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Using Dopants to Tune Oxygen Vacancy Formation in Transition Metal Oxide Resistive Memory Hao Jiang, and Derek A. Stewart ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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ACS Applied Materials & Interfaces

Using Dopants to Tune Oxygen Vacancy Formation in Transition Metal Oxide Resistive Memory Hao Jiang♠,§ and Derek A. Stewart♠* ♠ San Jose Research Center, HGST, a Western Digital Company, San Jose, California, 95119 § Department of Materials Science and Engineering, University of Wisconsin-Madison, Wisconsin, 53706

ABSTRACT: Introducing dopants is an important way to tailor and improve electronic properties of transition metal oxides used as high-k dielectric thin films and resistance switching layers in leading memory technologies, such as dynamic and resistive random access memory (ReRAM). Ta2O5 has recently received increasing interest because Ta2O5-based ReRAM demonstrate high switching speed, long endurance, and low operation voltage. However, advances in optimizing device characteristics with dopants have been hindered by limited and contradictory experiments in this field. We report on a systematic study on how various metal dopants affect oxygen vacancy formation in crystalline and amorphous Ta2O5 from first principles. We find that isoelectronic dopants and weak n-type dopants have little impact on neutral vacancy formation energy. P-type dopants can lower the formation energy significantly by introducing holes into the system. In contrast, n-type dopants have a deleterious effect and actually increase the formation energy for charged oxygen vacancies. Given the similar doping

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trend reported for other binary transition metal oxides, this doping trend should be universally valid for typical binary transition metal oxides. Based on this guideline, we propose that p-type dopants (Al, Hf, Zr, and Ti) can lower the forming/set voltage and improve retention properties of Ta2O5 ReRAM.

KEYWORDS: resistive RAM, tantalum oxide, dopant, oxygen vacancy, formation energy

1. INTRODUCTION The use of dopants to tune the electronic properties of semiconductors was essential for the development of key electronic devices (e.g. diodes and field effect transistors) and the subsequent birth of the vast silicon microelectronics industry. In order to maintain Moore’s law at small scales and also expand the suite of existing memory technologies, numerous groups are now also exploring ways to tailor and improve the electronic properties of oxides by introducing dopants. This has been an active area of research for high-k dielectric oxides used in dynamic random access memory (DRAM)1-2 and an emerging field for oxide-based non-volatile resistive RAM (ReRAM)3-5. Advances in this area have been challenging due to the presence of multiple defects in deposited films, dopant interactions with oxygen vacancies (VO), and difficulty in characterizing transport mechanisms in oxide stacks. Therefore, it is important to develop a clear atomic scale understanding of the impact of dopants on stoichiometric oxides with a particular focus on VO. Tantalum oxide has emerged as a frontrunner for the primary switching layer used in ReRAM6-8. In this study, we perform a systematic first principles study of how various metal dopants affect VO formation in Ta2O5 and discuss how this could impact ReRAM device performance.


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Oxide-based resistive switching devices or memristive devices have demonstrated the necessary fast switching speed and low power consumption required for future ReRAM.3-5 While some oxide ReRAM devices (e.g., metal doped SrTiO3 in low current limits) do exhibit interface-based switching (area scaling of the low resistance state)9-10, the vast majority of oxide based ReRAM devices typically demonstrate a lack of area scaling of the low resistance state that points to a highly localized switching mechanism. The prevailing explanation that has found increasing experimental support is that resistive switching is caused by the connection and disruption of a conductive filament inside the oxide under applied bias5, 9-13. The conductive filament consists of a region of high VO concentration with high electrical conductivity, and the formation/connection (disruption) of the conductive filament switches the device into the low (high) resistance state14-16. The microscopic energy landscape for VO formation and diffusion is critical for determining several key device properties like electroforming, switching voltages, retention, and endurance. Among many proposed memory switching binary oxides like HfOx, TiOx, TaOx, NiOx, and AlOx, tantalum pentoxide (Ta2O5) is a leading candidate material for resistive switching devices. Ta2O5 based ReRAM exhibit superior device characteristics (e.g. high switching speed, long endurance, and low voltage) compared to ReRAM based on other oxides6-8, 17. The robust performance of Ta2O5 based devices has been attributed to the low formation energy and migration barrier of VO in the materials, which facilitate the formation, connection, and disruption of the conductive filament18.

While these devices offer many

superlative properties, it is also important to note that Ta2O5 devices can suffer from problems related to broad distributions of high and low resistance states and challenges in achieving low current operation.


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Introducing dopants into the oxide layer can serve as a powerful tool for device performance optimization. It has been speculated that dopants can not only alter the energy landscape of VO but also modify the electrical conductivity of the filament19. Both of these changes can potentially lead to promising doped oxides with low operating voltages, good retention, high endurance, as well as high ON/OFF ratio. Many studies have been conducted to explore the doping effect on resistive switching device performance19-41. For example, Zhao et al.19-21 from Stanford University and Zhao et al.26 from Anhui University systematically studied the trend of VO formation energy in metal doped TiO2 and HfO2 using first principle calculations. The authors concluded that a larger difference in the number of valence electrons between the dopant and the host atom can cause more significant reduction in the VO formation energy near the dopants19. In addition, they also found that as the formation energy of VO decreases, the forming voltage and ON/OFF current ratio also decrease in metal-doped HfO2 resistive switching devices. Besides metal dopants, the doping effect of non-metal dopants such as H, N, C, and Si in memory switching oxides was also widely studied24, 30-36. Research into using metal dopants to modify the behavior of Ta2O5 ReRAM devices is still a relatively nascent field. To the best of our knowledge, only three references23-25 have reported on the doping effect of metal dopants (Zr, Ti, Al) in Ta2O5 based resistive switching devices. Tidoped Ta2O5 devices showed soft breakdown process with higher ON/OFF ratio and higher resistance compared to un-doped Ta2O5 devices23. This modification is attributed to the formation of thin conductive filaments due to the suppression of VO migration caused by inactive Ti − V − Ti complex. In another study24, doping Ta2O5-x/TaOy sputtered bilayers with Al for ten devices resulted in a lower, more consistent reset voltage as well as a tighter distribution of measured low resistance states. However, the distribution of forming voltages was greater


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than that observed in undoped devices. Most recently, a study25 on Zr-doped Ta2O5 showed decreased forming voltages as the doping concentration increase. The optimal resistance switching performance in terms of ON/OFF ration and stability was found to occur at a low doping concentration. Although there has been limited work on ReRAM devices based on doped Ta2O5, several groups have explored the use of dopants to increase the dielectric constant and reduce the leakage current in Ta2O5 high-k dielectric films used for nanoscale DRAM applications. The results are overall mixed. Prior studies found that Al42-43 or Hf1 doped films typically had higher leakage current than undoped Ta2O5 films. The observed higher conductivity could indicate that Al and Hf promote the formation of VO which leads to defect states and facilitates trap assisted tunneling. However, introducing Ti into Ta2O5 was found to reduce the leakage current1-2. W. S. Lau et al.2 argued that the reduced current was due to Ti forming electrically inactive complexes with existing impurities (C, Si) and oxygen vacancies (e.g. Si − V − Ti ).

It is important to note that if the deposited Ta2O5 film is sub-

stoichiometric, Ti dopants could passivate existing oxygen vacancies in the film and reduce the leakage current1-2. The ability of some dopants to passivate films could be beneficial in ReRAM applications for establishing more consistent high resistance states (HRS) and also improving the ON/OFF window. While these previous ReRAM and high-k dielectric films studies do show that dopants clearly impact the performance of Ta2O5-based resistive switching devices, a deeper atomistic understanding of how dopants affect VO formation in Ta2O5 is required to establish clear chemical trends and identify optimal dopants for robust ReRAM devices. To address this issue, we perform a systematic study of how metal dopants affect VO formation energies in Ta2O5. Formation energies of oxygen vacancies are closely related to the conductive filament forming


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process where VO are generated and driven to aggregate by an applied voltage8, 14-15. It has been speculated that lower VO formation energies can lead to lower formation voltage18-20. The formation energies of VO can also affect the stability of the conductive filament as well as migration of VO near dopants, thereby changing the retention and switching performance of devices. We also explore the doping effect on formation energies of VO in both crystalline and amorphous Ta2O5. In almost all Ta2O5 resistive switching devices, the deposited tantalum oxide layers are amorphous6-8. Amorphous oxides are easier to deposit using standard techniques (e.g., sputtering at low temperatures) and amorphous oxides are generally isotropic so the device to device variability is reduced. In contrast, it is more challenging to obtain crystalline oxides at annealing temperatures compatible with CMOS BEOL processing. In addition, anisotropic microstructure features such as crystal orientation or grain boundary distribution can also lead to large variance in the properties for small devices. Although Ta2O5 ReRAM devices are typically based on amorphous films, recent experimental results indicate that locally nano-crystalline regions do appear in the vicinity of the conductive filament in the amorphous Ta2O58. While the filament forming process initiates from the amorphous phase, resistive switching mainly involves vacancy dynamics in the local environment of the filament. Therefore, a clear understanding of the doping effect on the defect energy landscape in both crystalline and amorphous phases is essential. To address this issue, we first examine oxygen vacancy formation in crystalline and amorphous Ta2O5 in the absence of dopants. We consider the impact of disorder and local bonding on the distribution of vacancy formation energy at different oxygen sites in amorphous Ta2O5. We then perform a systematic study of how the valence state of dopants affects oxygen vacancy formation for both crystalline and amorphous Ta2O5. We compare our results to other


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theoretical and experimental data from other oxides and we also consider how the charge state of the vacancy affects vacancy formation near dopants. Finally, we discuss how this will impact general ReRAM device operation and provide some suggestions for optimal dopants in Ta2O5 based ReRAM devices. 2. EXPERIMENTAL METHODS Density functional (DFT) electronic-structure calculations were done using the Quantum Espresso package44 with ultrasoft pseudopotentials. Calculation parameters used in this work are similar to those in our previous paper on crystalline Ta2O518, and key parameters are reiterated here. Plane wave and charge density energy cutoffs were 110 Ry and 880 Ry, respectively. We used the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) for exchange and correlation. Force convergence threshold of 0.05 eV/Å was used for all structural optimizations. A 3 × 3 × 3 supercell (378 atoms) was employed throughout the study in order to capture the infinitely adaptive structure of λ-Ta2O545 as well as to obtain a reasonable amorphous structure. The Brillouin zone was sampled at the Γ point for the large supercell used in this study. Molecular dynamics simulations were conducted using the Atomistic Toolkit (ATK) developed by Quantum Wise46-48. The empirical potential developed by Trinastic et. al.49 was selected to describe the inter-atomic interactions in Ta2O5 for simulated annealing runs. 3. RESULTS Among several proposed crystalline structures45, 50-53, we choose the λ phase45 to represent crystalline Ta2O5. This is because the λ phase not only shows the lowest energy per formula unit45 but also agrees well with experimentally measured bond length distribution and bandgap54. In λ-Ta2O5, there are three distinct VO sites: 2 coordinated in-plane sites (2f), 3 coordinated inplane sites (3f), and 2 coordinated sites between the Ta-O planes (bwp). The formation energies


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of oxygen vacancies in these sites without dopants have been investigated in our previous study18. In ReRAM devices, sputtered amorphous Ta2O5 layers are typically used6-8 rather than crystalline films. Therefore, it is important to examine whether any conclusions drawn from the crystalline Ta2O5 study would still be valid for the amorphous phase. To test this, we have generated an amorphous Ta2O5 supercell to study how dopants affect vacancy formation at different bonding sites. We created the amorphous Ta2O5 supercell from a 3 × 3 × 3 crystalline supercell using a melt-and-quench

classical molecular dynamics

approach55-56 (see

supplementary materials for further details). The quenched amorphous structure was then further optimized by DFT structural relaxation at fixed volume. The radical distribution function (g(r)) of the amorphous Ta2O5 system is plotted in the sub-plot of Figure 1a. Short range order is maintained in the amorphous system as the first nearest neighbor Ta-O peak (1.94 Å) and O-O peak (2.71 Å) occur at the same positions as in crystalline phase. However, long range order no longer holds as g(r) converges to 1 when radius is larger than 4 Å, confirming the amorphous character of the system. To understand the vacancy formation energy distribution in the amorphous system, forty VO sites are randomly selected out of the total 270 oxygen atom sites in the supercell. Each of the selected VO sites is structurally optimized in neutral state by DFT calculations. The formation energies of neutral VO in amorphous system are calculated as

= − −

(1)

where is the supercell energy of the amorphous system with one VO, is the supercell

energy of the stoichiometric amorphous system (without VO), and is the oxygen chemical potential. The cumulative distribution function of Ef is plotted in Figure 1a. The statistical average formation energy of VO in amorphous system is 4.86 eV with a standard deviation of 0.94 eV. The formation energy falls into a range between 2.37 eV and 6.49 eV, which is much


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wider than the window between 4.99 eV and 5.74 eV in the λ phase18. A similar wide distribution range was also reported in Ref.57 and this can be attributed to the deviation of local atomic structures in amorphous phase from the crystalline phase. In addition, we found a strong correlation between the VO formation energy and the coordination number of neighboring Ta atoms of VO, as shown in Figure 1b. The definition of the coordination index in Figure 1b is discussed in detail in the supplementary materials. Here an intuitive physical interpretation of the index is that positive (negative) coordination index means neighboring Ta atoms of a VO are initially over-coordinated (under-coordinated) with respect to the coordination number of 6 in the crystalline phase45. A correlation factor of -0.68 is found between coordination index and VO formation energies (value of -1 indicates a perfect negative correlation and value of 0 indicates no correlation). Therefore, in general, over-coordinated Ta atoms promote lower vacancy formation energies at neighboring oxygen sites. This finding agrees with the statement in Ref.58 that VO tends to form at the position where the O atom and its first neighboring Ta atom have high coordination number. This implies that in the initial forming process, oxygen vacancies are generated first in locally oxygen-rich regions as the formation energies in these regions are lower. As more vacancies form and the oxide becomes oxygen deficient, the formation energy increases and it becomes thermodynamically more difficult to form new vacancies. We now consider the effect of various dopants on oxygen vacancy formation in crystalline and amorphous Ta2O5. Ten metal dopants including Mg, Al, Ga, Ti, Zr, Hf, Nb, Mo, Re, and Ru are selected to cover dopants with valence electrons ranging from 2 to 8. Our calculations show that substitutional sites are more energetically favorable than interstitial sites for these dopants in λ phase (see supplementary materials for details). Given that all selected dopants prefer to occupy substitutional sites, each single dopant is introduced into the crystal and amorphous supercell by


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replacing one Ta atom in the first nearest neighbor Ta site of VO. Three vacancy sites (2f, 3f, and bwp) in λ-Ta2O5 and three VO sites in amorphous Ta2O5 were selected to investigate how formation energies are modified by dopants. The three VO sites in amorphous Ta2O5 were chosen to represent local coordination environments that are lower, equivalent, and higher than that found in crystalline Ta2O5. The local bonding environments for the three vacancies are shown in the supplemental materials. Although ideally one would analyze the impact of dopants at numerous vacancy sites in the amorphous oxide, given the high computational cost associated with these calculations, the current comparison should be sufficient to suggest key trends. The formation energy of VO is calculated as

= − − + (" #$ + "% + ∆)

(2)

()*( ()*( where is the supercell energy of the doped system with one VO, + * is the supercell

energy of the doped system without VO, is the oxygen chemical potential for oxygen rich conditions, is the charge state of VO, " #$ is the energy of valance band maximum (VBM), "% is the Fermi level measured from VBM, and ∆ is the change of VBM induced by defects. Our calculation shows that ∆ is within 0.02 eV, which is negligible in formation energy calculations18. We assume that deposition takes place under oxygen rich conditions. In this case, oxygen atoms that leave the vacancy sites will go on to form oxygen molecules in the ambient atmosphere. It is important to note that even if the oxygen chemical potential changes (e.g., oxygen poor environment), the trend of vacancy formation energy, Ef, with respect to various dopants will be preserved since changing µO only provides a constant shift to the trend. For our case of a single dopant in a 378 atom supercell, the doping concentration in our calculations is approximately 2×1020 cm-3 or 0.9 at%. This dopant concentration is comparable to those used in some experimental investigations24-25, 37. In general, a low concentration of dopants is preferable


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for promoting oxygen vacancies in ReRAM oxide layers. A high dopant concentration could lead to the formation of separate stoichiometric oxide regions (e.g. large regions of Al2O3 embedded in Ta2O5) and a potential reduction in the overall resident vacancy population.

Figure 1. The formation energies of neutral VO in amorphous Ta2O5. a, the cumulative distribution function of VO formation energies, the inset is the radial distribution function of the amorphous Ta2O5 system; b, formation energies of VO as a function of coordination index of neighboring Ta atoms, as defined in supplementary materials.

Figure 2. The formation energies of neutral VO next to isolated metal dopants in Ta2O5 with respect to the number of valence electrons of dopants. a, in λ-Ta2O5; b, in amorphous Ta2O5; c, comparison of VO formation energies at different oxygen sites are shown for doped λ-


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/amorphous-Ta2O5 in the current study (black squares), doped monoclinic-HfO2 19-20 (red circles) and doped rutile TiO219-20 (blue triangles) by researchers from Stanford University, and doped monoclinic-ZrO238 (green diamonds) by researchers from Peking University. Relative valence state shows the differences in the number of valence electrons between dopants and host atoms. VO formation energies are scaled between 0 and 1 with respect to the maximum and minimum values reported in each study. Substitutional metal dopants are considered in these studies. The wide distribution window at each valence column comes from different VO formation energies calculated for different oxides. The formation energies of neutral VO next to metal dopants in λ-Ta2O5 and amorphous Ta2O5 are plotted with respect to the valence charge state of dopants in Figure 2a and 2b. These figures highlight a strong dependence of VO formation energy on the number of dopant valence electrons. We find that as the difference in the number of electrons between p-type dopants and host atom (Ta in this case) increases, the formation energies of VO near dopants is reduced. Nb has the same number of valence electrons as Ta, and the formation energy of VO in Nb-doped system at all 3 sites is comparable (< 0.1 eV difference) to the formation energies in the undoped system. Ti, Zr, and Hf are weak p-type dopants that have one less valence electron compared to Ta. The formation energies of VO next to these three dopants are comparable and are approximately half of the values of formation energies in the un-doped system (2.18-2.93 eV vs. 4.89-5.53 eV). As the number of valence electrons further decreases, strong p-type dopants such as Mg (2 valence electrons) and Al (3 valence electrons) can significantly decrease the formation energies of VO to zero or even lower. Oxygen ions have two extra electrons compared to an oxygen atom. When strong p-type dopants are added to Ta2O5, they introduce two or more holes that can readily convert an oxygen ion into an oxygen atom. This makes the formation of


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VO energetically favorable. This agrees with our calculations where the formation energies of VO next to dopants are zero or negative. Negative formation energy indicates that the Ta2O5 system with the dopant and neighboring oxygen vacancy is more stable than the Ta2O5 system with only the dopant. Such low formation energies can lead to a resident population of neutral vacancies formed thermally during annealing, even without the application of an electric field. These neutral oxygen vacancies provide sites for trap-assisted tunneling in the HRS and they can also serve as seed sites during the creation of conductive filaments. Weak p-type dopants introduce one hole so they reduce formation energies approximately by half. N-type dopants, Mo and Re, with six and seven valence electrons, respectively, do not cause any appreciable reduction in VO formation energy. When an oxygen atom is removed to create a neutral vacancy, the defect state is still filled with the two electrons that originally formed bonds with the removed oxygen atom. Therefore, since the state is filled, the presence of additional valence electrons from n-type dopants will have little impact on the local electron structure and little impact on the formation energy as demonstrated in Figure 2a and 2b. It is interesting to note that Ru with eight valence electrons [Kr] 4d75s1 causes a significant drop in the neutral vacancy formation energy and does not follow the trend observed in other n-type dopants. This is due to the fact that Ru prefers a +4 oxidation state (stable form RuO2) where only four valence electrons participate in bonding over the +8 oxidation state (highly volatile RuO4)59-60. This would shift Ru into the p-type 4 valence electron dopant column. From Figure 2a, it is clear that Ru doping results in a reduced formation energy comparable to four valence dopants, Hf, Zr, and Ti. This indicates that while the formal valence state can serve as a straightforward rule for dopant selection, exceptions may occur if the local environment of a dopant leads to a different


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effective charge. This indicates that in some cases, the dominant oxidation state could serve as a more predictive measure for the impact of dopants on VO formation energy. Comparing Figure. 2a and 2b, we found that the trend of formation energy dependence on dopant valence charge state in amorphous system is very similar to that in crystalline system. In Figure. 2b, we also find a general trend where the vacancy formation energy is lowest at overcoordinated sites and highest at under-coordinated sites regardless of the dopant considered. Therefore, both the valence configuration rule and coordination effect identified from crystalline and un-doped amorphous system are still valid in doped amorphous Ta2O5 system. This finding provides an important bridge between theoretical investigations on dopant effects in crystalline oxides phases and experimental work on amorphous oxide films in ReRAM. In general, predictions of promising dopants based on simulations of crystalline phase can provide guidance for experimental works on amorphous oxides. This can help expedite material and dopant screening for resistive switching applications because simulations of crystalline phase are significantly faster than amorphous phases. The atomic structure of dopant-vacancy complexes in λ-Ta2O5 and analysis on the structures of complexes in amorphous phase are presented in Supplementary Materials. A similar doping trend has also been reported in the literature for other binary oxides for resistive switching application, such as crystalline HfO219, 26, TiO220 and ZrO238, as shown in Figure 2c. Relative valence states and scaled formation energies are employed here to eliminate differences in VO formation energies among materials, phases, and methods. Regardless of the wide distribution window at each valence state due to various VO sites in the different oxides, it is clear that dopants with the same valence charge state as the host cations, and weak n-type dopants have minimal impact on the formation energy of neutral VO. In contrast, p-type dopants


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with one less valence electron compared to host atoms can lower VO formation energies by approximately 50%. P-type dopants with 2 or 3 less valence electrons can significantly decrease VO formation energies to 0-30% of the pristine formation energy. The similarity in doping effect on VO formation energies among these oxides implies that the identified doping trend is a universal feature for crystalline and amorphous binary transition metal oxides. The physics behind the doping trend can be explained by the number of valence electrons of dopants. P-type dopants promote VO formation because of the weaker metal-oxygen interaction due to the lack of valence electrons. Given that the local electron density of metal-oxygen bonds are relatively unchanged in host-like and n-type doped system due to the equivalent or excessive number of valence electrons, the formation energies of VO do not change significantly. This doping trend can serve as a universal guideline for dopant selection in binary transition metal oxides to tune VO formation energies.

Figure 3. The formation energy of +2 charged oxygen vacancy in doped λ-Ta2O5. In addition to neutral VO, previous studies also found that +2 charged VO are energetically favorable over a wide range of Fermi energy in λ-Ta2O518. Therefore, the formation energy of +2 charged VO at all 3 distinct sites in λ-Ta2O5 are calculated to resolve the general doping trend for


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charged VO. The formation energy of +2 charged VO at oxygen sites neighboring metal dopants in λ-Ta2O5 are plotted with respect to the valence charge state of dopants in Figure 3. Again, we see a strong dependence of VO formation energies on the valence state of the dopants. However, now the peak of VO formation energies shifts to dopants with 7 valence electrons (e.g., Re), instead of 5 (e.g. Nb). For the case of +2 charged oxygen vacancies, this shows that using n-type dopants (Mo, Re) can actually lead to higher formation energies which could potentially reduce ReRAM performance. The general trend of vacancy formation can be explained in terms of the dopant’s contribution to the electronic structure.

N-type dopants will provide additional

electrons to the system and raise the overall charge density. Given that the system now has a resident population of itinerant electrons, it will be much more difficult energetically to form an unfilled +2 charged vacancy. If the additional electrons remain localized near the dopant, this would also lead to stronger metal-oxygen bonding and higher +2 VO formation energies near the n-type dopants. Adding n-type dopants leads to a relatively small drop in formation energies for neutral VO and higher formation energies for +2 charged VO. This suggests that the effect of ntype dopants in facilitating conductive filament formation and improving ReRAM performance should be limited. Instead, p-type dopants are more promising in facilitating electroforming due to their ability to lower formation energies for both neutral and +2 charged VO. It is important to note that we did not calculate formation energies of charged VO in doped amorphous system. However, due to the excellent agreement in the dopant trends on neutral vacancies for both crystalline and amorphous phase, we expect the trend observed for charged oxygen vacancies in crystalline Ta2O5 to hold in the amorphous phase as well. 4. DISCUSSION


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It has been theoretically predicted and experimentally shown that low VO formation energies in oxides can facilitate electroforming in metal oxide ReRAM19-20, 26-27. This is because low VO formation energies can enhance the formation of VO thermally or under bias voltage, making it easier to connect conductive filaments. Following in this spirit, we predict p-type dopants (e.g., Al, Ti, Hf, and Zr) can facilitate electroforming in Ta2O5 ReRAM. To our best knowledge, only three studies on metal doped Ta2O5 ReRAM were reported in the literature, and the results are reviewed here. A very recent experimental study for Zr-doped Ta2O5 provides an important comparison with our predictions. In this study25, Ta2O5 films with different concentration of ZrO2 were grown by atomic layer deposition and further annealed at 800 °C in nitrogen for 10-30 min. Resistance switching in these films shows a clear trend that as Zr doping concentration increases, the forming voltages drops. In the nearly un-doped Ta2O5 films, the forming voltage is rather high at up to 4 V for 30 nm thick films. In contrast, 0.2 at% and 0.9 at% Zr-doped Ta2O5 films with similar thickness show lowered forming voltage of 2.5 eV and 2.0 eV, respectively. In addition the forming voltages in Zr-doped films are comparable to subsequent set/reset switching voltages. These results show excellent agreement with our prediction that p-type dopants can facilitate electroforming. The lower VO formation energies induced by p-type dopants can promote a high population of VO during thermal annealing, and thereby significantly reduce the forming voltage or even make devices electroforming free. The same phenomenon was also found in an earlier experimental results for Ti-doped Ta2O5 ReRAM device. In that study23, Ta2O5 thin films with 5 at% Ti dopant were deposited at 300 oC and annealed at 650 oC for 5 min in ambient oxygen. The initial state of the device was found to be HRS and resistance switching was achieved without the need for a higher voltage forming operation. In another study24, Ta2O5/TaOx bilayer ReRAM devices with ion implanted Al dopants demonstrated either


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comparable or lower forming voltages than undoped Ta2O5 ones. This implies that Al doping can benefit electroforming in some cases which agrees with our prediction. However, the wider distribution of forming voltages could be due to variations in the Al dopant profile created during the implantation process and short thermal anneal (10s).

The bilayer structure could also

promote the diffusion of Al ions to passivate charged oxygen vacancies in the sub-stoichiometric TaOx layer. Additional experimental investigations of how the dopant approach (e.g. implantation, co-sputtering, alloy targets) affect performance of doped Ta2O5 ReRAM devices would be extremely helpful in clarifying this issue. Though limited data exists for metal doped Ta2O5, there are several experimental works on HfO2, ZrO2, and TiO2 that support our conclusions on the general doping trend and low forming voltage induced by p-type dopants. For instance, reduced forming voltages were measured in Al doped HfO2 and Ni doped HfO2 ReRAM devices.19, 27 Al doped ZrO2 ReRAM was also found to show reduced forming voltage compared to undoped ZrO2 devices.38 Similarly, a recent study reported decreased forming voltage in Al doped TiO2 ReRAM with respect to undoped TiO2 ReRAM37. This agrees with the doping trend identified in Figure 2c as Al (valence = 3) and Ni (valence = 2) are p type dopants for these oxides. Doping and the corresponding low VO formation energies can also benefit retention of ReRAM. It has been established that dopants and VO usually form dopant-VO complex23, 26-27. For example, the p-type dopant Hf can be considered as negatively charged substitutional atoms, -. → -.0 + ℎ . The negatively charged substitutional atoms can attract positively charged to form defect complex such as -.0 − − -.0 . Dopant-VO complexes can trap local

VO and connections through these complexes can lead to conductive filaments. Therefore, improvement in conductive filament stability and retention should be expected in doped


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systems27, and enhanced retention has been reported in Ti-doped Ta2O5 ReRAM devices23. The ability of p-type dopants to passivate impurities and vacancies in slightly sub-stoichiometric deposited oxide layers should also lead to more consistent and robust HRS in ReRAM devices. In addition, since p-type dopants promote oxygen vacancy formation, this can also modify the resistance of ReRAM, especially in the HRS. There is strong experimental and theoretical evidence that the leakage current in the HRS is dominated by trap assisted (Poole-Frenkel) tunneling61-62, and populated oxygen vacancies can increase the density of defect states within the gap. However, extremely low or negative formation energies may not always benefit device performance. Negative VO formation energies make it energetically unfavorable to reintroduce an oxygen atom in a vacancy site. This can result in greater energy being required to disrupt the conductive filament. As dopant-VO complexes trap local VO, the overall mobility of VO is also reduced. This may inhibit VO kinetics related to recombination and therefore undermine resistance switching of the devices. In addition, different valence state p-type dopants will also lead to different size dopant-vacancy complexes. For example, 3 valence Al dopants may lead to dopant-vacancy pairs ( Al 45 − V ) or conceivably two vacancy-one dopant cluster ( V − Al 45 − V ), while 4 valence Hf dopants result in two dopant-vacancy clusters, Hf45 − V − Hf45 , as noted earlier. While it is beyond the scope of this current work, the relatively stability of

these complexes and their effective electronic defect state could impact device performance and should be explored in future works. The complex interaction of dopants and vacancies underscores the need to consider trade-offs among various device characteristics, including electroforming, resistance switching, and retention when selecting dopants to tune resistance switching memory.


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We would like to emphasize that in this work, we have focused on the energetics of isolated oxygen vacancies in the presence of dopants due to their importance in the formation and set process for conductive filaments and the lack of research in this area for Ta2O5. For the formed conductive filaments, the energetics of neutral vacancy clusters will also be crucial for the reset process and retention properties of ReRAM devices. Developing a good understanding of the nature of these clusters and their interaction with dopants is important. However, considering the potential complexity of vacancy clusters in terms of size and cluster structure, this topic will require a much larger supercells and computational resources and is best suited for a future comprehensive study. Besides dopant type and concentration, different doping methods have also been shown to affect ReRAM characteristics differently. For example, Al-doped HfO2 ReRAM by co-sputtering Al in the deposition process of HfO2 shows lower set/reset voltage compared to undoped devices19. However, Al-doped HfO2 ReRAM by atomic layer deposition of AlOx in HfO2 shows increased set/rest voltage40-41. The major difference in doped HfO2 between the two methods is the distribution of dopants. Co-sputtering can result in uniform distribution of dopants in the materials while atomic layer deposition can lead to locally concentrated regions. This implies the distribution of dopants in the materials can also be an important factor in tuning ReRAM performance. In summary, developing a better understanding of the effect of doping on ReRAM device characteristics calls for the systematic investigation of several factors including dopant type, concentration, and doping approach. 5. CONCLUSION We investigated doping effect on oxygen vacancy formation energies in both crystalline and amorphous Ta2O5. Substitutional cation dopants have the same valence charge state (e.g. Nb)


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with host atoms, and weak n-type dopants (e.g. Mo, Re) have little effect on the neutral oxygen vacancy formation energy. P-type cation dopants, however, significantly lower the formation energy for neutral and charged oxygen vacancies by introducing holes into the system. N-type dopants can increase the formation energy for charged oxygen vacancy formation. It is important to note that this tendency holds for both crystalline and amorphous Ta2O5. Given that similar doping trends have been observed in other binary crystalline transition metal oxides (e.g., HfO2, TiO2, and ZrO2), we propose that this doping trend is universally valid for crystalline and amorphous binary transition metal oxides. The formation of dopant-vacancy complexes could also lead to tighter distributions of HRS in ReRAM devices. Based on this guideline, we propose p-type dopants such as Al, Hf, Zr, and Ti promote lower forming voltages and improve retention properties of Ta2O5 ReRAM.

Supporting Information. The supporting information contains a discussion of technical details used to generate the amorphous Ta2O5 supercell, the coordination index used to describe bonding in the amorphous oxides, the calculations to determine whether dopants are substitutional or interstitial, and analysis of atomic configuration for various dopant-vacancy complexes. It also contains a table of the coordination index and formation energies for all 40 oxygen vacancies calculated in amorphous Ta2O5. The following files are available free of charge. supporting_info_dopant_effects_on_vacancies_in_tantalum_oxide.pdf (PDF)

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AUTHOR INFORMATION Corresponding Author *Email [email protected] (D. A. S.)


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TABLE OF CONTENTS GRAPHIC


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Using Dopants to Tune Oxygen Vacancy ... - ACS Publications

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