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Article

Enhanced Transparency and Resistive Switching Characteristics in AZO/HfO2/Ti RRAM Device via Post Annealing Process

by
Yuseong Jang
,
Chanmin Hwang
,
Sanggyu Bang
and
Hee-Dong Kim
*
Department of Semiconductor Systems Engineering, Convergence Engineering for Intelligent Drone, and Institute of Semiconductor and System IC, Sejong University, 209, Neungdong-ro, Gwangjin-gu, Seoul 05006, Republic of Korea
*
Author to whom correspondence should be addressed.
Inorganics 2024, 12(12), 299; https://doi.org/10.3390/inorganics12120299
Submission received: 18 October 2024 / Revised: 19 November 2024 / Accepted: 20 November 2024 / Published: 21 November 2024
(This article belongs to the Special Issue Optical and Quantum Electronics: Physics and Materials)
Browse Figures
Figure 1
<p>(<b>a</b>) Schematic structure, (<b>b</b>) cross-section FE-SEM images, and (<b>c</b>) transmittance at wavelengths of 200 nm to 1100 nm of the proposed T-RRAM.</p> "> Figure 2
<p>Resistive switching characteristics of proposed T-RRAM (<b>a</b>) without RTA and (<b>b</b>) after RTA at 450 °C for 60 s in a nitrogen atmosphere. (<b>c</b>) Retention and (<b>d</b>) endurance characteristics of T-RRAM without RTA and after RTA.</p> "> Figure 3
<p>XPS spectra of the Ti 2p region of the Ti top electrode without RTA and after RTA.</p> "> Figure 4
<p>XRD patterns of as-deposited HfO<sub>2</sub> and Ti films and HfO<sub>2</sub> and Ti films after RTA, and (inset) average grain size of the HfO<sub>2</sub> and Ti films.</p> "> Figure 5
<p>SCLC mechanism at the positive bias of proposed T-RRAM (<b>a</b>) without RTA and (<b>b</b>) after RTA.</p> "> Figure 6
<p>Band diagram of proposed T-RRAM (<b>a</b>) in the low-voltage region, (<b>b</b>) medium-voltage region and (<b>c</b>) high-voltage region. (The orange arrows indicate the direction of the electric field).</p> "> Figure 7
<p>Nyquist plot of proposed T-RRAM at (<b>a</b>) HRS and (<b>b</b>) LRS.</p> "> Figure 8
<p>Equivalent circuit of proposed T-RRAM at (<b>a</b>) HRS and (<b>b</b>) LRS.</p> ">
Versions Notes

Abstract

:
As interest in transparent electronics increases, ensuring the reliability of transparent RRAM (T-RRAM) devices, which can be used to construct transparent electronics, has become increasingly important. However, defects and traps within these T-RRAM devices can degrade their reliability. In this study, we investigated the improvement of transparency and reliability of T-RRAM devices with an AZO/HfO2/Ti structure through rapid thermal annealing (RTA) at 450 °C for 60 s in a nitrogen atmosphere. The device without RTA exhibited a low transmittance of 30%, whereas the device with RTA showed a significantly higher transmittance of over 75%. Furthermore, the device operated at lower current levels after RTA, which resulted in a reduction in its operating voltages, and the forming, setting, and reset voltages changed from 3.3, 2.4, and −5.1 V, respectively, to 2, 1, and −2.7 V. This led to an improvement in the endurance characteristics of the device, which thereby suggests that these improvements can be attributed to a reduction in the defects and trap density within the T-RRAM device caused by RTA.
Keywords:
T-RRAM; TCO; AZO; RTA; trap

1. Introduction

With the growing interest in transparent electronics, such as transparent displays, transparent RRAM (T-RRAM) devices based on metal oxide semiconductors (MOS), or transparent conductive oxides (TCO) like indium tin oxide (ITO), indium gallium zinc oxide (IGZO), and aluminum zinc oxide (AZO) are emerging with significant industrial potential [1]. Additionally, recent studies on photosensitive and optoelectronic neuromorphic memristors, which utilize the photogenerated charge carriers and optoelectronic properties of these MOS and TCO materials [2,3,4], are further expanding the potential application fields of these materials. ITO and IGZO are representative TCO materials that are widely studied in T-RRAM due to their high conductivity and transmittance [5]. However, the limited availability of indium, its extraction being geographically restricted, its high cost, and its toxicity [6] necessitate research into alternative materials for T-RRAM devices. AZO is, in particular, a TCO material doped with Al in order to compensate for the low conductivity of ZnO, which makes it a strong candidate to replace indium-based TCO. AZO offers a promising alternative, which is due to its lower cost and abundance compared to indium-based materials, but the trade-offs with regard to conductivity and performance stability present challenges in its widespread adoption. Recent efforts have focused on optimizing the properties of AZO via various techniques, such as doping concentration adjustments and post-deposition treatments [7], which are shown in Table 1, but the most recent studies heavily rely on the simultaneous use of AZO and ITO. It is, therefore, difficult to regard these studies as focused on replacing ITO in T-RRAM.
Furthermore, even if the transparency of the device is achieved, a large number of defects and traps within the device can degrade its reliability [8], significantly reducing its commercial viability as a transparent device. This is especially problematic in applications requiring long-term stability, where defects and trap formation can lead to premature device failure or inconsistent performance. Addressing these reliability concerns is critical for ensuring the commercialization of transparent electronic products. Therefore, efforts to ensure the stability of devices through annealing under various conditions are ongoing to guarantee the reliability of RRAMs based on transparent materials. For example, Zhao et al. reported improved reliability of TaOx-based RRAM after rapid thermal annealing (RTA) at 300 °C for 120 s in an oxygen atmosphere [9].
Table 1. Summary of the recently reported AZO-based transparent RRAM devices.
Table 1. Summary of the recently reported AZO-based transparent RRAM devices.
DeviceSwitching TypeThreshold VoltageON/OFF RatioTransparencyRef.
AZO/SiOx/ITObipolar7.2 V1685%[4]
AZO/ZnO/ITObipolar4.5 V1480%[5]
ITO/SiCN/AZObipolar6.4 V>100085[6]
ITO/SiCN/AZObipolar6.4 V>100085[10]
AZO/CeO2/ITObipolar−7 V>1092.5[11]
RTA can be a key technology in regard to enhancing the performance of T-RRAM among the various annealing methods. RTA exposes thin films to high temperatures for short durations, which promotes crystallization, improves electrical conductivity, and enhances optical transmittance [12]. The crystallization process increases the order within the thin films and also reduces the number of grain boundaries, which are often the sites for defect and trap formations [13]. RTA can greatly minimize electron scattering and improve the overall current flow within the device by reducing grain boundaries and defects [14]. Moreover, the ability of RTA to be precisely controlled in terms of temperature and duration allows for the fine-tuning of material properties, which ensures that the process can be adapted in order to meet specific performance requirements across various applications [15]. In other words, the short processing time of RTA minimizes thermal damage as well as quickly improves the device’s performance, which makes it a crucial factor in T-RRAM processing [16].
In this paper, we propose a T-RRAM device using an AZO/HfO2/Ti structure. The proposed device initially showed a low transmittance of 30% in the as-deposited state. However, after RTA at 450 °C for 60 s in a nitrogen atmosphere (hereafter referred to as ‘after RTA’ for short), the transmittance increased to over 75%. Furthermore, the T-RRAM after RTA exhibited improved switching operation and stability characteristics. The X-ray diffraction (XRD) results confirmed that crystallization occurred, with a significant increase in grain size for both the HfO2 and Ti films after RTA. Additionally, X-ray photoelectron spectroscopy (XPS) analysis revealed a transformation of the Ti top electrode into TiN following RTA. Subsequently, we investigated how RTA impacts the conduction mechanism and impedance characteristics of the proposed T-RRAM device. Although all devices were found to be governed by the SCLC conduction mechanism, slight variations in behavior were observed, which we attributed to the reduction in defects and trap density due to RTA. Additionally, we established equivalent physical models for both the low resistance state (LRS) and high resistance state (HRS), and we compared the parameters of their components.

2. Results and Discussion

First, our proposed T-RRAM structure is illustrated in Figure 1a, and as shown in Figure 1b, the AZO/HfO2/Ti structure was confirmed through field-emission scanning electron microscopy (FE-SEM). After RTA, the thickness of the Ti layer decreased from 10 nm to 7 nm, and the thickness of the HfO2 layer decreased from 20 nm to 13 nm. This reduction in thickness is likely due to RTA-induced crystallization, which reduced the grain boundary density in the films [17]. After that, to evaluate whether the proposed AZO/HfO2/Ti structured T-RRAM is suitable for transparent applications, we measured its transmittance, as shown in Figure 1c. The proposed T-RRAM device features a top Ti layer with a circular pattern applied using a shadow mask. As a result, in the case of the device without RTA, the regions without Ti deposition exhibit high transmittance, while the localized areas with Ti dots exhibit transmittance below 30% due to the presence of the metal layer. On the other hand, the device after RTA exhibited transmittance of over 75% across all areas of the device. This marked improvement in optical transparency highlights the effectiveness of RTA in enhancing the structural and optical qualities of the T-RRAM, making it more applicable to transparent electronics [18]. The improved transmittance of our device may be attributed to the transformation of the Ti top electrode to TiN through RTA in a nitrogen atmosphere and the crystallization of the film layers. TiN has higher transmittance than Ti, and previous studies have shown that thin TiN films can be effectively used as transparent electrodes in devices [19,20]. Additionally, RTA-induced crystallization reduces film thickness and grain boundary density, minimizing optical scattering and enhancing transmittance [17]. To investigate this further, we performed XPS analysis on the Ti top electrode before and after RTA, as well as XRD analysis on the Ti and HfO2 films. Further details on these analyses are provided in the following sections.
In order to analyze the effect of RTA on the resistance switching characteristics, we analyzed the I–V curve of T-RRAM without RTA and after RTA. Figure 2a,b show the I–V curves of the T-RRAM without RTA and after RTA condition, respectively. In both cases, the log(I)–V curves were plotted to capture the distinct changes in resistance as the voltage bias was swept across both positive and negative directions. The DC voltage sweep was applied in the following sequence: 0 → 4 → 0 → −8 → 0 V (for the device after RTA, the negative voltage was limited to −4 V). Each device shows bipolar RS characteristics, where the resistance of HfO2 changes depending on the direction of applied bias [21]. This bipolar behavior is a hallmark of HfO2-based RRAM, demonstrating its ability to switch between LRS and HRS depending on the applied voltage polarity [22]. The initial device shows HRS since the HfO2 layer acts as an insulator with high resistance. When a voltage is applied to HfO2-based RRAM, the conduction filament (CF) formation in the HfO2 layer at a specific voltage condition causes an abrupt increase in current. This process is well known as forming, and it typically needs a higher voltage than the setting voltage. The forming voltage is critical for initiating the resistive switching mechanism, as it enables the initial formation of conductive paths through the insulating HfO2 layer, which can later be modulated by subsequent voltage sweeps [23].
In the first positive voltage sweep, the forming voltage was 3.3 V and 2 V without and after RTA, respectively. After the initial forming process, in order to observe the RS characteristics, we applied a DC bias sweep from a negative to a positive direction. When a bias sweep was applied in the negative direction, the reset point was observed at −5.1 V for the device without RTA and at −2.7 V for the device after RTA, indicating a transition from the LRS to the HRS due to the partial or complete rupture of the CF [24]. Following that, to change HRS to LRS, the bias sweep was applied in a positive direction, and the setting point was observed at 2.4 V and 1 V, which shows an abrupt increase in current due to the reformation of the CF [25]. After that, we applied a positive voltage sweep again and confirmed the changed resistance state with an increased current level.
This varying conductive state mechanism is mainly attributed to the formation and rupture of CF consisting of oxygen vacancies in the HfO2 layer [26]. In the initial forming process, a high voltage is applied to the top electrode Ti, resulting in oxygen vacancies (Vo2+) generated in the insulator layer. With the increased amount of Vo2+ within the HfO2 layer, they can act as a conductive path between the Ti and AZO, resulting in LRS facilitating electron transport through the HfO2 layer [27]. In the reset process, it needs a high negative voltage to the bottom electrode AZO, inducing the HRS with the rupture of the CF [28]. During the setting operation to LRS transition, a positive voltage is applied to the top electrode, leading to the reformation of CF with Vo2+ aggregation [29].
After RTA, we observed that each forming, setting, and reset voltage, as well as the overall current level, decreased. The forming, set, and reset voltages for each device are shown in Table 2 below. This could be due to the decrease in defects and trap density and enhanced crystallization with the annealing process [30]. The correlation between RTA, smaterial phases, and trap density is explained in detail in a later section.
To evaluate our device as a non-volatile memory (NVM) device, we measured retention and endurance data, as shown in Figure 2c,d. The on/off ratio was compared with resistance differences between HRS and LRS at a Vread of 0.5 V for devices without and after RTA. Retention measurements were conducted for 10,000 s at Vread to assess the stability and reliability of both devices over time. The retention test is conducted to ensure that the device can maintain its state (either LRS or HRS) without significant drift in resistance, which is crucial for long-term data storage applications [31]. The retention was measured in both the LRS and HRS states, and to further investigate the retention capabilities of the two devices, Vread pulses were applied to each device 1000 times with a 10 s delay between pulses. This process simulates the read operations that would occur in a typical memory application, ensuring that the device can maintain its state even after multiple read cycles [32]. As a result, both devices were confirmed to perform reliably in terms of retention, maintaining stable resistance values over the testing period. The ability to retain distinct resistance levels over time and under repeated read conditions indicates that both without RTA and after RTA devices have the potential to function effectively as NVM devices [33].
After that, the endurance characteristics were evaluated by measuring 100 endurance cycles, as shown in Figure 2d. During the endurance measurement, the device without RTA exhibited unstable endurance characteristics along with an on/off current ratio (CR) of approximately 1.5. The lower CR indicates a smaller difference between the high HRS and LRS, making it more difficult to reliably distinguish between the two states during read operations. This unstable endurance further suggests that the device without RTA may suffer from wear and degradation over time, potentially leading to reduced reliability in memory applications [34]. Conversely, the device after RTA shows a significantly increased CR of approximately 10, along with improved endurance characteristics. This higher CR provides a clearer distinction between HRS and LRS, which is essential for reliable read/write operations in non-volatile memory devices [35]. The enhanced endurance indicates that the device after RTA can withstand more switching cycles, suggesting that the RTA process strengthens the device’s structural and electrical stability [36]. By reducing defects and trap density while stabilizing CF, RTA enhances both the CR and the endurance of the device, making it more robust for long-term use [37]. In contrast, the device without RTA is more prone to errors or data loss due to its lower CR and unstable endurance, making it less suitable for reliable memory applications [38]. When voltage is applied to the device, it induces the formation and dissolution of CF, altering the material’s resistance [39]. However, if defects and traps are present in the material, unwanted leakage current may occur, leading to degraded device performance [40]. In other words, the RTA process helps reduce defect and trap density in the T-RRAM [41], which in turn lowers leakage current, operating voltage, and current, thereby promoting stable operation. This is consistent with findings from other studies [42,43,44].
Figure 3 shows the Ti 2p spectra of the Ti top electrode without and after RTA. In the Ti 2p spectrum prior to RTA, two peaks at 458.5 eV and 464.3 eV, corresponding to the oxidized state Ti4+ (Ti-O), were identified, indicating that Ti underwent oxidation upon exposure to air. After RTA, the full width at half maximum (FWHM) of the Ti4+ peaks increased, and their intensity decreased. Additionally, new peaks associated with nitridation emerged: Ti4+ (Ti-O-N) at 457.1 eV and Ti4+ (Ti-N) at 454 eV. These results suggest that, during RTA in a nitrogen atmosphere, inhomogeneous defects and structural distortions formed within the top electrode, indicating the conversion of the Ti top electrode to a nitride state [45]. As noted earlier, this nitridation of Ti is believed to have contributed to the enhanced transmittance observed in our device.
As shown in Figure 4, we measured and analyzed the XRD patterns of the HfO2 and Ti films before and after RTA to confirm the crystallization effect induced by RTA. For the HfO2 film, no peaks were observed before RTA, but after RTA, peaks were found at 24.2°, 28.34°, 31.6°, 34.38°, 36.12°, 41.26°, 49.44°, 50.6°, 55.78° and 61.84°, corresponding to the (011), (−111), (111), (020), (200), (102), (211), (022), (−220), (221) and (113) planes of crystalline HfO2 [46,47]. In the case of the Ti film, peaks were present both before and after RTA at 34.4°, 38.4°, 40.14°, 53.08°, 62.94°, 70.56°, and 76.3°, but sharper and narrower peaks were observed after RTA. These correspond to the (100), (002), (101), (102), (110), (103), and (112) planes of crystalline Ti [48,49]. This result is consistent with previous studies that show a significant improvement in diffraction intensity with increasing annealing temperature [50]. The sharpening and narrowing of the diffraction peaks with RTA indicate the formation of larger grains due to a high level of crystallinity and grain growth mechanisms. The average grain size (D) was calculated using Equation (1).
D = n λ β c o s θ
Here, n represents the Scherrer constant (0.90), λ is the wavelength of the incident X-rays (0.15406 nm), β is the full width at half maximum (FWHM), and θ is the Bragg angle [51]. As a result, for HfO2, no peaks were observed without RTA, making it impossible to calculate the grain size. However, after RTA, the grain size was calculated to be 22.88 nm. In the case of Ti, the grain size was found to be 19.03 nm without RTA and increased to 24.08 nm after RTA
Based on these results, it can be concluded that before RTA, the small grain size leads to the formation of numerous grain boundaries, causing a probabilistic process related to the distribution of oxygen vacancies, which results in uneven and high forming and operating voltages [52]. However, after RTA, grain growth reduces the grain boundaries, which can accelerate the diffusion of oxygen ions along these boundaries. Then, when an external bias is applied, the oxygen ions preferentially migrate along the grain boundaries, forming CF rich in oxygen vacancies [53]. In other words, the localization of grain boundaries through RTA reduces variability in CF formation sites, thereby lowering the forming and operating voltages and improving device reliability [54,55].
We plotted the I–V curves of the devices and analyzed them using various conduction mechanisms to gain a more detailed understanding of the crystallization effect induced by RTA on T-RRAM [56]. As a result, the I–V curves of both without and after RTA devices aligned well with the SCLC mechanism. Figure 5a,b show the I–V curves replotted on a double logarithmic scale for the positive voltage region of the device across cycles. The fitting results match the measured data, indicating that the conduction mechanism of the proposed T-RRAM is SCLC [57]. In the low-voltage region below 0.5 V, the current density follows Ohm’s law, exhibiting a linear I–V relationship proportional to the applied voltage. However, in the high-voltage region above 0.5 V, the slope increases to 1.31 for the device without RTA and to 2.3 for the device after RTA, resulting in a nonlinear I–V relationship [58]. This behavior suggests that, as described above, the current in various voltage regions is well explained by the SCLC mechanism. The SCLC mechanism is modeled using the equation in Equation (2).
J = θ θ + 1 × 9 8 ε μ V 2 L 3
where θ = N c N t exp ⁡ E c E t / K T demonstrates the ration to free to trap electrons. Here, Nc and Nt are defined as the effective density of states in the conduction band and the number of effective electron traps, respectively, while ε and μ correspond to the permittivity and electron mobility of HfO2. V represents the applied voltage, and L is the thickness of the HfO2 film. Furthermore, in SCLC, the transition voltage (VTR) and the trap-filled limit voltage (VTFL) are two important parameters that can be derived from the I–V curve, providing information about the characteristics of the proposed T-RRAM. In the low-voltage region below VTR, the electric field is uniformly distributed across the HfO2, and since there is no space-charge region in HfO2, the current density follows Ohm’s law [59]. The energy band diagram illustrating the carrier transport behavior in this region is shown in Figure 6a. When a higher electric field is applied, the previously unoccupied trap sites in HfO2 become filled, and a space-charge region is created. Once the applied voltage exceeds VTR, the time required for externally injected charge carriers to pass through the HfO2 becomes very short. At this point, the thermally generated charge carriers in the HfO2 are no longer sufficient to mitigate or reduce the transit time of the externally injected carriers, as shown in Figure 6b [60]. As a result, the current density increases significantly because the externally injected carriers move through the material faster, encountering less interference from thermally generated carriers. Consequently, the Fermi level of HfO2 rises above the trap level, and all traps in the dielectric film become filled, as shown in Figure 6c [61]. The voltage at this point is referred to as VTFL, and it can be calculated using the equation in Equation (3) [62].
V T F L = q N t L 2 2 ε
To calculate Nt, the static dielectric constant of amorphous HfO2 was assumed to be 25 [63]. As a result, Nt values of 1.66 × 10 13 / c m 3 for the device without RTA and 8.99 × 10 12 / c m 3 for the device after RTA were obtained, confirming a reduction in both bulk and interface trap density due to RTA [64]. This phenomenon is also reflected in the energy band diagram shown in Figure 5.
Impedance spectroscopy (IS) is a highly effective characterization technique for investigating the electrical properties of dielectric thin films, where the films can be modeled using electrical components such as resistance, inductance, and capacitance [65]. By integrating IS analysis with DC I–V characteristics, we can gain a deeper understanding of the RS behavior and current conduction mechanisms in both the HRS and LRS [66]. Z can be defined as Equation (4).
Z w = Z ′ − j Z ″
Here, Z′ represents the real part and jZ″ represents the imaginary part of the complex impedance, while ω is the angular frequency. Figure 7a presents the Nyquist plots of the proposed T-RRAM in the HRS state, both without and after RTA. The semicircular shape observed in the HRS state indicates that the memory state resistance of the RRAM device can be modeled as a parallel combination of a resistor and a capacitor.
Based on the Nyquist plots, the equivalent electrical circuits for the two devices in the HRS state can be represented as shown in Figure 8a [67]. The equivalent circuit can be equivalent to a parallel component (RC). Therefore, the Z in the HRS can be expressed by Equation (5).
Z H R S = R s + R H R S 1 + ( w R H R S C H R S ) 2 − j w R H R S 2 C H R S 1 + ( w R H R S C H R S ) 2
We assumed that Rs has a value of 0 to simplify the calculations. This is exclusively associated with all the resistance provided by the interfacial layer between the dielectric and the top electrode [67]. Furthermore, in the HRS, the relationship between RHRS and CHRS can be expressed by Equation (6).
C H R S = 1 R H R S w m a x
In the case of HRS, the RHRS values for the device without RTA and the device after RTA were calculated as 453.89   Ω and 199.2 × 10 3   Ω , respectively, while the CHRS values were calculated as 8.79 × 10 − 10   F and 3.99 × 10 − 13   F , respectively.
Figure 7b shows the Nyquist plots of the proposed T-RRAM in the LRS state, both with and without RTA. The fact that the semicircular curve shape is not entirely lost in the LRS state indicates that the RC parallel circuit continues to influence the device’s behavior. However, the progressive increase in the positive Z” direction with increasing frequency suggests that inductance, caused by the formation of oxygen-vacancy-based CF, begins to play a significant role [68]. The equivalent circuit can be equivalent to a parallel component (RC) and L in series. Therefore, the Z in the LRS can be expressed by Equation (7) [67].
Z L R S = R L R S 1 + w R L R S C L R S 2 + j ( w L L R S − w R L R S 2 C L R S 1 + w R L R S C L R S 2 )
In the case of LRS, the RLRS values for the device without RTA and the device after RTA were calculated as 179.65   Ω and 301.99   Ω , respectively, while the CLRS values were calculated as 1.84 × 10 − 5   F and 2.43 × 10 − 5   F , respectively. Additionally, in the case of LRS, the inductance component, LLRS, was added and calculated as 8.49 × 10 − 8   H and 6.43 × 10 − 8   H , respectively. Compared to HRS, the formation of CF in LRS resulted in a reduction in RLRS, an increase in CLRS, and the appearance of the LLRS component [69]. The presence of LLRS could be attributed to the weak polarization caused by oxygen ions at the contact resistance between the HfO2 film and the top electrode Ti due to CF formation [67]. Additionally, this contact effect is likely due to the small RLRS and the broad frequency range [70]. Therefore, it can be considered that the conduction mechanism in LRS follows Ohmic behavior, which aligns well with our previous results [67]. Finally, the calculated values of the circuit parameters for HRS and LRS for each device are presented in Table 3 below. R, C, and L represent the parameters of the device without RTA, while R’, C’, and L’ denote the parameters of the device after RTA.

3. Materials and Methods

3.1. Fabrication of AZO/HfO2/Ti T-RRAM

First, the AZO-coated quartz substrate was sequentially cleaned with acetone for 10 min, methanol for 10 min, and deionized (DI) water for 10 min. After cleaning, a 20 nm-thick HfO2 layer was deposited on AZO-coated quartz as a resistive switching layer using radio frequency (RF) sputtering (KVS-2000L) at 100 W in an Ar ambient of 20 sccm. After that, a 10 nm thick Ti top electrode (TE) was deposited using RF sputtering in an Ar ambient of 20 sccm with a metal shadow mask with a circular pattern. After deposition, the RTA process was carried out using a MILA-3000 (RTA, ULVAC, MILA-5000) for 60 s in a nitrogen atmosphere at 450 °C.

3.2. Analysis of Proposed T-RRAM Before and After RTA

To evaluate the optical properties of the proposed T-RRAM, transmittance was measured using a UV–Vis spectrophotometer (Cary 5000 UV–vis–NIR spectrophotometer, Agilent Technologies Inc., Santa Clara, CA, USA) over a spectral range of 200 nm to 1100 nm. The measurements were taken across this wide spectral range to ensure a comprehensive understanding of the device’s transparency across both the ultraviolet and visible light regions. We also investigated changes in the grain size of the Ti and HfO2 films without and after RTA using XRD. Following this, we utilized XPS to assess the chemical and material composition of the Ti top electrode and evaluate RTA-induced transformations. In addition, the electrical properties of the device were characterized. To measure the current–voltage (I–V) and impedance characteristics, a Keithley 4200A-SCS (current: ±0.025%, voltage: ±0.012%) parameter analyzer was utilized. The I–V measurements provided insights into the switching behavior and conduction mechanism of the device, and the impedance analysis provided insights into the frequency-dependent properties.

4. Conclusions

In this study, we demonstrated that RTA at 450 °C for 60 s in a nitrogen atmosphere significantly enhances both the transparency and reliability of AZO/HfO2/Ti-based T-RRAM devices. The device without RTA exhibited a low transmittance of 30%, while the RTA-treated device showed a much higher transmittance of over 75%. Additionally, RTA resulted in the device operating at lower current levels, with reduced forming, set, and reset voltages from 3.3, 2.4, and −5.1 V to 2, 1, and −2.7 V, respectively. These changes improved the endurance characteristics of the device, indicating that the reduction in traps and defects within the T-RRAM structure, induced by RTA, plays a key role in the observed performance improvements.

Author Contributions

Conceptualization, H.-D.K. and Y.J.; methodology, Y.J. and H.-D.K.; validation, Y.J.; investigation, Y.J.; resources, H.-D.K.; data curation, H.-D.K., Y.J., S.B., and C.H.; writing—original draft preparation, Y.J.; writing—review and editing, H.-D.K.; visualization, Y.J., S.B., and C.H.; supervision, H.-D.K.; project administration, H.-D.K.; funding acquisition, H.-D.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported in part by the Basic Science Research Program via the National Research Foundation of Korea (NRF) funded by the Ministry of Education under grant NRF-2022R1F1A1060655 and in part by the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) via the Competency Development Program for Industry Specialists under grant P0020966.

Data Availability Statement

The data are contained within the article.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

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Inorganics 12 00299 g001
Figure 1. (a) Schematic structure, (b) cross-section FE-SEM images, and (c) transmittance at wavelengths of 200 nm to 1100 nm of the proposed T-RRAM.
Figure 1. (a) Schematic structure, (b) cross-section FE-SEM images, and (c) transmittance at wavelengths of 200 nm to 1100 nm of the proposed T-RRAM.
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Figure 2. Resistive switching characteristics of proposed T-RRAM (a) without RTA and (b) after RTA at 450 °C for 60 s in a nitrogen atmosphere. (c) Retention and (d) endurance characteristics of T-RRAM without RTA and after RTA.
Figure 2. Resistive switching characteristics of proposed T-RRAM (a) without RTA and (b) after RTA at 450 °C for 60 s in a nitrogen atmosphere. (c) Retention and (d) endurance characteristics of T-RRAM without RTA and after RTA.
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Figure 3. XPS spectra of the Ti 2p region of the Ti top electrode without RTA and after RTA.
Figure 3. XPS spectra of the Ti 2p region of the Ti top electrode without RTA and after RTA.
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Figure 4. XRD patterns of as-deposited HfO2 and Ti films and HfO2 and Ti films after RTA, and (inset) average grain size of the HfO2 and Ti films.
Figure 4. XRD patterns of as-deposited HfO2 and Ti films and HfO2 and Ti films after RTA, and (inset) average grain size of the HfO2 and Ti films.
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Figure 5. SCLC mechanism at the positive bias of proposed T-RRAM (a) without RTA and (b) after RTA.
Figure 5. SCLC mechanism at the positive bias of proposed T-RRAM (a) without RTA and (b) after RTA.
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Figure 6. Band diagram of proposed T-RRAM (a) in the low-voltage region, (b) medium-voltage region and (c) high-voltage region. (The orange arrows indicate the direction of the electric field).
Figure 6. Band diagram of proposed T-RRAM (a) in the low-voltage region, (b) medium-voltage region and (c) high-voltage region. (The orange arrows indicate the direction of the electric field).
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Figure 7. Nyquist plot of proposed T-RRAM at (a) HRS and (b) LRS.
Figure 7. Nyquist plot of proposed T-RRAM at (a) HRS and (b) LRS.
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Inorganics 12 00299 g008
Figure 8. Equivalent circuit of proposed T-RRAM at (a) HRS and (b) LRS.
Figure 8. Equivalent circuit of proposed T-RRAM at (a) HRS and (b) LRS.
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Table 2. The forming, setting, and reset voltages for each device.
Table 2. The forming, setting, and reset voltages for each device.
DeviceFormingSettingReset
Without RTA3.3 V2.4 V−5.1 V
After RTA2 V1 V−2.7 V
Table 3. Calculated parameter values of equivalent circuit elements.
Table 3. Calculated parameter values of equivalent circuit elements.
ComponentHRSLRS
R 453.89   [ Ω ] 179.65   [ Ω ]
C 8.76 × 10 − 10   [ Ω ] 1.84 × 10 − 5   [ F ]
L- 8.49 × 10 − 8   [ H ]
R’ 199.2 × 10 3   [ Ω ] 301.99   [ Ω ]
C’ 3.99 × 10 − 13   [ F ] 2.43 × 10 − 5   [ F ]
L’- 6.43 × 10 − 8   [ H ]
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Jang, Y.; Hwang, C.; Bang, S.; Kim, H.-D. Enhanced Transparency and Resistive Switching Characteristics in AZO/HfO2/Ti RRAM Device via Post Annealing Process. Inorganics 2024, 12, 299. https://doi.org/10.3390/inorganics12120299

AMA Style

Jang Y, Hwang C, Bang S, Kim H-D. Enhanced Transparency and Resistive Switching Characteristics in AZO/HfO2/Ti RRAM Device via Post Annealing Process. Inorganics. 2024; 12(12):299. https://doi.org/10.3390/inorganics12120299

Chicago/Turabian Style

Jang, Yuseong, Chanmin Hwang, Sanggyu Bang, and Hee-Dong Kim. 2024. "Enhanced Transparency and Resistive Switching Characteristics in AZO/HfO2/Ti RRAM Device via Post Annealing Process" Inorganics 12, no. 12: 299. https://doi.org/10.3390/inorganics12120299

APA Style

Jang, Y., Hwang, C., Bang, S., & Kim, H. -D. (2024). Enhanced Transparency and Resistive Switching Characteristics in AZO/HfO2/Ti RRAM Device via Post Annealing Process. Inorganics, 12(12), 299. https://doi.org/10.3390/inorganics12120299

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