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Effect of aluminum interfacial layer in a niobium oxide based resistive RAM
Solid State Electronics Letters 1 (2019) 52–57
Contents lists available at ScienceDirect
Solid State Electronics Letters journal homepage: http://www.keaipublishing.com/en/journals/solid-state-electronics-letters/
Effect of aluminum interfacial layer in a niobium oxide based resistive RAM Vishal Jain Manjunath∗, Andrew Rush, Abhijeet Barua, Rashmi Jha Department of Electrical Engineering and Computer Science, University of Cincinnati, Cincinnati, OH 45221, USA
a r t i c l e
i n f o
Article history: Received 22 January 2019 Revised 16 August 2019 Accepted 20 September 2019
Keywords: Resistive random access memory (ReRAM) Niobium oxide Resistive switching Aluminum Gibbs free energy
a b s t r a c t Resistive RAM (Random Access Memory) has good scalability with high switching speed and low operating voltage making it one of the promising emerging nonvolatile memory technologies. Interfacial layer between the electrode and metal-oxide interface in a Resistive RAM (ReRAM) could either enhance or deteriorate the switching performance of the device. In this study, we investigate the role of aluminum (Al) as an interfacial layer under the top electrode (TE) layer in a niobium oxide (Nb2 O5 ) based ReRAM. We compare the Current-Voltage (I-V), Capacitance-Voltage (C-V) characteristics and endurance of the Nb2 O5 based ReRAM with an Al interfacial layer below the tungsten (W) TE and a control sample without the Al interfacial layer to contrast the performance of each type. Additionally, we connect the tested device behavior with the enthalpy, entropy, and Gibb’s free energy to illustrate that aluminum is an inefficient interfacial layer in the niobium oxide ReRAM. © 2019 KeAi Communications Co., Ltd. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/)
1. Introduction Emerging nonvolatile memories (NVMs) namely magneto resistive RAM (MRAM) and Phase change RAM (PRAM) have a very promising future [1–3], but still have a long way in replacing flash memory as they have major issues such as large current, high cost in MRAM, energy loss during phase change and quenching, and large size in PRAM [4,5]. The desired characteristics of Emerging NVM’s are high endurance and retention compared to Flash memory [6] and scalability beyond 10 nm [7]. An exciting new circuit element, memristor was proposed in early 1970 s by Leon Chua [8]. Research on ReRAM gained significant traction following the physical realization of ReRAM by research group at Hewlett-Packard labs in early 20 0 0 s [9]. ReRAM is a two terminal device in which a switching layer (usually a transition metal oxide layer) is sandwiched between top and bottom electrodes. A filament is formed upon biasing the electrodes which is responsible for conduction leading to a low resistance state (LRS). It can be ruptured by changing applied voltage leading to low conductance or high resistance state (HRS). Apart from filamentary switching, there are other switching methods namely homogenous switching, threshold switching and redox based switching that have been reported in prior research [10,11]. Extensive research has been carried out in determining an ideal switching layer for ReRAM. Nb2 O5 has desirable features such as switching behavior with thickness less than 50 nm [12]. Nb2 O5 film has distinct binding energy spectra due to strong refractory properties compared to other metals when subjected to metrology which makes identifying the composition of Nb2 O5 material post switching an interesting phenomenon [13]. This makes it relatively easier to study the crystal structure of Nb2 O5 and identify crystal defects, surface topology and interface effects using metrology [14]. 2. Experiment Initially, silicon dioxide (SiO2 ) was grown thermally with a thickness of 100 nm on a boron doped silicon wafer (B: Si). An interfacial layer of 3 nm of titanium (Ti) was deposited to promote adhesion. Ruthenium (Ru) of 40 nm thickness was deposited as the bottom ∗
Corresponding author. E-mail address: [email protected] (V.J. Manjunath).
https://doi.org/10.1016/j.ssel.2019.09.001 2589-2088/© 2019 KeAi Communications Co., Ltd. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/)
V.J. Manjunath, A. Rush and A. Barua et al. / Solid State Electronics Letters 1 (2019) 52–57
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Fig. 1. a) Stack of the device with W TE and b) stack of Al/W TE device (Al interfacial layer capped with W TE). Both the devices have 15 nm Nb2 O5 as the switching layer and Ru BE layer which makes it easier to isolate the effect of Al layer on the performance of the Nb2 O5 ReRAM.
electrode (BE) on the Ti/SiO2/Si substrate by DC sputtering at room temperature. Thereafter, Silicon Nitride (Si3 N4 ) layer with thickness of 50 nm was deposited via reactive RF sputtering of a Si target in a N2 environment, as an insulator for patterning the device area. This Si3 N4 layer was then patterned and etched to expose squares of 100 × 100 μm2 of Ru. Then 15 nm Nb2 O5 was sputtered by RF Magnetron Sputtering under a 1:6 ratio O2 /Ar gas mixture. During the deposition of the Nb2 O5 thin films, the RF power and substrate temperature were maintained at 100 W and 475 °C respectively. Finally, for the first sample, tungsten (W) is DC sputtered as the TE with thickness of about 70 nm. For the second sample, 20 nm Al is deposited as interfacial layer and capped with 50 nm W. Tungsten and Ruthenium are chosen as electrodes since they are inert and exhibit lower rate of oxidation. The solid state characterization of the devices was performed with Keithley 4200 Semiconductor Characterization System. The focus of this study is to explore the performance of Nb2 O5 based ReRAM in depth under the influence of two top electrode configurations. One stack utilizes Tungsten layer as TE and the second configuration is modified with an additional Aluminum interfacial layer under the Tungsten TE. We study the effect of Al interfacial layer on the resistive switching properties of Nb2 O5 layer. We compare Virgin CurrentVoltage (I-V) and Capacitance-Voltage (C-V) characteristics, Capacitance Density of devices, Endurance and Gibbs Free Energy to determine role of an Al interfacial layer. This report sheds some interesting light on the physics behind such interfacial-dependent switching (Fig. 1). 3. Results and discussion 3.1. Virgin state I-V and C-V characteristics The Virgin I-V characteristics of the two types of devices were extracted by applying a positive bias to the TE with the BE grounded. All samples were measured with a cell size of 100 μm × 100 μm. W TE sample shows low leakage current in the order of 16–55 nA (Fig. 2a) whereas it can be clearly seen that the Al/W TE samples are leaky under a Read Voltage of 0.5 V, conducting current in the order of 150–300 μA (Fig. 2b). By observing the C-V of both the samples, it is observed that W TE sample shows almost uniform capacitance density across devices with a variation between 5.9 to 6.2 fF/μm2 (Fig. 3a) whereas Al/W TE sample show a non-uniform variation between 45 and 88 fF/μm2 (Fig. 3b). In RF Sputtered samples, linear increase of capacitance with the applied bias could imply the possible occurrence of a spacecharge accumulation; wherein a continuum of charges is distributed along the W/ Nb2 O5 interface rather than being distinct point charges [15]. However, the capacitance in Al/W TE sample decreases with applied voltage, which indicates a Schottky contact between Al and Nb2 O5 [16]. By comparing the capacitance densities of both the samples in (Fig. 4), we can deduce the capacitance of sample with Al TE in HRS state is lower than its virgin state value. This indicates that there is a possibility of formation of a thin oxide layer along the interface between aluminum and niobium oxide after switching. We can find the value of the capacitance of the layer formed at interface of Al/Nb2 O5 by difference in the values of capacitance of Al/W sample prior and post switching.
1 1 1 = + CS Cvirgin CAl2 O3
(1)
where Cs is the Capacitance of the device after switching, Cvirgin is the capacitance during its virgin state, CAl2 O3 is capacitance of Al2 O3 layer.
CAl2 O3 =
εo εr A t
(2)
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Fig. 2. a) Virgin I-V curves of W TE sample depicting a low initial conductance b) Virgin I-V curves of Al/W TE sample depicting a high initial conductance indicative of a leaky behavior. In both cases, the dimensions of the devices 1 through 5 are 100 μm × 100 μm, which represents the area of the TE.
where ε 0 is absolute permittivity given by 8.85 × 10−12 F/m. ε r is the relative permittivity of Al2 O3 which is ∼10, A is area of the Al/W TE sample given by 100 × 100 μm2 and t is the thickness of the Al2 O3 layer formed [17]. After calculating the capacitance of the oxide layer formed at the Al/ Nb2 O5 interface, we find the thickness of the aluminum oxide layer formed which is found to be 3.72 nm by extracting values from Fig. 4) and substituting in ((1) and (2). This indicates the oxidation of Al to form a thin Al2 O3 layer at Al/Nb2 O5 interface. 3.2. Switching and endurance characteristics of the samples I-V curves of W TE sample has relatively low VSET , VRESET and is consistent (Fig. 5a). The Al/W TE sample had relatively high values of VSET and VRESET of 1.4 V and −2 V respectively with a high reset compliance current of 50 mA shown (Fig. 5b). A thin Aluminum oxide (Al2 O3 ) layer of 3.72 nm which is formed in Al/Nb2 O5 interface as discussed earlier could have a permanent breakdown due to the applied voltage, which is observed in the Al/W TE sample. This requires a high current to reset the device (Fig. 5b). Endurance depicts the efficiency of the device over the long run, which is performed by applying repeated positive and negative pulses to the TE over a long period. Due to the Al2 O3 layer formed at the Al/Nb2 O5 interface, we anticipated the performance of this sample to deteriorate with time which is proven by the endurance of the Al/W TE sample (Fig. 6b) which fails after 32,0 0 0 cycles when compared to W TE sample which lasts up to 106 cycles with an on/off ratio of ∼100 (Fig. 6a). 3.3. Gibbs free energy of formation To fully understand the mechanisms behind the set/reset behavior of the two samples, Gibbs Free Energy analysis is conducted. Nb2 O5 has a work function of 5.2 while W and Al have a work function of 4.5 and 4.3 respectively [18]. While the work function has a role in determining the barrier height and the nature of contact between the metal/Nb2 O5 interface, it fails to explain if the metal layer of the TE is oxidized to form an interfacial metal oxide layer affecting the conductivity of the ReRAM. From the capacitance calculations earlier, we can clearly infer that the Al interfacial layer oxidized during switching. The reactions that would occur at the interface of metal/ Nb2 O5 layer in both the samples are given below.
W ( s ) + O2 ( g ) ↔ WO2 ( s )
(3)
2 2 W ( s ) + O2 ( g ) ↔ WO3 ( s ) 3 3
(4)
4 2 Al(s ) + O2 (g ) ↔ Al2 O3 (s ) 3 3
(5)
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Fig. 3. a) Capacitance density variation of W TE sample depicting linear increase with applied bias that signifies possibility of space charge accumulation b) Capacitance density variation of Al/W TE sample depicting a decrease in capacitance with applied bias that corresponds to a Schottky contact. In both cases, the dimensions of the devices 1 through 5 are 100 μm × 100 μm, which represents the area of the TE.
Fig. 4. Capacitance density comparison of W TE sample and Al/W TE sample in their virgin states, after a single switching cycle, and after 10 switching cycles. Capacitance density of Al/W TE sample is lower after HRS switching compared with its virgin state indicating formation of an oxide layer along the Al/ Nb2 O5 interface.
The tendency of metals to oxidize can be demonstrated by the change in Gibbs Free Energy ࢞G, which also indicates the stability of the metal oxide.
G = H − T S
(6)
where ࢞H is change in enthalpy, T is the temperature and ࢞S is the change in entropy. According to the Second Law of Thermodynamics, a spontaneous reaction occurs when ࢞G is less than zero, which is observed in the case of oxides (Table 1). The entropy change in the reaction is calculated by multiplying the stoichiometric co-efficient of the reactants and the products with the equivalent values of entropy mentioned in Table 2. The enthalpy of the metal and oxygen is zero. Hence, the net enthalpy change is given by enthalpy of formation of the metal oxide After extraction of the entropy change, free energy of these metal oxides is calculated as shown in Table 3 assuming room temperature. The negative value of ࢞G depicts the free energy released upon oxide formation, and since the system tends to reduce its energy, oxide formation will spontaneously occur to make the system thermodynamically stable.
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Fig. 5. a) Butterfly curves of W TE sample depicting low VSET within 0.8 V, VRESET of −1.4 V, forming voltage of 1.8 V and low ICC of 5 mA needed to reset the device. b) Butterfly curves of Al/W TE sample depicting a high VSET of ∼1.4 V, VRESET of −2 V, forming voltage of 3.6 V and higher ICC of 30 mA needed to reset the device.
Fig. 6. a) Endurance plot of W TE sample depicting an endurance cycle lasting 106 cycles with an on/off ratio of ∼100; b) Endurance plot of Al/W TE sample which fails after ∼104 cycles with an on/off ratio of ∼20. A read voltage of 0.1 V is used for measuring the endurance in both the samples. In both cases, the dimensions of the devices 1 through 5 are 100 μm × 100 μm, which represents the area of the TE. Table 1 List of key comparison factors between the W TE sample and Al/W TE which indicates that W TE sample has better overall performance. Parameters
W TE sample
Al/W TE sample
Forming voltage VSET VRESET Set ICC Reset ICC Endurance HRS/LRS
1.8 V 0.8 V −1.45 V 800 μA 5 mA 106 cycles ∼100
3.6 V 1.4 V −2 V 7 mA 30 mA 104 cycles ∼20
Table 2 Entropy values of the reaction. Metal oxide WO2 WO3 Al2 O3
Entropy of Metal −1
−1
45.6 J mol K 32.6 J mol−1 K−1 28.3 J mol−1 K−1
Entropy of Oxygen −1
Entropy of Metal oxide
−1
50.5 J mol−1 K−1 75.9 J mol−1 K−1 50.9 J mol−1 K−1
205 J mol K 205 J mol−1 K−1 205 J mol−1 K−1
Table 3 Free energy values of the metal oxides with the equivalent change in the enthalpy and entropy of the reaction. Metal oxide WO2 WO3 Al2 O3
࢞H (Enthalpy) −1
−590 kJ mol −842.9 kJ mol−1 −1675.70 kJ mol−1
G (Free energy)
࢞S (Entropy) −1
−1
−200.18 J mol K −176.16 J mol−1 K−1 −208.75 J mol−1 K−1
−530.34 kJ mol−1 −790.40 kJ mol−1 −1613.49 kJ mol−1
V.J. Manjunath, A. Rush and A. Barua et al. / Solid State Electronics Letters 1 (2019) 52–57
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Fig. 7. . Ellingham diagram depicting the variation of free energy of metal oxides with temperature; showing a linear trend and similar slope.
When we consider the Gibbs Free Energy for formation of oxides given by Table 3, we can observe that Al is highly susceptible to formation of an oxide layer with a free energy value of −1613.5 kJ mol−1 . The other sample containing only W TE has a relatively lower free energy of oxidation, −530.3 kJ mol−1 for WO2 and −790.4 kJ mol−1 for WO3 respectively, which indicates that W is less likely to form a metal oxide layer whilst the Gibbs free energy of Nb2 O5 is −725.4 kJ mol−1 . The variation of free energy of metal oxides with temperature is depicted in an Ellingham diagram after calculations (6). The slope in Ellingham diagram (Fig. 7) is uniformly linear for all oxides, since the entropy change in the reaction is approximately equivalent to the entropy of oxygen, as the metal reactant and the oxide product are both in solid state. 4. Conclusion In this work, the effect of the Aluminum interfacial layer was investigated on a W/ Nb2 O5 /Ru ReRAM device. The Al/W TE sample has high conductance in its virgin state which implies a leaky behavior. I-V curves of W TE sample has relatively lower VSET , VRESET and is consistent. The capacitance density of the Al/W TE sample has a lower value compared to its virgin sample. It has been experimentally proven that a layer (3.72 nm) of aluminum oxide (Al2 O3) is formed at the Al/Nb2 O5 interface. This is considerably detrimental in the longtime performance of a Nb2 O5 ReRAM as was proven by Endurance testing of the samples. Al/W TE sample fails after 32,0 0 0 cycles whereas W TE sample lasts over 106 cycles with a higher on/off ratio. Free energy of formation of metal oxide explains the mechanism behind formation of an oxide layer. Reactions at the interface of the metal/metal-oxide layer are established. After free energy calculations the Ellingham diagram is plotted which depicts the likelihood of formation of metal oxide layer. Hence, Al is more likely to form an oxide layer as it is thermodynamically more stable than WO2 or WO3 . The Al2 O3 layer created at the interface between Al/Nb2 O5 affects resistive switching characteristics through the presence of higher number of oxygen ions trapped in the interface requiring high current to reset the device. We conclude that an Al interfacial layer may not be an efficient interfacial TE layer in an Nb2 O5 ReRAM. Declaration of Competing Interest None. References [1] Jagan Singh Meena, Simon Min Sze, Umesh Chand, Tseung-Yuen Tseng, Overview of emerging nonvolatile memory technologies, Nanoscale Res. Lett. 9 (1) (2014) 526. [2] J.M. Slaughter, R.W. Dave, M. DeHerrera, M. Durlam, B.N. Engel, J. Janesky, N.D. Rizzo, S. Tehrani, Fundamentals of MRAM technology, J. Supercond. 15 (1) (2002) 19–25. [3] Geoffrey Burr, Matthew J. Breitwisch, Michele Franceschini, Davide Garetto, Kailash Gopalakrishnan, Bryan Jackson, Bülent Kurdi, et al., Phase change memory technology, J. Vac. Sci. Technol. B Nanotechnol. Microelectron. Mater. Process. Meas. Phenom. 28 (2) (2010) 223–262. [4] Ludovic Goux, David Tio Castro, G.A.M. Hurkx, Judit G. Lisoni, Romain Delhougne, Dirk J. Gravesteijn, Karen Attenborough, Dirk J. Wouters, Degradation of the reset switching during endurance testing of a phase-change line cell, IEEE Trans. Electron. Dev. 56 (2) (2009) 354–358. [5] G.R. Fox, F. Chu, T. Davenport, "Current and future ferroelectric nonvolatile memory technology, J. Vac. Sci. Technol. B Microelectron. Nanometer Struct. Process. Meas. Phenom. 19 (5) (2001) 1967–1971. [6] Chih-Yuan Lu, Kuang-Yeu Hsieh, Rich Liu, "Future challenges of flash memory technologies, Microelectron. Eng. 86 (3) (2009) 283–286. [7] Linda Wilson, International Technology Roadmap For Semiconductors (ITRS), Semiconductor Industry Association, 2013. [8] Leon Chua, "Memristor-the missing circuit element, IEEE Trans. Circuit Theory 18 (5) (1971) 507–519. [9] Dmitri B. Strukov, Gregory S. Snider, Duncan R. Stewart, R. Stanley Williams, The missing memristor found, Nature 453 (7191) (2008) 80. [10] R. Waser, Redox-Based resistive switching memories – Nanoionic Mechanisms, prospects, and challenges, Adv. Mater. 2 (2009) 2632–2663. [11] T. Mikolajick, et al., Nonvolatile memory concepts based on resistive switching in inorganic materials, Adv. Eng. Mater. 11 (4) (2009) 235–240. [12] Kyooho Jung, Yongmin Kim, Woong Jung, Hyunsik Im, Baeho Park, Jinpyo Hong, Jiyeong Lee, Jongku Park, Jeon-Kook Lee, Electrically induced conducting nanochannels in an amorphous resistive switching niobium oxide film, Appl. Phys. Lett. 97 (23) (2010) 233509. [13] Andrew J. Rush, Rashmi Jha, NbOx synaptic devices for spike frequency dependent plasticity learning, in: Proceedings of the 75th Annual Device Research Conference (DRC), IEEE, 2017, pp. 1–2. [14] Ligang Gao, Pai-Yu Chen, Shimeng Yu, NbOx based oscillation neuron for neuromorphic computing, Appl. Phys. Lett. 111 (10) (2017) 103503. [15] Keiichi Miyairi, Anomalous dielectric dispersion in tantalum oxide films prepared by RF sputtering, Jpn. J. Appl. Phys. 37 (9R) (1998) 4900. [16] Fu-Chien Chiu, Conduction mechanisms in resistance switching memory devices using transparent boron doped zinc oxide films, Materials (Basel) 7 (11) (2014) 7339–7348. [17] James Kolodzey, Enam Ahmed Chowdhury, Thomas N. Adam, Guohua Qui, I. Rau, Johnson Olufemi Olowolafe, John S. Suehle, Yuan Chen, Electrical conduction and dielectric breakdown in aluminum oxide insulators on silicon, IEEE Trans. Electron Devices 47 (1) (20 0 0) 121–128. [18] Sheela Devi, A.K. Jha, "Structural, dielectric and ferroelectric studies of tungsten substituted barium strontium titanate, Ferroelectrics 402 (1) (2010) 168–174.
Contents lists available at ScienceDirect
Solid State Electronics Letters journal homepage: http://www.keaipublishing.com/en/journals/solid-state-electronics-letters/
Effect of aluminum interfacial layer in a niobium oxide based resistive RAM Vishal Jain Manjunath∗, Andrew Rush, Abhijeet Barua, Rashmi Jha Department of Electrical Engineering and Computer Science, University of Cincinnati, Cincinnati, OH 45221, USA
a r t i c l e
i n f o
Article history: Received 22 January 2019 Revised 16 August 2019 Accepted 20 September 2019
Keywords: Resistive random access memory (ReRAM) Niobium oxide Resistive switching Aluminum Gibbs free energy
a b s t r a c t Resistive RAM (Random Access Memory) has good scalability with high switching speed and low operating voltage making it one of the promising emerging nonvolatile memory technologies. Interfacial layer between the electrode and metal-oxide interface in a Resistive RAM (ReRAM) could either enhance or deteriorate the switching performance of the device. In this study, we investigate the role of aluminum (Al) as an interfacial layer under the top electrode (TE) layer in a niobium oxide (Nb2 O5 ) based ReRAM. We compare the Current-Voltage (I-V), Capacitance-Voltage (C-V) characteristics and endurance of the Nb2 O5 based ReRAM with an Al interfacial layer below the tungsten (W) TE and a control sample without the Al interfacial layer to contrast the performance of each type. Additionally, we connect the tested device behavior with the enthalpy, entropy, and Gibb’s free energy to illustrate that aluminum is an inefficient interfacial layer in the niobium oxide ReRAM. © 2019 KeAi Communications Co., Ltd. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/)
1. Introduction Emerging nonvolatile memories (NVMs) namely magneto resistive RAM (MRAM) and Phase change RAM (PRAM) have a very promising future [1–3], but still have a long way in replacing flash memory as they have major issues such as large current, high cost in MRAM, energy loss during phase change and quenching, and large size in PRAM [4,5]. The desired characteristics of Emerging NVM’s are high endurance and retention compared to Flash memory [6] and scalability beyond 10 nm [7]. An exciting new circuit element, memristor was proposed in early 1970 s by Leon Chua [8]. Research on ReRAM gained significant traction following the physical realization of ReRAM by research group at Hewlett-Packard labs in early 20 0 0 s [9]. ReRAM is a two terminal device in which a switching layer (usually a transition metal oxide layer) is sandwiched between top and bottom electrodes. A filament is formed upon biasing the electrodes which is responsible for conduction leading to a low resistance state (LRS). It can be ruptured by changing applied voltage leading to low conductance or high resistance state (HRS). Apart from filamentary switching, there are other switching methods namely homogenous switching, threshold switching and redox based switching that have been reported in prior research [10,11]. Extensive research has been carried out in determining an ideal switching layer for ReRAM. Nb2 O5 has desirable features such as switching behavior with thickness less than 50 nm [12]. Nb2 O5 film has distinct binding energy spectra due to strong refractory properties compared to other metals when subjected to metrology which makes identifying the composition of Nb2 O5 material post switching an interesting phenomenon [13]. This makes it relatively easier to study the crystal structure of Nb2 O5 and identify crystal defects, surface topology and interface effects using metrology [14]. 2. Experiment Initially, silicon dioxide (SiO2 ) was grown thermally with a thickness of 100 nm on a boron doped silicon wafer (B: Si). An interfacial layer of 3 nm of titanium (Ti) was deposited to promote adhesion. Ruthenium (Ru) of 40 nm thickness was deposited as the bottom ∗
Corresponding author. E-mail address: [email protected] (V.J. Manjunath).
https://doi.org/10.1016/j.ssel.2019.09.001 2589-2088/© 2019 KeAi Communications Co., Ltd. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/)
V.J. Manjunath, A. Rush and A. Barua et al. / Solid State Electronics Letters 1 (2019) 52–57
53
Fig. 1. a) Stack of the device with W TE and b) stack of Al/W TE device (Al interfacial layer capped with W TE). Both the devices have 15 nm Nb2 O5 as the switching layer and Ru BE layer which makes it easier to isolate the effect of Al layer on the performance of the Nb2 O5 ReRAM.
electrode (BE) on the Ti/SiO2/Si substrate by DC sputtering at room temperature. Thereafter, Silicon Nitride (Si3 N4 ) layer with thickness of 50 nm was deposited via reactive RF sputtering of a Si target in a N2 environment, as an insulator for patterning the device area. This Si3 N4 layer was then patterned and etched to expose squares of 100 × 100 μm2 of Ru. Then 15 nm Nb2 O5 was sputtered by RF Magnetron Sputtering under a 1:6 ratio O2 /Ar gas mixture. During the deposition of the Nb2 O5 thin films, the RF power and substrate temperature were maintained at 100 W and 475 °C respectively. Finally, for the first sample, tungsten (W) is DC sputtered as the TE with thickness of about 70 nm. For the second sample, 20 nm Al is deposited as interfacial layer and capped with 50 nm W. Tungsten and Ruthenium are chosen as electrodes since they are inert and exhibit lower rate of oxidation. The solid state characterization of the devices was performed with Keithley 4200 Semiconductor Characterization System. The focus of this study is to explore the performance of Nb2 O5 based ReRAM in depth under the influence of two top electrode configurations. One stack utilizes Tungsten layer as TE and the second configuration is modified with an additional Aluminum interfacial layer under the Tungsten TE. We study the effect of Al interfacial layer on the resistive switching properties of Nb2 O5 layer. We compare Virgin CurrentVoltage (I-V) and Capacitance-Voltage (C-V) characteristics, Capacitance Density of devices, Endurance and Gibbs Free Energy to determine role of an Al interfacial layer. This report sheds some interesting light on the physics behind such interfacial-dependent switching (Fig. 1). 3. Results and discussion 3.1. Virgin state I-V and C-V characteristics The Virgin I-V characteristics of the two types of devices were extracted by applying a positive bias to the TE with the BE grounded. All samples were measured with a cell size of 100 μm × 100 μm. W TE sample shows low leakage current in the order of 16–55 nA (Fig. 2a) whereas it can be clearly seen that the Al/W TE samples are leaky under a Read Voltage of 0.5 V, conducting current in the order of 150–300 μA (Fig. 2b). By observing the C-V of both the samples, it is observed that W TE sample shows almost uniform capacitance density across devices with a variation between 5.9 to 6.2 fF/μm2 (Fig. 3a) whereas Al/W TE sample show a non-uniform variation between 45 and 88 fF/μm2 (Fig. 3b). In RF Sputtered samples, linear increase of capacitance with the applied bias could imply the possible occurrence of a spacecharge accumulation; wherein a continuum of charges is distributed along the W/ Nb2 O5 interface rather than being distinct point charges [15]. However, the capacitance in Al/W TE sample decreases with applied voltage, which indicates a Schottky contact between Al and Nb2 O5 [16]. By comparing the capacitance densities of both the samples in (Fig. 4), we can deduce the capacitance of sample with Al TE in HRS state is lower than its virgin state value. This indicates that there is a possibility of formation of a thin oxide layer along the interface between aluminum and niobium oxide after switching. We can find the value of the capacitance of the layer formed at interface of Al/Nb2 O5 by difference in the values of capacitance of Al/W sample prior and post switching.
1 1 1 = + CS Cvirgin CAl2 O3
(1)
where Cs is the Capacitance of the device after switching, Cvirgin is the capacitance during its virgin state, CAl2 O3 is capacitance of Al2 O3 layer.
CAl2 O3 =
εo εr A t
(2)
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Fig. 2. a) Virgin I-V curves of W TE sample depicting a low initial conductance b) Virgin I-V curves of Al/W TE sample depicting a high initial conductance indicative of a leaky behavior. In both cases, the dimensions of the devices 1 through 5 are 100 μm × 100 μm, which represents the area of the TE.
where ε 0 is absolute permittivity given by 8.85 × 10−12 F/m. ε r is the relative permittivity of Al2 O3 which is ∼10, A is area of the Al/W TE sample given by 100 × 100 μm2 and t is the thickness of the Al2 O3 layer formed [17]. After calculating the capacitance of the oxide layer formed at the Al/ Nb2 O5 interface, we find the thickness of the aluminum oxide layer formed which is found to be 3.72 nm by extracting values from Fig. 4) and substituting in ((1) and (2). This indicates the oxidation of Al to form a thin Al2 O3 layer at Al/Nb2 O5 interface. 3.2. Switching and endurance characteristics of the samples I-V curves of W TE sample has relatively low VSET , VRESET and is consistent (Fig. 5a). The Al/W TE sample had relatively high values of VSET and VRESET of 1.4 V and −2 V respectively with a high reset compliance current of 50 mA shown (Fig. 5b). A thin Aluminum oxide (Al2 O3 ) layer of 3.72 nm which is formed in Al/Nb2 O5 interface as discussed earlier could have a permanent breakdown due to the applied voltage, which is observed in the Al/W TE sample. This requires a high current to reset the device (Fig. 5b). Endurance depicts the efficiency of the device over the long run, which is performed by applying repeated positive and negative pulses to the TE over a long period. Due to the Al2 O3 layer formed at the Al/Nb2 O5 interface, we anticipated the performance of this sample to deteriorate with time which is proven by the endurance of the Al/W TE sample (Fig. 6b) which fails after 32,0 0 0 cycles when compared to W TE sample which lasts up to 106 cycles with an on/off ratio of ∼100 (Fig. 6a). 3.3. Gibbs free energy of formation To fully understand the mechanisms behind the set/reset behavior of the two samples, Gibbs Free Energy analysis is conducted. Nb2 O5 has a work function of 5.2 while W and Al have a work function of 4.5 and 4.3 respectively [18]. While the work function has a role in determining the barrier height and the nature of contact between the metal/Nb2 O5 interface, it fails to explain if the metal layer of the TE is oxidized to form an interfacial metal oxide layer affecting the conductivity of the ReRAM. From the capacitance calculations earlier, we can clearly infer that the Al interfacial layer oxidized during switching. The reactions that would occur at the interface of metal/ Nb2 O5 layer in both the samples are given below.
W ( s ) + O2 ( g ) ↔ WO2 ( s )
(3)
2 2 W ( s ) + O2 ( g ) ↔ WO3 ( s ) 3 3
(4)
4 2 Al(s ) + O2 (g ) ↔ Al2 O3 (s ) 3 3
(5)
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Fig. 3. a) Capacitance density variation of W TE sample depicting linear increase with applied bias that signifies possibility of space charge accumulation b) Capacitance density variation of Al/W TE sample depicting a decrease in capacitance with applied bias that corresponds to a Schottky contact. In both cases, the dimensions of the devices 1 through 5 are 100 μm × 100 μm, which represents the area of the TE.
Fig. 4. Capacitance density comparison of W TE sample and Al/W TE sample in their virgin states, after a single switching cycle, and after 10 switching cycles. Capacitance density of Al/W TE sample is lower after HRS switching compared with its virgin state indicating formation of an oxide layer along the Al/ Nb2 O5 interface.
The tendency of metals to oxidize can be demonstrated by the change in Gibbs Free Energy ࢞G, which also indicates the stability of the metal oxide.
G = H − T S
(6)
where ࢞H is change in enthalpy, T is the temperature and ࢞S is the change in entropy. According to the Second Law of Thermodynamics, a spontaneous reaction occurs when ࢞G is less than zero, which is observed in the case of oxides (Table 1). The entropy change in the reaction is calculated by multiplying the stoichiometric co-efficient of the reactants and the products with the equivalent values of entropy mentioned in Table 2. The enthalpy of the metal and oxygen is zero. Hence, the net enthalpy change is given by enthalpy of formation of the metal oxide After extraction of the entropy change, free energy of these metal oxides is calculated as shown in Table 3 assuming room temperature. The negative value of ࢞G depicts the free energy released upon oxide formation, and since the system tends to reduce its energy, oxide formation will spontaneously occur to make the system thermodynamically stable.
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Fig. 5. a) Butterfly curves of W TE sample depicting low VSET within 0.8 V, VRESET of −1.4 V, forming voltage of 1.8 V and low ICC of 5 mA needed to reset the device. b) Butterfly curves of Al/W TE sample depicting a high VSET of ∼1.4 V, VRESET of −2 V, forming voltage of 3.6 V and higher ICC of 30 mA needed to reset the device.
Fig. 6. a) Endurance plot of W TE sample depicting an endurance cycle lasting 106 cycles with an on/off ratio of ∼100; b) Endurance plot of Al/W TE sample which fails after ∼104 cycles with an on/off ratio of ∼20. A read voltage of 0.1 V is used for measuring the endurance in both the samples. In both cases, the dimensions of the devices 1 through 5 are 100 μm × 100 μm, which represents the area of the TE. Table 1 List of key comparison factors between the W TE sample and Al/W TE which indicates that W TE sample has better overall performance. Parameters
W TE sample
Al/W TE sample
Forming voltage VSET VRESET Set ICC Reset ICC Endurance HRS/LRS
1.8 V 0.8 V −1.45 V 800 μA 5 mA 106 cycles ∼100
3.6 V 1.4 V −2 V 7 mA 30 mA 104 cycles ∼20
Table 2 Entropy values of the reaction. Metal oxide WO2 WO3 Al2 O3
Entropy of Metal −1
−1
45.6 J mol K 32.6 J mol−1 K−1 28.3 J mol−1 K−1
Entropy of Oxygen −1
Entropy of Metal oxide
−1
50.5 J mol−1 K−1 75.9 J mol−1 K−1 50.9 J mol−1 K−1
205 J mol K 205 J mol−1 K−1 205 J mol−1 K−1
Table 3 Free energy values of the metal oxides with the equivalent change in the enthalpy and entropy of the reaction. Metal oxide WO2 WO3 Al2 O3
࢞H (Enthalpy) −1
−590 kJ mol −842.9 kJ mol−1 −1675.70 kJ mol−1
G (Free energy)
࢞S (Entropy) −1
−1
−200.18 J mol K −176.16 J mol−1 K−1 −208.75 J mol−1 K−1
−530.34 kJ mol−1 −790.40 kJ mol−1 −1613.49 kJ mol−1
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Fig. 7. . Ellingham diagram depicting the variation of free energy of metal oxides with temperature; showing a linear trend and similar slope.
When we consider the Gibbs Free Energy for formation of oxides given by Table 3, we can observe that Al is highly susceptible to formation of an oxide layer with a free energy value of −1613.5 kJ mol−1 . The other sample containing only W TE has a relatively lower free energy of oxidation, −530.3 kJ mol−1 for WO2 and −790.4 kJ mol−1 for WO3 respectively, which indicates that W is less likely to form a metal oxide layer whilst the Gibbs free energy of Nb2 O5 is −725.4 kJ mol−1 . The variation of free energy of metal oxides with temperature is depicted in an Ellingham diagram after calculations (6). The slope in Ellingham diagram (Fig. 7) is uniformly linear for all oxides, since the entropy change in the reaction is approximately equivalent to the entropy of oxygen, as the metal reactant and the oxide product are both in solid state. 4. Conclusion In this work, the effect of the Aluminum interfacial layer was investigated on a W/ Nb2 O5 /Ru ReRAM device. The Al/W TE sample has high conductance in its virgin state which implies a leaky behavior. I-V curves of W TE sample has relatively lower VSET , VRESET and is consistent. The capacitance density of the Al/W TE sample has a lower value compared to its virgin sample. It has been experimentally proven that a layer (3.72 nm) of aluminum oxide (Al2 O3) is formed at the Al/Nb2 O5 interface. This is considerably detrimental in the longtime performance of a Nb2 O5 ReRAM as was proven by Endurance testing of the samples. Al/W TE sample fails after 32,0 0 0 cycles whereas W TE sample lasts over 106 cycles with a higher on/off ratio. Free energy of formation of metal oxide explains the mechanism behind formation of an oxide layer. Reactions at the interface of the metal/metal-oxide layer are established. After free energy calculations the Ellingham diagram is plotted which depicts the likelihood of formation of metal oxide layer. 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