Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films
© Chiu et al; licensee Springer. 2012
Received: 24 November 2011
Accepted: 8 March 2012
Published: 8 March 2012
In this work, bipolar resistive switching characteristics were demonstrated in the Pt/ZnO/Pt structure. Reliability tests show that ac cycling endurance level above 106 can be achieved. However, significant window closure takes place after about 102 dc cycles. Data retention characteristic exhibits no observed degradation after 168 h. Read durability shows stable resistance states after 106 read times. The current transportation in ZnO films is dominated by the hopping conduction and the ohmic conduction in high-resistance and low-resistance states, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in ZnO were identified.
KeywordsZnO resistive switching reliability electrical parameters
Resistance random access memory [RRAM] has attracted a great deal of attention because of its good compatibility with the complementary metal-oxide semiconductor [CMOS] process, nonvolatility, low power consumption, low cost price, high switching speed, high durability, small cell size, simple cell structure, and multistate switching [1–4]. There are several types of materials used in RRAM, such as perovskite-type oxides [1, 3], binary metal oxides [2–4], solid-state electrolytes , organic compounds , and amorphous Si . Among the RRAM materials being studied, binary metal oxides are most favorable because of their simple constituents, compatible with CMOS processes, and resistive to thermal/chemical damages [2, 4, 7].
Zinc oxide [ZnO] has the properties of wide bandgap (approximately 3.4 eV), adjustable doping, and low synthetic temperature. Therefore, the ZnO thin films have been investigated for the applications of transparent electrodes, light-emitting devices, photodiodes, thin film transistors, sensors, solar cells [8, 9], and piezoelectric devices . Recently, the resistive switching behaviors of ZnO have been reported [11–15]. Although the resistive switching characteristics and reliability were studied, the spacing between trap sites, the trap energy levels, as well as the electron mobility in ZnO films have not been addressed in detail. In this work, the behavior of bipolar resistive switching in Pt/ZnO/Pt metal-insulator-metal [MIM] structure was demonstrated. An exponential relationship between the switching voltage and the ac pulse width [Wac] was observed for low Wac (10-7 to 100 s), while for large Wac (>1 s), a critical switching voltage is approached. Reliability characteristics of ac/dc cycling endurance, data retention, and read durability were measured. The dominant conduction mechanism in ZnO films are the hopping conduction and the ohmic conduction in high resistance state [HRS] and low resistance state [LRS], respectively. Therefore, the trap energy level, the trap spacing, and the electron mobility in ZnO films were determined.
In this work, Pt/ZnO/Pt MIM diodes were fabricated. The ZnO films of 25 nm were deposited on Pt/Ti/SiO2/Si substrates at room temperature using radio frequency [rf] magnetron sputtering of a ceramic ZnO target in Ar ambient. The rf power was 40 W. The flow rate of argon was 25 sccm. The working pressure during deposition was 5 mTorr. To achieve the MIM structure, a Pt top electrode was deposited by rf magnetron sputtering with a round area patterned by the shadow mask process. Because of the defect issue, the failure probability is higher for the samples with a larger dielectric area. Hence, a relatively large device is more critical to monitor the production yield in future nanoscale nonvolatile memory applications. In this work, the device area is 1.27 × 10-3 cm2. The electrical characteristics of the fabricated ZnO-based resistive memory devices were measured by Agilent 4156C semiconductor parameter analyzer (Agilent Technologies, Santa Clara CA, USA), Agilent 8110A pulse pattern generator (Agilent Technologies), and Barth 4002 transmission line pulse generator (Barth Electronics, Inc., Boulder City, NV, USA). All the measurements were performed under dark condition.
Results and discussion
where q is the electronic charge, a is the mean spacing between trap sites (i.e., the hopping distance), n is the electron concentration in the conduction band of the dielectric, v is the frequency of thermal vibration of electrons at trap sites, T is the absolute temperature, k is Boltzmann's constant, and Φ t is the energy level from the trap states to the bottom of conduction band [EC] in ZnO. Therefore, the trap spacing in ZnO is determined to be about 2.0 nm according to Figure 8a. Besides, the trap energy level is determined to be about 0.46 eV according to the temperature dependence of current density, as shown in Figure 8b. The trap energy level of 0.46 eV in HRS may come from the defect state of interstitial zinc  which may be produced during the initial forming process. Note that the hopping conduction is not the electrode-limited conduction mechanism but the bulk-limited conduction mechanism. The bulk-limited conduction mechanism depends only on the properties of the dielectric itself.
In summary, reliability characteristics and conduction mechanisms in ZnO-based RRAM devices were studied. Bipolar resistive switching characteristics were demonstrated in the Pt/ZnO/Pt structure. The dependence of ac voltage pulse on the switching voltages was characterized. Reliability tests indicate that the memory cells consisting of Pt/ZnO/Pt possess good ac cycling endurance (>106 cycles), data retention (>168 h), and read durability (>106 times). However, the dc switching cycling suffers the serious reliability issue. Based on the I-V measurements, the dominant conduction mechanisms in ZnO films are the hopping conduction and the ohmic conduction in HRS and LRS, respectively. Therefore, the trap spacing (2 nm) and the trap energy level (0.46 eV) in HRS are obtained. In LRS, the Fermi level in ZnO (0.4 eV) and the temperature dependence of electron mobility, as well as the effective density of states in conduction band in ZnO are also obtained.
The authors would like to acknowledge the late Prof. Tai-Bor Wu of National Tsing-Hua University. This research was supported by National Science Council, Taiwan, Republic of China under contract no. NSC 98-2221-E-130-027-MY2.
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