Resistive switching behavior and mechanism of HfOx films with large on/off ratio by structure design

Xianglin Huang(黃香林), Ying Wang(王英), Huixiang Huang(黃慧香), Li Duan(段理), and Tingting Guo(郭婷婷)

School of Materials Science and Engineering,Chang’an University,Xi’an 710061,China

Keywords: HfOx film,resistive switching,structure design,interface modulation

1.Introduction

For decades,great efforts have been devoted to seeking a novel non-volatile memory as the alternative to flash memory since its device size can not be reduced continuously to meet the requirements of portable devices with high storage density,considering the issue of increased leakage current.Recently,much attention has been paid to resistive random access memory (RRAM) as one of the most competitive candidates owing to its merits such as simple device structure, high storage density, and good compatibility with conventional complementary metal-oxide-semiconductor (CMOS) process, in which the resistances can be changed by an external electric field or magnetic field.[1–3]Through the study of a series of switching characteristics, several kinds of switching mechanism for RRAM have been proposed, such as the conductive filament model, space charge limited current effect, Schottky emission,and Poole–Franck effect.[4,5]Although the origin of resistive switching of RRAM device is still under investigation and debate, the significant role of the migration of ions and the redox reactions,which result in formation and rupture of conductive paths in switching layer, on the state of resistances has been widely accepted.[6,7]For binary oxide-based RRAM, migration of oxygen ions leaves an oxygen-deficient region in the film,which is mainly responsible for the resistive switching.[8–10]Research indicates that HfOxmaterials have shown some advantages as a promising switching layer for future nonvolatile memory due to their large band gap with high dielectric constant, easily controlled composition, nice scalability, and good switching performance.[11,12]However,similarly to most switching materials, HfOx-based RRAMs face several challenges in their applications,including random switching behaviors and an ambiguous switching mechanism.A clear understanding of microscopic physical mechanisms is the key point for improving the switching performance.Recently, employing bilayer instead of single-layered switching materials has attracted increasing attention owing to the low power consumption,more stable and controllable resistive switching.[13,14]Methods such as designing bilayer structures or inserting a buffer layer as an oxygen reservoir have been attempted to modulate the defects(mainly oxygen vacancies)in the films or at the interface,[15,16]which would be beneficial to suppressing the random formation of conductive path and to improving the device reliability owing to the modulation of defects.Nevertheless,the switching mechanism and effective method for improving the uniformity and reliability of switching behaviors have been under exploration.

In this work, different device structures of HfOx/Ti and HfOx/TiOxare designed to improve switching performance and understand switching mechanisms of hafniumbased RRAM.The chemical composition and oxygen vacancies of HfOx-based samples are analyzed by x-ray photoelectron spectroscopy (XPS) technology.The switching characteristics in terms of memory window,distribution of switching parameters, and reliable properties are comparatively investigated.The switching mechanisms are illustrated based on the formation and rupture of oxygen vacancy filaments with the variation of barrier height.

2.Experimental details

In our experiment,the n+Si wafers were used as the bottom electrode and soaked in HF solution firstly to remove the oxides at the surface before cleaning, then cleaned by alcohol, acetone, and deionized water sequentially.Then HfOxfilms were deposited on Si substrates by radio-frequency magnetron sputtering using an Hf metal target as the source of Hf atoms with O2as the reactive gas.During the deposition,the Ar/O2was 12/3, the working pressure was 0.3 Pa, and the sputtering power was 70 W.Two different bilayer structures were designed for HfOx-based memory devices.After the fabrication of HfOxfilms, a thin Ti metal layer or TiOxlayer as an oxygen reservoir was deposited on HfOxfilms to form HfOx/Ti and HfOx/TiOx(15 nm/5 nm)structures.Ti layer and TiOxlayer were prepared with the gas flux for Ar of 15 SCCM and Ar/O2of 15/3, respectively.In addition, singlelayer HfOxfilms(20 nm)were also fabricated for comparison.All prepared films were post-annealed at 200°C for 10 min in a nitrogen atmosphere.Finally, the top electrodes of Cu were evaporated with a metal mask to pattern the size to form an MIM structure for measurement.The chemical composition of the films was characterized by XPS technology.The electrical measurements were carried out by an Agilent 4155C semiconductor parameter analyzer using a two-probe method.The voltage was applied on Cu electrode with the Si bottom electrode always grounded.

3.Results and discussion

Figure 1 shows the resistive switching characteristics of HfOx, HfOx/Ti, and HfOx/TiOxsamples.The initial states of the fresh samples are in a high resistance state (HRS) and an electroforming process is required to initiate the switching behavior, as presented in Fig.1(a).Compared with the HfOxsample, the forming voltage is reduced by designing a bilayer structure, which may be due to the modulation of defects at the interface.After the forming process,the reversible bipolar switching behavior can be realized by applying the voltage in a counterclockwise direction, as indicated by arrows in Fig.1(b).The currents increase abruptly at the set voltage, switching the sample from HRS to a low resistance state(LRS).During the reset process, the resistance switches back to HRS.The switching behaviors of HfOx,HfOx/Ti,and HfOx/TiOxsamples are displayed in Figs.1(b) and 1(d), respectively,and the corresponding cross-sectional TEM images are shown in the insets.The enlarged memory window and much better repeatability ofI–Vcurves can be observed for the HfOx/Ti and HfOx/TiOxsamples compared to the HfOxsample.It is worth noting that the lowest current for all samples is observed at non-zero voltage but the negative voltage.As seen from Fig.1(b),the minimum current in the HRS of the HfOxsample is not at 0 V,behaving an open circuit voltage of 0.06 V,which is likely due to the fixed charge or accumulated electrons at the surface of the Si substrate.In addition, the open circuit voltage increases for the bilayer structure,as seen in Figs.1(c) and 1(d).This phenomenon may be due to the displacement of current and related to the bulk heterojunction composite and the capacitance of the sandwich structure,[17,18]and further investigations are needed.

To analyze the chemical bonding states of the prepared samples, the XPS measurements were carried out.All peaks were calibrated by Au 4f peak(83.8 eV).Figure 2(a)shows the XPS spectrum of Hf 4f in the bulk of HfOxfilm,which can be fitted as a double peak of Hf7/2and Hf5/2peaks,corresponding to Hf–O bond.[19]In Fig.2(b),the O 1s peak in the bulk of HfOxfilm can be deconvoluted into two peaks,lattice oxygen with lower binding energy(530.58 eV)and non-lattice oxygen with slightly higher energy(532.28 eV).[20]The concentration of oxygen vacancies is qualitatively estimated by non-lattice oxygen.[21,22]The XPS depth profile of the HfOx/Ti structure is performed to analyze the O 1s peak near the interface of the HfOx/Ti structure, as presented in Fig.2(c).In Fig.2(c),the fitted result of the O 1s peak is similar to that in Fig.2(b).The calculated result of oxygen vacancies near the interface of the HfOx/Ti structure(8.18%)is larger than that in the bulk of HfOxfilm(6.72%), which can be attributed to the absorption of oxygen atoms from the HfOxlayer to Ti layer.The inset of Fig.2(d)shows the XPS spectrum of Ti 2p in TiOxfilm.The Ti 2p spectrum can be fitted as a double peak of Ti 2p3/2and Ti 2p1/2with the binding energies of 458.25 eV and 464.25 eV,in agreement with the reported Ti4+/Ti3+.[23,24]In addition,the oxygen vacancies in TiOxfilm (10.27%) are larger than those in HfOxfilm, as shown in Fig.2(d).It can be inferred that the oxygen vacancies near the interface of the HfOx/TiOxstructure can be modulated as well.

Figure 3(a)shows the distributions of switching voltages for HfOx,HfOx/Ti,and HfOx/TiOxsamples.Compared to the HfOxsample, better distributions ofVsetandVresetcan be obtained for HfOx/Ti and HfOx/TiOxsamples.However,no obvious reduced switching voltages are observed for the bilayer structure, except for the reset voltage of the HfOx/Ti sample,which may be due to the formation of an interfacial layer although more oxygen vacancies are created near the interface.Since defects can be produced by a higher operating current which is the primary source for the device breakdown,[14]the variations of reset currents are also studied, as displayed in the inset of Fig.3(a).As can be seen, decreasing reset current can be observed for bilayer structure samples,particularly for the HfOx/Ti sample,which is beneficial to the decrease of power consumption.Moreover, the HfOx/Ti sample exhibits a much-scattered distribution of reset current.The distributions of resistances in LRS and HRS for all samples are presented in Fig.3(b).Excellent uniformity of resistances can be observed for HfOx/Ti and HfOx/TiOxsamples compared to the HfOxsample.Furthermore, the memory window also improves greatly (＞100) by designing the bilayer structure,which mainly results from the increasing resistance in HRS and may be related to the formation of an interfacial layer in the bilayer structure.Compared to the similar research,[25,26]a larger memory window and smaller switching voltages can be observed,indicating a promising application in the future.

Fig.1.(a) The forming process of the prepared samples and the typical I–V curves of (b) HfOx, (c) HfOx/Ti, and (d) HfOx/TiOx samples.The insets in(b),(c),and(d)show the cross-sectional TEM images of HfOx,HfOx/Ti,and HfOx/TiOx samples,respectively.

Fig.2.The XPS spectrum of(a)Hf 4f and(b)O 1s in HfOx film,(c)O 1s near the interface of the HfOx/Ti structure,and(d)O 1s in TiOx film.The inset of(c)shows the XPS depth profile of the HfOx/Ti structure.The inset of(d)shows the XPS spectrum of Ti 2p in TiOx film.

Fig.3.The distributions of switching parameters of HfOx,HfOx/Ti,and HfOx/TiOx samples: (a)switching voltages,(b)resistances.The inset of(a)shows the corresponding distribution of reset current.

Fig.4.The fitted I–V curves of HfOx,HfOx/Ti,and HfOx/TiOx samples.

To explore the current conduction mechanism of the prepared samples,the correspondingI–Vcurves are replotted and fitted in a double-log scale,as displayed in Figs.4(a)–4(d).For the LRS of the HfOxsample,the current and voltage can be fitted as a straight line with a slope of about 1 in a low-voltage region, as shown in Fig.4(a), exhibiting ohmic conduction behavior.In the high-voltage region, the current is proportional to the square of the voltage,following Child’s law.For HRS, the conductive mechanism is slightly complicated and includes three regions: slope～1 at low voltage, slope～2 at higher voltage,and the current increased abruptly with the increased voltage(slope＞3)due to the formation of conducting paths in the switching layer.Such carrier conduction behaviors can be well understood by the SCLC effect,[4,27,28]and are attributed to the trapping and de-trapping process of defects in the film,which are most likely associated with the oxygen vacancies.Similar fitted results can be observed for HfOx/Ti and the HRS of HfOx/TiOxsamples,as presented in Figs.4(b)and 4(c).However,it is found that the region of abruptly increasing current at high voltage can not be observed for the HRS of the HfOx/Ti sample.Other possible conduction mechanisms have been attempted, and the curve of the HfOx/Ti sample in HRS can be well fitted by Schottky emission, which was an interface-related mechanism, as indicated in Fig.4(d).The higher barrier height may lead to a larger memory window.Moreover,it is observed that the results of the fittedI–Vcurve in LRS for the HfOx/TiOxsamples are also different and the slope is much larger (in Fig.4(c)), which may be ascribed to the fact that partial traps are unfilled and the current density of unfilled ones varies more steeply with the voltage,[29]and the further investigation is required.Overall, the formation and rupture of oxygen vacancy filaments are responsible for the switching behavior of HfOx-based memory.

To further investigate the reliability of the prepared samples,the endurance and retention properties are demonstrated,as presented in Figs.5(a) and 5(b).In Fig.5(a), large fluctuations are observed for the HfOxsample during switching cycles,which may be related to the random formation of oxygen vacancies during fabrication, and the least memory window is only 10.Better cycling characteristics can be observed for HfOx/Ti and HfOx/TiOxsamples,in which the resistances in two states can steadily switch over 120 cycles with the on/off ratio larger than 104,mainly attributed to the higher off states due to the higher barrier heights suppressing the electron leaps.Results show some performance advantages of our samples especially in on/off ratio by comparing with other similar bilayer-structured devices.[30–33]Good retention properties can be observed for all prepared samples, as displayed in Fig.5(b).The currents in LRS and HRS can maintain for 104s without obvious degeneration,showing the non-volatile property of the samples.

Fig.5.(a)Endurance and(b)retention properties of HfOx,HfOx/Ti,and HfOx/TiOx samples.The read voltage was at 1 V.

Fig.6.The physical model of(a)HfOx,(b)HfOx/Ti,and(c)HfOx/TiOx samples.

Based on the above analysis, the physical models of the switching behaviors of HfOx, HfOx/Ti, and HfOx/TiOxsamples are proposed.For HfOxsamples, the movement of oxygen ions towards the anode under positive voltage leads to the creation of oxygen vacancies which form the oxygen vacancy chains connecting the top and bottom electrodes,assisting the transition of electrons and turning the samples into LRS,as indicated in Fig.6(a).By applying a negative voltage,the oxygen vacancy chains recovered with oxygen ions,rupturing the conductive paths,and switching the sample back to HRS.Owing to the random formation of oxygen vacancies in the film,the formation and rupture of filaments during each cycle are stochastic, resulting in the poor uniformity of switching parameters.

For the bilayer structure of HfOx/Ti and HfOx/TiOxsamples, due to the interface modulation, the resistive switching characteristics are mainly dominated by a redox reaction or Joule heat near the interface, and the schematic diagrams are presented in Figs.6(b)and 6(c).Ti and TiOxlayers can act as an oxygen-reservoir layer.During the set process,the oxygen ions move towards the anode under the driving force of a positive electrical field and left oxygen vacancies in the film, especially creating more oxygen vacancies near the interface of the HfOx/Ti sample,as shown in Fig.6(b).The filaments are formed through the accumulation of oxygen vacancies gradually at the set process and then rupture at the weak points near the interface during the reset process.Furthermore,the higher barrier height was achieved during the reset process, which was contributed to the enlarged memory window.For the HfOx/TiOxsample,the different filaments are formed in TiOxand HfOxfilms,[34]the asymmetric conductive filaments lead to the weak points near the interface.On the other hand, due to the different dielectric constants of HfOxand TiOx,the filaments at the interface may be the weaker ones.[35]Therefore,under the negative voltage, larger currents at the weak point lead to the first rupture of filaments by the recovery of oxygen vacancies, accompanied by Joule heat at the same time,as displayed in the reset process of Fig.6(c).In addition, for bilayer structure,it is inferred that the filaments are not completely ruptured during the reset process and the residual filaments can act as lightning rods and promote the re-formation of fixed filaments along the last path during the subsequent set process.Therefore, much uniform and reliable switching performance can be observed for bilayer structure, which is consistent with Huang’ study that the heterostructure and its interface can improve the switching behaviors in multilayer structures.[20]Overall,the results indicate the improvement of switching performance by designing the device structure.

4.Conclusions

In summary, better switching performance including a larger on/off ratio (～104), uniform distribution of switching parameters, and lower reset current is obtained in the bilayer structures, especially in the HfOx/Ti sample due to the modulation of oxygen vacancies near the interface and barrier height.Different conduction mechanisms of Schottky emission are observed for the HfOx/Ti sample, which are closely related to the large memory window and the uniform distribution of the switching parameters.A filamentary model is proposed to clarify the switching behaviors of HfOx-based samples.Since the weak points of the filaments near the interface are firstly ruptured by the recovery of oxygen vacancies, the residual filaments can act as lightning rods to promote the reformation of fixed filaments, and better endurance properties can be observed for bilayer structure.Results indicate that the growth of oxygen vacancy filaments can be better controlled by designing a bilayer structure.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China(Grant No.51802025)and the Natural Science Basic Research Plan in Shaanxi Province of China(Grant No.2020JQ-384).