Bipolar resistance switching characteristics with opposite polarity of Au/SrTiO3/Ti memory cells
© Sun et al; licensee Springer. 2011
Received: 1 September 2011
Accepted: 23 November 2011
Published: 23 November 2011
Two types of bipolar resistance switching with eightwise and counter eightwise polarities are observed to coexist in Au/SrTiO3/Ti memory cells. These two types of switching can be induced by different defect distributions which are activated by controlling the electric process. The analyses of I-V and C-V data reveal that the resistance switching with eightwise polarity originates from the change of Schottky barrier at the Au/SrTiO3 interface caused by trapping/detrapping effects at interface defect states, while the switching with counter eightwise polarity is caused by oxygen-vacancy migration.
Resistance switching between a high-resistance state [HRS] and a low-resistance state [LRS] by voltage pulses has recently attracted intensive attention for their potential application in the next-generation nonvolatile memory . Many perovskite-type transition metal oxides, especially titanates [2–6], zirconates [7, 8], and manganites [9–11], have been investigated as resistance switching materials. The resistance switching effect can be classified into two types: bipolar and unipolar [12, 13]. Perovskite-type metal oxide devices generally exhibit bipolar resistance switching, in which the resistance state depends on the polarity of voltage. Two types of polarity behavior under the same bias voltage exist in the bipolar resistance switching. For the positive bias voltage, one is eightwise polarity, which changes resistance from a HRS to a LRS; the other is counter eightwise polarity, which converts a LRS into a HRS [14, 15]. Up to now, the underlying mechanism for bipolar resistance switching is still a controvertible problem, and various models, such as Schottky-like barrier alteration [3, 10], voltage-driven oxygen-vacancy migration , charge trapping in trap states , and so on, have been proposed to explain the switching behavior.
For bipolar resistance switching, an important issue is the physical origin of the switching polarity and their respective drive mechanism involved. Its clarification will be beneficial to get a comprehensive understanding of the switching mechanisms. These two switching types with eightwise and counter eightwise polarities occurring in the same medium have been discussed in the literatures [14–16]. We note that both kinds of polarity could be induced by choosing different top electrodes or modulating the range of applied voltage. These results actually imply the existence of different switching mechanisms in the same medium. Yang et al. reported that the redox reaction of the top electrode's oxide layer results in the bipolar switching with counter eightwise polarity, and the generation/annihilation process of the oxygen vacancy located at an oxygen-deficient layer at a metal/oxide interface contributes to the eightwise polarity . Shibuya et al. showed that the conversion of switching polarity is due to the modification of the Schottky-like barrier and electron-trapping effect at the interface by applied fields . Muenstermann et al. suggested allocating these two types of polarity to the filamentary or homogeneous conduction on the basis of conductive-tip atomic force microscopy topography . Although these descriptions are not the same, they all recognize that the defect states are very important to the polarity conversion of these two switching types. The electric process can bring impacts about the defect density in the insulator/semiconductor [3, 14]. The relationship between the resistance switching and the electric process is investigated in this paper in order to further understand the switching mechanism.
SrTiO3 [STO] films are considered as an n-type semiconductor due to the presence of oxygen vacancies . In this work, we investigated the resistance switching performance of STO films which sandwiched between the Au top electrode and the Ti bottom electrode. The electric measurement results clearly show that the bipolar resistance switching originates from the Schottky junction formed at the Au/STO interface. Two types of bipolar switching with eightwise and counter eightwise polarities can be activated under different electric processes. The involved physical mechanisms and their relationship with defect states are investigated and discussed.
Results and discussion
The above analysis shows that the bipolar resistance switching with eightwise polarity originates from the presence of the Schottky barrier near the Au/STO interface, in which there exist trapping states, such as oxygen vacancies and impurity states. The Schottky barrier width/height is changed due to the trapping/detrapping effect in the depletion layer [10, 14]. When a large forward-bias voltage is applied to the Schottky junction, electrons are discharged from the trapping states, resulting in unoccupied trapping states in the Schottky barrier. The increased density of positively charged trapping states in the depletion layer reduces the Schottky width/height, which results in the large leakage current simultaneously. So the Au/STO/Ti cell is switched to the LRS. In the lower-bias-voltage region, electrons can tunnel easily through the reduced Schottky barrier. When a large reverse-bias voltage is applied, electrons are captured in the trapping states, resulting in the reduction of the net positive charge in the depletion layer. This makes the barrier wider and higher, and the cell returns to the HRS. The trapped electrons seem to be released during a coming positive bias sweeping. The electroforming process is not required to realize this eightwise-polarity switching. This is because the high concentration of defects exists in the pristine junction due to the STO films prepared in an anoxic environment.
As stated above, the bipolar resistance switching with eightwise polarity originates from the change in the Schottky-like barrier height and/or width by trapping/detrapping effects at interface defect states, and the counter eightwise-polarity switching is caused by oxygen-vacancy migration. The current transition in Figure 4a is needed to convert the eightwise polarity to counter eightwise polarity. These findings can be explained as follows: In the initial state of the Au/STO/Ti cell, there exist trapping states in the Schottky junction. A strong electric field is applied to the depletion layer because the Schottky width is very small, so the trapping/detrapping effect of the defect states, which results in the change of barrier height/width, occurs at the depletion layer. Hence, the switching with eightwise polarity is realized. After the current transition, the Schottky barrier collapses, and the extended defects within the active thin film work as fast migration paths for oxygen vacancies, so the oxygen-vacancy migration is significantly enhanced along the extended defects. Thereafter, the cell exhibits counter eightwise-polarity switching based on an oxygen-vacancy-migration-related switching mechanism. In short, the switching mechanism is dominated by the defect state density of the active film, and the defect states can be controlled by controlling the electric process.
In summary, we have investigated bipolar resistance switching characteristics of the Au/STO/Ti cell treated with different electric processes. The experiment results demonstrate that two types of bipolar resistance switching coexist in the same cell. The switching with eightwise polarity originates from the change in the Schottky-like barrier height and/or width by trapping/detrapping effects at the interface defect states, and the switching with counter eightwise polarity originates from oxygen-vacancy migration. The conversion from the eightwise polarity to the counter eightwise polarity is caused by the different defect distributions in the films which can be changed by different electric processes.
This work has been supported by the National Natural Science Foundation of China (grant number. 60976016) and the Specialized Research Fund for the Doctoral Program of Higher Education (grant number 20094103110001).
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