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Efficient and Robust Oxide Switching

Periodic Reporting for period 1 - EROS (Efficient and Robust Oxide Switching)

Período documentado: 2021-05-01 hasta 2022-10-31

The probkem being addressed is to achieve efficient and robust materials for non-volatile memory and neuromorphic computing by materials innovation.
The probkem is very important both for new forms of computing with much greater power and speed and also for achieving operation at much low power.

The objectives are:

1. Translate high performance key model memristor system to industry platform film to meet industry performance metrics. The industry platform films will be comprised of sub-5 nm, columnar structured, ionic thin films (preferably of HfO2, but other systems if their performance is better) with conducting column boundaries. The work is geared mainly towards RRAM
2. Use enirely new hybrid/VAN composites to enable unprecedented scaling, uniformity and robustness, for both RRAM and neuromorphics, as well as to enable a simpler RRAM device configuration.
Plasmon-enhanced spectroscopy was undertaken on doped HfO2 films, Hf0.5Zr0.5O2 (HZO), to learn about mechanisms of resistive switching. This work with Dr. Giuliana DiMartino,
Having a plasmonic field confined in a region where agglomeration of atomic defects can be triggered by applying an external voltage, allowed us track in-operando ion migration within the switching material enabling to resolve the drift of just few hundreds of ions just by aid of visible light, making it possible to gather information on defect formation (nano-PL) and crystallography (nano-Raman) on thin films which are just few atomic layers. This allows switching in real-time and in-situ during device operation.

We achieved a breakthrough in the understanding of fundamental physical effects governing switching (Fig. 1), and oxide resistive memory .We viewed nanoscale direct tracking of oxygen vacancy migration in 5nm HZO during a pre wake-up stage and showed a structural phase change which influences the device wake-up and which leads to device fatigue. Those effects alter the remanent polarisation value of a device (i.e. increasing during wake-up and decreasing during fatigue), causing the expected device performance to vary over consecutive electronic switching cycles (Fig. 1). A paper has been submitted on this,
As well as exploring HfO2, other systems were also studied (new resistive switching oxides which are CMOS compatible and related to HfO2). We have been collaborating widely with groups across the EU and outside to make strong advances. Hence, we have published on differently doped HfO2 materials, ZrO2. We have also done basic studies on the influence of protons and defects on RS in SrTiO3.

WE also focused whether we can achieve a memristor by exploiting a Schottky-to-Ohmic transition. With such a transition, the memristor is expected to exhibit a significantly larger on/off ratio compared to other structures, e.g. conventional ferroelectric tunnel junctions (FTJs). We made ionic HfO2 by doping in 5% Y to induce oxygen vacancies. We made model epitaxial films od very high quality, 4.5-nm-thick Y-doped HfO2 (YHO) film by pulsed laser deposition (PLD) on LSMO-buffered Nb-STO substrates. The ferroelectricity of the bare films was investigated by X-ray diffraction (XRD) (Fig. 2) and piezoresponse force microscopy (PFM) (Fig. 3). The films are ferroelectric. Current-voltage (J-V) characteristics of the YHO films were carried out in two-terminal device configuration (Fig. 4). We found a Schottky-to-Ohmic transition with a large on/off ratio of more than 500, much larger than the on/off ratio of conventional FTJs.

The semiconducting nature of our YHO films, results in radically different current–voltage characteristics to standard tunnel junctions or memristors which are formed from fully depleted insulating ferro-electric barriers. Hence, we have a polarization-modulated transition from Schottky-barrier-controlled charge transport to Ohmic conduction. Hence, in our semiconducting FE, there is imperfect screening of fixed ferroelectric polarization charges by free charges in the electrodes, and so free charge carriers inside the ferroelectric also act as screening charge. Thus, accumulation or depletion of screening charge density ρ(f) supplied by the semiconducting ferroelectric in a depletion layer in the film, below the electrode, controls the interface transport regime. A polarization state -Pr leads to a depletion of negative free charge density -ρ(f) at the Au interface, resulting in Schottky-barrier controlled conduction. Polarization reversal from -Pr to +Pr leads to accumulation of negative free charge density -ρ(f) at the Au interface, which changes the Schottky tunnelling barrier to an Ohmic contact (Muller M, et al. Applied Physics Letters,Aug. 2022; 121 09350).

We also explored other oxygen ion conducting and inorganic ferroelectric systems, i.e. new complex ferroelectric/multiferroic perovskites, and yttria stabilised zirconia. These help to formulate our overall understanding of combining ionic and ferroelectric effects.

The high operation voltages and the random nature of filamentary switching still hinders the robust accomplishment of real-world tasks. We explored oxide thin films with defined ion channels to achieve robust resistive switching (RS). We explored NaxWO) that has CMOS compatible properties and by regulating the amount of Na+ dopant ions in the system and their direction of transport (similar to ion channel flow in synapses), the formation of different crystalline structures and conductivities is possible. We studied the effect of different Na concentrations with formula unit AxWO3 where x varies from 0.1 to 0.28. The NaxWO3 layers on Si, were grown with top electrodes (TE) of Cr/Au on top of the NaxWO3 active layers. The most promising characteristics were observed in the double active layer device: NbSTO/Na0.2WO3/Na0.28WO3/Cr/Au where the Na0.28WO3 showed a polycrystalline nature with (200) preferred orientation (5 a). During I-V electrical measurements an abrupt increase in current has not been observed, implying that we don’t have filamentary RS that covers the first aim for robust RS (Figure b). EDS on the top active layer shown that the regions of higher sodium content promote perpendicular crystallite growth (Figure 6). That made us assume that the robust RS observed during I-V characterization is due to the predefined Na+ paths. To the best of our knowledge there is no existing report that investigates the effect of doping on PLD grown NaxWO3 for memristive devices.

To further investigate t Na intercalation and movement as the driving mechanism, in-situ Raman was conducted during pulsed voltage application. The measurements shown a shift only on ~880 Na peak found on Na doped samples. Figure 7 shows the in-situ RAMAN of undoped and Figure 8 the doped WO3 samples with their corresponding applied input on the left. The results indicates that Na movement is the driving mechanism during low voltage application, permitting control of RS under low voltage operation. A manuscript is in preparation.

We also explored other composition ion channel structures, as a prelude to WP2 and 4.
Progress beyond state of art:

New CMOS materials of different tyoes showing exemplary resisttive switching performance.
New ferroelcetric memristor mechanism
all figures