Effect of Al doping on the retention behavior of HfO2 resistive switching memories

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Effect of Al doping on the retention behavior of HfO2 resistive switching memories

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Jacopo Frascaroli a,b,⇑, Flavio Giovanni Volpe a, Stefano Brivio a, Sabina Spiga a,⇑

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a b

Laboratorio MDM, IMM-CNR, via C. Olivetti 2, 20864 Agrate Brianza (MB), Italy Department of Physics, University of Milano, via Celoria 16, Milano, Italy

a r t i c l e

i n f o

Article history: Received 21 February 2015 Received in revised form 27 March 2015 Accepted 7 April 2015 Available online xxxx Keywords: RRAM Hafnium oxide HfO2 Doping Retention

a b s t r a c t The retention behavior of HfO2-based resistive switching memory cells (RRAM) is characterized as a function of Al doping, which was previously reported to be a viable method for the improvement of the switching uniformity. While the low resistance state (LRS) does not exhibit any major variation up to 106 s for all the tested devices, two retention loss mechanisms can be identified for the high resistance state (HRS). The main HRS trend follows a temperature-activated gradual decrease of the resistance, which also depends on the oxide doping concentration. In addition, tail bits of the population distribution show a very fast retention loss process that strongly depends on the doping concentration. Ó 2015 Published by Elsevier B.V.

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1. Introduction

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RRAMs based on metal oxides are considered one of the best candidates for the next generation of memory devices, given their fast switching time, low power consumption and high scalability [1]. However, one of the main concerns is the high variability between repeated cycles [2] and among different cells, mainly related to the intrinsically stochastic processes involved in the formation and partial dissolution of the conductive filaments responsible for the resistance switching [3–5]. Several methodologies were developed to improve the uniformity. As an example, the introduction of an Al buffer layer between the HfO2 and the TiN electrode improved the resistance and switching voltage distributions due to the Al atoms diffusing in the HfO2 layer [6]. In a similar way, the oxide doping with trivalent ions was identified as an effective method to reduce the switching non-uniformities and improve device performances [7–9]. This finding was theoretically associated with a localization of the oxygen vacancies constituting the conductive filament in the HfO2 film due to a reduction of their formation energy in correspondence of the doping sites [10,11]. In this respect, the in situ doping during atomic layer deposition (ALD) film growth brings the advantage of a uniform incorporation of the Al atoms across the film thickness together

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⇑ Corresponding authors. Tel.: +39 039 6035938; fax: +39 039 6881175. E-mail addresses: [email protected] (J. Frascaroli), sabina. [email protected] (S. Spiga).

with the optimal control over the film quality and the deposited film thickness typical of the ALD growth process. However, while doping with Al atoms was found to induce the formation of more stable filaments [12] and improve the LRS retention [13], a deterioration of the HRS retention was observed upon Al doping [9]. In this work, the retention characteristics are reported as a function of baking temperature and doping concentration for times up to 106 s, identifying the major mechanisms responsible for the retention loss of the HRS in order to address the trade-off between memory performance and HRS retention.

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2. Experimental details

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HfO2 and Al:HfO2 films of 5.5 nm were deposited on the sputtered TiN bottom electrode by ALD at 300 °C in a Savannah 200 reactor (Cambridge Nanotech Inc.) by employing (AlCH3)3 (TMA) as the Al source, MeCp2HfMe(OMe)Hf (HfD-04) as Hf precursor, and H2O as oxygen source [14]. For the Al:HfO2 films, the depositions were carried out by alternating Al2O3 and HfO2 cycles with 1:12 and 1:6 ratios, obtaining oxide films with different Al doping concentrations (4% and 7%, respectively). The thickness of the deposited films was inspected by means of Spectroscopic ellipsometry, while the actual doping concentration was evaluated from high resolution X-ray Photoemission spectra of the samples (data not shown). X-ray diffraction analysis didn’t reveal any crystallization feature for the Al-doped HfO2 films, while the un-doped HfO2 film is partially crystalline after deposition [15,16]. Pt top

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http://dx.doi.org/10.1016/j.mee.2015.04.043 0167-9317/Ó 2015 Published by Elsevier B.V.

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electrodes were patterned by standard optical lithography procedures obtaining capacitor structures of 40  40 lm2. Electrical measurements were performed in DC mode using a B1500A parameter analyzer with the bias applied to the top electrode, while the bottom electrode was grounded. In order to obtain fully operational resistance switching properties, an initial electroforming step was performed by current-controlled I–V sweeps up to 1 mA, obtaining the switching of the cells from the initial state to the LRS. After forming, at least 10 consecutive reset/set DC cycles were operated to check for cycling repeatability and ensure cell functionality. For the LRS, the current compliance was set to 1 mA, while the HRS was obtained by ramping the voltage up to 2 V. Given the lower ON/OFF ratio obtained with the highly doped devices with 7% Al concentration, we also tested an HRS achieved by increasing the reset stop voltage until the same HRS resistance of the HfO2 and 4% Al-doped devices was obtained. After setting the memory cells to either the HRS or LRS, the retention characteristics were acquired by baking at least 20 cells at temperatures between room temperature (23 °C) and 180 °C and measuring the memory state by DC voltage sweeps up to 10 mV at defined time intervals.

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3. Results and discussion

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After an initial forming cycle (not shown), all the analyzed devices exhibit bipolar switching behavior, with positive reset and negative set polarities, as shown in Fig. 1. According to previous studies on similar transition metal oxide-based systems, upon trivalent ion doping an enhanced uniformity of repeated cycles is achieved [8–10]. However, a high doping concentration (7% Al:HfO2 sample of Fig. 1) leads to a reduced ON/OFF window for the same reset stop voltage, due in particular to a lower HRS resistance. This observation is in agreement with previous results for high doping concentrations [9,12]. On the contrary, for a moderate Al concentration of 4% no window closure is observed. Temperature-accelerated retention tests reveal that no major modification of the LRS can be found up to 106 s for all the doping concentrations at testing temperatures up to 180 °C, as demonstrated by the stable LRS resistances in Fig. 2. On the contrary, two main mechanisms are responsible for the HRS retention degradation for all the three groups of devices. The HRS retention characteristics of Fig. 2 show a main trend characterized by a gradual resistance lowering as a function of time, while a number of bits exhibit a complete loss of the HRS between two consecutive measures (labeled failure bits in Fig. 2). Most of the previous works on HfO2-based memories focus only on the phenomenon associated with sharp resistance transitions

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Reset 10-3

C u rrent (A)

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Voltage (V) Fig. 1. Bipolar resistive switching cycles for the un-doped and Al-doped devices.

[17], which lie just in the tail of the device distribution. As will be explained in more details later, this effect strongly depends on the doping concentration and is particularly relevant for the 7% Al-doped devices. Since in these devices 50% of the memory cells exhibit sudden transitions after 106 s and it constitutes the predominant retention loss mechanism, these devices will deserve a separate consideration. Testing the retention behavior up to 106 s, we were able to identify also a gradual lowering of the average resistance. This latter phenomenon strongly depends on temperature, as shown in Fig. 3: the HRS decay rate of the un-doped and 4% doped devices increases with baking temperature. From data of Fig. 3, the time in which the cells lose half of their initial resistance value (s1/2) can be extracted. The associated Arrhenius plot reported in Fig. 4 is typical of a temperature-activated process (s1/2 = A exp[Ea/kT]). It is worth noting that half of the resistance value corresponds to a small variation of the resistance window and does not prevent to reliably distinguish the HRS from the LRS. For the un-doped cells, an activation energy (Ea) of 1.5 ± 0.1 eV can be derived from the slope of the fitting line of Fig. 4a, while for the 4% Al-doped samples a lower Ea of 1.1 ± 0.1 eV is extracted (Fig. 4b). These values are effective activation energies obtained at equilibrium, with no applied field, for the process of gradual filament recovery after reset. While in the literature scattered values are reported based on both theoretical and experimental approaches, the obtained Ea are compatible with the energy barrier for oxygen self-diffusion in HfO2 films [18,19]. Since during reset a gap is created rupturing the conductive filament [20], the retention loss can be explained by oxygen vacancies diffusing in the gap region driven mainly by the concentration gradient, gradually restoring the broken filament. Given the lower Ea determined for the 4% Al-doped samples, the faster retention loss is indicative of an enhanced oxygen self-diffusion, which can be associated with a different atomic configuration in the gap. The presence of defects induced by the trivalent Al atoms is likely to enhance the oxygen diffusivity. Additionally, the local stoichiometry of the film, together with its structure and defect density, are important factors in determining the effective energy barrier for the diffusion of oxygen species [21]. Recent calculations for amorphous HfO2 films reported a strong Ea variation as a function of the local atomic structure, reporting an enhanced activation in presence of oxygen vacancies and other defects in the film [22]. In order to evaluate the long-term retention properties, we estimated the 10 years retention goal, defined as the temperature at which, after 10 years, the average HRS resistance reaches a value compatible with the LRS resistance (complete closure of the ON/ OFF window). The time to failure can be extrapolated from the average resistance trend as a function of time, while the retention goal can be evaluated from a graph similar to Fig. 4 in which the time to failure is plotted instead of s1/2. For the un-doped devices, the 10 years retention goal occurs at 170 °C. Despite the reduced activation energy, for the 4% Al-doped devices this goal is only slightly reduced to 164 °C. Considering now the bits in the tail of the device distribution, they exhibit a rapid resistance decrease which leads to a complete failure of the cell. For this reason, they constitute a much more severe issue in terms of data retention. The number of failure bits is strongly dependent on the doping concentration, while no major correlation was found with the baking temperature. In the undoped devices, about 10% of the memory cells exhibit rapid failure, while in the 4% Al-doped devices this number increases to 15–20%. This allows to clearly distinguish between two different HRS decay regimes. Conversely, the highly doped devices with 7% concentration constitute a particular case, since about 50% of the cells lies in the distribution tail (Fig. 5); hence, the failure bits constitute the

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Fig. 2. Comparison between the retention characteristics at 150 °C of the un-doped and Al-doped samples. The full points represent the average value of the cells. In panel (c) the reset stop voltage was increased in order to obtain the same initial HRS as of the cells in panel (a) and (b).

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High resistance (Ω)

Fig. 5. Distribution of the RHRS for the 7% Al:HfO2 devices before and after baking at 150 °C for 106 s. (a) HRS obtained with a reset stop voltage of 2 V. As inset, Weibull distribution of the time to failure for the failure bits. (b) HRS obtained by incrementing the reset stop voltage in order to obtain the same initial resistance value of the un-doped samples.

Fig. 4. Arrhenius plot of the extracted decay time for the HfO2 (a) and 4% Al:HfO2 samples (b). s1/2 represents the time in which the cells lose half of their initial resistance value.

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predominant retention loss mechanism in case of high doping concentrations. In Fig. 5a, the circles belonging to the cumulative distribution of the as programmed cells lie on a single straight line corresponding to a lognormal distribution [3]. After baking the memory cells at

150 °C for 106 s, two different slopes can be identified (red squares). About 50% of the cells exhibits the same initial slope shifted at a lower resistance, which is related to a gradual lowering of the cells resistance. On the other hand, the remaining half lies on a line with a lower slope (region included in the dashed circle) and can be attributed to failure bits. In the 7% Al:HfO2 devices, the time to failure of the failure bits follows the Weibull distribution (inset of Fig. 5a) with a shape parameter k = 0.19 ± 0.01, comparatively lower than values reported for un-doped systems [23]. This means that the failure rate is variable in time and follows a power law / tk-1. Since the failure probability rapidly decreases over time, this provides some direct evidence of the early decay of the analyzed cells. By extrapolating the Weibull plot to 10 years, we expect that 84% of the cells will have completely lost its state by sudden transitions, proving that sudden resistance transitions constitute the main failure mechanism in highly doped samples. A possible explanation for the increase of sharp resistance transitions as a function of doping can be found in the clusterization effect of oxygen vacancies around the Al impurities due to the preferential formation of the oxygen vacancies around the doping

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sites and their positive attraction [11,12]. For high Al concentrations, the reset process only partially disconnects the cluster of vacancies constituting the conductive filament, leaving preferential conduction paths inside the film that can be easily reconstructed by the movement of a limited number of oxygen vacancies from the near proximity. Fig. 5b displays the 7% Al:HfO2 device distribution attained with the same initial HRS of the HfO2 and 4% Al-doped devices. Even if the initial HRS distribution exhibits a flat trend, the final HRS distribution obtained after baking at 150 °C for 106 s shows the same resistance broadening over two decades and the pronounced distribution tail of Fig. 5a, irrespective of the higher initial HRS. Considering only the bits displaying gradual resistance lowering, the 10 years retention goal is projected in this case for a storage temperature of 160 °C, only slightly lower than for the less-doped samples. However, even assuming that the same initial HRS of the less-doped samples might correspond to a similar gap extension, the defects induced by the Al introduction determine a large number of failure tail bits and they still represent the main concern for highly doped HfO2 films.

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4. Conclusion

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We studied the retention behavior of Al-doped HfO2 RRAM devices. While no major modification of the LRS was detected, we distinguished between two concurrent HRS retention loss mechanisms. For the un-doped and 4% Al-doped HfO2 devices, the prevalent effect is a gradual HRS resistance lowering. This was related to a lowering of the activation energy for oxygen species diffusing in the filament gap region, gradually restoring the LRS. Additionally, the bits in the tail of the device distribution show a rapid resistance lowering that completely erases the HRS. Since Al doping induces a sensible increase of the tail bits, for high doping concentrations this effect becomes the predominant retention loss mechanism. Tailoring of the Al concentration allows to optimize the switching performance, avoiding an excessive deterioration of the HRS retention.

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Acknowledgments

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This work was partially supported by Fondazione Cariplo (MORE Project n 2009-2711). The authors thank Dr. Claudia

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Wiemer for useful discussions about X-ray diffraction analysis and Dr. Elena Cianci for the ALD growths.

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