Unipolar resistive switching behavior in Pt/HfO2/TiN device with inserting ZrO2 layer
and its 1 diode-1 resistor characteristics
Dai-Ying Lee, Tsung-Ling Tsai, and Tseung-Yuen Tseng
Citation: Applied Physics Letters 103, 032905 (2013); doi: 10.1063/1.4816053
View online: http://dx.doi.org/10.1063/1.4816053
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/3?ver=pdfcov Published by the AIP Publishing
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Unipolar resistive switching behavior in Pt/HfO
2/TiN device with inserting
ZrO
2layer and its 1 diode-1 resistor characteristics
Dai-Ying Lee, Tsung-Ling Tsai, and Tseung-Yuen Tsenga)
Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan
(Received 18 April 2013; accepted 29 June 2013; published online 18 July 2013)
Transition of resistive switching (RS) behavior from bipolar to unipolar is observed in Pt/ZrO2/HfO2/TiN device. Due to the lower oxygen vacancy concentration of the HfO2 layer,
formation/rupture of the conducting filament is confined in the HfO2layer. To fulfill one diode and
one resistor (1D1R) structure, the electrical relation between the RS device and diode is investigated. A Pt/InZnO/CoO/Pt/TiN oxide diode is fabricated to provide enough forward current and large forward/reverse current ratio to achieve unipolar RS behavior. The 1D-1R structure with Pt/ZrO2/HfO2/TiN resistive random access memory shows robust retention and nondestructive
readout property at 85C.VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4816053]
Resistive random access memory (RRAM) has attracted significant attention in recent research because of its advan-tages over present flash memory for next generation nonvo-latile memory applications. These advantages include simple structure, low-power consumption, high-speed operation, high-density capacity, and easy-integration processing.1 It uses two distinguishable resistive states (a low resistive ON state and a high resistive OFF state) to store digital data in the memory cell. By applying an appropriate electric field on the cell, two memory states can be continuously switched back and forth.
Detailed resistive switching (RS) mechanisms are still arguable and unclear, where material properties and fabrica-tion processes have considerably influence on them. However, the most acceptable and popular RS mechanism was believed to be the formation/rupture of conducting fila-ment formed by oxygen ions/vacancies migration within the RRAM device.2–6 In our previous reports, the bipolar RS mechanism was proposed that the region where the conduct-ing filament formation (as a field driven process)/rupture (as a current driven process) would be confined near the inter-face between active top electrode and dielectric film.3,4 However, for the unipolar RS (one polarity of electric field is required to ON and OFF), the conducting filament was formed and ruptured in the middle of RS layer, which pro-duced a high temperature during RS process.5Unipolar de-vice is preferred compared to the bipolar one to reduce the circuit complexity and also easily implemented in one diode and one resistor (1D1R) for high density memory arrays.7–9 Moreover, the major benefits of comparing metal oxide p-n diode over the traditional Si base p-n diode are the oxide diode can retain better thermal budget and much process flexibility for its room temperature processing.
In this letter, we report the fabrication and unipolar RS behavior of Pt/HfO2/TiN device by introducing ZrO2layer.
A Pt/InZnO (IZO)/CoO/Pt/TiN oxide diode is integrated the Pt/ZrO2/HfO2/TiN device for 1D1R structure to avoid
mis-reading during operation.8 A Pt/InZnO (IZO)/CoO/Pt/TiN
oxide diode is fabricated with an enough forward current (IF)
and a large reverse current (IR) ratio (F/R ratio) of 7 10 3
at |2| V. The repeatable operation for our 1D1R structure is demonstrated.
In this experiment, various films were deposited on the TiN/Ti/SiO2/Si substrates as described below. 4-nm-thick
HfO2layer on TiN bottom electrode was deposited at 250C
by atomic layer deposition (ALD). The rf magnetron sputter-ing was used to deposit 4-nm-thick ZrO2, and 10-nm-thick
CoO and IZO films for oxide diodes, using base pressure less than 2 105Torr and working pressure 10 mTorr. The 30-nm thick Pt top electrode with 150 lm diameter is evapo-rated on the dielectric layer using metal shadow mask. The thickness of the Pt/ZrO2/HfO2/TiN structure is confirmed by
cross-sectional transmission electron microscopy (TEM) image in Fig.1(a). The schematic structure of the integrated Pt/IZO/CoO/Pt/TiN oxide diode and Pt/ZrO2/HfO2/TiN
RRAM device to fabricate 1D1R structure is shown in Fig.
1(b). Agilent 4155C semiconductor parameter analyzer was used to measure the current-voltage (I-V) curves.
Fig.2(a)exhibits the typical bipolar RS behavior of the Pt/HfO2/TiN device. An abrupt increase of current is observed
when sweeping bias to1 V (VON) with a 5 mA current
com-pliance, and the memory state is switched to ON state. In sequence, by sweeping the voltage to 0.7 V (VOFF) without
any current compliance, the current drops abruptly and the device is switched back to OFF state. On the other hand, the Pt/ZrO2/HfO2/TiN device performs successive unipolar RS
operations over 200 cycles as shown in Fig.2(b), where VON
is about 2 V, which is close to forming voltage (VF). The Pt/
ZrO2/HfO2/TiN device shows more stable unipolar RS cycles,
but Pt/HfO2/TiN device exhibits poor bipolar RS cycles with
around two cycles. In the bipolar RS model, oxygen ions migration acts as the conduction mechanism.3,4After forming process, the conducting filament, composed of oxygen vacan-cies, is formed in series with the TiN-induced interface layer (TiON).10 While performing bipolar RS, the interfacial oxy-gen migration causes the redox reaction within the conducting filament near the interface layer, leading to the formation/rup-ture of the conducting filament.
a)
To investigate the compositional depth profile of the Pt/ ZrO2/HfO2/TiN device, X-ray photoelectron spectroscopy
(XPS) and energy-dispersive X-ray (EDX) are performed. We define the three regions just like the inset of the Fig.
1(a). Regions 1, 2, and 3 are from the top layer of the ZrO2
to the HfO2 layer, respectively. The compositional depth
profile is demonstrated in Fig.3(a) and the EDX profile of the region 2 is shown in Fig.3(b). The mixture of ZrOxand
HfOylayer is observed from the both XPS and EDX signals
at the interface of two oxide layers (region 2).
Figs.3(c)and3(d)show the Hf 4f and O 1s spectra in the different regions. Peak binding energy of Hf 4f7/2 is red shifted from 16.8 eV to17.3 eV due to the stronger charge holding capability of Ti than Hf and also lead to the peak binding energy of O 1 s increase to 531.8 eV in the region 3 of Fig.1(a).11Because of the higher binding energy of oxygen in region 3, the oxygen vacancy concentration induced in the HfO2layer is less than that in the ZrO2layer (Fig.1(c)). After
forming process, the generation and alignment of oxygen vacancies take place to form the conducting filament,12 con-necting between Pt top electrode and TiN bottom electrode. Memory state is switched from original state to ON state as
indicated in Fig. 1(d). Due to the different oxygen vacancy concentration between the HfO2and ZrO2layers, the physical
dimension of the conducting filament of the HfO2layer should
be narrower than that of the ZrO2layer.13The OFF process is
dominated by the competition between the diffusion force by O2 concentration gradient and the drift force by electric field.14,15 Therefore, during OFF process as shown in Fig.
1(e), the O2 ions drifted from the HfO2/TiN interface
layer(TiON)10to reoxidize the narrow filament in the HfO2by
the Joule heating effect to switch the device into high resist-ance state (HRS), while the filament still existed in the ZrO2
layer. For the ON process, the oxygen vacancies are created to form the filament in HfO2layer and the device is switched to
low resistance state (LRS). Due to the effective reduction and localization of the RS region within the HfO2layer and the
for-mation/rupture of the filament within this lower layer, the Pt/ ZrO2/HfO2/TiN device exhibits stable unipolar RS behavior.
15
To fabricate p-n metal oxide diode, CoO with Eg 1.9 eV is employed as a p-type material with IZO (Eg 3.3 eV) as an n-type material.8,16 Figure 4(a) shows the I-V curves of the Pt/IZO/CoO/Pt/TiN oxide diode with 10th, 50th, and 100th cycles without any degradation. The F/R ratio measured at |2| V is about 7 103as shown in the inset of Fig.4(a). Moreover, the forward I-V characteristics of this diode can be described as follows:
I expðqV=nkTÞ; (1) where q is the electron charge, n the ideality factor, k the Boltzmann constant, and T the absolute temperature. The slope of Pt/IZO/CoO/Pt/TiN oxide diode at RT is 8 as shown in the inset of Fig.4(a), where n (estimated to be 5) is similar to previous study for oxide diode.17
For 1D1R integration, a suitable RS device with oxide diode is necessary to perform RS behavior. However, no RS
FIG. 1. (a) Cross sectional TEM image of the Pt/ZrO2/HfO2/TiN resistive
switching memory device. Inset shows the regions 1, 2, and 3 of the ZrO2/
HfO2 layers for compositional
analy-sis, as shown in Fig. 3(b). (b) Schematic diagrams of 1D1R structure by integrating the Pt/IZO/CoO/Pt/TiN oxide diode and the Pt/ZrO2/HfO2/TiN
device. (c), (d), and (e) Hypothetical diagrams show the unipolar RS mecha-nism of the Pt/ZrO2/HfO2/TiN device.
FIG. 2. (a) Typical bipolar I-Vswitching curve of the Pt/HfO2/TiN device.
(b) Unipolar I-V switching characteristics of the Pt/ZrO2/HfO2/TiN device
during continuous RS cycles.
behavior is observed in the Pt/HfO2/TiN device connected
with our oxide diode. It can be explained as (1) most of the voltage drop across the diode under reverse bias (the reverse resistance, RR) during ON process or (2) IR is considerably
smaller than IOFF during OFF process. Oppositely, the ON
state resistance (RON) of the Pt/ZrO2/HfO2/TiN device is
well matched for the forward resistance (RF) of our oxide
diode. Therefore, the Pt/ZrO2/HfO2/TiN device is
success-fully integrated with the Pt/IZO/CoO/Pt/TiN oxide diode.
Typical I-V characteristics and endurance (inset figure) are demonstrated in Fig.4(b), where the unipolar RS behavior is observed under forward bias. However, the operation vol-tages (VONand VOFF) are increased in our 1D1R structure as
compared to the Pt/ZrO2/HfO2/TiN device due to the
resist-ance from the diode element.8During ON process, the for-ward resistance of the oxide diode (RF 9 kX at 0.9 V) is in
series with the resistance of Pt/ZrO2/HfO2/TiN device
(ROFF 9 kX at 0.9 V). When the voltage larger than 1.2 V,
FIG. 4. I-V curves of (a) Pt/IZO/CoO/ Pt/TiN oxide diode and (b) 1D1R structure having 10th, 50th, and 100th cycles. Inset of (a) show F/R ratio (top) and curve fitting of I-V curve (bottom) and (b) show the endurance characteristics read at 1 V. (c) Retention and (d) nondestructive read-out properties of our 1D1R structure measured at RT and 85C.
FIG. 3. (a) XPS composition depth profile of Pt/ZrO2/HfO2/TiN memory
stack. (b) EDX profile of the region 2 of TEM. (c) XPS profiles of Hf 4f elec-trons, and (d) O 1s electrons at differ-ent regions.
RF decreases abruptly, leading to most of the voltage drops
across the Pt/ZrO2/HfO2/TiN device. Finally, 1D1R structure
is switched to ON state, in which VONis higher than that of
the Pt/ZrO2/HfO2/TiN device. During OFF process, VOFFis
increased due to the same basis for the RFin series with the
Pt/ZrO2/HfO2/TiN device when reset current reaches to IOFF.
Retention characteristics of the 1D1R structure measured at room temperature (RT) and 85C are shown in Fig. 4(c). Both ON and OFF state show no data loss for more than 105s even at 85C. Fig.4(d)shows both ON and OFF state ratio almost fixed under 1 V voltage stress at RT and 85C, respectively, without any observable degradation over 104s. Hence, our 1D1R structure possesses robust ON and OFF state for nonvolatile memory applications.
A nonlinearity factor a determines how many word lines (N) and bit lines in implemented memory arrays to ensure that there is sufficient read margin. For higher a, more num-ber of word lines can be fulfilled in a square array.18Fig.5
illustrates the leakage current path in cross-array at read volt-age (Vread) and the equivalent circuit of the worse case (all
unselected cells in ON state). The nonlinearity factor a can be expressed as F=R ratio
1þRON RF
, where F/R ratio is Iforwardat Vread/
Ireverse at Vread, RON is the LRS resistance of the RRAM
cell and RFis the resistance of the diode at forward bias, the
higher F/R ratio is required to obtain a higher a. However, value of the RON/RFcannot be too small. If RONis
consider-ably smaller than RF, RFis in series with RON, leading to a
significantly increase in VOFF at the constant IOFF. When
RON is considerably larger than RF, a is extremely
sup-pressed. According to above reasons, an adequate RON/RF
ra-tio and a high F/R rara-tio are necessary to perform well RS characteristics and can be applied in high density memory arrays, respectively.
The bipolar RS mechanism of the Pt/HfO2/TiN device is
proposed to be related to the formation/rupture of the con-ducting filament near the interface between HfO2and TiN
layers. However, by inserting a ZrO2 layer in the Pt/HfO2/
TiN device, the unipolar RS behavior is revealed that the conducting filament forms/ruptures within the HfO2 layer
due to its narrower filament. For 1D1R structure, a Pt/IZO/ CoO/Pt/TiN oxide diode exhibits a sufficient IFand a good
F/R ratio. Integrating the Pt/HfO2/TiN device into 1D1R
structure, there is no RS characteristics because RRis
signifi-cantly larger than ROFF during ON process. Pt/ZrO2/HfO2/
TiN with 1D1R structure exhibits continuous and stable RS behaviors. For retention characteristics and nondestructive readout properties, both ON and OFF states remain stable at both RT and 85C. A higher F/R ratio and a suitable RON/RF
ratio achieve a large a and cause well RS behaviors, respectively.
This work was supported by the National Science Council, Taiwan, under project NSC 99-2221-E-009-166-MY3.
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FIG. 5. Schematic diagram of leakage current path and the equivalent circuit in a square cross-array. R2(under reverse bias) is much higher than R1and
R3(under forward bias) in 1D1R structure where R2is scaled with (N 1)2.
