Intrinsic SiOx-based unipolar resistive switching memory. II. Thermal effects on charge transport and characterization of multilevel programing

Abstract

Multilevel programing and charge transport characteristics of intrinsic SiOx-based resistive switching memory are investigated using TaN/SiOx/n++Si (MIS) and TiW/SiOx/TiW (MIM) device structures. Current transport characteristics of high- and low-resistance states (HRS and LRS) are studied in both device structures during multilevel operation. Analysis of device thermal response demonstrates that the effective electron energy barrier is strongly dependent on the resistance of the programed state, with estimates of 0.1 eV in the LRS and 0.6 eV in the HRS. Linear data fitting and conductance analyses indicate Poole-Frenkel emission or hopping conductance in the low-voltage region, whereas Fowler-Nordheim (F-N) or trap-assisted tunneling (TAT) is indicated at moderate voltage. Characterizations using hopping transport lead to hopping distance estimates of ∼1 nm in the LRS for both device structures. Relative permittivity values (εr) were extracted using the Poole-Frenkel formulism and estimates of local filament temperature, where εr values were ∼80 in the LRS and ∼4 in the HRS, suggesting a strongly polarized medium in the LRS. The onset of F-N tunneling or TAT corresponds to an observed “overshoot” in the I-V response with an estimated threshold of 1.6 ± 0.2 V, in good agreement with reported electro-luminescence results for LRS devices. Resistive switching is discussed in terms of electrochemical reactions between common SiO2 defects, and specific defect energy levels are assigned to the dominant transitions in the I-V response. The overshoot response in the LRS is consistent with TAT through either the Eγ' oxygen vacancy or the hydrogen bridge defect, both of which are reported to have an effective bandgap of 1.7 eV. The SET threshold at ∼2.5 V is modeled as hydrogen release from the (Si-H)2 defect to generate the hydrogen bridge, and the RESET transition is modeled as an electrochemical reaction that re-forms (SiH)2. The results provide further insights into charge transport and help identify potential switching mechanisms in SiOx-based unipolar resistive switching memory.

Discussion on device structures and hermetic encapsulation for SiOx random access memory operation in air

“…Only encapsulated bulk devices were able to switch in air. While the presence of a SiO x edge has often been considered necessary for SiO x -based devices to electroform and switch, 14,15 these results demonstrate that this is not the case. The reason why encapsulated edge devices fail to switch in air may be related to the detailed structure of the edge.…”

Review of Recently Progress on Neural Electronics and Memcomputing Applications in Intrinsic SiOx-Based Resistive Switching Memory

“…Although the state's stability still needs to be improved (no equal split of resistance states), the retention reliability test demonstrates ` operation by using different SET voltages, and no degradation is observed for more than 10 3 s, thus confirming the stable, nonvolatile nature of the SiO x -based 1D-1R devices. In recent studies, a possible proton exchange model consistent with the observed resistive switching I-V response has been proposed, as shown in Figure 2f [59,60]. Several studies have used transmission electron microscopy (TEM) to document the presence of Si nanocrystals within the CF [43,61,62], but it is not yet clear whether resistive switching (RS) is the result of an overall increase in nanocrystal size or whether switching occurs in "GAP" regions in between nanocrystals.…”

“…Adding a proton to Si-H-Si forms the nonconductive (SiH) 2 defect and proton desorption from (SiH) 2 reforms Si-H-Si, which are wellunderstood electrochemical reactions that could enable localized switching without incorporating ion diffusion or drift mechanisms into the model. The SET transition voltage from HRS to LRS occurs at ~2.5 V in the I-V response, and is very near the activation energy for proton desorption from SiH (~2.5 eV), thus making the defect transformation from (SiH) 2 to Si-H-Si a logical assignment for the SET transition [59,60]. In this model, the proton that is lost from (SiH) 2 reacts electrochemically with (SiOH) 2 , which is simply chemisorbed H 2 O, to form the fixed positive charged H 3 O + defect.…”

“…Incorporating known proton exchange reactions that can dramatically alter the conductivity of specific defects further leads to a model where the LRS has a large concentration of conductive defects within the switching region, and, conversely, when the device is programmed to the HRS, most of the defects are converted to their nonconductive form. The electrically conductive hydrogen bridge (Si-H-Si) is viewed as the most likely defect responsible for the LRS due to the location of its energy levels relative to the oxide conduction band and its small effective bandgap energy [59,60]. Adding a proton to Si-H-Si forms the nonconductive (SiH) 2 defect and proton desorption from (SiH) 2 reforms Si-H-Si, which are wellunderstood electrochemical reactions that could enable localized switching without incorporating ion diffusion or drift mechanisms into the model.…”

“…In this model, the proton that is lost from (SiH) 2 reacts electrochemically with (SiOH) 2 , which is simply chemisorbed H 2 O, to form the fixed positive charged H 3 O + defect. The transition from LRS to HRS is modeled as being initiated by electron injection into H 3 O + that induces proton release and electrochemical reaction with Si-H-Si to reform (SiH) 2 [59,60]. The localized proton exchange switching model can thus be written as (SiH) 2 + (SiOH) 2 ↔ Si-H-Si + Si 2 =O-H 3 O + , where a voltage drop of ~2.5 V across the switching is required to drive the reversible reaction.…”