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Ozone Treatment Improved the Resistive Switching Uniformity of HfAlO2 Based RRAM Devices
HfAlO2 based resistive random access memory (RRAM) devices were fabricated using atomic layer deposition by modulating deposition cycles for HfO2 and Al2O3. Effect of ozone treatment on the resistive switching uniformity of HfAlO2 based RRAM devices was investigated. Compared to the as-fabricated devices, the resistive switching uniformity of HfAlO2 based RRAM devices with the ozone treatment is significantly improved. The uniformity improvement of HfAlO2 based RRAM devices is related to changes in compositional and structural properties of the HfAlO2 resistive switching film with the ozone treatment.
Resistive switching phenomena in transitional metal oxide, such as HfOx, TaOx, are actively studied in order to apply them to the resistive random access memory (RRAM) [1–3]. RRAM is one of the most promising candidates for next generation of nonvolatile memory owing to its excellent performance, such as simple structure, low power consumption, fast switching speed, long retention time, and CMOS technology compatibility [4–6]. However, the key electrical characteristics of oxide-based RRAM devices still have random dispersion, such as operation voltage and high/low resistance values [7–12]. One of the main challenges that hinder RRAM devices from practical device application is exploring effective ways to suppress the fluctuations of key switching parameters, thus improving the resistive switching uniformity. Some technological methods have been presented to improve uniformity in oxide-based RRAM, such as using ion doping technical approach, specific top electrode materials, inserting interface layer between oxide and electrode, and applying certain operation model [9–12].
Ozone (O3) treatment can change the compositional and structural properties of the oxide films which may have an effect on the resistance switching behavior of RRAM [13, 14]. In this paper, we investigated the electrical characteristics of HfAlO2 based RRAM devices under different treatment processes and observed that the resistive switching uniformity of HfAlO2 based RRAM devices can be significantly improved under the ozone treatment condition. The improvement of resistive switching uniformity is discussed.
A 5 nm thick HfAlO2 layer was fabricated on the Pt/Ti/SiO2/Si substrate using atomic layer deposition by modulating deposition cycles for HfO2 (derived from Hf[N(CH3)(C2H5)]4 and H2O precursors) and Al2O3 (derived from Al(CH3)3 and H2O precursors) at 250°C. The deposition cycle ratio of HfO2 : Al2O3 was set 9 : 1. After the HfAlO2 film deposition, some samples were treated in O3 ambient at 100°C for 30 minutes; other samples without O3 treatment were kept as control samples. Subsequently, a TaN top electrode with an area of 100 μm × 100 μm was fabricated by magnetron sputtering through a liftoff process. Cross-sectional RRAM stacks are schematically shown in Figure 1. The electrical characteristics of the fabricated RRAM devices were measured using a Keithley 4200 semiconductor parameter analyzer with biased voltage top and grounded bottom electrodes at room temperature. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to evaluate the structure and compositional characteristics of the HfAlO2 film.
3. Results and Discussion
All the fresh HfAlO2 based RRAM devices show high resistance state (HRS). An electric forming process with a high voltage was needed to realize reversible resistive switching, which is a precondition for activation of the soft breakdown of the HfAlO2 film. After the electric forming process, stable reversible resistive switching between HRS and low resistance state (LRS) could be achieved in the HfAlO2 based RRAM devices. Figure 2 shows the typical current-voltage () characteristics of as-deposited HfAlO2 based RRAM devices without the ozone treatment. Typical bipolar resistive switching behaviors were observed in the RRAM devices. A current compliance (CC) during set process is necessary to protect the devices from damage. It can be seen from Figure 2 that the sharp set process from high resistance state (HRS) to low resistance state (LRS) takes place under positive voltage sweep and gradual reset process from LRS to HRS takes place under negative sweep.
The statistical distributions of key memory parameters of the HfAlO2 based RRAM devices were measured by direct current (DC) voltage sweeping mode. Figures 3(a) and 3(b) compared the statistical distributions of HRS resistance () and LRS resistance () and set voltage () and reset voltage () from cycle to cycle separately between the HfAlO2 based RRAM devices with and without the ozone treatment. Significantly reduced dispersions of both and and and are observed in the devices with ozone treatment. The resistance ratios of HRS to LRS in the HfAlO2 based RRAM devices with the ozone treatment are more than 10 during the 512 cycles of DC resistive switching test, as shown in Figure 4.
Figure 5 shows the SEM images of the HfAlO2 films with and without the ozone treatment. SEM measurement results revealed that surfaces of the as-deposited HfAlO2 films were featureless and smooth. The nanoparticles’ size of as-deposited HfAlO2 films is about 25 nm while it became larger to 30 nm after the ozone treatment. Figure 6 shows the X-ray photoelectron spectroscopy (XPS) of the O 1s and Hf 4f orbitals of argon sputtered HfAlO2 films with spectrum aligned to C 1s on different treatment conditions. XPS depth analysis on the samples with and without the ozone treatment were also carried out. For the as-deposited HfAlO2 film, the banding energy of Hf 4f7/2 and Hf 4f5/2 was around 16 and 18 eV, respectively, and O 1s was around 530 eV, consistent with the HfO2 composition . After the ozone treatment, the two distinct peaks corresponding to the Hf 4f7/2 and 4f5/2 of the Hf-O merged into one and the banding energy of O1s and Al 2s also increased, which could be attributed to the partial formation of new Hf-Al-O structure in the HfAlO2 resistive switching film .
Various mechanisms have been proposed to explain the resistive switching behaviors in oxide-based RRAM devices, among which the formation/rupture of nanoscale conductive filaments (CFs) that consisted of oxygen vacancies (Vo) in the resistive switching layer has been widely recognized . The origin of the switching performance variation may be due to the random formation of the CFs. Two possible reasons may be responsible for the improvement of resistive switching uniformity in the HfAlO2 RRAM devices with the ozone treatment. First, ozone treatment can increase the film crystallinity, which could provide easy migration of oxygen vacancies along the grain boundaries , so oxygen vacancy conductive filaments can be formed in a more orderly way. Second, ozone treatment may change the compositional characteristic of the HfAlO2 resistive switching layer and partially form the new Hf-Al-O structure, which is supported by the XPS results. Therefore, the improvement of resistive switching uniformity in HfAlO2 based RRAM devices with the ozone treatment is related to the changes in compositional and structural properties of the HfAlO2 resistive switching film after the ozone treatment.
HfAlO2 based RRAM devices were fabricated using atomic layer deposition. The effects of ozone treatment on the resistive switching uniformity of HfAlO2 based RRAM devices were investigated. Ozone treatment significantly improved the resistive switching uniformity of HfAlO2 based RRAM devices. The SEM and XPS measurement results indicate that the uniformity improvement of HfAlO2 based RRAM devices with the ozone treatment is related to the changes in compositional and structural properties of the HfAlO2 resistive switching film with the ozone treatment.
Conflict of Interests
The authors declare no competing financial interest.
This work was supported in part by 973 and NSFC (nos. 2011CBA00600, 61376084, and 60925015).