Resistance switching of thin AlOx and Cu-doped-AlOx films

Thin Solid Films 544 (2013) 24–27

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Resistance switching of thin AlOx and Cu-doped-AlOx films Ya-Ting Wu, Shyankay Jou ⁎, Ping-Jung Yang Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, ROC

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Available online 11 June 2013 Keywords: Resistive switching Alumina Cu doping Plasma oxidation

a b s t r a c t Thin copper-doped aluminum oxide (Cu-doped-AlOx) and AlOx films of about 5 nm thick were generated by oxidizing the surfaces of Al and Al-5 wt.% Cu (Al-Cu) films in an oxygen plasma. According to X-ray photoelectron spectroscopy analyses, these two oxide films were found to be deficient in oxygen and had gradient concentrations of Al and O. The oxide films were employed as resistor layers sandwiched between an Al top electrode and an Al or Al-Cu bottom electrode to form resistive memory devices. The devices demonstrated unipolar resistance switching between high resistance state and low resistance state (LRS), and their resistance ratios measured at +0.2 V were around 105. Furthermore, their current–voltage characteristics showed ohmic conduction with the resistance increasing with temperature, in LRS. Conductive filaments were thought to form inside the AlOx film and the Cu-doped AlOx film, causing resistive switching. The resistive memory device using the AlOx film had unstable switching behaviors during cyclic testing, whereas the device using the Cu-doped AlOx film demonstrated stable resistance switching during 100 cycles of testing. The presence of the Cu ingredient in the AlOx film is likely to facilitate the formation and rupture of conductive filaments and induced stable resistance switching. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Resistive random access memory (RRAM) devices are considered as the next-generation non-volatile memory devices [1–3]. RRAM has a simple structure comprising a thin resistor layer sandwiched between two electrodes and can be switched between high resistance state (HRS) and low resistance state (LRS) through voltage sweeps. In the recent past, the resistive switching of aluminum oxide (AlOx) films prepared by various techniques has been demonstrated [4–7]. Available data do suggest that some AlOx-based RRAM show scattered resistive switching properties. The insertion of additional metal layers or embedding metal nanocrystals inside AlOx films has been found to improve the performance of resistive switching of AlOx-based RRAM [8–10]. Adding an interfacial Ti layer between AlOx and electrode has also facilitated stable resistive switching of AlOx-based RRAM [11–13]. Filamentary paths of oxygen vacancies or Al inclusions have been found to be present inside the AlOx film in AlOx-based RRAM [4–6,8–11]. As a result, the formation and rupture of conductive filaments induces LRS and HRS in AlOx-based RRAM, respectively. On the other hand, filamentary paths of Cu inside Al2O3 have been realized for the resistive switching of RRAM using Al2O3 as a resistor film and Cu or Cu alloys as an active electrode [14,15]. Similar Cu filaments have also been observed in RRAM using SiO2, ZrO2 and Ta2O5 as resistor films [16–22]. In this study, the effect of Cu doping on resistive switching of AlOx-based RRAM was investigated. Thin AlOx resistor layers were generated by plasma oxidation [23] of Al and Cu-doped Al electrodes. ⁎ Corresponding author. Tel.: +886 2 27376665; fax: +886 2 27376544. E-mail address: [email protected] (S. Jou). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.05.115

Resistive switching properties of undoped and Cu-doped AlOx-based RRAM were compared. 2. Experimental setup Si(100) wafers were oxidized in O2 atmosphere to grow SiO2 films 500 nm thick and then sliced to 20 × 20 mm2 substrates. Crossbar devices with an area of 60 × 60 μm2 were fabricated on the SiO2-covered Si(100) substrates by sequential growth of the bottom electrode (BE), resistor layer and the top electrode (TE). Both the bottom and the top electrodes were prepared by sputter deposition of Al or Al-5 wt.% Cu alloy with 99.99% purity, through a shadow mask with stripe patterns of 60 μm wide. The sputter depositions were conducted in a vacuum system with a base pressure of 1.6 × 10-3 Pa, working pressure of 2.3 Pa, Ar (99.999% purity) flow of 20 cm3/min, dc power of 10 W to magnetron cathode and deposition time of 30 min. The surface of Al and Al-Cu bottom electrode were oxidized in a microwave plasma to form oxide resistor layers. The plasma oxidation was conducted with a microwave power of 100 W, working pressure of 1.3 × 103 Pa, gas flow of 100 cm3/min Ar (99.999%) and 40 cm3/min O2 (99.999%), process time of 7 min and substrate temperature of 140 °C. Surface morphologies of the bottom electrodes before and after the plasma treatment were inspected by a scanning electron microscope (SEM; JEOL JSM-6500 F) and an atomic force microscope (AFM; Digital Instrument D3100). Chemical structures of the oxide layers were characterized by X-ray photoelectron spectroscope (XPS; VG ESCA Scientific Theta Probe) using Al Kα (1486.7 eV) radiation. The binding energy values were corrected with C 1 s peak of 284.5 eV. The surfaces of the oxide layers were pre-cleaned using a 3-kV Ar ion beam for 10 s before

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acquiring XPS spectra. The XPS depth profiles of the oxide layers were obtained with assistance of sputter etch by a 3-kV Ar ion beam. Cross sections of the crossbar devices were inspected by a high-resolution transmission electron microscope (HRTEM; Philips Tecani G2 F20). Resistive switching behaviors were obtained by measuring the current– voltage (I–V) characterization of the crossbar devices using Agilent B1500A. The bottom Al or Al-Cu electrodes were grounded and the Al top electrodes were biased with staircase dc voltages sweeps of 40 mV steps, during the I–V measurements taken at room temperature. Further, temperature dependence of the resistance of the crossbar devices were characterized at a constant voltage of +0.2 V at elevated temperatures. Retention of LRS and HRS were characterized at +0.2 V, at 85 °C.

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3. Results and discussion Fig. 2. (Color online) XPS depth profiles of the oxidized (a) Al film and (b) Al-5 wt.% Cu film. Insets are Al 2p spectra of the surfaces of the oxidized Al and Al-5 wt% Cu films.

The as-deposited Al and Al-Cu bottom electrodes were about 250 and 300 nm thick, respectively. Fig. 1(a) and (b) show the surface morphology of the as-deposited bottom electrodes. The granular size was between 10 and 100 nm for the Al electrode and about 20 nm for the Al-Cu electrode. After performing the plasma oxidation, the surfaces of both electrodes retained the same granular morphology under SEM observation. The surface roughness values obtained by AFM measurements were 16.2 and 2.7 nm for the as-deposited Al and Al-Cu bottom electrodes and 18.6 and 2.6 nm after the plasma oxidation. As a consequence, the Al bottom electrode had a rougher surface than that of the Al-Cu electrode. The surface roughness of both Al and Al-Cu electrodes did not change after growing a thin oxide layer by utilizing the plasma oxidation process. XPS depth profile of the surface of the oxidized Al bottom electrode suggests that the AlOx layer has gradient concentrations of Al and O with less O in deeper places, as displayed in Fig. 2(a). The top surface of the oxidized Al electrode is Al-rich or oxygen-deficient compared to the stoichiometric Al2O3, and the atomic percentages of Al and O are 52% and 48%, respectively. The Al 2p spectrum comprises two peaks corresponding to elemental Al at around 73 eV and Al-O at around 75.6 eV, as shown in the inset of Fig. 2(a). As a result, the surface of the Al bottom electrode is composed of an Al-rich oxide layer (AlOx) with increasing concentration of Al with depth. Similarly, the surface of the oxidized Al-Cu bottom electrode is rich in Al and has gradient concentrations of Al and O across the oxide layer, as shown in Fig. 2(b). The atomic percentages of Al and O are 57% and 43% in the top surface of the oxidized Al-Cu, respectively. The Al 2p spectrum seen in the inset of Fig. 2(b) also indicates that elementary Al is present in this oxide layer. In addition, Cu is also found in the surface layer of the oxidized Al-Cu with a gradual increase of concentration with depth up to 1.9 at.%. Hence, a Cu-doped AlOx (AlOx-CuOx) layer was formed by the plasma oxidation of the Al-Cu surface. That less Cu was present in the upper oxide layer was

possibly caused by faster surface oxidation of Al than that of Cu, causing Al to diffuse towards the surface. Fig. 3 shows a cross-sectional TEM image of the Al(TE)/AlOx/Al(BE) specimen with an oxide layer formed by the plasma oxidation of the Al electrode. The AlOx layer is about 5 nm thick and has an amorphous structure. Similarly, the oxide layer in the Al(TE)/AlOx-CuOx/Al-Cu(BE) specimen is amorphous and about 5 nm thick, as shown in Fig. 4. Further inspection of the TEM image shows several dark spots in the Al-Cu(BE). These dark spots are possibly the clusters of the Cu-rich phase, dispersed in the Al-Cu matrix, owing to low solubility of Cu in Al, at room temperature. These clusters are similar to the Cu-rich phase, which has been observed in Al-Cu alloy films with 0.5 wt.% [24] and higher concentrations of Cu [25]. Conversely, dark spots are not observed in the AlOx-CuOx oxide layer. However, presently we cannot reveal whether the AlOx-CuOx oxide layer has local Cu- or CuOx-rich clusters. The Al(TE)/AlOx/Al(BE) device was swept with dc voltages form 0 to above +3.4 V for several times to initiate a forming condition. Subsequently, the device showed a unipolar switch of resistance upon dc voltage sweeps, as shown in Fig. 5. The lower I–V curve in Fig. 5 indicates that the device remained HRS until the applied voltage reached a SET voltage of +2.0 V, where the device changed to LRS. After conversion to LRS, the I–V characteristic follows the upper curve in Fig. 5 then jumps back to HRS at a RESET voltage of +0.7 V. As shown in the lower inset of Fig. 5, a slope of 0.99 in log(I)–log(V) plot for LRS (upper curve) suggests that the electrical conduction follows Ohm's law (I∞V). The device was further modulated to LRS to measure its resistance at +0.2 V and elevated temperatures. The result indicated an increase of the resistance with temperature in LRS, as shown in the upper inset of

Fig. 1. SEM micrographs of the as-deposited (a) Al surface and (b) Al-5 wt.% Cu surface.

Fig. 3. TEM micrograph of the Al/AlOx/Al device.

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Fig. 6. I–V curve demonstrating unipolar resistive switching behavior of the Al/AlOx-CuOx/ Al-Cu device. The upper inset is the temperature effect of resistance on LRS. The lower inset is the corresponding log I–log V plots in LRS and HRS. Fig. 4. TEM micrograph of the Al/AlOx-CuOx/Al-Cu device.

Fig. 5. Therefore, a metallic conductive path is suggested for the electrical conduction in LRS of the Al(TE)/AlOx/Al(BE) device, the same as those reported by Zhu et al. [5] and Song et al. [8]. Similarly, the Al(TE)/AlOx-CuOx/Al-Cu(BE) device also exhibits unipolar switch of resistance after conducting a forming step. As shown in Fig. 6, the SET and the RESET voltages of this device are about +1.9 V and +0.6 V, respectively. Also, the LRS is an ohmic conductor with a slope of 0.98 in log(I)–log(V) plot and has metallic conduction behavior whose resistance increases with temperatures, as shown in the lower and upper insets of Fig. 6. In order to reveal the advantage of Cu doping in AlOx, the RRAM devices were cyclically switched in unipolar manner. Fig. 7 shows the resistance for LRS and HRS of these two devices obtained at a reading voltage of 0.2 V at room temperature. The Al(TE)/AlOx/Al(BE) device has scattered distribution of resistance values over 5 orders of magnitude in both HRS and LRS (Fig. 7(a)). Although the resistance ratio of HRS to LRS (RHRS/RLRS) can be greater than 105, the Al(TE)/AlOx/Al(BE) device fails to switch after a few cycles of testing. By comparison, the Al(TE)/AlOx-CuOx/Al-Cu(BE) device has stable switching behavior with RHRS/RLRS ratios of about 105 for the whole 100 cycles of resistive switching, as shown in Fig. 7(b). The resistance values of the Al(TE)/AlOx-CuOx/Al-Cu(BE) device have narrow distribution from 4×106 to 8×106 Ω for the RHRS, and from 80 to 105 Ω for the RLRS, as shown in the inset of Fig. 7(b). Further, retention tests were carried out for both devices by measuring time-dependent resistance at a constant voltage and temperature of +0.2 V and 85 °C, respectively. Fig. 8(a) shows that the RLRS of the Al(TE)/AlOx/Al(BE) device is stable for 104 s of test duration, whereas the RHRS starts to scatter after 200 s.

Fig. 5. I–V curve demonstrating unipolar resistive switching behavior of the Al/AlOx/Al device. The upper inset is the temperature effect of resistance on LRS. The lower inset is the corresponding log I–log V plots in LRS and HRS.

In comparison, the RLRS of the Al(TE)/AlOx-CuOx/Al-Cu(BE) device is stable for 104 s of test duration at 85 °C, as shown in Fig. 8(b). The RHRS starts to increase from 7×106 Ω at 200 s to 1.5×107 Ω at 103 s, and then remains stable for the rest of the test duration. According to preceding analyses, electrical conduction in LRS of both Al(TE)/AlOx/Al(BE) and Al(TE)/AlOx-CuOx/Al-Cu(BE) devices were attributed to metallic filaments being possibly present in the thin oxide layers. Zhu et al. [5] found that a high concentration of Al in an Al-rich AlOx film yielded Al filaments, which showed ohmic conduction in LRS of AlOx-based RRAM. Song et al. [8] demonstrated that conduction of LRS in AlOx-based RRAM was attributed to metallic filaments, in which resistance increased with operating temperatures but decreased with compliance currents. Further, Kim et al. [6] observed local conducting spots corresponding to filamentary paths in AlOx-based RRAM by utilizing a conductive atomic force microscope. In addition, the stability of AlOx-based RRAM has been improved by adding metallic phases inside the AlOx films [8–10]. Embedding metallic nanoparticles in AlOx film would induce local enhancement of the electrical field thus promoting the formation of conducting filaments [9]. Accordingly, our oxide films were composed of Al-rich AlOx, as determined by the XPS analyses. The excess Al possibly promoted the formation of Al metallic filaments in the AlOx films. In this study, Cu doping of the AlOx also improved the performance of the AlOx-based RRAM, and the reasons for the improvement are discussed. Continuous Cu filaments, which

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Fig. 7. (Color online) Plots of resistance against cycle numbers of resistive switching in LRS and HRS of (a) the Al/AlOx/Al device and (b) Al/AlOx-CuOx/Al-Cu device at a reading voltage of +0.2 V. The inset in 7(b) shows the statistical distribution of resistance for 100 sweep cycles in LRS and HRS of the Al/AlOx-CuOx/Al-Cu device.

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filaments of mixed Al and Cu phases. The Al(TE)/AlOx-CuOx/Al-Cu(BE) device retained high resistance ratios of 105 during 100 cycles of resistance switching at room temperature, and its LRS and HRS were stable for 104 s of retention test at 85 °C. Acknowledgement This work was financially supported by the ROC National Science Council through grant no. NSC 100-2221-E-011-053-MY2. References

Fig. 8. (Color online) The retention characteristics of the LRS and HRS of (a) the Al/AlOx /Al device and (b) Al/AlO x -CuOx /Al-Cu device. The reading was done at a constant bias of + 0.2 V at 85 °C.

[1] [2] [3] [4] [5] [6] [7] [8]

were present inside Al2O3 films by a thermal diffusion or an electrical forming process, facilitated resistive switching of Cu-doped Al2O3 using pure Cu or CuTe as an active electrode [14,15]. Conductive filament (bridge) comprising mixed Cu and CuOx has previously been observed in a CuO-based RRAM after a forming procedure [26]. Because our AlOx-CuOx film has low concentration of the Cu ingredient, a continuous Cu filament might not form inside the AlOx matrix. However, the CuOx phase inside the AlOx matrix may be converted to clusters of metallic Cu or low-resistance mixtures of Cu and CuOx during the forming process. These Cu clusters could induce local enhancement of the electrical field, thus facilitating the formation of short Al filaments between them. Consequently, filaments with mixed Al and Cu were formed inside the AlOx-CuOx film. The Cu ingredient in the AlOx-CuOx film helped to form and rupture the metallic filaments, causing stable switching of resistance. 4. Conclusion

[9] [10] [11] [12] [13] [14]

[15] [16] [17] [18] [19] [20] [21] [22]

Thin films of AlOx and AlOx-CuOx formed by plasma oxidation exhibited unipolar switching of resistance. Electrical conduction of these RRAM devices in LRS followed Ohm's law with the resistance increasing with temperature. Hence, metallic filaments are thought to form inside the oxide films, causing resistive switching of these devices. Moreover, doping Cu in AlOx improved the stability of the resistive switching of the RRAM, possibly due to the formation of

[23] [24] [25] [26]

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