Dehydroxyl effect of Sn-doped silicon oxide resistance random access memory with
supercritical CO2 fluid treatment
Tsung-Ming Tsai, Kuan-Chang Chang, Ting-Chang Chang, Yong-En Syu, Kuo-Hsiao Liao, Bae-Heng Tseng, and Simon M. Sze
Citation: Applied Physics Letters 101, 112906 (2012); doi: 10.1063/1.4750235 View online: http://dx.doi.org/10.1063/1.4750235
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/101/11?ver=pdfcov Published by the AIP Publishing
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Dehydroxyl effect of Sn-doped silicon oxide resistance random access
memory with supercritical CO
2fluid treatment
Tsung-Ming Tsai,1,a)Kuan-Chang Chang,1Ting-Chang Chang,2,3,a)Yong-En Syu,3 Kuo-Hsiao Liao,1Bae-Heng Tseng,1and Simon M. Sze2,4
1
Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, 70 Lien-hai Road, Kaohsiung 804, Taiwan
2
Department of Physics, National Sun Yat-Sen University, 70 Lien-hai Road, Kaohsiung 804, Taiwan
3
Advanced Optoelectronic Technology Center, National Cheng Kung University, Taiwan
4
Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan
(Received 29 March 2012; accepted 21 August 2012; published online 14 September 2012) The tin-doped can supply conduction path to induce resistance switching behavior. However, the defect of tin-doped silicon oxide (Sn:SiOx) increased the extra leakage path lead to power
consumption and joule heating degradation. In the study, supercritical CO2fluids treatment was used
to improve resistive switching property. The current conduction of high resistant state in post-treated Sn:SiOx film was transferred to Schottky emission from Frenkel-Poole due to the passivation
effect. The molecular reaction model is proposed that the defect was passivated through dehydroxyl effect of supercritical fluid technology, verified by material analyses of x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.VC 2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4750235]
To overcome the technical and physical limitation issues of conventional charge storage-based memories,1–4the resist-ance random access memory (RRAM) composed of an insu-lating layer sandwiched by two electrodes is a great potential candidate for next-generation nonvolatile memory due to their superior properties such as low cost, simple structure, fast operation speed, and nondestructive readout.5,6
In our previous research, supercritical CO2 (SCCO2)
fluid technology was used to improve the dielectric proper-ties and performance of various thin film transistors (TFTs), such as hydrogenated amorphous-silicon TFTs and ZnO TFTs.7–13 The liquid-like and gas-like double properties of SCCO2fluids can be used to dissolve and transport H2O
molecules into the thin film and assist in oxidizing thin film at a low temperature. In addition, CO2 is a nontoxic,
non-flammable, and chemical-stable material. Although most RRAM devices have many superior properties of nonvolatile memory, the high operation current of RRAM during steady state is a major issue to nonvolatile memory for the applica-tion of portable electronic products. Therefore, the supercrit-ical CO2 is worthy to develop for improving the electrical
properties of RRAM switching layer.
The tin-doped can supply conduction path to induce resis-tance switching behavior.14However, the defect of tin-doped sili-con oxide (Sn:SiOx) would increase the extra leakage path lead
to power consumption and joule heating degradation. In this work, the resistive switching layer of Sn-doped silicon oxide (Sn:SiOx) was treated by SCCO2fluids to enhance its electrical
properties. The Pt/Sn:SiOx/TiN sandwiched devices were
fabri-cated to investigate resistive switching properties of Sn:SiOxafter
SCCO2treatment. In addition, the influence of SCCO2treatment
on resistive switching behaviors of Sn:SiOx was evaluated by
material and conduction mechanism analyses. Because the super-critical fluid has gaslike and high pressure properties to effi-ciently diffuse into nanoscale without damage,15the current of post-treated Sn:SiOxwas reduced obviously due to the trap
passi-vated by H2O molecule of the SCCO2fluids.
The experimental samples were prepared as follows: the Sn:SiOxthin film (about 30 nm) was deposited on the TiN/Ti/
SiO2/Si substrate by co-sputtering with the pure SiO2and Sn
targets. The sputtering power was fixed at RF power 200 W and 3 W for SiO2 and Sn targets, respectively. The
co-sputtering was carried out in argon ambient (Ar¼ 30 sccm) with a working pressure of 6 mTorr at room temperature. In contrast, the Sn:SiOxfilms were put into the reactive chamber
of supercritical fluid system and then the SCCO2fluid mixed
with 0.5 ml water were syringed into the reactive chamber to treat the sample. During the treatment, the water-mixed super-critical CO2 fluids were heated and pressured to 120C and
3000 psi in the stainless steel chamber of supercritical fluid system for 1 h. Finally, the Pt top electrode of 200 nm thick-ness was deposited on Sn:SiO2film to form electrical devices
with Pt/Sn:SiOx/TiN sandwich structures by DC magnetron
sputtering. The entire electrical measurements of devices with the Pt electrode of 250 lm diameter were performed using Agilent B1500 semiconductor parameter analyzer. In addition, the Fourier transform infrared spectroscopy (FTIR) measured by Bruker VERTEX 70v spectrometer in far infrared region and x-ray photoelectron spectroscopy (XPS) were used to ana-lyze the chemical composition and bonding of these insulator materials, respectively.
The “forming process” is required to activate all of the Sn:SiOx RRAM devices, using dc voltage sweeping with a compliance current of 2 mA. The leakage current of the Sn:SiOx RRAM devices after SCCO2 treatment was lower
than that of pre-treatment devices (Figure1(a)). This phenom-enon is attributed to the improvement on dielectric properties
a)Authors to whom correspondence should be addressed. Electronic addresses:
through SCCO2 treatment, which has been reported by our
previous study.8 After the forming process, the electrical current-voltage properties of the Sn:SiOxdevices were
com-pared before and after SCCO2 treatment (Figure 1(b)). The
current of Sn:SiOxdevices is reduced at 0.1 V reading voltage
after SCCO2 treatment. Figure 1(b) shows the electrical
current-voltage (I-V) properties of the Sn:SiOxRRAM
devi-ces before and after SCCO2 treatment. We can find that the
current of Sn:SiOx in high resistive state (HRS) is reduced
from 9 lA to 0.6 lA at 0.1 V reading voltage after SCCO2
treatment. The interesting phenomenon indicates that the in-crement of operation resistance for readout is about 15 times after SCCO2 treatment. To investigate the current reduction
mechanism, we analyzed the current conduction mechanism in HRS of Sn:SiOx with and without SCCO2 treatment as
shown in Figure2. The relationship in the curve of ln(I/V) versus the square root of the applied voltage (V1/2) is linear. According to the relationship of Frenkel Poole conduction, I¼qNcl d Vexp½ q kTð2 ffiffiffiffiffiffiffiffi qV 4peid q /BtÞ, where d, Nc, l, ei, and /Bt
are the insulator thickness, density of ionized traps, carrier mobility, dielectric permittivity, and trap barrier height, respectively. The Frenkel Poole conduction is due to emission of trapped electrons into conduction band. The supply of elec-trons from the traps is through thermal excitation. The barrier reduction is larger than in the case of Schottky emission by a
factor of 2, which can be obtained as compared with the slope of the plot of ln(I) versus (V1/2) based on the formula of Schottky emission, I¼ AAT2exp½kTqðqffiffiffiffiffiffiffiffi4peqVid /BÞ, where
/B, d, A, and A* are the Schottky barrier height, film thickness,
electrode area, and Richardson’s constant for thermionic emission, respectively.16The results revealed that the carrier transport of Sn:SiOxfilm was dominated by Frenkel Poole
conduction due to the trap in the film. After SCCO2
treat-ment, the current conduction mechanism will transfer to Schottky emission because of the improvement of dielectric properties. Therefore, we utilized the material spectra analy-ses to find out the reason of electrical transfer mechanism from Frenkel Poole conduction to Schottky emission. Com-pared the FTIR spectra of Sn:SiOx film with and without
SCCO2treatment (Figure 3), we found that the absorption
peak of Sn-O bond at 586 cm1was increased after SCCO2
treatment. The result implies that the density of Sn-O bond was increased in the Sn:SiOxfilm after SCCO2treatment. In
addition, the absorption of Si-O-Si stretch bond at 450 cm1 was also increased after SCCO2 treatment, illustrating the FIG. 1. (a) The forming current curves of the Sn:SiOxRRAM devices before
and after SCCO2treatment. (b) The black and red curves are the resistive
switching characteristics of Sn:SiOxfilm before and after SCCO2treatment,
respectively. The current in high resistance state of post-treated Sn:SiOxfilm
is reduced about 15 times from 9 lA to 0.6 lA.
FIG. 2. The current conduction curves in the Sn:SiOxfilm before and after
SCCO2treatment.
FIG. 3. The comparison of FTIR spectra of Sn:SiOxfilm before and after
SCCO2treatment. Both intensity of Sn-O and Si-O-Si bonds are increased in
Sn:SiOxfilm after SCCO2treatment.
content of silicon oxide bonding in the film also increased.17 According to XPS spectra analyses for Sn 3d5/2 core level
(Figure 4), the mole fraction of Sn-O bond was obviously risen but that of Sn element was decreased in Sn:SiOxfilm
after SCCO2 treatment. Besides, the mole fraction of Si-O
bond was substantially increased in contrast with that of Si-OH bond after SCCO2treatment in terms of the XPS spectra
analyses of Si 2p core level as shown in Figure4. Therefore, we infer that the level of oxidation would increase and accompany with dehydration in the post SCCO2-treated film.
These results were consistent with the above-mentioned FTIR analyses.
Based on the material analyses results, we proposed a reaction model to explain the current reduction mechanism of
Sn:SiOxfilm with SCCO2treatment as shown in Figure5. As
the sample was put into the water-mixed SCCO2 fluid
envi-ronment, the H2O molecule was carried into the dangling
bonds of amorphous Sn:SiOxfilm by SCCO2fluid, which is
attributed to the high penetration ability of SCCO2fluid. The
H2O molecule was approached to dangling bonds leading to
the hydration reaction in the Sn:SiOxfilm. Then,
monomolec-ular CO2 in supercritical fluids induces the dehydration of
neighbor hydroxyl groups so as to form Si-O-Si and Sn-O-Si cross-linking bonding in the film. Hence, the trap of Sn:SiOx
film can be passivated by SCCO2treatment, which can cause
the electrical current conduction in HRS of Sn:SiOxfilm
trans-ferred from Frenkel Poole conduction to Schottky emission. The phenomena will cause the improvement of dielectric
FIG. 4. XPS spectra of Sn 3d5/2and Si 2p
core levels in Sn:SiOxfilm before and after
SCCO2treatment. The mole fraction of
me-tallic tin and Si-OH bonds in Sn:SiOxfilm
is reduced obviously but that of tin oxide and silicon oxide bonds is increased after SCCO2treatment.
FIG. 5. The schematic diagram of passivation mechanism of SCCO2 treatment on Sn:SiOx
film. The schematic structure for each step repre-sents the situation of chemical bonding in the amorphous Sn:SiOxfilm before and after SCCO2
properties of thin film, which is demonstrated by our previous study.8
In summary, the operation current of Sn-doped silicon oxide RRAM device was decreased by supercritical fluid treatment in this study. The water molecular can be brought into the film by supercritical CO2fluid, which induce
dehy-droxyl effect to passivate the dangling bond in the amor-phous resistive switching layer. The operation resistance of RRAM can be increased due to the decrease of defect in the layer, which results in low power consumption. Therefore, supercritical fluid treatment can improve the properties of resistive switching layer of RRAM device.
This work was performed at National Science Council Core Facilities Laboratory for Nano-Science and Nano-Technology in Kaohsiung-Pingtung area and supported by the National Science Council of the Republic of China under Contract Nos. NSC 100-2120 -M-110-003 and NSC 100-2221-E-110-060.
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