6. Memristor effects in SiOx films

Resistive random-access memory (RRAM) [29, 30] is the highly promising candidate for the next-generation nonvolatile memory (NVM), because conventional charge-based memories, namely, dynamic random-access memory and flash memory, have too low capacitance after continuously downscaling into 1X-nm regimes. In addition, an RRAM array can be fabricated in the back end of the line of a complementary metal-oxide-semiconductor circuit, which makes such device an excellent candidate for embedded NVM (e-NVM) application. The typical write speed of RRAM device ranges from 100 ns to 1 μs, which is three to four orders of magnitude faster than flash memory. Such high-speed and process-compatible e-NVM can enable hardware technologies such as artificial intelligence and neuromorphic computing.

The conduction mechanism of RRAM, however, is not fully understood, and it is generally attributed to metallic filament conduction because of its metal-insulatormetal (MIM) structure, where the insulator is usually formed by metal oxide-based dielectric. The first RRAM that does not contain any metal in both the electrodes and dielectric insulator (nonmetal RRAM) is demonstrated here. To obtain RRAM device, a 15-nm-thick SiOx was deposited directly on a p+ -Si substrate by reactive sputtering. Then, a 15-nm-thick amorphous n<sup>+</sup> -Si layer was formed as the top junction electrode. The value x in SiOx was determined to be 0.62. Because no metal or metallic ions were present in the whole RRAM device, metallic filaments were not formed.

Figure 13(a) depicts the measured I-V characteristics of an n<sup>+</sup> -Si/SiO0.62/p<sup>+</sup> -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

Figure 11.

Figure 12.

24

from Si▬N4 tetrahedron.

Nanocrystalline Materials

Experimental XPS spectra of the Si 2p level in SiNx (solid black lines) and the results of theoretical modeling using the IM model (dashed red lines). Green line is peak from Si▬Si4 tetrahedron, and magenta line is peak

Schematic diagrams illustrating the proposed intermediate model of SiNx: (a) a two-dimensional diagram of SiNx structure showing (bottom) the regions of a silicon phase, stoichiometric silicon nitride, and subnitrides and (top) the energy band profile of SiNx in the A–A section (Ec is the conduction band bottom; Ev is the valence band top; Φ<sup>e</sup> and Φ<sup>h</sup> are the energy barriers for electrons and holes at the a-Si–Si3N4 interfaces, respectively; Eg

is the bandgap width). (b) The potential fluctuations in Shklovskii–Efros model.

Figure 13. (a) I-V characteristics of n+ -Si/SiO0.62/p<sup>+</sup> -Si RRAM device under forming, set, and reset operations. I-V dependences of (b) VS, (c) HRS, and LRS (d) currents of n+ -Si/SiOx/p<sup>+</sup> -Si RRAM and fitting curves of Shklovskii-Efros model.

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 current conduction mechanism is crucial for RRAM devices. To understand the conductive mechanism in this completely nonmetal RRAM, the measured I-V curves at different temperatures were further analyzed. Figure 13(b), (c), and (d) 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 percolation model [28]:

$$I = I\_0 \exp\left(-\frac{W\_\varepsilon - \left(\text{Ce}\frac{U}{d}aV\_0^r\right)^{\frac{1}{1+\gamma}}}{kT}\right) \tag{15}$$

and then, it can be assumed that the conducting channel is not continuous. Hence,


Figure 13(a) plots potential switching mechanisms. During the forming step,

2+. It is assumed that after generation of anti-Frenkel pairs, electrons are


, creating the LRS current pass in the SiOx layer.


the RRAM device was under sufficiently high positive voltage, soft breakdown in SiOx occurred and disrupted the covalent bonds [32], generating unbonded Si ions,

interstitial oxygen atoms are formed [30]. Because the atomic size of O is signifi-

SiOx 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

After application of a negative voltage, interstitial oxygen atoms moved away from

Data retention and endurance are necessary characteristics for NVM, and they are related to the nonvolatile behavior and lifetime of an RRAM device. Figure 14

The completely nonmetal RRAM device could achieve favorable retention with a slight resistive window decay from 1.9 <sup>10</sup><sup>4</sup> to 8.7 <sup>10</sup><sup>3</sup> at RT and 3.6 <sup>10</sup><sup>3</sup> to

device under set/reset pulses of +5/5 V for 1 μs. In this case, higher voltages were used than DC switching cases because the energy to disrupt the covalent SiOx bonds equals to the multiplication of I, V, and time. The resistance ratio between HRS and LRS decreased after increasing the pulsed cycles; however, the device exhibited excellent endurance with a resistance window of 89 after 10<sup>5</sup> pulsed

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 the forming process

redistributed to maintain charge neutrality, and new oxygen vacancies (Vo

cantly smaller than Si, the interstitial oxygen atoms and Vo

0

Figure 14 (right) depicts the pulsed endurance of the n<sup>+</sup>


(left) depicts the retention characteristics of the n+


0 ) and



<sup>0</sup> could migrate inside

2+ inside the SiOx layer [31]. When


<sup>0</sup> to rupture the conduction path—the


the simulated results demonstrate that the charge transport of the n<sup>+</sup>


Silicon Nanocrystals and Amorphous Nanoclusters in SiOx and SiNx: Atomic…


the current is conducted through the initial Vo

p+

model.

Figure 14.

characteristics of n<sup>+</sup>

(left) Retention characteristics of n+


DOI: http://dx.doi.org/10.5772/intechopen.86508

O<sup>2</sup>, and Vo

interface of top n<sup>+</sup>

the top n<sup>+</sup>

Vo

27

could transport through the Vo

<sup>0</sup> recombined in the reset process.

1.2 <sup>10</sup><sup>3</sup> at 85°C after 10<sup>4</sup> s retention.

switching cycles [33].

where I0, We, a, V0, C, and γ are the preexponential factor, percolation energy, space scale of fluctuations, energy fluctuation amplitude, numeric constant which is equal to 0.25, and critical index which is equal to 0.9, respectively. The simulation by the Shklovskii-Efros model gives reasonable model parameters to all resistance state (Figure 13(b–d)). The percolation energy decreases with decreasing resistance. The relation a � V<sup>0</sup> 0.52 = 1 � <sup>10</sup>�<sup>7</sup> cm eV0.52 does not change from resistance to resistance. This is due to the fact that decreasing resistance increases space scale of fluctuations a, but decreases energy fluctuation amplitude V0. In addition, it can be said that the Shklovskii-Efros percolation model is applicable to the LRS case,

Silicon Nanocrystals and Amorphous Nanoclusters in SiOx and SiNx: Atomic… DOI: http://dx.doi.org/10.5772/intechopen.86508

Figure 14.

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

conditions, respectively. All state the HRS and LRS currents adhere to the

<sup>I</sup> <sup>¼</sup> <sup>I</sup><sup>0</sup> exp �We � Ce <sup>U</sup>

The current conduction mechanism is crucial for RRAM devices. To understand the conductive mechanism in this completely nonmetal RRAM, the measured I-V curves at different temperatures were further analyzed. Figure 13(b), (c), and (d) depict the measured and modeled I-V curves in the virgin state (VS), HRS, and LRS

> <sup>d</sup> aV<sup>γ</sup> 0 � � <sup>1</sup>

kT !

where I0, We, a, V0, C, and γ are the preexponential factor, percolation energy, space scale of fluctuations, energy fluctuation amplitude, numeric constant which is equal to 0.25, and critical index which is equal to 0.9, respectively. The simulation by the Shklovskii-Efros model gives reasonable model parameters to all resistance state (Figure 13(b–d)). The percolation energy decreases with decreasing resis-

to resistance. This is due to the fact that decreasing resistance increases space scale of fluctuations a, but decreases energy fluctuation amplitude V0. In addition, it can be said that the Shklovskii-Efros percolation model is applicable to the LRS case,

1þγ




0.52 = 1 � <sup>10</sup>�<sup>7</sup> cm eV0.52 does not change from resistance

(15)

voltage once the RRAM switching function was established.


dependences of (b) VS, (c) HRS, and LRS (d) currents of n+

Shklovskii-Efros percolation model [28]:

tance. The relation a � V<sup>0</sup>

26

Figure 13.

(a) I-V characteristics of n+

Nanocrystalline Materials

Shklovskii-Efros model.

(left) Retention characteristics of n+ -Si/SiO0.62/p<sup>+</sup> -Si RRAM devices at RT and 85°C. (right) Endurance characteristics of n<sup>+</sup> -Si/SiO0.62/p<sup>+</sup> -Si RRAM devices.

and then, it can be assumed that the conducting channel is not continuous. Hence, the simulated results demonstrate that the charge transport of the n<sup>+</sup> -Si/SiO0.62/ p+ -Si RRAM in VS, HRS, and LRS are described by the Shklovskii-Efros percolation model.

Figure 13(a) plots potential switching mechanisms. During the forming step, the current is conducted through the initial Vo 2+ inside the SiOx layer [31]. When the RRAM device was under sufficiently high positive voltage, soft breakdown in SiOx occurred and disrupted the covalent bonds [32], generating unbonded Si ions, O<sup>2</sup>, and Vo 2+. It is assumed that after generation of anti-Frenkel pairs, electrons are redistributed to maintain charge neutrality, and new oxygen vacancies (Vo 0 ) and interstitial oxygen atoms are formed [30]. Because the atomic size of O is significantly smaller than Si, the interstitial oxygen atoms and Vo <sup>0</sup> could migrate inside SiOx 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<sup>+</sup> -i junction. Once the conduction path was formed, electrons could transport through the Vo 0 , creating the LRS current pass in the SiOx layer. After application of a negative voltage, interstitial oxygen atoms moved away from the top n<sup>+</sup> -i junction and recombined with Vo <sup>0</sup> 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 the forming process Vo <sup>0</sup> recombined in the reset process.

Data retention and endurance are necessary characteristics for NVM, and they are related to the nonvolatile behavior and lifetime of an RRAM device. Figure 14 (left) depicts the retention characteristics of the n+ -Si/SiO0.62/p<sup>+</sup> -Si RRAM device. The completely nonmetal RRAM device could achieve favorable retention with a slight resistive window decay from 1.9 <sup>10</sup><sup>4</sup> to 8.7 <sup>10</sup><sup>3</sup> at RT and 3.6 <sup>10</sup><sup>3</sup> to 1.2 <sup>10</sup><sup>3</sup> at 85°C after 10<sup>4</sup> s retention.

Figure 14 (right) depicts the pulsed endurance of the n<sup>+</sup> -Si/SiO0.62/p<sup>+</sup> -Si RRAM device under set/reset pulses of +5/5 V for 1 μs. In this case, higher voltages were used than DC switching cases because the energy to disrupt the covalent SiOx bonds equals to the multiplication of I, V, and time. The resistance ratio between HRS and LRS decreased after increasing the pulsed cycles; however, the device exhibited excellent endurance with a resistance window of 89 after 10<sup>5</sup> pulsed switching cycles [33].
