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Al structure
Vacuum 156 (2018) 91–96
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Impact of potential barrier on electronic resistive switching performance based on Al/TiOx/Al structure
T
Shangfei Wan, Yu Yan, Chen Wang, Zhengchun Yang, Jinshi Zhao∗ School of Electrical and Electronic Engineering, Tianjin Key Laboratory of Film Electronic & Communication Devices, Tianjin University of Technology, 391 West Binshui Road, Tianjin, 300384, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electronic bipolar resistive switching TiOx Endurance Space charge limited conduction
The thickness-dependent electronic bipolar resistive switching (eBRS) behaviors in Al/TiOx/Al (ATA) structure were examined based on the I-V sweep and endurance test. The trap mediated space charge limited conduction was confirmed through the fitting of the I-V curves. The ATA devices showed the different endurance trends and performance with the TiOx thickness. ATA with 80 nm TiOx maintained the longest endurance cycles compared with 50 nm and 110 nm_TiOx. For 110 nm_TiOx, the slope of the trap-filling region increased with RS cycles which differed from other two cases, suggesting the RS is closely related to the electron trapping and detrapping rather than oxygen ion. Overall, the endurance performance based on eBRS was affected by the formation of interfacial layer (IL) of AlOx during TiOx deposition, wherein the IL AlOx increased with increasing TiOx deposition time due to the higher oxidation potential of Al. The IL AlOx plays a role in determining the potential barrier height that adjusts the amount of trapped and detrapped electrons, is presented. In addition, the effect of the potential barrier at the interface of top electrode (TE) on endurance performance was also investigated by inserting a 1 nm_Al.
1. Introduction In semiconductor devices, TiO2 as a transition metal oxide (TMO) has very diverse configurations in terms of its thickness, resistivity, density, and dielectric constant depending on its purpose of use. Its functionality was recently extended to resistance switching random access memory (RRAM) [1,2]. RRAM has been researched and developed as one of the most promising candidates for information storage due to a feasible successor to instead of the NAND-type flash memory [3–5]. The commonly accepted RS mechanism in many TMOs is the formation and rupture of conducting filaments (CFs), which are either an aggregation of defects, such as oxygen vacancy (VO), the nano-scale conducting phase (e.g., the Magnéli phase in TiO2), or the metallic filament (e.g., Cu) in the electrochemical metallization (ECM) cell [6–9]. No matter what the detailed nature of these CFs is, the involvement of ionic defects, i.e., electric-field-induced defect generation and migration (assisted by Joule heating) as well as thermal motion (for rupturing in non-polar RS), is the critical factor of memory operation. These ionic defect mediated RS generally had a relatively high-power consumption, uniformity concerns and reliability issues, although several great improvements were also achieved during the past decade [10]. On the other hand, the more recently emerged eBRS mechanism has been
∗
reported in several systems [11,12]. To realize the eBRS operation, an asymmetric potential barrier must be presented [13,14]. The electron should be fluently injected from a cathode electrode which has a lower potential barrier interface, and the electron trapping is happened (set process). Subsequently, a negative bias is applied to the other electrode that maintains a higher potential barrier interface, and the electron injection should be suppressed, resulting in the trapped electron during set process can be detrapped during the reset process. Such mechanism has been explained in detail for the case of the TiO2 memory cell with Pt electrode [15]. Although Pt is a feasible electrode material for research, it is incompatible with mass production due to its difficult patterning and high cost [16]. Author also has been presented the eBRS based on a simple Al/TiOx/Al memory cell, which was conventional in the existing semiconductor fabrication process. The eBRS mainly depends on electron trapping/detrapping without ionic motion. Therefore, a low power consumption, high uniformity and high reliability of RRAM can be expected for eBRS. However, in fact, the expected merits were not achieved as the earlier reports on eBRS in the TMO system, especially at endurance performance. Therefore, a simple structure based on eBRS with a better endurance performance is still open to more extensive research. As mentioned, the asymmetric potential barrier plays a big role in the electron trapping/detrapping, is one of critical factors for the
Corresponding author. E-mail address: [email protected] (J. Zhao).
https://doi.org/10.1016/j.vacuum.2018.07.018 Received 24 May 2018; Received in revised form 29 June 2018; Accepted 12 July 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
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increasing thickness of TiOx due to the higher oxidation potential of Al, as shown in Fig. 2. Therefore, the potential barrier height at TiOx/Al (BE) should be varied with the different thickness TiOx. Furthermore, the thicker TiOx may be contained more defects due to its suffering long-term plasma damage. An attempt to improvement the endurance performance through controlling the potential barrier of TiOx/BE was explored in this paper. In addition, the potential barrier of TE interface was also adjusted by inserting a 1 nm_Al thin film, investigating the its effect on the electron trapping/detrapping. It also provided a more clearly evidence to prove that the electron trapping/detrapping is major mechanism in ATA structure through fitting the I-V curve under repeat cycles. 2. Experimental details Firstly, a 200 nm-thick Al thin film was electron-beam-evaporated on a Ti (5 nm)/SiO2/Si wafer as BE. Then, the TiOx thin films were deposited on BE Al under the different working pressure, the effect of working pressure on the resistive switching of the TiOx were observed as previous reports [17,18]. In this paper, the deposited TiOx film at 1 Pa working pressure showed more stable resistive switching properties. Therefore, the different thicknesses of TiOx films were sputtered on the blanket Al in O2/Ar mixed gas ambient at room temperature (1 Pa total pressure, 4% O2). For fabricating a sample with an Al insert layer, a 1 to 2 nm-thick Al layer was then sputtered on the TiOx/Al (BE) in an ion beam sputtering system using an Al target with Ar gas, and an ion beam power of 50 W at room temperature. Finally, the top electrode Al was evaporated on TiOx through a metal shadow mask. The XPS combined with 3 keV Ar+ depth profiling was performed to investigate the binding energy status of Ti 2p, Al 2p and O 1s in the Al(1 nm)/TiOx/Al sample. The incident angle of Ar+ maintains 50 degrees with the sample surface. The atomic concentrations were calculated from the
Fig. 1. XPS spectra of the TiOx film.
eBRS. However, there has been barely reported about the eBRS properties, which were improved through adjusting the potential barrier in a simple structure. In addition, it is also a generally challenge to prove that the RS mechanism is solely or mainly depend on the electron trapping due to the co-existence of electron and defect (VO) at a given ionic configuration. In author previous study, several measurements were implemented to support the Al/TiOx/Al structure was related to the electron trapping/detrapping rather than ionic motion (VO) [12]. The asymmetry potential barrier has been elucidate using HR-TEM in the author's previous studies, which is due to the formation of AlOx at Al (bottom electrode: BE) interface during TiOx deposition, but there is no formation of AlOx at the TE interface [12]. In present work, the thickness of the interfacial layer (IL) AlTiOx was controlled through varying the thickness of TiOx, and it should be increased with
Fig. 2. The low magnification TEM images of the (a) 80 nm_TiOx/Al and (b) 110 nm_TiOx/Al samples, and the high resolution TEM images of (c) 80 nm_TiOx/Al and (d) 110 nm_TiOx/Al samples. The EDS line scan data (count per second with an arbitrary scale) appended in the (c) and (d), respectively. 92
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V [19]. As the absolute voltage increased, slope became larger than 2, which is well coincident with the trap-limited space charge limited current (SCLC), can be express as
photoelectron peak areas using Shirley background subtraction and sensitivity factors provided by the spectrometer manufacturer Thermo Scientific (ESCALAB250Xi). The ratio of atomic concentration of Ti/O is ∼1/2 which was analyzed according to the survey spectrums of the TiOx film in Fig. 1. The morphologies of the films were observed through atomic force microscopy (AFM), and the microstructures of TiOx/Al (BE) were examined by the cross-section transmission electron microscopy (TEM). The I−V and endurance characteristics of the samples were measured in the voltage-sweeping mode with an Agilent B1500 A at room temperature. The Al top electrode was biased while the Al bottom electrode was grounded during the I-V sweeps.
JSCLC =
9 ε 0 εr μn θV 2/(1 + θ) d3 8
(1)
, where ε0 is the permittivity of vacuum, εr is the relative dielectric constant of media materials, μn is the electron mobility, θ is the ratio of free electrons and trapped-electrons, V is the applied voltage, d is the thickness of the film [20,21]. The fitting results of I-V of 50 nm_ATA in HRS was shown in Fig. 3(d), according to the space charge limited current theory. The slope of the trap-filling region is 3.62, 3.56 and 2.33 at 1 s t, 2nd and 401 s t cycles, respectively. The decreased slope relates to the decrease of trap density with RS cycles, suggesting the trapped electron cannot be effectively detrapped during the reset process [22]. While the TiOx thickness was increased to 80 nm, both RH and RL also decreased until 2 × 103 RS cycles, wherein the RH/RL ratio was still higher than 10, as shown in Fig. 3(b). After that point, it appeared that RH and RL were less influenced by increasing RS cycles. The effective working time of the 80 nm_ATA can be greatly improved compare with 50 nm_ATA, and it can keep working over 1.6 × 104 cycles RS. The slope also decreased with the RS cycles, but the slowing in the rate of slope decrease could indicate the electron detrapping is more efficiency than that of 50 nm_ATA. The endurance test result of 110 nm_ATA is shown in Fig. 3(c). It is interesting that the trend of RH and RL with the RS cycles are opposite to the former two. Both RH and RL increased continuously with RS cycles, even in the early stage of endurance test. The conduction mechanism also can well be fitted with SCLC theory. The slope of Fig. 3(f) did not decrease but increased, meaning that the trap density increased with RS cycles. It indicates that the electron detrapping is more active than the electron trapping. The ratio of RH/RL over 10 can be maintained less than 8 × 103 RS cycles, which is just as long as half of 80 nm_ATA. This finding indicates that the endurance performance seemed to be affected by the thickness of TiOx in the ATA system. In this work, 80 nm_ATA is an optimized thickness for endurance performance at a given switching condition (Vset: −4 V and Vreset: 3.5 V). The actual maximum endurance cycle number of pulse switch type might be much higher than I-V sweep method because a much higher stress was imposed on the sample using I-V sweep method [23]. Therefore, it is believed that the reliability of the device in this work could be even higher than demonstrated presently. The endurance degradation caused by the unbalance of electron trapping and detrapping has been elucidated in the previous paper [12]. In other words, the balance between electron trapping and detrapping could be regulated through modifying the potential barrier of the device. In this study, the thickness of TiOx film was controlled by the deposition time, and the influence of plasma-activated oxygen ions and radicals on Al BE increases with time, and it is possible to form the more stable and thicker IL AlOx for the thicker TiOx sample, as descripted in Fig. 2. ATA samples have an asymmetry potential barrier even using symmetrical electrode in Fig. 4. It is because the IL AlOx was formed at the interface of BE during TiOx deposition, and there was no IL at the TE interface. Therefore, the formation of AlOx at the BE interface can play a role of the barrier for the electron injection compared with the TE interface. From these experimental results, the following considerations are proposed to explain the thickness effect on endurance based on eBRS. Fig. 4 shows the energy band diagrams of SCLC mechanism according to the different thicknesses of TiOx. A relative high defect in TiOx work as the trap centers, and a lower potential barrier could be formed at TE interface due to the low work function of Al. Therefore, the fluent electron injection could be obtained when a negative bias was applied to the TE, and then the injected electron was trapped at the defect center, resulting in the transition from HRS to LRS (set process). In the case of 50 nm_TiOx, the trapped electron cannot be effectively detrapped due to more electron injection (set process) and
3. Results and discussion Fig. 2(a) and (b) show the low magnification TEM images of the 80 nm_TiOx/Al and 110 nm_TiOx/Al samples, respectively, and Fig. 2(c) and (d) show the high resolution TEM images of the same samples. The TiOx layers showed a mainly amorphous phase, which included several weakly crystallized regions. It was notable that the ILs were induced between TiOx and Al BE, wherein the IL is ∼7 nm (80 nm_TiOx) and ∼10 nm (110 nm_TiOx), respectively. The TEM analysis confirmed that the IL increased with increasing TiOx thickness. The energy dispersive spectroscopy (EDS) line scan data appended in the TEM images revealed that IL layer include Al-Ti-O. The Ti signals in the EDS data from both samples show abruptly decrease in the IL region, which suggest that AlOx layer was firstly formed, and then the Al-Ti-O was induced due to the reaction of the formed AlOx and TiOx. In this study, the IL is called AlOx to simplify the description. The 110 nm_TiOx sample shows a thicker Al-Ti-O and AlOx layer compared with 80 nm_TiOx sample. The interfacial AlOx layer has a higher band gap than that of TiOx. This provides the sample with an asymmetric potential barrier for the emergence of eBRS in this work. Therefore, the thicker thickness of IL could contribute to increase the potential barrier height at the BE interface due to its higher band energy. The surface morphologies of TiOx films on Al with different thicknesses were investigated using atomic force microscopy (AFM) (data is not shown here). The root-meansquare roughness value of 50 nm, 80 nm, 110 nm TiOx is ∼10 Å, indicating that the thickness of TiOx does not affect its surface morphology. The morphology of TiOx is insensitive to the thickness due to the amorphous phase of the deposited TiOx. From the results mentioned above, the surface morphology is not the factor of affecting the RS property. The I-V curves of the ATA with 50 nm, 80 nm and 110 nm TiOx thickness are shown in the insets of Fig. 3, respectively. The three samples initially have the insulate state and the initial resistance increases with the TiOx thickness. The electroforming process (black square symbols) must be performed to activate the 80 nm and 110 nm_TiOx samples, but the first voltage sweep curve (black square symbols) of 50 nm_TiOx sample is almost overlap with that of second (the red circle), meaning an electroforming free behavior. After the first voltage sweep, all the samples show a gradual I-V curve during the set/ reset process. The devices structure and RS feature are similar to the previous work, suggesting the RS mechanism in this work can be regarded as eBRS [12]. Fig. 3(a)-(c) show the endurance test results were measured by the repeated I-V sweep for the different TiOx thickness, respectively. It is noteworthy that the samples show a different endurance behavior according to TiOx thickness. Here, the sample is defined as a failure when the resistance value of high resistance state (RH)/the resistance value of low resistance state (RL) is less than 10 during endurance test. In the case of 50 nm_ATA, Fig. 3(a) shows that both RH and RL decreased with increasing the RS cycles (Ncy), and the continued decrease of RH is more obviously in comparison to RL. The sample had a fast failure after Ncy of 300. The logI-logV curve of the HRS according to the switching cycles were measured in the negative bias region, as shown in Fig. 3(d)-(f). In the absolute low voltage, all samples showed slope close to 1, suggesting an ohmic behavior with I ∝ 93
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Fig. 3. The endurance results and the SCLC fitting results from double-log scale for different repetition cycles of the ATA devices (a), (d) 50 nm, (b), (e) 80 nm (c), (f) 110 nm, respectively. The insets of (a)–(c) show the typical BRS I-V curves of the devices.
increasing TiOx thickness. These made the electron trapping is comparable to the electron detrapping, resulting in a better endurance performance in Fig. 3(b). In the case of 110 nm_ATA, Fig. 4(c) shows that a further increase in TiOx thickness has an effect on the increase of potential barrier height, which even causes that the amount of the detrapped electrons are larger than the trapped electrons due to the formation of the more stable AlOx. If there is an excess of the electron detrapping, it can cause the increase of the defect density and lead to the increase of the slope. This is consistent with the result of Fig. 3(c). Base on the above analysis, the electron detrapping can be controlled through adjusting the potential barrier at the interface of TiOx/
inefficient electron blocking of the thinner AlOx (reset process), resulting in a rapid endurance degradation as shown in Fig. 4(a). With increasing the TiOx thickness, the effective potential barrier height should be increased due to the formation of the more stable AlOx, as shown in Fig. 4(b) and (c). Hence, the electron injection from BE can be effectively suppressed when a positive bias was applied to TE (reset process). In other words, more frequent electron detrapping activity can be obtained with increasing TiOx thickness. With increasing the TiOx thickness to 80 nm, the ability of electron detrapping can be improved because the thicker AlOx can more effectively block the electron injection from BE. In addition, the injected electron also reduces with
Fig. 4. The energy band configurations of RS process of (a) 50 nm_ATA, (b) 80 nm_ATA and (c) 110 nm_ATA, respectively. 94
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Fig. 5. Core-level spectra of (a) Ti 2p, (b) Al 2p and (c) O 1s at the surface and the inside of TiOx after sputter etching.
similar to the Al/TiOx/Al. However, compared with 80 nm_ATA, it is noteworthy that endurance performance can be significantly improved by inserting 1 nm Al between TE and TiOx. As mentioned above, some of trapped electron cannot be effectively detrapped during the reset process, which is the reason for the endurance degradation. The formed Al2O3 at the interface of TE/TiOx plays a role of blocking electron injection during the set process, resulting in the decrease in the number of the trapped electron. In the case of Al/Al (1 nm)/TiOx (80 nm)/Al, the balance of electron trapping and detrapping could be further adjusted through decreasing the amount of injected electron. This again confirms that the IL AlOx at both interfaces can modify the potential barrier in the ATA structure and affect the performance of eBRS, such as endurance performance.
Al (BE), and which is more efficient in the thicker TiOx. An attempt was made to change the potential barrier of the interface of TE/TiOx, which was also done through inserting 1 nm_Al in 80 nm_ATA. Fig. 5 (a), (b) and (c) show the Ti 2p, Al 2p and O 1s core-level of XPS spectra for Al (1 nm)/TiOx/Al, respectively, where the upper is the surface of Al (1 nm)/TiOx/Al and the bottom is the inside position of TiOx after etching Al and a part of TiOx. The binding energy was calibrated using the surface carbon–carbon binding energy (284.8 eV). Ti4+ and Ti3+ were observed in Fig. 5(a), respectively, and there is no metallic Ti, wherein the Ti4+ is the mainly component of the TiOx layer. The area of Ti3+ of the inside is larger than that of surface which maybe caused by the high energy ion bombardment during the depth etching. In Fig. 5(a), the surface of Ti 2p showed a shift of the peak position to lower binding energy direction by 0.3 eV. This suggests the portion of TiOx contacted with Al layer is reduced. The O 1s spectra is shown in Fig. 5(c). The O 1s of the surface TiOx shows a broader position than that of inside, and the binding energies at 530.7 eV, 531.3 eV and 531.9 eV are coincided with the oxygen bound to the TiO2 (OTi), A2O3 lattice oxygen (OAl) and oxygen vacancies or defects (OV), respectively. There are only OTi (530.7 eV) and OV (531.7 eV) without OAl in the inside position. The O 1s of the inside TiOx can be separated into OTi (530.7 eV) and OV (531.7 eV), which shows an increasing of area ratio of OTi/OV from 0.56 to 1.47 in compared with that of surface TiOx. Therefore, the XPS results indicated that oxygen vacancies were induced at the Al (1 nm)/TiOx interface. It is closely to correlation with the oxidation of Al, and the Al-O bonding at 74.9 eV indicates the 1 nm Al was full oxidized to Al2O3, which was induced by extracting the oxygen from the surface TiOx [24]. Therefore, the increase of potential barrier height can be expected from the formed Al2O3 at TE/TiOx interface. Fig. 6(a) show I-V curve of the Al/Al (1 nm)/TiOx/Al, and which is
4. Conclusions In summary, this study examined the eBRS of ATA structure with different thicknesses of TiOx. The ATA showed a distinctive endurance performance according to the TiOx thickness, suggesting the potential barrier at TiOx/BE could be influenced due to the formation of interfacial layer AlOx. The thickness of interfacial layer AlOx increased with the deposition time of TiOx, and the frequency of electron trapping and detrapping was affected. Both 50 nm and 80 nm_ATA samples show a decreased RL and RH with the repeated RS cycle, but 110 nm_ATA sample has an increased trend of RL and RH. Whether electron trapping (50 nm_ATA) or electron detrapping (110 nm_ATA) is excessive-both samples show an endurance degradation. When a relative balance of electron trapping and detrapping can be maintained, the 80 nm_TiOx sample shows a longest endurance performance. In addition, the endurance performance of 80 nm_ATA was also improved through inserting 1 nm_Al into TE/TiOx interface because the balance of electron
Fig. 6. (a) Typical BRS I−V curve and (b) endurance results of Al/Al (1 nm)/TiOx (80 nm)/Al. 95
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trapping and detrapping was further adjusted. Although the actual situation is quite complicated, the fitting of I–V curves dependent I–V measurements showed the trap-mediated SCLC. The eBRS is strongly related to the electron trapping and detrapping rather than the oxygen motion, which was understand from the increased slope of the trapfilling region analysis according to the 110 nm_ATA. Although the eBRS performance in a sample ATA structure need to improve more, it is believed that this approach could provide valuable insights into electronic RS mechanism of other binary oxide.
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Impact of potential barrier on electronic resistive switching performance based on Al/TiOx/Al structure
T
Shangfei Wan, Yu Yan, Chen Wang, Zhengchun Yang, Jinshi Zhao∗ School of Electrical and Electronic Engineering, Tianjin Key Laboratory of Film Electronic & Communication Devices, Tianjin University of Technology, 391 West Binshui Road, Tianjin, 300384, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electronic bipolar resistive switching TiOx Endurance Space charge limited conduction
The thickness-dependent electronic bipolar resistive switching (eBRS) behaviors in Al/TiOx/Al (ATA) structure were examined based on the I-V sweep and endurance test. The trap mediated space charge limited conduction was confirmed through the fitting of the I-V curves. The ATA devices showed the different endurance trends and performance with the TiOx thickness. ATA with 80 nm TiOx maintained the longest endurance cycles compared with 50 nm and 110 nm_TiOx. For 110 nm_TiOx, the slope of the trap-filling region increased with RS cycles which differed from other two cases, suggesting the RS is closely related to the electron trapping and detrapping rather than oxygen ion. Overall, the endurance performance based on eBRS was affected by the formation of interfacial layer (IL) of AlOx during TiOx deposition, wherein the IL AlOx increased with increasing TiOx deposition time due to the higher oxidation potential of Al. The IL AlOx plays a role in determining the potential barrier height that adjusts the amount of trapped and detrapped electrons, is presented. In addition, the effect of the potential barrier at the interface of top electrode (TE) on endurance performance was also investigated by inserting a 1 nm_Al.
1. Introduction In semiconductor devices, TiO2 as a transition metal oxide (TMO) has very diverse configurations in terms of its thickness, resistivity, density, and dielectric constant depending on its purpose of use. Its functionality was recently extended to resistance switching random access memory (RRAM) [1,2]. RRAM has been researched and developed as one of the most promising candidates for information storage due to a feasible successor to instead of the NAND-type flash memory [3–5]. The commonly accepted RS mechanism in many TMOs is the formation and rupture of conducting filaments (CFs), which are either an aggregation of defects, such as oxygen vacancy (VO), the nano-scale conducting phase (e.g., the Magnéli phase in TiO2), or the metallic filament (e.g., Cu) in the electrochemical metallization (ECM) cell [6–9]. No matter what the detailed nature of these CFs is, the involvement of ionic defects, i.e., electric-field-induced defect generation and migration (assisted by Joule heating) as well as thermal motion (for rupturing in non-polar RS), is the critical factor of memory operation. These ionic defect mediated RS generally had a relatively high-power consumption, uniformity concerns and reliability issues, although several great improvements were also achieved during the past decade [10]. On the other hand, the more recently emerged eBRS mechanism has been
∗
reported in several systems [11,12]. To realize the eBRS operation, an asymmetric potential barrier must be presented [13,14]. The electron should be fluently injected from a cathode electrode which has a lower potential barrier interface, and the electron trapping is happened (set process). Subsequently, a negative bias is applied to the other electrode that maintains a higher potential barrier interface, and the electron injection should be suppressed, resulting in the trapped electron during set process can be detrapped during the reset process. Such mechanism has been explained in detail for the case of the TiO2 memory cell with Pt electrode [15]. Although Pt is a feasible electrode material for research, it is incompatible with mass production due to its difficult patterning and high cost [16]. Author also has been presented the eBRS based on a simple Al/TiOx/Al memory cell, which was conventional in the existing semiconductor fabrication process. The eBRS mainly depends on electron trapping/detrapping without ionic motion. Therefore, a low power consumption, high uniformity and high reliability of RRAM can be expected for eBRS. However, in fact, the expected merits were not achieved as the earlier reports on eBRS in the TMO system, especially at endurance performance. Therefore, a simple structure based on eBRS with a better endurance performance is still open to more extensive research. As mentioned, the asymmetric potential barrier plays a big role in the electron trapping/detrapping, is one of critical factors for the
Corresponding author. E-mail address: [email protected] (J. Zhao).
https://doi.org/10.1016/j.vacuum.2018.07.018 Received 24 May 2018; Received in revised form 29 June 2018; Accepted 12 July 2018 0042-207X/ © 2018 Elsevier Ltd. All rights reserved.
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increasing thickness of TiOx due to the higher oxidation potential of Al, as shown in Fig. 2. Therefore, the potential barrier height at TiOx/Al (BE) should be varied with the different thickness TiOx. Furthermore, the thicker TiOx may be contained more defects due to its suffering long-term plasma damage. An attempt to improvement the endurance performance through controlling the potential barrier of TiOx/BE was explored in this paper. In addition, the potential barrier of TE interface was also adjusted by inserting a 1 nm_Al thin film, investigating the its effect on the electron trapping/detrapping. It also provided a more clearly evidence to prove that the electron trapping/detrapping is major mechanism in ATA structure through fitting the I-V curve under repeat cycles. 2. Experimental details Firstly, a 200 nm-thick Al thin film was electron-beam-evaporated on a Ti (5 nm)/SiO2/Si wafer as BE. Then, the TiOx thin films were deposited on BE Al under the different working pressure, the effect of working pressure on the resistive switching of the TiOx were observed as previous reports [17,18]. In this paper, the deposited TiOx film at 1 Pa working pressure showed more stable resistive switching properties. Therefore, the different thicknesses of TiOx films were sputtered on the blanket Al in O2/Ar mixed gas ambient at room temperature (1 Pa total pressure, 4% O2). For fabricating a sample with an Al insert layer, a 1 to 2 nm-thick Al layer was then sputtered on the TiOx/Al (BE) in an ion beam sputtering system using an Al target with Ar gas, and an ion beam power of 50 W at room temperature. Finally, the top electrode Al was evaporated on TiOx through a metal shadow mask. The XPS combined with 3 keV Ar+ depth profiling was performed to investigate the binding energy status of Ti 2p, Al 2p and O 1s in the Al(1 nm)/TiOx/Al sample. The incident angle of Ar+ maintains 50 degrees with the sample surface. The atomic concentrations were calculated from the
Fig. 1. XPS spectra of the TiOx film.
eBRS. However, there has been barely reported about the eBRS properties, which were improved through adjusting the potential barrier in a simple structure. In addition, it is also a generally challenge to prove that the RS mechanism is solely or mainly depend on the electron trapping due to the co-existence of electron and defect (VO) at a given ionic configuration. In author previous study, several measurements were implemented to support the Al/TiOx/Al structure was related to the electron trapping/detrapping rather than ionic motion (VO) [12]. The asymmetry potential barrier has been elucidate using HR-TEM in the author's previous studies, which is due to the formation of AlOx at Al (bottom electrode: BE) interface during TiOx deposition, but there is no formation of AlOx at the TE interface [12]. In present work, the thickness of the interfacial layer (IL) AlTiOx was controlled through varying the thickness of TiOx, and it should be increased with
Fig. 2. The low magnification TEM images of the (a) 80 nm_TiOx/Al and (b) 110 nm_TiOx/Al samples, and the high resolution TEM images of (c) 80 nm_TiOx/Al and (d) 110 nm_TiOx/Al samples. The EDS line scan data (count per second with an arbitrary scale) appended in the (c) and (d), respectively. 92
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V [19]. As the absolute voltage increased, slope became larger than 2, which is well coincident with the trap-limited space charge limited current (SCLC), can be express as
photoelectron peak areas using Shirley background subtraction and sensitivity factors provided by the spectrometer manufacturer Thermo Scientific (ESCALAB250Xi). The ratio of atomic concentration of Ti/O is ∼1/2 which was analyzed according to the survey spectrums of the TiOx film in Fig. 1. The morphologies of the films were observed through atomic force microscopy (AFM), and the microstructures of TiOx/Al (BE) were examined by the cross-section transmission electron microscopy (TEM). The I−V and endurance characteristics of the samples were measured in the voltage-sweeping mode with an Agilent B1500 A at room temperature. The Al top electrode was biased while the Al bottom electrode was grounded during the I-V sweeps.
JSCLC =
9 ε 0 εr μn θV 2/(1 + θ) d3 8
(1)
, where ε0 is the permittivity of vacuum, εr is the relative dielectric constant of media materials, μn is the electron mobility, θ is the ratio of free electrons and trapped-electrons, V is the applied voltage, d is the thickness of the film [20,21]. The fitting results of I-V of 50 nm_ATA in HRS was shown in Fig. 3(d), according to the space charge limited current theory. The slope of the trap-filling region is 3.62, 3.56 and 2.33 at 1 s t, 2nd and 401 s t cycles, respectively. The decreased slope relates to the decrease of trap density with RS cycles, suggesting the trapped electron cannot be effectively detrapped during the reset process [22]. While the TiOx thickness was increased to 80 nm, both RH and RL also decreased until 2 × 103 RS cycles, wherein the RH/RL ratio was still higher than 10, as shown in Fig. 3(b). After that point, it appeared that RH and RL were less influenced by increasing RS cycles. The effective working time of the 80 nm_ATA can be greatly improved compare with 50 nm_ATA, and it can keep working over 1.6 × 104 cycles RS. The slope also decreased with the RS cycles, but the slowing in the rate of slope decrease could indicate the electron detrapping is more efficiency than that of 50 nm_ATA. The endurance test result of 110 nm_ATA is shown in Fig. 3(c). It is interesting that the trend of RH and RL with the RS cycles are opposite to the former two. Both RH and RL increased continuously with RS cycles, even in the early stage of endurance test. The conduction mechanism also can well be fitted with SCLC theory. The slope of Fig. 3(f) did not decrease but increased, meaning that the trap density increased with RS cycles. It indicates that the electron detrapping is more active than the electron trapping. The ratio of RH/RL over 10 can be maintained less than 8 × 103 RS cycles, which is just as long as half of 80 nm_ATA. This finding indicates that the endurance performance seemed to be affected by the thickness of TiOx in the ATA system. In this work, 80 nm_ATA is an optimized thickness for endurance performance at a given switching condition (Vset: −4 V and Vreset: 3.5 V). The actual maximum endurance cycle number of pulse switch type might be much higher than I-V sweep method because a much higher stress was imposed on the sample using I-V sweep method [23]. Therefore, it is believed that the reliability of the device in this work could be even higher than demonstrated presently. The endurance degradation caused by the unbalance of electron trapping and detrapping has been elucidated in the previous paper [12]. In other words, the balance between electron trapping and detrapping could be regulated through modifying the potential barrier of the device. In this study, the thickness of TiOx film was controlled by the deposition time, and the influence of plasma-activated oxygen ions and radicals on Al BE increases with time, and it is possible to form the more stable and thicker IL AlOx for the thicker TiOx sample, as descripted in Fig. 2. ATA samples have an asymmetry potential barrier even using symmetrical electrode in Fig. 4. It is because the IL AlOx was formed at the interface of BE during TiOx deposition, and there was no IL at the TE interface. Therefore, the formation of AlOx at the BE interface can play a role of the barrier for the electron injection compared with the TE interface. From these experimental results, the following considerations are proposed to explain the thickness effect on endurance based on eBRS. Fig. 4 shows the energy band diagrams of SCLC mechanism according to the different thicknesses of TiOx. A relative high defect in TiOx work as the trap centers, and a lower potential barrier could be formed at TE interface due to the low work function of Al. Therefore, the fluent electron injection could be obtained when a negative bias was applied to the TE, and then the injected electron was trapped at the defect center, resulting in the transition from HRS to LRS (set process). In the case of 50 nm_TiOx, the trapped electron cannot be effectively detrapped due to more electron injection (set process) and
3. Results and discussion Fig. 2(a) and (b) show the low magnification TEM images of the 80 nm_TiOx/Al and 110 nm_TiOx/Al samples, respectively, and Fig. 2(c) and (d) show the high resolution TEM images of the same samples. The TiOx layers showed a mainly amorphous phase, which included several weakly crystallized regions. It was notable that the ILs were induced between TiOx and Al BE, wherein the IL is ∼7 nm (80 nm_TiOx) and ∼10 nm (110 nm_TiOx), respectively. The TEM analysis confirmed that the IL increased with increasing TiOx thickness. The energy dispersive spectroscopy (EDS) line scan data appended in the TEM images revealed that IL layer include Al-Ti-O. The Ti signals in the EDS data from both samples show abruptly decrease in the IL region, which suggest that AlOx layer was firstly formed, and then the Al-Ti-O was induced due to the reaction of the formed AlOx and TiOx. In this study, the IL is called AlOx to simplify the description. The 110 nm_TiOx sample shows a thicker Al-Ti-O and AlOx layer compared with 80 nm_TiOx sample. The interfacial AlOx layer has a higher band gap than that of TiOx. This provides the sample with an asymmetric potential barrier for the emergence of eBRS in this work. Therefore, the thicker thickness of IL could contribute to increase the potential barrier height at the BE interface due to its higher band energy. The surface morphologies of TiOx films on Al with different thicknesses were investigated using atomic force microscopy (AFM) (data is not shown here). The root-meansquare roughness value of 50 nm, 80 nm, 110 nm TiOx is ∼10 Å, indicating that the thickness of TiOx does not affect its surface morphology. The morphology of TiOx is insensitive to the thickness due to the amorphous phase of the deposited TiOx. From the results mentioned above, the surface morphology is not the factor of affecting the RS property. The I-V curves of the ATA with 50 nm, 80 nm and 110 nm TiOx thickness are shown in the insets of Fig. 3, respectively. The three samples initially have the insulate state and the initial resistance increases with the TiOx thickness. The electroforming process (black square symbols) must be performed to activate the 80 nm and 110 nm_TiOx samples, but the first voltage sweep curve (black square symbols) of 50 nm_TiOx sample is almost overlap with that of second (the red circle), meaning an electroforming free behavior. After the first voltage sweep, all the samples show a gradual I-V curve during the set/ reset process. The devices structure and RS feature are similar to the previous work, suggesting the RS mechanism in this work can be regarded as eBRS [12]. Fig. 3(a)-(c) show the endurance test results were measured by the repeated I-V sweep for the different TiOx thickness, respectively. It is noteworthy that the samples show a different endurance behavior according to TiOx thickness. Here, the sample is defined as a failure when the resistance value of high resistance state (RH)/the resistance value of low resistance state (RL) is less than 10 during endurance test. In the case of 50 nm_ATA, Fig. 3(a) shows that both RH and RL decreased with increasing the RS cycles (Ncy), and the continued decrease of RH is more obviously in comparison to RL. The sample had a fast failure after Ncy of 300. The logI-logV curve of the HRS according to the switching cycles were measured in the negative bias region, as shown in Fig. 3(d)-(f). In the absolute low voltage, all samples showed slope close to 1, suggesting an ohmic behavior with I ∝ 93
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Fig. 3. The endurance results and the SCLC fitting results from double-log scale for different repetition cycles of the ATA devices (a), (d) 50 nm, (b), (e) 80 nm (c), (f) 110 nm, respectively. The insets of (a)–(c) show the typical BRS I-V curves of the devices.
increasing TiOx thickness. These made the electron trapping is comparable to the electron detrapping, resulting in a better endurance performance in Fig. 3(b). In the case of 110 nm_ATA, Fig. 4(c) shows that a further increase in TiOx thickness has an effect on the increase of potential barrier height, which even causes that the amount of the detrapped electrons are larger than the trapped electrons due to the formation of the more stable AlOx. If there is an excess of the electron detrapping, it can cause the increase of the defect density and lead to the increase of the slope. This is consistent with the result of Fig. 3(c). Base on the above analysis, the electron detrapping can be controlled through adjusting the potential barrier at the interface of TiOx/
inefficient electron blocking of the thinner AlOx (reset process), resulting in a rapid endurance degradation as shown in Fig. 4(a). With increasing the TiOx thickness, the effective potential barrier height should be increased due to the formation of the more stable AlOx, as shown in Fig. 4(b) and (c). Hence, the electron injection from BE can be effectively suppressed when a positive bias was applied to TE (reset process). In other words, more frequent electron detrapping activity can be obtained with increasing TiOx thickness. With increasing the TiOx thickness to 80 nm, the ability of electron detrapping can be improved because the thicker AlOx can more effectively block the electron injection from BE. In addition, the injected electron also reduces with
Fig. 4. The energy band configurations of RS process of (a) 50 nm_ATA, (b) 80 nm_ATA and (c) 110 nm_ATA, respectively. 94
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Fig. 5. Core-level spectra of (a) Ti 2p, (b) Al 2p and (c) O 1s at the surface and the inside of TiOx after sputter etching.
similar to the Al/TiOx/Al. However, compared with 80 nm_ATA, it is noteworthy that endurance performance can be significantly improved by inserting 1 nm Al between TE and TiOx. As mentioned above, some of trapped electron cannot be effectively detrapped during the reset process, which is the reason for the endurance degradation. The formed Al2O3 at the interface of TE/TiOx plays a role of blocking electron injection during the set process, resulting in the decrease in the number of the trapped electron. In the case of Al/Al (1 nm)/TiOx (80 nm)/Al, the balance of electron trapping and detrapping could be further adjusted through decreasing the amount of injected electron. This again confirms that the IL AlOx at both interfaces can modify the potential barrier in the ATA structure and affect the performance of eBRS, such as endurance performance.
Al (BE), and which is more efficient in the thicker TiOx. An attempt was made to change the potential barrier of the interface of TE/TiOx, which was also done through inserting 1 nm_Al in 80 nm_ATA. Fig. 5 (a), (b) and (c) show the Ti 2p, Al 2p and O 1s core-level of XPS spectra for Al (1 nm)/TiOx/Al, respectively, where the upper is the surface of Al (1 nm)/TiOx/Al and the bottom is the inside position of TiOx after etching Al and a part of TiOx. The binding energy was calibrated using the surface carbon–carbon binding energy (284.8 eV). Ti4+ and Ti3+ were observed in Fig. 5(a), respectively, and there is no metallic Ti, wherein the Ti4+ is the mainly component of the TiOx layer. The area of Ti3+ of the inside is larger than that of surface which maybe caused by the high energy ion bombardment during the depth etching. In Fig. 5(a), the surface of Ti 2p showed a shift of the peak position to lower binding energy direction by 0.3 eV. This suggests the portion of TiOx contacted with Al layer is reduced. The O 1s spectra is shown in Fig. 5(c). The O 1s of the surface TiOx shows a broader position than that of inside, and the binding energies at 530.7 eV, 531.3 eV and 531.9 eV are coincided with the oxygen bound to the TiO2 (OTi), A2O3 lattice oxygen (OAl) and oxygen vacancies or defects (OV), respectively. There are only OTi (530.7 eV) and OV (531.7 eV) without OAl in the inside position. The O 1s of the inside TiOx can be separated into OTi (530.7 eV) and OV (531.7 eV), which shows an increasing of area ratio of OTi/OV from 0.56 to 1.47 in compared with that of surface TiOx. Therefore, the XPS results indicated that oxygen vacancies were induced at the Al (1 nm)/TiOx interface. It is closely to correlation with the oxidation of Al, and the Al-O bonding at 74.9 eV indicates the 1 nm Al was full oxidized to Al2O3, which was induced by extracting the oxygen from the surface TiOx [24]. Therefore, the increase of potential barrier height can be expected from the formed Al2O3 at TE/TiOx interface. Fig. 6(a) show I-V curve of the Al/Al (1 nm)/TiOx/Al, and which is
4. Conclusions In summary, this study examined the eBRS of ATA structure with different thicknesses of TiOx. The ATA showed a distinctive endurance performance according to the TiOx thickness, suggesting the potential barrier at TiOx/BE could be influenced due to the formation of interfacial layer AlOx. The thickness of interfacial layer AlOx increased with the deposition time of TiOx, and the frequency of electron trapping and detrapping was affected. Both 50 nm and 80 nm_ATA samples show a decreased RL and RH with the repeated RS cycle, but 110 nm_ATA sample has an increased trend of RL and RH. Whether electron trapping (50 nm_ATA) or electron detrapping (110 nm_ATA) is excessive-both samples show an endurance degradation. When a relative balance of electron trapping and detrapping can be maintained, the 80 nm_TiOx sample shows a longest endurance performance. In addition, the endurance performance of 80 nm_ATA was also improved through inserting 1 nm_Al into TE/TiOx interface because the balance of electron
Fig. 6. (a) Typical BRS I−V curve and (b) endurance results of Al/Al (1 nm)/TiOx (80 nm)/Al. 95
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trapping and detrapping was further adjusted. Although the actual situation is quite complicated, the fitting of I–V curves dependent I–V measurements showed the trap-mediated SCLC. The eBRS is strongly related to the electron trapping and detrapping rather than the oxygen motion, which was understand from the increased slope of the trapfilling region analysis according to the 110 nm_ATA. Although the eBRS performance in a sample ATA structure need to improve more, it is believed that this approach could provide valuable insights into electronic RS mechanism of other binary oxide.
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