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Investigation of Electrical Properties of the Al/SiO_{2}/n^{++}-Si Resistive Switching Structures by Means of Static, Admittance, and Impedance Spectroscopy Measurements

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## Abstract

**:**

^{++}-Si structures is observed and studied by means of DC, small-signal admittance, and complex impedance spectroscopy measurements. Possible transport mechanisms in the high and low resistance states are identified. Based on the results of the applied measurement techniques, an electrical equivalent circuit of the structure is proposed. We discuss the effect of parasitic elements influencing the measurement results and show that a proper model can give useful information about the electrical properties of the device. A good agreement between the characteristics of the proposed equivalent circuit and the experimental data, based on different measurement procedures, confirms the validity of the used methodology and its applicability to the electrical characterization of RRAMs.

## 1. Introduction

_{2}, TiO

_{2}, and Ta

_{2}O

_{5}[10,11,12,13,14,15,16,17,18,19]. Silicon oxide has also been tested as a potential candidate for an RS layer in CBRAM and OxRAM devices [20,21,22,23,24,25,26,27]. It would be a promising candidate for these applications due to its well-known properties and fabrication techniques [28,29]. However, there is only a limited number of works related to SiO

_{2}as an RS layer, and further studies are needed.

_{2}/very highly doped Si(n) structures with the use of the admittance and impedance spectroscopy measurements. We show that a change in the compliance current results in different conductance levels, which is common for some types of RRAM devices [39,40]. Possible transport mechanisms in the high resistance state (HRS) and the low resistance state (LRS) are indicated and identified. An equivalent circuit of the structure is proposed, which can provide useful information about the RS layer. The results of the work give new insight into the possible origins of the RS effect in the investigated structures.

## 2. Materials and Methods

_{2}/Ar atmosphere in 400 °C for 30 min. The schematic cross-section of the investigated devices is presented in Figure 1.

## 3. Results and Discussion

#### 3.1. DC Measurements

_{2}/n

^{++}Si structure with a gate pad diameter of 156 μm (S1). Initial electroforming voltage is above 2.5 V. Structures were measured with the compliance current (CC) of 20 mA. For a given CC, the set voltage is above 1.2 V. We identify the transport mechanism as the space charge limited current (SCLC). In Figure 4, we show the slope of I–V curves at different states and voltage ranges. In the high-resistance state (HRS) of the SET cycle, we observe that initially, the current is proportional to the applied voltage (Ohmic conduction), and then it obeys Child’s quadratic law, which is related to partially filled traps [41,42]. In the high field region, a higher slope of the curve is observed, which is related to fully filled traps. In the low-resistance state (LRS), we have mainly Ohmic conduction. In the RESET cycle, the Ohmic conduction is mainly observed at low voltages. At higher voltage values, carrier transport through the dielectric is a mix of different types of transport mechanisms, and it is hard to identify it in a simple way.

#### 3.2. Small-Signal Measurements

_{PM}values at −0.5 V and +0.5 V, as marked in Figure 6a. The imaginary part of the measured admittance is presented in Figure 6b. In the HRS state, the susceptance is positive for negative gate voltages up to −1.0 V, indicating the capacitive behavior of the structure. While moving towards positive gate voltages, one can observe that susceptance becomes negative in the bias range above 0.65 V. The closer to the set voltage, the lower susceptance is observed, and the characteristic tends to the curve representing the susceptance in the LRS. In the LRS, the structure behavior is mainly inductive within the considered gate voltage range. Only in a limited voltage range close to 0.0 V, the structure exhibits capacitive behavior. In general, the admittance of a considered device can be described by a simple electrical equivalent circuit, which is presented in Figure 7.

_{P}) and resistance (R

_{P}) cannot be omitted when the resistance (R

_{RRAM}in Figure 7) in the parallel RC circuit of RRAM structure becomes low, and the capacitance C

_{RRAM}simultaneously becomes high. Such a situation occurs when a conductive filament inside RRAM dielectric is formed. As a result, the admittance of the electrical equivalent circuit presented in Figure 7 can be calculated as follows:

_{RRAM}) increases according to the increase in the absolute value of the applied voltage. It results from a shortening gap between the growing filament and the top electrode of the device. In such a situation, the parasitic inductance (L

_{P}) can dominate the numerator of Equation (3), imposing the negative value of the measured susceptance. A similar mechanism can be a reason for the negative susceptance of the investigated structures in LRS (Figure 6b).

#### 3.3. Complex Impedance Spectroscopy

_{ox}represents the capacitance related to the gate insulator of the device (its value directly corresponds to the approximately 5 nm thick SiO

_{2}layer and the gate electrode diameter of 156 um). Leakage resistance (R

_{leakage}) represents the gate leakage current that results from tunneling or other transport mechanisms, different than the transport through conducting filaments. The third branch, comprising series resistance and parallel RC circuits, represents the cumulative electrical behavior of conductive filaments. R

_{parasitic}describes the spread parasitic resistance brought in by the measurement setup and the series resistance of the device. L

_{parasitic}represents the uncompensated inductance of the wiring and the switching matrix in the measurement setup (Figure 2).

_{4}reflects the electrical behavior of the conducting filament(s).

_{parasitic}, L

_{parasitic}). During switching on, the filament increases its volume, so the resultant value of spread resistance of the device decreases. At the same time, the gap between the filament and the top electrode decreases substantially. It is represented by capacitance, the value of which is greater than the value of capacitances corresponding to filament structure at HRS.

_{OX}and R

_{leakage}were omitted in the electrical equivalent circuit in LRS because their values are negligible compared to C and R in a parallel RC network (Figure 10).

_{S}in Figure 2) of the measurement setup (wiring, switching matrix, etc.). In our considerations, a suitable agreement between the simulated and measured data was obtained for the parasitic inductance; its value of 3.5 μH is comparable to L

_{S}. This gives rise to the claim that the L

_{parasitic}value in our model is closely related to the measurement setup, rather than the conduction mechanism within the RRAM structure. Based on the obtained measurement, we believe that the studied structure is an OxRAM device. However, further studies are needed to confirm this claim.

_{PM}, C

_{PM}) whose values correspond to the measured ones marked in Figure 6a,b—real and imaginary parts of admittance, respectively. Static equivalent resistance (R

_{DC}) value extracted from the proposed equivalent circuits agrees with the differential resistance R

_{diff}values obtained from the measured static I–V characteristics for both considered bias voltages at HRS and LRS (Figure 12).

## 4. Conclusions

_{2}/n

^{++}Si RRAM structures are presented and analyzed. An influence of the compliance current is shown. The small-signal admittance and complex impedance measurements are used to characterize the structure and propose its electrical equivalent circuit. A suitable agreement between characteristics of the proposed equivalent circuit and the experimental data based on different measurement procedures (DC, small-signal admittance, complex impedance spectroscopy) confirms that the used methodology can be a useful technique for investigating electrical properties of RRAM devices, giving new insights into the origins of the resistive switching phenomenon.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 3.**Measured current-voltage characteristics of Al/SiO2/n

^{++}Si RRAM structure S1 with a gate diameter of 156 μm and CC = 20 mA.

**Figure 4.**Current-voltage characteristics of Al/SiO

_{2}/n

^{++}Si RRAM structure S1 with the gate diameter of 156 μm and CC = 20 mA at SET (

**a**) and RESET (

**b**) cycle with fitted curves of different slopes.

**Figure 5.**Measured current-voltage characteristics of Al/SiO2/n

^{++}Si RRAM structure S2 with the gate diameter of 74 μm and various compliance set currents.

**Figure 6.**Small-signal admittance components for S1 structure at the frequency f = 100 kHz in different resistance states versus the gate bias voltage. (

**a**) conductance; (

**b**) susceptance.

**Figure 7.**Electrical equivalent circuit of RRAM device with series parasitic components and its measurement parallel depiction.

**Figure 8.**Complex impedance spectra of investigated structure (S1) for a frequency range of 20 kHz–1 MHz at different gate bias voltage values in (

**a**) HRS and (

**b**) LRS.

**Figure 9.**Electrical equivalent circuit for the measured device in HRS at two bias points, (

**a**) V

_{g}= 0.5 V (

**b**) V

_{g}= −0.5 V.

**Figure 10.**Electrical equivalent circuit for the measured device in LRS at two bias points, (

**a**) V

_{g}= 0.5 V; (

**b**) V

_{g}= −0.5 V.

**Figure 12.**R

_{diff}values extracted from the measurements of S1 structure at different resistance states and considered gate bias voltages.

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**MDPI and ACS Style**

Wiśniewski, P.; Jasiński, J.; Mazurak, A.; Stonio, B.; Majkusiak, B.
Investigation of Electrical Properties of the Al/SiO_{2}/n^{++}-Si Resistive Switching Structures by Means of Static, Admittance, and Impedance Spectroscopy Measurements. *Materials* **2021**, *14*, 6042.
https://doi.org/10.3390/ma14206042

**AMA Style**

Wiśniewski P, Jasiński J, Mazurak A, Stonio B, Majkusiak B.
Investigation of Electrical Properties of the Al/SiO_{2}/n^{++}-Si Resistive Switching Structures by Means of Static, Admittance, and Impedance Spectroscopy Measurements. *Materials*. 2021; 14(20):6042.
https://doi.org/10.3390/ma14206042

**Chicago/Turabian Style**

Wiśniewski, Piotr, Jakub Jasiński, Andrzej Mazurak, Bartłomiej Stonio, and Bogdan Majkusiak.
2021. "Investigation of Electrical Properties of the Al/SiO_{2}/n^{++}-Si Resistive Switching Structures by Means of Static, Admittance, and Impedance Spectroscopy Measurements" *Materials* 14, no. 20: 6042.
https://doi.org/10.3390/ma14206042