Effects of sol aging on resistive switching behaviors of HfOx resistive memories

Physica B 508 (2017) 98–103

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Effects of sol aging on resistive switching behaviors of HfOx resistive memories

MARK

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Chih-Chieh Hsua,b,c, , Jhen-Kai Sunc, Che-Chang Tsaoc, Yu-Ting Chenc a b c

Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, Douliu 64002, Taiwan, ROC Department of Electronic Engineering, National Yunlin University of Science and Technology, Douliu 64002, Taiwan, ROC Graduate School of Electronic Engineering, National Yunlin University of Science and Technology, Douliu 64002, Taiwan, ROC

A R T I C L E I N F O

A BS T RAC T

Keywords: Semiconductor devices Electrical characteristic Thin films Solution process Aging

This work investigates effects of long-term sol-aging time on sol-gel HfOx resistive random access memories (RRAMs). A nontoxic solvent of ethanol is used to replace toxic 2-methoxyethanol, which is usually used in solgel processes. The top electrodes are fabricated by pressing indium balls onto the HfOx surface rather than by using conventional sputtering or evaporation processes. The maximum process temperature is limited to be 100 ℃. Therefore, influences of plasma and high temperature on HfOx film can be avoided. Under this circumstance, effects of sol aging time on the HfOx films can be more clearly studied. The current conduction mechanisms in low and high electric regions of the HfOx RRAM are found to be dominated by Ohmic conduction and trap-filled space charge limited conduction (TF-SCLC), respectively. When the sol aging time increases, the resistive switching characteristic of the HfOx layer becomes unstable and the transition voltage from Ohmic conduction to TF-SCLC is also increased. This suggests that an exceedingly long aging time will give a HfOx film with more defect states. The XPS results are consistent with FTIR analysis and they can further explain the unstable HfOx resistive switching characteristic induced by sol aging.

1. Introduction Binary transition metal oxides such as HfO2, Ta2O5, ZrO2, and TiO2 are highly compatible with a complementary metal oxide semiconductor (CMOS) process [1–6]. Among these materials, HfO2 has been intensively studied for NAND flash memory cells [7], metal-oxide– nitride-oxide–silicon (MONOS)-type memory [8], and advanced device technology [9]. Meanwhile, it could possibly be a solution to implement other novel devices [10]. HfO2 has also found to be a potential candidate for resistive random access memory (RRAM) applications [11–13]. Sol-gel processes are known to have advantages of simple process, low temperature and low cost, and they have been widely investigated for applications to semiconductor devices such as dyesensitized solar cells [14–16], perovskite solar cells [17,18], gas sensors [19], pressure sensors [20], and thin film transistors [21,22]. Recently, sol-gel processes are also found to be promising for fabricating RRAMs. Ramadoss et al. fabricated an Ag/HfO2/ITO RRAM by using a sol-gel HfO2 film as the active layer [23]. The spin coated HfOx films have smooth morphologies and good uniformity. Jang et al. used a solution process to develop a HfAlOx RRAM [24]. Effects of mixing of AlOx and HfOx solutions were studied. Impacts of post annealing on

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TiN/Ti/HfOx/TiN RRAM was explored in [25]. The RRAM performance influenced by the thickness of the solution deposited HfOx film was examined in [26]. A toxic solvent of 2-methoxyethanol was used to prepare the HfOx sol-gel solution. Sol aging time has been known to be critical to characteristics of sol-gel derived films. Guo et al. investigated effects of sol aging on a TiO2 compact layer and performance of a solar cell [27]. Influences of sol aging time on vanadium pentoxide (V2O5) conductivity was studied in [28]. They found the conductivity of V2O5 film was unaffected when the sol aging time was 30 days. Santos et al. studied effects of sol aging time of 12 h-30 days on calcium phosphates (CaP) powder [29]. Impacts of sol aging on ZnO structural and optical properties, and TiO2 photo-electrochemical characteristics were investigated in [30– 32]. In the reported literatures, influences of sol aging on properties and resistive switching behaviors of HfOx layers have never been discussed. Toxic solvents are usually required for fabricating RRAMs that use sol-gel derived films as the resistive switching layers [33–38]. In this work, a HfOx film derived by a sol-gel process was used as the resistive switching layer. The toxic solvent of 2-methoxyethanol, which is usually required for preparing sol-gel solutions, was replaced by a nontoxic ethanol and influences of sol aging on HfOx RRAM

Corresponding author at: Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, Douliu 64002, Taiwan, ROC E-mail address: [email protected] (C.-C. Hsu).

http://dx.doi.org/10.1016/j.physb.2016.12.023 Received 29 November 2016; Received in revised form 17 December 2016; Accepted 18 December 2016 Available online 19 December 2016 0921-4526/ © 2016 Elsevier B.V. All rights reserved.

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3. Results and discussion

performance were investigated. To avoid influences of high temperature and plasma damage on the sol-gel HfOx film caused by evaporation or sputtering processes, the top electrode was fabricated by pressing indium balls onto the HfOx surface. Meanwhile, the maximum process temperature for fabricating the RRAM was controlled to be 100 ℃. In this situation, effects of sol aging on composition, chemical bonds, and resistive switching behavior of the HfOx RRAM can be more clearly explored. Current conduction mechanisms of the HfOx RRAMs with different sol aging times were also studied. The correlation between sol aging and RRAM characteristic was analyzed by X-ray diffraction (XRD), an X-ray photoelectron spectroscopy (XPS), and a Fourier transform infrared spectroscopy (FTIR).

3.1. HfOx RRAMs prepared with sol aging times of 1–14 days The as-prepared HfOx sol-gel solution shown in Fig. 1(a) was opaque. It seems that the HfCl4 powder was difficult to be dissolved in ethanol. After aging at RT for 14 days, the solution became semitransparent [Fig. 1(b)]. For a further aging time of 60 days, it turns to transparent, as revealed in Fig. 1(c). By using Mo as the bottom electrode, a HfOx film can be spin coated on the Mo surface because Mo has a higher surface energy than that of the Si surface [39,40]. However, the electrical characteristic of the In/HfOx/Mo RRAM fabricated by using the solution aged for 14 days (RRAM14) revealed a poor insulation property and a negligible resistive switching behavior, as shown in Fig. 1(d). This may due to incomplete reaction of the solgel solution, which can be confirmed by using FTIR analysis, as will be discussed later.

2. Experiments A heavily arsenic-doped Si (100) wafer with a resistivity of 0.002– 0.005 Ωcm was cleaned by diluted buffer oxide etch and deionized (DI) water. Moisture was removed by baking at 100 ℃ for 3 min in an oven. Molybdenum deposited on the substrate by using a sputter was used as the bottom electrode. For preparation of the HfOx sol-gel solution, hafnium tetrachloride (HfCl4, 98%) powder was dissolved in 99.9% ethanol followed by addition of DI water and nitric acid (HNO3) with a mole ratio of HfCl4: C2H5OH: HNO3: DI water=1: 410: 5: 5. Then, this mixed solution was divided into four portions and they were stirred at room temperature (RT) for 14, 60, 90, and 150 days, respectively, for exploring effects of sol aging on RRAM performance. Three HfOx layers were stacked on the Mo surface as the resistive switching layer. The first HfOx film was spin coated on the Mo/N+ Si substrate with a first spin speed of 1000 rpm for 10 s and second spin speed of 3000 rpm for 30 s. Then, the sample was baked on a hotplate at 100 ℃ for 1 min. Then, second HfOx film was deposited on the first layer by using the same procedure. The third HfOx film was spin coated on the sample by using the same spin speed followed by a baking process on a hotplate at 100 ℃ for 10 min. To avoid the influences of plasma damage and high temperature on the sol-gel HfOx film caused by sputtering and evaporation processes, the top electrode was fabricated by pressing indium balls onto the HfOx film to obtain an In/HfOx/Mo RRAM device. RRAMs fabricated by using HfOx solutions aged at 14, 60, 90, and 150 days are named as RRAM14, RRAM60, RRAM90, and RRAM150, respectively. The crystallinity of the HfOx film was examined by XRD. The chemical bonds and composition of the HfOx film were examined by FTIR and XPS. The resistive switching behaviors of the RRAMs were measured by using Keysight B1500A.

3.2. HfOx RRAM prepared with a sol aging time of 60 days The measured I-V curves of the RRAM prepared by using the solution aged for 60 days (RRAM60) are displayed in Fig. 2(a). Compared with RRAM14 [Fig. 1(d)], RRAM60 exhibits a significant bipolar resistive switching behavior. The resistance switches from high resistance state (HRS) to low resistance state (LRS) at a set voltage of −1 V and switches back at a reset voltage of 1 V. The endurance plotted in Fig. 2(d) reveals a significant and stable resistance window of 2.7 for over 500 switching cycles. This resistance window and the number of switching cycles are higher than or comparable to 1.3 for one switching cycle of a solution processed HfO2 RRAM [23] and 3–5 for 120 switching cycles of the HfO2 RRAM that is prepared by reactive molecular beam epitaxy [41]. Ohmic conduction and trap-filled space charge limited conduction (TF-SCLC) are frequently used to explain carrier transport in RRAMs and semiconductor devices [42–44]. The conduction current density dominated by Ohmic conduction can be expressed as [42]

⎡ −(EC − EF ) ⎤ J = σE = qμENC exp ⎢ ⎥ ⎣ ⎦ kT

(1)

where J is the current density, E is the electric field, σ is the electrical conductivity, q is the electron charge, μ is the carrier mobility, NC is the effective density of states of the conduction band, EC is the conduction band edge, EF is the Fermi energy level, k is Boltzmann constant, and T is the Kelvin temperature. The equation of

J=

9 V2 εi μθ 3 8 d

(2)

is used to describe the TF-SCLC behavior [45–47], where εi is the

Fig. 1. Pictures of (a) as-prepared HfOx sol-gel solution, (b) HfOx solution aged for 14 days, and (c) HfOx solution aged for 60 days. (d) I-V curves of the In/HfOx/Mo RRAM prepared with the HfOx solution aged for 14 days (RRAM14).

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Fig. 2. (a)–(c) I-V curves, (d)–(f) endurances, and (g)-(i) ln I vs. ln V plots of RRAM60, RRAM90, and RRAM150, respectively.

changes to TF-SCLC [42,51–54], which is described in Eq. (2). The Vtr can be theoretically described as

permittivity of the material, V is the applied voltage, and d is thickness of the material. θ is the ratio of free carrier density to total carrier (free and trapped) density and can be expressed as

n θ= n + nt

Vtr = (3)

8 qn 0 d 2 9 εi θ

(4)

where n0 is thermally generated free carrier density [51]. In Fig. 2(g), a clearly Vtr of 0.75 V is observed. When the voltage increases higher than this Vtr, the slope of the I-V curve changes from 1 to 2, which corresponds to the transition from Ohmic conduction to TF-SCLC [Eq. (2)]. When the RRAM switches to LRS, formation of conductive filaments cause that the conduction mechanism is fully dominated by Ohmic conduction [23,55].

where n is the free carrier density and nt is the trapped carrier density. Tunneling mechanism is also frequently used for explaining conduction currents of devices [48,49]. However, because the HfOx thickness in this work is higher than 80 nm, the tunneling process is less likely to occur [49,50]. Fig. 2(g) plots ln I vs. ln V diagram of RRAM60. In HRS, the slope of the linear fitting line in low electric field region is 1.1, which is in accordance with Eq. (1). This means that the injected carrier density is lower than thermally generated carrier density and the transit time of injected excess carriers is longer than the dielectric relaxation time [8,39]. The injected carriers will redistribute themselves to reestablish the quasi-neutrality in the HfOx layer and will have no chance to travel across the HfOx thin film [42,51,52]. The thermally generated carriers are predominant over the injected carriers and the conduction current can be described by Ohmic conduction, as depicted in Eq. (1). When the voltage increases to a transition voltage (Vtr), the injected carrier density starts to exceed the thermally generated carrier density and the transit time of the injected carriers becomes equal to or shorter than the dielectric relaxation time [42,51–54]. The injected carriers can fill the defect states and the conduction mechanism

3.3. HfOx RRAM prepared with a sol aging time of 90 days I-V curves of the RRAM using the sol-gel solution aged at RT for 90 days (RRAM90) are shown in Fig. 2(b). A clear bipolar switching characteristic can also be obtained. However, endurance plotted in Fig. 2(e) shows that the resistances of the HRS and LRS become unstable. The average resistances for HRS and LRS are 10.6 kΩ and 3.2 kΩ, respectively. They are found to be higher than those of RRAM60. It implies that the resistance of the HfOx film increases with increasing aging time. Notably, the Vtr of RRAM90 is increased to 1.44 V, which is significantly higher than 0.75 V of RRAM60. From Eqs. (3) and (4), a higher Vtr indicates a lower θ corresponding to a higher 100

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trapped carrier density. This means that more defect states exist in RRAM90 and a higher voltage is required for injecting more carriers to fill the traps. Thus, onset of the transition from Ohmic conduction to TF-SCLC occurs at a higher Vtr. Comparison of defect densities in the RRAMs is studied by XPS and FTIR analyses, as will be discussed later. In LRS, the conduction mechanism is also dominated by Ohmic conduction due to formation of conductive filaments, which is the same as RRAM60. 3.4. HfOx RRAM prepared with a sol aging time of 150 days We further investigated the resistive switching characteristic of the HfOx RRAM that was prepared by using the sol-gel solution aged at RT for 150 days. Although a bipolar resistive switching characteristic still can be found, the I-V curves exhibit a high instability, as observed in Fig. 2(c). The resistances of HRS and LRS of RRAM150 are further increased to 15.4 kΩ and 11.3 kΩ, respectively, and the resistance window is decreased to 1.6 [Fig. 2(f)]. Moreover, unstable resistances in HRS and LRS are observed. Different from RRAM60 and RRAM90, the conduction mechanism in HRS of RRAM150 becomes purely dominated by Ohmic conduction. The transition from Ohmic conduction to TF-SCLC is absent. This implies that RRAM150 has more traps and the injected carriers are difficult to fill them completely.

Fig. 4. FTIR spectra of the sol-gel HfOx films by using the sol-gel solutions aged for 14, 60, and 150 days.

characteristic, as observed in Fig. 2(c). 3.6. FTIR analysis The FTIR analysis is utilized for further exploring effects of aging time on HfOx films. Fig. 4 reveals the FTIR spectra of the HfOx films. Wavenumber of 650–770 cm−1, 970–1250 cm−1, 1220–1550 cm−1, and 1628 cm−1 relate to O-H bending (out of plane), C-O stretching, C-O-H bending vibrations, and C-C stretching, respectively [61–65]. For RRAM14, observable chemical bonds are not found in the HfOx film. It is difficult to obtain a sol-gel film without any O-H or carbon related bonds by using a solvent of ethanol [66,67]. The FTIR spectrum of RRAM14 may suggest an incomplete sol-gel reaction. This explains the poor resistive switching characteristic of RRAM14, as revealed in Fig. 1(d). For RRAM60, O-H and carbon related bonds, which are usually obtained by using a sol-gel process [66–69], are found. When the aging time increases to 150 days, extremely higher absorbance caused by these bonds are observed (RRAM150). The extraordinary increases in these bonds are believed to be caused by an exceedingly long aging time and environmental contaminations [67]. These bonds can act like defect states in the forbidden bandgap of the HfOx film [70–72]. When more defect states exist in the HfOx film, a higher voltage is required to inject enough carriers to fill the defect states [73]. This explains that RRAM90 needs a higher voltage of 1.44 V than 0.75 V of RRAM60 for the transition from Ohmic conduction to TFSCLC, as revealed in Fig. 2(h). For RRAM150, these considerable defect

3.5. XRD and XPS analyses From the XRD analysis (not shown), diffraction peaks at 2θ=28° and 31.3° corresponding to HfO2(111) and (111) [23] are not found. This indicates an amorphous structure of the HfOx film. Fig. 3(a) and (b) reveal Hf 4f and O 1s XPS spectra of RRAM60 and RRAM150. Fig. 3(a) shows that the Hf 4f spectra of RRAM150 clearly shift to a lower binding energy. Charge transfer is known to be a dominant mechanism for the binding energy shift. Removing an electron from the valence orbital leads to a higher potential of core electrons and subsequently cause a positive shift of binding energy. Thus, the negative shift revealed in Fig. 3(a) infers that more un-oxidized Hf atoms exist in the HfOx film of RRAM150 [56,57]. This suggests that an exceedingly long aging time will give a HfOx film with more un-oxidized Hf atoms. Typically, the O 1s spectra are deconvoluted into three peaks. Two peaks near 531.2 eV and 532.3 eV are attributed to Hf-O bond and non-lattice oxygen (α-O) [56,58], respectively. A peak at 533.2 eV is caused by O-C and O-H bonds [58–60]. Significant shifts of these peaks to lower binding energies are also found, which is consistent with the observation in Hf 4f spectra and further confirms that the more unoxidized Hf atoms in RRAM150 [56]. This explains that a long aging time gives a deteriorated HfOx layer and an unstable resistive switching

Fig. 3. (a) Hf 4f and (b) O 1s XPS spectra of RRAM60 and RRAM150.

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states, as analyzed in Fig. 4, are difficult to be completely filled. Thus, the transition from Ohmic conduction to TF-SCLC cannot be observed, as shown in Fig. 2(i). These defect states also explain that an exceedingly long aging time will result in a HfOx RRAM with an unstable resistive switching characteristic and a smaller resistance window.

[16]

[17]

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4. Conclusion

[19]

Effects of long-term sol aging on the sol-gel HfOx RRAMs that were prepared by using a nontoxic solvent of ethanol were investigated in this work. The solution aged for 14 days gives the HfOx film with a poor insulation property. For the solution aged for 60 days, the HfOx RRAM with a stable resistive switching behavior can be obtained. In the HRS, conduction mechanisms in low and high electric field regions are dominated by Ohmic conduction and TF-SCLC, respectively. The transition voltage is 0.75 V. When the aging time increases to 90 days, the transition voltage significantly increases to 1.44 V. For the further aging time of 150 days, the transition from Ohmic conduction to TFSCLC is not observed. The XPS and FTIR analyses indicate more unoxidized Hf atoms, C-O, C-OH and C-C bonds in the HfOx film with an exceedingly long sol aging time. This explains the observation that long aging times of 90 and 150 days exhibit higher transition voltages and unstable resistive switching behaviors.

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Acknowledgement [27]

This research was supported by the Ministry of Science and Technology of Taiwan, ROC, under the contract No. MOST 105– 2221-E-224-027.

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