarization in the BFO film. Here a double-voltage sweep as -4 -> 5V was applied to the sample under test combined with compliance currents of 10 nA, 100 nA, 1 A, 10 A, 0.1 A, respectively. Interestingly, we found a significant difference respect to the voltage controlled experiment shown in Fig. 3a. When the backward sweep is performed the current remains constant at the compliance level over an extra voltage beyond V* (see Fig. 6), producing a resistance state lower than it would correspond in a voltage controlled experiment with a SET voltage equal to V*. Here V* is defined as the crossing voltage between the forward and backward characteristics. Such V increases with the strength of the compliance as shown in the inset. According to Lee et al. 24  In summary, we have demonstrated multilevel tunability of resistance states of Bi deficient Pt/BFO/SRO heterostructures by using either the SET, RESET voltage, or the current compliance as well. We suggest that the mechanism behind is that of Schottky barrier height modulation provided by the fine level of control for the relative proportion of up and down domains, which is the basis for a multilevel storage ferroelectric nonvolatile memory.