Mechanism of power consumption inhibitive multi-layer Zn:SiO2/SiO2 structure
resistance random access memory
Rui Zhang, Tsung-Ming Tsai, Ting-Chang Chang, Kuan-Chang Chang, Kai-Huang Chen, Jen-Chung Lou, Tai-Fa Young, Jung-Hui Chen, Syuan-Yong Huang, Min-Chen Chen, Chih-Cheng Shih, Hsin-Lu Chen, Jhih-Hong Pan, Cheng-Wei Tung, Yong-En Syu, and Simon M. Sze
Citation: Journal of Applied Physics 114, 234501 (2013); doi: 10.1063/1.4843695 View online: http://dx.doi.org/10.1063/1.4843695
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/114/23?ver=pdfcov Published by the AIP Publishing
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Yong-En Syu, and Simon M. Sze
1
School of Software and Microelectronics, Peking University, Beijing 100871, People’s Republic of China 2
Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
3
Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan 4
Advanced Optoelectronics Technology Center, National Cheng Kung University, Tainan 700, Taiwan 5
Department of Electronics Engineering and Computer Science, Tung-Fang Design Institute, Kaohsiung, Taiwan
6
Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan
7
Department of Chemistry, National Kaohsiung Normal University, Kaohsiung, Taiwan 8
Department of Electronics Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
(Received 11 April 2013; accepted 25 November 2013; published online 16 December 2013) In this paper, multi-layer Zn:SiO2/SiO2 structure is introduced to reduce the operation power
consumption of resistive random access memory (RRAM) device by modifying the filament formation process. And the configuration of multi-layer Zn:SiO2/SiO2structure is confirmed and
demonstrated by auger electron spectrum. Material analysis together with conduction current fitting is applied to qualitatively evaluate the carrier conduction mechanism on both low resistance state and high resistance state. Finally, single layer and multilayer conduction models are proposed, respectively, to clarify the corresponding conduction characteristics of two types of RRAM devices.VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4843695]
I. INTRODUCTION
For next generation nonvolatile memory,1–7 the resist-ance random access memory (RRAM) was widely discussed and investigated recently due to its superior properties such as low cost, simple structure, fast operation speed, and non-destructive readout.8–13Among numerous resistive switching materials, the silicon-based oxide thin films were intensively investigated for the applications in RRAM for portable elec-trical devices owing to its compatibility in integrated circuit (IC) processes and relative stability compared with metal ox-ide materials.14–26
Filament formation and rupture is considered to be the reason of resistive switching in RRAM devices.27–29 Thus, the filament shape and formation process affects the magni-tude of working current, which in turn influences the power consumption of the device. Meanwhile, low power consump-tion has always been one target of the semiconductor indus-try not only for the commercial interests but also for the capability of high density integration. The formation process of filament has great impact on the RRAM switching and operating properties. And it can be modified by adjusting the thickness of switching layer, types of dopants, ways of elec-trical operation, etc.30–33 In this paper, multi-layer stacking technology is applied to modify the filament formation process.
Contemporary binary oxide materials have been reported having low-power RRAM switching properties by different types of structures and electrical operation.34–36 But world-wide researchers mainly focus metal oxide system like HfOx, 34 TiOx, 35 and TaOx. 36
By reducing the device dimen-sion and tuning the dopant concentration, we can achieve low operation voltage in metal oxide based RRAM devices. However, most power-saving operations are based on the strictly control of compliance current or the low operation voltage is the intrinsic material property, and if the compli-ance current is not restricted severely, the device will suffer unrecoverable hard breakdown. Here in this research, we focus zinc doped silicon oxide RRAM owing to the relative mature background of zinc, especially in the fabrication of thin film transistor display37–43and the wide applied low-cost silicon oxide. Multilayer structure RRAM device is fabricated to evaluate the merits of multi-layer stacking technology in the cycling process, from which obvious reduction of operat-ing current can be observed even the compliance current is 10 mA. Conduction current fitting combined with varied tem-perature measurement are used to investigate the resistance switching mechanism. Finally, single layer and multilayer conduction model are proposed to clarify the switching char-acteristics and the reason of working current reduction.
II. EXPERIMENTAL DETAILS
In this study, zinc doped SiO2 (Zn:SiO2) by magnetic
co-sputtering at room temperature is taken to form the multi
a)Authors to whom correspondence should be addressed. Electronic addresses:
resistance switching layer Zn:SiO2/SiO2structure. To make
comparison, single Zn:SiO2 layer device is also fabricated.
To the multi-layer Metal-insulator-metal (MIM) capacitor structure, each Zn:SiO2thin film layer is deposited by
sput-tering pure SiO2and Zn targets in argon ambient (4 mTorr)
on a patterned TiN/Ti/SiO2/Si substrate. To the patterned
substrate, The 200 nm TiN is deposited as bottom electrode and it is processed with standard lithography process to form T-shape bottom electrode. Then 300 nm low temperature sili-con oxide (LTO) is grown on the whole wafer and etched to expose TiN bottom electrode and form via, in which switch-ing layer is deposited. After the etchswitch-ing process, photoresist (PR) is spin-coated and etched to expose the via and the area where switching layer and top electrode will be deposited. The whole view and cross-sectional view of the device are shown in Figs.1(a)and1(b). The total film thickness of the active layer in the via is 30 nm and the corresponding thick-ness of each layer for multi-layer sample is marked out in Fig.1(c). The single layer device is fabricated with the same parameter as the Zn:SiO2layer in multilayer sample and the
thickness of switching layer is also 30 nm (Fig.1(d)). Then, the top electrode is made of Pt by sputtering with a thickness
of 200 nm on multi-layer sandwich structure. Finally, the electrical device cells were fabricated through lithography and lift-off techniques. The device area is 1 lm 1 lm and the whole electrical measurements on single layer Zn:SiO2
and multi-layer Zn:SiO2/SiO2 RRAM devices are done by
Agilent B1500 semiconductor parameter analyzer.
III. RESULTS AND DISCUSSION
To verify the structure configuration of multi-layer Zn:SiO2/SiO2 film, Auger Electron Spectroscopy (AES) is
applied to obtain the zinc AES spectra, which is shown in Fig.2(a). From the AES spectra, the 7-layer stacking struc-ture is corroborated by zinc peaks distribution.
After AES analyzing, Fourier transform infrared spec-troscopy (FTIR) is used to investigate the chemical bonding of the Zn:SiO2film. Fig. 3 shows that the Si-O-Zn stretch
bonding is found in the Zn:SiO2film at 1132 cm1. In
addi-tion, the anti-symmetric stretch mode and the symmetric FIG. 1. Device schematic structure of (a) whole view (b) cross sectional view. (c) and (d) are the corresponding switching layer structure for multi-layer and single-multi-layer RRAM. The switching layer is marked with purple in Figure1(b).
FIG. 2. Auger electron spectrum for multi-layer Zn:SiO2/SiO2structure.
FIG. 3. FTIR spectra of Zn:SiO2film measured in middle infrared region.
The XPS spectra of Zn 2P3/2, Si 2P3/2, O 1s core levels in Zn:SiO2film are
shown in the insets, respectively.
stretch mode of Si-O-Si bonds are discovered at 1042 cm1 and 796 cm1, respectively. To further analyze the chemical composition of Zn:SiO2film, X-ray photoelectron
spectros-copy (XPS) of Zn 2P3/2, Si 2P3/2, and O 1s peaks are
per-formed and shown as the insets in Fig.3. The mole fraction of Zn:Si:O in the Zn:SiO2 film was 4.9%:24.9%:70.2%.
Furthermore, compared with the areas of deconvolution peaks of Zn 2P3/2 core levels, we find the mole fraction of
ZnO:Zn was 45.1%:54.9% in Zn:SiO2film.
After material analysis confirmation, electrical measure-ments are performed to investigate the performance of both types of devices. Before current-voltage sweeping, all the devices are activated by electro-forming process with a com-pliance current of 10 mA, as shown in Figs.4(a) and 4(b). After that, DC sweeping is applied to analyze the standard RRAM resistive switching behavior. Fig.5shows the bipolar switching behavior of single Zn:SiO2layer and multi-layer
Zn:SiO2/SiO2structure RRAM devices by applying bias on
TiN bottom electrode (inset of Fig.5). Compared with single layer RRAM devices, it can be observed that multi-layer structure devices exhibit lower operation current on both low
resistance state (LRS) and high resistance state (HRS). If the reading voltage is set with 0.1 V, the corresponding current is 300 lA in LRS and 20 lA in HRS.
To better understand the properties of both types of devices, we apply conduction current fitting, as shown in Figs.6and7. Insets of Figs.6and7are their corresponding fitting curve. From Fig.6, we can find that the HRS of single layer devices exhibit Poole-Frenkel conduction mechanism owing to the existence of defects between ruptured filament and bottom electrode, which act as the conduction media of HRS leakage current. Nevertheless, the HRS of multi-layer devices also reveal Poole-Frenkel conduction mechanism, and this is ascribes to the carrier transfer through the hetero-junction defects produced between zinc extended-precipitate and silicon material in the inserted SiO2layers during
form-ing process. Accordform-ing to the Poole-Frenkel equation, which
is J/ Eiexp q UB ffiffiffiffiffiffiffiffiffiffiffiffiqV=pdei p kT , we obtain InðJ VÞ / In d qUB kT þ q ffiffiffiffiffiffiffiffiffiffiffiffi qV=pdei p kT . As In d qUB kT is constant, by drawing out the curve with axis of InðJ
VÞ and
ffiffiffiffi V p
we can get FIG. 4. (a) and (b) are the forming curve of multi-layer and single-layer devices.
FIG. 5. The bipolar behavior of the RRAM devices using the single Zn:SiO2
layer and multi-layer Zn:SiO2/SiO2structure. The schematic of RRAM
de-vice is shown in the right bottom inset.
FIG. 6. The HRS current fitting of single Zn:SiO2 layer and multi-layer
the fitting curve slope of two devices (insets of Fig.6). m1
and m2are the slope of single layer and multilayer device
fit-ting curve, respectively. As m1¼ 1.3, m2¼ 4, m /
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi const=dei
p
, and the permittivity ratio between SiO2 and
ZnO is eZnO:eSiO2¼ 11:3.9,44,45 we can estimate the ratio of
switching layers’ thickness between these two devices, which is d1:d2¼ 3:1.
While to the LRS of both devices, it is noted that multi-layer device owns hopping conduction, which is quite differ-ent from normal Ohmic conduction in single Zn:SiO2layer
device. The common Ohmic conduction ascribes to high concentration metal phase zinc precipitation in Zn:SiO2thin
film, leading to the formation of metal filaments. Together with the XPS analysis of Zn concentration, we think the fila-ment is made of high concentration metal. The carriers con-duct through relative complete metal filament, which is in turn registered as Ohmic conduction.29,38The Ohmic current conduction mechanism has also been investigated in our pre-vious study.46To the multilayer device, the hopping conduc-tion is due to electron carriers surpassing the energy barrier
height of each blocking SiO2layer. By surpassing the
spac-ing SiO2 layer, carriers conduct in a manner of hopping
conduction.
In order to further confirm the current conduction mech-anism of LRS for these two types of RRAM devices, we measure the I-V characteristics under vary-temperature envi-ronment. In Fig.8, the on state currents of LRS of both types of devices are measured at low temperature with a range from 100 K to 298 K. According to the experimental data, the current of single layer structure RRAM decreases with the rising of the temperature (Fig. 8(a)), which is consistent with Ohmic conduction mechanism property. On contrast, the relationship between conduction current and temperature of multi-layer structure RRAM devices on LRS exhibits the reverse trend (Fig.8(b)), which complies with hopping con-duction equation J¼ anvq expðU
kTþ qaE
kTÞ.
To clarify the conduction mechanism, single layer con-duction model and multilayer concon-duction model are pro-posed, respectively, as shown in Figs. 9 and 10. In single FIG. 7. The LRS current fitting of single Zn:SiO2 layer and multi-layer
Zn:SiO2/SiO2devices. The insets are the corresponding matching degree.
FIG. 8. I-V curves of LRS in single Zn:SiO2layer and multi-layer Zn:SiO2/SiO2devices under vary temperature measurement.
FIG. 9. The schematic diagram of redox reaction in single Zn:SiO2layer
RRAM device.
layer device, after electro-forming process, conduction fila-ment is formed. By reducing and oxidizing the tip of metal filament, LRS and HRS can be switched. That is also the rea-son why LRS exhibits Ohmic conduction mechanism while HRS changes into Poole-Frenkel conduction, as relative complete filament acts as the conduction media in LRS while defects facilitated carrier emission-capture dominates after the filament is oxidized. Compared with single layer RRAM, filament formation process in multilayer devices is restricted with the Zn:SiO2layers. Owing to the relative incomplete
fil-ament in each layer, it is less easily for carriers conduct through, which in turn contributes to the reduction of con-duction current density.37 But there is also an interesting phenomenon, which is the higher operation current for multi-layer RRAM below0.7 V. This is mainly due to the thinner switching layer, in which the alleviated oxidation effect in reset process happens. Comparatively, drastic oxi-dation combined with severe thermal effect in the single layer sample dominates, which results in intensely rupture of the filament and thus lower current. Furthermore, we also conducts retention experiments, and the resistance values of LRS and HRS of multilayer Zn:SiO2/SiO2 RRAM devices
can remain almost constant even after 104s retention per-formance test at 85C, which implies the multilayer device has stable non-volatile memory characteristics.
Normally, when the compliance current is high enough, for example 10 mA, it is not easy to control the over-formation of metal filaments as massive metal ions triggered by electrical and thermal energy migrate to form precipitates. Large quantities of precipitates will lead to the formation of metal filaments, which induce current over-shooting phenomenon in the sweeping process. And this phe-nomenon is even more familiar in metal oxide based RRAM devices due to the high concentration of metal ions.29,38,46 Thus, with the combination of silicon oxide base and insert-ing metal-free SiO2layers into the resistive switching layer,
filament formation process is effectively restrained, from which working current reduction and over-shooting elimina-tion can be achieved.
current and current over-shooting phenomenon. Conduction current fitting was performed to analyze the carrier conduc-tion property and vary temperature experiment was applied to confirm the conduction mechanism. Meanwhile, models for both types of devices were proposed to clarify the resis-tive switching characteristics. Multi-layer stacking silicon oxide based structure RRAM is a promising candidate for future low electrical power consumption non-volatile mem-ory devices.
ACKNOWLEDGMENTS
This work was performed at National Science Council Core Facilities Laboratory for Science and Nano-Technology in Kaohsiung-Pingtung area, NSYSU Center for Nano-science and Nano-technology and supported by the National Science Council of the Republic of China under Contract Nos. NSC-102-2120-M-110-001 and NSC 101-2221-E-110-044-MY3.
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