Reset First Resistive Switching in Ni1-xO Thin Films as Charge Transfer Insulator Deposited by Reactive RF Magnetron Sputtering.

Reset-first resistive random access memory (RRAM) devices were demonstrated for off-stoichiometric Ni1-xO thin films deposited using reactive sputtering with a high oxygen partial pressure. The Ni1-xO based RRAM devices exhibited both unipolar and bipolar resistive switching characteristics without an electroforming step. Auger electron spectroscopy showed nickel deficiency in the Ni1-xO films, and X-ray photoemission spectroscopy showed that the Ni3+ valence state in the Ni1-xO films increased with increasing oxygen partial pressure. Conductive atomic force microscopy showed that the conductivity of the Ni1-xO films increased with increasing oxygen partial pressure during deposition, possibly contributing to the reset-first switching of the Ni1-xO films.

1. Introduction

Resistive random access memory (RRAM) [1] has been widely studied as a candidate for next-generation non-volatile memory to overcome the limitations of conventional memories, such as flash memory and dynamic random access memory (DRAM). RRAM has a relatively low operation voltage with excellent program and erase speed [2]. In addition, the device could be fabricated in a simple metal–insulator–metal (MIM) [3] structure, enabling the high-density cell structure of a cross-bar array with 4F2 [4,5]. It was reported that numerous transition metal oxides, including Al2O3 [6,7], HfO2 [8,9,10], NiOx [11,12,13,14], TiOx [15,16], TaOx [17,18], Nb2O5 [19,20], and Pr1−xCaxMnO3 [21,22,23] show resistive switching (RS) characteristics. Moreover, various deposition techniques, such as sputtering [24,25,26,27,28], atomic layer deposition (ALD) [29] and pulsed laser deposition (PLD) [30] were used for the formation of such oxides. Notably, nickel oxide (NiO) film is one of the most widely studied oxides and is reported to have low operation power, a high on/off resistance ratio and is compatible with the CMOS fabrication process [31,32]. NiO has a rock salt structure composed of Ni2+ and O2− and is a member of the strongly correlated 3d transition metal oxides that exhibit charge-transfer insulator behavior [33,34]. It is an insulating oxide with a wide bandgap (Eg ≈ 4.3 eV) due to the charge transfer gap caused by “Hubbard U” between the 2p and 3d states [34,35]. Therefore, the pristine state of NiO is typically the insulating state in RRAM [36,37]. The RS phenomenon in NiO has been mainly described as the formation and rupture of conductive filaments. This reversible resistance transition between the high-resistance state (HRS) and low-resistance state (LRS) is caused by applying electrical stress after an “electroforming” step [38]. It was suggested that oxygen atoms are migrated by the electric field, leaving oxygen vacancies (Vo2+) at the vacated sites during the electroforming step; the adjacent Ni2+ atoms are changed to Ni0 to compensate for the charge state, resulting in a Ni filament [39,40,41]. The electroforming process degrades the chemical and physical properties of devices of MIM structure, affecting their reliability. The characteristics of RS uniformity also deteriorate because of non-uniform filament formation among MIM devices [42]. Moreover, electroforming requires additional high-voltage circuits, significantly reducing the device density. Therefore, research on devices that can be operated without an electroforming step is essential for realizing RS memories [43,44,45].

This study investigated the RS characteristics of off-stoichiometric Ni1−xO films for unipolar and bipolar RSs (URS and BRS, respectively). Particularly, it was demonstrated that nickel-deficient Ni1−xO films deposited under excessive oxygen partial pressure exhibit a reset-first RS without an electroforming step. An RRAM device with a reset-first RS could be an alternative to overcome the limitations of RRAM requiring an electro-forming step.

2. Experimental

MIM devices with Pt/NiO/Pt and Pt/NiO/TiN stacks were fabricated for electrical characterization. First, Ti/TiN adhesive layers with thicknesses of 10–50 nm were deposited onto SiO2 on a Si substrate using DC magnetron sputtering. Pt or TiN films were then deposited as bottom electrodes (BE). BE with various areas of 0.18~4.0 µm2 were formed to investigate the area-dependence of the electrical characteristics. After BE formation, off-stoichiometric Ni1−xO films with a thickness of 10 nm were deposited via reactive RF magnetron sputtering using a Ni target under various O2 partial pressures. During sputtering, the base and working pressures were less than 3 × 10−3 and 3 mTorr, respectively. During deposition, the RF power and temperature of the substrate were main-tained at 100 W and 400 °C, respectively. The fraction of the O2 partial pressure in the mixture of Ar and O2 varied from 10% to 50% for deposition. Finally, Pt top electrodes (TEs) with a thickness of 100 nm were formed using DC magnetron sputtering and a lift-off process. The electrical characteristics of the device were characterized using a Keysight B1500A analyzer at 21~23 °C. RS under DC bias was measured with a com-pliance current of 10 mA to avoid hard breakdown of the Ni1-xO films. The spatial distribution of conductivity in the pristine state was investigated using conductive atomic force microscopy (C-AFM) (Park Systems, XE-100) with a measurement bias of 3 V [46,47]. Grazing incidence X-ray diffraction (GI-XRD, Rigaku SmartLab), Auger electron spectroscopy (AES, PHI-700, ULVAC-PHI), and X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo U. K.) analyses were conducted to investigate the crystallinity, composition, and valence states of Ni in the Ni1−xO films, respectively.

3. Results and Discussion

XRD analysis was conducted to investigate the crystallinity of Ni1−xO films. The XRD patterns of Ni1−xO films deposited under various O2 fractions are illustrated in Figure 1a. The peaks of NiO (111), NiO (200), NiO (220), and NiO (311) imply a polycrystalline structure [48]. NiO films, deposited with an O2 partial pressure fraction of 50% showed lower intensity with a more comprehensive full-width half maximum (FWHM), implying poorer crystallinity of NiO films. The XRD peak of the (111) plane shifted to lower diffraction with increasing O2 partial pressure, indicating an increase in the lattice constant with increasing O2 partial pressure, as shown in Figure 1b. The increase in the lattice constant could be ascribed to the increased strain effect as Ni vacancies increase with excessive O2 partial pressure [48,49,50]. Figure 1c shows the composition of Ni and O, estimated from AES analysis of the Ni1−xO films with various O2 partial pressures during deposition. The volume of Ni is gradually reduced with increasing O2 partial pressure, resulting in a Ni-deficient Ni1−xO film. The compositions of nickel oxide at 10% and 50% O2 partial pressures were estimated to be Ni0.89O and Ni0.86O, respectively.

Figure 1

(a) XRD patterns of Ni1−xO films deposited with various oxygen partial pressures. (b) Lattice constant of Ni1−xO, estimated from (111) peak position, as a function of oxygen partial pressures. (c) Nickel and oxygen composition in Ni1−xO by AES.

Figure 2a shows the typical behavior of Pt/Ni1−xO/Pt stacks. The pristine Ni1−xO films deposited under an O2 partial pressure fraction of 10% offered an initial high resistivity [51] at an applied voltage of 1.77 V (1.4 MV/cm) on the TE. The film resistance changed from HRS to LRS during the forming step. The resistance state was changed back to HRS at 0.64 V (0.5 MV/cm) during the subsequent bias application, exhibiting reversible switching for the positive bias on TE. The difference between the forming voltage (Vform) and set voltage (Vset) was approximately 0.57 V. In contrast, pristine Ni1−xO films deposited under the 30% or 50% O2 ratio showed low resistance in the pristine state without the electroforming step and reset-first RS behavior, where the initial LRS state was changed to the HRS state, as shown in Figure 2b,c. While Vset is similar to that of Ni1−xO films for the O2 partial pressure fraction of 10%, the IHRS/ILRS ratio decreased because of the overall high current level in the HRS state. In particular, the IHRS between these oxygen partial pressure fractions showed that the 50% O2 ratio was 10 times higher than that of 30% O2. The I-V curves of TiN/Ni1−xO/Pt stacks are plotted in Figure 2d–f. The Ni1−xO film deposited under a 10% O2 partial pressure fraction show BRS [52] characteristics, as shown in Figure 2d. The pristine Ni1−xO film showed high resistivity, and the resistance state changed to LRS after the electroforming step with a negative bias on TE. The difference between Vform (−4.0 V) and Vset (−0.7 V) was approximately 3.3 V. On the contrary, the Ni1−xO film deposited under the 30% or 50% O2 partial pressure fraction showed reset-first BRS behavior for a positive voltage on the TE, as shown in Figure 2e,f.

Figure 2

I−V characteristics of Ni1−xO devices with a bottom electrode of 2 × 2 μm2. URS characteristics of Ni1−xO films deposited with partial oxygen pressure of (a) 10%, (b) 30% and (c) 50%. BRS characteristics of Ni1−xO films deposited with oxygen partial pressure fraction of (d) 10%, (e) 30% and (f) 50%.

Figure 3 shows the electric currents at 0.64 V of the Pt/Ni1−xO/TiN stacks in the LRS and HRS states, where Ni1−xO films were deposited at various O2 partial pressures. The mean values of IHRS and ILRS (red line) increased with the O2 ratio, suggesting that the Ni1−xO film conductivity depends on the O2 partial pressure, as shown in Figure 3a. The Ni1−xO films with a 10% O2 fraction required electroforming for resistive switching, but the Ni1−xO films with a 30% O2 fraction or higher showed reset-first RS behavior without electroforming. Figure 3b shows the electrical currents at 0.64 V in the LRS states, which has a similar tendency to the IHRS with O2 partial pressure, but the slope was lower than that of the IHRS state. The IHRS and ILRS showed the highest values for Ni1−xO films deposited under the 50% O2 partial pressure fraction.

Figure 3

Influence of oxygen partial pressure on (a) IHRS of Ni1−xO films and (b) ILRS of Ni1−xO films.

To understand the nature of resistance switching, HRS and LRS resistances were measured from devices with BE of 0.18, 0.38, 2.00, and 3.69 μm2 at a bias of ±0.48 V. Figure 4a shows the area dependent resistance for BRS device with Ni1−xO films deposited by 10% O2 partial pressure fraction. The resistance of the HRS remained almost constant with decreasing geometric device area, while that of the LRS is almost independent of the device area. These area-independent characteristics imply that resistance switching through the device occurs in local regions, such as filament paths, rather than homogeneously distributed switching paths [53,54,55,56,57]. Meanwhile, the resistances of reset-first RS devices with Ni1−xO films deposited at 50% O2 partial pressure showed increased dependence on the device area, as shown in Figure 4b. Because the area dependence of the LRS for Ni1−xO films with 50% O2 partial pressure is close to that of Ni1−xO films with 10% O2 partial pressure, the nature of the RS is filamentary in the local area. The significant dependence of HRS on the Ni1−xO films with 50% O2 partial pressure is attributed to the reduced resistance of the Ni1−xO films, as shown in Figure 4b.

Figure 4

Area dependence of HRS and LRS resistances for Pt/Ni1−xO/TiN stacks (a) with Ni1−xO films, deposited with oxygen partial pressure fraction of 10%, with electroforming (b) with Ni1−xO films that are deposited with oxygen partial pressure fraction of 50%, with reset-first BRS without electroforming.

The DC, and AC endurance characteristics of the Ni1−xO device are shown in Figure S1. DC endurance in Figure S1a was measured at a read voltage (Vread) of ±0.25 V under a compliance current of 10 mA. The measured IHRS/ILRS ratio is higher than 101 even after 103 cycles. Figure S1b shows the AC endurance under pulse, which is measured with a set pulse of −0.95 V with 180 ns, a reset pulse of 1.2 V with 180 ns, and a Vread of 0.3 V conditions. The device has a uniform IHRS/ILRS ratio even after 105 cycles, which results in a stable RS property.

C-AFM measurements investigated the two-dimensional (2D) variation of the Ni1−xO film conductivity. Figure 5a illustrates the scheme of the C-AFM measurement. NiO/Pt and NiO/SiO2/Pt stacks were simultaneously formed on a sample to compare the differences during the current image mapping. Cross-sectional TEM images of the Ni1−xO films for C-AFM measurements are shown in Figure 5b. The sample-to-sample variation in the Ni1−xO thickness on the SiO2/Pt stacks was estimated to be within 15%. Therefore, we ignore the difference in conductivity due to thickness variation. Figure 5c–e show the current mapping images at a bias of 3 V from Ni1−xO films deposited under various O2 partial pressures. The left region of each mapping image represents the reference of the insulating SiO2 between the BEs and Ni1−xO films. The regions on the right represent the Ni1−xO films on the Pt BEs in their pristine state. Similar to the I-V characteristics of MIM devices, C-AFM showed an increased current through the Ni1−xO films with increasing O2 partial pressure. The conductive regions in the Ni1−xO film regions increased with increasing O2 partial pressure fraction, as shown in Figure 5d,e. In particular, the current distribution is relatively uniform in Ni1−xO film with a 50% O2 fraction. In contrast, films deposited under 10% O2 partial pressure fraction showed improved resistivity, as shown in Figure 5c.

Figure 5

(a) Schematic diagram of the C-AFM measurement. (b) Cross-sectional TEM image of Ni1−xO films deposited at various oxygen partial pressure. C-AFM current mapping images of the pristine Ni1−xO films under oxygen partial pressure fraction of (c) 10%, (d) 30%, and (e) 50%.

The effect of the O2 partial pressure on the chemical bonding states in the Ni1−xO films is investigated through XPS analysis. Figure 6a–c show the Ni 2p3/2 peaks of Ni1−xO films deposited with various O2 partial pressures. Ni0, Ni2+ and Ni3+ states with binding energies of 852.5, 853.7, and 855.5 eV, respectively, are used for deconvolution of Ni 2p3/2 peaks [58,59].

Figure 6

XPS peaks of Ni 2p3/2 of Ni1−xO films with oxygen partial pressure fraction of (a) 10% (b) 30% (c) 50%.

The proportion of the Ni3+ state was estimated from the ratio of the Ni3+ peak area to the Ni2+ peak area. The Ni3+ valence state increased while the fraction of Ni2+ ions decreased with increasing O2 partial pressure (Figure 6a–c). The Ni3+ ratio in the film grown under 10% and 50% O2 partial pressure was estimated at 14.0% and 23.9%, respectively. Meanwhile, the Ni0 state at the 852.5 eV peak was not observed in our Ni 2p2/3 peak analysis, although it was considered a conductive path in previous studies [39,40,41]. Conventionally, Ni vacancies form in Ni-deficient NiO films with relatively excessive oxygen. It was reported that nickel deficiency could promote the further oxidation of Ni2+ ions, which can be expressed with Kröger–Vink notation, as follows [48,49]: where , , , represent Ni2+, Ni3+, O2−, and ionized Ni vacancies, respectively. Ni2+ ions react with oxygen to generate ionized nickel vacancies and two Ni3+ ions, which affect the conductivity of the nickel oxide films. Therefore, it is shown that the increase in Ni3+ in Ni1−xO films is related to the increase in the current in the HRS state of MIM devices and C-AFM. It is expected that Ni deficiency in Ni1−xO films grown under high O2 partial pressure causes a high Ni3+ concentration, leading to a highly conductive state and possibly the reset-first RS behavior with reinforced localized conductive paths [39,60,61]. Further investigation is required to understand how excess Ni3+ ions produce the reset-first resistive switching behavior in Ni1−xO films.

4. Conclusions

In this study, the reset-first RS characteristics of off-stoichiometric Ni1−xO films were investigated. The RS behavior without the electroforming step was observed in the unipolar and bipolar off-stoichiometric Ni1−xO films. Ni3+ distribution contributes significantly to the conductivity of the pristine Ni1−xO films. The conductivity and Ni deficiency of pristine Ni1−xO films increased as the O2 partial pressure increased during a deposition as revealed by the C-AFM and AES results. Moreover, Ni2+ was further oxidized to Ni3+ as the O2 partial pressure increased, as revealed by the XPS results.

The Ni2O3 bonding by Ni3+ ions is related to the reset-first RS behavior without the electroforming step. This is advantageous in terms of device scale-down, making Ni1−xO films promising candidates for memory applications by overcoming the limitations of the electroforming step in RRAM.