Wear-out and breakdown of Ta2O5/Nb:SrTiO3 stacks
Introduction
The current flowing through a gate oxide layer (or multi-layer) is strongly related with the quality of the stack (density of interface states and bulk traps), and it causes long-term reliability issues, where electrical parameters shift (wear-out) which eventually escalates to oxide breakdown (BD) [1]. The understanding of degradation mechanisms through the insulating layers and their relationship with the reliability are important for the application of new technologies. A common challenge in such studies stems from the back electrode necessary for the measurement. When a semiconductor such as silicon is used as the back electrode, usually an interface layer of SiOX or silicates is formed, regardless of the material and deposition method [2], [3]. Even a SiOX layer as thin as 1 nm can dramatically change the insulating properties, which may cause significant errors in the interpretation of the leakage behavior, sometimes overestimating the performance of the studied materials. Moreover, such interface layers are highly sensitive to small variation in the process conditions; this in turn introduces significant variability between process runs, further impeding reliable understanding of the physics.
In this light, it could be expected that metals may provide an alternative to semiconductors in the role of the back electrode. However, many metals feature a thin native oxide layer. Thus, depositing an oxide on top results in a similar issue as with semiconductors: the metal electrode’s native oxide serves as an insulator in series, which may obstruct an accurate interpretation of the leakage. Native oxides can further result in a surface dipole [4] which further influences the physics and interpretation of the problem. Metals that do not readily form surface oxides, such as the noble metals, pose different challenges of adhesion and wettability when one tries to deposit an oxide on top.
Therefore, a conductive back electrode with a clean, well-defined surface, is important for the study of conduction through oxides. We recently proposed single-crystalline niobium-doped strontium titanate (Nb:SrTiO3, Nb:STO), as a useful back electrode for such studies [5], and implemented it to study the leakage currents through Ta2O5 [6]. This was particularly selected given the high interest triggered by its usage in resistive switching devices [7], [8], [9], [10]. As a corollary of such previous studies, we obtained an experimental picture of the band alignment at the Ta2O5-Nb:STO interface and its leakage mechanisms. Regarding reliability, several studies have reported results on Ta2O5-based devices. Atanassova et al. [11] studied the conduction mechanisms of amorphous and crystalline Ta2O5 films (>25 nm) on Si. In both cases, they report interfacial ultrathin SiO2 layer. They also studied how the effect of oxygen annealing improves the breakdown characteristics, which is reflected in a higher breakdown electric field for higher annealing temperatures. Tsai et al. [12] investigated the time-dependent dielectric breakdown statistics of multilayered electrode MIM capacitors with Ta2O5 as insulator layer. There are also many works that study how the set-reset transition is affected by oxide degradation, but they generally use TaOX as gate oxide [13], [14]. Here we leverage on this well-defined interface to promote our understanding of the reliability of Ta2O5-Nb:STO stacks, focusing on the wear-out phase and the dielectric breakdown event. Electrical breakdown and degradation dynamics of the devices are characterized by current–voltage (IV) sweeps and constant voltage stress (CVS) experiments.
Memristors have been intensively investigated due to their great potential for next generation nonvolatile information storage devices and synaptic devices [15], [16]. As one of the most competitive memristor materials, TaOX has shown prominent performance such as high switching endurance, fast switching speed, and low switching energies [17]. Here we focus on nearly-stoichiometric Ta2O5, meaning that it has very little oxygen deficiency. While oxygen deficiency is useful for resistive switching devices [18], [19], the use of stoichiometric films ensures the consistency of our interpretation and allows benchmarking of this system and our analytical approach. These could later be generalized to various degrees of defects, and additional material systems.
Section snippets
Experimental
The samples are composed of doped (0 0 1) STO substrates (0.01 % and 0.5 % wt. Nb) with TiO2 surface termination, followed by a 4 or 10 nm amorphous Ta2O5 layer reactively sputtered, and 50 nm thick circular Pt top contact. A 300 nm thick Al layer serves as the back contact.
Amorphous tantalum oxide layers were deposited by reactive sputtering (AJA international ATC 2200) at room temperature and 3 mTorr. TaOX films are sputtered from a Ta2O5 target with an Ar:O2 flow of 50:5 standard cubic
Results
Electrical characterization experiments were conducted on Pt/STO samples, finding a clear rectifying behavior, with low currents for V less than 0 but with a strong dependence on the doping level of the substrate. The forward bias characteristic on a semi-log scale shows a linear increase with bias consistent with thermionic emission over the Schottky barrier (not shown here), similar to those reported by Rana et al. [21]. It is relevant to highlight that the influence of the Nb:STO resistivity
Summary
Metal-oxide–semiconductor capacitors composed of a Pt contact, a Ta2O5 insulating layer and a Nb:STO substrate with 0.01 % and 0.5 % wt. doping were used to characterize electrical degradation and breakdown of Ta2O5. The use of STO as a substrate presents a clean way to study Ta2O5, as there is no native oxide. The dynamics of the progressive breakdown was studied, differentiating charge trapping from oxide degradation. Current transients were measured, showing that the breakdown dynamics of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
S. M. Pazos is currently with the Physical Science and Engineering Division, King Abdullah University of Science and Technology. This work has been funded by MINCyT (contracts PICT2013/1210, PICT2016/0579 and PME2015-0196), CONICET (project PIP-11220130100077CO) and UTN.BA (projects PID-UTN EIUTIBA4395TC3, CCUTIBA4764TC, MATUNBA4936 and CCUTNBA5182). Work at the Technion was supported by the Israeli Science Foundation (ISF Grant No. 1351/21). The samples were fabricated in the Technion’s
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