Effect of Ti doping concentration on resistive switching behaviors of Yb2O3 memory
cell
Somnath Mondal, Hung-Yu Chen, Jim-Long Her, Fu-Hsiang Ko, and Tung-Ming Pan
Citation: Applied Physics Letters 101, 083506 (2012); doi: 10.1063/1.4747695 View online: http://dx.doi.org/10.1063/1.4747695
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/101/8?ver=pdfcov Published by the AIP Publishing
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of Yb
2O
3memory cell
Somnath Mondal,1Hung-Yu Chen,1Jim-Long Her,2Fu-Hsiang Ko,3and Tung-Ming Pan1,a) 1
Department of Electronics Engineering, Chang Gung University, Taoyuan 333, Taiwan
2
Division of Natural Science, Center for General Education, Chang Gung University, Taoyuan 333, Taiwan
3
Department of Materials Science and Engineering, Institute of Biological Science and Technology, National Chiao-Tung University, Hsinchu 300, Taiwan
(Received 6 July 2012; accepted 8 August 2012; published online 23 August 2012)
We investigate the resistive memory switching behaviors of Yb2O3 thin films for different
Ti-dopant concentrations. A higher doping concentration of 9.4% of Ti atom into Yb2O3thin film
causes the switching mechanism to change from bipolar to unipolar behavior. This is ascribed to different chemical compositions of the filament through the oxide film. The reset mechanism is associated with the annihilation of oxygen vacancies and other ionic and electronic defects within or near the interface area of oxide film for bipolar switching, while it is believed to be due to rupture of the conducting filament by local Joule heating effect for unipolar resistive switching. Furthermore, the incorporation of Ti atom into the Yb2O3 memory device exhibits improved
electrical performances including low set/reset voltages and good endurance and retention characteristics.VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4747695]
Resistive random access memory (ReRAM) is an emerging storage device for future nonvolatile memory applications because of its simple design, excellent scalabil-ity, high switching speed, and compatibility with comple-mentary metal–oxide–semiconductor process.1–7 Based on the electrical field induced change of resistivity in some metal oxides, ReRAM devices switch between two different resistance states namely a high-resistance state (HRS) and a low-resistance state (LRS). A variety of metal oxides, including TiO2, HfO2, Al2O3, ZrO2, and Yb2O3,8–12 have
widely been investigated for ReRAM device applications. Although resistance-switching phenomena in several binary metal oxides have been known from decades, the details of the switching mechanisms and the nature of the different resistive states are still under debate. However, two domi-nant resistive switching mechanisms have been classified into the “interface type” and “filament type” to clarify the driving mechanism of resistive switching. The application of Yb2O3thin film in ReRAM device had testified and
demon-strated the formation and rupture of filamentary path in resis-tive switching.12 In order to use the Yb2O3-based ReRAM
for the practical implementation, its switching behavior needs to be fully controlled. The stochastic formation and rupture of the filaments are one of the major challenges that need to be minimized to improve the stability of resistive switching characteristics. So far, one approach of negative bias stress after a standard forming process has been demon-strated to improve the resistive switching in Yb2O3 thin
film.12
In this study, we explored the influence of Ti doping concentration on the resistive switching characteristics of Yb2O3-based ReRAM device. It has been reported that the
switching endurance and data retention suitable for future nonvolatile memory applications. Besides, our works focus on the driving mechanism of resistive switching of the mem-ory cell with different Ti-dopant concentrations. The switch-ing behavior changes from bipolar to unipolar resistive switching with increasing doping concentration of Ti atom (above 5%) into the Yb2O3film. A plausible mechanism is
proposed to explain the influence of Ti doping on resistive switching behaviors of Yb2O3-based ReRAM devices.
The TaN bottom electrode was deposited on SiO2/Si
wa-fer substrates by dc sputter deposition technique. A 30-nm Ti doped Yb2O3(Ti:Yb2O3) film with different Ti atomic
con-centrations was deposited on the TaN electrode through reac-tive rf sputtering from both Yb and Ti targets at room temperature.15 The chamber working pressure was main-tained at 10 mTorr, and the Ar/O2flow was set at 3/1. The
atomic concentration of the Ti dopant was examined by Au-ger electron spectroscopy (AES) analysis on the ReRAM devices. The AES analysis was performed on the selected devices at LRS condition after 10 resistive switching cycles. The physical thickness and film behaviors of the oxide layer were confirmed by cross-section transmission electron mi-croscopy (TEM) image (not shown here). The chemical bonding of the oxide films was analyzed by x-ray photoelec-tron spectroscopy (XPS). Finally, a 100-nm Ni top electrode was deposited on the oxide film through shadow mask by thermal evaporation technique. The electrical measurements were performed in semi-automated cascade system using Agilent E5260A high speed semiconductor parameter analyzer.
To investigate the structural and compositional changes of the Ti doped Yb2O3 thin films with different Ti doping
concentrations in the Yb2O3film. Adventitious hydrocarbon
C 1s binding energy at 285 eV was used as a reference to correct the energy shift of O 1s core levels due to differential charging phenomena. Each fitting peak followed the general shape of the Lorentzian–Gaussian function. In the three sets of spectra, the O 1s peaks at 529.5, 530.5, and 531.7 eV rep-resent the Yb–O,16Ti–O,17and OH (Refs.18and19) bonds, respectively. The intensity of O 1s peak corresponding to OH bond decreases with increasing the Ti doping concentra-tion, while the intensity of O 1s peak corresponding to TiO2
increases accordingly. The high Ti content in Yb2O3 film
effectively decreases the nonlattice oxygen ions from the ox-ide film.
Figure 2(a) shows the electro-forming process of the Yb2O3(0% Ti) and Ti:Yb2O3(3.5% Ti and 5.0% Ti)
mem-ory devices with a current compliance of 100 lA. The Ti-doped Yb2O3 memory device exhibited a lower forming
voltage than the Yb2O3memory device. The lower forming
voltage can be attributed to the easy formation of the field induced oxide defects into the Yb2O3film due to lower
for-mation energy of the Ti-O compound.14 After a standard electro-forming process, typical bipolar resistive switching characteristics of the Ti:Yb2O3(3.5% Ti and 5.0% Ti)
mem-ory devices are shown in Figs. 2(b) and 2(c), respectively. During set process in Ni/Ti:Yb2O3/TaN with Ti atomic
con-centration of 3.5% and 5.0%, a negative dc bias sweep with a current compliance of 1 mA causes the device suddenly transformed from HRS to LRS. The high current during set operation is guided by the broken filamentary path in the ox-ide, which can overwhelm the uncontrollable breakdown of the oxide thin film. In a subsequent positive sweep in reset process, the device can be switched again to HRS and a bipo-lar resistive switching is achieved. In contrary, Fig. 2(d)
exhibits the unipolar resistive switching behavior of the memory device with Ti atomic concentration of 9.4% in pos-itive bias sweep without any electroforming process. A sta-ble and reproducista-ble resistive memory switching can be performed more than 1000 sweeping cycles in the Ti:Yb2O3
(5.0% Ti and 9.4% Ti) memory devices, while a failure from LRS to HRS transition after few cycles occurs for device with a low Ti content (3.5% Ti) or no Ti doping concentra-tion (not shown).
The driving mechanism of the resistive switching char-acteristics in Ti:Yb2O3 thin film with different Ti doping
concentrations was investigated. The HRS and LRS resist-ance values as a function of the device size for Ti:Yb2O3
FIG. 1. (a) AES depth-profiling analysis of the Ni/Ti:Yb2O3/TaN interface with
differ-ent Ti concdiffer-entrations. (b) XPS line-shape analysis of O 1s spectra for the Ni/Ti: Yb2O3/TaN memory devices with different
Ti concentrations.
FIG. 2. (a) Electro-forming process in the Yb2O3 and Ti:Yb2O3 memory devices for
different Ti atomic concentrations. Repro-ducible resistive switching behavior of the ReRAM cells with different Ti doping concentrations (b) 3.5%, (c) 5.0%, and (d) 9.4%.
memory devices with different Ti concentrations are plotted in Fig.3(a). The LRS values of the Ti:Yb2O3memory
devi-ces seem to be insensitive to the cell size. Therefore, the for-mation and rupture of percolated filamentary paths by oxygen vacancies or metal ions/defects are preferred as the driving mechanism of resistive switching.20 We consider that the percolation of localized filament in the devices is associated with the oxygen vacancies and other ionic and electronic defects within or near the interface area for bipolar resistive switching, while it is accompanying with the oxy-gen migration between two metal electrodes for unipolar resistive switching behavior. During set process in bipolar resistive switching, the large number of nonlattice oxygen ions and oxide defects drift towards opposite polarity of elec-trode making a conducting filament into the oxide thin films. The devices transform from HRS to LRS. The oxygen ions are absorbed by the anode (TaN) to form a very thin con-ducting interfacial layer which, acts as oxygen reservoir21 and/or modifies the electrical contact of the interface.13 By applying opposite polarity bias of set process, the absorbed oxygen ions are released from the oxide/anode interface causing the rupture of conducting filament and the devices switches from LRS to HRS. Fig.3(b)shows the temperature dependence resistance in LRS for memory devices with dif-ferent Ti concentrations. For lower Ti atomic concentration (5% Ti), the LRS increases with increasing the tempera-ture, indicating the formation of metallic filament by perco-lation of oxygen vacancies and other ionic and electronic defects within or near the interface area.12 The unipolar resistive switching behavior in device with higher Ti atomic concentration (>5% Ti) can be attributed to formation and rupture of filament by oxygen migration between two metal electrodes and Joule heating effect, respectively. It has been reported that Ni as one of the electrodes in some oxide sys-tems shows unipolar resistive switching behavior because of the diffusion of Ni ions during electroforming process into the oxide film.22,23The Ni/Ti:Yb2O3/TaN memory cell with
9.4% Ti atomic concentration shows no Ni diffusion during resistive switching as evidenced by AES spectra in Fig.1(a). This may be due to the fact that the formation of unipolar
are chemisorbed at the grain boundary or penetrate through the Ni electrode. The resistance value of LRS in unipolar memory device (9.4% Ti) remained almost unchanged upon increasing temperature, as shown in Fig.3(b). The semicon-ducting behavior of the LRS conduction process corroborates well with the filament formation mechanism by field induced oxygen vacancies in the oxide film.12 The transition from LRS to HRS is believed to be related to the rupture of the conducting filament by local Joule heating. The temperature dependence HRS is shown in the inset of Fig.3(b). The HRS resistance decreases with increasing temperature, indicating oxide or semiconducting conduction mechanism in the mem-ory devices. Furthermore, the bipolar memmem-ory devices (3.5% Ti and 5.0% Ti) exhibit a higher value of LRS than the uni-polar memory device (9.4% Ti). This may be due to the for-mation of interfacial layer by absorbed oxygen ions at the oxide/TaN interface, which acts as a series resistance to the LRS value.24
Significant changes of the resistive switching behaviors are observed with increasing Ti atomic concentration into Yb2O3 thin film. The set/reset voltages of the Ti-doped
Yb2O3memory devices are shown in Fig.4. The reset
vol-tages decrease gradually with increasing Ti atomic concen-tration in the Ni/Ti:Yb2O3/TaN memory devices. Moreover,
the devices exhibit more stable switching in set/reset process with higher Ti atomic concentration and the distribution of set/reset voltages decreases significantly. The lower enthalpy of the Ti-O compound may induce the oxygen vacancies (Yb0) and compensate the defects in the oxide, which effec-tively improve the set/reset voltages of the device.10,14,15
ory devices for different Ti concentrations. (b) Temperature dependence LRS value of Ni/Ti:Yb2O3/TaN memory cells for
differ-ent Ti concdiffer-entrations. Inset: temperature de-pendence HRS value of Ni/Ti:Yb2O3/TaN
memory cells for different Ti concentra-tions. The LRS and HRS values of the memory devices with 5% Ti and 3.5% Ti concentrations were measured in bipolar mode, whereas the device with 9.4% Ti was tested in unipolar mode.
To consider the Ni/Ti:Yb2O3/TaN devices for ReRAM
applications, endurance and retention tests of the devices were performed. Fig.5(a)illustrates the endurance character-istics of the Ni/Ti:Yb2O3/TaN devices under pulse voltages
for different Ti doping concentrations. Both bipolar and uni-polar resistive switching devices can be switched over 105 cycles with RHRS/RLRSratio of about 103. But, the failure of
resistance switching from LRS to HRS happens after few hundreds cycles in the bipolar Ni/Ti:Yb2O3/TaN device for
3.5% Ti atomic concentration. We believe that the localized Ti ions in Ti-doped Yb2O3 films help to the formation of
conducting filaments through the oxide film easily, resulting in an improved resistive switching behavior of the devices.14 Furthermore, the retention characteristics of all the Ti-doped memory devices after 105switching cycles are shown in Fig.
5(b). No degradation in retention behavior of the memory devices are observed after 105s, measured at 100C.
In conclusion, the influence of Ti doping on resistive switching behaviors in Yb2O3-based resistive random access
memory has been investigated. The resistive switching char-acteristics of Ni/Ti:Yb2O3/TaN memory device depend on
the Ti-dopant concentration in oxide film. The resistive switching behavior changes from bipolar to unipolar resistive switching as the Ti atomic concentration in the Ti:Yb2O3
film changes to higher value. The driving mechanism of ferent resistance switching behaviors can be attributed to dif-ferent chemical compositions of the filament through the oxide. The reset process in the bipolar memory devices is attributed to the annihilation of oxide defects in the oxide film, whereas the disruption of filament through Joule heat-ing effect is responsible for unipolar resistive switchheat-ing devi-ces. Furthermore, the high Ti content in the Yb2O3memory
device exhibits improved electrical performances including reduced set/reset voltage and good endurance and retention characteristics.
This work was supported by the National Science Council (NSC) of Taiwan under Contract No. NSC-98-2221-E-182-056-MY3.
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FIG. 5. (a) Pulse induced resistive switch-ing endurance characteristics of the Ni/Ti:Yb2O3/TaN memory devices for
dif-ferent Ti concentrations. (b) Retention behavior of the Ni/Ti:Yb2O3/TaN memory
cells for different Ti concentrations meas-ured at 100C.
