Ozone Treatment Improved the Resistive Switching Uniformity of HfAlO 2 Based RRAM Devices

HfAlO 2 based resistive random access memory (RRAM) devices were fabricated using atomic layer deposition by modulating deposition cycles for HfO 2 and Al 2 O 3 . Effect of ozone treatment on the resistive switching uniformity of HfAlO 2 based RRAM devices was investigated. Compared to the as-fabricated devices, the resistive switching uniformity of HfAlO 2 based RRAMdevices with the ozone treatment is significantly improved. The uniformity improvement of HfAlO 2 based RRAM devices is related to changes in compositional and structural properties of the HfAlO 2 resistive switching film with the ozone treatment.


Introduction
Resistive switching phenomena in transitional metal oxide, such as HfO x , TaO x , are actively studied in order to apply them to the resistive random access memory (RRAM) [1][2][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][5][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][8][9][10][11][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][10][11][12].
Ozone (O 3 ) 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 HfAlO 2 based RRAM devices under different treatment processes and observed that the resistive switching uniformity of HfAlO 2 based RRAM devices can be significantly improved under the ozone treatment condition.The improvement of resistive switching uniformity is discussed.

Experiment
A 5 nm thick HfAlO 2 layer was fabricated on the Pt/Ti/SiO 2 / Si substrate using atomic layer deposition by modulating deposition cycles for HfO 2 (derived from Hf[N(CH 3 )(C 2 H 5 )] 4 and H 2 O precursors) and Al 2 O 3 (derived from Al(CH 3 ) 3 and H 2 O precursors) at 250 ∘ C. The deposition cycle ratio of HfO 2 : Al 2 O 3 was set 9 : 1.After the HfAlO 2 film deposition, some samples were treated in O 3 ambient at 100 ∘ C for 30 minutes; other samples without O 3 treatment were kept as control samples.Subsequently, a TaN top electrode with an area of 100 m × 100 m 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 HfAlO 2 film.

Results and Discussion
All the fresh HfAlO 2 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 HfAlO 2 film.After the electric forming process, stable reversible resistive switching between HRS and low resistance state (LRS) could be achieved in the HfAlO 2 based RRAM devices.Figure 2 shows the typical current-voltage (-) characteristics of as-deposited HfAlO 2 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 HfAlO 2 based RRAM devices were measured by direct current (DC) voltage sweeping mode.Figures 3(a 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 HfAlO 2 films with and without the ozone treatment.SEM measurement results revealed that surfaces of the as-deposited HfAlO 2 films were featureless and smooth.The nanoparticles' size of as-deposited HfAlO 2 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 HfAlO 2 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 HfAlO 2 film, the banding energy of Hf 4f 7/2 and Hf 4f 5/2 was around 16 and 18 eV, respectively, and O 1s was around 530 eV, consistent with the HfO 2 composition [4].After the ozone treatment, the two distinct peaks corresponding to the Hf 4f 7/2 and 4f 5/2 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 HfAlO 2 resistive switching film [13].
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 [15].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 HfAlO 2 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 [16], so oxygen vacancy conductive filaments can be formed in a more orderly way.Second, ozone treatment may change the compositional characteristic of the HfAlO 2 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 HfAlO 2 based RRAM devices with the ozone treatment is related to the changes in compositional and structural properties of the HfAlO 2 resistive switching film after the ozone treatment.

Conclusions
HfAlO 2 based RRAM devices were fabricated using atomic layer deposition.The effects of ozone treatment on the resistive switching uniformity of HfAlO 2 based RRAM devices were investigated.Ozone treatment significantly improved the resistive switching uniformity of HfAlO 2 based RRAM devices.The SEM and XPS measurement results indicate that the uniformity improvement of HfAlO 2 based RRAM devices with the ozone treatment is related to the changes in compositional and structural properties of the HfAlO 2 resistive switching film with the ozone treatment.

Figure 3 :
Figure 3: Measurement of devices for continuous 100 cycles in DC sweeping mode on different treatment conditions: (a) distribution of the  HRS and  LRS as a function of switching cycles, (b)  set and  reset voltage distribution.

Figure 4 :Figure 5 :
Figure 4: The variation of resistance ratios of HRS to LRS in the HfAlO 2 RRAM devices with the ozone treatment.

Figure 6 :
Figure 6: XPS O1s and Hf4f spectra of argon sputtered resistive switching films on different treatment conditions: (a) and (b) without the ozone treatment and (c) and (d) with the ozone treatment.