All Nonmetal Resistive Random Access Memory

Traditional Resistive Random Access Memory (RRAM) is a metal-insulator-metal (MIM) structure, in which metal oxide is usually used as an insulator. The charge transport mechanism of traditional RRAM is attributed to a metallic filament inside the RRAM. In this paper, we demonstrated a novel RRAM device with no metal inside. The N+-Si/SiOx/P+-Si combination forms a N+IP+ diode structure that is different from traditional MIM RRAM. A large high-resistance/low-resistance window of 1.9 × 104 was measured at room temperature. A favorable retention memory window of 1.2 × 103 was attained for 104 s at 85 °C. The charge transport mechanism of virgin, high- and low-resistance states can be well modeled by the single Shklovskii-Efros percolation mechanism rather than the charge transport in metallic filament. X-ray photoelectron spectroscopy demonstrated that the value of x in SiOx was 0.62, which provided sufficient oxygen vacancies for set/reset RRAM functions.


Results
depicts the measured I-V characteristics of an N + -Si/SiO x /P + -Si RRAM device. During the forming step, the device was first subjected to a 6 V and 100 μA compliance current stress to attain the LRS. The same device was reset into HRS after a negative voltage bias. Then, the device was set to LRS again under a positive voltage bias. However, the positive set voltage was lower than the forming voltage once the RRAM switching function was established.
The charge transport mechanism is crucial for RRAM devices. To understand the charge transport mechanism in this completely nonmetal RRAM, we further analyzed the measured I-V curves at different temperatures. Figure 2(a-c) depict the measured and modeled I-V curves in the virgin state (VS), HRS and LRS conditions, respectively. All state the HRS and LRS currents adhere to the Shklovskii-Efros (S-E) percolation model: www.nature.com/scientificreports www.nature.com/scientificreports/ where I 0 , W e , a, V 0 , C and ɣ are the preexponential factor, percolation energy, space scale of fluctuations, energy fluctuation amplitude, numeric constant and it is equal to 0.25, critical index and it is equal to 0.9, respectively. The simulation by the S-E model gives reasonable model parameters to all resistance state (Fig. 2). The percolation energy decreases with decreasing resistance. Also, in the S-E model for LRS, the active contact area reduction of the charge involved in the transport is taken into account. The relation a × V 0 0.52 = 1 × 10 −7 cm·eV 0.52 does not change from resistance to resistance. This is due to the fact that with decreasing resistance increases space scale of fluctuations a but decreases energy fluctuation amplitude V 0 . In addition, it can be said that the S-E percolation model is applicable to the LRS case, then it can be assumed that the conducting channel is not continuous. Hence, the results demonstrate that the charge transport of the N + -Si/SiO x /P + -Si RRAM in VS, HRS and LRS are described by the S-E percolation model. For more details on other models and their inapplicability to HRS, see the ref. 24 .
To further understand the device characteristics, material analyses were performed. Figure 3 displays the cross-sectional transmission electron microscope (TEM) image of this RRAM device. As depicted, the RRAM device was fabricated directly on a P + -Si substrate, followed by a 15-nm thick SiO x dielectric layer and a N + -Si top electrode. The SiO x layer was further analyzed using X-ray photoelectron spectroscopy (XPS). The sample surface was pre-sputtered to ensure that the native oxide did not influence the measurements. Figure 4 displays the XPS spectrum. From the peaks of O1s, Si2s, and Si2p, the mole fraction x in SiO x was determined to be 0.62. Because no metal or metallic ions were present in the whole RRAM device, metallic filaments were not formed [13][14][15][16] . In accordance with XPS experimental data certain fraction of vacancies exist in dielectric immediately after synthesis. The migration of oxygen vacancies plays an important role for current conduction. Figure 5 plots potential switching mechanisms. During the forming step, the current conducted through the initial V o 2+ inside the SiO x layer 14 . When the RRAM device was under sufficiently high positive voltage, soft breakdown in SiO x occurred and disrupted the covalent bonds 25 ) and interstitial oxygen atoms are formed 29 . Because the atomic size of O is significantly smaller than Si, the interstitial oxygen atoms and V o 0 could migrate inside SiO x under the applied electric field. At the end of the forming process, the interstitial oxygen atoms were attracted to the positive voltage and accumulated at the interface of top N + I junction. Once the conduction path was formed, electrons could transport through the V o 0 creating the LRS current pass in the SiO x layer. After application of a negative voltage, interstitial oxygen atoms moved away from the top N + I junction and recombined with V o 0 to rupture the conduction path-the reset process. After a positive voltage was applied again, the set process behaved as the forming process to form a conduction path, but under a lower positive voltage than the forming voltage due to not all generating in forming process V o 0 recombined in reset process. Data retention is the necessary characteristics for NVM, and they are related to the nonvolatile behaviour and lifetime of an RRAM device. Figure 6 depicts the retention characteristics of the N + -Si/SiO x /P + -Si RRAM device. The completely nonmetal RRAM device could achieve favourable retention with a slight resistive window decay from 1.9 × 10 4 to 8.7 × 10 3 at RT and 3.6 × 10 3 to 1.2 × 10 3 at 85 °C after 10 4 s retention.  www.nature.com/scientificreports www.nature.com/scientificreports/

Conclusion
A completely nonmetal RRAM device was demonstrated for the first time. A large resistance window of 1.9 × 10 4 at RT was measured. An excellent retention resistance window of 1.2 × 10 3 was obtained for 10 4 s retention at 85 °C. In addition, the charge transport of the N + -Si/SiO x /P + -Si RRAM in VS, HRS and LRS are described by the S-E percolation model. And the V o 0 migration played an important role in the set/reset functions.

Methods
The RRAM device was made on a highly doped P + -Si substrate with a resistance lower than 0.01 Ω per square, which was also used as a bottom electrode. After standard RCA clean, the native oxide on P + -Si wafer was removed by a dilute hydrofluoric (HF) acid (HF: H 2 O = 1:100) solution for 60 sec. Then, a 15-nm-thick SiO x was deposited by reactive sputtering. The composition ratio inside the SiO x was determined using XPS. Then, a 15-nm-thick amorphous N + -Si layer, was formed as the top junction electrode. The diameter of the fabricated device was 120 μm. The I-V characteristics was measured using an HP4155B parameter analyzer. The voltage was applied on the N + -Si (top electrode) side and P + -Si (bottom electrode) were grounded. The sweep rate is 0.5 V/s. A Thermo K-alpha system with an X-ray spot size of 400 μm was employed for XPS measurements. The cross-sectional image of the RRAM device was measured using a JEOL 2010F high-resolution TEM. The modeled data for HRS and LRS were fitted under positive and negative voltage bias, respectively.