Enhanced Switching Stability in Ta2O5 Resistive RAM by Fluorine Doping



Sedghi, N ORCID: 0000-0002-2004-6159, Li, H, Brunell, IF, Dawson, K ORCID: 0000-0003-3249-8328, Guo, Y, Potter, RJ ORCID: 0000-0003-0896-4536, Gibbon, JT ORCID: 0000-0003-1548-0791, Dhanak, VR, Zhang, WD, Zhang, JF
et al (show 3 more authors) (2017) Enhanced Switching Stability in Ta2O5 Resistive RAM by Fluorine Doping. Applied Physics Letters, 111 (9).

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Abstract

The effect of fluorine doping on the switching stability of Ta2O5 resistive random access memory devices is investigated. It shows that the dopant serves to increase the memory window and improve the stability of the resistive states due to the neutralization of oxygen vacancies. The ability to alter the current in the low resistance state with set current compliance coupled with large memory window makes multilevel cell switching more favorable. The devices have set and reset voltages of <1 V with improved stability due to the fluorine doping. Density functional modeling shows that the incorporation of fluorine dopant atoms at the two-fold O vacancy site in the oxide network removes the defect state in the mid bandgap, lowering the overall density of defects capable of forming conductive filaments. This reduces the probability of forming alternative conducting paths and hence improves the current stability in the low resistance states. The doped devices exhibit more stable resistive states in both dc and pulsed set and reset cycles. The retention failure time is estimated to be a minimum of 2 years for F-doped devices measured by temperature accelerated and stress voltage accelerated retention failure methods. There is increasing interest in resistive random access memory (RRAM) as a potential substitute for existing non-volatile memory (NVM). In RRAM devices incorporating metal oxides, switching between different resistive states takes place by field-assisted diffusion of defect sites, usually oxygen vacancies (Ovac), producing a conductive filament (CF), which is formed, ruptured, and restored by applying the appropriate voltages.1,2 Control of the Ovac profile in the oxide film, therefore, is crucial in improving the device performance. The key requirements are a large switching memory window for noise immunity and stability of resistive states. Atomic doping with different elements such as N, Ti, Au, Cu, Gd, and Si has been reported as an efficient method to modify the oxygen vacancy profile and their movement in forming the conductive paths.3–11 Recently, the doping of dielectrics with fluorine has attracted attention because its electronegativity can significantly influence the energy of mid bandgap states which affects charge trapping and mobility.12–16 In this work, we have used atomic layer deposition (ALD) to fabricate fluorine doped Ta2O5 films, which shows a widening of the switching memory window by up to four orders of magnitude together with improved resistive states stability, endurance, and retention time. This improvement is realized by reducing the high resistance state (HRS) current, due to the elimination of excess conduction paths by passivating oxygen vacancies with fluorine dopant atoms. The amorphous tantalum oxide based metal-insulator-metal (MIM) structures were fabricated on Corning glass or silicon wafer substrates. The devices comprise a bottom electrode of 50 nm Pt, with a 10 nm Cr adhesion layer, deposited by dc magnetron sputtering, the oxide layer, 15 nm thick Ta2O5 or F-doped Ta2O5 deposited by conventional ALD, using the precursor Ta(OC2H5)5 and H2O as an oxidant, and the top electrode of 30 nm Ti, with a 60 nm Pt capping layer, deposited by rf sputtering. In-situ F doping was achieved using an aqueous NH4F solution as a co-reagent and as a substitute for pure water in the ALD deposition process. Substituting 100% of the co-reagent cycles with the NH4F solution results in an upper F atomic concentration of 1%, as estimated from X-ray photoelectron spectroscopy (XPS). Conventional photolithography and metal lift-off were used to define the top and bottom contacts and device active area of 512 overlapping and 512 cross-line square devices on each sample, with dimensions of 2–150 μm. The plan view optical microscopy image of a group of 4 devices and a transmission electron microscopy (TEM) cross sectional image of a device are shown in Fig. 1. The dc set-reset cycles were performed by return sweeps of a dc voltage from 0 V to a positive value, applied to the Ti contact, for set and to a negative value for reset, using either HP 4155A or Agilent B1500A Semiconductor Parameter Analyzers. The low resistance state (LRS) resistance was programmed by setting the current compliance during the set cycle. The resistance at each resistive state was calculated at a “read” voltage of −0.1 V. The temperature accelerated retention failure measurements were performed at elevated temperatures up to 250 °C on a heated stage using a Signatone S-1060 temperature controller. The variation of state resistance with time, up to 105 s at logarithmically spaced intervals, was calculated from the measured current in the sampling mode at a read voltage of −0.1 V. The applied voltage was kept at 0 V between the sampling intervals to minimize the disturbance of the state current with voltage. For constant voltage stress (CVS) accelerated failure measurement, the appropriate hold voltage was applied between sampling intervals. The pulsed set and reset endurance measurements, up to 1 × 106 cycles, were performed using an Agilent 33522A function generator, an Agilent DSO7012B oscilloscope, and a multi-gain transresistance amplifier with current limit circuit, designed in-house. The statistical variability of each programmed state was analyzed after performing 30 alternate set and reset cycles, for dc sweeps, and up to 1 × 106 cycles, for pulsed sweeps, and calculating the cumulative distribution function (CDF) of resistance at each state.

Item Type: Article
Depositing User: Symplectic Admin
Date Deposited: 26 Jun 2017 10:13
Last Modified: 21 Jan 2021 09:56
DOI: 10.1063/1.4991879
Related URLs:
URI: https://livrepository.liverpool.ac.uk/id/eprint/3008137