Voltage-induced recovery of dielectric breakdown

APPLIED PHYSICS LETTERS 98, 023504 共2011兲

Voltage-induced recovery of dielectric breakdown „high current resistance switching… in HfO2 F. El Kamel,1,a兲 P. Gonon,2 C. Vallée,2 V. Jousseaume,3 and H. Grampeix3 1

Laboratory for the Organization and Properties of Materials, El Manar University, Tunis 1060, Tunisia Microelectronics Technology Laboratory, FJoseph Fourier University, French National Research Center, 17 Avenue des Martyrs, 38054 Grenoble Cedex 9, France 3 CEA-LETI MINATEC, 17 Avenue des Martyrs, 38054 Grenoble Cedex 9, France 2

共Received 31 August 2010; accepted 27 December 2010; published online 13 January 2011兲 Metal/HfO2 / Pt stacks 共where the metal is Au, Ag, Co, Ni, Cr, or In兲 are voltage stressed to induce a high-to-low resistive transition. No current compliance is applied during stressing 共except the 100 mA limit of the voltage source兲. As a consequence very high conductance states are reached after switching, similar to a hard breakdown. Samples conductance after breakdown can reach up to 0.1 S, depending on the metal electrode. Despite the high postbreakdown conductance level, the samples are able to recover an insulating state by further voltage biasing 共“high current resistance switching”兲. © 2011 American Institute of Physics. 关doi:10.1063/1.3541961兴 Insulation degradation of thin oxide films under bias stressing has been the subject of intense research in microelectronics. Research is driven by the need to improve the reliability of metal-oxide-semiconductor gates and metalinsulator-metal 共MIM兲 capacitors. Insulation degradation is broadly classified as “hard breakdown” and “soft breakdownn.”1 The latter includes current spikes 共digital breakdown兲 and the progressive increase in leakage currents which are observed under bias stressing.2 Soft breakdown arises from field-induced defects, for instance Si–O bond breaking in SiO2.1 Recovery of soft breakdown is possible by extended annealing.1 On the contrary, hard breakdown manifests itself by an abrupt and large increase in leakage current. Hard breakdown is thought to be due to the percolation of a large density of defects. Once hard breakdown is reached, damage is usually considered to be irreversible.1,2 However, recovery of hard breakdown was recently observed by Wu and co-workers3 in HfO2 gate stacks 共for which leakage currents after breakdown are as high as several mA at 1 V兲. This result challenges the common idea that hard breakdown is irreversible. Following breakdown, gate metal was detected in silicon,3 suggesting that the gate metal diffuses throughout the oxide thickness to create a metallic filamentary path. According to the same authors, metal diffusion occurs along an oxygen vacancy path which originates from the washing-out of oxygen in the gate dielectric.4 Recovery was observed during post-breakdown voltage scans and was related to the melting of the metallic filament by Joule heating.3 Independently from breakdown studies, in the past voltage-induced resistance changes 共“resistance switching”兲 have been observed in various oxides.5 Resistance switching is now attracting considerable interest for the fabrication of nonvolatile memories.6 As for hard breakdown, resistance switching manifests itself by a large increase in conductivity at a certain voltage. Conductance increase 共resistance switching兲 is ascribed to the creation of an oxygen vacancy path across the oxide. Oxygen vacancies originate from oxide reduction at the anode.6 At an oxygen vacancy site, the rea兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0003-6951/2011/98共2兲/023504/3/$30.00

moval of an oxygen atom leaves a reduced metal atom.7 Once they percolate, reduced metal atoms lead a conducting filamentary path.7 Recovery of an insulating state proceeds by applying a subsequent voltage scan whose role could be to reoxidize the material 共annihilation of the oxygen vacancy path兲 or to melt the conducting filament by Joule heating.6,7 When comparing resistance switching to hard breakdown, phenomena appear to have the same roots 共percolation of oxygen vacancy related defects, leading to a conduction path across the oxide thickness兲. Resistance switching differs from breakdown in the fact that current is intentionally limited during the formation of the conducting path. The current limit 共“current compliance”兲 is intended to avoid irreversible breakdown, i.e., to avoid the formation of a “too strong” conducting filament. In a previous work we studied resistance switching in HfO2.8 A current compliance of 0.1 mA was applied during the SET stage 共high to low resistance transition兲. The low resistive state 共LRS兲 was found to be nonmetallic with a very low thermal activation energy 共hopping conduction through oxygen vacancy defects兲. RESET 共low to high resistance transition兲 was observed by applying a reverse voltage 共bipolar behavior兲 and supposed to be linked to a redox mechanism. In the present work, we investigate resistance switching under the absence of current compliance 共apart the current limit of 100 mA imposed by the voltage source capacity兲, i.e., using conditions which usually lead to a hard breakdown. Under such extreme conditions, it is shown that samples are still able to recover an insulating state. Moreover, the LR state is probed by ac impedance spectroscopy. A metallic LR state is clearly observed 共inductive behavior兲. Finally, conductance of the LR state is evidenced to be strongly dependent on the electrode nature. HfO2 films 共10 nm兲 were grown at 350 ° C by atomic layer deposition using alternate cycles of H2O and HfCl4 precursors 共1 Torr兲 on Pt/Ti/Si wafers. The films crystallized in the monoclinic phase. Different top metal electrodes 共Au, Ag, Co, Ni, Cr, and In兲 were thermally evaporated through a patterned shadow mask to constitute planar MIM capacitors of 1.77 mm2 area. Capacitance 共C兲 and conductance 共G兲 were measured 共test voltage of 0.01 Vrms @ 1 kHz兲 as a

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© 2011 American Institute of Physics

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Bias voltage (V) FIG. 2. 共Color online兲 C-V and G-V characteristics of Ag/ HfO2 / Pt device in the HR and LR states. The HRS-to-LRS transition (open symbols) is accompanied by a remarkable drop of the capacitance toward negative values.

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FIG. 1. 共Color online兲 Conductance-voltage 共G-V兲 plots for six M / HfO2 / Pt 共M = Au, Ag, Co, Ni, Cr, In兲 devices. Measurements were conducted at room temperature with a test voltage of 0.01 Vrms 共@ 1 kHz兲.

function of the dc bias using a HP 4284A precision LCR meter. The dc source bias of the LCR meter is used to SET the devices 共dc voltage sweep speed of 0.05 V/s兲. The dc source is able to output a maximum current of 100 mA. No other current limit was imposed during dc biasing. Positive bias was applied on the bottom electrode so that the top electrode 共M兲 is the cathode. All the electrical measurements were performed in air. Figure 1 shows the conductance—dc bias plots for six M / HfO2 / Pt devices. The advantage of using an ac signal superimposed to the dc bias is that we can probe the conductance 共at 1 kHz兲 when the dc source is at saturation 共100 mA兲. We measure a low ac conductance on all the MIM devices, which varies at zero bias from 0.8 共Au/ HfO2 / Pt兲 to 200 ␮S 共In/ HfO2 / Pt兲, depending on the cathode nature. The conductance abruptly increases at VSET and the devices switch to a LRS. As outlined above, as there is no current limit 共apart the 100 mA saturation limit of the dc source兲, the LRS is the state of the samples after breakdown. Following breakdown, we measure conductances varying in the 10−1 – 10−3 S range, depending on the electrode 共highest for Au and lowest for Cr, see Fig. 1兲. Note that the ac conductance measured in Fig. 1 is not limited by the ac source. Indeed, G = 0.1 S at 0.01 Vrms corresponds to 1 mA, well below the ac source current limit of 20 mA. In a preceding study 共where a current compliance of 0.1 mA was applied兲,8 the ac conductance of the LRS was shown to be constant up to 1 kHz so that the ac conductance measured in Fig. 1 also corresponds to the dc conductance after breakdown. The LR state is maintained when the dc bias is swept back to 0 V. When a bias sweeping of the same polarity 共unipolar resistive switching兲 is applied to the LRS, the conductance falls at VRESET and the films revert to a high resistance state 共HRS兲. It is quite remarkable that despite the very high conductance of the LRS 共G = 0.1 S at Vdc ⬎ 1 V corresponds to Idc ⬎ 100 mA兲 devices are able to recover the

HRS. This agrees with breakdown recovery reported by Wu and co-workers in HfO2 gate stacks.3 The LR state after breakdown is clearly metallic in nature. This is evidenced by the negative capacitance measured in LRS 共Fig. 2兲, indicative of an inductive behavior. The LRS conductance decreases as the dc bias is increased 共Figs. 1 and 2兲. This is due to Joule heating which lowers the conductance of the metallic path. Joule heating is responsible for the RESET 共melting of the conducting filament兲, as already suggested.3 When samples are heated, while in the LR state, the conductivity decreases 共Fig. 3兲, further confirming the metallic nature of the LRS. Above 350 ° C, HRS 共␴ ⬃ 55 nS/ m兲 is recovered in absence of dc bias 共Fig. 3兲. Concomitantly, the capacitance returns to positive value 共dielectric behavior兲. Combination of the ac signal 共0.01 Vrms兲 and the high temperature lead to the melting of the metallic path. The impedance spectroscopy 共Z⬙ − Z⬘兲 was employed to study the electrical properties of the HfO2 device in virgin, low resistance, and recovered, states. In virgin state 关Fig. 4共a兲兴, the impedance spectrum shows a single arc indicating high resistance governed by the bulk property. The device can be thought of an equivalent circuit only composed of parallel connection of resistance and capacitance. However, after the SET process, Fig. 4共b兲 shows the existence of high conducting path in the HfO2 matrix. So, according to the impedance plot, the leaky device can be modeled by a series 1x10 1x10 1x10 1x10 1x10 1x10

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Appl. Phys. Lett. 98, 023504 共2011兲

El Kamel et al.

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Temperature (°C) FIG. 3. 共Color online兲 Temperature dependence of the capacitance 共c兲 and conductivity 共o兲 measured on Ag/ HfO2 / Pt device in LRS. During these experiments a test voltage with an amplitude of 0.01 Vrms 共@ 1 kHz兲 was used.


Appl. Phys. Lett. 98, 023504 共2011兲

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Im Z (MΩ)

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virgin device σg = 12.7 nS/m recovered device σg = 158 nS/m σgb = 52.2 nS/m σep = 4.18 nS/m

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Re Z (Ω) FIG. 4. 共Color online兲 Complex impedance spectra 共Z⬙ − Z⬘兲 of the HfO2 device in virgin and recovered states 共a兲 and in the low resistance state 共b兲.

connection of resistance and inductance. The measured conductivity in the LRS device varies from 1.41⫻ 10−4 to 7.06 ⫻ 10−4 S / m when temperature varies from 25 to 325 ° C. Finally, the recovered device 关Fig. 4共a兲兴 displays three semicircular arcs. The presence of more than one semicircle certainly demonstrates that the conduction arises from the grain bulk 共158 nS/m兲, grain boundaries 共52.2 nS/m兲 and electrode/oxide interface 共4.18 nS/m兲. In addition, this provides clear evidence for the role of grain boundaries, which are the favored locations for the defects migration and accumulation. Based on the literature, it seems that conducting paths (current channels) are formed easier in polycrystalline films, where grain boundaries provide fast diffusion paths for oxygen vacancies as well as for other metallic defects. Migration of the metal gate through the oxide was evidenced by Wu and co-workers. We can assume that the same occurs here, i.e., the metallic path consists of metal atoms which have diffused from the top electrode. The following scenario is proposed. As the dc bias is increased oxygen vacancies are created at the anode 共bottom electrode in our case兲. Once the vacancy path joins the cathode 共top electrode兲 a conducting path is created through the oxide 共“resistance switching”兲. However, as no current compliance is ap-

plied, a large electronic current density is allowed from the cathode to the anode. This in turn leads to an electromigration of the metal cathode through the oxide 共“breakdown”兲. In Fig. 1 it was already noticed that the conductance after breakdown is dependent on the metal cathode. Closer examination 共Fig. 5兲 shows that the LRS conductance could be related to the heat of electrode oxidation 共−⌬Hox兲. Generally, the lower the −⌬Hox is, the denser or thicker oxide interlayer will form. It is though that oxide formation at the M / HfO2 interface more or less prevents metal electromigration. As −⌬Hox is increased, a denser or thicker oxide interlayer is formed, less metal migrates in HfO2 and the conductance after breakdown decreases 共Fig. 5兲. In conclusion, HfO2 thin films were voltage stressed while allowing high current compliance 共100 mA兲. After the high-to-low resistance transition the samples depict a high conductance level 共0.1 S兲. This can be assimilated to a “hard breakdown” or to a “high current resistance switching.” Contrary to expectation, the samples are able to recover an insulating state. Though the current levels are too high for memory applications, the devices can be used for fuse related applications. Regarding the reliability of HfO2 based MIM capacitors, the present results show that postbreakdown recovery of the devices is possible. 1

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